(551e) Mechanisms of Rubidium and Krypton Release from Zeolite Getters

Preventing the release of radioactive krypton (⁸⁵Kr) to the atmosphere is a major obstacle in nuclear fuel reprocessing. One of the methods used to store radiokrypton is to use zeolites with small “windows” between pores, such as sodalite (framework code SOD), zeolite 5A (LTA), and zeolite 13X (FAU). Systems based on this concept load krypton into the zeolite at high temperature and pressure, which is then rapidly cooled. The resulting decrease in the window size encapsulates the gas, in principle preventing its release. However, the decay product of ⁸⁵Kr, ⁸⁵Rb, is highly corrosive and can damage the metal containers in which the zeolites are stored. For example, researchers at Oak Ridge National Laboratory recently investigated the content of five canisters of zeolites that were used years ago to encapsulate radiokrypton, finding two of these canisters cracked and the insides severely corroded. Such severe corrosion suggests the presence of rubidium. This raises a question: did the krypton decay while still encapsulated followed by diffusion of the rubidium out of the sodalite cages, or did krypton leave the sodalite cage and subsequently decay to rubidium? We answer this question by studying the rate at which krypton atoms, rubidium atoms, and rubidium ions traverse the six-membered rings of sodalite cages by way of density functional theory calculations and the nudged elastic band method combined with classical transition state theory. We also investigate how the presence of non-sodium charge-compensating cations or water affects the rates of diffusion.