As mentioned before, BWRs have the feature that the sets of a cruciform control rod and four surrounding assemblies are regularly arranged in the core. In the case of no large change in dimensions of fuel assemblies and fuel rods, the main investigation point in core design set-up is to determine how many fuel assemblies to load. When altering the dimensions of fuel assemblies or fuel rods in order to improve the core performance, it is important to investigate the relationships of each design param­eter to the core performance and to other design parameters.

[1] Fuel inventory

Technical terms and their definitions [7] concerning fuel loading amount in

core are as follows.

(i) Fuel inventory (W): total mass amount of fissionable materials in reactor core [kg or ton].

(ii) Specific power (Ps): the thermal power produced per unit fuel inventory [kW/kg or MW/t].

(iii) Fuel discharge burnup (Bd): the total energy produced per unit mass of initial fuel until the fuel materials are discharged from the core [MWd/t]; there is a restriction from the viewpoint of mechanical design of fuel.

(iv) Operating cycle length (D): the length of the cycle of continuous opera­tion after refueling [days or months].

(v) Fuel batch size (n): the reciprocal of the discharged fuel fraction in one refueling.

For a reactor thermal power of Q, the fuel batch size n and the specific power PS are given by Eqs. (3.1) and (3.2).

Ps = Q/W=Bd/(nxD) (3.2)

The fuel batch size is deeply related to the fuel economy. A large batch size leads to a small number of fuel assemblies to be discharged in refueling. The fuel assemblies remain in the core for a long period and reach a high burnup. Therefore, the fuel cycle cost is reduced. On the other hand, a small batch size shortens the burning period of fuel assemblies in the core and leads to a low burnup. Hence, the fuel cycle cost is increased. The typical batch size of BWRs is about 4, namely, about one of four fuel assemblies is discharged and replaced in one refueling.

If a small amount of fuel is loaded in the core, that is, if a high specific power is intended, the fuel batch size becomes small. For a given fuel discharge burnup and operating cycle length, it is necessary to set the specific power so that the fuel batch size does not become extremely small. Since the specific power is equivalent to the measure of energy produced per unit volume of pellets, it is related to the linear heat generation rate of fuel rods and the heat flux density on the fuel rod surface as well. Therefore, the specific power is designed considering fuel integrity and coolant heat removal.

Based on the discussions above, the specific power of a BWR is set roughly as 25 kW/kgU as a criterion. The specific power and fuel batch size are important indicators in estimating the fuel inventory for a given reactor power or in evaluating the validity of the fuel inventory estimated from the specifications of fuel rods and assemblies.