Analytical aspects of flexible operation

Flexible operation may challenge the underlying assumptions within the licensing basis of an existing nuclear power plant or of generic design and licensing of a new plant, as well as the plant technical specifications (TSs) and the safety analysis report. As such, a comprehensive assessment of the plant licensing basis is required prior to undertaking flexible operation. This includes the initial safety assessments [39] and periodic safety reviews [40].

In some cases, depending on the Member State regulatory requirements, prior review and approval by the regulatory body are required before implementing flexible operation. The subsections below briefly discuss the potential impacts of flexible operation on key analysis supporting the licensing basis as well as SSC performance.

The analysis associated with particular phenomena and components is further discussed in the sections describing the phenomenon and component aspects.

TABLE 3. AGGRAVATED PHENOMENA BY FLEXIBLE OPERATION AND AFFECTED PLANT SYSTEMS, STRUCTURES AND COMPONENTS

Component (section of this publication)

Fuel rods (5.3.1) Fuel assemblies (and fuel channels for boiling water reactors) (5.3.2) Control rods (5.3.2) Control rod drive systems (5.3.4) Core detectors (5.3.5) Core shrouds (5.3.6) Boiling water reactor jet pumps (5.3.7) Reactor coolant pumps (5.3.7) Steam dryers (5.3.8) Pressurizers (5.3.9) Steam generators (5.3.10) Chemical and volume control systems (5.3.11) Valves and pipes (5.3.12) Turbine components (5.3.13) Other conventional island mechanical components (5.3.14) Conventional island electrical components (5.3.15) Instrumentation and process control components (5.3.16)

Phenomenon (section of this publication) Fatigue

(5.2.1) X X X X X X X X X X X X X

Erosion/corrosion

(5.2.2) X X X X

Wear and tear

(5.2.3) X X X X X X X

Core power redistribution

(5.2.4) X X

Pellet–cladding interaction

(5.2.5) X

Creep induced channel distortion

(5.2.6) X X

Extended operating cycle

(5.2.7) X X X X X

Chemical impurities

(5.2.8) X X X X X X

Ageing

(5.2.9) X X X X X

Note: ‘X’ denotes the aggravated phenomena.

5.1.1. Safety analysis 5.1.1.1. Accident analysis

As described above, a nuclear power plant licensing basis captures the boundary of plant operation and response of SSCs designed to mitigate the consequences of AOOs and DBAs to ensure safe operation. However, a presumption of baseload operation may exist in the plant licensing basis, and this would affect the scope of the safety analysis report. For example, the licensing basis of a plant may limit the safety analysis of AOOs and DBAs only to reactor startup conditions (subcritical up to hot zero power) and full power conditions. This limitation is based on the assumption that a plant does not operate for a significant duration at intermediate power conditions. If that is the case, flexible operation will challenge this underlying assumption in safety analysis, as plant manoeuvring will likely result in being off those specified nominal conditions for reactor power redistribution, xenon oscillation, steam generator (SG) and pressurizer liquid levels, core flows, control rod insertion, etc.

Therefore, the plant’s response to AOOs and DBAs initiated from previously off-nominal conditions (i.e.initiated at intermediate power conditions) may become necessary due to a higher probability of occurrence or a more severe consequence, resulting from longer and more frequent operation at those conditions. With those power levels where operational transients may occur, other associated initial plant conditions (e.g. the possible deformation of the shape of the axial power distribution by control rod manoeuvring) have to be taken into account.

The amplitude of such perturbations has to be limited in order to avoid a violation of the OLCs after power changes.

Consequently, if the results of the analysis show more severe consequences, other plant protection and control requirements, as well as additional OLCs, might be needed.

For example, the occurrence of design basis seismic and loss of coolant accident (LOCA) transients initiating during operation at reduced power plateaus may not have been fully addressed in the design bases for components.

In such cases, the impacts of new plant initial conditions resulting from flexible operation that were previously not part of normal operation on existing safety analysis space need to be reconsidered. During initial design of plants, on the analysis space, the design basis may not have included analysis from intermediate power levels in all cases, considering the scenario is a limited time plant state, where the plant would be operated at full RTP for the vast majority of the time. As a result, as higher temperatures and higher power would be more limiting in terms of consequences, the design basis analysis for seismic, LOCA forces and LOCA short term mass/energy releases could be performed only at full power conditions. If that is the case, the seismic, LOCA forces and LOCA short term mass/energy release analysis at full power conditions have to be revised to specifically address operation at reduced power for an extended period of time with a reduced hot leg temperature, as it becomes a part of normal operations. Consequently, these evaluations may require the component and hot leg piping support gap conditions to be addressed, as the physical gap between a component and its support may deviate from the ‘zero gap’ assumption based on operation at full power for the majority of the time.

Additionally, for a plant design or configuration that has protection and control systems optimized for baseload operation, there might be the possibility of new plant transients being created or the probability of consequences of previous transients being increased. Those transients that were not previously evaluated would require analysis, either to determine readjustment of protection and control system parameters (e.g. process set points or the specific range) for which they have been tuned, or to demonstrate the acceptability of consequences. This may be an issue particularly for those plants that are being converted from baseload to flexible operation.

To minimize reload design costs, many plant owners/operators have opted for cycle independent reload safety analysis, which establishes an enveloping analysis space based on bounding input and assumptions, instead of reperforming safety analysis for each core reload design. Instead, the validity of this envelope is confirmed for each reload design by targeted parameter checks, also referred to as the reload checklist process, by comparing cycle specific core physics and reload parameters to bounding parameters that form the bases of the safety analysis (the ‘analysis of record’). This approach reduces core reload design efforts; however, it also reduces operating margin and fuel management flexibility. Flexible operation requires larger operating margins (e.g. a wide band on the allowable axial power distribution and control rod insertion limits) than baseload operation. Thus, in order to support flexible operation margins, the plant owners/operators who utilize cycle independent bounding analysis may need to migrate to cycle specific core reload safety analysis based on as-built core loading and as-burned reload depletions. This approach improves the operating margins and fuel management flexibility; however, it increases the reload design cost. Planned power manoeuvring (daily load following, end of cycle coastdown to

manage timing of refuelling outages, etc.) could be built into core reload depletion and safety analysis. However, unplanned power manoeuvring may alter power and burnup profiles, change core physics parameters and necessitate additional reanalysis.

5.1.1.2. Component design analysis

Component design analysis demonstrates that all plant SSCs which are important to safety conform to the design requirements under all operational modes throughout their intended lifetime. They provide a demonstration that all relevant functional and regulatory requirements have been considered adequately. For systems important to safety, this demonstration is supported by the assessments of design function performance under potential operational conditions. The assessments for overall reliability complement these assessments by considering the applicable failure modes and effects.

Any increase in thermal and mechanical cycling as a result of flexible operation will adversely affect evaluation for components with respect to fatigue, wear, erosion/corrosion, ageing, etc. For systems important to safety, the deviations from the existing component design assumptions, failure modes and effects that demonstrated insufficient system and design capacity to perform the safety functions throughout the intended lifetime in all operational modes must be reviewed and addressed. These include the expected response to static and dynamic mechanical loads and their behaviour, and a description of the effects of flexible operation on the ability of the SSCs to perform their safety functions adequately over the lifetime of the plant, including any changes for human and machine reliability.

Similarly, for systems not important to safety, evaluations have to ensure that the system changes due to flexible operation preclude the possibility of affecting safety system performance, as well as efficiency and availability. In particular, the operating conditions of secondary system components will change, thus affecting their design assumptions, such as turbine efficiency. These include changes in the efficiency of each component, as well as the overall heat balance of the secondary system, which may require reanalysis of heat balance calculations for various power levels, considering that the percentage change in REO will be neither equal nor proportional to the percentage change in RTP in the proposed operational spectrum.

Although in most cases, the extent of cycling is bounded by conservative lifetime assumptions, they have to be confirmed, and measures have to be taken to monitor that they will remain bounded. The effects of service on the performance of design functions, including surveillance, inspection and maintenance programmes, need to be described.

In addition, as noted above in the accident analysis, the component design analysis can be affected by the creation of new conditions if the plant’s design or configuration basis assumptions are based on the response and performance of protection and control systems that are optimized for baseload operation. These assumption and design conditions would require evaluations either to determine the readjustment of protection and control system parameters (e.g. process set points of the specific range) for which they have been tuned, or to demonstrate the acceptability of component performance under these new design conditions.

5.1.1.3. Risk informed analysis

In addition to the traditional deterministic safety analysis, nuclear power plants may utilize probabilistic risk analysis to ‘risk inform’ plant O&M. Such risk analysis models usually calculate core damage frequency and large early release frequency.

Flexible operation has the potential to increase the probability of equipment failure (e.g. transformers or feedwater control systems), plant transients (e.g. loss of feedwater or power) and reactor trips, which may affect the predicted core damage frequency and large early release frequency. A systematic review of inputs to the probabilistic risk assessment and to the assumptions and supporting sensitivity analysis to address the potential impacts on equipment and operations may be necessary to support flexible operation.

5.1.1.4. Prediction of core physics parameters

Core reload depletions and core physics calculations provide input to downstream safety analysis, plant monitoring and protection systems, and operator guidance. Physics inputs include reactor startup predictions for

core parameters (e.g. critical boron concentration and rod position), reactor kinetics, fuel temperature coefficients, moderator temperature coefficients, control rod and bank worth, and power peaking factors. Predicted core physics parameters are based on predetermined plant operation schemes and plans. Unplanned power manoeuvring during flexible operation may alter the power and burnup profile across the core (relative to the predictions). As such, core reload depletions and physics calculations may need to be continuously checked and evaluated following power manoeuvring, depending on the instrumentation utilized for monitoring.

5.1.1.5. Prediction of instrumentation and control system settings

Flexible operation will increase the use of I&C systems to accommodate power changes. Operation at lower powers will utilize the I&C systems for long periods of time beyond the specific range for which they have been tuned. This may require design evaluations to determine the effects on instrument uncertainties and set points.

Furthermore, for existing plants being converted to flexible operation, there may be changes to plant control systems (e.g. reactivity, temperature and pressure control systems) that are associated with the changes to plant operation. These are further discussed in the sections on phenomenological and component aspects, below.

In addition, continuous operation at less than full power may extend the operating cycle beyond the existing calibration period for the various plant instruments considered in the design calculations that determine the overall instrument uncertainty and define field set points. An increase in the calibration period may negatively affect these calculations and alter the analytical set point credited in the accident analysis.

Therefore, design I&C calculations (and any necessary changes to safety analysis) to bound anticipated cycle length, operation range and operating schemes with flexible operation need to be reviewed and revised, if needed, with respect to uncertainties, set points and calibration.

5.1.2. Analytical models

Validated analytical models are used to demonstrate the performance of SSCs designed to mitigate the consequences of AOOs and postulated DBAs. The ability of these models to accurately simulate the SSC response to plant events may be affected by flexible operation.

Plant system models are based on the first principles of thermohydraulics. However, variables within the model algorithms are used to calibrate the model predictions to known operating conditions. For example, primary to secondary heat transfer is adjusted to achieve operating SG pressure in a PWR. Many of these operating parameters are power dependent and will need to be adjusted to support flexible operation. Another example is calculation of the SG liquid inventory, which is an important parameter for the postulated main steam line break DBA, and which increases at lower power levels in U-tube SG designs.

Fuel rod thermal and mechanical models are used to demonstrate compliance with regulatory performance requirements. These models are usually calibrated against a large empirical database of separate effects testing (e.g. irradiated mechanical property measurements) and in-reactor data (e.g. in-pile fuel temperature measurements or rod puncture fission gas release measurements). Flexible operation involving frequent power manoeuvring, especially when driven by control rod movement, may challenge the accuracy of these fuel performance models.

Data from power ramp testing, prototypical of the power oscillations experienced during flexible operation, are used to confirm the accuracy of these models.

5.2. PHENOMENOLOGICAL ASPECTS OF FLEXIBLE OPERATION