On the Reactivity Effects of Nuclear Fuel Fragmentation with Reference to the Chernobyl Accident

The reactivity effects caused by fragmentation of nuclear fuel and by simultaneous cooling of the fragments are described. A series of light water reactor (LWR) cases and three speculative scenarios for the Chernobyl accident are considered. Calculations were carried out with the LWR cell burnup code CASMO-HEX. Fragmentation is described by increasing the number of fuel pieces while decreasing their diameter. Cooling is considered to occur as quasi-stationary. Relative movement of the fragments and the coolant is taken into account by varying the water/fuel ratio. Under certain circumstances, substantial reactivity increases are found to occur in both reactor types. These may have contributed significantly to the severity of the Chernobyl accident.