14.2.1 Overview

Figure 14.1 shows the reactor pressure vessel (RPV) of the RBWR. The common plant specifications of the RBWR and the latest commercial BWR, the ABWR, are listed in Table 14.1. The rated thermal power, electric power, diameter of the RPV, and core pressure are identical for both reactor plants. Figure 14.2 shows a hori­zontal cross-sectional view of the RBWR core configuration, which is composed of 720 hexagonal fuel bundles and 223 Y-type control rods. The axial configuration uses the parfait core concept in which an internal blanket of depleted uranium oxide is placed between the upper and lower fissile zones of the TRU oxides.

Various design concepts of the RBWR core have been proposed. Recent core designs have focused on TRU management. The RBWR-AC is the break-even reactor that can burn depleted uranium by using TRUs extracted from the spent fuel bundles of LWRs without decreasing the amount of TRUs. The RBWR-TB is the TRU burner that can fission almost all the TRUs, leaving only the minimum critical mass of TRUs, by repeating their recycling and collecting. The RBWR-TB2 is a modified version of the TRU burner. The RBWR-TB2 is designed to be able to burn

Fig. 14.1 Reactor pressure vessel of the resource — renewable boiling water reactor (RBWR) [3]

Reactor pressure vessel

Core (showing some fuel bundles)

Control rod

Table 14.1 Plant specifications [3]

Item RBWR ABWR Thermal power (MWt) 3,926 3,926 Electric power (MWe) 1,356 1,356 RPV diameter (m) 7.1 7.1 Core pressure (MPa) 7.2 7.2 Number of fuel bundles 720 872 Fuel lattice type Hexagonal Square Lattice pitch (mm) 199 155 Number of control rods 223 205 Control rod type Y-type Cross shape

Fig. 14.2 Horizontal cross-sectional view of the RBWR core configuration [3]

Break-even	TRU-burner
BWR/PWR (for RBWR-TB2)
Fig. 14.3 Utilization concept of the RBWR-AC, — TB, and — TB2

TRUs from LWR spent fuels, whereas the RBWR-TB is designed as a burner for the TRUs from the RBWR-TB itself, assuming the RBWR-TB would be utilized when the TRU usefulness is exhausted and almost all should have been fissioned. Figure 14.3 shows the utilization concept of the RBWR-AC, — TB, and — TB2.

In core designs for the RBWR-AC, — TB, and — TB2, keeping charged TRU composition preserved at every operation cycle is mandatory. This criterion ensures the multi-recycling capability, fission, and recycling process of TRUs can be continued while maintaining the criticality and fulfilling the various operation constraints, such as sufficient reactor shutdown margin and negative void reactivity coefficient. As mentioned in the Introduction, the multi-recycling capability is achieved by hardening the neutron energy spectrum and promoting the transmuta­tion of 238U to fissile plutonium using the hexagonal tight fuel lattice, which has a H/U less than that of the conventional BWR square fuel lattice. Figure 14.4 shows the relationship between the volume ratio of water to fuel and the breeding ratio in the RBWR-AC, — TB, — TB2, and the conventional BWR. Because the RBWR-AC and — TB need to continue operation cycles without feeding fissile materials other than those contained in the discharged fuel from themselves, the volume ratios of water to fuel are set lower than those of the RBWR-TB2 and the conventional BWR.

In the following sections, the core calculation method is described first, and then each type of RBWR is described.