Investigation of helium and hydrogen deposition in an accelerator based neutron source

Abstract

One of the major obstacles to be surmounted in the design of a controlled thermonuclear reactor (CTR) is the requirement of structural materials which can tolerate the intense radiation environment surrounding a fusion reactor core. The long lead time required for the development of suitable new alloys underscores the need for an intense source of energetic neutrons which can “simulate” the radiation environment associated with a fusion plasma. Among the various accelerator based neutron sources which have been proposed for such a simulation, the spallation neutron source (SNS) is considered to be very promising [1,2]. However, before the technical feasibility of SNS as a tool for simulation of radiation damage characteristics of a CTR can be established we have to compare various neutron damage parameters for samples irradiated in CTR and SNS. Several theoretical studies have been conducted for gas deposition characteristics of different designs of CTRs [3,4]. The aim of the present paper is to study helium and hydrogen deposition in samples irradiated in an SNS and compare the results with those of a CTR. As a reference design for an SNS we have chosen a proposed spallation neutron source called “EURAC” which is based on a 600 MeV beam of protons impinging on a liquid lead target. For this design of SNS we have carried out detailed evaluation of helium and hydrogen deposition in samples of Fe. We have also evaluated neutron and proton fluxes and their spectra using the high energy transport code HETC and studied the effect of variation in sample position on hydrogen deposition. The results of these calculations show that the helium and hydrogen deposition are very sensitive to the high energy part of the neutron spectrum in EURAC. It is also shown that more than half of the total helium production in the irradiation of a sample of Fe is due to neutrons above 40 MeV even though these neutrons constitute only about 3% of the total neutron flux. The results also show that most of the hydrogen deposition is due to the slowing down of secondary protons in the sample.