PWR fuel has been improved for high burnup from the viewpoint of high economy in fuel cost and lowered amount of spent fuel to be handled and stored. The limit of the maximum fuel assembly burnup was 39,000 MWd/t in the beginning, and increased to 48,000 MWd/t in Step I and 55,000 MWd/t in Step II fuels. Further high burnup is planned [39]. Such a high fuel burnup has the following effects on core and plant facilities and measures against them are discussed below.

(i) Neutron spectrum hardening

(ii) Enrichment increase (increase in 235U)

(iii) Fission product accumulation increase

(1) Impact of high enrichment and burnup

A high enrichment and burnup leads to an increase in thermal neutron absorbing materials such as 235U, plutonium and FPs and a decrease in thermal neutron flux. Therefore, the thermal neutron absorption by the rod cluster control assemblies (RCCAs) and soluble boron in primary coolant is reduced and their reactivity worth is decreased slightly.

As a measure of the reactivity worth reduction of RCCAs, a proper arrangement of fuel assemblies loading pattern in the core can secure the

reactivity shutdown margin. The number of RCCAs should be increased if insufficient.

As a measure of the reactivity worth reduction of soluble boron, the boron concentration can be increased for reactor shutdown if necessary. The boron concentration in the refueling water storage tank (the boric acid water stored in the refueling water storage tank could be injected into the reactor at abnormal transients or accidents in which the boron injection function of the emergency core cooling system is expected to work) can also be increased.

The increase of enrichment also leads to lower subcriticality margin of fuel storage facilities. Since the subcriticality evaluation of fuel storage facilities has been performed for fresh fuel with initial enrichment of 5.05 wt%, a new measure is not necessary if the enrichment is lower than 5 wt%.

(2) Impact of high burnup

A high burnup increases the amount of fission products and therefore increases the decay heat of spent fuel. Spent fuel storage cooling system are enhanced in case of insufficient cooling capability, considering heat load to spent fuel pit.