Nuclear reactor operation management is based on a long-term operation plan of 3-5 years. The plan includes startup date of each operating cycle, shutdown period for periodic inspection, and load factor in operation. It also includes intermediate shutdown planned in each operating cycle. After the long-term operation plan, the cycle burnup is obtained from reactor operating days and load factor of each cycle, and the plan of fuel exchange to be performed during the periodic inspection period is investigated. Core analysis is done for each cycle to evaluate the plan. Proper fuel loading pattern of each cycle is inves­tigated and fresh fuel amount and types are determined.

Nuclear and I hermal-Hydraulic Calculation

and Cycle Burnup Calculation

Cycle Length

YKS

Design Criteria and Operation Limits^

This long-term burnup plan is used to systematically procure fuel which is ordered from the supplier at the very latest 2 years before delivery to the plant; this is based on considerations of procurement of enriched uranium and clad­ding and fuel fabrication times from pellet to fuel assembly.

Main tasks in the reload core design are to determine fresh fuel amount necessary for the planned operating cycle and its arrangement in the core and to make a basic operation plan.

Figure 3.28 shows a typical process flow for the reload core design in which an optimal reload core is designed by repeated investigations with combina­tions of available fuel assemblies after the number and arrangement of fuel assemblies loaded into the core and control rod pattern are guessed first.

Safety parameters for the BWR reload core include the reactivity shutdown margin, maximum linear heat generation rate, MCPR, maximum fuel assembly

burnup, channel stability, core stability, and control rod irradiation lifetime, as discussed in the beginning of this section.

During the periodic inspection period, fuel exchange is performed based on the prepared fuel loading pattern. Usually, about 1/4 of the core fuel assemblies are replaced with fresh ones and the other fuel assemblies are, if necessary, moved to different locations in the core (fuel shuffling). This refueling opera­tion is a major step to determine the periodic inspection period and therefore a plan with less movement of fuel assemblies, namely, a plan with less fuel shuffling is important from the viewpoint of an improved capacity factor by shortening the periodic inspection period. In the fuel loading pattern of the equilibrium cycle core shown in Fig. 3.20, fuel assemblies stay in almost the same location until the third cycle since they were first loaded, and then they are moved to the outermost region of the core or the control cell region for the fourth cycle. This results in the flat power distribution as well as less fuel shuffling.

Possible fuel loading patterns are innumerable, even considering core symmetry, and search for an optimized loading pattern in combination with control rod patterns is inevitably limited by limited computational resources. Recently, a tool based on accumulated operation knowledge was developed to automatically plan and optimize the fuel loading and control rod pattern [28].

After the periodic inspection, various performance factors are tested before startup, including the reactor shutdown margin in core and safety in fuel reloading.