This paper introduces the purpose, technical background and framework of preliminary secondary water chemistry guidelines for Japanese PWRs. Addition, the differences in bases of parameter settings between the Japanese and overseas guidelines are discussed.
Trang 1Shunsuke Uchidae, Takayuki Mizunof, Yusa Muroyag, Yasuo Tsuzukih, Ryuji Umeharai,
a Central Research Institute of Electric Power Industry, Japan
b Mitsubishi Heavy Industry, Ltd, Japan
c Institute of Nuclear Safety System, Inc, Japan
d University of Tokyo, Japan
e Tohoku University, Japan
f Mie University, Japan
g Osaka University, Japan
h Japan Nuclear Safety Institute, Japan
i Japan Nuclear Safety Institute, Japan
j Central Research Institute of Electric Power Industry, Japan
A R T I C L E I N F O
Keywords:
Guidelines
PWR
Secondary water chemistry
System component integrity
Steam generator
Stress corrosion cracking
Flow accelerated corrosion
A B S T R A C T
In the more than 40 years of operational history of pressurized water reactors (PWRs) in Japan, sustainable development of water chemistry technologies has resulted in the world's highest secondary system component integrity; additionally, secondary system components, especially steam generator (SG) tubing, with excellent material integrity have been developed to prevent leakage of radioactive contamination from the primary to the secondary system and to maintain the heat removal function of the secondary system Although reasonable control and diagnostic parameters for water chemistry are utilized by each PWR owner, the specific values are not shared
To ensure reliable PWR operation and to achieve the highest safety level, relevant members of the Standards Committee and the related committee organized by the Atomic Energy Society of Japan (AESJ) decided to establish water chemistry guidelines for PWRs The Japanese PWR secondary water chemistry guidelines provide strategies for improving material integrity and the heat removal function The guidelines also provide reasonable
“action levels” for control parameters and “control values” and “diagnostic values” for multiple parameters, and they stipulate the responses when these levels are exceeded Specifically, “conditioning parameters” are adopted
in the guidelines Good operational practice conditions are also discussed with reference to long-term experi-ence
This paper introduces the purpose, technical background and framework of preliminary secondary water chemistry guidelines for Japanese PWRs Addition, the differences in bases of parameter settings between the Japanese and overseas guidelines are discussed
1 Introduction
To increase the safety and reliability for the operation of light
water-cooled nuclear power plants, careful and reliable water chemistry
control is one of the key issues For this, plant water chemistry should
be controlled by the water chemistry experts based on the suitable
water chemistry guidelines There are many water chemistry guidelines
prepared by many organizations, e.g., Electric Power Research Institute (EPRI) in US (Fruzzetti, 2004), Vereinigung der Groβkesselbesitzer (VGB) in Germany (Neder et al., 2006) and Électricité de France (EDF)
in France (Odar and Nordmann, 2010)
On the other hand, the major features of a plant's water chemistry depend on its' unique construction materials and operational histories This means that the water chemistry guidelines should include common
https://doi.org/10.1016/j.pnucene.2019.01.027
Received 28 October 2018; Received in revised form 20 December 2018; Accepted 25 January 2019
∗Corresponding author
E-mail address:kawamuh@criepi.denken.or.jp(H Kawamura)
Available online 14 March 2019
0149-1970/ © 2019 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/BY-NC-ND/4.0/)
Trang 2features while ensuring theflexibility required for each plant.
In the Standard Committee of the Atomic Energy Society of Japan
(AESJ), the water chemistry guidelines have been prepared Those for
BWR water chemistry and PWR primary water chemistry are now in
press Those target values and technical background have been
in-troduced in the previous paper (Kawamura et al., 2016)
The water chemistry guidelines for PWR secondary water chemistry
is now on thefinal stage of editing processes, which will be published
after public review One of the major objectives and roles of PWR
secondary water chemistry control are to ensure the secondary coolant
system components integrity, especially steam generator (SG) tubing, to
prevent leakage of radioactive contamination from the primary to
secondary systems and to maintain the heat transfer efficiency for
steam generation In the PWR secondary coolant system, the structural
materials contact with the secondary water under a high-temperature
and high-pressure environments Degradation in the system component
are due to corrosion affected by the following secondary water
para-meters, e.g., pH, conductivity, presence of impurities, and dissolved
oxygen content Notably, if inappropriate water chemistry management
occurs for a prolonged time, stress corrosion cracking (SCC) propagates
through the wall of the SG tubes, and coolant with radioactive species
may leak from the primary system to the secondary system in the SG
and potentially leak out of the station Some of corrosion products
might accumulate on heater tube surface, which results in decreasing
heat transfer efficiency and then decreasing plant efficiency In
addi-tion, wall thinning of secondary coolant pipes caused by
flow-ac-celerated corrosion (FAC) is an important safety issue for plant workers
because wall thinning will cause possible large amount of steam leakage
from the secondary coolant piping
However, changes in water chemistry as a material corrosion
con-trol technique often result in change in the integrity of the various
secondary system component materials due to different corrosion
me-chanisms Thus, the various issues must be solved harmoniously
through a comprehensive understanding of the plant system Due to the
complexity of the water chemistry, which affects several corrosion
mechanisms of secondary system components, various sustainable
de-velopments and improvements in water chemistry technologies have
been applied to commercial PWRs based on plant systems, material
design and operational experiences to achieve high-reliability
perfor-mance of secondary system components and highly effective
heat-ex-change performance Those backgrounds are also involved in the PWR
secondary water chemistry guidelines
“Control values”, “diagnostic values” and “action levels” for
mul-tiple parameters are also provided in the Japanese PWR secondary
water chemistry guidelines The concept of these values are same as the
Japanese PWR primary water chemistry guidelines (Kawamura et al.,
2016) Specifically, the concept of a “conditioning parameter”, such as
the hydrazine (N2H4) content and pH of the feed water, is adopted in
the Japanese PWR secondary water chemistry guidelines These
guidelines lead to the optimum water chemistry parameters and
pro-tocols for Japanese PWRs to assist in self-discipline and sustainable
safety improvements and to provide strategies to improve material
in-tegrity and heat-exchange performance A further goal is to create more
human resources for developing water chemistry experts, including
those of the next generation
This paper introduces the purpose, technical background and
fra-mework of the secondary water chemistry guidelines for Japanese
PWRs Additionally, the differences and the bases of parameter settings
between the Japanese and the EPRI and VGB guidelines (Fruzzetti,
2004), (Neder et al., 2006) are discussed
2 PWR secondary water chemistry guidelines
2.1 PWR secondary coolant
The secondary water in the PWR is an alkaline solution
supplemented with chemical additives To scavenge dissolved oxygen and maintain an adequate reducing condition in the secondary coolant system, hydrazine (N2H4) is added to the secondary water Ammonia or ethanol amine (ETA) is also injected into the coolant to maintain a suitable pH and increase the corrosion resistance of the secondary system components
Recently, sophisticated chemical injection control has been carried out using multiple pH-control agents such as ETA, dimethylamine (DMA) and 3-Methoxypropylamine (MPA) in US PWRs to ensure the long-term integrity of the secondary system material However, Japanese PWR utilities emphasize reliability rather than efficiency, and therefore simple operation using single pH-control agents has been carried out Concurrent achievement of reliability and efficiency is targeted by eliminating copper-based alloys and adapting high pH op-eration
2.2 Objectives of water chemistry PWRs have experienced various corrosion problems, such as inter-granular attack (IGA), SCC of nickel-based alloy tubing in the SG and stainless steel piping, and FAC of carbon steel
To overcome these problems, it has been widely recognized that secondary water chemistry is very important for the safe and reliable operation of PWRs
The primary objectives of PWR secondary water chemistry control are as follows:
(1) To mitigate coolant-assisted corrosion and ensure the material in-tegrity of the secondary system components
(2) To maintain heat exchange performance 2.3 Necessity of water chemistry guidelines PWR secondary system management is charged with generating safe, reliable, and low-cost electric power Management is periodically faced with a choice of either keeping a unit available to generate power
to meet short-term system demands or maintaining good control of chemistry to help ensure the long-term integrity of the secondary system components, and to improve power generation and balance-of-plant (BOP)
Based on the PWR operational history of more than 40 years in Japan, Japanese PWR utilities have made huge efforts to maintain re-actor and component integrity and improve power generation as well as
to pursue corrosion risk reduction
Japanese PWR utilities have voluntarily implemented secondary water chemistry precautions to obtain the highest reliability In the implementation process, secondary water chemistry experts have dis-cussed the operating rules for PWR secondary water chemistries based
on state-of-the-art scientific understanding as well as the field experi-ences of Japanese PWRs
To ensure secondary system safety, assurance of the material in-tegrity of the secondary system components, particularly, the SG tubing integrity, to prevent radioactive contamination from the primary to secondary leakage and to maintain the heat-removal function of the primary system are related strongly to the nuclear safety principles of
“confining sources of radiation risks” and “protecting people and the environment from radiation” Pipe wall thinning control is related to labor safety principles Both sets of principles are important to the operation of PWRs from the viewpoint of safety and reliability Therefore, to ensure nuclear safety, continuous integrity of the sec-ondary system component material based on appropriate water chem-istry control techniques is required
However, changes in the water chemistry as a material corrosion control technique should be performed to maintain the integrity of the various different secondary system component materials, which have different corrosion mechanisms Thus, the various issues must be solved
Trang 3The objectives of the secondary water chemistry guidelines are to
simultaneously assure the material integrity of the secondary system
components and improve the heat-exchange performance To achieve
these objectives, suppressing material corrosion in the secondary
system and reducing corrosion product release and deposition on the SG
tubes are key issues
The AESJ PWR Secondary Water Chemistry Guidelines are
applic-able only to the recirculating SG and cover the secondary coolant
system and make-up water system.Fig 1shows the targets of the PWR
secondary water chemical system in the guidelines Similar definitions
are used for the chemistry parameters in all systems The parameters
can be categorized as control, conditioning, and diagnostic parameters,
and they are set for all PWR operation modes
The following framework was used to establish the parameters
● Each control parameter has three action levels that are defined to
ensure the long-term integrity of the secondary system materials
● The conditioning parameter is defined by the hydrazine (N2H4)
content and pH of the feed water
● The diagnostic parameters are defined to complement the overall
products which are affected by changes in coolant temperature and reactor pressure
2.4.1 Plant status of PWR With respect to the water chemistry parameters, these guidelines
define the plant status in the four operating modes shown inTable 1, and they consider the thermal and hydraulic conditions and their effects
on the chemical environment Typical plant status modes and opera-tions in Japanese PWRs are shown inFig 2
2.4.2 Concept of control, conditioning and diagnostic parameters
As mentioned above, the key purpose of the secondary water chemistry is the elimination of impurities except for chemical additives
to reduce corrosion product and to ensure secondary coolant system component integrity
Fig 3shows an example of the concept of control, conditioning and diagnostic parameters for the SG blowdown water and feed water and
at the outlet of condensate water during power operation according to the PWR secondary water chemistry guidelines The concept of control, conditioning and diagnostic parameter definitions are the same as that
in the PWR primary water chemistry guidelines (Kawamura et al.,
Fig 1 Targets of the PWR secondary water chemical system in the guidelines
Trang 42016) In the guidelines, the ideas of recovering from deviations in the
control parameters are the same to ensure secondary coolant system
component integrity and to improve heat-exchange performance
2.4.3 Control parameters
The control parameters are selected to ensure that overall water
chemistry allows optimal plant operation, and the parameters are the
water quality limits for ensuring the long-term reliability of the
mate-rials In addition, the control parameters are selected based on their
importance according to the state-of-the-art scientific understanding
and extensive Japanese PWR field experience Moreover, the
para-meters are selected based on the availability of detection methods that
are reliable, sensitive and accurate when used in PWR secondary
sys-tems When the water quality is outside the safe limits, suitable
coun-termeasures should be taken to maintain plant system reliability For
the control parameters, the following values are defined
(1) Action Levels
Three action levels are defined as the chemical conditions that
re-quire immediate evaluation and corrective actions The definitions of
the action levels are the same as that in the PWR primary water
chemistry guidelines (Kawamura et al., 2016) When the water chem-istry parameters deviate from the action levels, water chemchem-istry experts should ensure that the optimal water chemical conditions are recovered within the set time Requirements for the action levels are the same as that in the PWR primary water chemistry guidelines (Kawamura et al.,
2016)
(2) Recovering from Action Levels and Requirements The basic concept of recovering from the secondary water chemical deviations is the same as that in the PWR primary water chemistry guidelines (Kawamura et al., 2016) If it is foreseeable that the values will go down immediately within Action Level 3 by a power descent, then the operating status can be continued
2.4.4 Conditioning parameters Conditioning parameters are key parameters associated with che-mical additives, such as the pH and N2H4used to maintain appropriate feed water quality Conditioning parameters are not stipulated in the EPRI and VGB guidelines (Fruzzetti, 2004), (Neder et al., 2006)
Table 1
Operational status modes in a PWR secondary coolant system
Plant Status Reactor Condition Remarks
Start-up Critical to power operation Covers the period of increasing pressure before power operation.
Power Operation Reactor critical Covers the period from power up to the beginning of the shutdown process.
Shutdown Power descent to shutdown Covers the period from power descent to heat removal using an SG.
Outage/Wet Layup (Clean-up) Shutdown to coolant temperature < 100 °C Covers the period from shutdown to start-up.
Purification of the feed water and condensate water systems and deaeration before start-up are also included in this period.
Fig 2 Typical plant status conditions and operations for Japanese PWR secondary systems
Trang 52.4.5 Diagnostic parameters
The concept of diagnostic parameters is the same as that in the PWR
primary water chemistry guidelines (Kawamura et al., 2016)
2.4.6 Recommended values
The concept of recommended values is the same as that in the PWR
primary water chemistry guidelines (Kawamura et al., 2016)
2.4.7 Monitoring frequency
The concept of monitoring frequency is the same as that in the PWR
primary water chemistry guidelines (Kawamura et al., 2016)
2.5 Example of PWR secondary water chemistry guideline values and
settings for control parameters and recommendations
As mentioned previously in the paper, action level 1 and
re-commended values are defined for self-disciplined safety improvement
In this section, some examples of action levels and recommended values
are shown for PWR power operation
2.5.1 SG blowdown water during power operations
Table 2 shows the control and diagnostic parameters and
re-commended values for SG blowdown water during power operations
pH can be a harmful parameter that adversely affects the long-term
integrity of secondary system components via general corrosion and
FAC of the carbon steel used for the SG support plates, feed water
systems, and bleeding and drain lines, and corrosion product release
and deposition due to material corrosion in the secondary system
Ammonia attack of condenser tubes made of copper alloy has been
observed
Fig 4shows SCC initiation mapping for SG tubing made of alloys
MA600 (mill-anneled alloy 600), TT600 (thermally treated alloy 600),
and TT690 (Yashima, 1995) SCC initiates at pH300C< 5 or
pH300C> 10.Fig 5shows an example of the effect of pH on magnetite
(Fe3O4) dissolution Fe3O4easily forms on the surface of carbon steel
and stainless steel under the reducing conditions present in a PWR
secondary system The dissolution of Fe3O4increases at pH < 9.8 at
25 °C, as shown inFig 5 Based on the data, action level 2 for pH is set
to < 8 at 25 °C, as shown inTable 2 The pH represents the balance
between the anion and cation concentrations in the secondary coolant
Action levels 1 and 3 are not stipulated for pH because the direct effect
of protons in the secondary coolant has not been clarified When the pH does not recover from action level 2, impurities must be identified, and remedial action should be taken The monitoring frequency is weekly because the pH depends on the feed water pH, which is checked daily,
as shown inTable 4 The secondary coolant properties can affect intergranular attack and stress corrosion cracking (IGA/SCC) and pitting corrosion in the pre-sence of small amounts of oxygen and/or oxidant IGA/SCC has ap-peared on the secondary side of nickel-based alloys within SG tubes/ tube support plate (TSP) crevices and SG tubes/tube sheets (TS) be-cause sodium and sulfate ions (Na+and SO4 −) are concentrated in the crevices (Tsuruta et al., 1995), (Shoda et al., 1996), (Kawamura and Hirano, 2000) The impurities are slightly dissolved in the secondary coolant due to ion exchange resin degradation in the condensate pol-isher Sodium and chloride can also be present in the secondary coolant due to sea water ingress from a leaking condenser tube Chloride can form acid chlorides in crevices, and acid chlorides may be a major factor in the denting of SG tubes and pitting corrosion on ferric mate-rials (EPRI, 1983a), (EPRI, 1982), (Von Nieda et al., 1980) The pre-sence of oxidants can promote the formation of acidic conditions in crevices Thus, the sodium, sulfate, and chloride concentrations are stipulated as control parameters because they are harmful species that adversely affect the long-term integrity of the nickel-based alloys used for SG tubes and other structural materials
According to the crevice calculation code provided by Mitsubishi Heavy Industry, the relationship between the pH300Cand sodium con-centration for crevice concon-centration factors of (a) 107and (b) 105at a simulated SG tube support plate crevice is shown inFig 6 The sodium concentrations are 5μg/L and 50 μg/L for concentration factors of 107
and 105, respectively, at a drilled-type and a broached egg crate (BEC)-type TSP crevice with pH300C= 10, respectively Based onFigs 4 and 6, action levels 1 and 2 for sodium are set to > 5μg/L (> 5 ppb) and >
50μg/L (> 50 ppb), respectively Action level 3 is set to > 300 μg/L (> 30 ppb) for concentration factors of 105at a BEC-type TSP crevice with pH300C= 10.5, which was calculated considering thermally treated alloy 600 (alloy TT600) Even if condenser leakage occurs, the feed water can be demineralized within 24 h after sodium detection using a condensate demineralizer system On the other hand, sodium contamination has been caused by human error during chemical Fig 3 Example of the concept of control, conditioning and diagnostic parameters in the PWR secondary water chemistry guidelines
Trang 6additive injection, leading to a rapid increase in the sodium
con-centration in the SG blowdown water and plant shutdown in some
overseas PWRs Large amounts of sea water leakage can be detected by
a salt detector When the sodium concentration does not recover from
action level 2, appropriate remedial actions should be taken The
re-commended values for sodium are≤1 μg/L (≤1 ppb) because data do
not show an adverse effect on the coolant system component integrity
at this level The monitoring frequency is daily because sodium is
dependent on the feed water pH and conductivity, as shown inTable 3 Additionally, to check the water quality trend, continuous monitoring should be recommended if an on-line monitoring system is im-plemented The frequency should be increased if sea water ingress into the secondary system is noted
According to the crevice calculation code provided by Mitsubishi Heavy Industry, the relationship between pHtand sulfate concentration for 105as a concentration factor at a simulated SG tube support plate crevice is shown in Fig 7 As shown in the figure, the sulfate con-centration is 120μg/L (120 ppb) for a concentration factor of 105at crevice pH300C= 5 The maximum concentration factor of sulfate is estimated to be 105in the test solution with sulfuric acid (Tsuruta et al.,
1995), (Shoda et al., 1996), (Kawamura and Hirano, 2000) Based on Figs 4 and 7, action level 2 for sulfate is set to > 100μg/L (> 100 ppb) Action level 1 is set to 1/10 of action level 2, i.e > 10μg/L (> 10 ppb) Action level 3 for sulfate is not stipulated because the effect of sulfate
on SG material corrosion is not clear When sulfate does not recover from action level 2, it is necessary to identify the impurities, and take appropriate remedial actions The recommended value for sulfate
is≤ 2 μg/L (≤2 ppb) because there are no data showing an adverse effect on coolant system component integrity at this level The mon-itoring frequency of sulfate is daily for the same reason as for sodium Fig 8shows the effect of chloride on the corrosion rate of alloy 600 using a boiling heat transfer test loop In Japanese PWRs (Atomic Energy soceity Of Japan, 2000), pitting corrosion cannot easily occur because the SG secondary side is maintained under reducing conditions Even if some oxidants are present in the SG secondary side, the effect of
Table 2
Control and diagnostic parameters for SG blowdown water during power operations in the Japanese, EPRI, and VGB guidelines
Period Japanese Guideline EPRI Guideline ( Fruzzetti, 2004 ) VGB Guideline ( Neder et al., 2006 )
Start-up to 100% Reactor Power > 30% Reactor Power 100% Reactor Power
Parameters Action Levels Recom-mended value Frequency Action Levels Frequency Action Levels
< 9.5
Cation Conductivity, mS/m (μS/cm) 1 - a – – 1 – Continuous 1 > 1
Sodium, μg/L 1 > 5 b ≤1 Daily d 1 > 5 Continuous 1 > 50
Sulfate, μg/L 1 > 10 ≤2 Daily f 1 > 10 Daily Normal operating values:
< 10
Chloride, μg/L 1 > 10 ≤2 Daily d 1 > 10 Daily Normal operating values:
< 10
2 > 100
b
2 > 50
3 > 2000
b
3 > 250
Note
a In Japanese PWRs, Na, SO4and Cl concentrations in SG secondary water should be monitored instead of cation conductivity because cation conductivity reflects the sum of the effects of impurities
b These values are defined based on experimental corrosion data
c When the value is over action level 2, the cause should be sea water ingress If ingress can be detected and the leakage line can be isolated, the polluted secondary water can be cleaned up by condensate demineralizer within 24 h
dContinuous monitoring should be recommended if a continuous monitoring system is implemented Ion chromatography is semicontinuous monitoring and an optional analysis method The frequency should be increased if evidence of sea water ingress is noted
e Action level 2 of sulfate is treated in the same manner as for chloride
f
Continuous monitoring should be recommended if a continuous monitoring system is implemented Ion chromatography is semicontinuous monitoring and an optional analysis method
g Continuous monitoring should be adopted if using an SG blowdown water monitor
Fig 4 SCC initiation region for SG tubing (Yashima, 1995)
Trang 7chloride ions on alloy 600 corrosion is very small with a maximum
chloride level of 0.1 mg/L (0.1 ppm) On the other hand, the possibility
of pitting corrosion increases for chloride levels over 2 mg/L, as shown
inFig 8 Based on the test results, action levels 2 and 3 are set to >
100μg/L (> 100 ppb) and > 2000 μg/L (> 2000 ppb), respectively
Action level 1 is set to 1/10 of action level 2, i.e., > 10μg/L
(> 10 ppb) When chloride does not recover from action level 2, it is
necessary to identify the impurities and take remedial action The re-commended values for chloride are≤2 μg/L (≤2 ppb) because there are no data showing an adverse effect on the coolant system component integrity at this level The monitoring frequency of chloride is daily for the same reason as for sodium
Total radioactivity is an important parameter and can be used as an index to check primary coolant leakage The total radioactivity is Fig 5 Relationship between the Fe ion concentration and pH (JIS B 8223: 2015)
Table 3
Control and diagnostic parameters for feed water during power operations in the Japanese, EPRI and VGB guidelines
Period Japanese Guideline EPRI Guideline ( Fruzzetti, 2004 ) VGB Guideline ( Neder et al., 2006 )
Start-up to 100% Reactor Power > 30% Reactor Power 100% Reactor Power
Parameters Action Levels Recom-mended value Frequency Action Levels Frequency Action Levels
Hydrazine, μg/L 1 < 50 – Daily a 1 < 8xCDP [O 2 ] or < 20 Continuous Normal operating values:
> 20
Dissolved Oxygen, μg/L 1 – – Weekly b 1 > 5 Continuous 1 > 5
pH control agent, mg/L 1 – Plant-specific e Appropriate
f
1 Plant-specific Daily 1 < 9.8
Conductivity, mS/m (μS/cm) 1 – Plant-specific e Daily a 1 – – Normal operating values
> 1.5 (> 15) g
Note
a
Continuous monitoring should be recommended Inlet water monitoring at a deaerator is an alternative analysis method
b Weekly monitoring is enough to detect the water quality changes because the vacuum in the condenser, the temperature of the deaerator, Do in the condenser pump, and the hydrazine concentration at the outlet of the high-pressure feed water heater are monitored daily
c
If copper alloys are used in the secondary system, such as condenser tubes
dLead material is not used in Japanese PWR systems, although it is known to be the cause of PbSCC
e Plant-specific administrative limits should be established Values are defined based on the plant design, component material and water chemistry
f Required as appropriate during power operation
g Cation conductivity is set to > 0.2μS/cm in the VGB guidelines
hValue is defined based on the plant design, component material and water chemistry
i Dispersant is not employed in Japanese PWRs
Trang 8stipulated as a diagnostic parameter because it does not affect
sec-ondary system material integrity The monitoring frequency is monthly
for the same reason However, continuous monitoring should be
recommended if using an SG blowdown water monitor
Table 2also shows the control and diagnostic parameters for SG blowdown water during power operation in the EPRI (Fruzzetti, 2004) and VGB guidelines (Neder et al., 2006)
The pH is not stipulated within control parameters in the EPRI guidelines and is stipulated as normal operating values, i.e < 9.5, in the VGB guidelines because multiple pH control agents such as ETA, dimethylamine (DMA) and 3-methoxypropylamine (MPA) are em-ployed in US and EU PWRs In these guidelines, the cation conductivity should be stipulated as the control parameter instead of pH monitoring and monitored continuously In Japanese PWRs, on the other hand, cation conductivity is not stipulated as a control and/or diagnostic parameter, and action level 2 for pH is stipulated based on the SCC initiation mapping for the nickel-based alloy (Fig 4) and the effect of
pH on Fe3O4dissolution (Fig 5) When the pH is over action level 2, the cause should be sea water ingress and impurities in the chemical ad-ditives because sea water is used as the condenser coolant in all Japa-nese PWRs pH monitoring is also categorized in the JapaJapa-nese guide-lines to check for the ingress of impurities other than sodium, sulfate and chloride into the secondary coolant
Ingress can be detected using a salt detector at the condensate hot well When sea water leaks into the secondary coolant system from the condenser tube, the leakage line should be isolated and the polluted secondary water should be cleaned up using condensate demineralizer within 24 h
In the VGB guidelines, the sodium concentration was stipulated as the control parameter, and both sulfate and chloride in SG secondary water were set as normal operating values, i.e., < 10μg/L (< 10 ppb)
On the other hand, in the EPRI and Japanese guidelines, the impurity concentrations should be monitored separately, i.e., sodium, sulfate and chloride, instead of cation conductivity because cation conductivity reflects the sum of the effects of the impurities, and it is difficult to separate the effects of each impurity In the EPRI guidelines, the action levels of sodium, sulfate and chloride are stipulated based on thefield experience in US PWRs In the Japanese guidelines, the action levels of sodium and chloride are larger than those in the EPRI guidelines, but they are defined based on many kinds of experimental corrosion data (Figs 4, 5, 7, and 8) The Japanese guidelines stipulate daily checking frequencies for sodium, sulfate and chloride for SG blowdown water, and continuous monitoring should be recommended using some kinds
of semicontinuous monitoring, such as ion chromatography and op-tional analysis The monitoring frequency should be increased if evi-dence of sea water ingress is noted
In the Japanese guidelines, the total radioactivity is stipulated as a diagnostic parameter and should be monitored continuously using the
Table 4
Conditioning parameters for feed water during power operations
Conditioning Parameters Conditioning Value Frequency
pH at 25 °C Plant-specific a Daily b
Hydrazine, μg/L Plant-specific b Daily b
Note
a Plant-specific administrative limits should be established Values are
de-fined based on the plant design, component material and water chemistry
b Continuous monitoring should be recommended Inlet water monitoring at
the deaerator is an alternative analysis
Fig 6 Relationship between the pH300Cand Na concentration for
concentra-tion factors of (a) 107and (b) 105at a simulated SG tube support plate crevice
Fig 7 Relationship between the pHtand SO4concentration in a NaOH solution
for a 10−5mist carry-over rate at a simulated BEC-type tube support plate
crevice
Fig 8 Relationship between the corrosion rate of alloy 600 and the Cl con-centration (Atomic Energy Society of Japan, 2000)
Trang 9SG blowdown water monitor In the EPRI and VGB guidelines, the total
radioactivity is not stipulated within the control parameters and/or
diagnostic parameters
2.5.2 Feed water during power operations
Table 3 shows the control and diagnostic parameters and
re-commended values for the feed water during power operations The
water is sampled at the outlet of a high-pressure feed water heater
Hydrazine is stipulated as a control parameter because hydrazine is
injected into the PWR secondary coolant as an oxygen scavenger to
reduce oxygen levels in the secondary coolant and to suppress SG tube
corrosion Fig 9shows the effect of hydrazine on the corrosion
po-tential (electrochemical corrosion popo-tential, ECP) of alloy 600
(Fruzzetti, 2000) Based on the data, action level 1 is set to < 50μg/L
(< 50 ppb) because this value is the limit to suppress oxidant formation
in the secondary system Action levels 2 and 3 and recommended values
are not stipulated The monitoring frequency is daily because reducing
conditions should be maintained during power operation Continuous
monitoring should be recommended if an on-line monitoring system is
implemented Inlet water monitoring at the deaerator can be an
alter-native analysis method
The dissolved oxygen (DO) content is set as a control parameter
because it is a harmful parameter that adversely affects IGA/SCC and
pitting and crevice corrosion of SG tubes by increasing the corrosion
potential of nickel-based alloys, as shown in Fig 10(Kishida et al.,
1987) Action level 2 is set to > 5μg/L (> 5 ppb) because this
con-centration is the monitoring limit of the implemented continuous
monitoring system Action levels 1 and 3 and the recommended values
are not stipulated The monitoring frequency is weekly to check the
reducing conditions Continuous monitoring should be recommended if
an on-line monitoring system is implemented
The copper concentration is also stipulated as a control parameter
because copper ions and copper oxide increase the ECP of carbon steel,
stainless steel, and nickel-based alloys as oxidants, and copper is a
harmful species that adversely affects the long-term integrity of these
materials Based on test results (Kishida et al., 1987) and Japanese PWR
operating experiences, action level 1 is set to > 1μg/L (> 1 ppb) The
value may be stipulated if copper alloys are implemented in the
sec-ondary system Action levels 2 and 3 and recommended values are not
stipulated The monitoring frequency is weekly because the
con-centration changes in copper are very small during power operation
Continuous monitoring should be recommended if an on-line
mon-itoring system is implemented
A control parameter for lead is also stipulated to monitor
con-tamination in the SG even though lead-induced SCC (PbSCC) has not
been experienced and lead materials are not installed in Japanese
PWRs On the other hand, PbSCC has been experienced in some over-seas PWRs due to lead shielding blocks left in the secondary system after a refueling outage Experimental data for PbSCC indicate that lead levels should be as low as possible (Takamatsu et al., 1997), (Staehle,
2005), (Fruzzetti, 2006a), (Fruzzetti, 2006b) However, the effect of lead on the SCC has not been clarified The lead level should be re-commended to be as low as possible.Fig 11shows the effect of lead on SCC of nickel-based alloys (Staehle, 2005) A lead concentration < 0.1 mg/L (< 0.1 ppm) is not harmful for PbSCC In an SG crevice, the concentration factor of lead is estimated to be 5 for 1% of the SG blowdown rate in a commercial Japanese PWR In the guidelines, the concentration factor is set to 10 as a conservative estimate Based on this knowledge, action level 1 is set to > 10μg/L (> 10 ppb) Action levels 2 and 3 and recommended values are not stipulated The mon-itoring frequency is monthly because the dominant changes in lead concentration are caused by leaving lead shielding blocks in place after periodic inspections The lead concentration in the feed water should be checked when the NH3chemical additive is changed to another man-ufacturing lot because lead may be present in the additive
The concentration of a pH control agent such as ammonia (NH3) or ethanol amine (ETA) is stipulated as a diagnostic parameter because an Fig 9 Effect of hydrazine on the corrosion potential of alloy 600 (Fruzzetti, 2000)
Fig 10 Effect of oxidant on the ECP of alloy 600 (Kishida et al., 1987)
Trang 10adequate concentration is needed in the feed water The pH control
agent should be selected according to the plant design, component
material and water chemistry The recommended value should also be
defined as a plant-specific administrative limit based on the plant
de-sign, component material and water chemistry The monitoring
fre-quency should be appropriate to check the concentration of the pH
control agent injected into the secondary coolant during power
opera-tion
Conductivity is stipulated as a diagnostic parameter to monitor the
concentration of the injected pH control agent The recommended value
should be defined as a plant-specific administrative limit according to
the plant design, component material and water chemistry The
mon-itoring frequency is daily to check the concentration of the pH control
agent injected into the secondary coolant Continuous monitoring
should be recommended at the inlet of the deaerator Inlet water
monitoring at the deaerator is an alternative analysis method
Iron oxide affects the heat transfer of SG tubes via scale adhesion on
the tube surface and scale blockage within the TSP crevice A diagnostic
parameter for iron is stipulated to suppress the above phenomenon The
recommended value is set to≤5 μg/L (< 5 ppb), and the value is
de-fined based on the plant design, component material and water
chem-istry The monitoring frequency is weekly to check the iron
con-centration in the feed water
Table 3also shows the control and diagnostic parameters for the
feed water during power operation in the EPRI (Fruzzetti, 2004) and
VGB guidelines (Neder et al., 2006)
Hydrazine is stipulated as a control parameter in the EPRI and
Japanese guidelines However, action level 1 for hydrazine is higher in
the Japanese guidelines than in the EPRI guidelines because < 50μg/L
(< 50 ppb) is the limit to suppress oxidant formation in the secondary
system In the EPRI guidelines, action level 1 for hydrazine is set to
maintain reducing conditions and to maintain hydrazine at greater than
eight times the condensate dissolved oxygen in the condensate polisher
demineralizer (CDP) or 20μg/L (< 20 ppb) The value is set based on
field experience in US PWRs On the other hand, hydrazine is stipulated
as normal operating values, i.e., > 20μg/L (< 20 ppb), in the VGB
guidelines
Action level 2 for DO in the Japanese guidelines is set to > 5μg/L
(> 5 ppb) and is more conservative than in the EPRI and VGB
guide-lines
Action level 1 for copper is the same value in the EPRI and Japanese
guidelines
Lead is not stipulated as a control parameter in the EPRI and VGB guidelines because the lower limit of the lead concentration that affects the PbSCC of nickel-based alloys has not been clarified Lead is not included as an additive in any materials in Japanese PWR systems However, a negligible amount of lead may be included in the NH3
chemical additive when the additive is changed to another manu-facturing lot
The pH control agent is stipulated as a control parameter in the EPRI and VGB guidelines to check the concentration of the injected pH control agent In the Japanese guidelines, on the other hand, the pH control agent is stipulated as diagnostic parameter because the pH control agent should be selected according to the plant design, com-ponent material and water chemistry
Conductivity is not stipulated as a control and/or diagnostic para-meter in the EPRI guidelines In the VGB guidelines, on the other hand, conductivity is stipulated as a control parameter to check the con-centration of the injected pH control agent In the Japanese guidelines, conductivity is stipulated as a diagnostic parameter to monitor the concentration of the injected pH control agent
Iron is stipulated as a control parameter in the EPRI guidelines to check the iron concentration In the Japanese guidelines, on the other hand, iron is stipulated as a diagnostic parameter because the value is defined based on the plant design, component material and water chemistry, and the recommended value, i.e.,≤5 μg/L (< 5 ppb), is the same as in the EPRI guidelines In the VGB guidelines, iron is not sti-pulated as a control and/or diagnostic parameter
The dispersant is stipulated as a control parameter in the EPRI guidelines because the dispersant is employed in US PWRs In the Japanese guidelines, on the other hand, the dispersant is not stipulated
as a diagnostic parameter because dispersant is not employed in Japanese PWRs
The conditioning parameters in these guidelines are original and are not stipulated in the EPRI and VGB guidelines The pH and hydrazine concentration are stipulated as conditioning parameters because they are additives in the secondary coolant and should be controlled based
on the plant design, component material and water chemistry Table 4shows the conditioning parameters for the feed water during power operations
pH is a parameter that adversely affects the general corrosion and FAC of the carbon steel used for the feed water system and bleeding and Fig 11 Effect of lead on the SCC of a nickel-based alloy (Staehle, 2005)