Fission-fusion correlation by fission reactor irradiation with improved control

Abstract

The necessity for the elimination of exposure to neutrons at lower temperatures during start-up and shut-down of the reactor is confirmed by experiments which compare the result of irradiation of metals with conventional and improved temperature control in JMTR. Only several percent of exposure to neutrons at lower temperatures is found to result in a one hundred percent difference of radiation induced microstructures in some cases. All differences can be understood from the microstructural development mechanisms, i.e. from the temperature dependence of the stability of point defect clusters and from the relationship of the transient temperature to the temperature for nucleation and growth. Fission neutron irradiation data with improved control are compared with fusion neutron irradiation data from RTNS-II. The differences of vacancy and interstitial clusters formed directly from cascades, observed in samples irradiated as thin foils, are understood when the difference in the primary recoil energy spectrum and the thermal stability of the clusters are taken into consideration. The defect structures which are developed and/or modified by the reactions of freely migrating point defects, such as vacancy clusters interstitial type dislocation structures and voids, observed in samples irradiated as bulk, are remarkably different in the two cases. Factors to be applied to the fission neutron irradiation dose to introduce microstructures equivalent to those by fusion neutrons are found to range widely, depending on the kind of microstructure, materials and irradiation temperature, from a value less than one up to 30 in the scaling of damage energy per atom.