SAFETY RISKS IN SPENT NUCLEAR FUEL AIR TRANSPORTATION - A 'BLACK SWAN' ANATOMY

Drawing upon an actual case the repatriation by air of the spent fuel of the VVR-S nuclear research reactor at `Horia Hulubei' National Institute of Physics and Nuclear Engineering, IFIN-HH, Bucharest, currently in decommissioning, the paper points at several aspects of the safety assessment involved in the authorization of such operations. Addressed are especially issues relating to plausible severe scenario construction; the consequent inference of accident source terms; the environmental dispersion particulars; the possible radiological consequences and their territorial distribution. On special request from the Regulator, the analysis which came on top of the regular Emergency Response Plan took up the assumption that the postulated mishap was severe enough to exceed the original, experimentally-verified terms of reference offered by the fuel Transporter, thus placing a risk of some sizeable magnitude on the population potentially exposed. While debatable in principle from the perspective of the conventional Probabilistic Risk Assessment, this posture was eventually found justifiable in the light of the results obtained and also considering the current trend in the realm of emergency preparedness to go beyond design basis assumptions towards a more comprehensive acceptance of low-probability-high-risk ('black swan' [1]) abnormal occurences in handling radioactive inventories of all conceivable origins.