VII — 1. Introduction

The Reduced-Moderation Water Reactor (RMWR) is a light-water cooled high-conversion reactor that is being developed by the Japan Atomic Energy Research Institute (JAERI) with collaboration from the Japanese industries. The design study conducted so far has indicated that the RMWR can realize the favourable characteristics of high conversion ratio of more than one, high burn-up, long operation cycle, and multiple recycling of plutonium. The design is characterized by the use of the ‘double flat core’ which consists of two flat core parts and three blanket parts in the vertical direction. This geometry is adopted to increase the neutron leakage from the core to make the void reactivity coefficient negative. The fuel assembly consists of the MOX fuel rods tightly arranged in the triangular lattice with the gap width of typically 1.3 mm to increase the fuel-to-coolant volume ratio.

Although several types of the RMWR systems have been investigated, the current study focuses mainly on the BWR type due to advantages regarding the core performances and the system simplification. Among the BWR type RMWRs, two system designs have been developed for different core powers. The larger one is the 1300 MW(e) RMWR based on the ABWR design (IAEA — TECDOC-1391). Table VIII-l summarizes the major characteristics of this system, which has 900 fuel assemblies, each of which consists of 217 fuel rods with the outer diameter of 13.7 mm arranged in the triangular lattice in gap width of 1.3 mm (see Figures VIII-1 and VIII-2). The double flat core consists of the lower and upper core parts with the height of 205 and 195 mm, and the lower, medium, and upper blanket parts with the height of 190, 295, and 220 mm as shown in Figure VIII-3. The fissile plutonium-enrichment of the MOX fuel is 18% for the reload fuel at the equilibrium core. The blanket material is depleted UO2.

900 fuel assemblies & 283 control rods

228 mm

Number of fuel rods 217

FIG. VIII-1. Core configuration for the 1300 MW(e) RMWR.

7,600 m m

FIG. VIII-3. Schematic of axial core
configuration for the 1300 MW(e) RMWR.

The smaller one is the 330 MW(e) RMWR system designed to seek benefits of the low initial capital cost and the flexibility in the plant installation corresponding to the power demand (IWAMURA, T. et al., 2002). To overcome the economic disadvantage for small reactors, the system was simplified by adopting natural circulation core cooling and passive safety features. Since the RMWR utilizes the flat and short length core and operates under lower core flow conditions, the steam velocity in the chimney region is smaller than for the conventional BWR. This characteristic allows further simplification: the steam/water separator and the steam dryer may be eliminated since the gravitational steam/water separation is expected to be possible at the free surface. The major characteristics of the reactor are listed in Table VIII-2. A breeding ratio of 1.01, negative void coefficient and natural circulation cooling of the core were realized under the discharged burn-up of 60GWd/t. The core consists of 282 of hexagonal fuel bundles: each has 217 fuel rods arranged in triangular lattice of 1.3 mm in gap width as listed in Table VIII-2. The core height and outer diameter are 1300 and 4140 mm, respectively, as shown in Figure VIII-4. Figure VIII-5 summarizes the plant system concept. The passive safety features include the accumulator injection system, the isolation condenser, and the passive containment cooling system.

VII — 2. Description of natural circulation core cooling for the 330 MW(e) RMWR

The natural circulation loop consists of the core, the divided chimney, the downcomer, and the lower plenum, which is similar to the other natural circulation cooling BWR concepts such as the SBWR. There are, however, several differences in the characteristics comparing to the SBWR, which includes:

1) One-order smaller absolute value for the void reactivity coefficient, which makes the neutronics — thermal-hydraulic coupling to be almost negligible,

2) Smaller frictional pressure drop across the core because of the shorter core length and the lower core flow rate despite the smaller core flow area

3) Hexagonal cross-sectional shape of the divided chimney

4) Elimination of the conventional separator.

The first characteristic makes the evaluation of the instability problem much simpler. Thus, the characteristic can be regarded as a benefit in view of the thermal-hydraulic analysis. The second one is also considered as a benefit for the realization of the natural circulation system. The third one causes the prediction of the void fraction in the chimney to be difficult because the previous studies are almost nonexistent for this geometry. However, the thermal-hydraulic relationships among the pressure, vapor and liquid flow rates, and void fractions are supposed to be basically the same as those for the circular geometry. So this characteristic is not an essential problem but just requires confirmation tests using the actual geometry of the system to get the relationships for the design finalization. Since the free-surface separation was utilized for the old type natural circulation BWR, the fourth characteristic does not create a new problem. Careful consideation, however, is necessary because the conditions are not completely the same between the old BWRs and the RMWR. The mockup test will be required to confirm especially the phase separation characteristics on the free surface.