Studies on the optimized use of the spectral conditions in BWR-tape reactors with new fuel concepts

Most commercial nuclear power plants are LWR-type reactors. In the BWR-type reactor the coolant starts boiling during heat uptake producing vapor. Therefore the fuel usage in the upper parts of the fuel elements is very inefficient and the almost fresh highly enriched uranium is disposed. In order to reduce the respective cost the following measures are used: less enriched uranium in the upper parts of the fuel elements, reduction of several fuel element pins or longer water column along the fuel elements. All these strategies increase the heat transfer from the fuel elements to the coolant and reduce the cost. On the other hand the reduced reactivity in the reactor core reduces the vapor quality and shortens the operational cycle. The simulations show that with an adequate selection of the fuel composition the fast spectra in the upper part of BWR-type reactors can be used to release a larger reactivity potential. Conventional MOX elements with axially inhomogeneous plutonium content were studied. The results indicate also more favorable reactor physical safety properties.