Development of advanced-RCCA in PWR (1). Basic specifications and critical experiment of B4C with 90% enriched 10B

The control rod worth should be enhanced in order to have flexibility of extended burn up fuels and large MOX fuels loading capability in PWR. The large number of RCCAs would be one of solutions, but it would not meet the requirement of reduction of construction costs for future plants. Therefore, Japanese PWR utilities and Mitsubishi Heavy Industries, Ltd. had investigated the advanced-RCCA which the individual control rod worth can be considerably increased. B4C with 90% enriched 10B is adopted as the advanced-RCCA absorber to enhance control rod worth, and the B4C/Ag-In-Cd axially hybrid design is selected to increase the weight of control rod to a reasonable value. With the advanced-RCCA, the required number of RCCAs can be reduced for the same MOX capability. And the duplex cladding design can increase the reliability against the RCCA-wear and result in reduction of inspection cost. In order to confirm the integrity of the advanced-RCCA, we performed the RCCA drop test to get the scram time data, the RCCA vibration test to confirm that the wear would not be larger than the current RCCA. The hot cell PIE data of the B4C pellets irradiation performance was acquired in order to verify the margin of the design evaluations. And, the critical experiments were carried out for the purpose of investigating the neutron absorption of enriched B4C in PWR condition and verification of nuclear design method for it. In this paper, the basic specifications of the advanced-RCCA, merit of it and the critical experiments of enriched B4C are described. (author)