Hydrogen generation in transuranic waste storage containers

In addition to emitting various types of radiation, nuclear waste materials are commonly mixtures of different compounds and even different phases. Energy transfer between phases and interfacial chemistry will affect the yields and types of products formed in these systems. Interfacial effects in radiation chemistry have long been observed, but the detailed mechanisms involved are not understood [3-5], Recent studies of water adsorbed on ceramic oxides clearly show that energy can migrate from the solid oxide phase to the water phase and lead to excess production of H2 [6, 7], This process complicates dosimetry because energy 

The production of H2 in the radiolysis of water has been extensively re-examined in recent years [8], Previous studies had assumed that the main mechanism for H2 production was due to radical reactions of the hydrated electron and H atoms. Selected scavenger studies have shown that the precursor to the hydrated electron is also the precursor to H2. The majority of H2 production in the track of heavy ions is due to dissociative combination reactions between the precursor to the hydrated electron and the molecular water cation. Dissociative electron attachment reactions may also play some role in y-ray and fast electron radiolysis. The radiation chemical yield, G-value, of H2 is 0.45 molecule/100 eV at about 1 microsecond in the radiolysis of water with y-rays. This value may be different in the radiolysis of adsorbed water because of its dissociation at the surface, steric effects, or transport of energy through the interface. 

The migration of energy between phases will also have an effect in the radiolysis of mixed polymeric systems not associated with transuranic waste. For instance, the radiolysis of polymers attached to silica particles or the radiolysis of rubber in steel belted tires will probably be affected by energy deposited in the non-organic phase. Energy migration to the polymeric phase may lead to the need for lower overall doses than initially anticipated for a 

The yield of H2 in the radiolysis of polymers with y-rays is well known for several types of polymers [2], However, transuranic waste materials are a-particle emitters. The radiation chemistry induced by a-particles can be very different than that due to y-rays because of the difference in energy deposition density [13], The high linear energy transfer (LET, equal to the stopping power) of heavy particles leads to an increase in second order reactions, which may change the yields of some products. 

The H2 yield from polystyrene irradiated with y-rays is two orders of magnitude less than that in polyethylene. The H2 yields increase with increasing LET for all the polymers shown in Fig. 2, but the increase is not linear. There is a considerably greater increase for polystyrene than polyethylene. A 5 MeV helium ion, a-particle, gives a G-value for H2 of 4.6 molecules/100 eV from polyethylene and 0.15 molecule/100 eV from polystyrene [11], The large increase in H2 yield for polystyrene suggests that this material is not as radiation inert as typically thought. The use of yields determined with y-rays for heavy ion radiolysis would clearly underestimate the production of H2 in transuranic waste materials. More experiments coupled with sophisticated models are required to predict H2 yields in other unexamined polymers and in complex mixtures. 

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HYDROGEN GENERATION IN TRANSURANIC WASTE STORAGE CONTAINERS... 

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