Many materials used and produced in the nuclear industry include fissionable radionuclides; that is, on interaction with neutrons, the nuclei of such radionuclides can fission (split) into lighter nuclei. During a fission event, energy and further neutrons are released. If sufficient fissionable material is present under appropriate conditions, a self-sustaining nuclear chain reaction (or criticality) may occur, with neutrons released by fission leading to fission of other nuclei. A chain reaction can release large amounts of radiation and energy and, therefore, controls need to be imposed to ensure criticality safety in the management of such materials.
GSL has developed a track record in criticality safety assessment through supporting many clients in the nuclear industry. Our experience includes the development and review of many criticality safety assessments, covering the transport and disposal of radioactive materials. We have particularly strong experience in the assessment of the criticality safety of fissile materials during the post-closure evolution of geological disposal facilities. We are active members of the UK Working Party on Criticality and our staff are trained and experienced in the use the neutron transport modelling code MCNP. Project experience includes:
- Criticality safety assessment for all phases of radioactive waste management operations;
- Development of safe fissile material limits for a wide range of waste packages; and
- Review of criticality safety cases and assessments.
Generic Waste Package CSAs
GSL has led the development of all of the generic criticality safety assessments (CSAs) used by Radioactive Waste Management Limited (RWM) and waste producers in the UK. The work involved identification of generic intermediate-level waste (ILW) categories that capture a broad range of potential UK radioactive wastes requiring geological disposal. Four categories of waste covering irradiated natural uranium, low-enriched uranium, highly-enriched uranium and plutonium contaminated material were identified and generic CSAs were produced for each category. The generic CSAs involved consideration of conditions during disposal facility operations (normal and accident conditions) and conditions after disposal. A cautious deterministic approach was taken to identify modelling configurations for operational and post-closure conditions. Post-closure criticality scenarios were identified that focused on conditions at the waste package scale as well as configurations involving the migration and accumulation of fissile material from many waste packages. Neutron transport calculations were undertaken to identify fissile material limits for different types of waste package, taking account of appropriate criticality safety margins. The result of this work was to provide waste package ‘criticality screening levels’ for different categories of waste by which RWM can judge whether waste packages are acceptable for disposal.
The generic CSAs are available from RWM (RWM bibliography) – search for ‘criticality safety’.
CSA for Wastes Packaged in Robust Shielded Containers
GSL has also produced a generic criticality safety assessment (CSA) for wastes packaged in Robust Shielded Containers (RSCs) for Radioactive Waste Management Limited (RWM). RSCs are thick-walled containers intended to provide a high degree of waste containment and package performance during all phases of waste package management without needing to take credit for specific characteristics of the wasteform. A generic CSA for wastes packaged in RSCs was required by RWM to complement the existing suite of generic CSAs, because the assumptions underpinning the existing CSAs did not reflect typical RSC designs.
Different types of RSC were considered and conditions during disposal facility operational and after facility closure were evaluated. Neutron transport calculations using MCNP were undertaken to identify safe package fissile material limits.
The generic CSA is available from RWM (RWM bibliography) – search for ‘criticality safety’.
Likelihood of Criticality in a Geological Disposal Facility
Radioactive Waste Management Limited (RWM) is responsible for implementing geological disposal of the UK’s higher-activity radioactive wastes. The presence of fissile materials in these wastes requires an assessment of the potential for criticality. As waste packages begin to degrade after disposal, fissile and other materials may be mobilised and this could affect the potential for criticality. In response to the need to meet particular regulatory requirements, RWM commissioned GSL to lead a multi-year research project to develop, document and communicate an evaluation of the probability of criticality after closure of a geological disposal facility.
GSL undertook the majority of the project technical work, which involved consideration of the features, events and processes (FEPs) that could affect nuclear reactivity after GDF closure. The FEP analysis led to the construction of post-closure criticality scenarios and these scenarios were evaluated using the GoldSim radionuclide transport model. This probabilistic modelling approach enabled the effects of parameter value uncertainties to be evaluated in the criticality scenario analysis. Understanding the radioactive waste inventories, the disposal concepts for the different waste types and geological settings, and the expected evolution of conditions in the disposal facilities was of fundamental importance to the criticality FEP and scenario analysis. Disposal concepts for intermediate-level waste (ILW), depleted, natural and low-enriched uranium (DNLEU), spent nuclear fuel (SF), high-level waste (HLW), separated plutonium (Pu) and highly-enriched uranium (HEU) were considered.
The work was innovative and comprehensive in its approach to evaluating the likelihood of criticality in a geological disposal facility, combining the GoldSim probabilistic modelling capabilities with the results of MCNP neutron transport calculations to evaluate the reactivity of evolving systems. The work has provided RWM with the analysis required to meet regulatory requirements on understanding the potential for post-closure criticality. In addition, the tools developed in the project are now being used by RWM to assess the disposability of fissile waste packages.
The likelihood of criticality reports are available from RWM (RWM bibliography) – search for ‘likelihood’.
CSA for the New Low Level Waste Disposal Facilities at Dounreay
GSL developed the post-closure criticality safety assessment for the New Low-Level Waste Facilities (NLLWF) at Dounreay on behalf of Dounreay Site Restoration Ltd (DSRL). The most significant fissile radionuclide that will be present in the LLW disposal facilities is 235U and the criticality safety assessment derived limits on the 235U content of waste packages that will ensure large criticality safety margins at the time of disposal and in the long term after disposal. MCNP was used to determine package fissile material limits based on consideration of conditions at and after disposal, and criticality handbook data were used in assessments of the reactivity of credible fissile material accumulation scenarios. The post-closure criticality scenarios were identified and evaluated based on consideration of the expected evolution of the disposal facilities. Material degradation and mass transfer mechanisms under expected groundwater flow and geochemical conditions were considered.
Based on the assessment, and the derived 235U limits on waste packages, it was judged that the occurrence of a criticality event after closure of the disposal facilities is not credible. The criticality safety assessment has been reviewed and accepted by the Scottish Environment Protection Agency (SEPA) as part of the licence application for the NLLWF.
UK Waste Package Disposability Assessment
GSL provides criticality safety support to Radioactive Waste Management Limited (RWM) as part of the disposability assessment (Letter of Compliance) process for waste packages intended for geological disposal in the UK. This support involves reviewing packaging submissions from waste producers, such as Sellafield Ltd and DSRL, against specific disposability requirements, and, if required, reviewing or developing package-specific criticality assessments.
For example, GSL developed a package-specific CSA for waste packages containing Dragon reactor fuel. The Dragon reactor was used to undertake research into high temperature reactor technology. The fuel comprises highly-enriched uranium and the proposed disposal route is to package the fuel in 500 litre stainless steel drums. The CSA involved consideration of conditions during waste package transport and disposal facility operations, and after geological disposal. A cautious deterministic approach was taken to assess normal and accident conditions during waste package transport, consistent with regulatory requirements, including International Atomic Energy Agency (IAEA) transport regulations. Disposal facility operational and post-closure conditions were also analysed using a deterministic analysis. Neutron transport calculations were undertaken using MCNP to identify waste package fissile material limits based on consideration of each phase of waste package management, taking account of appropriate criticality safety margins. The CSA has been crucial in supporting decisions on the acceptability of the proposed package for disposal.
Spent MOX Fuel Disposability Assessment
On behalf of the Nuclear Decommissioning Authority (NDA), GSL produced a criticality safety assessment in support of an assessment of the disposability of spent mixed oxide (MOX) fuel, if generated in the UK. MCNP calculations were undertaken covering conditions during package transport and disposal facility operations, and conditions after geological disposal. The importance of neutron poisons and/or multiple waste barriers during waste package management and the significance of fuel burnup credit in post-closure assessments were considered. The work has been used to inform the nuclear industry on the viability of using MOX fuel in future nuclear reactors in the UK.
Review of Post-closure CSA Methodologies
GSL provided a review of approaches to assessing repository post-closure criticality safety for the Environment Agency. The review considered the methodologies used in different radioactive waste disposal programmes worldwide for demonstrating criticality safety of a range of waste types after geological disposal. Guidance was provided on the identification and assessment of post-closure criticality scenarios.
CSA for Spent Fuel Disposal in Sweden
On behalf of SKI (now SSM) in Sweden, GSL led a review of the post-closure criticality safety analysis for SKB’s spent fuel repository. The project included criticality safety assessment calculations for spent fuel contained in copper canisters under disposal conditions, and involved consideration of the effects of fuel irradiation on nuclear reactivity. The approach taken by SKB to evaluate post closure criticality scenarios involving accumulation of plutonium and uranium in and surrounding degrading waste packages was reviewed.