Overview of work on improving existing LWR core designs and developments for reactors of the next generation

There is an interest in improving current reactors through minor and localized modifications to reactor system components. Improved safety and performance for current and future reactors is the main goal. The underlying motivation is to maintain the option for nuclear power in the future. It can be seen that significant improvements are potentially available through relatively minor modifications. Specific areas for improvement and an illustrative example are discussed in this paper. In this specific example, external assembly bypass water (an important moderator) was adjusted during normal operation to keep the reactor critical at hot full-power. It has been found that a reduction of about 10% in assembly average enrichment is achievable. Neither gadolinium nor control rods need to be used during normal operation. Reactivity coefficients (negativity) are increased. Great operational flexibility and core performance are promised but at the cost of a more complex control rod drive mechanism. (author) 2 figs., 7 refs