ANALYSES OF HYPOTHETICAL NUCLEAR CRITICALITY EXCURSIONS IN 10- AND 20-MW FREEZER SUBLIMER VESSELS/

A theoretical model is used to predict the consequences of a postulate d hypothetical nuclear criticality excursion in a freeezer/sublimer (F /S). Previous work has shown that an intrusion of water into a F/S may result in a critical configuration. A first attempt is made to model the neutronic and thermal-hydraulic phenomena occurring during a criti cality excursion involving both uranium hexafluoride (UF6) and uranyl fluoride (UO2F2) solution, which is present in the F/S during upset co nditions. The model employs point neutronics coupled with simple therm al hydraulics. Reactivity feedback from changes in the properties of t he system are included in the model. The excursion is studied in a 10- MW F/S with an initial load of 3500 kg of 5% weight enriched UF6 and i n a 20-MW F/S with an initial load of 6800 kg of 2% weight enriched UF 6. The magnitude of the fission release determined in this work is 5.9 3 x 10(18) fissions in the 10-MW F/S and 4.21 x 10(18) fissions in the 20-MW F/S. In order to demonstrate the reliability of the techniques used in this work, a limited validation study was conducted by compari ng the fission release and peak fission rate determined by this work w ith experimental results for a limited number of experiments. The agre ement between calculations and experiments in the validation study is considered to be satisfactory. The calculational results for the hypot hetical accidents in the two F/S vessels appear reasonable.