Biological hazard issues from potential releases of tritiated dust from ITER

Abstract

Tritiated dust will be generated during the operation of ITER. Several dedicated studies, involving in vitro and in vivo experiments on metal tritide and carbon tritide dust, have raised some concerns about the protection guidelines for workers exposed to tritiated dust, because they are based on the radiotoxicity of tritium in form of tritium gas (HT), tritiated water (HTO) or organically bound tritium (OBT). While the behaviour of HT, HTO and OBT in the human body is well understood, the same is not fully true for tritiated dust, because of the size distributions of the particles and the variety of base materials. The in vivo and in vitro studies on tritiated dust have shown the dependence of the tritium clearance and retention in the human body on the physico-chemical parameters (mainly the particle size and density). Some recent studies on JET dust relative to graphite and CFC dust have dealt with physico-chemical characterisation and with in vitro tritium dissolution studies. Some uncertainties still remain and further testing is necessary (in vitro and in vivo), restricting if possible the investigation to relevant parameters. In the working areas of ITER the radioactive dust concentration in air will be kept at a very low level. However, in the event of an accident, the atmosphere inside the buildings, including operative areas as well, might get contaminated with small amounts of tritiated dust and hence inhalation risks cannot be excluded. The present paper summarizes the results from previous investigations on the subject which are relevant in deriving sound dose conversion factors for tritiated dust. At the same time, it proposes a dose conversion factor for dust which is conservative in order to envelope all ITER relevant materials.