The Effect of Hydrogen Back-Diffusion on the Transport of Tritium in an MSBR

The effect of hydrogen back-diffusion on the transport behavior of tritium in a 1000 Mw(e) molten-salt breeder reactor has been examined with reference to the distribution of the tritium flow in the purge gas systems, the cell enclosures and the steam system. In the mathematical model chosen the hydrogen was added to the steam system and maintained there at a given steady state concentration, and its appearance elsewhere was calculated according to the diffusion equations used. The fluorides of hydrogen and tritium were considered to be nondiffusing through the metal walls. The results of some seven typical runs showed that: 1. The back-diffusion of hydrogen does not inhibit the transport of tritium to the steam until hydrogen concentrations in the steam of at least 50 ppm are reached. At lesser concentrations the hydrogen aids the tritium transport, reaching a maximum at about 5 to 50 ppb by weight. 2. The reason the hydrogen aids, rather than inhibits, the tritium transport is the overriding effect of its concentration in the fuel on the reaction of both hydrogen and tritium with UF4 to form TF and HF. It is the marked reduction of TF formation with increasing hydrogen concentration that makes so much tritium available as HT for transport to the steam system. 3. As previously pointed out, the assumed reaction of hydrogen with UF4 to form UF3 and HF affects the tritium transport importantly, and the conditions of this reaction need to be established firmly in order to validate the conclusions of this study. 4. The diffusive interaction of hydrogen and tritium at the metal walls, assumed here, is based largely on theoretical considerations and may not be quantitatively valid. Experiments are needed to establish the true effect.