Hydrogen and helium gas formation and their release kinetics in tungsten rods after irradiation with 800 MeV protons

In the Accelerator Production of Tritium Program, thermalized neutrons produced from multiplication and moderation of spallation neutrons will be absorbed in ³He gas to produce tritium. The spallation neutrons will be generated by the interaction of high energy (∼1 GeV) 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 higher exposures. Tungsten rods irradiated with 800 MeV protons in the Los Alamos Neutron Science Center (LANCE) to rather high exposures have been sectioned to produce small specimens suitable for measurement of both helium and hydrogen. Hydrogen evolution was measured both by dropping the specimen into a small ceramic crucible at 1200°C and also by subjecting the specimen to a simulated temperature ramp from ∼200 to ∼1200°C, as has been calculated to be induced by loss of cooling and subsequent afterheat in an APT plant. The latter technique showed four distinct hydrogen release peaks at temperatures of approximately 500, 800, 1000 and 1200°C, 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. For both gases, release measurements were conducted using mass spectrometric techniques. 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.