Peculiarity of Zr in the Neutron Absorption Cross-section and Corrosion Resistance in Water

Abstract

The elements Be, Bi, Mg, Pb, Zr, Al, Ca, Na, Sn, Rb and Ce are the metallic elements of small thermal neutron absorption cross section. Except Be and Zr, they are all soft metals. But the reasons for these small thermal neutron absorption cross-sections are not known. To clarify its mechanism, the thermal neutron absorption cross-sections of elements were plotted on the TC-YM diagram. They lie on a line connecting the elements of low Young’s modulus on the TC-YM diagram. The author at first considered that the neutron absorption characteristics of elements relate to the neutron multiple number which means the number of neutrons per proton in the nucleus. The absorption cross-section of elements roughly increases with increasing neutron multiple number. Among the elements of small neutron absorption cross section, only Zr and Be are the elements of high melting temperature. Zr should not show the small neutron absorption cross section inherently. But in fact, Zr is exceptional both on the TC-YM diagram and in the neutron multiple number. Zr has a small absorption cross-section against the general trends in both the TC-YM diagram and neutron multiple number. On the other hand, the corrosion resistance of Zircaloy is given by the anodic protection provided by the precipitates of metallic compounds containing Fe and Cr. It is fortunate that Fe and Cr belong to the element group of the metals nobler than Zr and that of no solubility in Zr on the TC-YM diagram.