Reactor kinetics calculated in the summation method and key delayed-neutron data

The point-reactor kinetics after a step reactivity insertion to a critical condition is solved directly form fission-product (FP) data (fission yields and decay data) for the first time. Numerical calculations are performed with the FP data in ENDF/B-VI. The inhour equation obtained directly from the FP data shows a different behavior at long periods from the one obtained from Tuttle's six-group parameter sets. The behavior is quite similar to the one obtained from the six-group parameter sets in ENDF/B-VI, that were obtained from FP data in a preliminary version of ENDF/B-VI. To identify the erroneous FP data, we examine the asymptotic form of the inhour equation at an infinitely long period. It is found that the most important precursors for long reactor periods are found 137I, 88Br and 87Br. They cover more than 60% of the reactivity. It is remarkable that 137I alone covers 30-50% depending on the fissioning system. In addition to the three precursors, 136Te is found a candidate precursor for the peculiarity from the time dependence of the delayed neutron activity. It is recommended that the precision of their Pn values should be improved experimentally. For 137I, 88Br, and 87Br, the relative uncertainty, dPn/Pn, should be decreased down to 2% and for 136Te to 5%. (author)