Various physical properties such as thermal conductivity, thermal expansion coefficient, density, elastic constant, and so on are of interest for nuclear

applications. Hence, it is important to understand the irradiation effects on the physical properties. Before we embark on discussing these, we must accept that irradiation can cause various changes in the structure of the materials and thus there may not be a general trend — but the effects will depend on particular situa­tions. So, one needs to be prudent while analyzing these conditions and drawing inferences.

6.3.1 Density

Calculations indicate that vacancy-interstitial pairs should cause substantial changes in the density of the irradiated material as they would increase the volume ~1.5 times theoretically. However, experimental observations show very little or no change in the density (which should decrease due to the generation of Frenkel pairs), except in the radiation swelling regime where volume increase occurs through the creation of voids/bubbles (discussed in Section 6.1.2.2). It is thought that due to the greater mobility of interstitials and their clusters, they would diffuse even at homologous temperatures of 0.15-0.20 and get trapped or annihilated. This implies that we would expect to see little or no change in density of metals/alloys irradiated near or below room temperature. Some exceptions have been seen in very high melting metals such as refractory metals. In such cases, at lower temper­atures, the interstitials are not that mobile, resulting in some significant changes in volume and in turn resulting in decreased density, as observed from lattice parame­ter measurements.

6.3.2