The core concept of the RBWR-TB2 was initiated by an Electric Power Research Institute (ERRI)-organized team of three universities in the United States [6] to compare its core performance values with those of the ABR, which is the SFR having the same purpose [7]. Although the RBWR-TB is assumed to be utilized in the final stage of the nuclear power phase-out scenario, the RBWR-TB2 is assumed to be utilized to control the amount of TRUs during the period while LWRs are being operated as base load power sources.

The axial configuration of the RBWR-TB2 (Fig. 14.9) and it is similar to that of the RBWR-TB. The RBWR-TB2 also does not have a lower blanket because breeding of fissile plutonium is not needed. The upper and internal blanket zones of depleted uranium oxide have heights of 20 and 560 mm, respectively; the upper and lower fissile zones have heights of 224 and 221 mm, respectively. The RBWR — TB2 also uses a lower neutron absorption zone in which the number of neutron absorber rods is 19. This number of the neutron absorber rods is sufficient to keep the void reactivity coefficient negative in the RBWR-TB2.

Figure 14.10 shows the horizontal configuration of the RBWR-TB2. The fuel bundle of the RBWR-TB2 is composed of the uniform fissile plutonium enrichment of 25 wt%. As the RBWR-TB2 fuel includes TRUs from LWRs, the fissile plutonium enrichment becomes higher than that of the RBWR-TB, which uses TRUs from itself and other RBWR-TBs in the equilibrium core. Geometries of the channel box and the control rods are the same as those of the RBWR-TB. 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.

The fuel rod diameter and gap of the RBWR-TB2 are 7.2 and 2.2 mm, respectively; these values result in a larger moderator-to-fuel ratio and a softer neutron energy spectrum than those of the RBWR-TB. Because the fissile compo­sition of the RBWR-TB2 is larger than that of the RBWR-TB, fissile TRUs need to be fissioned with a relatively larger rate to preserve TRU isotopic composition for multi-recycling. The number of fuel rods is the same as that of the RBWR-TB to make the averaged linear heat-generating rate almost the same.

The main core specifications and performance values of the RBWR-TB2 in the equilibrium core were shown earlier in Table 14.2. The core coolant flow is 2.4 x 104 t/h at a subcooling of 10 K at the entrance and has a steam quality of 36 % 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-TB: 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 1.28 after the control rod scheduling are achieved. The RBWR-TB2 has a void reactivity coefficient of -4 x 10-4 Ak/k/%void.

The fission efficiency of TRUs in the RBWR-TB2 is 45 %. This value corre­sponds to about twice the production efficiency of TRUs, 22 %, in the ABWR. Here, the production efficiency of TRUs is defined with the opposite meaning of the fission efficiency of TRUs, that is, the net increase in TRUs divided by the total amount of fissioned actinides through the total fuel residence time in the core. As the electricity output of the RBWR-TB2 is the same as that of the ABWR, this means accumulation of TRUs would be suppressed by introducing one RBWR-TB2 for two ABWRs.

Table 14.3 summarizes TRU compositions and weights of fissile plutonium and TRU of charged and discharged fuels in the RBWR-AC, — TB, and — TB2. In evalu­ation of the discharged fuel compositions, a 3-year cooling time after discharge from the core is considered. Because the RBWR-AC and — TB satisfy the multi-recycling criteria under the condition that both reactors charge TRUs that were discharged from themselves, the TRU compositions of the RBWR-AC and — TB are kept the same in the charged and discharged fuels. The weights of fissile plutonium and TRU increase slightly in the discharged fuel in the RBWR-AC, the break-even reactor, whereas they decrease in the discharged fuel in the RBWR-TB, the TRU burner. As TRUs from LWR spent fuels are added to TRUs discharged from the RBWR-TB2 itself, the TRU composition of their mixture is to be the same at every operation cycle with the constant mixing ratio of TRUs discharged from the RBWR-TB2 and LWR. The weight of TRU decreases in the discharged fuel in the RBWR-TB2.

14.2 Conclusion

The specific design and core characteristics of the RBWR were summarized from a review of published studies. The RBWR is categorized as a low moderation LWR. By utilizing a tight lattice fuel and two-phase flow of coolant, the latter of which is a feature of BWRs, the moderator-to-fuel ratio of the RBWR can be reduced to a small enough value as to achieve multi-recycling of TRUs.

Different RBWR cores have been designed for different purposes. The RBWR — AC is a break-even reactor with a Pu breeding ratio more than 1.0. The RBWR-TB and RBWR-TB2 are TRU burners that can fission TRUs at a rate more than twice the rate of TRU production by the ABWR. Each of the reactor types achieves the foregoing performances under the condition requiring negative void reactivity coefficient and multi-recycling capability. With the multi-recycling capability, the RBWR-AC/TB/TB2 can continue to fission or recycle TRUs while maintaining the criticality and fulfilling the various constraints, such as sufficient core shutdown margin and negative reactivity coefficient.

The RBWR appears to be a promising candidate energy source that responds to the needs for energy security, for reducing greenhouse-gas emissions, and for mitigating the negative environmental impact of TRUs.

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