Pitting Studies Under Anoxic Conditions on Candidate Container Materials AISI 316L hMo and UHB 904L for The Disposal of
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3LWWLQJ6WXGLHV8QGHU$QR[LF&RQGLWLRQVRQ&DQGLGDWH&RQWDLQHU0DWHULDOV $,6,/K0RDQG8+% /IRU7KH'LVSRVDORI+/:LQ$UJLOODFHRXV)RUPDWLRQV Bruno Kursten and Frank Druyts SCK•CEN, The Belgian Nuclear Research Centre Boeretang 200, B-2400 Mol, Belgium $%675$&7 Stainless steel is being envisaged as the primary candidate container material for the final disposal of vitrified HLW in deep geological argillaceous formations in Belgium. The impact of an evolving underground repository environment, i.e. a progressive change from oxic to anoxic conditions (due to the consumption of entrapped oxygen), on the pitting behaviour of austenitic stainless steels AISI 316L hMo and UHB 904L was studied. CPP-experiments were performed in synthetic solutions, which are representative for the near-field chemistry of an underground repository. The solutions contained various amounts of Cl- (100-50,000 mg/L) at near-neutral pH. Experiments were conducted at 16 and 90°C. AISI 316L hMo and UHB 904L will not be subjected to immediate pitting problems neither under oxic, nor under anoxic conditions. However, AISI 316L hMo could present long-term pitting problems under oxic conditions. Pits are much easier initiated on AISI 316L hMo, for both oxic and anoxic conditions. The pits propagate in a rather similar manner under oxic conditions for both alloys, whereas under anoxic conditions the pits formed on AISI 316L hMo are much deeper. AISI 316L hMo is more susceptible to crevice attack.. ,1752'8&7,21 During the geological disposal of vitrified HLW, the isolation between the radioactive waste and the environment is realised by the multibarrier principle. This principle is based on the complementary nature of various natural and engineered barriers which separate the buried waste from the biosphere. One of the main engineered barriers is the metallic container (canister, overpack) that encloses the radioactive waste. In the current Belgian repository concept, stainless steel (with AISI 316L hMo type stainless steel as a primary choice) is being envisaged as the most suitable candidate overpack material. The most likely cause of failure of this engineered barrier is localised attack (e.g. pitting corrosion, crevice corrosion, stress corrosion cracking, hydrogen embrittlement, ...) with pitting corrosion being considered the most probable. In Belgium, the Boom Clay sediment is studied as a potential host rock formation for the final disposal of vitrified HLW. In the present Belgian repository concept, the space between the overpack and the gallery lining will be backfilled with prefabricated segments made of a mixture of bentonitic FoCa swelling clay (60%), sand (35%), and graphite (5%). The latter component is added to improve the thermal conductivity of the mixture, and so dissipate the heat emitted by the waste more efficiently [1]. FoCa is a sedimentary clay from the Paris Basin, extracted in a site near to the Vexin region. The major component of the clay fraction is an interstratified clay made up of 50% calcium beidellite and 50%
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