Dose Calculation Due To Underground Exposure: The Tav Tunnel In Valle Di Susa

Radiological impact due to indoor exposure, particularly due to permanence underground, is considered by Italian and European laws, which suggest accurate measurements and define action levels. Risk connected to exposure is circumscribed by setting limits on indoor air concentration of nuclides and on consequent doses. Particularly, inhalation of radon – a radiological decay product of uranium - and its progeny can lead to neoplasms damaging the respiratory apparatus. Harmful concentrations of gas in indoor air can be reached because of its continuous production from uranium, and its tendency to stratification and other particular conditions. The estimate of radon potential concentrations and absorbed doses is useful to verify whether dose limits can be respected or not. In every underground building, there could be radioprotection problems, and such situations should be controlled. RESRADBUILD (RESidual RADioactivity in BUILDings) is a computer model designed for evaluating radiation exposures within structures. Natural radiation exposures in an underground building will be illustrated through a case study. In this work, the RESRAD-BUILD model is used, in fact, to evaluate the dose received by an individual being inside a contaminated underground tunnel: the TAV (Traforo Alta Velocità, High-speed Rail Tunnel) which will be situated in Valle di Susa (Piedmont). In the area we are considering, the presence of natural radionuclides slightly exceeds the worldwide mean value, particularly the concentration of 238U. The scheme of the tunnel is made considering a 15 meters length portion, 4x4 meters section, made of three compartments. The three compartments are in series, and inside them there are nine sources and one receptor. Parametric studies have been performed by means of this model, checking the influence of different parameters on the radon indoor concentration and the consequent dose to the individual. RESRAD-BUILD final results show that an air flux of 0.1 m3/h, entering the first compartment, can be sufficient to lower the dose received by the exposed receptor below 1 mSv/y, which is the population dose limit. An air flux of 0.1 m3/h is a quite small value: it means that the building`s inner air should be totally changed every 2400 hours (100 days). This air exchange can be obtained easily both in the construction and exercise phases. Results show the order of magnitude of the dose the exposed workers receive, notwithstanding the simplifications adopted. They can be useful to make an early estimate of radiological risk. The proposed practical application shows how limits imposed by regulations can be respected in the presence of concentrations of radionuclides slightly exceeding the world average, by means of modest air exchanges.