JAERI 1287 JNDC Nuclear Data Library of Fission Products Fir

JAERI 1287 3. Comparison of Calculated and Measured Decay Powers 135
and the ENDF/B-IV library are compared with the measurement in Figs. 3.2.1 through 3.2.3
for the thermal neutron fission of 235 U, 233 U and 239 Pu. The ratios of the calculated values
to the measured results are also shown in the figures.
It is seen in these figures that the improvement of accuracy by the present library is small
at the short cooling times less than 100 s after the finite irradiation of 2 X 10 4 s (~5.6 h).
At cooling times greater than 100 s, the calculations by the present library are greater
than those by the other two libraries. This is because of the over prediction of gamma decay
power for 10 3 to 10 4 s after burst fission as discussed in Section 3.1. The calculated decay
powers by JNDC library agree with experimental results within the discrepancies of+ 10, —6%
for 235 U, +6, -4% for 233 U, and +0, -10% for 239 Pu. The discrepancies between calculations
and measurements are larger for 235 U and smaller for 239 Pu in the JNDC library than in the
other two libraries. Accuracies of both calculations and measurements should be examined.
For calculations the over-estimation of decay power at cooling time of 10 3 to 10 s s should
be checked out, and for measurements the accuracies at short cooling times less than 100 s
and the normalization factor for fission rate should be checked out.
3.3 Comparison with UTT Experiment
Akiyama et al. 12 " 14 ^ measured the decay power of fission products for fast neutron
fission of 235 U. Samples of 1.6 mg were irradiated for 10 to 300 s in the core of the fast
neutron source reactor YAYOI of the University of Tokyo. The beta- and gamma-rays emitted
from the sample were measured for times-after-fission of 11 to 26000 s. The data were obtained
for beta- and gamma-rays separately as the spectral distributions. For beta ray, the
spectra were obtained by using a plastic scintillation detector combined with a transmission
type proportional counter to eliminate gamma-ray effects; and for the gamma-ray data, the
spectra were obtained by using a Nal(Tl) scintillation detector. These spectra were integrated
over beta and gamma energy to provide total yield and energy integrals as a function of time
after fission pulse in the range of 19 to 24000 s. The uncertainties (Iff) are estimated to be 5%
for both beta and gamma decay powers.
The measured decay powers are compared with the computational results using the
nuclear data libraries of JNDC, Tasaka and ENDF/B-IV in Figs. 3.3.1 through 3.3.3. The
ratios of the calculated decay powers to the measured ones are also given in the figures.
It is seen in the figures that the calculated decay powers, especially beta decay powers
by the present JNDC library are in much better agreement with the measured ones than the
calculation by the other two libraries of Tasaka and ENDF/B-IV at cooling times less than
10 3 s.
At cooling times greater than 4000 s calculation by the JNDC library underestimates
the beta decay powers by about 9%, whereas the other two libraries do so by 5%. For cooling
times greater than 10 3 s the JNDC library overestimates the gamma decay powers and the
total decay powers by about 19% and 6%, respectively. This trend is the same as that of the
discrepancies between the calculated decay powers and the experimental data of 23S U at
ORNL described in Sec. 3.1. Thus, the present JNDC library overstimates the gamma decay
power and underestimates the beta decay power compared with the measured data for cooling
times greater than 10 3 s. These discrepancies should be resolved in future work.