Patent Number: 051026180
Section: summary

This invention relates to an improved vapour phase catalytic exchange (VPCE) reactor for catalytic equilibration of hydrogen isotopic concentrations between water and hydrogen gas. In a water-moderated nuclear power plant, heavy or light water serves as a moderator to slow down high speed neutrons, as a reflector to deflect slowed-down neutrons back towards the fuel, and also as a coolant. Heavy water is the preferred moderator as it is about 300 times less absorbent to neutrons than light water and so the critical quantity of uranium is reduced. Whereas enriched uranium must be used in the case of a light water moderator, heavy water permits the use of naturally occurring uranium. In a heavy water reactor tritium is produced in the course of time by the absorption of neutrons by the deuterium nuclei, with consequent depletion of the heavy water. It is necessary to remove the highly radioactive tritium gas, and in addition it is necessary to remove the light water component continuously to prevent dilution of the heavy water content. In order to remove the tritium and excess light water, a combined tritium and hydrogen extraction plant was constructed at Grenoble, France. The plant was designed to extract tritium at the rate 160,000 Ci annually, and light water at the rate 160 l. annually. The plant employs the vapour phase catalytic exchange process, which process has been described in various papers. For example, Damiani et al described the VPCE process, and the design and construction of the plant at Grenoble in considerable detail [Sulzer Technical Review No. 4, 1972 - "Tritium and Hydrogen Extraction Plants for Atomic Power Reactors".] Sood and Kveton described another tritium removal system for use with a CANDU reactor [Ontario Hydro--"Tritium Removal System for Pickering", 1981]. Davidson et al described the commissioning and operating experience at a tritium removal plant ["Fusion Technology" 1988, pp 1373-1380]. In these systems the vapour phase catalytic exchange process has proved effective. In this process, which is a multistage process for the catalytic equilibration of hydrogen isotopic concentrations between water and hydrogen gas, the two exchange media, hydrogen and water, flow countercurrently from stage to stage and within each stage are first mixed with co-current contact in the presence of a platinum catalyst, then separated in a condenser for countercurrent advancement to opposite contacting stages. Each stage comprises essentially four components, namely, an evaporator in which tritium-rich water is converted into steam, a superheater in which the mixture of vapour and hydrogen is heated to approximately 200.degree. C., a catalytic reactor in which isotopic equilibration is completed, and a condenser from which water flows to the subsequent stage for contact with leaner hydrogen and the enriched hydrogen flows to the preceding stage for contact with richer water. A number of such stages may be cascaded. Thus, each stage of the vapour phase catalytic exchange process has four major components. The four components are necessarily interconnected by piping and valves, and in a multistage system the stages are themselves interconnected. The space taken up by the various components is very considerable. Furthermore, the extensive piping requires a large number of pipe joints which are potential sources of leaks. To reduce the likelihood of leakage of radioactive gas and vapour, stringent safety measures have to be taken, and in consequence the space occupied by the system is further increased. The present invention provides an improved system which occupies considerably less space than a conventional system of the same capacity. This is achieved by integrating the components of each stage of a multistage apparatus into a single structure.