JAERI 1287 JNDC Nuclear Data Library of Fission Products Fir

JAERI 1287 151
4. Calculation of Decay Power of Fission
Products
The decay power of fission products has been calculated by the DCHAIN code using
the JNDC nuclear data library described in Sec. 2 for the instantaneous fission pulse, a year's
irradiation and the infinite irradiation. A year's irradiation is considered to be a typical irradiation
time in a light water reactor operation. Thermal neutron fission of 235 U, 239 Pu, 241 Pu,
233 U, fast neutron fission of 235 U, 238 U, 239 Pu, 232 Th, and 14 MeV neutron fission of 235 U,
238 U were considered in the analysis. Neutron capture transformations of fission products
during operation were neglected in the calculation. The cooling time after irradiation ranged
from 0 to 10 13 s.
The calculations of decay power in this section aim at comparison of the present results
with other evaluations and, furthermore, at application for the best-estimate calculation of
LOCA. In order to compare the present library with others and to study the sensitivities of
fission products, contributions of important nuclides to the decay power are evaluated for
the thermal neutron fission of 23S U.
The calculated results have been fitted to an analytical function with thirty-one exponential
terms to apply the present results conveniently to a LOCA analysis and other
applications.
4.1 Tables of Calculated Results
Calculations of decay powers for the instantaneous burst fission, a year's irradiation, and
the infinite irradiation are given in Tables 4.1.1, 4.1.2, and 4.1.3, respectively. Contribution
of each fission-product nuclide to the decay power is shown for the thermal neutron fission
of 235 U in Tables 4.1.4, 4.1.5, and 4.1.6, respectively, for the instantaneous fission, a year's
irradiation, and the infinite irradiation.
Methods are given subsequently for obtaining the decay power Pit, T) for an arbitrary
reactor power history without neutron capture by fission products. Neutron capture in fission
products has only a small effect upon the decay power for the cooling time less than 10 3
s s9) . However, the direct summation calculation is necessary for the accurate evaluation of
the decay power, especially for long cooling times after long-term and high-neutron flux
irradiation.
The decay power Pit, T) for an arbitrary irradiation history PoiT) can be obtained
either from fit) for the instantaneous fission, or Fit) for the infinite irradiation. From
fit) , the decay power Pit, T) can be calculated as
D ( r X^ / \
Pit,T) = fJ^Q—f(t + T-T')dr (4.1.1)
where
t = cooling time after irradiation (s)
T = total irradiation time (s)
Pa = reactor power (MeV/s)
Q = fission energy (MeV/fission)
fit) = decay power after one fission [(MeV/s)/fission].
From Fit) ,Pit,T) can be calculated as