[1] Reactivity for reactor operation

The reactivity of nuclear reactors is defined in terms of the effective multipli­cation factor kef as follows.

P ~ (heff l)/keff (3«18)

Figure 3.13 shows a typical variation in reactivity with nuclear reactor operation. Nuclear reactors operate at the critical state of keff = 1, namely, p = 0. The effective multiplication factor and reactivity change with variations in conditions such as reactor pressure, fuel temperature, coolant temperature, and void fraction. As mentioned in Sect. 3.2.1, LWRs are designed so that the reactivity decreases when fuel and coolant temperatures rise and coolant void occurs or enlarges. Since neutron absorbers such as the FPs Xe and Sm are accumulated immediately after reactor startup, the reactivity is also reduced. It is, therefore, essential to give an excess reactivity corresponding to the expected reactivity decrease before reactor startup in order to maintain the critical state at a rated power operation. Moreover, it is also necessary to provide an excess reactivity compensating for the reactivity decrease with burnup after reactor startup in order to operate the reactor at the rated power during the operation period because the accumulation of FPs as neutron absorbers leads to a reactivity decrease while the amount of fissile nuclides in fuel decrease with reactor operation.

Temperature

Void Fraction Change

Accumulation of Xe and Sm

Operating Cycle

Operating Period

Fig. 3.13 Long-term variations in reactivity with nuclear reactor operation

Hence, a larger fuel amount than a critical one is loaded into the reactor and an excess reactivity necessary for reactor operation is provided considering variation in temperature, boiling effect, decrease in fissionable material with burnup, accumulation of FPs, and so on. The reactor should be designed to safely operate with proper control of the excess reactivity during the reactor operation period. Table 3.9 presents an example of the excess reactivity and reactivity worth of control elements for BWRs, which are designed to have a capability for reactivity control larger than the total excess reactivity for core shutdown even with one control rod stuck in the fully withdrawn position, including a calculation error.

The long-term variations in reactivity with burnup are controlled by control rods, coolant flow rate in core, and burnable poisons added to fuel pellets as shown in Fig. 3.13. Since the amount of burnable poisons cannot be adjusted during reactor operation, the reactor startup and shutdown are performed by

Stainless Steel Tube

(Neutron Absorption Hod> Sheath Roller

^.Iron

Wool

.End Plug

Detail Drawing of

Stainless Steel Pipe
і Neutron Absorption Rod>

Coupling Socket

Fig. 3.14 BWR control rod (B4C type of ABWR)

controlling control rods and coolant flow rate. A control rod scram is activated to shut down the reactor at an emergency and a boric acid solution injection system is provided as a control element for backup shutdown.