Disposal canister shock absorber tests and analysis

In the Finnish final disposal plan the spent fuel canister will be transferred to an underground repository at 400-450 meters below ground surface by a vertical lift. This paper considers the postulated accident scenario where the disposal canister falls during transportation. In case of free fall, the 25-ton disposal canister can reach a velocity of 90 m/s before impacting the shock absorber. The shock absorber is designed to consist of cohesionless granular lightweight expanded clay aggregate (LECA) material that will decelerate the disposal canister in a controlled manner. By utilizing the IMPACT test facility at VTT Technical Research Centre of Finland, laboratory scale shock absorber tests have been carried out. A rigid scale model of the disposal canister was shot into a shock absorber pipe in a horizontal setting with a realistic velocity. A total of 21 tests have been carried out with velocities ranging from 38 m/s to 97 m/s using disposal canister scale models with diameters from 63 mm to 150 mm. Other dimensions in the testing were scaled correspondingly. The aim of the experiments and numerical analysis was to assess the behavior of LECA as shock absorbing material and determine both the required depth of the shock absorber and loading subjected to the disposal canister. LECA used in the tests had grain size of 4-10 mm. The test results show a clear trend of penetration distance increasing as the impact velocity increases and some scatter. No distinctive effect of the test scale was seen in the results.