CLUSTER DYNAMICS STUDY OF EFFECT ON THE TEMPERATURE CHANGE ON THE NEUTRON EMBRITTLEMENT OF α-IRON
The actual issue of using data of neutron embrittlement of reactor materials received under the study of witness-samples, installed at selected points of the atomic reactor, to the assessment of the actual neutron embrittlement of the reactor is considered. Significant difference in the mechanical properties, including neutron embrittlement, of the predicted values based on the
results of examination of witness-samples and the results of examinations of the reactor pressure vessel of the nuclear power plant Graiswald (Germany) after its closure in 1990, was found experimentally in 2010-2013 years. Here, the mechanical properties have been measured and the small-angle neutron scattering method has been used to determine the size and density of nano-defects with a size of up to 3 nm, namely clusters of point defects (vacancy clusters and clusters of interstitial atoms), copper precipitates, and clusters composed of point defects and alloying elements of the reactor pressure vessel steel. In the proposed study, one of the possible causes of the mentioned discrepancy is considered, namely, the difference in the surveillance temperature of the reactor (300 °C) and the temperature of the witness-sample (123 °C), which is irradiated for about one year in the reactor, and then extracted from it during routine shutdowns, for example, when replacing nuclear fuel. In order to simplify, we study the effect of temperature change not on reactor steel, but on commercial pure α -iron (carbon content is less than 30 ppm). The distribution functions of vacancy clusters and clusters of self-interstitial atoms respect to the number of monomers have been calculated by means of cluster dynamics calibrated by experimental data of small-angle neutron scattering, transmission electron microscopy and positron annihilation spectroscopy of commercially pure α - iron neutron irradiated in the research reactor BR-2 (Belgium, Mol). The mean cluster size and the number density of clusters are used to estimate the increase of the yield strength of Δσ of α - iron due a neutron irradiation. This value is usually interpreted as a quantitative characteristic of neutron embrittlement. It is shown that the contribution of vacancy clusters to the value of Δσ of the studied α -iron can be neglected while the contribution from clusters of self-interstitial atoms decreases with increasing time during which the temperature of the reactor was reduced.
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