Disposal programme for UK radioactive waste Jun 2020

As of 1 April 2019, the UK radioactive waste inventory stood at 133,000m3. It is forecast that following the decommissioning of all current facilities, it will stand at more than 4 million m³ (4,560,000m³ total volume)⁽¹⁾.

Illustrative representation of a geological disposal facility Credit: © NDA, 2018

The UK classifies its waste according to the level of radioactivity and the heat generation it possesses and in the categories: high-level (HLW), intermediate-level (ILW), low- and very low-level (LLW/VLLW). Of these, high level and intermediate level waste is known as higher activity waste (HAW). In England and Wales the policy for the long-term management of this type of waste, along with a small proportion of low-level waste is disposal in a deep underground geological disposal facility (GDF)⁽²⁾. Conversely, Scottish Government policy favours near-surface disposal of waste. In Northern Ireland, where no higher activity radioactive waste exists, there are currently no plans to site a GDF.

High level waste (HLW) results from the vitrification of fission products from the reprocessing of spent nuclear fuel and currently (as of 1^st April 2019) contains about 95% of the inventory radioactivity in ~1% of the total waste volume. Intermediate level waste (ILW) arises from nuclear fuel cladding, a wide range of nuclear power plant items, equipment, materials and flocculants, all of which are encapsulated in cement. A small amount of low-level waste (LLW) that is deemed unsuitable for near surface disposal is also to be disposed of in a GDF.

The GDF will be constructed at a depth of between 200m and 1,000m below the ground surface. It will be designed as an engineered facility, within a stable geological setting, to provide containment and isolation of radionuclides from the biosphere for upwards of 100,000 years. The GDF will be designed to contain the waste and enable the radioactivity to decay to very low levels. Within the GDF, the goal is for the radionuclides contained within the radioactive waste to be surrounded by a number of engineered barriers designed to inhibit the release of radionuclides. These barriers include the wasteform that contains the radionuclides, the canisters that contain the wasteform, the buffer materials that surround the canister and GDF backfill materials.

Illustrative layout of a facility in a lower strength sedimentary rock Credit: © NDA, 2016

In England and Wales three principal geological settings have been identified in which the GDF could be located. The geological settings of interest are higher strength rocks (HSR), lower strength sedimentary (LSSR) rocks, and evaporites. HSRs generally exhibit high compressive strength but demonstrate a propensity for the formation of fracture networks and include lithologies such as intrusive and extrusive igneous rocks and low-grade metamudstones. LSSRs are fine grained sedimentary rocks formed by the deposition of grains, generally underwater, and are mechanically weaker than HSRs, but have self-sealing or creep tendencies. Evaporites are water soluble mineral salts that resulted from the evaporation of saline bodies of water. This geology creeps readily and provides an extremely arid, low permeability and low porosity environment.

Radioactive Waste Management Ltd (RWM) is the organisation created by the UK Government to be responsible for delivery of the GDF in either England or Wales. For each geological setting RWM has developed or adopted a number of generic repository designs⁽³⁾. Each design considers the challenges in excavating that particular geology along with the waste streams to be emplaced. Both considerations are reflected in the respective GDF footprints. The maximum extent of a GDF in HSR is anticipated to span an area of approximately 7.6km². In evaporites, the area is up to 10.6km² and in LSSR, the illustrative design has a footprint of 15.3km². This assumes that at some time in the future spent nuclear fuel (SNF), the irradiated fuel removed from a nuclear reactor at the end of its useful life, is declared as a waste. If spent fuel is declared a waste it will not be reprocessed and placed in a canister for direct disposal. Currently, there is no disposal route for the UK’s legacy or future SNF. At present, SNF resides in robust surface storage facilities to allow for radioactive decay and cooling prior to any form of disposal. Meanwhile the likes of Finland and Sweden have opted not to reprocess their SNF and will disposal of their SNF in a GDF after a suitable period on interim storage, the outcomes of which may aid the UK in making a decision on its own SNF.

The illustrative designs developed and/or considered by RWM consider the disposal of high heat generating wastes (HHGW), namely HLW and SNF and low heat generating wastes (LHGW), including ILW and LLW, in the same facility. This is reflected in the minimum separation of the disposal galleries for the cemented ILW zone of the repository and vitrified HLW and SNF zones. Irrespective of the geological setting, this minimum separation is set at 500 m.

Generic design for emplacement of waste in a rock disposal facilityCredit: © NDA, 2016

The heat output of HLW may be of detriment to a number of the GDF engineered barrier materials. In the case of cemented ILW the chemical conditions promoted in the ILW zone, as the wasteform and its container breakdown, could impact on the long-term behaviour of vitrified wasteforms. These potential interactions raise questions as to whether the co-location of multiple waste streams in a single repository, as opposed to the construction of multiple GDFs, is a viable solution. Though RWM’s current preferred policy is to build only one GDF, the possibility of having single facilities for particular waste streams has not been ruled out⁽⁴⁾.

The geological disposal of HAW is a long-term programme of more than 100 years and will involve the construction of surface and sub-surface facilities.

In the construction of the underground geological disposal facility or facilities, a combination of construction and excavation methods are likely to be used at various stages of the programme. These include cut-and-cover, TBM, roadheader and drill and blast techniques. The lining required for excavations is an important safety consideration and its choice will depend on the geological setting chosen for the GDF. Excavation support is required to be maintained and serviceable for up to 160 years throughout construction and operation. Steel mesh and rock bolts are likely to be employed in all three settings. In HSRs and LSSRs, shotcrete has been developed for incorporation into the lining, which exhibits low accessible gas porosity and permeability of ~10^-17 m².

Overview of the UK process for siting a GDFCredit: © NDA, 2020

The process to search for a suitable GDF site has been launched in England and Wales. In 2020 RWM released its guide as to how it will evaluate potential host sites in England⁽⁵⁾. This guide addresses:

• legal concerns;
• siting process and land use planning requirements;
• siting factors; and
• evaluation considerations.

It also shows how RWM plans to move through the siting process with interested parties. The process is built on the premise of voluntarism whereby only those communities expressing an interest in hosting a GDF will engage in the process of site identification and selection.

Initial discussions with interested parties will be informed by data gathered as part of the National Geological Screening process⁽⁶⁾. It is expected that RWM will form a number of working groups, along with willing interested parties, to identify the geographical areas suitable for a GDF. At present RWM is engaging with a number of communities in the hope of developing working groups in multiple locations from which community partnerships will emerge.

Although the UK has been a pioneering force in the nuclear sector, it is some way behind nations such as Finland, Sweden and France in the delivery of its GDF for radioactive waste. There is now, nevertheless, a real drive to deliver a GDF, with RWM gearing up to become an effective delivery organisation. Progress however will be determined by the success of voluntarism and the willingness of communities to play host to the GDF.