A Re-Analysis of Historical Los Alamos Critical Assembly Reaction Rate Measurements

Starting in the 1950s and continuing into the early 1970s, a number of foil irradiations and fission chamber measurements were made in a variety of Fast critical assemblies at Los Alamos National Laboratory. These include (i) Godiva, a bare HEU spherical assembly; (ii) Flattop-25, a spherical assembly consisting of an HEU core and a natural uranium reflector; (iii) Jezebel, a bare Pu-239 assembly; and (iv) Flattop-Pu, a spherical assembly consisting of a Pu-239 core and a natural uranium reflector. In most instances the irradiations occur at or near the center of the assembly, but in selected instances data were obtained for a radial traverse extending into the Flattop reflector region. Measurements were made for a number of threshold reactions, including Sc-45(n, 2n) (44)mSc, V-51(n, alpha) 48Sc, As-75(n, 2n) As-74, Y-89(n, 2n) Y-88, Zr-90(n, 2n) Zr-89, Rh-103(n, 2n) (102)gRh, Ag-107(n, 2n) (106)mAg, Tm-169(n, 2n) Tm-168, Lu-175(n, 2n) Lu-174, Ir-191(n, 2n) Ir-190, Au-197(n, 2n) Au-196, Tl-203(n, 2n) Tl-202, Pb-204(n, 2n) Pb-203 and U-238(n, 2n) U-237. Fission ratio data for U-238(n, f)/U-235(n, f) and Pu-239(n, f)/U-235(n, f) were also obtained. We report C/E values from MCNP6 calculations using ENDF/B-VII. 1 and IRDFF-v1.03 cross section data.