Identifying, Predicting and Preventing Localized Corrosion in Kr-85 Storage Canisters from the Reprocessing of Nuclear Fuel

Abstract

During the reprocessing of nuclear fuel radioactive gases are released from the fuel matrix. These gases need to be separated and stored to prevent release into the environment. It is proposed that certain risk factors, which are elucidated herein, caused accelerated corrosion of the interior canister walls breaking containment of legacy storage canisters. The main risk factor of concern is the effect of Rb-85. Rb-85 is produced from the radioactive beta minus decay of Kr-85. The main question is whether an environment containing Rb as a decay product causes corrosion of the canister materials at an accelerated rate. If this does occur, the the corrosion threat will grow with time as more rubidium is produced.Based on the environment within the canister, and the risk factors of the system, a corrosion lifecycle has been developed and is briefly discussed. Stage 1 represents the canister at high temperature of 600°C after it has been sealed and hot isotactic pressed (HIP) for 4 hours. In this stage it is possible for oxides to form on the canister walls due to the residual oxygen in the canister from loading. It is possible that the presence of Rb will interact with the canister material and disrupt the formation of iron and chromium oxides. These Rb oxides, if present, may not inhibit corrosion thus causing an increase in the corrosion rates. Stage 2 represents the canister cooling from 600°C to ambient. In this stage wet thermal oxides will form on the canister walls as the temperature drops below the deliquescence point and water droplets appear, setting up the conditions for aqueous corrosion, like stage 1 the possible formation of RbxCryOzwhich may not be as protective as other oxides, can cause an increase in corrosion rates. Stage 3 represent oxide and environmental interactions. In this stage the environmental risk factors, such as the presence of cathodic reactants oxygen, hydron and peroxide, will create conditions to enable aqueous corrosion. Stages 4 through 8, describe the stages of pitting, uniform corrosion, and galvanic corrosion. Because the material chosen for the canister is 4130 low alloy steel, and the welding material used was 304 stainless steel, there is the possibility of galvanic corrosion. This can increase the uniform corrosion rate, and lead to more pitting events causing depassivation of the surface, or deeper pit penetration. The chosen canister materials are also at risk of pitting attack, the risk of which is increased as the canister is likely driven to alkalinity from the products of the cathodic reaction. Pitting is especially dangerous to the canister which needs to maintain containment for 100 years.This work looks to test the risk factors and develop a mechanistic understanding of the canister’s corrosion process with the goal of recommending a material that will be able to maintain containment for the needed 100 years if nuclear fuel were to start being repressed in the United States.