The fuel inventory (W) can be given by

W=pp xNBx Nrod x Lrod x ИДУ2)2 (3.3)

where NB is the number of fuel assemblies, Nrod is the number of fuel rods per fuel assembly, Lrod is the active height of fuel rod (= active core height), and Dp and pp are diameter and density of the fuel pellet, respectively. The fuel rod diameter Drod is determined from the pellet diameter DP, cladding thickness, and gap between cladding and pellet. The core volume Vcore and the equivalent core diameter Dcore can be calculated by

V^ore = Nb x LBx LBx Lrod (3:4)

Dm = 2x(NBxLBxLB /n)1/2 (3.5)

where LB is the fuel assembly pitch.

For a constant fuel inventory in the core, a long active core height Lrod reduces the number of fuel assemblies NB. This is desirable from the viewpoint of fuel management, but not from the viewpoints of nuclear and thermal — hydraulic design and mechanical design of fuel. A low neutron leakage leads to an effective usage of neutrons and enhances the efficiency from the

viewpoint of nuclear design. It is desirable that the surface area of the core is small, namely, the ratio between the active height and equivalent diameter of the core Lrod/Dcore is close to 1.0. From the viewpoint of thermal-hydraulic design, a long fuel rod causes a high pressure drop in the fuel assembly and has an effect on the primary coolant pump design. Since it also increases the buoyancy of the fuel assembly, a design consideration is needed so that it does not rise. From the viewpoint of mechanical design, a long fuel rod is easily bent and therefore that influences the fuel loading characteristics. The typical design has about 4 m core height and about 3.6-3.7 m active height of the fuel assembly including fissionable materials.

A large-sized fuel assembly reduces the number of fuel assemblies to be replaced in refueling and that leads to an improvement in reactor capacity factor. It is desirable to make the factor large to the extent possible. From the safety viewpoint for handling fuel assemblies outside the core, however, the fuel assembly cannot be sized too large. The fuel assembly is required to maintain subcriticality, even if it is flooded in fresh water, for which the fuel enrichment is an important parameter. The fuel enrichment of 5 % is considered as an upper limit and its corresponding fuel assembly size is limited to about 220 mm when not considering the burnable poison effect on reactivity control. The practical size of fuel assembly is about 135 mm.