Authors & Affiliations
Grabezhnoi V.A., Dulin V.A., Dulin V.V.
A.I. Leypunsky Institute for Physics and Power, Obninsk, Russia
The neutron coincidence method is widely used to determine plutonium parameters in samples. Their acquisition is based on the approximation of the point kinetics model, i.e. on the assumption of small geometrical sizes of the samples and, thus, their low mass and neutron multiplication. It is quite obvious that in the course of mass measurement of extended samples with significant neutron multiplication, it is necessary to take into account the spatial effect, i.e. the dependence of probability (efficiency) of detector recording on its position and the position of a generated neutron. At the same time, the state of these samples can be far from critical. This fact does not allow the standard approach to be used for consideration of spatial effects and calculation, with making use of the solutions of adjoint conventionally-critical homogeneous equation, how it is usually done, for example, for subcritical states that are not deep (≤ βeff). Experimental determination of 239Pu mass fraction and Pu mass, with sets of disks of various isotopic compositions and with the use of method of double and triple neutron coincidences was implemented by means of a highly effective active well neutron coincidence counter AWCC. The aim of this work was to determine the following: the solution of what equation is the most applicable for the analysis of the results of measurements of multiplying media parameters with significant neutron multiplication, namely, an adjoint nonhomogeneous equation, whose solution shows the detector probability to record (detect) a neutron appeared in point - x,; an adjoint nonhomogeneous equation that considers neutrons from induced fissions of media nuclei, or the solution of adjoint conventionally-critical homogeneous equation.
Plutonium, transfer equation, neutron-neutron coincidences, point kinetics model, subcritical state, spatial effect, neutron multiplication
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