Uranium-saving technology is intended to attain a high burnup with the least increment of fuel enrichment by effectively burning the uranium loaded in the core. Only a simple increase in fuel enrichment raises the uranium and enrichment cost. For an economic high burnup, therefore, it is important to save the uranium simultaneously. The basis of uranium-saving technologies used practically in BWRs is the development of zirconium liner fuel in which the inside of the zircaloy-2 cladding is lined with soft and pure zirconium. The zirconium liner fuel mitigates the pellet-clad mechanical interaction (PCMI) and increases fuel integrity against a rapid power rise. Hence, it eliminates restrictions on the PCIOMR and makes it unnecessary to take an excessive design margin for the linear heat generation rate. The thermal margin obtained from the power distribution flattening for improvement of the capacity factor can be applied to the uranium saving.

(1) Utilization of power peaking

Arrangement of the high enrichment fuel in the proper locations with high

thermal neutron fluxes can increase the core reactivity. High enrichment

zoning in the peripheral region of a fuel assembly causes bias of the pin power distribution and large local power peaking. In the early core design, low enrichment zoning in the peripheral region was employed to improve the thermal margin and to mitigate the restrictions on the PCIOMR. How­ever, since the zirconium liner fuel has been employed and the axially two-zoned core has reduced the axial power peaking as mentioned, the core reactivity can be improved by allowing a larger local power peaking. The installation of natural uranium blankets on the upper and lower end parts and the arrangement of low enrichment fuel assemblies in the core periph­ery region cause a large axial and radial power peaking of the core. However, the power peaking margin from the axially two-zoned core can give a reactivity gain.

(2) Reduction in gadolinia residual

The reduction in gadolinia residual is the technology to minimize gadolinia embers in the core upper and periphery parts and to increase the core reactivity. The placement of natural uranium blankets on the upper and lower end parts also contributes to the reactivity gain by replacing the corresponding gadolinia.

(3) Spectral shift operation

The spectral shift operation is the technology to get a reactivity gain by changing the neutron spectrum through a change in the void fraction and distribution which are features of the BWR core. The void fraction change can be accomplished by the coolant flow rate control to decrease the core coolant flow rate during the first half of the operating cycle, which leads to a high void fraction in the core, less neutron moderation, and acceleration of 238U conversion to plutonium, and then to increase the core coolant flow rate during the latter half of the operating cycle, which leads to a burning of the plutonium accumulated in the first half. The void distribution change can be performed by the axial power distribution control to distort the axial power distribution downward, which leads to a downward shift of the void generation point and an increase of the void fraction in the core, and then oppositely to extend the axial power distribution upward, which leads to a decrease of the void fraction.

(4) Optimization of H/U ratio

High enrichment of uranium for high burnup decreases the ratio between H and 235U atoms, and hardens the neutron spectrum. It is, therefore, neces­sary to set the proper H/U ratio in order to effectively use fuel and to efficiently utilize thermal neutrons. The H/U ratio can be raised by increas­ing the number of water rods in the fuel assemblies or by enlarging their cross-sectional area as mentioned in Sect. 3.2.3.