A. L. Nichols*

International Atomic Energy Agency, Nuclear Data Section,
Department of Nuclear Sciences and Applications,
Vienna, Austria

Lectures given at the
Workshop on Nuclear Reaction Data and
Nuclear Reactors: Physics, Design and Safety
Trieste, 25 February — 28 March 2002

LNS0520003

Abstract

A sound knowledge of the time-dependent energy release resulting from the decay of the radioactive nuclides formed in the reactor core is extremely important in formulating safe procedures for the operation of nuclear facilities and the handling of irradiated fuel. Accurate estimates of this resulting decay heat are needed for a wide range of applications, including safety assessments of all types of nuclear plant, the handling of fuel discharges, the design and transport of fuel-storage flasks, and the management of the resulting radioactive waste. More specifically, the nuclear power community must ensure accurate and reliable calculations of the decay heat of irradiated fuel in order to maintain credibility and confidence in the safe and reliable performance of the various nuclear fuel cycles.

Neutron cross sections, fission yields and radionuclidic decay data represent the input to the summation calculations used to determine the release of decay heat over an extended period of time after reactor shutdown (i. e., following termination of neutron-induced fission). Nuclear data requirements for these summation calculations have been assessed, and a summary is given of their status. Associated uncertainties are examined, and specific inadequacies explored.

1. INTRODUCTION

Neutron-induced fission within the fuel of a reactor core, and the subsequent conversion of mass to energy constitutes a principal means of generating power. The resulting energy arises from the following phenomena:

kinetic energies of the fission products and neutrons;

prompt gamma radiation from highly-excited fission fragments, including short-lived isomeric states;

gamma and beta energy released through the delayed natural decay of the various radioactive products, particularly the fission products.

The last energy source contributes approximately 8% to 12% of the total energy generated through the fission process, and is commonly referred to as “decay heat”.

The author has focused on the decay heat that continues to be generated after the fission process has been terminated; the fission process is not considered directly, and the reader is referred to a number of dedicated publications on this phenomenon (Gonnenwein, 1991; Wagemans, 1991; Denschlag, 1997). The prompt sources of energy decline rapidly when a reactor is shutdown, but radioactive decay continues to heat the reactor core. Hence, coolant operation needs to be maintained after termination of the fission process, on the basis of reliable decay-heat calculations. Decay heat varies as a function of cooling time, and can be determined in theory from known nuclear data, based on computations of the inventory of the resulting radionuclides (primarily fission products, and actinides) created during the fission process and after reactor shutdown, and their radioactive decay characteristics.

Decay heat has been reviewed in detail by others from a technical perspective and also through the use of decay-heat equations as standards (ANS, 1979; Tobias, 1980; GNS, 1990; Dickens et al, 1991; Tasaka et al, 1991); the reader is referred to these publications for authorative assessments of the analytical procedures. Rather, the author has focused on the basic nuclear data of relevance to the decay heat generated in fission power-reactor systems, including the means of defining the initial radioactive inventory of controlled nuclear fission and other modes of decay within the core.

Cross-section, fission-yield and decay-data libraries are maintained for national and international usage. While this article avoids recommending specific sources of such data, Appendix A provides the reader with a brief summary of the means of accessing the most relevant data files via the acquisition of CD-ROMs or through the Internet. The latter method has become increasingly powerful, and provides the user with an extremely rapid route to virtually all of the highest quality nuclear data.