Determination of Gamma-Ray Exposure Rate from Short-Lived Fission Products under Criticality Accident Conditions.

Abstract

For the assessment of γ -ray doses from short-lived fission products (FPs) under criticality accident conditions, γ -ray exposure rates varying with time were experimentally determined in the Transient Experiment Critical Facility (TRACY). The data were obtained by reactivity insertion in the range of 1.50 to 2.93$. It was clarified from the experiments that the contribution of γ -ray from short-lived FPs to total exposure during the experiments was evaluated to be 15 to 17%. Hence, the contribution cannot be neglected for the assessment of γ -ray doses under criticality accident conditions. Computational analyses also indicated that γ -ray exposure rates from short-lived FPs calculated with the Monte Carlo code, MCNP4B, and photon sources based on the latest FP decay data, the JENDL FP Decay Data File 2000, well agreed with the experimental results. The exposure rates were, however, extremely underestimated when the photon sources were obtained by the ORIGEN2 code. The underestimation is due to lack of energy-dependent photon emission data for major short-lived FP nuclides in the photon database attached to the ORIGEN2 code. It was also confirmed that the underestimation arose in 1,000 s or less of time lapse after an initial power burst.

Assessment of Human Body Surface and Internal Dose Estimations in Criticality Accidents Based on Experimental and Computational Simulations

“…A pulse operation of TRACY can create a criticality accident situation of up to 1Â10 18 fissions. Excess reactivity can be added into the TRACY core up to 3$ in three modes: rapid or ramp withdrawal of a transient rod from the core, and ramp feed of fuel solution beyond the critical solution level.…”

“…In the eigenvalue calculation, the fluences of neutrons and prompt gamma-rays (including neutron-capture gammarays) were converted from those per neutron history into those per fission using the mean number of neutrons generated per fission, 2:38AE0:01, which was calculated simultaneously. 14) In the fixed-source calculation, the fluence of delayed gamma-rays was also converted from that per photon history into that per fission using the mean number of photons emitted from FPs per fission, 1:74AE0:03, which was calculated for the 22 s period after an initial power burst 14,18) using a code for generation and depletion of isotopes, ORI-GEN2, 19) with a decay and photon data file based on the JENDL FP Decay Data File 2000. 20) The photon source was assumed to have a uniform volumetric distribution in the fuel solution.…”

Evaluation of Gamma-Ray Dose Components in Criticality Accident Situations

“…The density distribution of the photon source was assumed to be a uniform volumetric distribution in the fuel solution. The intensity of the photon source (i.e., the number of delayed photons per fission) was calculated by Yanagisawa et al 8,18) using a code for generation and depletion of isotopes, ORIGEN2 19) together with updated decay and photon databases based on the JENDL FP Decay Data File 2000. 20) In this calculation, decay chains of FPs generated by the fission reaction of 235 U were traced, because the ratio of the number of 235 U fissions to that of the total fissions is 99.5% in the fuel solution of TRACY.…”

“…The calculated intensity of the photon source was verified to be correct by comparison with an experimental result. 8,18) In the present calculations of the delayed gamma-rays, an interpolating polynomial for the calculated intensity was employed to approximate the value of the intensity at a given duration of criticality. The photon energy spectrum integrated over the 60 s period after the initial power burst was employed as a typical spectrum for the TRACY pulse operation, because there was little difference between the energy spectra integrated over periods of 20 or of 60 s. The relative difference between the gamma-ray absorbed doses calculated with these two spectra was within 4%.…”

“…7) However, Yanagisawa et al ascertained that this assumption was not always valid for gamma-ray dosimetry, performing an experiment wherein the contribution of delayed gamma-rays from the decay of unstable fission products (FPs) was 16AE1% of the total gamma-ray dose in the 18 second period after an initial power burst. 8) The gamma-rays, whose total dose is roughly compa-rable to the neutron dose in free air in criticality accident situations, 6) continue to be emitted both during criticality and after its termination. Understanding of such emission behavior of gamma-rays is thus required to plan and attempt postaccident measures at actual criticality accidents.…”