T.R. England

Delayed Neutron Data and Group Parameters for 43 Fissioning Systems

AbstractThe quality and quantity of delayed neutron precursor data have greatly improved over the past decade and a half. Supplementation of the data with model calculations and the use of models to extend the number of precursors to 271 is now practical. These data, along with other improved fission product parameters, permit direct calculations of aggregate behavior for many fissioning nuclides. The results can still be approximated using a few (usually six) temporal groups, including corresponding spectra, as in past practice for reactor physics. An extensive effort to provide a complete set of evaluated data is summarized, with an emphasis on its use to generate the temporal approximations; precursor data and group values are intended for inclusion in ENDF/B-VI.

Evaluation of delayed-neutron emission probabilities

Delayed-neutron precursor data are being compiled in preparation of a new ENDF/B evaluation of delayed-neutron data. The probability of delayed-neutron emission, P /SUB n/, has been updated using ENDF/B-VM fission product yields and ENDF/B-V fission product decay data. Evaluations of 77 percursors are presented along with their effect on fission yields, P /SUB n/ systematics, and calculational models.

The Influence of Isomeric States on Independent Fission Product Yields

A simple one-parameter model is presented for calculating the distribution of independent yield strength between ground and isomeric states of primary fission products formed by neutron-induced fission of actinide nuclei. Yield branching ratios are calculated as a function of neutron energy (thermal, fast, and 14-MeV) for 33 cases that span 144 fission product nuclei having isomeric states with known spins.

Aggregate delayed neutron intensities and spectra using augmented ENDF/B-V precursor data

The evaluated nuclear data files (ENDF/B-V) have been used for 105 precursors, augmented with experimental spectral data for 29 emitters and recent model calculations for the remaining 76 emitters, in aggregate summation calculations. The equilibrium-delayed neutron intensities and spectra in the conventional six time groups for 11 fissionable nuclides were calculated. Normalized experimental neutron spectra for 29 emitters were supplied by Rudstam. Augmentation of the spectra uses data calculated with a recently developed code, BETA. Fission yields of the precursors and their delayed neutron branching fractions, P /SUB n/ , are from ENDF/B-V. Results are compared where possible with corresponding group and total evaluations of aggregate measurements. The intent of these calculations is to examine the adequacy of precursor data for inclusion in ENDF/B-VI. Most of the results, particularly with the added model parameters, are sufficiently accurate to be of direct interest to the reactor community. The total delayed neutron yield per neutron absorption, v /SUB d/ , for /sup 238/U and /sup 232/Th suggests that either an improvement in ENDF/B fission yields is needed or the evaluated experimental v /SUB d/ values are in significant error.

DANDE-A Linked General Code System for Core Neutronics/Depletion Analysis

DANDE—a modular neutronics, depletion code system for reactor analysis—is described. It consists of nuclear data processing, core physics, and fuel depletion modules, and it allows one to use diffusion and transport methods interchangeably in core neutronics calculations. This latter capability is especially important in the design of small modular cores. Additional unique features include the capability of updating the nuclear data file during a calculation; a detailed treatment of nuclide inventories and aggregate properties of burnable poisons, fission products, and actinides in the fuel; and the ability to make geometric changes such as control rod repositioning and fuel relocation in the course of a calculation. The detailed treatment of reactor fuel burnup, fission product creation and decay, as well as inventories of higher order actinides is a necessity when predicting the behavior of reactor fuel under increased burn conditions. The operation of the code system is illustrated by two actual problems.

Application of evaluated fission-product delayed-neutron precursor data in reactor kinetics calculations

Abstract Evaluated fission-product yield and decay data have been used to describe 105 delayed neutron precursors explicitly in point reactor kinetics calculations. Results calculated for 235U thermal fission show that rod-drop reactivity values obtained from kinetics calculations with 6-group precursor data are considerably higher than those calculated with explicit delayed-neutron precursor data. The calculated kinetics associated with positive reactivity steps are significantly different.

Application of detailed fission-product decay gamma spectra in the calculation of photoneutron spectra from d(γ, n) reactions in PWR fuel

Abstract The magnitude, average neutron energy, and spectrum of the photoneutron source in a PWR fuel is calculated for shutdown times to 100 y using CINDER radionuclide inventory calculations, ENDF/B-V γ line decay spectra, MCNP γ transport calculations, and PHONEX D(γ, n) reaction physics calculations.

Compilation of neutron precursor data

Delayed neutron precursor data (yields and spectra) are being compiled in preparation of a new ENDF/B evaluation of delayed neutron data. Precursor Pn values have been updated using ENDF/B-VM yields, ENDF/B-V decay data, and consensus Pn values. Weighted averages of these Pn values will be presented along with their effect on fission yields, Pn systematics, and model code calculations.

Delayed neutron spectra and intensities from evaluated precursor data

Recently evaluated and model estimated precursor data are used with preliminary ENDF/B-VI fission yields to calculate total and time-group anti nu/sub d/ and spectra. 8 refs.