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Report No.
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Impact of revised thermal neutron capture cross section of carbon stored in JENDL-4.0 on HTTR criticality calculation

Goto, Minoru; Shimakawa, Satoshi; Nakao, Yasuyuki*

In the past, benchmark calculations of criticality approach for the HTTR, which is a Japanese HTGR, were performed by research institutes in several countries, and almost all of the calculations overestimated the excess reactivity. In Japan, the benchmark calculations performed by JAEA also resulted in overestimation. JAEA improved the calculations by revising the geometric model and replacing the nuclear data library with JENDL-3.3, which was the latest JENDL at that time. However, the overestimation remained and this problem has not been resolved until today. We performed calculations of the HTTR criticality approach with several nuclear data libraries, and found that slight difference in the capture cross section of carbon at thermal energy among the libraries causes significant difference in the $$k$$$$_{eff}$$ values. The cross section value of carbon was not concerned in reactor neutronics calculation because of its small value of the order of 10$$^{-3}$$ burn, and consequently the cross section value was not revised for a long time even in the major nuclear data libraries: JENDL, ENDF/B and JEFF. We thought that the cross section value should be revised based on the latest measurement data in order to improve the accuracy of the neutronics calculations of the HTTR. In April 2010, the latest JENDL;JENDL-4, was released by JAEA, and the capture cross section of carbon was revised. JENDL-4 yielded 0.4%$$Delta$$$$k$$-0.9%$$Delta$$$$k$$ smaller $$k$$$$_{eff}$$ values than JENDL-3.3 in the calculation of the HTTR critical approach, and consequently the problem of the overestimation of the excess reactivity in the HTTR benchmark calculation was resolved.

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