Preliminary forensic engineering study on aggravation of radioactive releases during the Fukushima Daiichi accident

In this unit its blowout panel, provided for over-pressure protection against a main steam pipe breach in the secondary confinement building, was inadvertently activated before a leakage of radioactive effluent from the PCV.

Contents lists available atScienceDirect

Nuclear Engineering and Designjournal homepage:www.elsevier.com/locate/nucengdes

Preliminary forensic engineering study on aggravation of radioactive

releases during the Fukushima Daiichi accident

of core debris has been achieved thanks to effective accident management This should help greatly in theretrieval of the core debris by removing the top head of RPV

Under these circumstances the author has conducted a forensic engineering study (i.e., different fields ofscience working together to collect and integrate independent evidences) to clarify the most likely accidentscenarios of the Fukushima Daiichi accident Through this study the author identified that a large portion of theland contamination observed at the north-west direction is mostly the result of the accident that occurred at Unit2

In this unit its blowout panel, provided for over-pressure protection against a main steam pipe breach in thesecondary confinement building, was inadvertently activated before a leakage of radioactive effluent from thePCV Its activation is believed due to the hydrogen explosion in Unit 1 which occurred on March 12, next day ofthe Fukushima accident initiation By losing the confinement function, the radioactive effluent leaked from the1F2’s PCV and would have been discharged without mitigation This accident scenario explains the series ofleakage events identified in two of the 24 monitoring posts which had been installed by the FukushimaPrefectural Government A series of six large releases were repeated between March 15 and 16, behaving like aperiodical actuation of the safety valves for the PCV Such multiple release events were very likely induced bythe overpressure release of the PCV due to leakage of the dry wellflange This leakage should have been inducedthrough discharging steam and hydrogen due to the activation of Safety and Release Valves (SRV) into thesuppression pool (SP) water Unfortunately the wet well atmosphere must have been that of air, since there was

no nitrogen charge line to the atmosphere of the SP surface water The resultant air-hydrogen mixture resulted in

an“internal hydrogen explosion” which should have deformed the flange The recent robotic inspection insidePCV revealed that a gigantic water splash appears to have occurred at the bottom of the PVC dislodging thegratings installed over the platform

Next to the series of large releases from Unit 2, Unit 1 also induced two large releases on March 12.Fortunately, these releases left more than 3 orders of magnitude less soil contamination compared with the series

of releases from Unit 2 Unit 3 also released a significant amount of radioactive species twice on March 13,resulting in a very small soil contamination The main constituent of the radioactivity is likely radioactive noblegases in this unit

http://dx.doi.org/10.1016/j.nucengdes.2017.08.002

Received 28 February 2017; Received in revised form 31 July 2017; Accepted 7 August 2017

E-mail address: sajig@bd5.so-net.ne

Nuclear Engineering and Design 324 (2017) 315–336

Available online 21 September 2017

0029-5493/ © 2017 The Author Published by Elsevier B.V This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

MARK

1 Introduction

1.1 Global accident sequence

The accident at the Fukushima Daiichi nuclear power station in

Japan is one of the most serious in operating history for a commercial

nuclear power plant (Nuclear Emergency Response Headquarters, June

and September 2011: National Diet of Japan, 2012; (National

Government's) Investigation Committee, 2012; TEPCO's Investigation

Report, 2012; Atomic Energy Society of Japan, 2015; RJIF, 2014) The

author has also published Post Accident Safety Analysis Report of the

Fukushima Accident – Future Direction of Evacuation: Lessons Learned

(Saji, 2013)

The tsunami, which arrived at 15:37 on March 11, 2011 brought the

plants into an unprecedented severe accident status of prolonged SBO

(NUREG/CR-5850, 1994) combined with the“loss of ultimate heat sink

(LUHS)” and further aggravated through the “loss of I & C strumentation and Control) power.” By losing all safety provisions tocontrol the troubled reactors, a series of environmental release eventsfollowed during the active phase of the accident which took placeduring the course of the first week Unfortunately, the amount ofradioactive species and timing of the large environmental releases arestill not known, since all of the environmental monitoring stationssurrounding the site boundary were wiped out due to the“loss of I & Cpower” An overall accident scenario is illustrated inFig 1in the event-tree formalism

(In-1.2 Breakdown of fundamental safety approach– Significance of theFukushima severe accident

Assuming a LOCA as its DBA following the well-accepted hypothesisfrom the 1970s, the fundamental approach for safety assurance of the

Nomenclature

1F1∼1F4 Fukushima Daiichi Unit 1∼4

BFL basement-floor level

DBA/E design basis accident/event

DID defense-in-depth

ECCS emergency core cooling system

FDA Fukushima Daiichi accident

HPCI high pressure coolant injection

I&C Instrumentation and Control

LOCA loss of coolant accident

LPCI low pressure coolant injection

LUHS loss of ultimate heat sink

PCV Primary Containment Vessel

R/B reactor buildingRCIC reactor core isolation cooling systemRHRS residual heat removal systemRPV reactor pressure vesselSBO station blackout (loss of all AC power)S/C suppression chamber

S/P suppression poolSGTS standby gas treatment systemSRV Safety/Release ValveSPDS safety parameter display systemNPP nuclear power plant

PCV Primary Containment VesselTEPCO Tokyo electric power companyT/H Turbine Hall

W/W wet well (suppression pool)

Fig 1 Simplified accident sequence for Fukushima Daiichi Unit 1 to Unit 3 Note: The disaster was triggered by the gigantic earthquake which induced the loss of offsite power Although DGs started as designed, they all failed 51 min later due to the tsunami, which submerged seawater pumps disrupting the discharge of residual heat to the ocean and induced LUHS Resultant degradation of core cooling and loss of the hydrogen removal function induced a hydrogen explosion which devastated the reactor buildings.

Fukushima Daiichi has been deployed In this approach, the DBA-LOCA

should envelope a spectrum of accidents induced by the malfunction of

equipment and human errors This practice was imported by referring

to the many plants with a firm construction and performance record

developed in the Mid- to Eastern US where the seismic events are not

dominant “Design Basis Events (DBE) are defined as conditions of

normal operation, including anticipated operational occurrences,

de-sign basis accidents, external events, and natural phenomena for which

the plant must be designed to ensure functions.“ (NRC 50.49)

Historically “defense-in-depth (DID)” is the basic approach for

prevention of the occurrence of DBEs, as well as for protection from the

further evolution of the events and mitigation of their consequences

The concept of the DID has been fortified by incorporating lessons

learned from accidents and operational experiences One of the

re-spected textbooks on DID is the IAEA’s INSAG-10 which classifies five

levels of defense as extracted below (IAEA, 1996):

Level 1: Prevention of abnormal operation and failures

Level 2: Control of abnormal operation and detection of failures

Level 3: Control of accidents within the design basis

Level 4: Control of severe conditions including prevention of

acci-dent progression and mitigation of the consequences of a severe

accident

Level 5: Mitigation of the radiological consequences of significant

external releases of radioactive materials

In referring to this ranking, thefirst 3 layers related to the DBE were

wiped out upon the tsunami’s arrival Nevertheless there should have

been some means to control severe accidents and/or to mitigate their

consequences in relation to Level 4 of DID This is the main objective of

this report It is to provide feedback as to why the TEPCO’s

com-mendable accident management effort could not sufficiently prevent

gross release of radioactive species to the environment This induced

prolonged off-site counter measures (Level 5) resulting in prolonged

evacuation of the general public1and more than 2000“disaster-related

premature early deaths” due to stresses and deterioration of Quality of

Life of the evacuees, although there were no radiological health death

toll

Due to the difficulties of predicting natural phenomena, occurrence

frequency and prediction of consequences from earthquakes and

tsu-namis, the Fukushima Daiichi did not specify a well defined design base

for tsunamis Instead, TEPCO referred to historical experiences for

tsunamis which indicated OP + 5.7 as the maximum tsunamis,

al-though they raised the level to 6.1 m on 2006 The ground level of the

site was set at 10 m above average sea level (OP Onahama Pail)

However, the OP + 13.1 m tsunami ran up the site andflooded the D/G

and electrical panels, resulted in unprecedented prolonged SBO This

indicates the difficulties in predicting the natural phenomena and

protecting the site against such phenomena The author published a

new approach for incorporation of risk for earthquakes and tsunamis

(Saji, 2014)

1.3 Forensic engineering studies on severe accidents

Recently forensic engineering practices are being compiled

espe-cially in the field of civil engineering (Terwel et al., 2012) Formally

forensic engineering also involves testimony on the findings of these

investigations before a court of law or other judicial forum, when

re-quired A similar approach for the evaluation of the 1979 Three Mile

Island Unit 2 accident was performed by James O Henrie without legal

implication (Henrie, 1989) Unlike a computer simulation of an event,

forensic engineering is the evaluation of the recorded data and damages

as well as examining the surviving components after an event to

determine progressive causes of the event Since many law suits arebeing raised after the Fukushima Daiichi Accident (FDA) againstTEPCO, the Government Investigation Committee reports should haveconsidered such forensic engineering investigation Instead, the testi-mony collected during such investigation is not being disclosed in order

to weigh more on an investigation of the truth of the accident details.For example, nine months after the accident, the Fukushima NuclearAccident Independent Investigation Commission was established by aunanimous resolution of both the House of Representatives and theHouse of Councilors of the National Diet, which represent the people ofJapan (National Diet of Japan, 2012) In the preface of thefinal report,

Dr K Kurokawa explains the objectives of the committee which cludes the following statement.“To investigate what was at the center

in-of this accident, we could not but touch upon the root in-of the problems

of the former regulators and their relationship structure with the erators.” In line with the committee’s indication the new regulatoryframework (i.e., Nuclear Regulatory Agency (NRA)) was established byabolishing the previous Nuclear Safety Commission (NSC) Note thatthe forensic engineering studies are not in the basis for the investigationcommittee’s survey

op-However Dr J.O Henrie’s approach in investigating the TMI dent is impossible to apply in the study on FDA, since the process dataare not available unlike in the case of TMI accident In addition, Dr.Henrie focused on H2 generated by the reaction of zirconium withwater, by stating that“H2generated by radiolysis was probably insig-

acci-nificant” The author has theoretically investigated the root cause ofhydrogen generation during the FDA and found that the“radiation-induced electrolysis” is more likely than radiolysis (Saji, 2016) Itclarifies that the hydrogen generation may not have been from the hightemperature zirconium– steam reaction A short overview of the hy-drogen generation mechanisms is summarized in Annex B Rather it ismore likely due to“radiation-induced electrolysis” occurring with a

“different radiation cell” configuration (Saji, 2017) With this chanism, a large amount of hydrogen is generated before the loss of thereactor water level It also explains the root cause of the hydrogenexplosion that occurred in Unit 4, where all of the fuel assemblies in thereactor core were evacuated to the spent fuel pool for special main-tenance2at the time of the accident

me-Since there is a high possibility that hydrogen generation may ther be through the zirconium-steam reaction nor radiolysis, more ro-bust evidence of the footprints for the accident is necessary In view ofthis, diverse evidence were collected and integrated in this forensicengineering study, although the author does not intend to go into thelegal implications Typical data included:

nei-•Water samples acquired from the T/H Detailed characteristic dataobtained by TEPCO and in which chemical and radiological analyseswere performed by JAEA soon after the accident

•Independent radiation monitoring data obtained around the iphery regions of the Daiichi site by the Fukushima MunicipalGovernment

per-•Remote aerial 131I measurement data taken by USDOE and covered by JAEA in the soil contamination data

re-•Chronological data compiled by TEPCO from March 11–16 Thedata contains detailed operator actions, local indications of in-strumentation and observations of the actual state of components

•TEPCO’s robotic inspection data soon after the accident as well asthe recent robotic inspection data taken inside the PCVs and 1F1torus room

•Identification of “core debris” inside the Unit 3 PCV by introducing asubmarine type robot in July 2017

1 Even after 5 yeas there remain as many as 128,000 evacuees in Fukushima.

2 Replacement of its reactor core shroud which is a stainless steel cylinder surrounding

a nuclear reactor core It helps by directing this cool water towards the reactor core, providing stability to the nuclear reactions.

•Muon imaging data taken and analyzed jointly by several research

institutes

•Hamaoka Unit 1 accident analysis reports

•The author’s study on “radiation-induced electrolysis” (Saji, 2016,

2017)

2 Unique features of the Fukushima Daiichi accident

2.1 Fukushima disaster: un-experienced type of severe accident

The Fukushima accident is an un-experienced type of severe

acci-dent with the loss of the ultimate heat sink (LUHS) and prolonged SBO

which was further aggravated by the loss of I & C power With these

un-experienced accident initiators, the author believes that attention

should not be focused mainly on the current core status based on the

TMI experience Rather it is essential to investigate whether the various

safety provisions, notably containment vessels and ECCS, which are

designed based on DBA-LOCA, have some residual safety functions even

under LUHS and prolonged SBO This accident is very different from

both TMI (small LOCA aggravated by inappropriate operator actions)

and Chernobyl (reactivity accident due to intrinsically unsafe graphite

moderated light water cooled reactor configuration with a positive void

coefficient) The extrapolation of the knowledge on the degraded core

status obtained from these earlier severe accidents may not be

applic-able

Currently a “melt-down” scenario has been hypothesized and is

practically the consensus in Japan Although it has not been

inter-nationally defined as such by IAEA, this appears to mean that the

re-actor fuels have molten together with in-core structural materials

forming a volume of“corium.” Due to high decay heat during the active

phase of the severe accident (as long as 5 days in Daiichi), the corium is

considered to have melted through the lower head of the RPVs and

relocated at the bottom of PCVs

Based on this hypothesis, most of TEPCO’s decommission activities

have been directed towards identifying the location and properties of

corium, which should be a lava-like mixture of nuclear fuel and other

structural materialsfirst observed after the Chernobyl accident In spite

of their effort, TEPCO has not been able to confirm its existence at the

bottom of the PCVs, where the corium should have melted through the

lower heads of the RPVs

The “melt-down” scenario was developed through mechanistic

analyses by running severe accident analysis codes (e.g., MELCORE,

MAAP and SAMPSON) Unfortunately no data from the SPDS (safety

parameter display system) was available to verify the code predictions

during the active phase of the FDA The SPDS was developed based on

the TMI experience and installed in most NPPs around the world It was

also installed at the Fukushima Daiichi, however it was not available for

accident management and little data was recorded due to the loss of the

I & C’s power and failure of its local detectors (Toshiba, 2012)

Nevertheless, since these codes predicted a series of melt through of

the lower reactor vessel heads, current efforts are directed towards

robotic inspection of the bottom of the heads with remote cameras as

well as dose and temperature measurements of the environment inside

of the PCVs In spite of their effort, the observed data does not support

the existence of the large radiation heat sources attributable to corium

In addition recent cosmic ray muon imaging results from 1F2

con-cluded that most of the core debris should still be retained in the lower

head as well as on the Reactor Support Plate These results were

re-leased on July 26, 2016 by a team from TEPCO, IRID, KEK, Tsukuba

University and Tokyo Metropolitan University (TEPCO, 2016) The

re-sults imply that the mechanistic analyses applying the severe accident

analysis codes may not be reliable without plant data when performing

the reverse (unfolding) analysis

However, the “meltdown” hypothesis should be reconsidered in

view of the recent robotic inspection results inside of the PCVs (TEPCO,

2017a) If the core debris has molten through the bottom head of the

RPVs, the injected water should leak down to the PCV Since theflowrate of the injected water is still as much as 3 m3/h (TEPCO, 2017b), thePCVs should have beenfilled with the injected water However, such adifficulty has not been experienced In addition, the temperature of theaccumulated water in the PCV that is at an ambient temperature Thisobservation is not consistent with the melt-through scenario, since thedecay heat, estimated by TEPCO is 70/90/90 kW for Units 1–3, re-spectively (TEPCO, 2017b) Their water temperatures are around 17 °Cwhereas the temperature at the bottom of the RPV is 21.5/27.3/24.8 forUnits 1–3 respectively

More recently, starting July 19, 2017, TEPCO released a series ofresults of investigation of Unit 3 PCV by employing a newly developedsubmarine type (ROV) robot (TEPCO, 2017c) The most remarkablefinding is that they finally identified “core debris” drooping down fromthe bottom head of RPV like a stalactite and then piled up inside of theaccumulated water at the bottom of PCV In their photos it is remark-able to observe that there is no steaming, although the estimated totaldecay heat is as large as 127 kW in 1F3 after 2000 days since the reactortrip If the debris contains some fraction of the core materials, steambubble should be visible This observation suggests that the currentlyobserved debris should consist of solidified molten metals More de-tailed discussion is included in Section5.3

Under this situation, what is needed is an unfolding method toidentify the root cause of the accident based on the footprints left by theaccident This is the basic motivation for writing this paper

2.2 Unit-specific accident progressionDuring the Fukushima Daiichi accident, which occurred at 15:37 onMarch 11, 2011, each unit revealed its own peculiar accident pro-gression as summarized below

•1F1: Very early leakage of radioactive species into the reactorbuilding, as early as 21:51 on March 11, followed by the initialhydrogen explosion, which occurred the next day, March 12 Thismotivated many analysts to assume the very early onset of a zirco-nium-hydrogen reaction

•1F2: the largest leakage of highly contaminated water into thebasement of the Turbine Hall (T/H) This unit is likely responsiblefor the severe soil contamination with radioactive cesium deposited

in a northwest direction from the plant It also contaminated thenearfield with radioactive iodine to the south of the plant

•1F3: the most severe hydrogen explosion among these 4 units:however, there is no trace of radioactive contamination attributable

to this unit Nevertheless un-heating solidified debris has beenidentified recently

•1F4: hydrogen explosion occurred even though its reactor core hadbeen evacuated to the spent fuel pit No serious radioactive releases

Even after 6 years since the accident, the exact accident scenariosthat explain these morphologies remain unknown

2.3 GE’s assessment on performance of Mark I containments at FukushimaDaiichi

Fukushima Daiichi Units 1–4 are BWRs were constructed by portingthe technical bases of Mark I containments (GE Report, 03/19/

im-2011) The following statements are quoted from GE’s summary page,although the author is not necessarily in agreement with their entirestatement

“Early reports regarding Units 1–3 stated plant operators used safetyrelief valves to relieve pressure in the reactor pressure vessel Inaddition, when the fuel rods became uncovered, hydrogen formed inthe core (due to zirconium/water reaction) and was also transportedinto the wet well when the reactor vessel safety relief valves opened.The combination of steam and hydrogenflowing into the wet well

increased the temperature and pressure Since there was no on- or

off-site power available, there was no means of cooling the wet well

water Over time, the pressure in the primary containment rose over

the design pressure To avoid a containment breach, venting became

necessary Upon venting it is believed that vented hydrogen gas

caused the explosions at these units.”

GE noted the following points:

•Concurrent long-term loss of both on-site and off-site power for an

extended period of time is a beyond-design-basis event for the

pri-mary containment on any operating nuclear power plant

•The Mark I containment vessels appeared to have held pressure to

well above the design pressure

•The response of the reactor pressure vessel and reactor in general

agree with the severe accident management studies performed in

the 1980s and early 1990s

However, a similar statement of “a beyond-design-basis event”

made by TEPCO was thoroughly criticized by the Japanese public and

has not exonerated them from blame for the accident The author

in-tends to clarify why the radiation exposure to the public could not have

been evaded, in spite of TEPCO’s commendable accident management

activities under very restrictive situations

2.4 Bypass leak in Unit 2 through drywellflange leakage

GE’s early assessment as quoted above indicates that the Mark I

containment vessels appeared to have held pressure to well above the

design pressure However, the environmental monitoring data which

will be discussed in Chapter 4 indicates that Unit 2’s dry well flange

leaked several times on March 15–16, behaving like a mechanical safety

valve resulting in a series of puff releases (i.e., several short duration

releases due to overpressure in the PCV) which were notfiltered nor

confined for removal of aerosol through precipitation on the walls

Such a release during the severe accident was preventable as stated

in the IAEA’s Safety Guide No NS-G-1.10 (IAEA, 2004) The following

basic requirement is quoted from Section 6.4 of IAEA’s Safety Guide

“For existing plants, the phenomena relating to possible severe

acci-dents and their consequences should be carefully analyzed to identify

design margins and measures for accident management that can be

carried out to prevent or mitigate the consequences of severe

acci-dents.” Additionally, the blowout panel of the R/B was inadvertently

opened which was said to be due to the hydrogen explosion in nearby

Unit 1, which occurred on March 12 With the large hole in the R/B, it

was unable to serve as a secondary confinement function resulting in a

bypass leakage directly from the reactor vessel Note that the SGTS,

which was supposed to filter the effluent leakage before discharging

from the stack, was not available due to SBO

With an unfiltered bypass leakage, the main cause of the

environ-mental contamination to the NWN direction from the plant induced

three orders of magnitude more severe land contaminations brought on

by the events at Units 1 and 3 Apparently the dry wellflange did not

have the necessary safety margins for the FDA

3 Amount of radioactive releases

3.1 Radioactivity leaked into the basement of T/H

The forward analysis to predict the amount released to the

en-vironment is extremely difficult, however an indirect estimation can be

made from the sampling data from the accumulated water as explained

in this section The pure-water/seawater injection was initiated during

the early phase of the accident and continued during the removal of

decay heat Since the injected amount exceeded the necessary amount

for decay heat removal with evaporation, a large portion of the water,

containing radioactive species, accumulated in the basements of the R/

B as well as the T/H The injected water carried the radioactive speciesand leaked out of the RPV into the PCV, untilfinally accumulating intothe basements of T/Hs The continued water injection gradually raisedthe water level, on top of the tsunami water, both of whichflooded thebasements of these buildings When the accumulated water purificationsystems were constructed and began operating around June 17, aportion of the water volume as well as radioactive species were re-moved There is a further complexity to the volume of accumulatedwater, due to an unidentified amount from an underground watersource which seeped into the original amount

In spite of these uncertainties, it should be possible to estimate theleakage of radioactive species from the PCV by knowing the con-centration of radioactive species and total volume of the accumulatedwater on the same sampling date The amounts of release not accountedfor are: (1) direct release from the R/B to the environment, such asthrough the hydrogen explosions, venting events through the operator’sactions, as well as several“spontaneous venting” events; (2) an amount

of radioactivity deposited inside of the R/B but not in the accumulatedwater in the basement of the T/H; (3) sludge of the insoluble species(e.g., refractory species, such as Zr, Mo, Ce, Np, Pu, Cu, U and inter-mediate species such as Sr, Ru, Ba) likely deposited at the bottom of thebasement

On May 22, 2011, TEPCO released “Results of Analysis ofAccumulated Water in the Turbine Building (JAEA)” (TEPCO, 2011a) assummarized in Table 1 The sampling was made under a very highradiationfield, with contact dose rates of nearly 1 Sv/h Although itmay not be representative with just one set of samplings, the data arevery precious

In principle, by knowing the total volume of accumulated water, thetotal amount of radioactive species leaked in this pathway can be es-timated Unfortunately, the levels of the accumulated water only be-came available months after sampling For this reason, the total amount

of accumulated water was assumed to be close to the estimated totalamount of injected water up to the sampling dates as shown inTable 2

By multiplying the values inTables 1 and 2, and adjusting the sults to the shutdown activity on March 11, the total radioactive in-ventory in Bq are summarized inTable 3

This table is converted to a fraction of radioactive species with spect to the shutdown inventory inTable 4

The results indicate that the release fraction (i.e., the ratio of leased/shutdown inventory of radioactive species) into the

re-Table 1 Results of nuclide analysis by JAEA (Bq/ml).

mm.dd

2011.4.13.12.50 2011.4.13.18.34 2011.4.14.21.00

Duration of Counting (s)

I-131(Bq/

ml) 8.04d 3.00E+04 2.00E+06 1.60E+05

Cs-134 2.06a 1.20+05 2.60E+06 1.40E+05 Cs-137 30.0d 1.60+05 2.80E+06 1.60E+05

Sr-89 50.5d 5.70E+01 7.00E+05 8.60E+04 Sr-90 29.12a 2.10E+01 1.40E+05 1.50E+04

Table 2 Estimated volume of accumulated water.

Location 1F1 T/H BFL 1F2 T/H BFL 1F3 T/H BFL

Total injection (m 3 ) 2633 9824 4225

accumulated water ranges from 14% for 1F2 to a fraction of a percent

for“volatile” species (i.e., I and Cs) in 1F1 and 1F3 Those for

“inter-mediate” species (i.e., Ba, La, and Sr) are even smaller in magnitude

This suggest that the major pathway of radioactive release from the fuel

cladding should be from a puncture in the fuel cladding due to high

temperature ballooning failure The ballooning failure occurs through

the softening of fuel cladding at a high temperature combined with

overpressure induced with gaseous and volatile species

The large release fraction for 1F2 indicates that the release passage

of 1F2 may be different from both 1F1 and 1F3 The difference suggests

an involvement of the S/C where the steam, containing the radioactive

species, is discharged from the RPV into the S/C pools The radioactive

species are decontaminated in the pool water by a factor of two orders

of magnitude of reduction In addition, the large explosive sound

oc-curred near the S/C between 6:00∼6:10 on March 15 may indicate

another“internal hydrogen explosion” event in the S/C, which resulted

in the leaking of the highly contaminated suppression pool water into

the R/B through degradation of theflange of PCV Further discussion

will be made in Section5.2

Also, the behavior of the release fraction of Cs-134 and Cs-137 are

strange since the former depends strongly on the burn-up through a fuel

management sequence whereas the latter is relatively independent In

Fukushima Daiichi, the reactor cores contain fuels with a variety of fuel

management histories, some of which date back to more than 10 years

There is a high possibility that radioactive releases are more from the

aged fuel assemblies, many of which may have punctured due to their

higher internalfission gas pressure It is necessary to calculate the

ac-tivation data of each assembly, instead of a core average, when the

status of an individual fuel assembly would be confirmed years from

now

The release fractions indicate that a large portion of volatile species

(as well as rare gases) have been released from the core into the reactor

water at least in 1F2 Although the estimation of radioactive species

into the accumulated water may become a source term for the marine

environment release, it does not provide any source term information

for the atmospheric releases that produced widespread land

con-tamination around the Fukushima Daiichi

3.2 Aerial releases of radioactive cesium

The Japanese regulatory body was expecting that the environmental

radiation monitoring data could have been transmitted from Fukushima

Daiichi to the Emergency Response Center to assist in performing quickdispersion calculations by SPEEDI for accident management However,due to the SBO’s creating further adverse conditions which were in-duced by the“loss of I & C power”, neither the environmental radiationmonitoring nor plant data from process computers were available.Due to this lack of info related to the event, one of the most preciousdata was obtained via remote monitoring by the US DOE/NNEA, wherethefirst survey data was disclosed on March 22 (US DOE/NNEA, 2011).More detailed data were collected byfixed wing and helicopter surveyflights at altitudes ranging from 150 to 700 meters The cesium de-position was determined from aerial and ground-based measurements.Using US technology, the Ministry of Culture, Education and Sports(MEXT) produced wider land contamination maps, which were verifiedwith detailed in-situ soil measurements.Table 5has been updated byusing the new set of data for the integration of the total amount of137Csdeposited on the land The results show approximately a factor of 0.4less than the author’s previous results (Saji, 2013) The difference wastraced down to the dose range of Zone VIII (previously Zone V), wherethe upper bound was reduced by an order of magnitude Due to theuncertainties in the remote monitoring soon after the accident with thestrong gamma spectra from radioactive iodine isotopes, it is likely thatthe DOE assumed a large safety margin in the most highly contaminatedzone The in-situ soil sampling data now revealed that the highest doserate in the most contaminated region was 3000 kBq/m2

In updatingTable 5, the recently obtained wider (> 100 km fromthe Fukushima Daiichi) land contamination map3was used to includeabove 10 kBq/m2zones, although some decrease in dose rate due toweathering (approximately 40%/y) is anticipated Because of thisconcern, the earlier August 30, 2012 data4were used for the regionwithin 100 km

With detailed land contamination density data now available, it was

a straightforward process to estimate the total amount of the cesiumsimply by calculating the areas of each zone The estimated gross landcontamination of approximately 3PBq in137Cs indicates that the en-vironmental release is much less than 75–86 PB (1996 estimation) re-ported for the Chernobyl accident (UNSCEAR, 2000) The estimatedenvironmental release fraction of137Cs, which is the ratio of the en-vironmental releases to the shutdown core inventory, amounts to only1.5% of the1F1 core inventory or 1.2% for 1F2 and 1F3 The best es-timate for the environmental release is 1–2% of the shutdown coreinventory of137Cs, considering uncertainties in the land contaminationdensity maps

This value should not be confused with a fuel failure rate since thecontainment system had a large decontamination factor The actual fuelfailure can be an order or so larger in the reactor vessel These valuesindicate the following land contamination characteristics:

(1) The accident resulted in a severe land contamination (zone VII and

Table 3

Total amount of radioactive species in accumulated water.

Species Half Life 1F1 T/H BFL 1F2 T/H BFL 1F3 T/H BFL

Cs-134 2.06a 3.16E+14 2.55E+16 5.92E+14

Cs-137 30.0d 4.21E+14 2.75E+16 6.76E+14

Ba-140 12.7d 8.93E+12 1.43E+16 3.84E+14

La-140 40.3 h 6.52E+17 1.78E+21 5.93E+19

Sr-90 29.12a 5.53E+10 3.69E+14 3.95E+13

Table 4

Ratio of released/shutdown inventories.

Species Half Life 1F1 T/H BFL 1F2 T/H BFL 1F3 T/H BFL

Cs-134 2.06a 3.57E−04 1.67E−02 3.88E−04

Cs-137 30.0d 3.76E−03 1.42E−01 3.49E−03

Ba-140 12.7d 3.41E−06 3.16E−03 8.50E−05

Sr-90 29.12a 5.57E−07 2.15E−03 2.31E−04

Table 5 Cs-137 deposition and areas.

Zone Cs-137 deposition Area (km 2 ) Cs-137(PBq)

VIII) covering an area of 380km2, where rehabilitation will be

prohibitive without proving a substantial reduction in the dose rate

of greater than a factor of 10

(2) The total area of high concentration (> 100 kBq/m2) covers an area

of up to 2900 km2, although most of the contaminated area is

lo-cated in the Abukuma Mountain Chains

4 Environmental monitoring of radioactive species

4.1 Analysis of environmental monitoring data

One of the unique features of the Fukushima Daiichi accident is in

the simultaneous evolution of its accident sequences, which resulted in

the environmental radiation releases from one unit after another The

resultant releases left significant land contamination especially to the

northwest of the damaged plants In order to appreciate the scientific

implication of the areas marked with the footprints of land

con-tamination, it is necessary to clarify whether they were left with one

dominating release event or the superposition of multiple releases from

different units at different times The land contamination maps indicate

that the radioactive plumes passed over the area thereby contaminating

the soil Therefore people who resided in the area at the time of the

plume passage may have inhaled the radioactive effluent, especially in

the form of131I and133I

Although an atmospheric dispersion assessment has been used in the

investigation of the environmental release and consequence assessment

studies (Chino et al., 2011), this approach is not applicable in the case

of the FDA The reason why an atmospheric dispersion assessment

would not be dependable in the case of the FDA is due to the following

limitations:

(1) No actual source term data was available to begin with

(2) No release height data was known

(3) No local wind velocity and on-site wind direction data could be

transmitted for assessment due to SBO

(4) The terrain of the nearby Abukuma Mountain range is not

in-corporated in the dispersion model with aflat landscape

assump-tion For example, Iidate-mura is located 500 meters above sea level

and is one of the most highly contaminated highland stretching in a

NW direction from the Daiichi The village looks like a gorge

through which the plumes released at different times passing

through thisflat With such a terrain, it is necessary to consider a

change in the atmospheric pressure due to the height which may

have induced the radioactive rainfall

TEPCO’s monitoring stations installed along a semicircular

boundary of the site (MP-1∼ MP-8) and on the stack should have been

the reliable source of information, however their data were not able for accident management purposes due to SBO As a quick mea-sure, the on-site data was collected by TEPCO’s monitoring car5, whichshowed the changing wind directions every few hours during thefirstweek of the accident as shown inFig 2(TEPCO, 2011b)

avail-This plot was made by extracting the wind direction data fromTEPCO’s archived monitoring data, recorded from March 11–31, 2011.Due to the wake (i.e., a track of atmospheric turbulence) of on-sitebuildings, this wind direction may not be the true direction of theelevated release through the hydrogen explosions

The wind direction is shown in a clockwise azimuth with 0 (or 12)pointing to the North and 3 o’clock pointing to the East The originaldata was obtained by TEPCO’s two monitoring cars, one mostly parkednear the Front Gate located to the WSW of the Daiichi and the othernear MP-4 in a WWN direction The wind direction indicated by eachdot is an average value from the previous dot, since the original datawere recorded every 2∼ 5 ∼ 10 ∼ 30 ∼ 60 min depending on thechange in dose rates When the wind direction crosses the 12 o’clockdirection, additional data points were included before and after Thissmoothing of the trend graph was performed to indicate a global trendfor the hourly wind direction Even with this processing of the data, thewind direction changed too frequently to identify preferential winddirections that were stable for a few hours

When the wind direction value is larger than 6 o’clock, the wind isblowing from land to the Pacific Ocean At the time of the hydrogenexplosion in 1F1 (15:36 on March 12), the wind direction was from theocean to land in a SSE direction This wind direction is consistent withthe TV news video where a semi-spherical cloud is moving in a north-erly direction Another“internal hydrogen explosion” occurred in 1F2with the suppression chamber pressure “down-scaled” (off-scaled tozero, at 06:14 on March 15) The wind direction was also from theocean to land Prior to this event, at 11:01 on March 14, the hydrogenexplosion occurred in the Reactor Building of 1F3 At the time of thisevent, the recorded wind direction was from land to sea However, thisinformation is not necessarily consistent with video coverage at thetime of the explosion, as it shows that a tall mushroom cloud was tra-veling in a southern direction

In addition to TEPCO’s monitoring posts, the Fukushima PrefecturalGovernment had 25 monitoring posts scattered around the Fukushimasite Their data was also not available for accident management pur-poses due to the tsunami or failure of their telemetry system, in spite oftheir battery and engine power backups Fortunately, most continued to

Fig 2 Wind direction observed at the Fukushima Daiichi site during the active phase (March 11–16, 2011).

5 Data from the monitoring car substituted the eight failed stationary monitoring tions by periodically measuring doses near the location of each station In addition, a portable survey meter was placed near the front gate, located to the west of the Daiichi There was also meteorological equipment in the car.

work silently and automatically stored the data measured during the

active phase of the accident on their own hard drives On September 24,

2012, the Fukushima Prefectural Government recovered the radiation

monitoring data and posted it on their homepage only in Japanese

(Fukushima Prefectural Government, 2012)

These data were plotted for each monitoring station by the author

and displayed in a form of a“mandala” (pictorial disambiguation)6as

shown in Fig 3 The figure was highlighted with the following

screening criteria to select eight representative points from the original

23 data sets released from the Fukushima Prefectural Government

home page, namely;

•One representative location from each of the 8 sectors, covering the

western half of the Daiichi

•Including hourly data at least until March 15, 2011

These representative data were plotted to cover the most active

phase of the accident over six days from March 12 to 18

A zoom-in view ofFig 3is shown inFig 4, by focusing on the

Yamada Station, Futaba-machi (#16; 4.1 km WNW from the Daiichi),

since it contained the most representative overall dose information

recorded during the plume passage as well as the ground shine after

land contamination in the WNW to NW direction

SinceFig 4fails to show that another large release event occurred

at midnight to the early morning of March 15 in a SWS to S direction,

Fig 5is also selected to supplement the missing information

In the following explanation, the terminologies (e.g., plume passage

dose, ground shine and land contamination) are illustrated inFig 6

In thesefigures, a sharp increase in the dose rate curve represents a

plume passage dose which decays quickly after its passage After

reaching the peak, the dose rate decreases with an asymptotic decay

curve which levels off at a larger background than before the plume

passage This increase represents a ground shine dose due to land

contamination This step-wise increase in the ground shine represents a

contamination of soil due to radioactive materials The initial two large

releases from 1F1 occurred on March 12 left an order of magnitude

larger ground shine than prior to the arrival of the plume

Thefirst release is likely due to the overpressure leakage through

the dry well (D/W)flange of the PCV’s top head, whereas the second

release is due to the venting performed at 9:15 on March 12 The issue

of the dry wellflange leakage is discussed in Section5.5 There is also a

possibility that thefirst peak is due to the “internal hydrogen explosion”

at the wet well vent pipe induced a small crack since a leakage of water

has been identified in the suppression pool room as explained in Section

6.2 The effect of venting is indicated only by a plume passage dose

without an increase in ground shine The heavily contaminated corridor

stretching to the northwest is likely the superposition of the large

re-leases from 1F1 on March 12 (shown inFig 4) and from 1F2 during

March 15–16 (shown inFigs 4 and 5) The latter induced an additional

increase in contamination by 3 orders of magnitude on top of the

ground shine left from Unit 1 These correlations are identified by

collating with the chronology of the accident as summarized inTable 6

in comparison with the environmental contamination maps

The large release occurred on March 15 at the time when the

ex-plosive sound occurred in Unit 2’s suppression pool as shown inFig 5

This graph is connected with the three large release events recorded in

Fig 4on March 16 The wind direction changed from SWS on March 15

to WNW on March 16 Unit 2 appears to have released effluent

re-peatedly for more than a day, thereby heavily contaminating the near

field of the Daiichi The radioactive iodine was the main constituent of

the release This is a unique feature of the releases indicating a different

release mechanism Further explanation will be provided in Section4.3

4.2 Chronology of the Fukushima accident

The chronology as summarized in Table 6 is extracted from ference (TEPCO's Investigation Report, 2012) to highlight those eventsdirectly related with the environmental releases

re-(1) March 11: Dose rate increased in 1F1 R/B only 6 h after the arrival

of the tsunami

(2) March 12: Two large peaks in dose rate curve recorded by themonitoring station located at Yamada station (Fig 4), occurredfrom that morning until noon They were also recorded at othermonitoring stations located to the NW (#19 Kamihatori, 5.6 kmfrom Daiichi) and NWN (#22 Namie, 8.6 km from Daiichi) as shown

inFig 3 These releases induced severe land contamination Unit 1

is very likely responsible for these events These peaks occurredbefore the venting operation, indicating leakage from PCV’s flangewithout the scrubbing effects of the suppression pool water.(3) March 13: Two large releases (Fig 4) were from 1F3, although theydid not leave significant land contamination, since the step-wiseincrease in ground shine is only 1.0 nSv/h They were the result ofthe venting from S/C in which scrubbing of the effluent should havebeen effective

(4) March 14: Hydrogen explosion in Unit 3 It left no remarkablecontamination (Figs 3 and 4), since they are not correlated witheither the venting operation or the explosive phenomena It meansthat the aerosol was deposited onto the inner wall of the reactorbuilding by the time of the hydrogen explosion occurred TEPCO’son-site meteorological data was indicating that the wind was to-wards the in-land direction; therefore the plume should have beendetected, if it reached one of the monitoring posts

(5) March 15: Three large peaks in the dose rate curve (Fig 5) arerecorded in the monitoring station located in the WNW (#16 Ya-mada, 4.1 km from Daiichi), leaving behind severe land con-tamination Unit 2 is very likely responsible for these events Inparticular the spontaneous depressurization of D/W, which oc-curred around noon, appears to have been the result of effluentreleases without scrubbing by the suppression pool water Theleakages occurred three times since step-wise increases in theground shine after the plume passages are repeated three times.These phenomena are likely due to leakages from the D/Wflange ofthe PCV

(6) March 16: Following the large releases that occurred on March 15from 1F2 (Fig 5), two additional large leakages continued throughthe next day as shown inFig 4 The amount of these releases areamong the largest, and likely from 1F2 Unfortunately TEPCO’stimeline does not cover March 16 and no data is available for fur-ther study

(7) The exact timing of the hydrogen explosion in 1F4 is not knownalthough the large explosive sound and quaking was felt at 06:14 onMarch 14 at the Unit 4 side ceiling of Units 3/4's common controlroom It is likely due to this event, although, damage to the 5thfloor of the R/B was visually confirmed at 06:55 on March 15.4.3 Reconstruction of land contamination densities due to iodine

In general, retrospective reconstruction of131I map is very difficultdue to its short half-life However on June 27, 2013, JAEA (JapanAtomic Energy Agency) gave a press release on their successful re-construction of the131I land deposition maps based on the spectral datataken by the US DOE through their aerial remote monitoring operationperformed from April 2–4, 2011 (Torii, 2013)

In their analysis, Dr T Tori performed a reverse (unfolding) MonteCarlo Simulation by incorporating the attenuation of the gamma rayfrom the source to the detector as well as the detector characteristics

6 The term “mandala” is a religious picture often used in Buddhism to illustrate the

spiritual world as in the case of a Russian icon in Christianity In the picture, Buddha is

usually surrounded with the images of saints.

since the peak energy (365keV) was barely detected.Fig 7is copied

from their press release The figure shows a comparison of the land

contamination density (Bq/m2) of131I (left) and134Cs (right) as of June

16, 2011 In comparison with the134Cs map the131I map indicates:

•Large nearfield deposition of131I occurred in the southern direction

as far as 20 km away from the Daiichi;

•134

Cs induces highly contaminated corridor stretching from

ap-proximately 20–30 km NW from the Daiichi more heavily than131I

It has often been assumed that the radioactive iodine was released

in the form of CsI The author has confirmed the chemical stoichiometry

by investigating the numerous reported soil contamination samplestaken in the highly contaminated region stretching in the NW directionfrom the Daiichi However the 131I map created from the newly de-veloped method indicates that iodine is not necessarily released as CsI

Dr Torii’s131I map is more reasonable since the estimated shutdowninventories of131I/134Cs at the time of the accident are 2292/170 PBq,respectively, for both Units 2 and 3 The elemental iodine is scarcelysoluble in water, although it is a strong oxidizing agent Therefore, theFig 3 Survey map of environmental radiation monitoring data recovered by the Fukushima Prefectural Government.

radioactive release as CsI may indicate that the release is from a water

environment, such as from the suppression pool water, in which CsI has

been dissolved The release of mostly iodine appears to indicate that it

is directly from the D/W of the PCV without going through the

sup-pression pool water

By correlating the 131I land deposition map (Fig 5) with the

chronology (Table 6), the internal hydrogen explosion occurred in Unit

2 S/P at 06:14 on March 15 which induced a large release of131I to the

SWS direction of the Fukushima Daiichi site The potentialflow of the

plume with the high concentration of131I in a southern direction from

the Daiichi is a new finding and important to public safety Also the

new131I land contamination map is puzzling from the point of view of

its release mechanism The highly contaminated corridor stretching in a

NW direction from the Daiichi is likely the result of numerous plumes

released from both Unit 1 and 2 and occurring at various times This is

implied by the nearfield (< 5 km)134Cs land contamination map as

shown inFig 8 In thefigure, the routes of the numerous plume

pas-sages were more clearly reproduced These footprints indicate that the

plumes traveled in accordance with the thermo-hydraulic effects (i.e.,temperature of the initial released steam containing radioactive speciesand release height as well as wind direction and velocity) of atmosphereand not a simple dispersion mechanism applying Gauss’s atmosphericturbulence model

4.4 Summary of environmental releases in correlation with chronology

The review of the environmental dose rate correlated with thechronology of each unit clarified the following points:

(1) A series of venting operations after scrubbing with the suppressionpool water (W/W venting) left minor soil contamination in 1F1 and1F3

(2) A series of“spontaneous venting” events appear to have occurred inUnit 2 after the hydrogen explosion involving S/C The explosiondeteriorated the dry well flange seals leading to the effluentleakage When the overpressure in the PCV is reduced through thespontaneous venting, it leads to the reduced pressure boiling of thesuppression pool water releasing a large amount of dissolvedradioactive species contained in the S/P

(3) Although the devastation of the secondary containment system (i.e.R/B) should have been prevented, it did not result in a large releaseupon the explosion in 1F3 It is likely due to the precipitation ofparticles in the aerosol before the explosion As a matter of fact therubble of the building was reported to be highly contaminated

Fig 4 Aerial doses at Yamada, 4.1 km WNW of Fukushima Daiichi.

Fig 5 Aerial doses at Matsudate, 14.2 km SWS of Fukushima Daiichi.

Fig 6 Mechanism of changing dose rate curves during the plume passage at each

monitoring station.

Table 6 Simplified chronology directly related to environmental releases.

Day Unit Time and Events

3/11 All 14:46 Earthquake 14:48 Reactor trip.

All 15:37 Tsunami induced SBO.

1 21:51 Dose rate increased in 1F1 R/B.

3/12 1 02:30 D/W pressure reached 840 kPa (abs).

1 04:23 0.59 µSv/h at front gate D/W head flange leakage started(?).

1 09:15 Vent valve (MO) was manually opened.

1 14:30 D/W confirmed depressurized.

15:29 1015 μSv/h at MP4

1 15:36 Hydrogen explosion in 1F1 R/B 3/13 3 08:41 vent line configured to the rupture disk

3 08:56 882 μSv/h at MP4 (8:41 venting started)

3 09:08 RPV depressurized and water injection through DDFP.

3 09:20 DW is judged likely vented

3 14:15 905 μSv/h at MP4

3 14:31 Dose rate inside R/B 100 ∼ 300 mSv/h.

3 21:10 Confirmed pressure decrease in D/W 3/14 4 06:14 Large explosive sound and shaking of the ceiling at the Unit 4

side of Unit 3/4 common control room.

3 07:20 D/W pressure stabilized at 0.5 MPa (abs)

4 10:30 High dose rate in 1F4 prevented worker entry to the R/B.

3 11:01 Hydrogen explosion in 1F3 R/B

3 11:15 RPV pressures 0.195/0.203 MPa (abs) at Chanel (A/B) D/W Pressure 0.330 MPa (abs) and S/C pressure 0.390 MPa (abs) Both RPV and PCV were judged sound

2 23:46 D/W pressure 750 kPa (abs)

3/15 2 00:05 D/W Pressure 740 kPa (abs) Unable to vent (D/W head

flange leakage started?)

2 06:14 Large explosive sound and floor quaking D/W pressure

130 kpa(abs), S/C pressure 0 kPa Evacuation of staff.

4 06:55 Damage on 5th floor of R/B confirmed

5 Hydrogen generation and explosions

5.1 Early pipe break and hydrogen release (1F1)

As listed inTable 6, an increase in dose rate inside of the 1F1 R/B

prevented the entry of workers as early as 21:51 on March 11,

ap-proximately 6 h after the arrival of the tsunami Entry to the R/B was

restricted at 23:05 in consideration of the high dose rate This resulted

in a serious restriction for accident management purposes such as

venting by manually opening the vent valve Nevertheless TEPCO’s staff

entered the R/B for venting, resulting in a dose > 100 mSv even when

wearing an “air set” (respirator) This phenomenon motivated some

Japanese scientists to suspect an earthquake-induced pipe break or

failure of the Isolation Condenser (I/C) leading to a very early core melt

down event However this author believes that it is due to an“internal

hydrogen explosion” which is preceded by the earlier event at the

Hamaoka Unit 1 in Japan followed with Brunsbüttel BWR in Germany,

both of them occurred in 2001 This mode of pipe break could have

been prevented if more in-depth studies had been completed as

ex-plained below A brief summary of this accident is appended in the

Annex C The Hamaoka accident is important due to the fact that a

hydrogen explosion inside of the primary steam-water coolant may

induce an “internal hydrogen explosion” when dissolved hydrogen is

separated from the primary coolant This separation mechanism should

have also existed in the W/W vent pipe during the course of the

acci-dent in which the suppression pool water which reached temperatures

as high as 160 °C

The Hamaoka accident occurred in a part of the ECCS (RHRS) at the

top portion of the steam piping from the RPV During operation, this

portion of the stand pipe is open to the steam of the primary circulation

water at its inlet side, however, valves are kept closed at the exit to the

RHRS Heat Exchangers which is at an ambient temperature Due to this

closed configuration, this portion of the pipe is left cold, allowingcondensation of steam from the RPV during normal operation Thecondensation separated the dissolved hydrogen and oxygen gases gen-erated due to the radiological decomposition of the reactor water Uponcommencement of the routine ECCS test, the mixed gas exploded andshattered the OD 165 mm pipe with an 11 mm wall thickness Since theisolation valves were closed automatically 30 s after the breach, theaccident was terminated without developing into a small LOCA In spite

of the extensive follow-up experiments performed with closed pipecontaining mixed gas and steam, only 4 cases exploded out of 104 testsperformed with various noble metal concentrations at the surface Theauthor suspects that radiation may become an ignition source, since thehigh energy charged particles produce“spurs” which is an agglomera-tion of the secondary charged particles before being thermalized Insidethe“spurs” the temperature should be extremely high at the molecularscale

The Hamaoka hydrogen explosion issue was left unresolved at thetime of the FDA The regulatory body (NISA at that time) instructed theowners of the 14 BWRs with similar ECCS designs to remove the ac-cumulated gas and water before conducting the monthly ECCS tests Forthe removal of gas it was necessary to cool down the piping by closingthe isolation valves This preparatory operation usually took severaldays but the ECCS operation without the removal of the accumulatedhydrogen was feared to be risky It is a strange directive since accidents

in need of the activation of ECCS cannot be scheduled At the time ofthe FDA, the circulation of reactor water was terminated, which shouldhave induced a more favorable situation for the accumulation of hy-drogen and oxygen gas such as in the RCIC and HPCI steam turbinepiping

Unfortunately, direct evidence of this mode of pipe rupture has yet

to be confirmed in 1F1 Nevertheless a steam jet ejection was observed

on June 4, 2011 from the pipe sleeve penetrating thefloor near the wallFig 7 Comparison of land contamination density (Bq/m 2 ) of 131 I (Left) and 134 Cs (Right) as of June 16, 2011 including soil sampling data taken at each spot.