| Popis: |
Irradiation growth results are reported for annealed α-uranium at 373 K under 3.5 MeV proton bombardment. Two such experiments were performed at damage rates of 6.9 × 10−8 and 9.3 × 10 −8 dpa/s to doses of 0.0072 and 0.0077 dpa, respectively. In each case the growth rate remained constant throughout the experiment. The respective damage normalised growth rates were 5.6 × 10 −3 and 7.1 × 10 −3 dpa−1. Comparison between proton growth rates and published in-reactor growth rates is made by converting the more usual fuel damage parameters, such as burn-up, to dpa. Damage calculations, using the NRT damage model, are presented which indicate that, in uranium, each fission event produces 100 000 displacements. The reported growth rate of annealed, polycrystalline α-uranium at 353 K, during thermal neutron irradiation, represents a damage normalised growth rate of 9.6 × 10 −3 dpa −1 , which is not substantially different from the present proton results. This similarity of proton and fission growth rates appears to be contrary to the earlier finding of Thompson (1960), who deduced that proton bombardment produced two orders of magnitude less growth than fission fragments. Thompson concluded that thermal spikes played a dominant role in irradiation growth. Thompson's results and analysis are reassessed in the light of recent range data and damage models and found to be consistent with the present results in both magnitude and direction. The results are also inconsistent with Buckley's original model to the extent that thermal spikes were thought to play an important role. From a consideration of primary recoil spectra it is shown that the concept of the anisotropic aggregation of point defects to form vacancy and interstitial clusters, which is at the centre of that model, remains viable. Furthermore, similar though slightly less growth would be expected during proton bombardment. This was indeed found to be the case, the growth rate with protons being about half that with fission fragments. |
