Airborne Release from Transport and Storage Casks Due to the Impact of an EFP

There is the likelihood that a terrorist sabotage attack with explosively formed projectiles (EFP) against transport and storage casks will cause fragmentation of the (brittle) radioactive inventory and the formation and release of aerosol-borne radioactivity. The assessment of the radiological consequences requires the characterization of the damage pattern and the determination of the source term, i.e. the release fraction defined as the fraction of the inventory released as (respirable) aerosols. In order to quantify the release fraction and to understand the basic release mechanisms, a series of small scale experiments was performed. Pre-manufactured flyer plates of 24 mm caliber were impacted at a constant velocity of 1400 (m/s) against a small-scale mock-up (160 l) of a transport cask equipped with stainless- steel wall segments of variable thickness (1 – 10 mm). Chemically doped quadratic ceramic plates were used as surrogates for brittle radioactive inventory. The mass of two aerosol size fractions < 5 µm and < 10 µm formed inside the casks and released into the outside was measured by proper size and time-resolved aerosol diagnostics making use of the chemical tracer of the inventory. Furthermore, temperature and pressure inside the cask were recorded in order to model the potential outflow of gases. First results show that the release fractions are well below 1 % of the inventory surrogate. They are independent of the thickness of the steel panel penetrated by the projectile. The remaining impact velocity of the projectile on the target was in all cases sufficiently large for complete fragmentation of the targets. Further experiments were carried out with mock-ups of fuel elements filled with non- radioactive ceramic pellets. The small-scale data serve as input for the development of a predictive model to assess the consequences of an interaction of an EFP with a transport and storage cask.