Commercial BWRs in Japan have had an operating experience of over 40 years since the Tsuruga Nuclear Power Plant Unit 1 was started in 1970. The core and fuel have been improved on the basis of accumulated operating experiences and tech­nological progress and Fig. 3.21 shows the history [20] of improvements in it.

In the beginning, there were various improvements related to security and reliability of fuel safety; these included enhanced moisture management in fuel fabrication, application of the Pre-Conditioning Interim Operating Management Recommendation (PCIOMR) [21], and reduction in the average linear heat gener­ation rate by using a larger number of fuel rods (changing from the 7 x 7 type to the 8 x 8 type lattice). The PCIOMR sets restrictions on reactor operation. These include limits on the linear heat generation rate for free operation of control rods to mitigate pellet clad interaction (PCI); a mild power increase by coolant flow rate control when the power is increased over the allowable linear heat generation rate; and keeping the power at a level for a fixed period after which the power may be freely changed within the level. It is very effective as a measure of the PCI, while it

Fig. 3.21 Improvement of BWR core and fuel in Japan

leads to a decrease in the reactor capacity factor. Core improvement was undertaken to actualize flattening of the core power distribution for the purposes of improving the capacity factor under the PCIOMR application and simplifying the reactor operation, thus securing fuel integrity. The development of improved cores such as the axially two-zoned core and control cell core considerably enlarged the core thermal margin, and improved the reactor operation and capacity factor by reducing the PCI load. Since the 1980s, high burnup and high economy cores have been developed to further raise the economy of nuclear power and to lighten the burden of fuel cycle, such as by lowering the amount of spent fuel and high level wastes. The development of the Pu-thermal core (MOX-fueled core) and its fuel have also proceeded to establish a fuel recycling by using plutonium in LWRs. Moreover, the operation cycle length has been prolonged to improve the capacity factor and an increase in the rated reactor power has been planned from the viewpoint of advanced use of existing power plants. It is important to continue to develop core and fuel corresponding to those improvements.

[1] High burnup and long operation cycle length

High burnup increases the total energy (discharge burnup x fuel inventory) produced from fuel assemblies from the time they are loaded into core until they are discharged. It can significantly extend the operating cycle length without increasing the number of fuel assemblies to be exchanged in refueling, and then improve the capacity factor, which helps to reduce power generation costs. Since high burnup also increases the total energy per fuel inventory and reduces the spent fuel amount per unit energy generation, it is possible to reduce

Operation Cycle Length [Months] 00 Cycle Burnup Bc

the reprocessing and waste disposal costs in the fuel cycle cost. The natural uranium and enrichment conversion cost in the fuel cycle cost can be reduced by decreasing natural uranium resources necessary for unit energy generation, namely, by improving fuel economy. The following describes the effect on fuel economy of the high burnup and long operation cycle length.

In the linear reactivity model based on the assumption that the reactivity decreases linearly with burnup, when it is considered that enriched fuel assem­blies loaded into core stay during n operation cycles and 1/n fuel assemblies are replaced in refueling, namely, the batch size is n, the cycle burnup Bc (n) and the discharge burnup Bd (n) can be given by [7]

Bc(n)=2xB0/(n+l) (3.19)

Bd (n) =nxBc (n) =2 nx B0/ (n + l) (3.20)

Bd{n)+Bc{n)=2B0 (3.21)

where B0 is the achievable discharge burnup of fresh fuel when it burns without replacement and it is a constant depending on fuel enrichment. In the case of a change in the refueling batch size from n1 to n2 to extend the operation cycle length, the discharge burnup change can be written by

ABd U) =Bd U2) — Bd Ui) = 2B0 U2 — n1)/(n2 + 1) Ui + 1) (3.22)

When the operation cycle length is extended by increasing the number of refueling fuel assemblies (n1 > n2) without changing the average enrichment (B0 is constant), the discharge burnup decreases and the fuel economy is compromised as shown in Fig. 3.22. A large B0 , namely, a high average enrichment is required to prolong the operation cycle length [large Bc (n)] by

increasing the number of refueling fuel assemblies while maintaining the discharge burnup (Bd (n) is constant).

Figure 3.23 shows the relation [8] between the average enrichment of a fuel assembly and the discharge burnup or the feed component which is the natural uranium resources necessary for fabrication of 1 kg of enriched uranium. The increment of the feed component by increasing the average enrichment is smaller than that of the discharge burnup. The increase of the average enrich­ment leads to a higher infinite multiplication factor of the fuel assembly, which can meet the critical condition in the core even after burning to a low infinite multiplication factor. In other words, the high burnup obtained by increasing the average enrichment can reduce the necessary natural uranium resources per unit energy generation.