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Application of the International Commission on Radiological Protection System to the disposal of long-lived solid

Annie SUGIER Institut de Protection et de Sûreté Nucléaire, BP n° 6, 92265 Fontenay-aux-Roses Cedex,

1. INTRODUCTION

The objective of waste disposal is to protect current and future generations.

The International Commission on Radiological Protection is a reference in the field of radiological protection in the European Union, as ICRP recommendations constitute the basis for the regulatory provisions of the European Union in this domain. This is why, in the context of the conference on "radioactive waste management strategies and issues" it appeared important to describe the new ICRP policy concerning radioactive waste disposal.

ICRP provides recommendations and guidance on specific topics ranging from the protection of workers to potential exposures, exposures in accident situations etc. However, the strength and coherence of the Commission’s System of Protection lies in its ability to cover a wide range of circumstances in a consistent way. The basic elements of ICRP policy are defined in its general recommendations published in 1990 (ICRP 60).

As concerns the disposal of radioactive waste, the ICRP published a document entitled "Radiation Protection Principles for the Disposal of Solid Radioactive Waste" in 1985.

Since then, in-depth consultations have been conducted in different national and international organisations on issues such as the ethical implications of the protection of future generations, the search for indicators of environmental protection, the principles of sustainable development, the precautionary principle and the review of assessment methods and ways of building confidence.

It is in this context that in 1998 ICRP issued Publication 77 entitled "Radiological Protection Policy for the Disposal of Radioactive Waste". This report had been prepared by a Task Group of the Main Commission itself. The report reaffirms the Commission’s current policy of radiological protection, in particular its policy on public exposure, and aims to clarify the practical application of that policy to the disposal of radioactive waste.

In September 1999, two reports were adopted by the Main Commission and should be published in 2000:

• Protection of Public in Situations of Prolonged Radiation Exposure (Task Group chaired by Abel Gonzalez),

• Radiation Protection Recommendations as Applied to the Disposal of Long-Lived Solid Radioactive Waste (Task Group chaired by Annie Sugier).

This communication deals mainly with this second report. However, it emphasises, as a starting point, the conceptual framework of the Commission’s system of protection.

2. CONCEPTUAL FRAMEWORK

The primary aim of ICRP policy is to provide an appropriate standard of protection for man without limiting the beneficial practices giving rise to radiation exposure.

Two kinds of effect have to be taken into account: deterministic effects which can be avoided by restricting the doses to individuals below well-known thresholds, and stochastic effects which cannot be completely avoided because it has not been demonstrated that there are any thresholds for them.

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ICRP considers that some residual risk is acceptable as long as it has been limited by all reasonable means.

The system of Radiological Protection divides exposure situations into practices and interventions. Practices are human activities that are undertaken as a matter of choice despite the increase in exposure. This increase can be avoided by refraining from undertaking the practice or controlled by taking precautions. Sometimes, however, exposures result from situations that already exist. If any action is required to reduce exposures or to remove sources, the action is called intervention. It is clear that the principles of protection for practices apply to the disposal of radioactive waste, as long as it is a planned operation with the source under control.

The conceptual framework for practices developed by ICRP is based on three principles, namely justification of a practice on the grounds that it produces sufficient benefit to offset the radiation detriment that it may cause; optimisation of the protection, in relation to any particular source within a practice, economic and social factors being taken into account; and finally limitation of the exposure to which an individual may be subjected from the combination of all the relevant practices.

In Publication 60, the Commission distinguishes between "normal exposure" and "potential exposure". "Normal exposures" are those that are virtually certain to occur and which have a magnitude which is predictable, albeit with some uncertainty. The term "potential exposures" refers to situations where there is a potential for exposure but no certainty that it will occur, i.e. the type of situations of concern in the long term following closure of a solid radioactive waste disposal facility.

The principles and objectives of the 1990 Recommendations, as they relate to potential exposures, are developed in Publication 64, "Protection from Potential Exposure: a Conceptual Framework" (ICRP, 1993), which is intended to provide a basis for the preparation of more detailed guidance relating to specific practices, including radioactive waste disposal.

In Publication 60, the policy is set out in general terms, covering all sources of radiation. ICRP 77 deals more specifically with the specification and application of the Commission’s protection policy in the context of the disposal of radioactive waste.

ICRP 77 is particularly important as it clarifies the terminology and existing strategies in the field of waste disposal.

In the context of the Commission’s recommendations, waste is any material that will be or has been discarded as being of no further use. Waste includes liquid and gaseous effluents as well as solid materials such as process residues. Waste storage is the temporary retention of waste. Waste disposal is the discarding of waste with no intention of retrieval. The term disposal covers the discharge of effluents and solid waste disposal.

Waste disposal strategies can be divided into two conceptual approaches: Dilute and Disperse or Concentrate and Retain (see Fig. 1). Early or deferred releases of radionuclides to the environment would inevitably result from either of these strategies and therefore an objective of no release in not feasible. Both strategies are in common use and are not mutually exclusive. Where there is a choice, the balance between the two strategies is a radiological protection issue involving, inter alia, consideration of the decay of radionuclides during the period of containment and of the associated risk of elevated exposures from disruptive events which is an inescapable consequence of the decision to concentrate waste in a disposal facility rather than diluting or dispersing it.

The recommendations of the ICRP on long-lived solid waste disposal deal with the radiological protection of members of the public using the concentrate and retain strategy. It covers options including shallow land burial and deep geological disposal. The recommendations made in this report apply to new disposal facilities where there is the opportunity for their implementation during the site selection, design, construction and operation phases; they should also be taken into account in justification decisions involving practices generating waste.

The clarifications provided by ICRP Publication 77 concerning the application of the radiological protection system consist in emphasising that the main issue is the exposure of members of the public. They

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Publication also identifies the principles that are truly useful as well as the quantities that should preferably be used.

The principle of justification applies to practices as a whole. It is wrong to regard disposal of waste as a free-standing practice, needing its own justification. Waste management and disposal operations should therefore be included in the assessment of the justification of the practice generating the waste.

The Commission’s dose limit applies to the sum of doses from specified practices, but not to the sum of doses from all sources of exposure. Although the Commission continues to recommend dose limits, it recognises that dose Figure 1 IPSN WASTE PRODUCTION SYSTEM AND MANAGEMENT OPTIONS

Environment

NUCLEAR FUEL CYCLE IMMEDIATE Dilute/ te Disposal RELEASES Operations s Disperse a

w Atmospheric

al Liquid pos s i D Concentrate retain

REPOSITORY Dilution/ DELAYED LLW/ILW surface Dispersion HLW underground RELEASES

99-IPSN/169201/GG

limits for public exposure are rarely limiting in practice. Furthermore, it considers that the application of dose limits to waste disposal has intrinsic difficulties and that control of public exposure through a process of constrained optimisation will obviate the direct use of the public exposure dose limits in the control of radioactive waste disposal.

Thus, the principal means of achieving protection of the public is through a process of constrained optimisation, taking account of the Commission’s recommended upper value for the dose constraint of 0.3 mSv per year or its risk equivalent.

The dose constraint is an important component of the optimisation of protection. The term is used by the Commission to mean a source-related individual dose used exclusively in the optimisation of protection to exclude from further consideration any protection options that would cause the dose to a member of the critical group to exceed the constraint. A constraint is thus used prospectively and it should not be confused with a dose limit. The magnitude of the constraint is specific to the source and situation; the Commission does, however, as mentioned above, recommend a value of no more than about 0.3 mSv per year for the dose constraint resulting from radioactive waste disposal activities for members of the public.

The development of optimisation has given rise to questions, either essentially concerned with quantities

3 T-18-1, P-5-334 in the implementation of analysis/benefit type techniques or regarding assessments of a more qualitative nature. Preference is given to the latter approach in Publication 77. The optimisation of protection has the broad interpretation of doing all that is reasonable to reduce doses. Much of the Commission’s emphasis has been on the qualitative specification of the optimisation of protection. The basic role of the concept of optimising protection of protection is to engender a state of thinking in everyone responsible for controlling radiation exposures so that they continually ask themselves the question: have I done all that I reasonably can to reduce these radiation doses? Thus, the Commission’s policy on optimisation is judgmental and in essence is summarised in Paragraph 117 of Publication 60. "If the next step of reducing detriment can be achieved only with a deployment of resources that is seriously out of line with the consequent reduction, it is not in society’s interests to take that step". Conventionally, collective dose was one of the factors taken into account in an optimisation decision essentially based on differential cost benefit analysis. However, the Commission recognises the problems of estimating the collective dose over long periods of time in the future.

In the context of protecting future generations, the relevant indicators are the annual individual dose to a critical group for normal exposure and the annual individual risk to a critical group for potential exposure, and these "will together provide an adequate input to a comparison of the limiting detriment to future generations with that which is currently applied to the present generation".

Finally, another important aspect of ICRP 77 is the discussion of policies for the disposal of toxic wastes other than that of the ICRP one .

ICRP Publication 77 mentions that the term "best available technology under prevailing circumstances" is often used in connection with optimisation and that "there has been increasing pressure for the adoption of policies described by labels such as best available technology or best available technology not entailing excessive cost. The term best available has usually implied best from the environmental viewpoint, regardless of cost. The addition of not entailing excessive cost brings the concept closer to the Commission’s recommendation to keep doses as low as reasonably achievable, but involves costs only when they are becoming excessive. However, ICRP considers that these policies fall short of achieving the optimisation of protection".

3. DISPOSAL OF LONG-LIVED SOLID RADIOACTIVE WASTE

3.1. General considerations

The Commission’s system of protection is directly applicable to the dilute and disperse strategy. Exposures are estimated in order to place adequate control on the source of exposure. The characteristics and habits of exposed individuals and populations are taken into account. Furthermore, in these situations it can be verified to a great extent that protection is being achieved by measuring releases into the environment and by taking action in the event of unexpected releases. In the case of disposal of long-lived solid radioactive waste using the concentrate and retain strategy, the main protection issue concerns exposure that may or may not occur in the distant future, i.e. a situation of potential exposure. Even if the disposal system is well designed and properly managed, there may be releases into the environment. However, in the case of long-lived waste, an effective waste disposal system will retain the waste during the period of greatest hazard with only residual radionuclides entering man’s environment in the distant future. Any corresponding estimates of doses to individuals and populations will have growing associated uncertainties which increase as a function of time due to incomplete knowledge of the future disposal system behaviour, of geologic and biospheric conditions and of human habits and characteristics. Furthermore, as knowledge of a waste disposal system may eventually be lost, verification that protection is being achieved, in the same manner as for current releases, cannot be assumed. Neither can it be assumed that effective mitigation measures will necessarily be carried out. Nevertheless, the Commission’s system of protection can be applied to the disposal of long-lived radioactive waste.

The Commission published recommendations for the disposal of solid radioactive waste in Publication 46 (ICRP, 1985a). The aim of the new publication is to supplement, update and clarify those recommendations in the light of more recent developments, including the Commission’s policy for disposal of all types of radioactive waste (ICRP, 1998).

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3.2. Main issues

An essential conclusion of the report of the Task Group is that the exposures resulting from the disposal of long-lived radioactive waste are potential exposures.

In the context of exposures in the distant future, special consideration has to be given to the following issues: protection of future generations, critical group, design of the system, application of the optimisation principle, demonstration of compliance with the criteria.

• radiological criteria: ICRP 77 emphasises that the relationship between dose and detriment is uncertain at times in the distant future. However, the protection of future generations implies the use of the current quantitative dose and risk criteria derived from considering the associated health detriment. Thus, estimations of doses or risks for periods of time longer than several hundred years can be made and compared with the appropriate criteria provided that they are considered as indicators of safety rather than predictions of future health detriment.

• critical group: due to the long timescales under consideration, the habits and characteristics of critical group(s), as well as of those of the relevant environment can only be assumed. Any such critical group has to be hypothetical and derived on the basis of reasonably conservative assumptions, considering currently available site or region specific information, taking into account the assumed biosphere. The use of stylised approaches will become more important for the longer timescales.

• disposal system : it cannot be assumed that future generations will have any knowledge of disposal undertaken by the current generation. Therefore, the protection of future generations from the disposal of radioactive waste should be achieved primarily by passive measures at the repository development stage, and should not rely unduly on active measures taken in the future. However, the Commission recognises that institutional controls maintained over a disposal facility after closure may enhance confidence in the safety of the disposal facility particularly by reducing the likelihood of intrusion. The Commission feels that there is no reason why these controls may not continue for extended periods of time and, therefore, may make a significant contribution to the overall radiological safety of shallow disposal facilities in particular. Furthermore, for surface or near surface disposal of mill tailings, these controls may be relied on for long periods of time in situations where, if the controls fail, the consequences will be generally lower than those associated with other long-lived radioactive wastes.

Moreover, for many disposal systems, steps could also be taken during the disposal system development process to preserve choices for future generations (e.g. retrievability) but under no circumstances should these impair disposal system safety.

• application of the optimisation of protection : two broad categories of exposure scenario should be distinguished: natural processes and human intrusion. Natural processes include gradual degradation of the repository and other less likely natural processes that may disrupt or otherwise affect the performance of this disposal system. Human actions in the future may also disrupt a waste disposal system. The degree of difficulty in estimating both components of the risk is not the same for the two broad categories of exposure situations. Two main difficulties have to be dealt with (see Fig. 2):

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Figure 2

IC R P /TG OPTIMISATION / RADIOPROTECTION CONSIDERATIONS

formal techniques constrained optimisation potential exposure judgemental process

aggregated approach natural processes dissaggregated approach dose / likelihood SCENARIOS LIKELY/LESS LIKELY

Dose / risk constraint

reduce the possibility human intrusion limit consequences likelihood ? Risk constraint not appropriate barriers by passed

Generic Interventions Levels 99-IPSN/196809/GG

♦ general approach: there are no formal techniques for optimisation in the context of potential exposure ("optimisation of protection against potential exposures is still largely unresolved" ICRP 76 §62) - See also the following quotations from ICRP 46 and 64.

“Disposal of radioactive waste leads to a radiation source which may exist for extremely long periods. This poses methodological problems connected with the assignment of probabilities to events and processes for potential exposure assessment” (ICRP 64).

“The methods for optimisation of radiation protection range from simple common sense to complex quantitative techniques (see Publications 37 and 55 ; ICRP 1983, 1989). Optimisation of protection against potential exposures is still largely unresolved” (ICRP 76).

“The role of potential exposure in risk assessment for long-lived radionuclides is not yet clear” (ICRP 77).

This is why the Commission has favoured a judgmental and disaggregated approach of the optimisation rather than an aggregated one (recommended by ICRP 46): the assessed individual risks are compared with the appropriate constraints by separate considerations of the dose and the associated likelihood of occurrence of the exposure.

An aggregated approach is conceptually satisfying but requires a comprehensive evaluation of all relevant exposure situations and their associated probabilities within the time period under consideration.

In coherence with ICRP 77, the Commission has recommended an upper limit for the dose constraint of 0.3 mSv per year to be applied in normal exposure situations. This corresponds to a risk constraint of the

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order of 10-5 per year.

♦ specific treatment of human intrusion: it is not appropriate to include the probabilities of the occurrence of human intrusion in a quantitative performance assessment. Comparison with the risk constraint is not relevant as there is no scientific basis for quantifying the likelihood of exposure. Moreover, the constraint is not applicable when evaluating the significance of H.I because by definition, intrusion will have bypassed the barriers which were considered in the optimisation of protection for the disposal facility.

This is why the Commission has considered it more adequate to make a clear distinction between natural scenarios (gradual or disruptive evolution) and human intrusion, rather than between gradual and disruptive, the latter including natural evolution (seismic etc.) and human intrusion as in ICRP 46.

A criterion for the appreciation of the resiliency of the repository to human intrusion is proposed, based solely on the appreciation of the consequences of stylised scenarios.

The Commission considers that in circumstances where human intrusion could lead to doses sufficiently high that intervention on current criteria would almost always be justified, reasonable efforts should be made to reduce the likelihood of human intrusion or to limit its consequences. In this respect, the Commission has previously advised that an extant annual dose of around 10 mSv may be used as a generic reference level below which intervention is not always likely to be justifiable. Conversely, an existing annual dose of around 100 mSv per year may be used as a generic reference level above which intervention should be considered almost always justifiable. Similar considerations apply in situations where the thresholds for deterministic effects in relevant organs are exceeded.

• Technical and managerial principles :

The Commission recommends that technical and managerial principles for potential exposure situations (ICRP, 1993) should be applied during the disposal system development process to enhance confidence that radiation safety will be maintained throughout the post-closure period. These principles should be applied to disposal systems in a manner consistent with the inherent hazard level of the waste, as well as with the level of residual uncertainty identified in the assessment.

One of the key principles is the concept of defence in depth which provides for successive passive safety measures which enhance confidence that the disposal system is robust and has an adequate margin of safety. Defence in depth is primarily applied in waste disposal by using multiple barriers which provide a combination of different lines of defence against potential challenges to the safety of the disposal system.

Other technical and managerial principles also contribute to enhancing confidence that the required level of safety will be achieved.

In addition to the technical principles, an essential managerial principle for all individuals and organisations involved in the repository development process is to establish and maintain a consistent and pervading approach to safety which governs all their actions. This principle has been referred to as a "safety culture" and was first defined in the context of nuclear safety.

• Compliance with radiological criteria

Demonstration that radiological protection criteria will be met in the future is simply a matter of a straightforward comparison between estimated doses/risks and constraints. Proof that the disposal system satisfies criteria cannot be absolute because of the inherent uncertainties, especially in understanding the evolution of the geological setting, biosphere, and engineered barriers over the long term. Adequate assessments should be scientifically sound, accommodate reasonable conceptual understanding of system behaviour, use stylised approaches and reasonably conservative assumptions as appropriate, and typically be peer reviewed by consulting experts. These assessments should also address the remaining uncertainties by means of suitable presentations of their results (e.g. as ranges of numbers or bounding estimates). Thus, a decision on the acceptability of a disposal system should be based on reasonable assurance rather than on an absolute

7 T-18-1, P-5-334 demonstration of compliance. Evaluating whether there is compliance with the constraints requires a judgement. The dose/risk constraints should increasingly be considered as reference values for time periods farther into the future. The constraints provide a basis for judgements. Numerical compliance alone should not compel acceptance of a proposed safety case in the absence of adequate evidence of the quality of the supporting data and analyses, as well as an assessment that the overall design and construction of the disposal system complies with the technical and managerial principles cited above. By the same token, the fact that the constraint values are exceeded does not necessarily oblige rejection of a proposed safety case, merely because their value is estimated to be excessive. The unquantified conservatism likely to be incorporated into a performance estimate should be recognised in evaluating performance estimate as the time frame increases; some allowance should be made for assessed dose or risk exceeding the dose or risk constraint. This must not be misinterpreted as a reduction in the protection of future generations and, hence, a contradiction with the principle of equity of protection, but rather as an adequate consideration of the uncertainties associated with the calculated results. However, any excess must be justified and the system’s safety must be corroborated by other evidence or the reasons for the excess must be assessed to determine whether additional measures would result in improved protection.

3.3. Conclusion

In the Commission’s view, provided reasonable measures have been taken both to satisfy the constraint for natural processes and to reduce the likelihood or the consequences of inadvertent human intrusion, and technical and managerial principles have been followed, then radiological protection requirements can be considered to have been complied with.

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REFERENCES

ICRP (1985a). Radiation Protection Principles for the Disposal of Solid Radioactive Waste. Publication 46, Annals of the ICRP 15 (4), Pergamon Press, Oxford and New-York.

ICRP (1985b). Principles of Monitoring for the Radiation Protection of the Population. Publication 43, Annals of the ICRP 15 (1), Pergamon Press, Oxford and New York.

ICRP (1991). 1990 Recommendations of the International Commission on Radiological Protection. Publication 60, Annals of the ICRP 21 (1-3), Pergamon Press, Oxford and New York.

ICRP (1997). Protection from Potential Exposure: A Conceptual Framework. Publication 64, Annals of the ICRP 23 (1), Pergamon Press, Oxford and New York.

ICRP (1997). Protection from Potential Exposures: Application to Selected Radiation Sources. Publication 76, Annals of the ICRP 27 (2), Pergamon Press, Oxford and New York.

ICRP (1998). Radiological Protection Policy for the Disposal of Radioactive Waste. Publication 77, Annals of the ICRP 27 Supplement 1997, Pergamon Press, Oxford and New York.

ICRP (1999). Protection of the Public in Situations of Prolonged Radiation Exposure. ICRP to be published.

IAEA (1986). Definition and Recommendations for the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter, 1972. 1986 Edition, Safety Series 78, IAEA Vienna (1986).

IAEA (1988). Basic Safety Principles for the Nuclear Power Plants. Safety Series 75 - INSAG-3, International Atomic Energy Agency, Vienna.

IAEA (1997). Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management. INFCIRC/546, International Atomic Energy Agency, Vienna, 24 Jan 1997.

NEA (1985). Review of the continued suitability of the dumping site for radioactive waste in the north- east Atlantic. Nuclear Energy Agency, Organisation for Economic Co-operation and Development, , France.

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