NUREG/CR-5625 (ORNL-6698) - Oak Ridge National Laboratory

Introduction
A more detailed description of the improved analysis a cooling time of 2 years, the computed heat rate is 3.632
method is given in Sects. S2.2.2 through S2.2.5 of the W/kgU. Had the heat rate result been determined from
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SCALE-4.0 documentation. Essentially, this expanded calculations using the maximum power of 40 kW/kgU, for
depletion model removes most of the conservatism from which the computed heat rate is 5.129 W/kgU, the result
the computed actinide decay heat rates and, also, provides would have been excessively conservative by 41%. Of
a procedure for calculating decay heat rates of spent fuel course the differences between the heat rates at these two
from BWRs as well as PWRs. Prior to generating the data powers is decreased considerably at increased decay times.
base of decay heat rates, the SAS2H analysis procedure
was validated using measured decay heat data obtained for The data base for a proposed guide revision has been
PWR and BWR spent fuel assemblies.
developed to encompass the defining characteristics of the
vast majority of spent fuel that is discharged from the
The current regulatory guide is formatted to provide a set mainstream of normal reactor operations. It was decided
of tables containing decay heat rates as a function of
not to include assemblies with atypical characteristics
parameter values that characterize a particular assembly. because it would force the guide to be overly conservative
Using the appropriate table and interpolation guidelines, for typical assemblies and/or significantly increase the
an appropriate decay heat value can be obtained from the computational effort and/or guide procedure.
tabular data base. This basic concept of interpolating a
reference data base to obtain the decay heat value has been 1.3 Overview of Report
continued in preparing the proposed revisions to the
current guide. However, the revised data base has been
improved significantly by incorporating computed decay
heat rates at six different burnups for each of three specific
powers (compared with one maximum specific power in
the current guide). Within each case, final decay heat
generation rates were computed at 20 different cooling
times in the range of 1 to 110 years. The ranges of the
BWR burnup and power were 20 to 45 MWd/kgU and 12
to 30 kW/kgU, respectively. The PWR burnup and power
ranges were 25 to 50 MWd/kgU and 18 to 40 kW/kgU,
respectively. Also, additional cases were computed in
235
which the U enrichment was either decreased or
increased by one-third from that of the standard case.
Thus, the calculated decay heat rate data were produced as
a function of burnup, specific power, cooling time, initial
235
fuel U enrichment, and assembly type (i.e., BWR or
PWR).
An example demonstrating the significance of the
improvement in using the actual specific power as opposed
to a single maximum power can be seen in the following
comparison. Consider a PWR assembly that has a burnup
of 30 MWd/kgU and a specific power of 18 kW/kgU. At
This introduction has provided a brief background of the
current guide and a discussion giving justification for a
proposed revision. The sources of the data used for the
analyses are presented in Sect. 2. Section 3 presents the
validation of the decay heat rate computational model
performed by comparison with measured calorimetric
data. A description of the cases producing heat rate data
for a guide revision is given in Sect. 4. The tabulated
data and complete procedure proposed for a revised guide
are presented in Sect. 5. This section is followed by a
detailed analysis of the method and equations. Finally,
Sect. 7 provides a brief description of the LWRARC code
(for use on a personal computer), which is an easy-to-use
code applying the data base and procedures presented in
Sect. 5. Also included are the addresses of two code
centers from which the LWRARC code may be requested.
The Appendix contains assembly design and operation
data, examples of the input for two of the tabulated cases,
a sample problem using the proposed procedure, plots
showing heat rates of major isotopes, and examples of the
LWRARC code printouts.
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