Diffusion of neptunium in simulated high level waste glass.

Abstract

Neptunium in high level radioactive wastes has to be retained in glasses through geological period from the point of biological toxicity for more than million years. Neptunium-237 diffusion in borosilicate glass with simulated wastes of 26.4% was investigated in the temperature range of 400–600°C by the use of α-degradation method. The energy loss rate dE/dx of α-particles, which is necessary in order to determine diffusion coefficients by the α-degradation method, was calculated for the waste glass. The penetration depth of α-particle with 4.787MeV from 237Np was 17 μm, which gives a limit in applying the α-degradation method in the waste glass. The temperature dependence of the diffusion coefficient of Np in the waste glass was given by D Np=3.67 exp(-55,900/RT) (cm2/s), in which the activation energy of the diffusion was 55.9 kcal/mol. It was clarified that Np is one of the elements with the lowest mobility in waste glasses.