Assessing the technical requirements for flexible operation of nuclear

The technical requirements that are requested by the grid system operators are input into the assessment to determine the nuclear power plant evaluation of whether the existing design/facility is capable of meeting those, or what changes to the design/facility need to be implemented. At this stage, an overall feasibility study on the plant capabilities is performed by the plant owners/operators, and several iterations may take place between the grid system operator and the plant owner/operator and designer, as well as the grid and nuclear regulators, to agree on what is requested and what can be provided. Table 2 gives a set of considerations to start the iterations.

TABLE 2. BASIC CONSIDERATIONS OF FLEXIBILITY IN RESPONSE TO GRID OCCURRENCES

Event Response Associated methods or parameters

Predicted daily demand

variation Load following Low power period

Power change rate

Number of occurrences per given time (seasonal, monthly, weekly) Duration at low power for longer period planned demand (extended low power operation)

Minimum power for secondary system efficiency Real time small demand

variation Frequency control Power change equivalent of frequency disturbance (amplitude, ramp rate, required control type, e.g. local/remote, manual/automatic)

Grid disturbances, large and

infrequent power variations Spinning reserves Ramp (amplitude, rate, initial power level) Step (amplitude, initial power level)

Minimum stable power level, house load capability

Instantaneous (a few per cent rated thermal power change, return to full power notice)

The key technical parameters of interest to the grid system operators for stabilizing the grid are: frequency response capability, power ramping and power output. Current practices from experiences in Member States among the stakeholders demonstrate the following:

— Frequency response capability is an important performance requirement for the grid system operator because its fast response helps to cope with the stochastic fast fluctuations of generation and demand. Grid system operators in some Member States are increasingly requesting such a capability, mainly due to the increasing share of renewable energy generating units that have intermittent and fluctuating output and that do not participate in frequency control. Those renewable energy generating units are also connected via power

inverters, which do not contribute inertia to the grid, as discussed in Section 3.2.2. On the other hand, in some Member States, frequency response is considered an ancillary service carried out by a specific market.

In those cases, the grid system operator may request the frequency response capability and the nuclear power plant owner/operator can control utilization by means of the price of providing support for frequency control.

— Power ramping is also an important performance requirement for the grid system operator when it is necessary to reduce (or increase) the generated output in balancing the electrical system and controlling the frequency if the system frequency gets too high (or too low). For compliance with this requirement, the grid system operator needs to achieve adequate power ramping, with the necessary timing, magnitude and speed.

Specifically, the grid system operator’s main criteria include the amount of power change per minute, the minimum and maximum power levels of stable operation and the allowable time of operation at minimum or intermediate power levels. However, the grid system operator trying to achieve this performance ought to also take into account the implications and impacts of power manoeuvring of a plant, especially in terms of how often it needs to be achieved (i.e. whether a few times per year, or on a weekly or daily basis), the possible ramp rate (i.e. whether it is a few or tens of per cent of RTP per hour) and the limiting duration of long term reduced power operations.

— The stabilization of the power output (steady state or after ramping) at the right level and at the right time is considered important by the grid system operator because the quality of the frequency depends also on correctly operating at the power levels scheduled. Requirements of better than ±1% accuracy and within

±3 minutes are commonly observed in Member States, as agreed by the grid and plant operators.

— The minimum electrical output at which a generating unit can operate (i.e. design minimum operating level or design minimum load point) is also an important parameter. Below that power output, in response to a grid event, the generating unit would not be able to make a rapid (typically within seconds) reduction in active power to a level that needs to be sustained for an undefined time. Lowering this minimum operation point may become necessary in the context of flexible operation of plants¹³, and for some plant designs, there could be technical challenges.

Additionally, nuclear power plants may have physical and operational limitations on the minimum operating power, particularly for:

— Extended low power operation (i.e. length of periods at low power level);

— Minimum reactor power level (i.e. minimum core power), below which a plant cannot maintain stable, long term operations without some risks to its SSCs, or during which some control and protection systems are outside their operating range.

These low power operation limitations are based on several factors, including turbine and secondary system efficiency, control and protection system ranges, and mechanical and hydraulic limits, such as vibrations in components or perturbations in the reactor parameters.

Only a few Member States have explicit nuclear power plant designs and configurations to operate in a wide range of flexibility with load following and frequency response. Specifically, France and Germany have accumulated many reactor-years of experience designing and operating their nuclear power plants in load following and frequency response modes, including AGC or AFC. Some of those plants were initially designed to have such capabilities [25], while others required modification to transition from their original baseload design in order to achieve higher capability load following and frequency response [26].

Figure 22 shows a German pressurized water reactor (PWR) flexibility scheme considered in the design phase of Konvoi technology, as an example [27]. The blue lines show the grid requirements at the time (1992) that were considered in the safety analysis and in establishing the design limits (red lines) for flexible operation. The reviewed and approved operational limits (green lines) by the regulatory body, together with the grid requirements, were confirmed and qualified during commissioning. The blue shaded area represents the typical practised range of load following and frequency control operations by German PWRs initially, providing large margins to the operational limits. The grid requirements that were based on the 1992 grid configuration and plans, meanwhile,

13 This issue is also very important for other types of generating units, particularly for coal fired units, gas turbines or combined cycle gas turbines.

have evolved, requiring expansion of the operational envelope and, in some cases, review and approval of the operational limits by the regulatory body. Currently, in some nuclear power plants in Germany, the operational envelopes have been expanded to load changes up to 8% REO with gradients up to 60% REO/min for frequency control and to load changes up to 70–80% REO with gradients up to 2% REO/min for load following, to meet the latest grid requirements (see Annex I for further details on the flexibility capabilities and requirements in Germany).

As discussed in detail in Section 5.2.5, following a refuelling outage of a PWR or a boiling water reactor (BWR), the nuclear fuel needs to be ‘conditioned’ based on the fuel design. In addition, the plant equipment must be calibrated following the extensive periodic maintenance that is typically performed together with the refuelling outage. Plants typically request about 7–14 days of no flexible operation to allow for fuel conditioning and equipment calibration. In addition, late in the operating cycle of a PWR reactor, the plant operator may limit flexible operation because boric acid, the neutron absorber used to control some reactors, becomes highly diluted. Reactor power changes late in core life result in large quantities of liquid (primary water) waste generation, which must be processed and discharged. This curtails flexible operation in the period towards the end of the fuel cycle and, depending on the core design, it may be up to a month before flexible operation can be performed again. Figure 23 illustrates an example of a French nuclear power plant where non-flexible operation periods are determined and communicated to the grid system operator.

Additionally, a few nuclear power plants in the United States of America and Europe [29, 30] have conducted studies and have experience of carrying out some load following manoeuvres in a planned manner, as described earlier in Section 2.2.1. Figure 24 shows a limited load following scheme currently practised in Belgium. Some plants in the UK have agreements with the grid system operator to carry out rapid load reductions that may be required under certain infrequent grid conditions, as described in Section 2.2.3. Annex III provides further studies and experiences of several Member States.

From operating experience, the common capabilities that are currently included in design and utilized in some nuclear power plant operations include [31]:

— A power (load) cycle between 100% and 20% RTP, sometimes on a daily basis;

— A power (load) cycle over a smaller range more frequently, or over a larger range less frequently;

— When power cycling, a ramp rate of 2% RTP per minute;

— Power adjustments of up to ±5% RTP, in the AGC mode;

FIG. 22. German pressurized water reactor flexibility requirements and designed capabilities (courtesy of H. Ludwig, AREVA GmbH, reproduced from Ref. [27]). NPP — nuclear power plant.

— Power adjustments of up to ±2% RTP within 30 seconds in the AFC mode;

— No flexibility at certain times, such as during fuel conditioning or at the end of a cycle (in PWRs);

— A minimum power for extended low power operations of 20‒40% RTP.