Effect of Water Radiolysis Caused by Dispersed Radionuclides on Oxidative Dissolution of Spent Fuel in a Final Repository

When released out of a canister, the radionuclides originally incorporated in the spent fuel can still deposit radiation energy (even more efficiently) into the pore water, cause water radiolysis, and produce oxidants in the buffering material. This phenomenon is termed secondary water radiolysis. The oxidants thus produced can possibly diffuse back to oxidize the spent fuel and to increase the oxidative dissolution rate of the fuel.The effect of the secondary water radiolysis has been identified and preliminarily addressed by a mass-balance model. To explore whether the effect is significant on spent-fuel dissolution, the upper-boundary limit of the effect has been set up by considering a scenario that is very unlikely to occur. Several extreme assumptions have been made: First, the canister fails completely 103 yr after deposition; second, the spent fuel is oxidized instantaneously; and third, the radionuclides considered are those that dominantly contribute to radiolysis between 103 to 105 yr. With these assumptions, the spent-fuel dissolution rate can be increased dramatically if 10% or more of the oxidants produced by the secondary water radiolysis diffuse back to oxidize the spent fuel. It thus indicates that the effect of the secondary water radiolysis could be significant with some extreme assumptions. With more realistic assumptions, the effect could possibly become minimal. The subject is worth further investigation