The axial fuel bundle configuration of the RBWR-TB is shown in Fig. 14.7. The axial configuration is similar to that of the RBWR-AC, but the RBWR-TB does not have a lower blanket because breeding of fissile plutonium is not needed. Other blanket and fissile zones have different heights from those in the RBWR-AC to enable multi-recycling of TRUs under the different neutron energy spectrum from the RBWR-AC. The upper and internal blanket zones of depleted uranium oxides

have heights of 20 and 560 mm, respectively; the upper and lower fissile zones have heights of 192 and 221 mm, respectively.

The RBWR-TB also utilizes the neutron absorber zones above and below the fuel zone. The upper neutron absorber zone has the same structure as that of the RBWR-AC. The number of neutron absorber rods in the lower neutron absorber zone is 91, which was determined so as to keep the void reactivity coefficient negative.

Figure 14.8 shows the horizontal configuration of the RBWR-TB. The fuel bundle of the RBWR-TB is composed of the uniform fissile plutonium enrichment of 13.9 wt%. The lattice pitches of the fuel bundles are 199.3 mm on the side with the control rod and 194.4 mm on the side without it. The channel box of the fuel bundle is hexagonal with an inner width of 189.6 mm and wall thickness of 2 mm. The control rod is 7.5 mm thick, and the gap between the rod outer surface and the channel box is 1.6 mm on each side. The gap between channel boxes on the side without the control rod is 0.8 mm. Geometries of the channel boxes and the control rods are slightly different from those of the RBWR-AC. However, because the center positions of the control rods are the same in the RBWR-AC and — TB and reactor internals fixed to the RPV, such as the core support plate, control rod guide tubes, etc., can be shared, their cores are easily exchanged with each other by changing the fuel bundles, control rods, and some attachments between the core support plate and fuel bundles.

Because the RBWR-TB equilibrium core has a shorter height than that of the RBWR-AC, the number of fuel rods of the RBWR-TB (397) is larger than that of the RBWR-AC (271) to keep the averaged linear heat-generating rate almost the same.

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The RBWR-TB aims at burning MAs by transmuting them into fissile isotopes using relatively low energy neutrons as well as by direct fissioning using relatively high energy neutrons. Both capture and fission reactions occur in a broad neutron energy range from thermal to fast. It is expected that the balance of these reactions at different neutron energies enables TRU burning while keeping the isotopic composition almost the same before and after burning, as mentioned in the next subsection.

The main core specifications and performance values of the RBWR-TB in the equilibrium core were shown earlier in Table 14.2. The core coolant flow is

3.8 x 104 t/h at a subcooling of 10 K at the entrance and has a steam quality of 21 % at the core exit. The concept of the loading pattern of fuel bundles in the equilibrium core is the same as that of the RBWR-AC: it adopts zone loading and the reflective boundary condition of 60 ° in the azimuthal direction. A maximum linear heat generation rate of 47 kW/m and an MCPR of 1.3 after the control rod scheduling are achieved. The RBWR-TB has a void reactivity coefficient of -2 x 10-4 Ak/k/%void.

The fission efficiency of TRUs in the RBWR-TB is 51 %. Here the fission efficiency is defined as the net decrease in TRUs divided by the total amount of fissioned actinides through the total fuel residence time in the core. This value indicates what amount of the TRUs can be used as fuel for generating electric power and is related to fissioning cost of the TRUs. As the fission efficiency of TRUs becomes higher, it is expected that the electricity-generating cost needed for burning the same amount of TRUs becomes smaller, if the other costs such as fuel fabrication cost are comparable.

Plenum and holder

500 mm

Plenum and neutronabscrberrod

500 mm

Plenum 300 mm Upper blanket 20 mm Upper fissilezone

224mm

Internal blanket

560 mm

Lower fissilezone

221 mm

Lower neutron absorption zone-0mm