The reactivity control in PWRs is done by control rod operation, adjustment of boron concentration in primary coolant (referred to as chemical shim) and if necessary using burnable poison rods.

(1) Design Principles for Reactivity Control

(i) The reactor core should be made subcritical without exceeding the allowable design limit of fuel from the hot standby or hot operation

condition. Core reactivity should be controlled at the hot condition by at least two independent control systems: the control rod control system and the chemical and volume control system. The latter adjusts the boron concentration in primary coolant.

(ii) Core reactivity is controlled usually through role sharing of the reac­tivity control equipment as follows.

• Rod Cluster Control Assemblies: Rod cluster control assemblies are used to control the reactivity variation (power defect) by the power variation from hot zero power to hot full power; i. e. they provide control of relatively fast reactivity variations.

• Boron concentration: Boron concentration is adjusted to control reactivity variations due to the primary coolant temperature varia­tion from cold temperature to hot zero power temperature, FP (Xe and Sm) concentration variation, and fuel burnup; i. e. they provide control of relatively slow reactivity variations.

• Burnable poisons: Burnable poisons are employed to partially con­trol the excess reactivity necessary for fuel burnup, which makes the moderator temperature coefficient negative at hot power operation by being able to reduce the boron concentration in the primary coolant.

Figure 3.38 shows a typical reactivity control scheme by the listed reactivity control equipment. The left part (i. e. time before shutdown) indicates the necessary excess reactivity for fuel burnup, being controlled by soluble boron and burnable poison, and the reactivity decline as burnup, being compensated by boron dilution, where the reactivity is set as zero at hot full power. Control rods are inserted to shut down the core (from hot full power to hot zero power) at time zero and to secure subcriticality. The Xe decay following Xe accumulation is compensated by increasing the boron concentration (boration) and the coolant temperature decrease from hot to cold is also compensated in a similar way.

Figure 3.38 indicates that the boron concentration changes during the period of the Xe decay and the coolant temperature decrease. In actual reactor operation, however, the operation of changing the boron concen­tration can be activated before that period and the reactivity control scheme does not necessarily correspond to the period.