Experimental Estimation of Effective Dose for Irradiation Geometries in Working Places According to ICRP 1990 Recommendations

Conversion coefficients for the equivalent dose in tissue or organ and the effective dose were estimated experimentally with BeO-TLDs inside a male RANDO phantom against external photon radiation. The experiments were performed for Superior-Inferior and Inferior-Superior geometries in cases of unusual irradiation conditions which were occasionally seen in high radiation areas. For these geometries, a parallel photon beam to the long axis of phantom was irradiated. To evaluate the shielding effect of legs in the Inferior-Superior geometry, measurements with and without legs were done by linking a leg phantom, made of tissue equivalent material. The difference of effective dose by the leg phantom was found to be about 20% in the energy range studied.

The effective dose was calculated from the equivalent dose in tissues or organs by modifying the tissue weighting factors given in ICRP Publ. 60 for males, and the effective dose equivalent according to ICRP Publ. 26 was also derived to be compared with the effective dose. In this experiment, the effective dose was estimated lower than the effective dose equivalent about 40% for Superior-Inferior geometry and about 20% for Inferior-Superior geometry due to the change in tissue weighting factors. However, there was no remarkable difference for Anterior-Posterior geometry.     

Calculations of Effective Dose and Ambient Dose Equivalent Conversion Coefficients for High Energy Photons

The conversion coefficients from photon fluence to ambient dose equivalent, H* (10) and effective doses were calculated for photons up to 10 GeV. A Monte Carlo code EGS4 was used for these calculations and secondary particle transports were considered. The calculated ambient dose equivalents were compared to the calculated effective doses. The comparison shows that the ambient dose equivalents at 1 cm depth, H* (10) underestimate the effective doses at the energy above 5MeV. H* (10) is not suitable operational quantity since it does not provide reasonable estimation of effective dose. It is difficult to define the operational quantity which can be consistently used for photons from low energy to high energy above 10 MeV. Instead of operational quantities, the maximum effective dose in various irradiation geometries can be used for shielding design calculations.     

Calculation of Effective Doses for External Neutrons

Monte Carlo calculations of the effective dose, on the basis of 1CRP Publication 60, were performed for external neutrons from thermal energy to 18.3 MeV for five irradiation geometries: AP, PA, RLAT, ROT and ISO. A unisex anthropomorphic phantom and the MORSE-CG code were used in conjunction with a nuclear data set based on the JENDL-3 library. The effective dose was found to be superior to the effective dose equivalent, the former quantity, for neutrons below about 1 MeV and inferior above this energy for all the geometries. The ambient dose equivalent based on the new Q-L relationship proposed in the Publication was found not necessarily to give a conservative estimate of the effective dose for the AP and PA geometries. The results obtained here were in good agreement with those calculated with a different computer code and a different nuclear data set.     

External Radiation Conversion Coefficients using Radiation Weighting Factor and Quality Factor for Neutron and Proton from 20 MeV to 10 GeV

Conversion coefficients from neutron and proton fluences to effective dose are calculated in the range of incident neutron energy from 20 MeV to 10 GeV. Two different versions of effective dose are treated, respectively calculated using: (a) the radiation weighting factor wR, and (b) the Q-L relationship given in ICRP 60. Monte Carlo calculations are performed applying the HETC-3STEP and the MORSE-CG/KFA in the HERMES code system. The calculations are based on a modified MIRDS anthropomorphic phantom, which is irradiated in anterior-posterior and posterior-anterior directions of beam. For effective dose calculation using the Q-L relationship, a database was compiled and added to the HETC-3STEP, to derive the average quality factor. The effective dose derived using wR proved to overestimate that obtained with the Q-L relationship by about 80% at 10 GeV incident neutron energy in the case of conversion from neutron fluence. For proton fluence, the corresponding overestimation reaches a maximum factor of 4.     

Simple Empirical Expression to Evaluate Effective Dose Equivalent for External Photon Irradiation

In the CABRI-FAST and CABRI-RAFT programs within a collaboration with the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) and Forschungszentrum Karlsruhe (FZK), five pulse-type transient overpower tests were performed in order to study fuel pin behavior and failure condition in the Unprotected Loss-of-Flow (ULOF) accident. In these tests, two types of low-smear-density fuels irradiated in the French Phénix reactor at different burn-up levels were used so that an experimental database extension from the former CABRI-1 and CABRI-2 programs can be obtained. Pin failure took place in three of these tests giving information on the failure threshold. In two tests, no pin failure took place and useful information related to the transient fuel behavior up to failure and failure mechanism was obtained. These test results were interpreted through detailed analysis of experimental data and PAPAS-2S code calculations. In these calculations, pretransient fuel characteristics obtained from the sibling fuels were reflected, such that the uncertainty of the boundary condition can be minimized. Through the comparison among these tests and formerly existing CABRI tests, generalized understanding on the transient fuel behavior was obtained. It was concluded that the low-smear-density fuel mitigates cavity pressurization, thereby enhancing the margin-to-failure. It was also understood that this failure-thresholdenhancing capability is dependent on the type of transient.