CRITICALITY STUDIES FOR THE TRANSPORT OF UNIRRADIATED PWR ASSEMBLIES

The transport of low enrichment unirradiated PwR fuel involves no criticality risk in the absence of water. However, an adequate sub-criticality margin must remain in the event of water leaking into the package, even taking into account the worst credible configuration to be anticipated as a result of any accident. TRACTEBEL has estimated the subcriticality margins in worst case situations anticipated as a result of a vertical 9 m drop of a FRAMA TOME RCC-3 or RCC-4 fresh fuel package, loaded with two 15xl5 or 17xl7 fuel assemblies and fitted with copper or borated stainless steel neutron absorber plates. In the worst-case scenario considered, both fuel assemblies have slid along their supports until their bottom nozzle legs came into contact with the end of the package. As a result of damage to the lower grid, the two outermost rows of pins were free to diverge outward. The reactivity increase led to a slight reduction in enrichment limit only with 12 ft fuel and borated stainless steel absorbers. Furthermore it was assumed that can damage would allow a number of peUet to pile up in the space between assemblies. The resulting reactivity increase became noticeable only with a large number of free pellets, much larger indeed than anything to be expected. The Monte Carlo code MONK, developed and distributed by the ANSWERS Service (AEA Technology, UK) was used. The versatility of its geometry package made it easy to set up a realistic 30 model of the postulated accident configurations.