JAEA-Conf 2011-002 - 日本原子力研究開発機構

decreases of (n,g) reaction rates, and then generations of curium isotopes are restricted. That
is why the sensitivities to fission cross sections are negative.
- The negative sensitivities to fission cross sections are large for concentrations of high order
curium isotopes such as Cm-245 and -246 since such nuclides are generated via a large
number of (n,g) reactions.
- As mentioned above, an increase of fission cross sections results in decreases of reaction rates.
It relatively increases contribution of the nuclide decay to concentration changes. For
example, the increase of fission cross sections promotes the Pu-241 decay and the Am-241
generation. Since Am-241 is a parent nuclide of Am-242m, which is a parent nuclide of
Cm-242, that results in an increase of the Cm-242 concentration. This means that there is a
positive component in the Cm-242 concentration sensitivity to fission cross sections. Note
that the Cm-242 concentration after 5-year cooling is quite small since Cm-242 is transformed
to Pu-238 during the cooling time.
- The sensitivity of the Cm-242 concentration to the Pu-239 fission cross section is positive.
Additional calculations show that this sensitivity is dependent on the cooling time. The
sensitivity changes to -0.09 without cooling time, and to +0.52 with 10-year cooling. That is
because contribution of the Am-242m decay during the cooling time to the Cm-242
concentration becomes larger as cooling time increases.
- Sensitivities to the U-238 capture cross sections are negative. If the U-238 capture cross
section increases, the Pu-239 concentration also increases. That results in a decrease of the
neutron flux level and restrict (n,g) reactions since the heating rate is constant.
- The concentrations of Cm-244, -245 and -246 have sensitivities to the Pu-242 capture cross
section since these nuclides are generated mainly via Am-243, which is a daughter nuclide of
Pu-242. On the other hand, the concentrations of Cm-242 and -243 have no sensitivities to
this cross section since these nuclides are generated mainly via Am-241.
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JAEA-Conf 2011-002
Fig. 2 One-group sensitivities of the curium concentrations to major actinide cross sections