JAERI 1287 JNDC Nuclear Data Library of Fission Products Fir

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3. Comparison of Calculated and Measured
Decay Powers
Decay powers of fission products were calculated by the DCHAIN 15 ^ code using the
JNDC nuclear data library of fission products described in Section 2, the library evaluated by
Tasaka 20 > and the ENDF/B-IV library for the thermal neutron fission of 233 U, 235 U, 239 Pu
and 241 Pu, and for the fast neutron fission of 23S U. The results are compared with the latest
decay power experiments at ORNL 3 " 6) , LANL 7>8) and University of Tokyo 12 " 14 >.
3.1 Comparison with ORIML Experiment
Dickens et al. 3 ' 6 ^ at ORNL measured the decay power of fission products for thermal
neutron fission of 23S U, 239 Pu and 241 Pu. Samples of mass 1 to 10 ptg were irradiated for 1
to 100 s using the fast pneumatic-tube facility at the Oak Ridge Research Reactor. The resulting
beta- and gamma-rays emitted by the samples were counted for times-after-fission of 2
to 14000 s. The. data were obtained for beta- and gamma-rays separately as spectral distributions,
N(Er) versus ET and N(Ep) versus Ep. For gamma ray, the spectra were obtained using
an Nal detector; and for beta ray, the spectra were obtained using an NE-110 detector with
an anticoincidence mantle. The raw data were unfolded to provide spectral distributions with
modest resolution. These distributions were integrated over Er and Ep to provide total yield
and energy integrals as a function of time after fission.
The rate of energy release after t seconds following an instantaneous pulse of fission
can be estimated by dividing the energy integrals by the counting time tcount . The cooling
time t is taken as
I— t wait ' U. D ^ tirr ad ' I count )>
where, twail = waiting time before counting after irradiation,
tirrad - irradiation time,
tcount = counting time.
The difference between the estimate and an "exact" value is very small for most measurements
since twail is greater than {tirrad + tcount )• It becomes large for short waiting times
compared with the irradiation and counting times and reaches a maximum at t = 2.7 s which
are approximately 4% and 2% for 239 Pu and 241 Pu, respectively, for both beta- and gamma-ray
energy-release data. For most of integrals, the uncertainties (la) are less than 4%, 4% and
5% for the fissions of 235 U, 239 Pu and 241 Pu, respectively.
Figures 3.1.1 (a) through 3.1.9 (a) show the comparisons of calculated and measured
decay heat powers of beta, gamma, and beta + gamma rays by thermal neutron fissions of
235 U, 239 Pu and 241 Pu. The abscissa is the time t after burst fission and the ordinate is the
decay power multipled by the time t. The ratios of the calculated decay powers to the measured
ones are also shown in the figures (b) for evaluating easily the percent discrepancies
between calculated and measured decay powers.
As seen from these figures the beta decay powers calculated by the present JNDC library
agree with the measured results within the accuracy of ± 7% for the fissions of the all three
nuclides 235 U, 239 Pu and 241 Pu. The accuracy has been much improved compared to the
accuracies of ± 20% and ± 28% attained by the Tasaka library and the ENDF/B-IV library,