Radiological Protection in Geological Disposal of Long-lived Solid Radioactive Waste

Consensus comments based on input and coordination of technical staff from the U.S. Department of Energy, U.S. Environmental Protection Agency, and U.S. Nuclear Regulatory Commission [redline / strikeout version]

Three sets of comments are provided regarding the ICRP draft document entitled “Radiological Protection in Geological Disposal of Long-Lived Solid Radioactive Waste.”  Set number 1 provides general comments to improve clarity and consistency throughout the document.  Set number 2 provides revisions to establish fundamental concepts for implementation of dose and risk constraints that in our opinion is critical to providing consistency and clarity in the document (discussion throughout the document should be reviewed and, as appropriate, be revised consistent with the concepts described in the set 2 comments).  The set 3 comments provide suggested revisions some of which represent revisions to be consistent with the fundamental concepts provided in comment set 2 – the set comments were not intended to be comprehensive, thus, the authors need to do a careful review of the document to ensure consistency throughout with the set 2 fundamental concepts.  We believe that agreement on the fundamental concepts, as outlined in the set 2 comments, will provides a strong basis and starting point for ensuring consistency and clarity throughout the document.

Set 1 Comments:      General Comments

The draft report is a potentially useful document addressing application of ICRP 103, “The 2007 Recommendations of the International Commission on Radiological Protection,” issued February 2008, to aspects of geological disposal and associated activities focusing on long‑lived radioactive waste. 

The report lacks consistency and clarity in the use of key exposure terms and their associations with the dose/risk constraint in the context of geological disposal facility oversight periods.  For example, the following paired terms were inconsistently used and should be reviewed for appropriate and consistent usage:

·                     high probability/low probability

·                     likely/less likely

·                     high/low exposure

·                     normal/potential exposures

·                     design basis/outside design basis

In addition, the report should clarify and present adequate discussions of the following specific terms, definitions, usages, and their related application issues—examples of inconsistencies are in the specific comments section that follows:

·                     Define what is normal versus potential (e.g., low probability and inclusion in the design basis).

·                     Discuss in more detail when dose versus risk constraint should be applied (e.g., dose applies to a normal exposure situation and risk to a potential exposure situation, or dose is used for design basis and risk for outside design basis).

·                     Explain in more detail when normal versus potential exposures can occur (e.g., during the period when the disposal facility is under direct, indirect, or no oversight condition).

·                     Clarify that the discussion concerns individual doses, not population doses.  When exposures are discussed, it should be clear whether they are public or worker doses, and whether the discussion applies to the operational phase (i.e., operational activities associated with surface handling activities) or the postclosure phase of the repository.

Set 2 Comments:      Establishment of Fundamental Concepts for Implementation of Dose and Risk Constraints

The discussion explaining important concepts (e.g., design basis and oversight) and the associated dose and risk constraints, appear in numerous places.  To fully reconcile the discussion of these elements throughout the document requires, a priori, clear agreement on which version represents the correct interpretation.  We strongly suggest that Table 1 is an appropriate place to consolidate this understanding; it claims to summarize all situations, is reiterated in the executive summary, and would serve as a natural anchor for the document.  Additionally, Section 4.3.5 is an appropriate place to provide explanatory text regarding Table 1.  After Table 1 and the accompanying explanatory text in Section 4.3.5 are finalized to accurately capture the key concepts for the document, it will be considerably simpler to review the remaining portions of the document for consistency.  Revisions to Table 1 and Section 4.3.5 are provided below. Reaching agreement on the concepts – and capturing them accurately in writing in the Table and Section 4.3.5 is the initial step and a prerequisite to efficiently and effectively reach a clear and consistent final document.

Revisions to Table 1, which appears first in the Executive Summary (page 8) and later in Section 4.3.5 (page 27)

1 The design basis is the envelope of both expected (normal exposures) and less likely (potential exposures) events (not anticipated to occur) that are used in planning the facility.  

2 At design: Normal exposures to workers subject to 20 mSv in a year dose limit to worker and dose constraint to be specified by operators.;  Public exposures from expected performance subject to 1 mSv in a year dose limit from all sources and 0.3 mSv in a year dose constraint for waste disposalthe public, in the case of for less likely events (not anticipated to occur)  within the design basis a risk constraint of 1× 10^-5 per year for the public is suggested.  

3 No worker dose is foreseen during the period of indirect or no oversight .  For potential exposures a risk constraint of 1×10^-5 per year for the public is suggested.  Comparisons with the risk constraint become increasingly less useful for compliance purposes at times further in the future.

4  Non-design basis analyses include very unlikely or extreme events that could be postulated to lead to significant exposure to people and the environment.  If comparisons to numerical criteria are considered appropriate, the reference levels defined for emergency and/or existing exposure situations are recommended.  For an emergency exposure situation a reference level between 20 and 100 mSv per year is recommended; for an existing exposure situation a reference level should be selected in the lower part of the band between 1 and 20 mSv per year, eg., in the range of a few mSv per year.

⁵ If inadvertent human intrusion were to occur and result in an emergency exposure situation it would only be expected to occur in the distant future and thus should be a considered potential exposure.  If comparisons to numerical criteria are considered appropriate, the reference levels defined for emergency and/or existing exposure situations are recommended. 

Revisions to Section 4.3.5 to provide further explanation of the fundamental concepts in Table 1 (Lines 999 – 1001).

(60) The application of the three exposure situations and of dose limits, dose constraints and reference levels as defined in ICRP Publication 103 during these 1000 timeframes is indicated in Table 1. Table 1 provides the dose or risk criteria applicable during different time periods for a disposal facility corresponding to different levels of safety oversight.  The period of direct oversight refers to the period of time when the facility is actively being monitored and personnel would be present to respond to accidents and emergencies – this is typically considered the operational period when waste is being received at the disposal facility and emplaced in disposal drifts, including some storage of waste as part of the disposal operations.  Direct oversight would also be present during the pre-operational period if some radioactive waste was present for testing purposes and during decommissioning of waste handling facilities.  Following permanent closure of the disposal facility it is anticipated that there would be some monitoring of the environment surrounding the disposal facility (period of indirect oversight) that would occur for a period of time.  However, it cannot be assumed that such oversight would continue indefinitely and at some point in the future oversight would cease (period of no oversight).   Thus, a high-level radioactive waste disposal facility is sited and designed to not rely on the presence of man to ensure safety. 

Performance or safety analyses can be used to evaluate facility performance under specific conditions.  The first column in Table 1 provides three specific categories of analyses (i.e., design basis evolution, non-design basis evolution, and inadvertent human intrusion) that are generally considered in geological disposal.   “Design basis” analyses consider a range of credible or reasonably likely processes, accidents and disturbing events and attempts to assure that these events are prevented if possible and/or the consequences are mitigated.  “Non-design basis” evolution involves very unlikely or implausible processes, accidents and disturbing events, potentially including extreme natural disruptive events. For such events, the probability of occurrence is so low that it may be neither relevant nor feasible to assess their impact on the disposal system.  Therefore, it is not appropriate for the regulatory authority to apply dose or risk constraints to non-design basis evolution as a condition of licensing, and the regulatory authority may develop a strategy to limit their consideration or eliminate them from the analysis altogether. Non-design basis evolution may be evaluated in a general way as a “what if” situation, i.e., as a means to understand better the robustness of the site and design.  Lastly, inadvertent human intrusion represents a “disturbing” event that typically is represented by a “stylized” or simplified analysis given the uncertainties associated with human behaviour in the distant future (note: inadvertent human intrusion is only considered relevant when oversight is no longer present at the site to prevent the intrusion).

The criteria presented in Table 1 cover both the operational period of a nuclear facility (direct oversight) and a period of time in the distant future (relative to a human lifetime) for the period of indirect and no oversight.  For those exposures that occur during the period of direct oversight and are a result of credible incidents (design basis evolution) the exposure situation is to be considered a planned exposure situation that may result in normal exposures (primarily to workers) or potential exposures.  Exposures should be compared to a dose constraint depending on the likelihood of the events considered; a risk constraint should be considered when less likely events are considered in the design basis analyses. Design basis analyses beyond the period of oversight (i.e., projected performance during periods of indirect and no oversight), should be considered to result in only potential exposures where a risk constraint of 1 × 10^-5 per year is considered appropriate.  Exposures outside the design basis -- i.e., non-design basis and inadvertent human intrusion -- represent extreme and very unlikely conditions where it is appropriate, at all timeframes, to use reference levels defined for emergency or existing exposure situations as comparative criteria. Application of numerical criteria to these situations should be viewed more as a qualitative exercise rather than a judgment of acceptability. However, all analyses must be mindful that the scientific basis for dose and risk assessments that project exposures situations into the distant future become more questionable the further out in time the analyses project and the strict application of any numerical criteria may be inappropriate.         

Set 3 Comments: Specific comments intended to assist clarity and consistency in the document.  However, these should not necessarily be considered exhaustive; we suggest a complete and thorough review to cross-check all language against the fundamental concepts as proposed in Set 2 Comments (above).

1.    Lines 211–218

In these sentences, we recommend use of “risk constraint” for doses that occur in the distant future  (see also changes to Table 1 provided in Set #2 comments). However, this does not address the role of the design basis and use of the dose and risk constraint. Paragraphs 45–48 suggest that the risk constraint is preferred for potential events and that any long-term releases from a geological disposal facility fall under the “potential” category (even those that are part of the design basis).  Paragraphs 929–938 recommend the use of dose or risk constraints for planned exposure situations.  We recommend the revisions provided in Set #2 comments for Table 1 be adopted and this approach be used consistently throughout the document – planned exposure situations within the design basis would include both normal and potential exposures when the exposures occur in the period of direct oversight, planned exposures during the period of indirect and no oversight are considered potential exposures.  Normal exposures are to be compared to the dose constraint and potential exposures, which are less likely than the normal exposures, are preferred to be compared to the risk constraint.  Exposures outside the design basis are of lower likelihood and may include very unlikely or extreme events that could be postulated to lead to significant exposure to people and the environment.  If comparisons to numerical criteria are considered appropriate for these non-design basis analyses, the reference levels defined for emergency and/or existing exposure situations are recommended.  However, regardless of the type of exposure situation, at some point in the very far future, even risk constraints would not apply and the results of projections or performance assessments should be considered only qualitatively, if at all.     

Thus, we recommend modifying lines 211–228 to read as follows:

In the distant future when institutional control can no longer be assumed to continue, the geological disposal facility might give rise to some releases to the accessible environment and the “safety case” has to demonstrate that such releases, should they be projected to occur, such releases need to beare compatible with regulation and radiological protection criteria. For these projected events resulting in potential exposures, the Commission continues to recommend a risk¹ constraint for a defined representative individual in the population of 1x10^-5 per year. In application of the optimisation principle, the dose constraint can also be applied qualitatively for comparison purposes. The reference radiological impact criterion for the design of a waste disposal facility recommended by ICRP is an annual (individual) dose constraint for the population of 0.3 mSv in a year [ICRP 103] for a normal exposure situation., without any weighting of doses in the far future.  For less likely events resulting in potential exposures, the Commission continues to recommend a risk¹ constraint for the population of 1x10^-5 per year. However, ICRP Publication 103 also warns that, given the long timeframes considered in waste disposal and the evolution of society, human habits and characteristics are is such that effective dose looses loses its direct connection to health detriment after the time span of a few generations.  At the same time, in the distant future, the geosphere and the engineered system and, even more, the biosphere will evolve in a less predictable way.  The scientific basis for dose and risk assessments at very long times into the future then becomes questionable, and the strict application of numerical criteria may be inappropriate.  Hence, the annual dose constraint of 0.3 mSv in a year is to be used for the sake of comparison of options rather than as means of assessing health detriment.

2.    Lines 245–248; suggested revisions to help clarify planned exposures in the context of geological disposal:

(g) The exposures arising from the operation and projected design basis evolution of the geological disposal facility are planned exposure situations as defined in ICRP Publication 103. They include both normal (anticipated to occur) and potential exposures (unlikely to occur) that are from events with low probability which have to be considered as part of the design basis. If severe disturbing events outside the design basis occur while there is still oversight (direct or indirect) of the disposal facility and which result in doses largely exceeding 0.3 mSv in a year, the ensuing situation will be considered as an emergency exposure situation followed by an existing exposure situation in case this emergency is resulting in a long lasting contamination of the environment. If a severe disturbing event occurs when there is no longer any oversight of the disposal facility, there is no certainty that a competent authority would be able to understand what is the source of the exposure and therefore, it is not possible to consider with certainty the implementation of relevant countermeasures to control the source. However, there is a need to evaluate the consequences within the scope of an existing exposure situation. Generally, as part of the design basis analyses, the normal exposures would be compared to the dose constraint and potential exposures would be compared to the risk constraint. However, the risk constraint should be used to guide design and optimization decisions when a projected disturbing event within the design basis is sufficiently unlikely and/or occurs in the distant future.  Analyses of events outside of the design basis and consideration of inadvertent human intrusion would be compared to the reference levels defined for emergency and/or existing exposure situations, if comparison to quantitative limits was considered appropriate for these types of events.    

3.    Lines 257–264; suggested revisions to help clarify emergency exposures in the context of geological disposal:

(h) The constraints as described above would apply to the operational period and to projections of future performance of waste disposal.  To guide design and assess the safety of the disposal strategy, an analysis is made to understand the potential consequences of projected disturbing events. We must also address the context if the projected disturbing events actually occur. For the “design basis” evolution, the dosimetric criteria relevant to planned exposure situations will be considered for assessing the safety and robustness of the disposal facility. For a severe disturbing eventIf a disturbing event comes to pass, the reference level to be considered at the time of its occcurence would be that for an emergency exposure situation would apply when relevant (i.e. reference level in the range of 20 to 100 mSv for the first year). Exposures at the level triggering emergency situations although improbable are possible during operations when waste is on the surface and the inventory of short lived radionuclides is largest.  In the distant future, after the repository is permanently sealed and the radioactivity decreases, the conditions that allow releases that could produce emergency exposure situations are limited to extreme events such as human intrusion and catastrophic natural events that are typically not considered directly in assessing planned exposures because of the degree to which they are uncertain. Nevertheless, the occurrence of such events cannot be entirely ruled out and they would be considered emergency of existing exposure situations (depending on the degree of institutional control and understanding) at the time they occur, should they come to pass. For optimization and design purposes, such events could be postulated to occur in periods of indirect or no oversight and the consequences could be considered in view of emergency reference dose levels.It is also necessary to evaluate the possible consequences of the occurrence of such events on the basis of the dosimetric criteria relevant for existing exposure situation as defined by ICRP (i.e. reference level of a few mSv per year). 

4.    Lines 299 - 306

As written, this is confusing that could be read to imply that we are handling these wastes for thousands of years.  Suggested revisions:

(2) This report deals with geological disposal of long-lived solid radioactive waste which is intended to isolate and contain especially high level waste, spent fuel and intermediate level wastes containing radionuclides with long half lives. These are concentrated wastes which that contain high specific activities; and if the waste were not disposed of in a geological repository and remained on the surface of the earth, special precautions would be needed to maintain safety such as remote handling while waste remained on the surface.they need to be handled remotely, for hundreds or thousands of years. The report does not address near surface disposal facilities because they differ from geologic disposal facilities in two key aspects: the isolation and containment function and the waste for which they are intended.

5.    Lines 321 - 323:  The comment that radioactivity will decrease with time is very simplistic.  Some EPA calculations anticipate that the radiation exposure risk from a geological repository could peak tens of thousands of years into the future due to the decay of certain isotopes.  However, the overall radioactivity will be decreasing.  Suggested revisions:

At great distance from the surface, such changes are particularly slow and, at the same time, the overall radioactivity will decrease with time (e.g., some have estimated radioactivity of high-level waste approaches that of a uranium ore deposit within tens of thousands of years).

6.    Lines 448–452

While there are some very long-lived radionuclides, it is also true that some of these same radionuclides have existed in nature since the formation of the earth.  This seems, once again, to paint a picture of significant concern.  Isn’t a repository comparable to a uranium ore deposit at some time in the future?  It might be useful to mention this in the text to provide some context for the “hazard.”  Suggested revisions:

(14) The initial composition of radionuclides contained into long lived radioactive waste evolves over time, changing the nature of the hazard. At the same time, even though the activity decreases with time, the halflife of some radionuclides, and the rate of ingrowth of others, are such that some of these wastes may never be considered as not being a hazard. However, the overall radioactivity of the waste will decrease with time (e.g., some have estimated radioactivity of high-level waste approaches that of a uranium ore deposit within tens of thousands of years).

7.    Lines 454 – 461; suggested revision:

 (15) Over the past three decadeslast decade, reflections on safety and societal issues associated with this long-term dimension clearly point out the complexity of the situation: on one hand it is not possible to envisage how the society will be organized in the far future while on the other hand the current generation has to take care of the possible future in order to design the waste management strategy. This is notably the core of the ethical reflections regarding the precautionary principle and sustainable development, in order to preserve the resources and the environment for the future generations.

8.    Lines 476 – 481; suggested revision:

(17) In the same vein, the obligations of the present generation toward the future are complex, involving, for instance, not only issues of safety and protection but also of transfer of knowledge and resources. Due to the technical and scientific uncertainties and to the evolution of society in the long-term, it is generally acknowledged that the capacity of the present generation to guarantee delivery of its obligationsensure societal actions will be taken in the future diminishes with distance in increasing time.

9.    Lines 635 – 636;  suggested revision:

The loss of oversight, by itself, does not result in a change of the protection capability of the disposal facility.

10.  Lines 812–819

Issue 1:

The report associates potential exposures with higher exposures without providing a basis.  This approach unnecessarily narrows the definition of potential exposure to only high exposures.  This is somewhat contradictory to the definitions on pages 36–37, lines 1437‑1449. 

We suggest modifying the subject paragraph to read as follows:

· “Planned exposure situations are everyday situations involving the deliberate introduction and operation of deliberately introduced sources including, decommissioning, disposal of radioactive waste, and including the post-closure phase and rehabilitation of the previously occupied land. Planned exposure situations may give rise both to exposures that are reasonably anticipated to occur (normal exposures) and to higher exposures that are not anticipated (less likely) to occur with a lower likelihood (potential exposures). These may arise following deviations from normal operating procedures, but are considered at the planning stage.”

11.  Lines 848 - 865;  suggested revision for consistency with other suggested revisions:

(45) Given the potentially vast time periods considered for the postclosure period eventual release of some radioactive substances is inherent in the concept of geological disposal. The exposures arising from the evolution of the geological disposal facility are planned exposure situations as defined in ICRP Publication 103. They include exposures that are anticipated to occur (normal exposures) and exposures that are not anticipated to occur (potential exposures) that are considered as part of the design basis. For the emplaced waste, a typical disposal facility safety assessment would suggest that significant releases are unlikely during the emplacement period and the period of time during which a competently sited, operated and sealed disposal facility is being actively observed and monitored. Therefore, any exposures would be categorised as part of the potential exposure subset of planned exposure. Given the potentially vast time periods involved in the period of no oversight, the possibility of an eventual release of some radioactive substances is inherent in the concept of geological disposal even if the system operates as intended (i.e., without deviations from procedures in operations, construction or accidents). These very lLong term potential releases of radioactive substances and subsequent exposures are assumed to result from a variety of scenarios. While they may be foreseen and perhaps assigned a probability they are still intrinsically uncertain. Evaluations of these exposures serve the purpose of comparing alternative facility design options and reaching a regulatory judgment regarding the capability of the system to isolate and contain the waste. Such evaluations are not considered to be predictions, nor are they intended to be used for the protection of specific individuals or populations. Such exposures may in fact be projected to occur at such distant times that traditional concepts such as dose and risk have to be used with caution.

12.  Lines 916 - 920;  suggested revision:

Possible approaches include establishing a probability value for which events with lesser probabilities are excluded from consideration in the risk-assessment process, optimizing site selection and engineering to minimize the probability of such events, or assessing specific events through stylized assessments.

13.  Lines 921 – 938; suggested revisions to help clarify planned exposures in the context of geological disposal:

52) Previously the Commission considered all natural events, disruptive or not, within the same framework (Publication 81). Now, the Commission recommends separate consideration of natural disruptive events which are included in the design-basis evolution from those which are not. For the first onesthose within the design basis, the Commission recommends application of the dose or risk constraints for planned exposure situation (includes both normal and potential exposures depending on the timeframe). If comparison to numerical criteria is considered appropriate for the projected consequences of events outside the design basis (including severe natural events), For the severe natural disruptive events not taken into account in the design-basis evolution, the Commission now recommends application of the reference levels for emergency or existing exposure situations., depending on the severity of the consequences. If the events were to occur, while there is still (direct or indirect) oversight of the disposal facility, the authorities should be in a position to implement adequate protection measures to deal with this situation as emergency or existing exposure situation. If such a disruptive event occurs when oversight of the disposal system has disappeared, there is no certainty about the possibility that an organisation would exist or could be aware of the disturbance and therefore, it is not possible to consider with certainty the implementation of protective measures. If the authorities eventually became aware of the disturbance they would treat the situation as an emergency exposure situation or an existing exposure situation depending on the severity of the disturbance.

14.  Lines 939 - 943; suggested revision to help clarify emergency exposures:

(53) For emergency exposure situations, the Commission recommends selection of a reference level in the range of 20 mSv to 100 mSv for the first year and development of protection strategies to reduce exposures to as low as reasonably achievable below the reference level taking into account economic and societal factors (Publication 109). Exposures at the level triggering emergency situations although improbable are possible during operations when waste is on the surface and the inventory of short lived radionuclides is largest.  In the distant future, after the repository is permanently sealed and the radioactivity decreases, the conditions that allow releases that could produce emergency exposure situations are limited to extreme events such as human intrusion and catastrophic natural events that are typically not considered directly in assessing planned exposures because of the degree to which they are uncertain. Nevertheless, the occurrence of such events cannot be entirely ruled out and they would be considered emergency of existing exposure situations (depending on the degree of institutional control and understanding) at the time they occur, should they come to pass.

15.  Lines 944 – 949; suggested revision based on lines 219 – 226:

(54) According to Publication 103, long-lasting exposures resulting from natural disruptive events (with or without an emergency phase) should be referred to as anFor existing exposure situations  and the recommended reference levels to be selected for optimizing protection strategies ranges between 1 to 20 mSv per year. In agreement accordance with the Commission's recommendations in Publication 111, a reference level should be selected in the lower part of the band, e.g., in the range of a few mSv per year.

16.  Lines 1030 - 1033

(63) Optimisation of protection has to deal with the main aim of disposal systems, i.e. to protect humans and the environment, now and in the future, by isolating the waste from man, the environment and the biosphere and by containing the radioactive and other toxic substances in the waste to the largest extent possiblewhen the waste is most hazardous.

17.  Line 1165

Please clarify or expand what is meant by the “indicative nature” of calculated effective dose.

(78) The assessment of the robustness of the disposal facility can contribute to system optimisation, because it provides insight, quantitative or qualitative, in the performance of the disposal facility and its components, in the relative contributions of the various components to the overall system. So, the value of such an assessment for the optimisation process is mainly through the insights it provides on the relative contributions of the various components to the overall system objective of isolation and containment, and how these contributions can be affected by disturbing events and processes or by remaining uncertainties. The indicative nature of calculated effective dose and risk in the very far future reduces their usefulness for the optimisation process.

18.  Lines 1263-1270:  Suggest deleting because we are unsure what the value of this paragraph is to the document.  The undisturbed geological repository does not contribute to a change in the biosphere.  In most instances, a change in the biosphere may have significant impacts on the flora and fauna (e.g., glaciations), but no impact on the repository.

(87) Over the long time frames that are considered in waste disposal, the biosphere is likely to change, and even change substantially. Such changes entail biosphere evolution with time that is either natural, or enhanced or perturbed through human action. Contributing factors may be, e.g., climate change including glaciations cycles, and land uplift or depression. Understanding different biospheres today and assessing impacts in such biospheres based on an approach involving Reference Animals and Plants, may guide our understanding of future biosphere changes also for the purpose of environmental protection.

19.  Lines 1366 - 1370

The second sentence of the current version is confusing and could be confused with optimization.  Reducing existing exposure and reducing risk of potential exposure is more closely associated with the optimization of protection, not justification of a practice.  Revise to read

· Justification: Any decision that alters the exposure situation should do more good than harm. It is the process of determining whether a planned activity involving radiation is, overall, beneficial, i.e. whether the benefits to individuals and to society from introducing or continuing the activity outweigh the harm (including radiation detriment) resulting from the activity.This means that, by introducing a new source, by reducing existing exposure, or by reducing the risk of potential exposure, one should achieve sufficient individual or societal benefit to offset the detriment it causes.

20.  Lines 1801–1807; revisions to improve clarity consistent with other revisions:

Both dose and risk constraints should be applied in planned exposure situations. For the specific situation of waste disposal dose constraints can be used for comparison of options for normal releases, which relate to expected performance of the disposal system within the design basis during the period of direct oversight of the facility.referred as “desing-basis evolution” of the depository facility. Events with lower probabilities and exposure situations in the future than the desing-basis evolution should be treated as potential exposures and would require the application of relevant risk constraints. Risk constraints will be applied either in an aggregated or a disaggregated way.