jeudi 23 mai 2013 à 16:00
Amphi G. Besse
During the nuclear reactor operation, fission products and radioactive inert gases, as 133Xe, are generated. For an exposure to radioactive noble gases, the external exposure is the only exposure mode taken into account to quantify the dose to the whole-body, the skin and the
lens of the eye received by workers [ICRP Publication 30, 1978].
Studies on the external exposure have been done for an immersion in an infinite cloud of 133Xe but not in a realistic environment such as the reactor building of a nuclear power plant
[Poston and Snyder, 1974; Piltingsrud and Gels, 1985; Eckerman and Ryman, 1993; ICRP Publication 119].
Consequently, in this work, the external exposure to 133Xe is studied for a worker located in the reactor building. The radiological risk is evaluated by considering the effective dose rate to the whole body, equal to 1.29x10-6 ± 0.01x10−6 Sv.h−1.MBq−1.m3, and the equivalent dose
rate to the lens, equal to 1.60x10-6 ± 0.32x10−6 Sv.h−1.MBq−1.m3.
The aim of the next step is to compare the effective dose rate to the whole body with the values published in the articles mentioned on the external exposure. The work consists in studying the external exposure to 133Xe for an immersion in an infinite cloud of 133Xe. The
effective dose rate to the whole body obtained by our study is equal to 7.53x10−6 ± 0.02x10−6 Sv.h−1.MBq−1.m3.
The study is conducted to update data on the exposure to 133Xe by using Monte-Carlo simulations based on the software GEANT4 [Agostinelli and al., 2003; Allison and al, 2006], the MIRD phantom [Cristy and Eckerman, 1987] and a realistic geometry of the reactor building.
In these simulations, the phantom MIRD has been adapted to the problem of the external exposure in a 133Xe cloud. Organs have been added inside the phantom, as esophagus, salivary glands, eyes, lens and its radiosensitive part [ICRP Publication 89, 2002; ICRP
Publication 110, 2009]. The volume of the thyroid has been simplified [Clairand and al., 1999].
The Monte-Carlo code GEANT4, developed by the CERN, is used to model the radiation interaction with matter. All the ionizing radiation resulting from disintegrations of 133Xe is tracked in the program to get the energy deposed in all the organs of the phantom. The
equivalent dose rate in the organs and the effective dose rate are determined by using the weighting factors [ICRP Publication 60, 1991]. The work can be separated in two parts: the first part is concentrated on the exposure to 133Xe in the reactor building and the second part is concentrated to the exposure in a semispherical cloud of 133Xe.
Two steps are involved to study the external exposure of a worker in a reactor building containing 133Xe particles. At first, we study the environment of the worker to find the place where the maximum dose rate is in the reactor building. For the second step, the phantom
stands up in this place to treat the 133Xe external exposure in the reactor building. The aim is to get the effective dose rate to the whole body and the equivalent dose rate to the lens of the eye to evaluate the radiological risk of an exposure to 133Xe.
Another study is completed on the external exposure to obtain the effective dose rate to whole body as function of 133Xe semi-spherical cloud radius. This study is based on simulations Monte-Carlo and on extrapolations of effective dose rates.