Development of Dose Assessment Code for Accidental Tritium Releases

A computer code ACUTRI has been developed to assess tritium doses to the general public. ACUTRI can calculate the doses for tritium gas (HT) and tritiated water (HTO) accidentally released from the facilities to the atmosphere. The models in this code consist of the tritium transfer model including oxidation of HT to HTO and reemission of HTO from soil to the atmosphere and the dose calculation model. The tritium concentrations in air and doses were calculated using the ACUTRI code under some conditions as a preliminary analysis. In order to validate the model, calculations were compared with the experimental results. INTRODUCTION Tritium, which is used as a fuel of the D-T burning fusion reactor, is the most important radioactive nuclide for the safety assessment of a nuclear fusion experimental reactor such as ITER. Tritium has a characteristic behavior in the environment as follows. In case of HTO release, after the dispersion in the atmosphere, a fraction of HTO deposits to soil and then the HTO is reemitted from soil to the atmosphere. In case of HT, a fraction of HT diffuses in soil and is oxidized to HTO by microorganisms in soil. The oxidized HTO is also reemitted to the atmosphere. Since a dose coefficient of HTO is 10,000 times as high as that of HT (1), the oxidation of HT to HTO followed by the reemission of HTO is a very important process in the dose assessment for HT release. So far, several codes have been developed, such as UFOTRI (2), ETMOD (3) and NORMTRI (4). These codes can calculate the tritium concentrations in the atmosphere and the doses for HTO and HT releases. The environmental tritium transfer models in the codes mostly include the oxidation of HT to HTO on soil surfaces, the migration of HTO into soil, the uptake of HTO to the vegetation, the reemission of HTO and so on. However, these codes do not comply with the methodology, in particular the treatment of meteorological data, prescribed in the guide of the Nuclear Safety Commission (NSC) of Japan. ACUTRI has been developed to assess doses to the public due to tritium released accidentally, according to the meteorological guide of the NSC. This report describes characteristics of ACUTRI, tritium transfer models used in the code and some results analyzed preliminary. Furthermore, calculations of ACUTRI are compared with results of the short-term HT release experiment carried out in Canada, in order to validate the model. MODEL DESCRIPTION ACUTRI has the following characteristics: 1) it can calculate an individual dose due to tritium using a tritium transfer model in the environment and a dose model, 2) the dose is conservatively assessed to prevent underestimation, and 3) it is possible to analyze statistically on meteorology in the same way as a conventional dose assessment method according to the meteorological guide of the NSC of Japan. Environmental pathways incorporated in the code are shown in Fig. 1. ACUTRI can simulate the environmental transfer of HT and HTO released from the facilities to the atmosphere. The Gaussian plume model is used for calculating the atmospheric tritium dispersion of the primary plume. The dry depositions of HT and HTO on soil are calculated using deposition velocities of HT and HTO. The HTO dry deposition can be also given by exchange velocity instead of the deposition velocity. In addition, wet deposition of HTO can be calculated using a washout coefficient. The HTO in a soil surface layer is reemitted to the atmosphere as the secondary plume after HT and HTO deposit. The reemission of HTO is calculated using a reemission rate represented a ratio of the amount of the HTO reemission per unit hour to that of the total HTO deposited on soil. The Gaussian plume model is also applied to the atmospheric dispersion of secondary plume of HTO reemitted from soil. It is assumed that the secondary plume does not deposit on a soil surface. Schematics of internal exposure due to the primary and secondary plumes are shown in Fig. 2. Terrestrial elements are used to calculate the amount of HT and HTO deposition and reemission of HTO as the secondary plume. The elements are defined by fan-shape areas divided by concentric circles and radial lines from the evaluation point. For each terrestrial element, different deposition or exchange velocities can be given as input parameters. ACUTRI can simulate the tritium transfer according to hourly changes of the meteorological conditions, such as changes of wind direction, not only during the primary plume release but also