Reactivity and parameter measurements in a coaxial uranium fuel-D2O moderated critical lattice

Reactivity and reaction rate parameters were measured for a 7-inch triangular pitch lattice of coaxial uranium fuel in a critical, D2O-moderated reactor. The results were compared with RAHABR computations using ENDF/B-IV cross sections and with an earlier subcritical exponential measurement of the same lattice. Measured and calculated reactivity are in good agreement, however, the calculated ratio of epicadmium 238U captures to subcadmium 238U captures (rho28) was 10% lower than measured. Indirect verification of the rho28 measurement was obtained by measurement of a new conversion ratio parameter (C+) defined as the ratio of 238U captures to total fissions. Agreement between measured and calculated inner-to-outer fuel activation ratios suggests that the discrepancy is caused by underprediction of epicadmium captures in 238U or by erroneously high measured values of rho28 and C+. However, any adjustment of 238U captures to match the measured rho28 will cause the calculated reactivity to be underpredicted