ORNL high-temperature fission product

Figure 7.7. ORNL vertical furnace device for high-temperature fission product release studies...

Similar experimental conditions to those in the Sascha experiments, that is a flowing steam-hydrogen atmosphere at atmospheric pressure, were used in the ORNL high temperature tests, the principal aim of which was to determine the impact of specific accident conditions on the behavior of fuel and fission products. In the early ORNL high-temperature experiments (HT series), fuel rod segments fabricated of power-reactor irradiated fuel and encapsulated in Zircaloy cladding were heated for a short time (a few minutes) to temperatures up to 1900 K in atmospheres of various composition. The results obtained in these tests were the main basis for the assessment of fission product release from overheated fuels made in the NUREG-0772 report (US NRC, 1981). 

Despite obvious differences in the experimental techniques between the Sascha experiments and the ORNL high-temperature tests, mainly with regard to the nature of the analytical specimens (simulant fuel vs. irradiated fuel, melt composition and geometry of the specimens), the results showed general consistencies in the behavior, in particular, of the less volatile fission products (Osborne and Lorenz, 1992). 

The main advantage of a two fluid system is that the processing-out of fission products from the fuel salt is simplified by the absence of fhorium. The prime method is known as vacuum distillation (ORNL 3791, 1966) and was developed in 1964. After removal of all UF4, the carrier salt would be evaporated off af low pressure and high temperature (1000°C) and recycled, leaving most fission products behind in the still bottoms. 

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