Verifying neutron data for plutonium isotopes in experiments on fast critical piles

Abstract

As is well known, integral experiments on critical piles are useful in verifying neutron data obtained in nuclear physics experiments. The ABBN-78 group constants for 23921'1pu were verified in [l]. New nuclear physics data forming the basis of the new ABBN constants have now been obtained [2]. Having been tested and named ABBN-93, the new system of constants has been recommended as the standard reference data for reactor and shielding calculations [3].On the basis of new methods of evaluating macroscopic experiments and additional experiments on BFS fast critical piles, experimental data useful in verifying and refining the constants of 24~ have been obtained. These data, together with the experiments in [ I, 4], constitute a representative set of experiments allowing the constants of plutonium for the new ABBN-93 system of group constants to be tested.In the present work, theoretical data obtained on the basis of the ABBN-93 constants are compared with experimental data on the ratios of the mean fission cross sections and the ratios of the reactivity coefficients for 239"240"241pu (with respect tO 235U). The characteristics of new critical piles are also briefly presented, along with methods of evaluating experimental data. EXPERIMENTS ON BFS CRITICAL PILESThe structure of the new BFS critical piles (Table 1) is traditional; see [4], for example. In contrast to other piles, the central insert of the BFS-56-1B pile consists of seven actual fuel rods that are similar in composition to those of a fast power reactor with uranium-plutonium fuel based on plutonium extracted from a VVER-1000 reactor. The BFS-56-ITs pile differs from the BFS-56-IA pile in that the uranium dioxide is replaced by graphite. The BFS-58-1 pile is obtained by replacing the empty shells of the sodium pellets with pellets containing sodium.The ratios of the mean fission cross sections fi/f235 are measured by means of fission chambers and the method of calibration through the thermal column. In addition, the ratio of the mean fission cross sections of 240"239pu (f240/f239) is obtained by absolute calculation in chambers with a known quantity of material and a known efficiency of fission fragment recording (-0.99). The ratios of reactivity coefficients of the materials ri/r235 are measured by means of periodic perturbation of pile criticality using small samples. The pile reactivity is recorded by means of a digital reactimeter [5]. All the experimental values are obtained as a result of averaging over the height of the pile's central cell. CONSTRUCTING ADEQUATE THEORETICAL MODELS OF THE EXPERIMENTSBefore comparing the experimental and theoretical data, models of the experiments must be constructed and the experimental data must be reduced to the conditions of these models.The experiments are relatively simple. One-dimensional cylindrical calculation models are chosen. (The leakage at the ends is taken into account in the B 2 approximation.) The systems are brought to a critical state by varying the thickness of the State Scientific Center, Russ...

Measurement of the ratios of the237Np and241Am capture cross-sections to the235U fission cross-section by the reactivity method

“…It can be seen from the data given in Tables 1 and 2, and previously in [4,5], that the discrepancies with calculation for lOp/235p are lower for assemblies with less 238U. It would appear that one can reduce the 238U capture cross section.…”

“…It was previously 20-30% [13] but decreased to 105 with a more complete evaluation of the experiment and the use of the BNAB-78 system of constants in which the 235U and 239pu capture and f'hssion cross-sections were corrected to take account of integrated experiments [4, 5,13] and whose use the enabled the lower-energy spectrum to be obtained. The discrepancies between the evaluated experiment and calculation for 1~ are given in Table 3.…”

“…*Spectral hardness, fraction of neutrons with energies below 10 keV, normally used by authors of present article [4,5].…”