With the Radiation (Emergency Preparedness and Public Information) Regulations 2001 due to be reissued in line with the 2013 EU BSS (See HSE note), interest turns to the discussion about how to determine the appropriate level of emergency preparedness for nuclear sites.
Currently REPPIR requires that the risks posed by the site are assessed, reported and periodically reviewed. If there are identified potential fault sequences that exceed thresholds of both probability and severity then off-site plans are required.
The threshold for probability is “reasonably foreseeable”. In the REPPIR guidance (para 50) it is stated that “In the context of a radiation emergency, a reasonably foreseeable event would be one which was less than likely but realistically possible”. ONR have avoided accepting a numerical value for the threshold of reasonably foreseeable (see, for example, para A20 -A22 in an ONR TAG).
ONR’s description of safety cases tells us that (Para 607) Design basis analysis (DBA) leads to an understanding of the plant and a design proven “so that safety functions can be delivered reliably during all modes of operation and under reasonably foreseeable faults”. Combining this with (Para A.9) “only faults with an initiating fault frequency (IFF) greater than 1 x 10-5/yr need to be considered for DBA” suggests that a frequency of 1 x 10-5/yr could be proposed as the limit of a “reasonably foreseeable” initiator.
1 x 10-5 per year is also implied in the ONR Tolerability of Risk Document ToR and in the national risk assessment policy sponsored by the Cabinet Office as a boundary between events that should be prepared for and those that are too unlikely for detailed planning.
The threshold for severity used in REPPIR 2001 is that of a Radiation Emergency. This is defined as a situation in which a member of the public could receive an additional radiation dose of 5 mSv in the year following initiation. There have been difficulties interpreting this requirement as the public dose assessment depends on the individual habits assumed.
There is a general principle in radiological protection and emergency planning that any action taken by authorities should to do more good than harm. In the UK we use the Emergency Reference levels (ERLs) to decide if a countermeasure is warranted by comparing the avertable dose with the relevant ERL. We can therefore state that the imposition of a countermeasure is not necessary, indeed not appropriate, where the avertable dose is below the ERL and a detailed plan to implement a countermeasure is therefore not needed where it is not reasonably foreseeable that the threshold will be exceeded. Thus the severity threshold for requiring a plan can be based on whether or not the existence of a plan would enable the imposition of prompt countermeasures which could avert an ERL’s worth of dose that could not otherwise be averted.
Concern about faults too unlikely to appear in the DBA but more severe than the reference accidents leads to the demand for the ability to extend countermeasures beyond the detailed plan if required. However, it is realised that spend to enable this quickly becomes grossly disproportionate to the potential gain.
While the situation analysis and decision making process provided by the detailed plan can consider a wider area or longer duration fault, the question is whether or not countermeasures can be applied rapidly enough over a wider area to be effective. This would depend on the nature of the fault including the amount of activity released and the time structure of that release.
Again the question of the probability of the limiting fault to use in extendibility scenarios arises. In various safety methodology documents IAEA talks about a “screening probability level” (SPL) of probability below which there is no point analysing faults. So far as I can find IAEA fails to suggest a value. There is a claim in the literature that the US DOE suggest 1 x 10-6 for aircraft crash onto nuclear facilities. It is suggested that future UK guidance on emergency planning recommends a SPL for emergency planning (extendibility) and that this value should not be lower than 1 x 10-6 or, at a pinch, 1 x 10-7.
Emergency planning then becomes a question of having detailed plans to implement those countermeasures that might avert more than an ERL of individual dose for the set of reasonably foreseeable faults – defined as being more frequent than 1 x 10-5 per reactor year (making allowances for reasonable cliff-edges) and outline plans for faults down to maybe 1 x 10-7 per reactor year. Estimations of avertable dose against downwind distance can determine a sensible limit to the countermeasure zone (which regulators may then choose to inflate within reason for non-technical purposes).