Reactivity control

Potential impacts of flexible operation: Reactivity management is an important issue for baseload and flexible operation. Frequent manoeuvring of the reactor will require a stricter adherence to reactivity management standards, as baseload operation provides a simpler and easier approach than flexible operation in managing the reactivity state of the reactor. Flexible operation, especially when achieved via control rod insertion/withdrawal, will alter the power and xenon distribution within the core, will affect key core physics parameters (e.g. critical boron concentration) and will involve rod pilot, bank position control, insertion limits and associated design analysis.

A deformation of the neutron flux distribution can cause inadvertent xenon oscillations. In the case of a power increase, such a deformation will cause a nearly instantaneous decrease of core safety margins for parameters such as maximum LHGR and DNBR/critical heat flux limits or PCI restrictions, which were discussed earlier.

The reactivity management standards can also be influenced by the regulatory environment. For example, in the USA, direct reactivity changes may only be made by a licensed reactor operator, as discussed in Section 2.2.2.

This would require the grid system operator or corporate energy planners to contact the plant’s main control room and communicate the desired power change and ramp rate to the plant operators. The operators would then perform the activity. In France, on the other hand, reactivity (power) changes can be the direct result of grid system operator actions. The role of the licensed reactor operator in overseeing plant reactivity changes cannot be overemphasized.

Safety analysis: The OLCs that preserve safety margins by means of the prescribed initial conditions assumed in the safety analysis include limits on various core power distributions (e.g. axial, radial or azimuthal). Reactivity management during power manoeuvring is essential for maintaining the reactor within these boundaries. In most cases, control room alarms will warn operators if these power distribution limits are approached. In some cases, a reactor trip signal is generated if the prescribed power distributions are exceeded.

Possible solutions: In PWRs, using reactor coolant soluble boron concentration to control the reactor power is advantageous due its global effect on reactivity. However, large quantities of CVCS wastewater would be generated during dilution operation, especially later in the core life. Alternatively, using existing, standard control rods (e.g. full length/full strength) to control reactor power has the advantage of minimizing plant modifications.

However, this approach requires careful monitoring of bank worth (shutdown margin) and prompt and accurate in-core instruments to measure core power distribution. Using grey control rods (e.g. full length/part strength) to control reactor power has the advantage of minimizing core power redistribution (relative to full strength), which is not generally included in shutdown margin requirements.³¹

The following solutions have been employed to control reactivity during flexible operation, and details of implementation of these solutions are provided in Annexes I and II.

For PWRs:

— Utilization of an optimized control rod manoeuvring programme to minimize transient overshoot of the peak power density in combination with modified part load diagrams. For example, the control rod manoeuvring programme in German PWRs utilizes standard control rod assemblies [25]. These control assemblies can be used for both regulation and shutdown of the reactor, which are, as per their function, grouped into two banks: the D-Bank (Doppler Bank, i.e. the power reactivity relevant control rod bank) and the L-Bank (control bank). Both banks are manoeuvred in a mutually compensating sequence in order to control the power distribution while maintaining a constant reactor power. The D-Bank, which is used for regulating the integral reactor power, comprises four sequentially moving groups. Each group consists of only four control rod assemblies, and therefore the distortion of axial power distribution is minimized. The L-Bank, which is the remaining majority of the control rod assemblies, can be used to back up reactor power control. The L-Bank is primarily designed to fine control the axial power distribution (only a few centimetres of movement

31 It should be noted that many nuclear power plants do not have grey rods or do not reserve excess shutdown margins to allow swapping of full strength control rods for part strength ones.

can provide an adequate effect). This control rod manoeuvring scheme, together with the correspondingly selected part load diagram (see Section 5.4.1.1) characterized by a constant plateau in the ‘upper power range’, minimizes the transient overshoot of the peak power density after a quick return to full power. As a result, the required operational margin, which has to be considered in the optimization of the core design, does not significantly depend on the plant operating mode (e.g. baseload or flexible). Consequently, advanced loading strategies for fuel economy can be implemented. It should be noted, however, that while this control rod manoeuvring concept enables rapid power changes, it may require prompt and accurate measurement of core power distribution. Therefore, German light water reactors use special in-core instrumentation made of in-core power distribution detectors, which are precisely calibrated, and the Aeroball Measuring System (see Section 5.3.5.2), which continuously monitors and assesses the 3-D flux and power density distribution.

— Utilization of part strength, or grey, control rods to adjust the reactor power with less of an impact on the power and xenon distribution. For example, in French PWRs, the reactor control is accomplished by different modes of rod operation. In ‘A mode’ (or ‘black mode’), four banks of rods are utilized to adjust power and temperature. In ‘grey mode’, one bank (black or heavy rods) is dedicated to temperature control while four banks (two grey rods plus two black rods, with turbine power dependent positions) are dedicated to power compensation. This mode requires a calibration of the position versus electric power at the beginning of a cycle, and every 2–3 months thereafter, to take into account variations in efficiency (i.e. control rod worth) during the cycle. In both modes of operation, additional shutdown safety rods, which are black, provide shutdown margins.

— Power dependent modification of the part load control system (the part load diagram) for provision of operation with a constant primary coolant temperature or a constant SG pressure (see Section 5.4.1.1).

For BWRs:

— Control of reactor power via an optimized recirculation control (i.e. adjust the reactor coolant flow rate). This approach equivalently combines, for BWR conditions, the benefits of the optimized control rod manoeuvring programme and the part load diagram of a PWR, as it has a global impact on core reactivity and less of an impact on power peaking (see Section 5.4.1.2).

5.4.1.1. Pressurized water reactor part load diagram

The part load diagram of a PWR displays the relationship between the set point of the average primary coolant temperature and the reactor power output. The part load diagram is a specification for how to control the parameters of the NSSS that depends on the power output. In principle, two schemes (and combinations of both) are possible for PWRs, as shown in Fig. 29. Mode (a) in Fig. 29 (solid lines) represents keeping the coolant temperature constant with increasing reactor power. This mode of operation requires a dethrottling and therefore a pressure decrease on the secondary side due to the given and unchanged conditions for heat transfer in the SG.

Mode (b) in Fig. 29 (dashed lines) allows the coolant temperature to increase with increasing reactor power.

Potential impacts of flexible operation: Operating a PWR in flexible operation means that the parameters of the primary coolant and main steam will vary. The fluctuations in average coolant temperature will have various relevant impacts on other parameters, such as:

— Thermal stress, which would induce fatigue (see Section 5.2.1).

— Reactivity changes in the reactor core, which will cause more and deeper movement of control rods and therefore more deformation of the power density distribution in the core.

— Changes to the stored heat (energy) of the RCS. (For increasing power, the components of the primary circuit will adopt the increased coolant temperature and the additional energy produced in the reactor). This may cause inadvertent overswing of the reactor power control.

— Changes to the coolant volume, which requires more effort for the CVCS, and waste management.

— Changes to the lithium (pH) level, which requires more effort for water chemistry control and regulation.

— Changes to the secondary system overpressure protection components and controls due to main steam pressure changes.

Safety analysis: The operation mode will not have a direct impact on accident analysis. However, it will affect the variation of safety relevant properties such as fatigue, core criticality and other core physics parameters (LHGR and DNBR).

More importantly, the fluctuations in the main steam pressure will have an impact on the system design of the secondary system (e.g. protection against overpressure).

Possible solutions: By combining and optimizing for the applicable power ranges, PWRs combine the benefits of modes (a) and (b) and minimize the drawbacks in the relevant power range for flexible operation.

The characteristics of the simplified scheme for the combined part load diagram that are adopted by German PWRs (see Annex I) can be summarized as follows:

— In the upper power range (e.g. 40–100% RTP), where the majority of load following is performed, the average coolant temperature is kept constant (i.e. mode (a) in Fig. 29 is applied). In addition, flexible operation is less constraining in this range as less restrictive requirements to core, fatigue, criticality and coolant volume management are applicable with fewer challenges to safety and operational margins. The benefit of this mode of operation is reduced control rod movements, because temperature feedback of reactivity during power manoeuvres is minimized. Additionally, the RCS volume remains essentially constant, resulting in less ingress/egress through the pressurizer surge line, which reduces the temperature variations on the components, as well as reducing the CVCS manoeuvres.

— In the lower power range (e.g. 0−40% RTP), mode (b), as shown in Fig. 29, is applied. In this power range, flexible operation is difficult, and the main steam pressure is kept constant. The benefit of this scheme is a reduced challenge to margins for the secondary side design pressure (e.g. overpressurization is minimized).

FIG. 29. Schematic (steady state) part load diagram for a pressurized water reactor.

5.4.1.2. Boiling water reactor recirculation flow curve

The reactor power in BWRs is controlled either by control rods (blades) or by the reactor coolant flow rate.

The latter, called recirculation control, is typically used as the speed of the forced circulation pumps is adjusted.

A reduction of coolant flow rate increases the amount of steam in the reactor core, reducing the moderator density and the reactivity; an increase in the coolant flow rate results in increasing moderator density and reactivity and thus an increase in power (Fig. 30). This approach has a global impact on core reactivity and less of an impact on power peaking.

Potential impacts by flexible operation: The operation mode will not have a direct impact on accident analysis.

However, flexible operation will have impacts on the corresponding changes of the core physics parameters as well as on the thermohydraulic parameters of the NSSS, as it will affect the variation of safety relevant properties such as fatigue, core reactivity and other core physics parameters (LHGR and DNBR). Thus, an optimized power control scheme is necessary to minimize the impacts, such as minimized movements of control blades.

Possible solutions: An optimized recirculation control can be used to achieve minimized impacts from flexible operation in BWRs. For example, optimization is utilized in German BWRs as it is suitable for flexible operation in the upper power range (approximately 60–100% RTP), because BWRs, in principle, are able to change power in this upper range with faster and fewer power distribution changes in the core. The optimized recirculation curve provides the reactor power as a function of the core flow for a constant control rod position. Recirculation only control is advantageous for flexible operation as load changes can be performed without manoeuvring the control rods, and, therefore, the relative power distribution in the core is not significantly affected [25]. This also minimizes stressing of the fuel rods and changes in fuel rod temperature caused by flexible operation. Power changes beyond the recirculation control range are made by manoeuvring the control blades.

FIG. 30. Typical boiling water reactor flow recirculation curve.

5.4.1.3. Reactor power monitoring

Nuclear power plant operators are required to monitor and comply with the plant’s licensed power limit.

Potential impacts of flexible operation: Several indications of reactor power are available for plant operators to monitor reactor power. For example, many PWRs have neutron flux, primary calorimetric and secondary calorimetric power measurement indications. Flexible operation creates two concerns with power monitoring:

(a) application of power dependent uncertainties within the monitoring and protection system set points in safety analysis and (b) calibration periodicity for these various power indications during and following flexible operation.

These issues may also affect RCS flow measurements.

Safety analysis: Each of the reactor power indications has unique, power dependent uncertainties, which must be accounted for within monitoring and protection system set points and safety analysis. For example, the uncertainty of the PWR secondary calorimetric power may be less than 1% RTP at full RTP; however, it increases to a few per cent as the power level decreases. This increase in uncertainty must be accounted for during power manoeuvring.

Furthermore, the periodicity of power calibration and surveillance requirements may be based on an assumption of baseload operation. For example, PWR TSs require reactor power and RCS flow measurements and calibrations at key power plateaus during initial power ascension and then periodical surveillance and recalibration at full power operation. Subsequent power manoeuvring may invalidate the TS bases for these surveillances and must be addressed.

Possible solutions: In order to address reactor power monitoring impacts, the power signal needs to be calibrated at regular intervals (monthly or more frequently) at steady state, and full power conditions to the thermal reactor power provided by the secondary heat balance. In German PWRs, for example, this balance is calculated every minute by the process computer and displayed to the operator. This enables the operator, even after a load variation and a subsequent stabilization period of approximately 15 minutes, to compare the power signal with its reference and to verify compliance with the acceptance criteria. A calibration at a reduced power level is only performed for prolonged part load operation, but in this case, holding points for calibration purposes are imposed during power ascension in analogy to the first startup after refuelling.