(vi) Advanced PWR (APWR) (adoption of neutron reflector)

The implementation of the low leakage core and gadolinia-added fuel is discussed next, and the APWR (neutron reflector), high burnup, MOX-fueled core, and power up-rating issues are mentioned following that. The long cycle length and load following operation were touched in the list [1] of Sect. 3.3.2 and [4] of Sect. 3.3.5 respectively.

(1) Low Leakage Core

Originally the PWR fuel loading pattern was generally an out-in pattern (fresh fuel is loaded in the outermost core region and burned fuel in the core inside region). This pattern can flatten the core radial power distribution and reduce the power peaking factor. However, the loading of fresh fuel in the outermost core region leads to a high power in the core periphery and large neutron leakage to the core outside, and therefore it gives a disad­vantage in reactivity. Hence, since the late 1980s, a new fuel loading

Fig. 3.58 Arrangement of gadolinia-addded fuel rods in fuel assembly [36] (17 x 17 type and 24 gadolinia-added fuel rods) (Copyright Mitsubishi Heavy Industries, Ltd., 2014 all rights reserved)

pattern, in which burned fuel assemblies are loaded in the core periphery within the design limit such as power peaking, has been implemented in refueling instead of the out-in pattern. The new fuel loading pattern is advantageous regarding reactivity because of small neutron leakage to the core outside. The higher burnup and larger number of burned fuel assem­blies loaded in the outermost core region can lead to higher reactivity and a smaller number of discharged fuel assemblies (i. e. smaller number of fresh fuel assemblies). For example, number of fresh fuel assemblies needed can be reduced by two applying such a low leakage loading pattern. Various fuel loading patterns are investigated for advantageous reactivity within the basic criteria of core design [35].