PNNL

Abstract

In the Accelerator Production of Tritium Program, thermalized neutrons produced from multiplication and moderation of spallation neutrons will will be generated by the interaction of high energy (~1 Ge V) protons with solid tungsten rods. An unavoidable byproduct of the spallation reactions is large amounts of helium and hydrogen gas generated in the rods. The release kinetics of these gases during various proposed off-normal scenarios involving loss of coolant and afterheat-induced rises in temperature are of particular interest. In addition, the magnitude of the gas generation cross sections and the fractional retention of these gases is necessary for extrapolation of these data to highter exposures. Tengsten rods irradiated with 800 MeV protons in the Los Alamos Neutron Science Center (LANCE) to high exposures have been sectioned to produce specimens suitable for measurement of both helium and hydrogen. Hydrogen evolution was measured both by dropping the specimen into a small ceramic crucible at 1200C and also by subjecting the specimen to a simulated temperature ramp from ~200 to ~1200C. The latter technique showed four distinct hydrogen release peaks at temperatures of approximately 500, 800, 1000 and 1200C, suggesting a variety of trapping sites with different binding energies. Helium release and total content were measured by subjecting the specimens to the same temperature excursion, followed later by melting to release the remaining helium. Approximately 99% of the helium was retained until melting occurred. The measured amounts of helium agreed well with predictions. The hydrogen measurements were somewhat lower than predicted. This may reflect diffusional losses from the rods or some needed modifications in the evaporation model employed in the LAHET code.