Evaluation of Time Response of Thermal Neutron Detectors in TRACY

Evaluation of Photon Shielding Capability for Epithermal Neutron Detection during Reactivity-Initiated Power Burst Experiments

“…The contribution of those photons is considered to be enlarged as the increase of the distance between the fission chamber and the core because of the expansion of core as indicated in an earlier report. 5) The time delay effect found in those photons is not negligible in comparison with the full width at a half of maximum of an initial power burst in TRACY, about 8 ms in minimum. 3) Just after the power peak of the initial burst, the power rapidly decreases at a level less than a thousandth of the peak power.…”

“…We must pay attention to the photons generated by the latter process because they may bring the same time delay effect as found in the thermal neutron detection. [4][5][6] To confirm the time delay effect, an additional calculation was performed for the model-1. Using the MCNP4B code, the photon and neutron fluence at the fission chamber position were traced as a function of time after neutron emission at the core, as shown in Fig.…”

“…In TRACY, a micro 235 U fission chamber with fast response current amplifiers had been used to cover a wide range of power (10 3 to 10 10 W) during the experiments. 1) It was, however, clarified from experimental and analytical studies [4][5][6] that the consideration of time delay arising from flight-time of neutrons from the core to the neutron detector was indispensable for accurate evaluation of power histories.…”

Evaluation of Power History during Power Burst Experiments in TRACY by Combination of Gamma-Ray and Thermal Neutron Detectors.

“…In earlier reports, 3,4,11) the effect of the time delay of the thermal neutron detection at the fission chamber was simulated numerically by using the Monte Carlo method from the viewpoint of the evaluation of the inverse period and feedback reactivity after the power peak. By using the same simulation method, the underestimation of the peak power and integrated power detected by the fission chamber could also be reproduced.…”

“…It was also confirmed that the time delay of the thermal neutron detection by the fission chamber was caused by the flight-time of neutrons from the core to the fission chamber. To avoid the effect of the time delay, it was indicated from an analytical study 4) that the fission chamber had to be positioned within 10 cm from the core tank surface. This arrangement is not, however, appropriate for neutron detection by the fission chamber in TRACY, because the ionization cur-rent of the fission chamber is caused by the γ rays, due to the high dose rates of γ rays at the position in the vicinity of the core, which is estimated to be 2×10 7 C/kg/h (=8×10 10 R/h) in maximum.…”