Experimental Evaluation of the Consequences of Uranium Bed Air-Ingress Accidents

To help resolve unknowns regarding consequences of air-ingress accidents in uranium beds, a series of experiments was conducted at Ontario Hydro Research Division with the participation of Princeton Plasma Physics Laboratory and the Idaho National Engineering Laboratory. These experiments involved exposure of uranium beds of various sizes to air, oxygen in helium, argon and Nitrogen. Beds of 5-gram to 3-kg uranium capacity were tested. Starting temperatures ranged from 294 K to 824 K. Results of these experiments showed that in every test the reaction was restrained with modest temperature excursions. Either surface films or gas blanketing may be responsible for quenching the reaction with air. In these tests the reaction appears to be stopped by a diffusive barrier film of reaction products that grows on the surface of the uranium grains. The only tritium emissions appeared to be due to thermal oscillation-driven gas expansion. Our conclusion is that the hazard associated with an air-ingress accident involving a uranium bed is smaller than we thought initially. With proper bed design, the energy release will be modest and should not result in damage to the bed structure. Tritium release can be minimized or prevented by keeping the bed only partially loaded.