Abstract
The JEFF neutron induced and spontaneous fission product yield evaluation is currently unchanged from JEFF-3.1.1, also known by its UK designation UKFY3.6A. It is based upon experimental data combined with empirically fitted mass, charge and isomeric state models which are then adjusted within the experimental and model uncertainties to conform to the physical constraints of the fission process. A new evaluation has been prepared for JEFF, called UKFY3.7, that incorporates new experimental data and replaces the current empirical models (multi-Gaussian fits of mass distribution and Wahl Zp model for charge distribution combined with parameter extrapolation), with predictions from GEF. The GEF model has the advantage that one set of parameters allows the prediction of many different fissioning nuclides at different excitation energies unlike previous models where each fissioning nuclide at a specific excitation energy had to be fitted individually to the relevant experimental data. The new UKFY3.7 evaluation, submitted for testing as part of JEFF-3.3, is described alongside initial results of testing. In addition, initial ideas for future developments allowing inclusion of new measurements types and changing from any neutron spectrum type to true neutron energy dependence are discussed. Also, a method is proposed to propagate uncertainties of fission product yields based upon the experimental data that underlies the fission yield evaluation. The covariance terms being determined from the evaluated cumulative and independent yields combined with the experimental uncertainties on the cumulative yield measurements.
Highlights
Reliable and complete libraries of fission product yields with specified accuracies are needed for many different nuclear reactor calculations, including those on decay heat, dosimetry, burn-up, fuel handling, nuclide inventory and safety.This paper describes the latest such UK evaluation of fission yields
Improvements include bringing the database of fission yield measurements up to date by including 11 newly published measurements and using the GEF model [6] to predict unmeasured mass and charge yield distributions for all fissioning systems rather than fitting the experimental data to a combination of semi-empirical models for each system
It is too early to decide if this slight improvement results from switching the previous JEFF-3.1.1 extrapolation of Zp model parameters to the more self-consistent GEF model, but at the very least the UKFY3.7 results are no worse than the previous JEFF-3.1.1 results
Summary
Reliable and complete libraries of fission product yields with specified accuracies are needed for many different nuclear reactor calculations, including those on decay heat, dosimetry, burn-up, fuel handling, nuclide inventory and safety. The possible future developments of the fission yield libraries are discussed. These include; (i) production of covariance terms so that useful uncertainties can be determined on fission product inventories, (ii) including non-traditional fission product yield measurements in evaluations and (iii) changing the fission product yield evaluations from a typical standard neutron spectra to an energy dependent description. – Statistical analysis of all available experimental data producing a recommended value and uncertainty for all measured yields. – For all important fissioning system, see Table 1, fill all missing mass distributions and independent yields (including the ternary yields) using an appropriate model that estimates both the value and it uncertainty. Experimental cumulative yields and their uncertainties, produce uncertainties for all the cumulative yields. – Produce an ENDF formatted file containing the data
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