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

Main comment on the Draft ICRP publication on the Radiological protection in geological disposal of long lived solid waste

IRSN

The main concern IRSN has to express on the draft document is on the criteria applied for the various phases (direct oversight, indirect oversight, no oversight) and categories of situations (design basis, or out of design basis)  mentioned on the table in § 4.3.5.

A first comment concerns the lack of clarity on the use of the criteria recommended by the commission: are they meant to guide the pre-operational activities (siting and designing of the disposal facility) or are they criteria to be applied by future generations if they encounter a situation involving a contamination of the environment due to the presence of the facility? IRSN considers that only the first option (guidance for design) is relevant and should be clearly mentioned in front of the document.

A second comment concerns the use of emergency criteria for the post-closure phase. The initial draft contained some ambiguities on whether the possibility to mitigate the doses by human intervention, in case of emergency exposures, was an option to consider when assessing the acceptability of accidental situations in the far future. It seems hardly acceptable to consider such option after the phase of direct oversight, the reason is it would totally contradict the main principle that is recommended internationally for designing geological disposal facilities (rely on passive feature without considering the possibility for intervention after closure). I seems also hardly possible either to consider intervention during indirect oversight because it is strictly impossible to foresee the possibility to maintain such oversight after a few hundred years, and one cannot imagine situations of exposures (if human intrusion is ruled out) occurring during these few hundred years that would lead to doses approaching emergency levels. If so, such situations would indicate severe misfits in the conception of the facility and would as such be considered unacceptable. Thus, IRSN recommends the highest caution in defining the use of emergency levels as criteria for radiological protection after the closure of the facility, and that such levels be used only in the sense of the maximum levels that can be accepted without engaging mitigation actions, and applicable for comparison purposes in only very particular situations (extreme events). The text should be crossed checked in this sense.

A third comment concerns the use of the notion of design-basis situations in the case of a geological disposal. If this notion can be almost fully transposed to a disposal facility during its operational phase, it requires some adaptation when considering the long term evolution of the repository. Design basis for other nuclear facilities imply considering low annual probability events against which the facility is designed so as to prevent or limit releases as far as possible. In general, the objective is to ensure by appropriate design and organisation that no measures for the protection of population would be required in case of releases due to these low probability events (which correspond to effective doses below emergency levels but that can be above 1 mSv per year).

-For events other than human intrusion relevant the post-closure phase of a disposal facility, a number of low annual probability disruptive events is accounted for in design and considered to be relevant to the “expected normal evolution of the disposal facility”, because the occurrence of these events during the lifetime of the facility may become likely given the very long time frames involved (several hundred thousand years). As such the corresponding situations should be categorised as “planned exposure situations” constrained at 0,3 mSv/yr.

There are other situations that are not relevant to the normal evolution of the facility but that nevertheless are considered in the design-basis of the disposal. These generally correspond to the same type of events considered in the normal case, but with amplitudes that are less probable to be reached. For these situations, it may be considered that higher releases than in the normal case would occur. However, this would in most cases result in a progressive contamination of the environment, leading to a situation that is comparable to “existing exposure situations” (in the sense of ICRP103), without the possibility to act on the source of contamination. For this reason, IRSN would suggest to use the criteria relevant to existing exposure situations as bases for appreciating the acceptability of the potential doses incurred. This is furthermore consistent with the objective of designing against events with the objective of limiting releases so that no protection measures are needed to be implemented (with sufficient margins with regard to emergency levels).

There is finally a third category of events that are considered extremely unlikely to happen during the life time of the disposal facility. These are generally categorised as “what if” scenarios used mostly to test the resilience of the facility to extreme events. It can be decided to rule out some of these events attended their extremely low probability (meteorite fall for example) and include others for assessments. In this case it is understandable that the band of doses to appraise the acceptability of the related exposures be enlarged to emergency levels.

-For human intrusion, it does not seem meaningful recommending a criteria of individual exposure in cases involving a direct intrusion in waste, bypassing all the barriers (borehole or inadvertent re-mining of the facility). The danger of incurring high doses in this case is the counterpart of the concentration strategy adopted internationally. The situations that are relevant to compare against radiological protection criteria are therefore those corresponding to intrusions in the vicinity of the disposal facility that have not bypassed all the barriers. These situations should be studied to test whether the barriers that are still operant are able to limit the releases. Though acknowledging for the difficulty in appraising the likelihood of different events of this kind, it is necessary that the operator screens which situations should be considered in the design bases and which should not, with sufficient justification. When this is done, the rationale mentioned above for relating events to various radiological protection criteria may be applied.

Finally, it must be recalled (which is done in the draft) that these criteria, when applied to the very long term, are just tools for assessing the capability of the disposal facility to protect man and the environment and not a measurement of the health detriment that may be caused by the releases. These are indicators among many others that must be assembled to acquire reasonable confidence that the disposal of the waste do not constitute a threat for the health of future generations, and not interpreted like individual risk limits that cannot be exceeded. In that sense, if design bases events and non design basis events are estimated to lead to doses estimation above recommended criteria for “existing situation exposures” or of the range of emergency levels, this does not necessarily disqualifies the conception of the disposal facility. Each situation must be appraised with regard to several lines of reasoning (likeliness, dose range, population involved, spread of contamination, margins taken in estimates, etc.) to make a judgment of acceptability.

In conclusion, IRSN proposes to amend the table in § 4.3.5 as following :

In any case IRSN suggests to the commission to give careful explanation of the rationale that led to establish the table in § 4.3.5 (and in the executive summary), possibly referring to the various arguments exposed above. In particular, § 52 needs to be revisited in this perspective.