As you are aware the current government position concerning the long term management for Intermediate Level Nuclear Waste (ILW) in a geological repository setting was to be similar to the KBS (Sweden) canisters or some as yet undisclosed variant - These are closed and sealed canisters.
Recent publications by the NDA advise that major advances have been made in conditioning waste prior to packaging and that a previous multi-step process has now been reduced to three steps resulting in huge savings. - Indeed these savings are relied upon in future DECC budgets. The NDA have now advised me that containers will be "vented" indeed they maintain this has always been the case. - Was that the understanding of the ONR? - I find it strange the NDA's preferred canister was that being proposed by SKB which is a closed container.
Given a vented container is now preferred by the implementers, radioactive gas in the form of radioactive CH4 (Methane) will be released to a repository void and it follows if that void is in hard rock as opposed to clay then the gas will find its way into the biosphere.
Can the ONR provide guidance to the public as to what levels of radioactive gasses by volume would be acceptable in the context of a repository setting, over what time scales and as to how any gas venting that is acceptable will be dispersed?
There have always been two types of canisters proposed for the GDF, and one of the differences between them is that they are vented and unvented. The ILW waste packages produced by NDA for disposal in a GDF are vented, as this is important to stop over pressurisation of the containers from gas released from the waste both during long-term interim storage and when in the repository. The wastes are processed in a way to minimise gas production, but it is impossible to avoid some gas being produced by corrosion reactions given the nature of the waste being disposed. The majority of the gas produced will be hydrogen, rather than methane, as the disposal of unreacted organic materials to the GDF is minimised.
The SKB canister concept, on the other hand, is for the disposal of spent fuel rather than ILW - these packages are sealed as there is not the same problem of gas productions from spent fuel if it is properly dried and sealed in the container.
In respect of the question what levels of radioactive gases by volume would be acceptable in the context of a repository setting, over what time scales and as to how any gas venting that is acceptable will be dispersed, this is best answered by the Environment Agency who are responsible for regulating the post-closure environmental safety case for the GDF. ONR's regulatory responsibility is for the safety of the construction and operation of the facility.
We have contacted the Environment Agency in respect of this query, and they have responded with the following information:
The Environment Agency has produced guidance to developers of geological disposal facilities on land for radioactive wastes - https://www.gov.uk/government/publications/geological-disposal-facilities-on-land-for-solid-radioactive-wastes. This document explains the requirements we expect a developer or operator to fulfil when applying to us for a permit to develop or operate such a facility. A developer will need to take account of gas generation in a geological disposal facility as part of its overall safety case for the facility, as noted in paragraphs 6.4.9, 6.4.20 and 6.4.21 of the guidance.
The EA's radiological protection requirements are explained in Section 6.3 of the guidance. These are based around ensuring the protection of people and the environment, and are expressed either as dose to a representative individual during the permitted period (0.3mSv/0.5mSv per year) or risk to a representative individual after the permitted period (10-6 per year). To ensure that doses from all relevant pathways are considered together for a representative individual we do not specify separate criteria for each pathway (such as gas). Instead, we expect the developer to identify representative individuals during the operational phase and after closure and to show that the combined (summed) dose or risk to them from all relevant exposure pathways will meet our criteria.
Additionally, EA requires that the radiological risks to individual members of the public and the population as a whole shall be as low as reasonably achievable, as explained in Section 4.4 of our guidance. This places a requirement on the developer and operator to optimise disposals of solid radioactive waste so as to minimise radiological risk to people.
For your further possible interest, available on gov.uk are:
the Environment Agency's 2008 report reviewing Nirex/NDA's work in this area 'Gas generation and migration from a deep geological repository for radioactive waste'; https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/296497/geho1108bozn-e-e.pdf