In line with these results, it is shown that replacing the graphite in the follower with nonmoderating materials reduces the void coefficient dependence on the number of absorber rods and is an economic method that may have potential in the void coefficient reduction program. It is found that the scram reactivity is positive and its size is remarkably insensitive to a wide range of axial flux distortions provided the flux shape is concave, which is characteristic of neutronic decoupling of the core. In contrast, the scram reactivity is negative when flux shapes are convex, i.e., those that are a characteristic of strong neutronic coupling. This indicates that unless there were a significant number of control absorbers present in the core just before the accident to provide a convex flux shape, the chances that some positive scram reactivity was inserted to initiate the power pulse are high. Abstract - The reactivity effects caused by fragmentation of nuclear fuel and by simultaneous cooling of the fragments are described.
Fragmentation is described by increasing the number of fuel pieces while decreasing their diameter.
Cooling is considered to occur as quasi-stationary. Under certain circumstances, substantial reactivity increases are found to occur in both reactor types. These may have contributed signifcantly to the severity of the Chernobyl accident. A complementary experiment with Chernobyl reactor conditions was performed in order to observe the fuel failure behavior and the resultant vessel pressure rise, etc. Also, generation of hydrogen from the fuel rod cladding and the consequent system pressure rise were estimated base on the experiments.