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According to the Swiss disposal concept, the safety of a deep geological repository for spent nuclear fuel (SNF) is based on a multi-barrier system. The disposal canister is an important component of the engineered barrier system, aiming to provide containment of the SNF for thousands of years. This study evaluates the criticality safety and shielding of candidate disposal canister concepts, focusing on the fulfilment of the sub-criticality criterion and on limiting radiolysis processes at the outer surface of the canister which can enhance corrosion mechanisms. The effective neutron multiplication factor (k-eff) and the surface dose rates are calculated for three different canister designs and material combinations for boiling water reactor (BWR) canisters, containing 12 spent fuel assemblies (SFA), and pressurized water reactor (PWR) canisters, with 4 SFAs. For each configuration, individual criticality and shielding calculations were carried out. The results show that k-eff falls below the defined upper safety limit (USL) of 0.95 for all BWR configurations, while staying above USL for the PWR ones. Therefore, the application of a burnup credit methodology for the PWR case is required, being currently under development. Relevant is also the influence of canister material and internal geometry on criticality, enabling the identification of safer fuel arrangements. For a final burnup of 55MWd/kgHM and 30y cooling time, the combined photon-neutron surface dose rate is well below the threshold of 1 Gy/h defined to limit radiation-induced corrosion of the canister in all cases. [ABSTRACT FROM AUTHOR]

• Anoxic uniform corrosion can often be monitored via analysis of accumulated hydrogen. • Rates as low as 0.01 nm/year can be monitored with periodic analysis. • Several years are required for test systems to reach steady state. • Carbon steel, in anoxic cementitious environments at 50 °C, corrodes <1 nm/year. • This information is of value to organizations responsible for nuclear waste. The safe long-term disposal of radioactive waste arising primarily from power generation is a complex proposition. Safety assessment modelling typically incorporates up to one million years, which is an engineering requirement without precedent. In order to perform this modelling, it is imperative that the behaviour of the materials comprising the repository are well understood over the timescales in question. This article describes a method developed for studying the behaviour of metals under anoxic conditions via the corrosion reaction end product, hydrogen. At its simplest, test cells are left for periods of time, up to several months, to accumulate hydrogen. This gas is then expelled from the cell and analysed using a hydrogen sensor. As such, the method is limited to characterising gas generation and consequently only uniform corrosion, but does permit direct monitoring of corrosion rates as low as 0.01 nm/year, which is necessary in order to accurately monitor the very low long-term corrosion rates expected in a deep geological repository. [ABSTRACT FROM AUTHOR]

Abstract An on-line gas monitoring has been conceived and implemented to study the evolution of the composition of the free gas phase in the Full-scale Emplacement (FE) experiment in the Underground Rock Laboratory at Mont Terri (Switzerland). The FE experiment is a trial run for a spent-fuel emplacement drift for a repository according to the Swiss concept for radioactive waste disposal. The monitoring of gas species such as He, Ar, Kr, Xe, N 2 , O 2 , H 2 , CH 4 , and CO 2 was performed successfully over several months. The partial pressures of gases relevant for the operational safety such as H 2 and CH 4 have been found to be below the concentration threshold for ignition. The combination of the on-line monitoring data and conventional noble-gas isotope measurements reveals rapid gas exchange between the pore space of the compacted bentonite granulate material used as backfilling and both the access niche and the host rock surrounding the FE tunnel (Opalinus Clay). Such fast gas exchange partly explains the disappearance of oxygen from the bentonite pore space detected by O 2 sensors even prior to sealing of the drift and the accumulation of a fraction of terrigenic gases such as 4He, 40Ar, CH 4 , and CO 2. Highlights • An on-line gas monitoring has been implemented for the FE experiment at Mont Terri URL. • The monitoring of gas species was performed successfully over several months. • Rapid gas exchange occurs between drift backfilling and FE niche/host rock. • Terrigenic gases (e.g., 4He, 40Ar, CH 4 , CO 2) accumulated in the backfill pore space. • Fast gas exchange partly explains the O 2 removal from the backfill pore space. [ABSTRACT FROM AUTHOR]

Abstract Radioactive waste is planned to be disposed in a deep geological repository in the Opalinus Clay (OPA) rock formation in Switzerland. Copper coating of the steel disposal canister is considered as an option to ensure complete waste containment for a period of 100'000 years. Sulphide is a potential corroding agent to copper. While background sulphide concentrations in the OPA are very low (ca. 10−11 mol/L), it cannot be ruled out that sulphide reducing bacteria (SRB) thriving at discrete locations within the engineered barrier system (EBS) of the repository could generate significantly higher dissolved sulphide concentrations. The aim of this study is to conservatively evaluate the potential for SRB to generate and sustain elevated sulphide concentrations in the repository near-field by considering specific geochemical bottle-necks, to quantify the maximum sulphide fluxes towards the canister, and to assess their potential to corrode the copper canister coating. To address these objectives reactive transport calculations are performed, and main conceptual and parametric uncertainties of the model are assessed. Highlights • A model for sulphide generation, consumption and transport in the near-field of a nuclear waste repository is presented. • The model allows pessimistic estimates of canister copper coating corrosion due to sulphide. • Results suggest that corrosion due to sulphide will not compromise the copper coating during hundreds of thousands of years. [ABSTRACT FROM AUTHOR]

In the past decade, TWI has been working together with Nagra (the Swiss national cooperative responsible for the disposal of radioactive waste) to develop design concepts for canisters for the disposal of Spent Fuel (SF) and High Level Waste (HLW) in a deep geological repository. The canisters must provide containment of radionuclides for more than 10,000 years. Several different canister designs have been investigated. A forged carbon steel canister is the current reference design. In the framework of options evaluation and optimisation, this study summarises the work carried out to determine the structural integrity of a closure weld during long-term disposal. An analytical study on the lid and weld design for the closure weld and final materials selection have been undertaken. The lid and closure weld designs were refined to avoid heating the nuclear waste above its allowable temperature during post-weld heat treatment (PWHT). Flaw assessments in accordance with the procedures and guidance given in BS 7910, along with finite element analyses (FEA), were used to identify the most suitable combination of PWHT and lid design to ensure no breach of containment within 10,000 years. • A proposed lid design for a nuclear waste canister is evaluated; the requirement is to ensure integrity for 10,000 years. • BS 7910 is successfully used to assess an atypical application. • To achieve this, FEA and ECA are combined in a complementary approach. [ABSTRACT FROM AUTHOR]

[Display omitted] • Initial anaerobic corrosion rates of steel in bentonite outpace the diffusion of water. • A hydrogen gas phase is temporarily formed close to the steel surface. • Gas and the lack of water desiccate the bentonite resulting in shrinkage microfractures. • Diffusing Fe2+ is oxidized by sorbed oxygen and precipitates in the porespace. • The safety relevant properties of bentonite are not jeopardized by steel corrosion. Experimental evidence related to the interactions between steel corrosion and bentonite in deep geological repositories is reconsidered. The released Fe2+ interacts with the smectite and precipitates as Fe3+ when oxidised by residual immobile oxygen. This leads to the development of coloured fronts in the bentonite. The evolution of these fronts can be reproduced by a reactive transport model that takes account of the competition between diffusion and precipitation. Furthermore, two-phase flow modelling indicates that the consumption of water by the corrosion outpaces the diffusion of water through the bentonite, leading to shrinkage microfractures in bentonite that act as preferential pathways for corrosion products. [ABSTRACT FROM AUTHOR]