Modelling and Simulation of Thermal and Closed-Loop Control Behaviour of Uranium-Bed Assemblies

Abstract

A theoretical model describes the thermal and closed-loop control behaviour of uranium-bed assemblies used for the storage of hydrogen or its isotopes. Storage is in the form of UH₃ at ambient temperature, and heating uranium-beds to 700-900 K can deliver the desorbed gas at pressures useful for transport to other process equipment. Modern compact uranium-bed assemblies include multiple-containment stainless-steel vessels to provide vacuum insulation and safety boundaries and to accommodate the heating and cooling systems. The thermal model describes the heat balance and the time evolution of the hydride powder temperature and includes radiation, dissociation and convection heat-transfer processes. The process controller simulation includes PID-type control/decision and adaptive algorithms. The model allows for simulations of the behaviour of the closed-loop system to demonstrate its effectiveness and to achieve controlled release of hydrogen, deuterium and tritium gas. A numerical example is presented which is based on a typical bed assembly. The simulation methodology used can easily incorporate more complex geometries, additional process components such as control valves, pumps and reservoirs as well as advanced control strategies as a means of predicting gas flow/pressure characteristics and bed safety.

This study was initiated after the first applications of uranium and other metal hydride beds in the active gas handling plant (AGH) of the Joint European Torus Project (JET), where (a) an in-depth knowledge of the bed control system and operation under tritium gas and a prediction of system parameters and tritium gas behaviour were needed, and (b) minimization of unnecessary testing and stressing of the uranium beds was required. The model can, in general, be applied to any hydride gas storage system and also has the capability to simulate the hydriding process.