nuclear-news

Nuclear waste containers: the problem of corrosion in copper canisters

The court said no to the application because it considered that there were problems with the copper canister that had to be resolved now and not later. 

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 Copper Corrosion Conundrum  No2Nuclear Power  http://www.no2nuclearpower.org.uk/wp/wp-content/uploads/2018/03/NuClearNews_No105.pdf

The Swedish Environmental Court has rejected the Nuclear Waste Company SKB’s license application for a final repository for spent nuclear fuel in Forsmark, Sweden. This is a huge triumph for safety and environment – and for the Swedish NGO Office for Nuclear Waste Review (MKG), the Swedish Society for Nature Conservation (SSNC), and critical scientists. Now it is up to the Swedish government to make a final decision.

The Environmental Court took into consideration viewpoints from all parties in the case, including scientists who have raised concerns about disposing spent nuclear fuel in copper canisters. During the legal proceedings, the Swedish NGO Office for Nuclear Waste Review (MKG) and the Swedish Society for Nature Conservation (SSNC) presented the shortcomings of this method of disposal. For many years, the environmental organisations have been arguing that the Nuclear Waste Company SKB need to listen to critical scientists, and investigate alternative disposal methods, especially the possibility of developing a very deep boreholes disposal system. (1) Johan Swahn, Director at MKG said:

“Several independent researchers have criticized both the applied method and the selected site. There is a solid documentation base for the Environmental Court’s decision. It is hard to believe the Swedish Government’s conclusions will be any different from the Court’s.”

MKG has made an unofficial translation into English of the Environmental Court opinion. (2)

The court said no to the application because it considered that there were problems with the copper canister that had to be resolved now and not later. The translation shows the courts judicial argumentation and why it decided not to accept the regulator – the Swedish Radiation Safety Authority’s (SSM’s) opinion that the problems with the integrity of the copper canister were not serious and could likely be solved at a later stage in the decision-making process. The court is quite clear in its statement and argumentation:

“The Land and Environmental Court finds that the environmental impact assessment meets the requirements of the Environmental Code and can therefore be approved. All in all, the investigation meets the high standards set out in the Environmental Code, except in one respect, the safety of the canister.” (Emphasis added)

“The investigation shows that there are uncertainties, or risks, regarding how much certain forms of corrosion and other processes can impair the ability of the canister to contain the nuclear waste in the long term. Overall, these uncertainties about the canister are significant and have not been fully taken into account in the conclusions of SKB’s safety analysis. The Land and Environmental Court considers that there is some leeway for accepting further uncertainties. The uncertainties surrounding certain forms of corrosion and other processes are, however, of such gravity that the Court cannot, based on SKB’s safety analysis, conclude that the risk criterion in the Radiation Safety Authority’s regulations has been met. In the context of the comprehensive risk assessment required by the Environmental Code, the documentation presented to date does not provide sufficient support for concluding that the final repository will be safe in the long term.” (Emphasis added)

The court says that the application is only permissible if the nuclear waste company SKB:

“…produces evidence that the repository in the long term will meet the requirements of the Environmental Code, despite remaining uncertainties regarding how the protective capability of the canister may be affected by: a. corrosion due to reactions in oxygen-free water; b. pit corrosion due to reaction with sulphide, including the contribution of the sauna effect to pit corrosion; c. stress corrosion due to reaction with sulphide, including the contribution of the sauna effect to stress corrosion; d. hydrogen embrittlement; e. radioactive radiation impact on pit corrosion, stress corrosion and hydrogen embrittlement.”

The main difference between the court’s and the regulator’s decision-making was that the court decided to rely on a multitude of scientific sources and information and not only on the material provided by SKB. It had also been uncovered that the main corrosion expert at SSM did not want to say yes to the application at this time that may have influenced the court’s decision-making. In fact there appear to have been many dissenting voices in the regulator despite the regulator’s claim in the court that a united SSM stood behind its opinion.

The court underlines in its opinion that the Environmental Code requires that the repository should be shown to be safe at this stage in the decision-making process, i.e. before the government has its say. The court says that some uncertainties will always remain but it sees the possible copper canister problems as so serious that it is not clear that the regulator’s limits for release of radioactivity can be met. This is a reason to say no to the project unless it can be shown that the copper canister will work as intended. The copper canister has to provide isolation from the radioactivity in the spent nuclear fuel to humans and the environment for very long time-scales.

It is still unclear how the process will proceed. The community of Östhammar has cancelled the referendum on the repository, as there will be no question from the government in the near future. The government has set up a working group of civil servants to manage the government’s handling of the opinions delivered by the court and SSM. SKB has said that it is preparing documentation for the government to show that there are no problems with the canister. Whether the government thinks this will be enough remains to be seen. This is likely not what the court had in mind. The government would be wise to make a much broader review of the issue. There is a need for a thorough judicial review on the governmental level in order to override the court’s opinion. Otherwise the government’ decision may not survive an appeal to the Supreme Administrative Court.

There are eminent corrosion experts who believe that copper is a bad choice as a canister material. There is also increasing experimental evidence that this is the case. The court’s decision shows the importance of democratic and open governance in environmental decisionmaking. It is important that the continued decision-making regarding the Swedish repository for spent nuclear is transparent and multi-faceted. (3)

Copper Canisters The canister has to enclose the nuclear waste for a very long; it is the final repository’s primary safety function. The canister has a 50 mm thick copper shell with an insert of cast iron. The canister must withstand corrosion and mechanical stress.

The investigation on the capability of the canister is extensive and involves complex technical and scientific issues. These include groundwater chemistry, corrosion processes, as well as creep and hydrogen embrittlement (this latter affects the mechanical strength of the canister). However, the parties taking part in the court proceedings disagreed on several issues crucial to the final repository’s long-term security.

The Land and Environmental Court considered the following uncertainties regarding the canister to be most important in the continued risk assessment:

• 1. General corrosion due to reaction in oxygen-free water. The parties have different views on scientific issues surrounding this kind of corrosion. The Court found that there is considerable uncertainty on this topic that has not been taken account of in SKB’s safety analysis
• .· 2. Local corrosion in the form of pit corrosion due to reaction with sulphide. The Court found that there is significant uncertainty regarding pit-corrosion due to reaction with sulphide. This uncertainty has not been included in the safety analysis. In addition, there is uncertainty about the sauna effect, which may have an amplifying effect on pit corrosion.
• · 3. Local corrosion in the form of stress corrosion due to reaction with sulphide. The Court found that there is significant uncertainty regarding stress corrosion due to reaction with sulphide. This uncertainty has not been included in the safety analysis. In addition, there is uncertainty about the sauna effect, which may have an amplifying effect on stress corrosion.
• · 4. Hydrogen embrittlement is a process that affects the mechanical strength of the canister. The Court found that significant uncertainty regarding hydrogen embrittlement remains. This uncertainty has not been taken account of in the safety analysis.
•  · 5. The effect of ionizing radiation on pit corrosion, stress corrosion and hydrogen embrittlement. There is significant uncertainty regarding ionizing radiation impact on pit corrosion, stress corrosion and hydrogen sprays. This uncertainty has been included to a limited extent in the safety assessment.

Meanwhile, 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 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.

World Nuclear News reports that 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. (4

March 14, 2018 - Posted by Christina Macpherson | Reference, Sweden, wastes