Treatment of canister corrosion in Posiva's safety case for the operating licence application

Abstract

Posiva Oy have submitted an operating licence application to the Finnish government which, if approved, would permit the disposal of spent fuel in a deep geological repository at Olkiluoto. The license application is based on 40 years of site investigations, design and development work by Finnish, Swedish, and international experts. The canister is a key component of the overall multi‐barrier system and the treatment of the long‐term corrosion performance in the safety case is described in this paper. The importance of relating laboratory observations to the expected environmental conditions in the repository is highlighted, as is the use of complementary models to build confidence in long‐term predictions. Canister lifetime prediction is based on a combination of corrosion allowances for processes that will occur to a limited extent and mass‐transport calculations for the long‐term corrosion by sulphide. Examples of canister lifetime predictions for various scenarios and climate lines of evolution are given.

Review of the Modelling of Corrosion Processes and Lifetime Prediction for HLW/SF Containers—Part 2: Performance Assessment Models

“…Then there will be a number of variant cases that may describe possible alternative evolutions, possibly involving failure or sub-optimal performance of a given barrier; for example, an initial through-wall defect in one or more containers or bentonite buffer emplaced with a density insufficient to suppress microbial activity. Variant cases may also be based on different future climate evolutions, such as an extended global warming scenario that results in a delay of the next glaciation event [3,4]. There may also be a class of disruptive scenarios that, while of low probability, would result in a significant consequence.…”

“…Similarly, there is uncertainty about whether localized corrosion of copper is possible under anaerobic conditions in the presence of sulfide. While SKB [21,32] and Posiva [3,31] elect to make an allowance for possible "micro-galvanic corrosion", albeit only if the flux of sulfide is high due to chemical erosion of the buffer, NWMO [11] and Nagra [33] do not. Thus, different WMOs assess the level of uncertainty in different ways.…”

“…In addition to the factors described in Section 3.1.2, the other major factor affecting the flux of sulfide is whether the bentonite buffer maintains diffusive conditions or not. For intact, as-emplaced bentonite, mass-transport is dominated by diffusion, with typical sulfide fluxes of the order of 10 −18 to 10 −16 mol cm −2 s −1 [3,40]. However, loss of buffer density by chemical erosion can permit advective transport, with sulfide fluxes of the order of 10 −16 to 10 −14 mol cm −2 s −1 or approximately two orders of magnitude higher than for intact buffer [3,40].…”

“…For intact, as-emplaced bentonite, mass-transport is dominated by diffusion, with typical sulfide fluxes of the order of 10 −18 to 10 −16 mol cm −2 s −1 [3,40]. However, loss of buffer density by chemical erosion can permit advective transport, with sulfide fluxes of the order of 10 −16 to 10 −14 mol cm −2 s −1 or approximately two orders of magnitude higher than for intact buffer [3,40]. In addition to the increased sulfide flux associated with a reduced buffer density, there is also the possibility of localized corrosion and biofilm formation and microbial activity in the deposition hole, as described above.…”

“…PA models are often based on simplifying assumptions or bounding calculations and depart from the level of determinism common in many process models. For example, the sulfide corrosion model for copper canisters in Posiva's safety case for the operating licence application (SC-OLA) [3,4] is partly based on the detailed mechanistic process models of microbial sulfate reduction [5][6][7], but with the assumption that all sulfate is reduced to sulfide regardless of the availability of electron donor (organic carbon or H 2 ) or of the kinetics of the microbial reduction reaction.…”