Abstract
The presence of fissionable and/or fertile elements such as natural uranium and thorium in ppm quantity in zircaloy is shown to have potentially significant effects on zircaloy corrosion in-reactor. Fission events in zircaloy oxide are known to affect ≃ 106 atoms per event and to transform a small volume of oxide around each fission track from monoclinic to cubic. Annealing of the cubic phase is also known to take place at high temperatures. The analysis in this report builds from a value of transformed volume per fission event deduced from X-ray data on monoclinic to cubic transformation and on two different track shape models (cylindrical and dumbbell), to yield the spacetime distribution of the density of the cubic phase within the zircaloy oxide as it corrodes in reactor water while exposed to a neutron flux. Unfortunately, the value of the annealing constant for the reverse (cubic to monoclinic) transformation is not yet known for reactor temperatures and had to be assumed in the present study.
As a result of this space distribution of cubic phase after any given exposure time, the local density of the oxide is increased significantly from its value for the monoclinic phase of a value weighted by the presence of higher density cubic phase. This density distribution in turn is hypothesized to bring about a significant relief of the compressive stress known to be present in the zircaloy oxide and believed to be related to the kinetics of growth and transition. An example of possible space-time distributions of cubic phase density and of local density of an oxide film growing in-reactor for the hypothesized case of 10 ppm of natural uranium and for three different values of annealing factor is presented.
