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Nuclear and radiation safety:Guidance for emergency response

Criteria have been established at the international level to guide decisionsabout protective actions after a nuclear or radiological emergency

involving radioactive materials inrecent years have had consequences for thehealth of the general public. These have rangedfrom the major accident at Chernobyl in 1986 toaccidental dispersion of medical and industrialradioactive sources.

Responses to these accidents differed be-tween countries. It later became apparent thatsome protective actions were taken that, in themost extreme cases, may have worsened, ratherthan improved, the well-being of the populationsinvolved and their environmental surroundings.In other cases, the actions led to large but unpro-ductive expenditures of national resources. Fur-ther, where the accident involved exposure ofpopulations across national boundaries, many in-stances occurred of contradictory national re-sponses either side of the national borders.

During the past decade considerable pro-gress has been made in developing internation-ally recognized principles for decisions on pro-tective measures following accidents involvingradioactive materials, and in providing quantita-tive guidance for applying these principles. Ef-forts have involved the IAEA, the InternationalCommission on Radiological Protection (ICRP),World Health Organization (WHO), Food and Ag-riculture Organization (FAO), Commission of theEuropean Communities (CEC), and Nuclear En-ergy Agency of the Organization for EconomicCo-operation and Development (OECD/NEA).

This article summarizes guidance on the ra-diation protection criteria that have been estab-lished with regard to responding to nuclear acci-dents or radiological emergencies, and the prin-ciples for establishing intervention levels. Theguidance was developed to assist those at na-tional and regional bodies and at nuclear facili-ties having responsibility for emergency res-ponse planning.

Mr. Cnck is a staff member in the IAEA Division of Radiationand Waste Safety, Department of Nuclear Safety.

Establishing international consensus

In 1985, the Agency published Safety SeriesNo. 72, which set out guidance on the principlesfor establishing intervention levels for the pro-tection of the public in the event of a nuclearaccident or radiological emergency. That guid-ance was aimed at assisting national and regionalauthorities having responsibility for emergencyresponse planning to specify levels of projecteddose at which it may be necessary to introducerelevant protective measures. It recognized aneed for practical quantities that could be readilycompared with the results of measurementsmade in environmental materials and in food-stuffs, so-called Derived Intervention Levels(DILs). Shortly after the accident at the Cher-nobyl nuclear power plant in 1986, the Agencypublished Safety Series No. 81, which addressedthe principles, procedures, and data needed toestablish these DILS. Guidance was also givenon the extent to which the supportive numericaldata and the illustrative DILs might have moregeneric application.

Additionally over the past decade, new rec-ommendations for radiation protection havebeen issued by the ICRP; the FAO/WHO CodexAlimentarius Commission published GuidelineLevels of Radionuclides in Food Moving in In-ternational Trade; WHO issued recommenda-tions on Derived Intervention Levels for Protect-ing the Public; and the International ChernobylProject made a number of important recommen-dations.

In 1991, the IAEA revised its Safety SeriesNo. 72 to clarify the guidance with respect tointervention, and provided illustrative examplesof how intervention levels are established inemergency plans. It stopped short of providingnumerical intervention levels that might havesome generic application.

The emergency response to the Chernobyl ac-cident underscored the need for a simple set of

byMalcolm Crick

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consistent intervention levels at the internationallevel. Such a set of values was considered desir-able to increase public confidence in authoritiescharged with dealing with the aftermath of anaccident. Additionally, since many countries donot have nuclear facilities and hence detailedemergency plans themselves, a simple interna-tionally agreed set can assist them in the eventof transboundary releases.

In the process of establishing internationalconsensus on the values of these generic inter-vention levels, the IAEA convened a number oftechnical meetings. The work led to the prepara-tion, in 1993, of Safety Series No. 109, Interven-tion Criteria in a Nuclear or Radiation Emer-gency. This Safety Guide, published in 1994,represents the international consensus reachedon principles for intervention and numerical val-ues for generic intervention levels. These princi-ples and values subsequently became the basis ofintervention guidance in the Basic Safety Stand-ards for Protection against Ionizing Radiationand for the Safety of Radiation Sources, whichhave been issued jointly by the IAEA, FAO,ILO, NEA, PAHO, and WHO.

Summary of guidance

Prompt and Delayed Radiation Effects onHealth. For most prompt (or deterministic) ef-fects on health, the severity is related to the levelof dose to the individual and there is a practicalthreshold radiation dose below which effects arenot clinically observable. The most severe con-sequence is death, which may occur in sensitiveindividuals, due to bone marrow failure, at dosesabove one gray (Gy) delivered promptly to thewhole body. Serious prompt effects may alsooccur in other organs. Most of the thresholddoses for these are above that for bone marrowand will be avoided if the dose to the whole bodyis below one Gy. However, some individual or-gans, such as the thyroid and the lung, may receivehigh doses due to breathing or swallowing certainradionuclides and must be considered separately.

Delayed (or stochastic) effects include a widerange of cancers and hereditary effects, for whichthe probability of occurrence (not the severity)increases with dose. They usually appear manyyears after exposure, and, although they do notoccur in every exposed individual, there is nothreshold for their induction. Because of the as-sumed linear (proportional) relationship betweendose and the probability of these effects, it ispossible to estimate the number expected to oc-cur in a large exposed population even if thechance of an effect is very small for most indi-viduals. Since other causes (mostly unidentified)

can give rise to the same effects, it will be usuallyimpossible to identify those caused by radiation.

Typically, even a severe accident will causehigh doses to relatively few and small doses tomany people. Most cancers and hereditary ef-fects will occur in large populations that receivesmall doses. These usually cannot all be avoidedand the objective of intervention is to reducetheir number as much as is reasonably possible.

Exposure Routes and Dose Projections.Although accidental releases may occur to air,water, or land, those most likely to require urgentprotective action are major releases to air. Fol-lowing such a release people may be exposed toradiation from the airborne radioactive cloud andthrough inhalation of radioactive dust from thecloud. As the cloud disperses, particles willslowly settle on the earth's surface or be depos-ited rapidly by rainfall. People then may be ex-posed to radiation from these deposits, from in-haling resuspended dust, or from contaminatedfood or water.

During an accident, potential doses to thepopulation will usually be estimated by well-qualified professionals. However, early on, thereare many uncertainties (e.g., in the amount andrate at which radioactive material is being re-leased and in the meteorological situation). Be-cause of this and the need to use simple mathe-matical models to obtain results soon enough tobe useful, there will be large uncertainties inearly dose estimates.

Decision-makers must be aware of this situ-ation and ensure that their expert advisors pro-vide an expression of uncertainties in early esti-mates of projected doses. They should not relyon "most likely" estimates alone (which couldlead to wrong conclusions with severe repercus-sions for the population) and must consider theuncertainties in arriving at a suitable decision onurgent protective action. Later, as the situationbecomes clearer, it will be possible to modifyand initiate protective actions with a much firmergrasp of projected doses.

Normal and Emergency Situations. Undernormal conditions, doses from man-madesources (e.g., from nuclear power or the practiceof medicine) are kept within specified levels.These are much lower than would prompt a needfor protective action; typically they are compara-ble to local variations in natural background ra-diation. They are achieved through the use ofcontrols on the radiation source and do not re-quire direct constraints on people.

In the event of an accident, radioactive ma-terial released into the environment is no longerunder control; doses can only be reduced throughprotective actions — such as evacuation, shelter-ing, relocation, resettlement, prophylatic use of

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iodine, and restrictions on food and water — allof which impose constraints on people's activi-ties. These actions may also incur additionalrisks. Therefore, in choosing the level at which aprotective action should be initiated, it is neces-sary to consider the effects of constraints onpeople's activities and any additional risks fromthe action itself.

For the above reasons, the levels of dose forintervention following an accident and the levelsfor control of doses under normal conditions willbe different and it is important to avoid confu-sion between the roles of these two differentkinds of levels.

Protective Actions. There are limited majoroptions available to protect the public after anaccident. The most important are the following:

For early, or urgent, response: 1) sheltering,through advising people to remain indoors andclose their doors and windows, usually for lessthan a day; 2) evacuation, the urgent removal ofpeople from a specified area for periods on theorder of days; and 3) prophylactic administrationof iodine, if high intakes of radioactive iodinehave occurred or are expected to occur.

For later phases of the response: 1) tempo-rary relocation of people to a new habitat, usuallyfor no longer than one to two years; 2) permanentresettlement of people in new or existing settle-ments for the foreseeable future; and 3) controlof food and water contaminated in excess ofspecified levels.

Principles and levels of intervention

Three principles have been agreed upon bythe international community as a general basisfor intervention. They may be paraphrased asfollows: 1) Intervention to avoid serious prompthealth effects should be carried out as a firstpriority. 2) Protective actions to avoid delayedhealth effects should be initiated when they willproduce more good than harm in the affectedpopulation; and 3) These actions should be intro-duced and withdrawn at levels that produce amaximum net benefit to the population.

The first principle is critical for response toan accident producing any high doses. It meansthat any immediate threat to individuals shouldbe countered through evacuation (or, rarely,sheltering) (and, when appropriate, iodine pro-phylaxis) as a first priority, and carried out to themaximum extent of immediately available re-sources. There may be rare cases when evacu-ation to satisfy this first principle is not appropri-ate because it could cause greater harm (e.g.moving people on life support systems, or in theface of a competing disaster).

Intervention levels for minimizing delayedhealth effects are based on the second and thirdprinciples. In applying these principles, the terms"good," "harm," and "benefit" include — inaddition to health and safety and the tangiblecosts of protective actions — unquantifiable fac-tors such as reassurance, stress, and attention tosocietal values. These are not within the primaryprofessional competence of the radiation protec-tion expert. They are more appropriately the re-sponsibility of the decision-maker. He or shemay choose to consider these factors, in additionto those addressed by this radiation protectionadvice, in arriving at decisions that will producethe maximum benefit in the affected population.

Furthermore, the second and third principlesaddress only the risk of delayed effects in thepopulation as a whole. This means that they donot explicitly limit individual risks. A signifi-cantly higher than normal risk of delayed effectsto even a few individuals may be an importantfactor in national decision-making. For this rea-son authorities may choose an action level toavoid unacceptably high individual risks.Whether intervention at such a level is alwayspossible will depend on the accident's severityand nature and the resources at the disposal of thecountry. Such action levels were not consideredin deriving the generic intervention levels inSafety Series No. 109 and may lead to lowervalues for intervention, particularly in the case ofprotective actions for later phases of a response.

Protective Actions for Early, or Urgent, Re-sponse. These actions must be applied promptlyin order to be effective. Delays may lead topopulation doses that could have been avoidedand in the worst cases could lead to prompthealth effects. Rapid decisions are difficult be-cause there is usually very limited early informa-tion about an accident and large uncertaintyabout its consequences. For this reason pre-plan-ning should be carried out wherever possible sothat decisions can be made rapidly based onfacility conditions and pre-arranged patterns forresponse, rather than just on measurements car-ried out and actions hastily organized during theearly course of an accident. In the case of fixedfacilities with well-understood characteristics,response plans should prescribe action to imple-ment urgent protective actions on the basis offacility conditions, rather than rely on confirma-tion of an actual release through measurements atthe facility or offsite, whenever it is reasonablyfeasible to do so.

Sheltering means staying in buildings to re-duce exposure to airborne contamination andsurface deposits, and closing doors and windowsand turning off ventilation systems to reduceinhalation of radioactive material from outside

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Genericintervention levels

in emergencyresponse

situationsAction

Urgent protective actions

Avertable dose (Generic intervention level)

Sheltering 10 mSv for a period of no more than 2 days

Iodine prophylaxis 100 mGy (committed absorbed dose to the thyroid)

Evacuation 50 mSv for a period of no more than 1 week

Generic action levels for foodstuffs(From the CODEX Alimentarius Commission guideline levels for radionuclides in food moving in international

trade following accidental contamination )

Radionuclides

Caesium-134, Caesium-137, Ru-themum-103, Ruthenium-106,

Strontium-89

lodine-131

Strontium-90

Americium-241, Plutonium-238,Plutonium-239

Foods destined for generalconsumption (kBq/kg)

1

0.1

0.01

Milk, infant foods, and drinkingwater (kBq/kg)

1

0.1

0.001

Long-term actions

Action Avertable dose (generic intervention level)

Initiating temporary relocation

Terminating temporary relocation

Considering permanent resettlement

30 mSv in a month

10 mSv in a month

1 Sv in a lifetime

air. Sheltering can also facilitate staging forevacuation and the prophylactic use of iodine.Because of the small penalties, sheltering may bejustified at low dose levels. However, its effec-tiveness decreases rapidly with time for moststructures (typically reducing doses to airborneparticulates by a factor of two or three in a fewhours) and is low for lightweight structures orthose with high air exchange rates. Further, thereis a limit to the time that populations can remainindoors without undesirable complications.

The generic intervention level for shelteringis 10 mSv. This value was selected based on themaximum anticipated period of sheltering (2days). Sheltering may be advised at lower levelsfor shorter periods or to facilitate other protectiveactions.

Sheltering can be effective if the exposure isof short duration and buildings are of densestructure and well sealed, as in some northerncountries. In many warm countries, however,most houses are made of light materials, andpeople cannot stay indoors in sealed houses forlong periods. These factors must be considered

when choosing between protective actionthrough sheltering versus evacuation.

Evacuation is the urgent moving of peoplefrom their normal housing for a limited period oftime. Its use should be based on the dose that canbe avoided by evacuation and would not beavoided by sheltering. The generic interventionlevel for evacuation is 50 mSv. This value hasbeen selected based on the maximum anticipatedperiod of evacuation (7 days). Evacuation maybe initiated at lower levels for shorter periods orwhen it can be carried out easily, e.g. for smallgroups of people. Under exceptional circum-stances (such as hazardous weather, or the pres-ence of a competing disaster) or where evacu-ation would be unusually difficult (for very largepopulations or in the absence of adequate trans-portation) initiation of prompt evacuation maybe deferred to a higher intervention level.

In cases before an actual release has started,and where projected doses exceeding this levelhave a relatively high probability of occurrence,preventive evacuation normally will be advis-able. Evacuation as a protective action is com-

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monly used when people are threatened by otherman-made hazards (e.g., fire or chemical spills)or by forces of nature (e.g., hurricanes, tornados,earthquakes, or floods). In most cases peoplereturn in a short period, typically one to twodays, if their homes do not require prolongedclean-up. Because of the short time involved,primitive accommodation in schools or otherpublic buildings is typical.

Prophylactic use of iodine is the administra-tion of stable (non-radioactive) iodine in order toblock the uptake of radioiodine by the thyroid. Itmust be carried out promptly to be effective(ideally several hours before and no later than afew hours following exposure). For this reasonthis protective measure is most commonly prac-tical only when emergency planning has in-cluded predistribution of stable iodine to thepopulation at risk. It will usually be coupled withevacuation or sheltering. The generic interventionlevel for prophylactic use of iodine is 100 mGy. Thislevel applies to the dose to the thyroid that wouldbe received from intake of radioiodine. Sincethere may be complications depending on localdiet and other factors, public health authoritiesshould be involved in implementing this measure.

Protective Actions for Later Phases of aResponse. Sheltering and evacuation are short-term protective measures. If measurements con-firm that doses warrant further action, temporaryrelocation or permanent resettlement, and con-trol of food and water may be necessary.

For early protective actions, the greatestbenefit is likely to accrue if action is taken withminimal delay, based on rough predictions ofhow the accident will develop. For long-termprotective actions, there will usually be a rathersmall radiological health penalty for delaying toobtain accurate measurements for projectingdoses. Moreover, the social and economic penal-ties for imprudent decisions can be high, owingto the long period protective actions may be ineffect. It is important that a decision to imple-ment these protective actions is carried out in asinformed a manner as possible, using best esti-mates for the consequences of different options.

Temporary relocation means the organizedremoval of people for an extended but limitedperiod of time (e.g., several months) to avoiddoses from radioactive material deposited on theground, including resuspended materials, and insome cases from local food or water. Peopletypically would be housed in temporary accom-modation of a reasonable minimum standard ofcomfort and privacy. The generic interventionlevels for initiating and terminating temporaryrelocation are 30 mSv in a month and 10 mSv ina month, respectively; i.e., people should be tem-porarily relocated if the dose avertable over the

next month is expected to be greater than 30mSv. They may return when the avertable dosefalls below 10 mSv in a month. However, if thedose accumulated in a month is not expected tofall below this level within a period of a year ortwo, the population should be permanently reset-tled. Two levels are specified because there arerelatively high penalties for initiating relocationcompared to maintaining it. It is also necessaryto specify the period of time it is reasonable tolive in temporary housing.

Permanent resettlement means complete re-moval of people from the area with no expecta-tion of return for at least several years. Peopletypically would be resettled in accommodationscomparable to those vacated. This may involveconstruction of new housing and infrastructure.The generic intervention levels for permanent re-settlement are 1 Sv in a lifetime or a dose exceed-ing 10 mSv per month that persists beyond oneor two years (i.e., that does not permit return fromtemporary relocation within one or two years). Itshould be recognized that projected dose.s belowthe intervention levels for evacuation or for ter-minating temporary relocation could also, over alifetime, become high enough (i..e., exceed 1 Sv)to warrant permanent resettlement.

Control of food and water may have to beconsidered under three different circumstances:where alternative supplies are available; wherealternative supplies are scarce; and for distribu-tion in international trade. Generic action levelshave been established for use by nationalauthorities when alternative supplies of food areavailable. (See table.) The values depend uponthe type of foodstuff and the type of contaminat-ing radionuclide. The radionuclides in questionare those most likely to be of concern in foodsfollowing an accident.

In situations where extensive restrictions onfood supplies could result in nutritional deficien-cies or, in the extreme, starvation, case-by-caseevaluations will be required. In most such situ-ations relocation will be indicated, and alterna-tive food made available. However, when this isnot possible, the radiation hazard must be con-sidered realistically in comparison to competinghealth hazards, and higher action levels shouldinvariably be adopted.

Following any event that may contaminatefoodstuffs, a variety of countermeasures may beinstituted at various stages in production andmarketing. These should be implemented to en-sure that, to the maximum extent practicable,foodstuffs are maintained below the action lev-els. The generic action levels for foodstuffs willalso satisfy the requirements for distribution offoodstuffs in international trade for consumptionin countries unaffected by an accident. •

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