Impact of capture cross-section of carbon on nuclear design for HTGRs

Shimakawa, Satoshi; Goto, Minoru  ; Nakagawa, Shigeaki  ; Tachibana, Yukio 

Capture cross section of carbon in thermal energy range has been regarded as unimportant in neutronics calculations on general reactor design, because of its infinitesimal value of only 3 mb at 2200 m/s. However, it is not negligible for design works for graphite-rich reactors, such as the High Temperature Gas-cooled Reactors (HTGRs). For the High Temperature Engineering Test Reactor (HTTR) of JAEA, five percent differences in capture cross section of carbon makes 0.24% change in thermal utilization factor of the four factor formula. This impact is for the HTTR with a core configuration of full-loaded core, named the packed core. In this case, change of multiplier factor will be equivalent to a change of thermal utilization factor. The impact of the cross section is dependent on an atomic number ratio of graphite/235-uranimu in reactor core. For more graphite-rich core such as the HTTR with ring core configuration, the five percent change of the cross section value makes a 0.47% on multiplier factor. From our studies in the HTTR analysis, a value of capture cross section at 2200 m/s has been revised to 3.86 mb in evaluated nuclear data library of JENDL-4. Comparing with the value of JENDL4, the values in other libraries are about 10-15% smaller as 3.36 mb in ENDF/B-VII, 3.36 mb in JEFF-3.1 and 3.53 mb in JENDL-3.3. It was observed that discrepancy of a multiplier factor between former calculation and experiment of the HTTR showed disagreement in the evaluation of the critical approach tests. Monte Carlo calculation results using JENDL3.3 are overestimated about 0.4% with packed core configuration and 1.0% with ring core, respectively. In this report, the improvement of excess reactivity calculation for the HTTR with newly JENDL-4 is described.