Component aspects of flexible operation

The majority of impacts discussed below for the component aspects primarily apply for the load following type of flexible operation. Based on the existing technologies and operating experiences, frequency control, especially primary frequency control, has a very limited impact on nuclear power plant components. For example, for a small frequency range, control rod movement may not even be necessary. This is because small frequency deviations typically do not cause significant cycling in pressure, temperature or flow, and do not require actuation of many components. An exception would be control systems such as the turbine controller module that helps the turbine change power on a frequency deviation. However, this is a general rule and the impacts from the frequency response may differ from one design, technology or operation strategy to another. Thus, stakeholders, providing frequency control above and beyond the currently allowed and practised range of frequency control operation, ensure that all potential impacts are considered and addressed.

5.3.1. Fuel rods

Potential impacts of flexible operation: The potential impacts on fuel design functions²⁴ depend upon many factors, including the fuel design, operating history (e.g. power levels or fuel burnup), operating conditions (power level, periodicity, rate and magnitude of the power change, insertion of control rods, etc.) and coolant chemistry.

Mainly, the impacts are the result of power density redistributions in the core during load following operation, which are caused by control rod movement as well as the feedback effects linked with reactor coolant conditions and xenon distribution. In flexible operation mode, this redistribution will occur frequently and throughout most of the fuel cycle, and is likely to occur at intermediate power levels and in non-steady-state conditions. Generally, the impacts of load following on the fuel could be one or more of the following:

— The load following operation will likely promote changes in local power density. This, in turn, affects the fuel safety limits for the LHGR and DNBR in the OLCs, and may necessitate investigation of these safety parameters for normal operations and in AOO and DBA analysis, as well as review of the operating procedures.

— As a result of fuel pellet thermal expansion, depending on the fuel and operating conditions, PCMI may produce an increase in cladding stress proportional to any increase in local power density.

24 Such functions include transferring heat generated within the fuel pellet to the reactor coolant and retaining fission products.

— High induced cladding stress combined with the aggressive chemical agents present in fission products (e.g.

iodine) may lead to cladding failure via SCC.

— Increased impurities may aggravate corrosion, necessitating stricter monitoring and control of increased impurities.

— Depending on the rate, depth and periodicity of the load changes, there is potential for increased thermal and mechanical cycles on the fuel rods, which may reduce their predicted lifetimes. In addition, fatigue calculations may be further affected by any increase in core residence time.

— Continuous operation at less than full power may extend the operating cycle, and this prolonged residence time may have a detrimental impact on fuel performance.

— Flow induced PWR grid to rod fretting and BWR debris fretting may be negatively affected by extended operating cycles resulting from flexible operation.

— As a result of local power changes during control blade movement (if load following is achieved via control blade insertion/withdrawal), following a longer period of operation with inserted control blades has the potential to promote local power increases leading to conditions amenable to PCI.

Possible solutions: Advanced fuel designs as well as fuel management strategies, power manoeuvring guidelines and operator assistance tools have proven effective at significantly reducing the risk of impact on fuel during flexible operation. However, if any impact is identified, this would require:

— Restrictions on flexible operation, such as no load following early in the fuel cycle until the fuel has been properly conditioned;

— Following periods of extended low power operations, proper fuel reconditioning during power ascension (e.g. accommodated power increase or power level holding periods);

— Fuel mechanical design calculation reviewed to account for any increase in thermal and mechanical cycling.

5.3.2. Fuel assemblies (including fuel channels for boiling water reactors)

Potential impacts of flexible operation: The following aspects of flexible operation are considered impacts on the design function:²⁵

— Flexible operation may necessitate more frequent control rod insertion, which would promote more channel distortion due to shadow corrosion. BWR operating experience has shown that without proper attention, control blade interference due to channel distortion may result in slow to settle/inoperable control blades and forced outages.

— While no operating experiences exist for PWRs (due to normal all rods out operation), Zircaloy guide tubes may be susceptible to shadow corrosion and enhanced growth (due to close proximity with stainless steel control rods). Excessive guide tube growth would result in hard contact with core support plates (e.g.

bottoming out of the top nozzle spring) and promote guide tube and assembly bow, which may eventually affect control rod insertion. Hence, flexible operation achieved with control rod insertion in PWRs may introduce this phenomenon, and its contribution towards guide tube growth would need to be evaluated.

— Fuel assembly component and fuel rod corrosion, as well as fuel assembly fatigue calculations, may be further affected by any increase in core residence time.

— Fuel assembly mechanical design calculations are based on an established residence time, which will change due to the longer residence time induced by flexible operation.

— Increased thermal and mechanical cycles on fuel assembly components may reduce each component’s predicted lifetime.

25 Maintaining the fuel rod array positions the fuel within the reactor core and provides a path for the coolant and control rods into the core.

— Fuel assembly components, such as spacer springs, hold down springs and guide thimbles, will be adversely affected by the alternating stresses and may need additional verification that they are safely below the fatigue limits.²⁶

Possible solutions: Use of advanced zirconium alloys that are less susceptible to shadow corrosion, channel materials with less irradiation induced growth, fuel management guidelines that limit fluence gradients and track control blade exposure, and core design strategies that rechannel the assemblies exhibiting excessive channel bow for early in life control for the subsequent reloads are all effective methods to control channel bow and its impacts, and are examples of practised solutions.

In addition, nuclear power plant operators need to ensure that the fuel vendor’s fuel mechanical design calculations account for any impacts from load following operations and improve foreign material management procedures to effectively reduce debris.

5.3.3. Control rods/blades

Potential impacts of flexible operation: The following aspects of flexible operation are considered impacts on the design function:²⁷

— Flexible operation achieved via control rod motion will likely reduce control rod lifetime. Control rods/

blades are designed to a specified mechanical and nuclear lifetime. Their mechanical lifetime is limited by swelling of the absorber material (e.g. silver–indium–cadmium or boron carbide), while the nuclear lifetime is related to the reactivity worth (e.g. loss of absorber strength). The control rod lifetime is usually expressed in terms of neutron fluence or absorber material depletion. BWR operating experience shows that, depending on design attributes (e.g. clearance between boron carbide capsule and absorber tube), control blades may be either mechanical or nuclear lifetime limited. BWRs and PWRs have experienced mechanical failures of control rods/blades due to excessive swelling of absorber material.

— Flexible operation may necessitate more frequent control rod insertion, which would promote more channel distortion due to shadow corrosion. BWR operating experience has shown that without proper attention, control blade interference due to channel distortion may result in slow to settle/inoperable control blades and forced outages. While no operating experience exists (due to normal all rods out operation), PWR Zircaloy guide tubes may be susceptible to shadow corrosion and enhanced growth (due to close proximity to stainless steel control rods). Hence, PWR flexible operation achieved with control rod insertion may introduce this phenomenon, and its contribution towards guide tube growth would need to be evaluated.

— Control rod/blade lifetime (e.g. fluence) must be carefully monitored during flexible operation. A through wall crack in the control blade absorber tube or control rod cladding may expose absorber material to the RCS coolant, resulting in leaching of the absorber material and eventual loss of negative reactivity (worth). Safety analysis requires a prescribed amount of shutdown margin, which is usually dictated in plant TSs. Leaching of absorber material (leading to loss of control rod/blade worth) is difficult to detect during operation.

— BWR control blade interference due to channel distortion that results in significantly increased friction must be avoided. For PWRs, any enhanced guide tube growth resulting from rod insertion must be accounted for in the fuel assembly design. Excessive guide tube growth would result in hard contact with core support plates (e.g. bottoming out of the top nozzle spring) and promote guide tube and assembly bow, which may eventually affect control rod insertion.

Possible solutions: The following solutions have been employed to increase control rod/blade lifetime:

— In high neutron flux regions of the control rod/blade (e.g. tip of the rod or end of the blade wing), replacing the absorber material with a material less susceptible to swelling.

26 Fuel vendor investigations and evaluations support the conclusion that alternating stresses in fuel assembly components are generally below the fatigue limits; however, this conclusion needs to be validated for any cases that are not considered in the fuel vendor evaluations.

27 The design function is to provide a means of global and local reactivity control and reactor shutdown. Control blades must remain operable to perform their design function.

— Increasing the tolerance between the absorber material and the outer tube wall to accommodate swelling.

— Increasing the end phase of the absorber to provide additional space for swelling.

— Improving the design methodology based on a correlation between the absorber swelling and the fast neutron flux (which is measured up to very high fluence values).

— Reducing conservatism in individual rod control cluster assembly design limits, which can be achieved by considering the rod control cluster assembly specific conditions, derived from one time measurements of hoop strain (equivalent to determining the as-built gap size). By using this methodology and applying the same strain criterion, the design lifetime can be increased.

5.3.4. Control rod drive systems

Potential impacts of flexible operation: Flexible operation achieved via control rod motion will likely reduce the lifetime of control rod drive system (CRDS)/CRDM components, as they are active components. More frequent control rod/blade motion will increase fatigue and wear on CRDS components. Failure of the CRDS/CRDM to perform its intended function may result in inoperable control rods, forced outages and loss of shutdown margin.

In the worst case, the rods/blades do not insert when called upon during a reactor trip or the scram insertion times exceed those assumed in the safety analysis. The additional fatigue and wear may result in a higher probability of plant transients (e.g. inadvertent control blade withdrawal or control rod drop).

Possible solutions: In order to ensure that the control blades/rods remain operable to provide a means of global and local reactivity control and reactor shutdown during flexible operation, control rod usage and remaining life have to be monitored and managed. Therefore, a monitoring system that counts and records control rod motion (i.e. number of steps) for comparison to design CRDS/CRDM lifetime and increased frequency of maintenance of CRDS/CRDM components is necessary during flexible operation. The following specific solutions have been employed in the plants that are performing (or designed to perform) flexible operation to avoid causing CRDSs/

CRDMs to exceed their lifetime:

— Reassignment of control rods in the control banks to rotate mission time among the CRDSs/CRDMs (German approach);

— Replacement of CRDSs/CRDMs prior to reaching end of life (French approach).

5.3.5. Core detectors

Ex-core detectors provide a measurement of total neutron flux and flux distribution. Signals from ex-core detectors are used to monitor reactor power and axial power distribution (e.g. power tilts) and to initiate alarms and trips. In-core detectors provide a measurement of three dimensional (3-D) neutron flux distribution in the core. Plant operators may need to consider the potential impacts of flexible operation on predicted core power distribution and safety margins.

5.3.5.1. Ex-core detectors

Potential impacts of flexible operation: Flexible operation, especially when achieved via control rod insertion/withdrawal, may alter the relationship between peripheral core power (as measured by ex-core detectors) and interior core power. Many plants rely on fast responding, safety grade ex-core detectors to measure the total core power level and to initiate a timely reactor trip signal if the power exceeds a prescribed set point.

In addition, ex-core detectors monitor axial power distribution to ensure that the limits assumed in safety analysis are preserved. These ex-core instruments are periodically calibrated to other measurements of reactor power. A change in the relationship between interior to peripheral core power would effectively change the calibration of the ex-core detectors. This issue may necessitate a change to the periodicity of the ex-core calibration during and following flexible operation.

Possible solutions: In order to address the change in calibration of ex-core detectors, a thermal reactor power signal is generated as a combination of the average loop temperature rise and the ex-core neutron flux signals. In

German PWRs, for example, the neutron flux signals are the inlet temperatures corrected and primarily used to speed up the rather slow temperature rise signal in case of transient perturbations. Therefore, the consequences of possible changes in calibration of ex-core signals are minimized. The axial power shape is continuously assessed and monitored by fixed in-core detectors (SPNDs), calibrated in absolute units (watts per centimetre) used to control axial power shape and limit peak power density in the upper and lower core halves.

5.3.5.2. In-core detectors

Potential impacts of flexible operation: Coupling coefficients are generated on the basis of the predicted power distribution and used to generate 3-D power/flux profiles of the core based on in-core instrument signals.

Power manoeuvring, especially unplanned manoeuvring relative to core depletions, may alter the power and burnup profile across the core (relative to these coupling coefficients). As such, core reload depletions and coupling coefficients may need to be continuously evaluated following power manoeuvring.

Detailed 3-D power/flux profiles are generated periodically to measure or confirm power peaking factors used in the monitoring of important safety margins, including maximum LHGR and DNBR/critical heat flux limits.

Plant monitoring and protection systems often include control room alarms and reactor trips used to preserve these safety margins. Flexible operation creates two concerns related to this issue: (a) the accuracy of coupling coefficients and the resulting power profile and peaking factors and (b) the periodicity for generating the 3-D power/flux profile during and following flexible operation. The second item becomes even more important for reactor designs without fixed in-core detectors (e.g. transverse in-core probes).

Possible solutions: The following solutions have been employed to address changes in calibration of in-core detectors in German light water reactors where the in-core instrumentation concept is an important part of the I&C structure. It combines two complementary systems that are linked by the calibration process (Annex I provides more details of these systems):

— In PWRs, a fast flux mapping system called the Aeroball Measuring System is used to assess the 3-D flux and power density distribution, verify core conformity, determine key safety parameters (e.g. local peak power density), determine the steady state DNBR and the margins to the OLCs under normal operating conditions, and calibrate the SPNDs. In addition, a monitoring system employing the SPNDs is used for continuous monitoring of the power density distribution. Their signals are provided for core surveillance, axial power shape control, monitoring signal generation to represent core conditions (e.g. peak power density and DNBR) and ensuring compliance with the OLCs by means of the limitation system, local core protection and termination of fast transient perturbations.

— BWRs use traversing in-core probe systems instead of the Aeroball Measuring System, with which the in-core power distribution detectors are regularly calibrated during operation.

5.3.6. Core shrouds

Potential impacts of flexible operation: Under cyclic loadings, fatigue of core shrouds can be aggravated by flexible operation, especially when large temperature transients occur in the material. However, the design of core shrouds anticipates a certain number of oscillations based on load cycling spanning the entire lifetime. Accordingly, the number of load cycles has to be included in the design by setting relatively high cycle counts to bound the anticipated value. The loadings associated with the respective load cycles have to be determined.

Possible solutions: The solutions that have been employed to address thermal loads and fatigue phenomena are discussed in Section 5.2.1. Accordingly, for the core shrouds:

— Fatigue loadings ought to be well controlled by a suitable design (i.e. material selection and dimensions) and by proper operation to minimize temperature changes of the core shrouds.

— By comparing the history of service conditions, such as load cycles or operational transients, with the design assumptions documented in the thermal load specifications, it can be ensured that the core shroud design imposes no restrictions on the flexible operation of the plants.

— If deemed necessary, additional periodic, non-destructive testing at specified intervals may be performed.

— Specific attention has to be paid to weld seams, especially if they are not worked over in an appropriate manner.

5.3.7. Boiling water reactor jet pumps/pressurized water reactor coolant pumps

Potential impacts of flexible operation: BWR jet pumps will be subject to higher load cycling due to flexible operation and can be exposed to increased wear and tear. In addition, increased chemical impurities (e.g. aluminium, calcium, magnesium or silica) due to the use of flexible operation may have an impact on PWR RCP seals.

Possible solutions: Increased periodic monitoring, visual inspection and non-destructive testing of jet pump assemblies (in BWRs) and pump seals (in PWRs) would be a good practice to follow the effects from fatigue or water chemistry. For example, in French PWRs, pump seals are inspected during periodic maintenance outages (i.e. every other refuelling outage).²⁸

Measures to control chemical impurities may be necessary if there is evidence of coolant chemistry effects.

5.3.8. Steam dryers

Potential impacts of flexible operation: BWR steam dryers are subject to higher load cycling during flexible operation, which may result in an increased probability of thermal fatigue cracking. High cycle fatigue may also become relevant due to fluctuating, intermittent thermal loads aggravated by flexible operation.

Possible solution: On-line inspection and monitoring for damage and wall thickness deterioration should be performed.

5.3.9. Pressurizers

Potential impacts of flexible operation: Pressurizers and associated components are subject to higher thermal cycling due to load following and frequency control. As high cycle fatigue may occur owing to fluctuating and intermittent thermal loads aggravated by flexible operation, this becomes particularly relevant for the components that are being operated in an unfavourable operating range. The specifically affected components are surge/spray lines and nozzles, as well as pressurizer welds.²⁹

Possible solutions: The solutions for increased thermal cycling as a result of flexible operation can be implemented in the pressurizer component design or by controlling the number of cycles in the following ways:

— Pressurizer level and pressure control system modification, which can reduce the number of cycles with set point modifications;

— Use of materials and thermal sleeves that are more durable for thermal cycling during the mechanical design

— Use of materials and thermal sleeves that are more durable for thermal cycling during the mechanical design