Effect of gas generation in matrices containing Ra-226 sources

Abstract

ABSTRACTSpent sealed radiation sources containing Ra-226 are immobilised either by the embedding into metal matrix materials or by welding enclosure in metal hermetic capsules. Encapsulation of sources ought provide safe conditions for storage for a long time period. Gaseous products (helium and radon) are generated due to the decay of Ra-226. The gas generation of Ra-226 sources can cause their rupture and consequent leakages of radionuclides. The higher the radioactivity of sources the higher the intensity of gas generation. The pressure of accumulated gas depends on many other factors therefore necessary limitations ought be determined for the encapsulation procedures. The process of gas generation is described herein in order to reveal the conditions of safe immobilisation of radium sources. A simple model describing immobilisation of sources in a metal host is proposed. Both helium and radon generation is considered followed by accumulation of gases near sources. Partial pressures of helium and radon are found depending on time. The diffusion of helium in the metal and decay of Rn-222 are taken into account. It is shown that the pressures of both radon and helium increase until some maximum values, after that the pressures slowly fall down. The maximum radon pressure being not very high is achieved during 65 days after immobilisation whereas helium increases its pressure during an extended period of time lasting for tens of years (137 years in the glass matrix) reaching very high magnitudes for powerful sources (137.5 MPa for a 100 GBq source in a glass capsule). Safe conditions of immobilisation of radium sources can be determined on the base of obtained data.