The major features [37] of the Advanced PWR core design are the large-size core and introduction of neutron reflector in comparison with conventional PWRs. Table 3.13 summarizes the main core parameters of the Advanced PWR in comparison with the conventional 4-loop core. The core became large-sized, with 257 fuel assemblies. The radial power distribution oscillation by xenon should be noted with the enlargement of core, but such a large-size core still maintains the oscillation convergent.

Conventional 4-Coop PWR

Fig. 3.61 Structures of PWR reflector [38] (Copyright Mitsubishi Heavy Industries, Ltd., 2014 all rights reserved)

Another feature of the Advanced PWR core is the stainless steel neutron reflector installed around outmost fuel assemblies for improvement in neutron economy, reduction of neutron irradiation on the reactor vessel, and reduction in the number of parts by simplification of structures. The neutron reflector is constructed of stainless steel ring blocks piled up to eight stages, enclosing the fuel assemblies. Figure 3.61 depicts the neutron reflector structure in compar­ison with that of the conventional 4-loop core. While the conventional PWR had a stainless steel baffle and water in the reflector region outside core, the Advanced PWR introduces the stainless steel reflector and its cooling water which reduces the size of the water region and effectively reflects fast neutrons to the core inside without slowing down the neutrons.

Experiments in Japan using TCA, the light water critical assembly, have also confirmed that the steel reflector with the smaller sized water region gives excellent reactivity characteristics. In that experiment, the effect of the stainless steel reflector was estimated using iron reflector. The contribution of the iron

Fig. 3.62 Reactivity effect of reflector [38]

(Copyright Mitsubishi Heavy Industries, Ltd., 2014 all rights reserved)

• Measurement □ Calculation (PHOENIX-P) $ Calculation (MCNP) [Error Bar ± Sa]

20 40 60 80 100 120 140 160 180

Thickness of Iron Plate [mm]

reflector to reactivity was investigated with different thicknesses of the iron reflector installed at both ends of the fuel region with the 15 x 15 fuel rod arrangement. Figures 3.62 shows the result. It is seen that there is a reactivity gain of about 0.38 % Ak/k for the iron reflector thickness of about 150 mm. It is also noted that this gives a reactivity gain of about +1.8 % Ak/k from —1.4 % Ak/k at 22 mm in thickness. The 22 mm corresponds to the thickness of the stainless steel baffle. Thus, the experiments confirmed that the steel reflector reduces neutron leakage from core and significantly increases core reactivity.