The behaviour of spent nuclear fuel in wet interim storage

Howett, Elizabeth and Boxall, Colin and Hambley, David (2019) The behaviour of spent nuclear fuel in wet interim storage. PhD thesis, Lancaster University.

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Abstract

The UK is currently transitioning from a closed nuclear fuel cycle to an open cycle. During this transition, wet interim storage of AGR spent nuclear fuel in the UK will be extended until a long term disposal facility becomes operational. The current THORP Receipt and Storage pond is to be converted into an interim storage facility. Spent fuel will be more densely packed in this pond to accommodate current and future spent nuclear fuel. There will a resulting rise in pond water temperature from 24 ◦C to between 45-60 ◦C. This necessitates further research into the long term stability of stored spent nuclear fuel. This thesis describes the corrosion behaviour of AGR fuel cladding and AGR spent fuel under conditions relevant to the lengthened storage periods. An electrochemical study of the corrosion behaviour of unirradiated and unsensitised 20/25/Nb stainless steel under alkaline spent nuclear fuel interim storage pond water conditions is presented. The results give preliminary indications that dosing pond water to a pH'11.4, with an expected chloride concentration of ∼1 ppm and hydrogen peroxide concentration of ∼1 ppm, provides corrosion protection to unstressed cladding samples at the temperatures studied, 24 ◦C (current conditions), 45 ◦C (projected future pond temperature) and 60 ◦C (predicted peak operating conditions). Analogues for fuel cladding affected by radiation induced sensitisation were developed by the National Nuclear Laboratory. For both heat treated 20/25/Nb stainless steel and heat treated 304H stainless steel, experiments provide initial evidence that for sensitised 20/25/Nb SS dosing pond water to a pH'11.4, with an expected chloride concentration of ∼1 ppm and hydrogen peroxide concentration of ∼1 ppm, will lead to the passivation of the cladding surface for pond water temperatures between 24 and 60 ◦C. However general corrosion rates are higher than those of the unsensitised counterparts and spent fuel stringers likely to be affected by RIS should be monitored more closely. In the unlikely event that the fuel cladding is perforated, the fuel itself will be exposed to pond water. Thus in order to assist in the generation of a thorough safety case, the corrosion of pure UO2 and AGR SIMFuels was studied under conditions typical of those found in interim spent fuel storage pond waters. With respect to the dissolution of spent fuel, it appears to be advantageous to dose the ponds to pH'11.4. The alkaline conditions in the pond water suppress the dissolution as UO2+ 2 which may otherwise occur. A U3O7/U4O9 oxide layer is predicted to be developing on the surface of the samples at the open circuit potentials, for the samples exposed to simulant pond water.