Figure 3.3.15 Reactivity worth of control group bank d (beginning of cycle, hot zero power, no xenon; example 4-loop core)

local power to the average power is defined as Fq) of the reactor is also important. The factors which affect the power distribution are the control rod movements and the xenon distribution.

In the currently operating PWR plants, the constant axial offset control method (CAOC operation method) is adopted in controlling the power distribution; this reflects the historical

development of PWRs, which included systematic increases of power density, reconsideration of criteria for the ECCS and engineering advances.

The CAOC operation method is aimed at preventing large distortions in the core power distribution by flattening the axial power distribution which depends mainly on the core conditions (the basic characteristics are illustrated in Figure 3.3.18).

Figure 3.3.16 Structure of primary neutron source assembly

Primary neutron source

Section B-B

Section A-A

Section B-B

Section A-A

Figure 3.3.17 Structure of secondary neutron source assembly

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AO., axial imbalance, Al (defined as: Al = upper half core output - lower half core output = AO.x core output), is used as the monitoring parameter of the core axial power distribution. The Al is observed by the out-of-core nuclear instrumentation system.

2. Core management

a. Basic design criteria of reload core

The number of fuel assemblies which are replaced by the new ones in each subsequent core is determined on the basis of fuel burnup history of the previous cycles and the amount of planned output for the new cycle. The reload core is designed by considering the following items. (D The required fuel burnup (the amount of required electric power).

2. The number of fresh fuel assemblies and burnable poison rods.

3. The nuclear design requirements for safety assurance (c.f* section 3.3.2 (2)-b).

Fig. No.2: In case of control rod insertion from state No.l, power distribution skewed to bottom

Figure 3.3.18 Effects of control rods position, power level, burnup distribution and Xe on power distribution

Fig. No,3: In case of power decrease after long time burnup at state power distribution skewed to top due to imbalance in moderator reactivity effect

Xe effect

Fig. No.5: In case of power restoration after 6 hours at state No.3, power distribution skewed to top due to xenon reduction at upper core, even after returning to the same power as state No.l

Relative power

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b. Refueling pattern

The following main conditions are considered for selection of the refueling pattern.

(D 'Hie quadrant symmetry of the core must be maintained.

(D Proper loading of fuel and burnable poison must consider the power distribution flattening over the core life.

3. A positive moderator temperature coefficient must be avoided through decreasing the critical boron concentration by loading either fuel assemblies containing gadolinium, or burnable poison rods. This is valid because at high critical concentration of boron, the moderator temperature coefficient becomes positive.

4. The maximum fuel burn-up must not exceed the specified limits.

3. Control rods must provide the required control capability. That is because the reactivity effect of the control rods depends on the characteristics of the newly loaded fuel assemblies. Figure 3.3.20 compares a typical fuel loading pattern for a reload core with an initial core loading pattern.

4. Monitoring of nuclear and thermal conditions and burnup control during the operation Thermal and nuclear conditions of the core are monitored by using various detectors installed in and out of the core. Out-of-core neutron detectors and thermocouples installed in the upper part of the core are capable of continuous monitoring of the core nuclear characteristics. In-core neutron detectors are a movable type and in principle, are used only once a month. The in-core neutron

Power

As shown in the left figure the expected Fq x P might exceed the design allowable limit due to big distorition of power distribution in case of non-CAOC operation.

Axial neutron flux offset (Al)

Limiting condition

Limiting condition of axial offset is determined so that Fq may not exceed the design allowable limit.

Constant axial offset control

As shown in the left figure, the expected Fq x P values stay within the design allowable limit in case of CAO C.

Figure 3.3.19 Essentials of constant axial offset control (CAOC) operation method

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Initial Core Loading Pattern (example 3-Loop Core)

Reload Core Loading Pattern (example 3-Loop Core)

Figure 3.3.20 Fuel loading patterns (examples)

detectors can be inserted into almost one third of the core fuel assemblies (c.f. Figure 3.3.21) and therefore, obtain a reliable core power distribution profile.

The burnup of each fuel assembly is calculated from the core power distribution thus obtained and the average core burnup which is obtained from the integrated core output

3. Fuel integrity control during plant operation

The fuel integrity is controlled by routine sampling of the coolant and determining its radioactivity. Special attention is given to the radioactivity buildup of iodine-131 in the reactor coolant, from which the integrity of fuel is confirmed. When the reactor load is changed, the fuel is subject to local power changes to some

extent; however, by applying the CAOC operation method, these local power changes can be maintained within a relatively small range. A more detailed discussion is given in section 3.3.1-(2) regarding the fuel integrity during normal operation of the plant, including the load changes periods.

4. Fuel inspection during the periodical inspection outage

During the periodical inspection outage of the plant, the fuel assemblies (either spent or partially spent fuel) are removed from the RV and are inspected by test equipment installed in the cask loading pit, the spent fuel storage pit or the fuel test pit. Two inspections are performed on the fuel assemblies. If it is necessary, the presence of any leaks in the assemblies are examined by sipping inspection. External defects and dimensional changes of the fuel assemblies are visually examined by underwater TV cameras. Not only the integrity of the fuel is confirmed by these inspections, but, to understand the fuel irradiation behavior in detail, some important data such as irradiation growth of fuel rods and inter­red spacing in fuel assembly are also measured by these underwater TV cameras. These data are evaluated and serve as design feedback data.

5. Trouble prevention measures related to fuel

The cause of the inter-rod spacing reduction in the fuel assembly is fuel rod bending due to an irradiation effect In Japan, the local integrity of fuel in the case of mutual contact of fuel rods is also considered. In order to avoid such mutual contact of fuel rods and to assure the fuel integrity during operation, visual and dimensional inspections of the fuel are made.

An effective method to reduce the fuel rod bending is to increase the number of the support grids and shorten the span length of fuel rods. In newly-constructed plants, the numbers of grids in fuel assemblies consisting of 14x14 (used in two- loop plants) and 17x17 {used in three- and four- loop plants) fuel rod arrays have been increased from 7 to 8 and from 8 to 9, respectively. The effect has been dramatic; no fuel rod bending defects have been seen in 14x14 type fuel assemblies with 8 grids and, only very slight fuel rod bending was found in 17x17 type fuel assemblies with 9 grids.

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O Instrumentation Thimbles

A,B,C,D,E In-core Neutron Detectors

CAL In-core Neutron Detector for Calibration

Figure 3.3.21 Arrangement of in-core neutron detectors (example 3-loop core)

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