Effect of gas generation in matrices containing Ra-226 sources
- PDF / 292,824 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 3 Downloads / 196 Views
II3.33.1
Effect of gas generation in matrices containing Ra-226 sources M.I. Ojovan Immobilisation Science Laboratory, Department of Engineering Materials, University of Sheffield, UK, E-mail: [email protected] ABSTRACT Spent 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. INTRODUCTION Sealed radiation sources containing Ra-226 (radium needles) were widely used in the past activities mainly for medical applications. After 1950s most of radium sources were replaced by more cheaper, safer and efficient sources containing artificial radionuclides. However radium sources were stored for long periods of time at the places of their applications rather than disposed of [1-3]. Currently in most countries there were created national inventories of radium sources and sources were collected for safe storage and disposal. Ra-226 is a long-lived radionuclide (T1/2=1600 y) hence the only one accepted disposal option is a deep underground repository. Until the final disposal sources are stored thus they need an emplacement into a container, a matrix, or both to ensure eliminations of radionuclides releases. Among methods to immobilise radioactive sources there were chosen two methods, which provide necessary isolation for a more or less prolonged period of time: either encapsulation into a metal container (capsule) or fixation into a metal block. The encapsulation is performed utilising stainless steel pre-fabricated caps
Data Loading...
