Measurement of the heat of $\alpha \to \beta$ transformation of plutonium under pulsed volumetric heating of samples in Yaguar pulsed nuclear reactor

The paper describes the procedure and results of measurement of one of the basic properties of unalloyed plutonium, namely, the heat of $\alpha \to \beta$ transformation. The measurements are performed under conditions of unsteady-state volumetric heating of plutonium in the process of neutron irradiation of samples in a pulsed nuclear reactor (PNR). The experiments are aimed at determining the preassigned degree of phase transformation (PT) of plutonium by the variation of electric resistance of sample measured at one and the same temperature of $\sim100^{\circ}$C in the state of $\alpha$ phase prior to neutron irradiation and in the state of metastable $\beta$ phase after neutron irradiation, completion of$\alpha \to \beta$ transformation, and cooling down to this temperature of measurement. The desired degree of PT is attained in the process of pulsed volumetric heating of plutonium under neutron irradiation of sample in the channel of the core of Yaguar PNR. The value of fission energy of plutonium nuclei released in the sample is determined by measuring the absolute heating of the working medium of the monitor-detector made of plutonium dioxide and the relative activity of indicators made of uranium-235 oxide and located on the working sample and on the monitor-detector. The results of measurement of energy input and of respective variation of electric resistance due to heating and to partial or complete $\alpha \to \beta$ transformation of plutonium in a sequence of experiments with neutron irradiation of sample in the PNR are used for constructing the dependence of variation of electric resistance of sample on energy input. This dependence is linear in the range of values of measured parameters corresponding to $\alpha \to \beta$ transformation of plutonium. The slope of straight line reflects the ratio of variation of electric resistance in the case of phase transformation to the heat of $\alpha \to \beta$ transformation. It follows from the experimentally measured slope and value of variation of electric resistance in the case of complete phase transformation that the heat of $\alpha \to \beta$ transformation is $\Delta H_{\alpha\beta} = 3.35 \pm 0.1$ kJ/mol, which agrees well with the adopted reference value of $\Delta H_{\text{tr}} = 3.45 \pm 0.08$ kJ/mol.