UK nuclear laboratory to review Canadian research

26 February 2018

The UK's National Nuclear Laboratory (NNL) is to carry out an expert peer review of a Canadian research programme on microbiologically influenced corrosion of canisters that will be used to dispose of used nuclear fuel. The NNL has been contracted by Canada's National Waste Management Organisation (NWMO) to review its work on the potential for corrosion of the copper-clad canisters.

The Canadian canister design (Image: NNL/NWMO)

The NWMO is responsible for designing and implementing the safe, long-term management of Canada's used nuclear fuel under a plan known as Adaptive Phased Management. This requires used fuel to be contained and isolated in a deep geological repository, with a comprehensive process to select an informed and willing host for the project.

The used fuel will be isolated from the environment using a series of engineered barriers. Fuel elements comprise ceramic fuel pellets, which are themselves highly durable, contained inside corrosion-resistant zircaloy tubes to make fuel elements. Bundles of fuel elements are placed into large, durable copper-coated steel containers which are designed to contain and isolate used nuclear fuel in a deep geological repository, essentially indefinitely. The canisters will be placed in so-called "buffer boxes" containing by bentonite clay, providing a fourth barrier.

The NWMO in 2014 developed a new container design optimised for used fuel produced by Candu power reactors. The canister, which will hold 48 used fuel bundles, is 2.5 m long, 0.6 m wide and will weigh about 2800 kg when fully loaded. It features a 3 mm thickness of copper applied directly by electrodeposition and cold spray onto a steel container. This offers improved structural performance over previous designs using wrought copper, which can deform under structural loading. The canister and its spherical head are designed to have the mechanical strength needed to withstand pressures from the overlying rock and also the loading from a glacier up to 3 km thick in the event of a future ice age.

Although copper is highly resistant to corrosion, under anoxic conditions - that is, where no oxygen is present - sulphate-reducing bacteria have the potential to produce sulphide, which can lead to microbiologically induced corrosion (MIC) of copper. Waste management organisations and regulators therefore need to understand the levels of sulphide that will be present in a geological disposal facility, to understand its potential to migrate to the canister surface and the potential for it to cause copper corrosion, the NNL said.

The NWMO has been actively developing computer models that will be used to evaluate the potential for MIC once a disposal site has been selected, and has selected the NNL to carry out a peer review of its work because of the UK laboratory's expertise in the biogeochemical processes that could affect repository performance and in developing computer modelling techniques that simulate the effects of sulphate-reducing bacteria. The work is linked closely with NNL's participation in the European Commission Horizon-2020 MIND (Microbiology in Nuclear waste Disposal) project.

The contract will run until March 2019, and both parties hope it will be the start of an ongoing collaborative relationship, NNL senior research technologist Liam Abrahamsen-Mills said. "We are really pleased to be working with NWMO on improving the understanding of how microbes can influence the complex biogeochemistry of a repository, and ultimately its long-term safety," he added.

"Review of our corrosion research programs by international experts is an important component of our plan to demonstrate the safety of our new canister design concept," NWMO senior scientist Jennifer McKelvie said.

The NWMO launched the process to find a suitable site for a Canadian repository in 2010, and has progressively narrowed down study areas from a list of 21 communities that registered interest. Five sites - all in Ontario - now remain: Ignace; Hornepayne; Huron-Kinloss; Manitouwadge; and South Bruce. The NWMO completed drilling of its first borehole at Ignace in January, and has said it expects to be able to select the preferred site for detailed site characterisation by around 2023.

Researched and written
by World Nuclear News