ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - MANAGEMENT OF RADIOACTIVE WASTES potx

High level wastes HLW are produced in the first cycle of reprocessing spent nuclear material and are strongly radioactive.. It is possible to operate the reactor with defects in a few fu

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MANAGEMENT OF RADIOACTIVE WASTES

RADIOACTIVE WASTE

Radioactive waste may be defined as solid, liquid, or gaseous

material of negligible economic value containing

radionu-clides in excess of threshold quantities High level wastes

(HLW) are produced in the first cycle of reprocessing spent

nuclear material and are strongly radioactive Intermediate level

wastes (ILW) can be divided into short lived, with half lives of

twenty years or less, and long lived, in which the half lives

of some constituents may be thousands of years Low level

wastes (LLW) contain less than 4 GBq/ton of alpha emitters

and less than 12 GBq/ton of beta and gamma emitters Very

low level waste (VLLW) contains activity concentrations

less than 0.4 MBq/ton

ACTIVITY AND EXPOSURE

The Becquerel (Bq) is the activity of one radionuclide having

one spontaneous disintegration per second One Curie (Ci)

is defined as 3.7 ⫻ 10 10 disintegrations per second The

Becquerel is the more commonly used unit The unit of

ion-izing radiation which corresponds to energy absorption of

100 ergs per gram is the rad (roentgen-absorption-dose)

The newer unit is the Gray (Gy), which is equal to 100 rads

The amount of radiation which produces energy dissipation

in the human body equivalent to one roentgen of X-rays is

the rem (roentgen-equivalent-man) One Sievert is equal to

100 rems and is the commonly accepted unit

Philosophy

The group of people engaged in management of

radioac-tive wastes has evolved from a small body of operators who,

originally with little or no expert knowledge, were engaged

in day-to-day solution of unpleasant problems They now

form a recognized profession, extending from whole-time

research scientists to field workers who in some countries

are conducting a profit-making industry

The members of the profession came mainly from Health Physics and brought with them the caution and “conserva-tive” attitude to radiation hazards characteristic of Health Physicists They regard their mission as being to ensure that members of the public, as well as workers in the field of nuclear energy, will not be harmed by the radioactive mate-rial for which they are responsible With their Health Physics background this sometimes leads to an attitude which indus-try regards as overrestrictive, although recent controversies have tended to cast them ironically in the role of particularly dangerous polluters of the environment

It is clear that any human activity that involves conversion

of something into something different must produce waste

Conversion of energy from one from to another is no excep-tion It is sometimes possible for an industry to recycle its waste products and to convert part of them to a useful form, but there is always some minimal residue which cannot be retained within the system This must find some place within the environment Usually the cheapest procedure is to dis-charge it in some way that will ensure a sufficient dilution

to make it innocuous If this is impracticable for technical or political reasons it must be confined, but usually the more effective the confinement, the higher the cost To say that a process must be conducted with now waste is equivalent to saying that the process may not be conducted at all, and to demand a certain level of confinement or restriction of wastes implies an acceptance of the cost of the waste management system as a necessary part of the cost of the process

Discharge of potentially noxious materials into the envi-ronment involves some risk, which may or may not be mea-surable Within very broad limits research in nuclear hazards enables us to forecast the effects of exposure of large groups

of people, for extended periods, to low doses of radiation

We can also estimate, with less accuracy, the probability that

an individual will suffer some harm from such exposure, and

we can say with much greater confidence what will happen

if an individual is exposed to larger doses—say 50 rem and upwards—in a single dose The nuclear industry, then, can provide some information on the probable consequences of environmental contamination extended over a lifetime, and

better information on the probable consequences of a major

nuclear accident which leads to high radiation exposure

In other words we can, within rather broad limits, estimate

the risks

The situation is different in most other industries The

consequences of acute doses of cyanide, lead, fluoride or

carbon tetrachloride are well known, and there is some

evi-dence for the effects from low doses received over a lifetime,

but who knows what effect to expect in humans from

benz-pyrene or nitrous oxide emitted from smoke stacks or from

the low levels of polychlorodiphenyls and mercury

com-pounds that are liberated into the environment? They affect

every age group in the population and are a potential

life-long hazard But nothing is known about the probability that

they will eventually do harm, and it is difficult to see how

such knowledge could be obtained in a human population

Every human activity is associated with some risk,

however small Normally we do not solemnly calculate the

risk, weigh it against the benefit we expect to obtain, and

then decide for or against the activity Yet to decide to do

something—such as driving a car, getting up in the

morn-ing, or going mountain climbing—must involve some sort of

conscious or unconscious weighing of risk against benefit

In deciding upon a particular waste management system,

or in deciding to license a particular kind of nuclear power

station, a much more deliberate weighing of cost vs benefit

must be undertaken There is, however, a fundamental

dif-ficulty which up to now has made it impossible to express

such a judgment in numbers It is characteristic of a ratio

that the numerator and the denominator must be in the same

units It should be possible to express most of the benefits of

nuclear power, for example, in dollars, but if we regard part

of the cost of nuclear power as an increase in the probability

that people will develop cancer or that they will experience a

shortened lifetime, how can that be expressed in dollars?

One benefit of nuclear power is the difference between

death and injury among uranium miners and processors and

the corresponding figure for equivalent energy production

by the coal mining industry This, again, cannot be expressed

in dollars To work out a true COST/BENEFIT ratio is thus

little better than a dream, and the people responsible for

approving a waste management system or a new power

sta-tion are therefore faced in the last analysis with a value

judg-ment, which is at least to some extent subjective It is not

a scientific decision In the broadest sense, the decision is

political

Controls

The responsibility for making decisions on matters related to

“dealing in”—i.e having anything to do with—radioactive

materials, machines capable of producing electromagnetic

radiation (expect for medical purposes) and certain scheduled

materials such as heavy water, usually rests with a national

atomic energy authority Typically, regulations are issued by

the authority that have the force of law Assistance is given

to the authority in assessing hazards of reactors and other

installations—including waste management systems—by

an independent advisory committee which can call on the ser-vices of an expert staff

In most countries regulations lay down the maximum per-missible exposure to radiation for workers in nuclear industry and also for the general population Maximum permissible doses (MPDs) have been recommended by the International Commission on Radiological Protection (ICRP), which have received worldwide acceptance as the fundamental basis for national regulations The ICRP has derived from the MPDs

a list of maximum permissible concentrations (MPCs) in air and water on the basis that if workers were to breathe air, or drink water, at the MPC for any particular radionuclide over

a lifetime they would not suffer any unacceptable harm

“Unacceptable” means “detectable”, in the sense that it could reasonably be regarded as caused by the radiation The ICRP has also laid down rules for calculating the MPC for mixtures of more than one radionuclide

The MPDs are constantly under review by the ICRP, which consists of people who have devoted their profes-sional lives to assessment of radiation hazards They drawn upon the work of large numbers of scientists throughout the world, many of whom are actively engaged in research on somatic and genetic effects of radiation Changes have been made from time to time in details of the ICRP recommenda-tions but it is remarkable that in such a rapidly developing field the necessary changes have been so few

The ICRP has consistently emphasized that the MPD

and its associated MPs are maximum permissible figures

The Commission has made another recommendation equal

in force and status to those on maximum permissible doses

This states that exposure to radiation must always be held down to the lowest PRACTICABLE dose The world “prac-ticable” was carefully chosen, after considerable debate If

“possible” had been used it could have been claimed that a single contaminated rat must be buried in a platinum box It

is our mission to see that all practicable steps are taken to protect mankind from exposure to radiation, and we can do that very effectively

SOURCES OF WASTES

Uranium Mining and Milling

Apart from the normal hazards associated with hard-rock mining, the workers in uranium mines are exposed to radon and the decay products which arise from the radium content of the ore These hazards can be controlled by sealing old work-ings and general “good house-keeping”, but more particularly

by installation of an efficient ventilation system and, where necessary, the use of respirators The ventilation air contains radioactive material and dust, some of which can be removed if necessary by filtration, but the radon remains The large volume

of air used for mine ventilation is ejected at high velocity from

a stack, which ensures adequate dilution into the atmosphere

The end products of the mill are uranium oxide and “tail-ings” The tailings, together with mine drainage water, contain most of the radium originally present in the ore Radium is

one of the most toxic of all radionuclides and presents a

serious potential hazard Various methods of treatment, such

as co-precipitation with barium, render most of the radium

insoluble But the water draining from tailings ponds often

contains more radium than is permissible in drinking water

Proper design of outfalls into suitable bodies of water can

ensure adequate dilution, but vigilance is necessary to

pre-vent rupture of the tailings ponds or improper practices that

will nullify or bypass the treatment system A monitoring

system for analysis of downstream water and fish is common

today, but in the early days of the industry the dangers were

little understood or ignored, with the result that lakes and

streams in uranium mining areas became contaminated

In Canada the existence of a problem was recognized

in time to avert a public hazard, but the Report of a Deputy

Minister’s Committee showed that action was necessary to

protect the environment in the Elliott Lake and Bancroft

areas This was particularly urgent as greatly increased activity

in uranium mining was anticipated within a few years

The size of the problem can be judged from the fact that

a Congressional Hearing was told that 12,000,000 gallons of

water containing nearly 10 g of radium was discharged daily

to the tailings ponds of American uranium mills

Processing of Uranium Oxide

The crude (70%) U 3 O 8 produced by the mills may be

con-verted to metal, to UO 2 or to UF 6 The hexafluoride is used in

separation of 235 U from 238 U A serious waste problem would

result from nuclear fission if a critically large amount of 235 U

were to accumulate accidentally in one place This is a rare

event, but is not impossible Otherwise, the wastes consist

of uranium chips and fines, contaminated clothing and

res-pirators and dust accumulated in air-cleaning systems The

uranium at this stage is practically free from radium so it is

hardly a radioactive hazard The toxicity of natural uranium

or 238 U is that of a toxic metal rather than of a radionuclide

Uranium metal is produced by converting the dioxide

to tetrafluoride which is then reduced to the metal at high temperature with magnesium The waste form this process—

magnesium fluoride slag and uranium metal fines from trim-ming the ingots—is a normal slag disposal problem since it

is sparingly soluble in water

Fuel Fabrication

There are many different kinds of fuel elements, but their manufacture produces little waste beyond dust and faulty pellets or fuel pins This material is usually recycled, par-ticularly if it contains added 235 U

Reactor Wastes

An operating reactor contains a very large inventory of fission products A 500 MW (thermal) reactor, after operating for

180 days, contains four hundred million curies for fission products, measured one day after shutdown This is equivalent

to the activity of about 400 metric tons of radium The fission products decay rapidly at first, leaving 80 million curies at the end of a week, and more slowly later After a month, the inventory is reduced to about 8 million curies

Nuclear power stations rated at 1000 MW (electrical)—

i.e 3000 to 5000 MW thermal—are not unusual At first sight it would seem that these plants would be enormous potential sources of radioactive wastes, but in practice this

is not the case (Figure 1) In an operating power reactor the fuel is contained within a non-corrodible cladding—usually zirconium or stainless steel—and the fission products cannot get out unless the cladding is ruptured

It is possible to operate the reactor with defects in a few fuel elements, but these sources of leakage make the primary cooling circuit radioactive It is impracticable to operate a station in the presence of high radiation fields, so the primary coolant is continually purified by ion exchangers Again, it is

SECONDARY CIRCUIT

TURBINE CONDENSER

WATER

COLD WATER

PRIMARY CIRCUIT REACTOR CORE

HOT WATER

STEAM

HEAT EXCHANGER

FIGURE 1 Schematic diagram of processes in nuclear power station Nearly all radioactivity remains inside the fuel, which is inside the core, which is inside the primary circuit

a practical necessity to renew the ion exchangers after they

have developed a certain level of radiation The net result of

these considerations is that for reasons of operator safety and

economics the presence of more than a small proportion of

ruptured fuel in a reactor will require its removal

Fuel removed from the reactor is normally stored on site

for a considerable time to permit decay of shortlived

radioac-tivity Storage facilities are usually deep tanks filled with water,

which acts simultaneously as coolant and radiation shield If

defective fuel is present the water will rapidly become

con-taminated, but even if there are no defects in the cladding the

water in cooling ponds does not remain free from radioactive

material This is because the cladding and the reactor structure

contribute neutron activation products (or corrosion products)

to the cooling water and the cladding itself always contains

minute traces of uranium, which undergoes fission in the

reactor Hence, the pond water must be purified, usually by

resin ion exchangers, so these resins also become a waste

If resins are regenerated, the regenerants (acids, alkalis,

or salts) will appear as a liquid waste for disposal Otherwise,

the resin will be handled within its original container or as a

powder or slurry

The radioactive content of gaseous effluents from reactors

depends upon the design of the reactor If air passes through

the core very large amounts of argon-41 may be emitted

from the stack Although 41 Ar is a hard gamma emitter it

has a short half-life (about two hours) so its effects are only

noticeable within or very near to the plant Radioactive

iso-topes of nitrogen and oxygen decay so rapidly that they do

not reach the stack in appreciable amount and the long-lived

carbon-14 is not produced in sufficient amount to be

hazard-ous at the present scale of nuclear power generation Some

concern has, however, been expressed that by the end of this

century the buildup of 14 C in the atmosphere might become a

significant source of radiation within the biosphere

More concern attaches to radioactive krypton, 85 Kr, with

a half-life of 10.4 years This, in contrast with 41 Ar and 14 C, is

a fission product It is liberated via fuel defects and by

diffu-sion through fuel cladding It is not a hazard from any single

plant, but with increasing numbers of nuclear power stations

it might become an ubiquitous source of low-level radiation,

though the source of most of the 85 Kr would be spent fuel

processing plants rather than power stations

Similar concern has been expressed regarding tritium,

the radioactive isotope of hydrogen, which is produced

within the fuel and by neutron activation of the heavy

hydro-gen in ordinary water or the D 2 O coolant and moderator of

heavy-water reactors It is also formed by neutron activation

of lithium, sometimes used as a neutralising agent in reactor

coolants, or of boron which functions as a “poison” in some

reactor control systems

Sometimes the significance of a “source” of radioactive

waste depends on whether one is considering the safety of

people within the plant, or the public outside For example,

ruptured fuel elements or ordinary day-to-day type

mechani-cal failures can produce air-borne radioactive iodines and

other fission products which are a nuisance to operators

because they have to work in plastic suits and respirators

The ventilation filtration system and the high dispersion capability of the atmosphere combine to make sources of this kind insignificant beyond the boundary of the exclusion area However, they may reduce efficiency and disrupt work schedules within the station very seriously, and give rise to significant disposals in the form of clean-up solutions, con-taminated clothing, mopheads and metal scrap

A noteworthy source of this nature is the tritium which builds up in the coolant and moderator of heavy-water reactors

In a 1000 MW (electrical) power station the equilibrium tritium concentration in the moderator is about 50 Ci/litre

This leads to stack discharges which are quite negligible, but any leaks in pump seals, valves or pipe joints within the station would produce operating problems for those respon-sible for the radiation safety of the staff On the other hand, material sent for waste disposal would be no problem, partly because heavy water is recovered for economic reasons and partly because the maximum permissible concentrations of tritium in air and water are much higher than those of most other radionuclides

In summary, in spite of the enormous potential source

of radionuclides within an operating power station the amount of waste generated is small compared with that arising from a research and development establishment, and minute in comparison with a plant fuel processing plant This statement covers normal operation, including the ordinary accidents and malfunctions expected in any well-designed plant It does not include the consequences

of the “Maximum Credible Accident” which is, in fact, so improbable that designers of waste management systems

do not normally make provision for it

However, the accident at the Chernobyl Nuclear Power Station in 1986 was particularly sensational A reactor exploded and caught fire, releasing an estimated 30 million Curies Half of the resulting fallout was within 30 kilometers

of the plant The remainder spread over much of Europe

There was great economic loss and many cancer deaths were attributed to the incident

Spent Fuel Processing

Wastes arising from processing of spent fuel account for more than 99.9% of the “waste disposal problem” Fuel which has been enriched with 235 U must be treated for recovery of unburned 235 U because the fission product load

of spent fuel reduces its efficiency as a source of energy It ceases to be economic as fuel long before the expensive 235 U

is exhausted

After removal from the reactor, and storage for sufficient time for decay of short-lived fission products, the fuel is de-sheathed and dissolved, usually in strong nitric acid (Figure 2).Uranium and plutonium are extracted into an organic solvent, and the acid solution of fission products left behind forms the high level or primary waste Washing

of the organic extractant produces Medium Level wastes, whereas Low Level waste consists of further washings, cooling water, scrubber water and liquids from other sources too numerous to catalogue

As long ago as 1959 fifty million gallons of High Level

wastes were stored in stainless steel tanks at Hanford (USA)

alone The radionuclides in solution generate so much

decay heat that many of the tanks boil, making the

provi-sion of elaborate off-gas cleaning systems necessary Some

high level waste tanks have ruptured, but since they are

constructed on a cup-and-saucer principle, with adequate

monitoring for spills, and spare tankage is kept available, no

unexpected contamination problems have arisen

Gases from the dissolvers and storage tanks contain tritium,

bromides, iodines, xenon, krypton and smaller amounts of

less volatile elements such as ruthenium and cesium After

storage for decay, scrubbing and filtration, off-gases can be

liberated from a tall stack As mentioned in the section on

reactors, proliferation of fuel processing plants in the future

might conceivably lead to local or even eventual world-wide

atmospheric contamination if improved containment is not

provided in time at spent fuel processing sites

Solid waste may include glasses or ceramics, used as a

means for fixing the activity in high-level liquid wastes, and

bitumen or concrete blocks containing less active material

Products of waste processing such as sludges, evaporator

bottoms, incinerator ash, absorbers, filters and scrap fuel

cladding are usually in the medium level category Worn and

failed equipment such as pipes, tanks and valves,

unservice-able protective clothing, cleanup material and even whole

buildings may have a variety of levels of contamination, by

numerous different radionuclides, which defies quantitative

assessment This is not a serious difficulty, except for

admin-istrative and recording purposes when quantitative reports

have to be made, because most of these wastes have to be

contained in some way and none of them are dumped into

the environment

The most difficult problem for the fuel processing industry

is not high or medium level waste, offgases or heterogeneous

contaminated scrap The real problem is very low level liquid

waste, because it arises in such enormous volume Coming from numerous different sources—e.g cooling and final wash waters, laundry and decontamination center effluents, floor drainage from cleanup operations, personnel shower drainage and effluent from the final stages of liquid waste purification plants—low level and “essentially uncontaminated but sus-pect” waste adds up to billions of gallons per year Although some countries (Sweden and Japan, for example) evaporate such effluents on a large scale they are usually discharged by some route into the environment

Research and Development

A wide variety of wastes arises in such research establishments

as Brookhaven (USA), Chalk River (Canada) or Harwell (UK) and the include many of the types mentioned under the head-ing of fuel processhead-ing In addition the research reactors usually produce very large quantities of radioisotopes which may be processed onsite However, the quantities involved are very much lower, especially in the high level category, and elaborate waste processing systems are seldom needed even at large research centers unless they are situated in built-up areas

or immediately over important aquifers

Hospitals and Biological Laboratories

Organic material and excreta makes wastes from these insti-tutions difficult to handle The radioactive content is usu-ally small, and limited to a restricted list of radionuclides

Those used as sealed sources seldom appear as waste, and the rest are practically confined to 131 I, 32 P, 59 Fe, 51 Cr, 35 S and

24 Na Other nuclides may be used in small amounts for spe-cial purposes such as specific location in certain organs The nature and amount of radionuclides used in these institutions are such that a high proportion of the waste can be handled safely by the municipal sewage and garbage systems

SPENT FUEL

STACK

PURIFICATION

OFF-GASES

NITRIC ACID

DISSOLVER

HIGH LEVEL WASTE MEDIUM LEVEL WASTE LOW LEVEL WASTE

SEPARATION OF PLUTONIUM AND URANIUM

WASH WITH ACID

ORGANIC LAYER EXTRACTS

Pu plus U

AQUEOUS LAYER

ORGANIC SOLVENT

FIGURE 2 Schematic diagram of fuel processing plant Showing origins of main waste streams Reactor fuel contains over 99.95% of the total radionuclides eventually disposed of as waste

Sealed sources are, however, a very difficult matter While

they remain sealed they are usually within heavy shielding

in teletherapy machines, which are only operated by

compe-tent people, or they are in the form of needles and plaques

for implantation, or instrumental standard sources used by

specialists However, the time comes when such sources

have decayed to the point where they are no longer useful

Sufficient activity remains for them to be highly dangerous

to the unwary, so they are dealt with in special ways, usually

after return to the supplier

Isotope Production Plants

These facilities are often associated with large reactors,

and wastes are similar to those generated in Research and

Development plants Processing of very large sources of

volatile elements such as iodine and tellurium necessitates

an elaborate ventilation cleaning system Manufacture of

large sources of 90 Sr, 137 Cs or the trans-uranic elements as

power sources may call for sophisticated remote handling

equipment in heavily shielded cells But the waste

prob-lems are difficult only in scale from those encountered in

an R and D plant

Some people have considered the separation of 90 Sr and

137 Cs from fuel processing wastes as a helpful step in their

management Removal of these nuclides leaves a mixture

which, during 20 years’ storage, would decrease in activity

by a factor of about 30,000 However, an industry handling

the fission products from 50 tons of 235 U burned in one year

would have to deal with 500,000,000 Curies of separated

90 Sr and about the same amount of 137 Cs It might be difficult

to find a market for sources of this scale unless they were

cheap, and it must be remembered that they would

eventu-ally come back as “waste.”

Industrial Applications

Use of radioisotopes in industry is not a significant source

of wastes Most industrial sources are sealed, and nearly all

unsealed sources are short-lived

Transportation

Ships are the only form of transportation using nuclear

reactors as a source of power They include naval ships, ice

breakers and merchant vessels They contain large amounts

of fission products within the reactors, but as a source of

waste they are not important, except possibly in some

har-bours and inshore waters

During start-up of the reactor the secondary coolant

expands and the limited space in submarines necessitates the

dumping of this expansion water In common with landbased

reactor coolant it contains radioactive corrosion products and

tritium The coolant is maintained at a low level of activity

by means of ion exchangers, which become waste eventually

Normally this material is disposed of on land, although it has

been shown by the Brynielsson Panel of the International

Atomic Energy Agency that resin from a fleet of as many as

300 nuclear ships could be dumped safely if this were done only on the high seas

Apart from these sources wastes from nuclear shipping consist of clean-up solutions, laboratory wastes, laundry effluent and other minor sources common to all reactor operations Except in submarines, practically all wastes can

if necessary be retained on board for disposal ashore

DISPOSAL PRINCIPLES There are two main procedures available for disposal—

Concentration and Confinement: or Dilution and Dispersion

a) If wastes are truly confined, in the sense that in

no credible circumstances could they be liberated into the environment, then the only additional requirement is “perpetual custody” to ensure that the confinement is never broken This is easier said than done In the field of high level wastes when we say “perpetual” we are speaking in terms of thousands of years Few private firms go back for 100 years, political regimes have seldom lasted for as long as 500 years, and there are few civilizations that have survived for 2000 years

In our own day forecasters tend to regard dates beyond 2000 AD as being in the distant future

What, then, can we do about “perpetual custody”

of wastes containing, for example, plutonium with

a half-life of 24,000 years?

This is not a fanciful dilemma A story from Chalk River will illustrate the point When the Canadians decided to concentrate on natural ura-nium heavy water reactors for power production

it became apparent that processing of spent fuel would be uneconomic until the price of uranium or plutonium rose considerably Processing was there-fore stopped, but the wastes accumulated during the pilot plant operation had to be disposed of

A considerable volume of medium level waste was mixed with cement in steel drums and enclosed within solid concrete monoliths below ground in the waste management area (Figure 3).The ques-tion then arose “What if some archeologist digs this structure up 1000 years from now and thinks

it is an ancient temple or tomb?” Eventually some-one suggested that its true nature should be inlaid

in non-corrodible metal on the top of the monolith

Dr A J Cipriani, who had listened to the debate in silence, then asked “In what language?”

The implications of this question are profound

Some of the wastes for which we are responsible will still be radioactive after our present civilization has disappeared and perhaps been forgotten So far as we know there is no practicable solution to the problem The best we can do is ensure that the nature, amount and location of all major disposals

are recorded in the nearest approximation we have

to a perpetual repository of archives—a government department Beyond that we can only rely on folk memory After all, farmers in Europe have been ploughing around Neolithic tumuli and prehistoric roads for thousands of years for no good reason known to them, except that it was accepted to be the right thing to do

b) Dilution and dispersion is the traditional method

that men have always used for dealing with their wastes Until recently it seemed to work fairly well unless populations became very concentrated, but

it is now becoming clear that there are so many people that the system is showing signs of break-ing down It depends upon the capacity of the environment to dilute or detoxify the wastes to a level that is innocuous to man and to organisms of interest to man We are still a very long way from contaminating our environment with radioactivity

to a point where radiation effects are observable, even in close proximity to nuclear enterprises, but

we must maintain vigilance to ensure that slow and subtle changes do not occur which escape our notice until it is too late

Safety in discharge to the environment depends upon three factors—(1) Dispersion by such means

as atmospheric dilution, mixing into big bodies of water, or spreading through large volumes of soil

(2) Fixation of radionuclides on soil minerals and organic detritus (3) Decay of radionuclides, dis-persed or fixed, before they are able to affect man

The principle of dispersion has one logical trap into which regulatory bodies have sometimes fallen In some countries the discharge of liquid and gaseous wastes is limited by the concentra-tion in the effluent pipe or the concentraconcentra-tion at the stack mouth This is based upon the assumption that if the concentration is limited to the maxi-mum permissible value, all will be well However, the “dilution capacity” of a river is a function of the number of Curies per day put into the river, divided by the daily flow of water If an operator wishes to dispose of double the amount of waste, and he is limited only by the concentration in the effluent pipe, he need simply double the amount

of water flowing in the pipe But the downstream effect will be a doubling in the concentration, unless he has doubled the flow in the river

concrete slabs surrounded with forms The forms were filled with concrete The monoliths were about 2 m below ground level

For this reason, limitations must be made in

Curies per unit time, not in micro-curies per mil-lilitre, and account must be taken of volume of river flow if this is seasonally variable Regulations set on the basis of concentration at the point of discharge only protect people close to the discharge point

DISPOSAL PRACTICES

Gases

Radioactive gases arise mainly in reactors, spent fuel

pro-cessing, isotope production, and research and development

facilities The general principles are the same for all procedures

that depend upon dispersion into the atmosphere

If we have a stack that is emitting Q Curies/sec., the

con-centration C at a given distance downwind will be KQ The

parameter K is a very complex function which depends upon

wind speed and direction, weather conditions, stack height,

topographical features, variability of temperature with height,

velocity and buoyancy of the effluent and other conditions

Values of K for a range of conditions can be calculated from

equations proposed by Sutton (1947), Pasquill (1961) and

Holland (1953) These equations have been used to calculate

the permissible emissions from stacks by inserting

appropri-ate numbers and parameters applying to unfavourable weather

conditions likely to obtain at the site The permissible emission

rate has been set at a value which would ensure that

popula-tions downwind would not be exposed to more than an agreed

maximum radiation dose rate

The classical equations have been based on statistical

theory with empirical values for the diffusion parameters

being obtained from experimental work which has

some-times had little relation to real emissions from actual stacks

Returning to the superficially simple equation C ⫽ KQ,

it is apparent that if we could observe, over a long period

of time, the maximum value of C ever attained per unit

emission rate, we could define a figure K max which was not

likely to be exceeded With a sufficient number of

obser-vations of C and Q, extended over a sufficient variety of

weather conditions, we could estimate the probability that

our value K max could ever be exceeded

When a maximum permissible concentration is set for a

noxious substance the decision really depends upon a belief

that the probability of damage is so low that it is acceptable

If, then, C is set at the MPC at a given distance from the

stack, and K max is known for that distance, then Q p , the

max-imum permissible release rate, is determined

It has been shown by Barry that K max is not very

depen-dent upon topography or climate, because it depends mainly

on rather large-scale behaviour of the atmosphere, and the

frequency of most adverse conditions normally experienced

do not vary grossly from one place to another

The maximum permissible emission rate—or in some

cases the MPC at the stack mouth—is given in the

regula-tions governing the plant or laboratory It is then the

respon-sibility of the operator to ensure that emissions are kept as far

below the permissible level as may be practicable Numerous

methods are available, other than variation of stack height, for achieving this end (Figure 4)

Filtration It is advisable to filter contaminated air near

to the source of the activity This reduces the amount of air to

be filtered and also cuts down the “plating-out” of radionu-clides on the duct-work, which can be a source of radiation fields with the plant

Filters must be suitable for the job they are supposed to

do They should be made of non-flammable material such

as glass or other fibre and should be tested before and after installation If fine (e.g “Absolute”) filters are used it is often necessary to precede them with a coarse filter to avoid rapid clogging with dust

Filters must be very efficient to be adequate for fuel processing plants and incinerators burning highly active waste For example, a sand filter at Hanford capable of pass-ing 10,000 m 3 /min had an efficiency of more than 99.5%, but this was inadequate The necessary efficiency of 99.99% was attained with a bed of glass fibers 100 cm thick

Electrostatic Precipitators Small airborne particles are

usually electrically charged The charge can be increased

by passing the air through a corona discharge, or through

a charged fabric screen The particles are attracted to a sur-face carrying the opposite charge, from which they can be removed mechanically It is possible to use the same prin-ciple by imposing a charge on filters

Steam Ejector Nozzles The most efficient air

clean-ing device other than “Absolute” filters consists of a nozzle

in which the air is mixed with steam and expelled into an expansion chamber where the steam condenses on the par-ticles After passing through a second construction into another expansion chamber, where the air is scrubbed with water jets, removal efficiency for 0.3 micron particles is 99.9%

Incinerator Off-gases The hot gas from an incinerator

carriers with it fly ash, tars and water vapour as well as particles

Tars may be removed and the gases cooled by water scrubbing devices Water droplets must then be eliminated by reheating or passage through a “cyclone” This is a cylinder with a conical bottom Gas injected tangentially at the top sets up a vortex which causes deposition of particles on the sides

In smaller incinerators the gases are cooled and some fly ash is removed by passage through a cooling chamber fitted with baffles After this stage a roughing or “bag” filter is used, followed if necessary by Absolute or charcoal filters

Processing Plant Gases The devices required for

clean-ing gaseous effluent depend on the nature of the process

Off-gas from boiling high level wastes must be passed through condensers and scrubbers to recover nitric acid as well as to remove volatile radionuclides However, these and other air cleaning equipment previously mentioned will not remove gases such as 85 Kr, nor hold back all of the radioactive halogens

Radioactive iodine in molecular form is fairly easily absorbed by alkaline scrubbers and copper or silver mesh filters, but in the form of methyl iodine it can only be arrested

by an activated charcoal filter These filters have to be kept cool, not only to remove the decay-heat of adsorbed halogens

but also because 85 Kr is absorbed much more powerfully by

cold charcoal This is the only practical means we have for

removal of radioactive noble gases

The very large dispersive capacity of a high stack usually

makes it unnecessary to remove 14 C (as 14 CO 2 ) or tritium

(mainly 3 H 1 HO) because their toxicity is very low However,

the coolant CO 2 in a gas-graphite reactor does contain

enough 14 C to require alkaline scrubbing, which removes

radioiodine as well

Liquids

Storage The necessity for long-term storage of very large

quantities (many millions of gallons) of high level, strongly acid

waste has led to the development of tankage and pipeline

sys-tems which have stood up to severe conditions for many years

Failures have occurred, but good design and carefully selected

materials have prevented environmental contamination

Tanks are constructed from material, often stainless

steel, which will not be corroded by the solutions to be

stored Secondary containment is provided by catch tanks or

drip trays and sufficient spare tankage is kept available for

rapid emptying of a ruptured tank Leakage is detected by

a monitoring system which alarms immediately if

radioac-tive liquid appears in the catch tank (Figure 5) Movement

of active liquid is effected by pumping rather than by gravity

to ensure that it is the result of deliberate action rather than accident

Evaporation The most straight-forward and apparently

the simplest method of treatment for radioactive liquid wastes is evaporation In a carefully designed evaporator with an efficient droplet de-entrainment system the radio-nuclide content of the distillate can be about one millionth

of that in the pot There is little about the design that is spe-cifically related to radioactivity except that shielding may have to be provided for the operator, and off-gases must be monitored and possibly treated in some way Unfortunately, evaporation is expensive because it consumes a large amount

of energy and the end product—the concentrate—is still a radioactive liquid waste Evaporation to dryness or to the point of crystallization has been practised, by the residue is

so soluble in water that without further processing it is not suitable for disposal

Where discharge of a large volume of low-level waste into the environment is unacceptable the cost of evapora-tion may be justified by its many advantages Practically all liquid wastes are treated by evaporation in Denmark and Sweden, and it is also widely used in Japan

Residues from evaporation may be mixed with cement, fused with glass frit or various ceramic mixtures, or incor-porated with melted bitumen The product is then handled as

a solid waste

CONTAINMENT:

IN CLADDING

IN PRIMARY CONTAINMENT

IN SECONDARY CONTAINMENT

IN EXCLUSION AREA

EMERGENCY COOLING

FILTER–ADSORBER SYSTEMS

ORNL– DWG 70– 9869

FIGURE 4 Reactor containment system Any leakage from fuel must pass through the cladding, the primary containment, and

either the secondary containment or the stack filters Contamination within the building can be removed by sprays and/or filters

Flocculation and Precipitation The cheapest and

sim-plest process for treatment of radioactive liquids is removal of

the activity on some kind of precipitate, either as an integral

part of the precipitated material, or adsorbed on its surface

In most waste tanks a sludge settles out which may

con-tain up to 90% of the activity, and a copious precipitate of

metallic hydroxides is formed on neutralization which may

carry down up to 90% of the remainder Further purification

of the clear effluent after separation of these sludges can be

achieved by addition of lime and sodium carbonate Up to

99% of the remaining activity can sometimes be removed by

this treatment Treatment with lime and sodium phosphate is

also very effective (Figure 6)

The treatment used depends upon the particular

radio-nuclides present in the waste, and also its gross composition

for example, the pH and salt content of the solution In some

cases ferric chloride, clay or other additives are introduced

at carefully chosen points in the process The selection of the

process, and modifications introduced as the composition of

the waste changes, require constant analysis and control by

specialized chemists

One problem common to all flocculation processes is how to deal with the sludge The floc settles very slowly and after it has been drained through filters or separated by centrifugation it is in the form of a thick cheese-like solid which, in spite of its appearance, still contains 80 to 90% of water In a successful British process the sludge is repeat-edly frozen and thawed The separation of pure ice crystals leaves behind a concentrated salt solution which coagulates the small particles of floc into a form which settles more rapidly and is less likely to clog vacuum filters

Ion Exchange The effluent from a flocculation process

may still contain too much activity for discharge to public waters It can then be passed through ion exchangers, which are expensive but very efficient They cannot be used eco-nomically on a solution with a high salt content because their ion-exchange capacity would rapidly be exhausted by absorbing the dissolved salts

The effluent from a well-controlled flocculation process has a low total-solids content and after filtration to remove traces of floc it can be passed through a cation exchanger

or mixed-bed resin suitable for removal of the radioactive

COOLING COIL RISER INSTRUMENT RISER

CONDENSER

FILTER (FIBERGLASS)

NOTE:

ALL WELDS ARE RADIOGRAPHED

STEEL WASTE TANK SHOTCRETE

GROUT

STEEL PAN

CONCRETE SLAB WATERPROOF

MEMBRANE

CEMENT PLASTER

SUPPORT COLUMN VERTICAL

COOLING COIL HORIZONTAL COOLING COILS

STEEL

WASTE

TANK

INLET

FIGURE 5 Structure of high level waste tank at Savannah River