Burnup (MWd/t)

Figure 3.3.13 Critical boron concentration vs. Burnup (hot full power (hfp), all rods out)

No. of Control Rod Clusters

A: Control Group Bank A 4 B : Control Group Bank B 8 C: Control Group Bank C 8 D: Control Group Bank D 9 SA Shutdown Group Bank SA 8 SB: Shutdown Group Bank SB 8 SC: Shutdown Group Bank SC 4 SD: Shutdown Group Bank SD 4

Figure 3.3.14 Arrangement of control rods (example 4-Loop core)

typical PWR core is given in Figure 3.3.15.

3. Burnable poison

The burnable poison rods consist of either borosilicate glass tubes or aluminum-boron mixed oxide pellets contained within a corrosion- resistant cladding (stainless steel or Zircaloy-4) and they are positioned in vacant control rod guide thimbles of the fuel assemblies. The poison rods in each fuel assembly are grouped and attached together at their top ends with a spider similar to the RCC assembly.

The burnable poison rods are used to maintain a negative moderator temperature coefficient by controlling part of the excess reactivity at the beginning of the core life. In other words, relatively large excess reactivity at the beginning of the core cycle requires high concentration of soluble boron in the coolant, which in turn, may shift the moderator temperature coefficient from negative values to positive. Employment of the burnable poison reduces the concentration of soluble chemical shim (¹⁰B in this case), and prevents this shift of the moderator temperature coefficient to positive. Furthermore, the burnable poison assists the function of the chemical shim in providing control for the change in excess reactivity due to fuel burn-up, by simultaneous burning out of neutron absorbing poison (in this case ¹⁰B), With same purpose, gadolinium-loaded fuel rods are also employed to enhance the core performance of PWR.

2. Neutron sources for startup

In order to appropriately control the criticality during the startup operation, neutron sources are loaded into the reactor core in advance. ²⁵²Cf, a spontaneous neutron emitter enclosed in a stainless steel cladding, is utilized as a primary neutron source during the first reactor core startup. After then, antimony-beryllium, irradiated in the same reactor, is used as a secondary neutron source during the subsequent core startup operations. The primary and the secondary neutron source structures are shown in Figures 3.3.16 and 3.3.17, respectively.

2. Power distribution control

In addition to the reactivity control, control of the power distribution (the ratio of the maximum

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