Impacts at plant level

In the absence of economically viable large scale energy storage — other than in pumped storage hydroelectric plants, where it is available — the ability of nuclear power plants to operate flexibly is limited by physical constraints and economic profitability. While there may be net benefits of increased plant flexibility at the grid and portfolio levels, the deterioration of a plant’s profitability is considered to be one of the major economic risks associated with flexible operation at the plant level. The first channel through which the profitability of a plant can be affected is related to the potentially higher plant costs associated with flexible operation. The literature distinguishes between four different cost categories that are likely to be affected when flexible operation, especially load following, mode is introduced. These cost categories are described schematically in Fig. 33.

36 The latest edition in this series publication of Ref. [57] is the 2016 edition.

6.2.1.1. Capital costs

Flexible operation will require capital costs associated with a design that is compatible with flexibility needs, or conversion of existing units to acquire those needs, owing to the technical and programmatic changes discussed in Section 5, if these have not already been considered in the design or installed in the facility. Capital costs associated with a design compatible with flexible requirements — or conversion of an existing facility — would accrue from additional studies and replacement equipment for the modified design/facility, as well as requalification, retesting and amended licensing processes.

If existing generating units were not configured for the proposed capability of flexible operation that was discussed in Section 4, retrofitting of equipment (operating procedures, as well as capital improvements) will be a major factor in capital costs. Additionally, grid system owners/operators and nuclear power plant owners/operators will make their decisions on which units to operate flexibly based on economic factors, including unit size, plant age, plant configuration and fuel costs. For economic reasons, plant owners are more likely to be inclined to continue operating existing units with marginal upgrades, instead of undergoing major equipment retrofits to improve plant flexibility.

Similarly, the purchasers of a new plant would prefer less change to the standard design to minimize their investment and financing. For the same level of flexibility, however, the associated capital costs may be different for a new design than for converting an existing facility. For example, additional investment could be needed in I&C to become eligible for operation in flexible mode, especially for earlier vintage nuclear power plants. An upgrade of existing plants — through retrofits and the installation of advanced control systems — would also be needed to provide better monitoring of physical wear, while these advanced control systems are readily available in the newer vintage of plant design. The capital costs will vary for each specific unit and cycling conditions. Figure 34 illustrates one example of capital costs for initial design and later design upgrades for a sample generating unit and their impacts on plant life and forced outage, in terms of the equivalent forced outage rate (EFOR).

In the case of a nuclear unit, there are additional issues that must be addressed and that do not come into play for fossil fuel plants: regulatory requirements and licensing commitments. These additional requirements and commitments may lead to major plant modifications, activities and programmes that might not be required for a fossil fuel generating unit and that will also add to the cost of flexible operation in nuclear power plants.

6.2.1.2. Operation and maintenance costs

A major cost that has been overlooked, while the initial capital costs are investigated, is the life cycle cost of operational changes. Life cycle costs include equipment and maintenance expenditures for component upgrades, equipment life reduction and potential derating due to equipment degradation associated with flexible operation

FIG. 33. Plant cost categories affected by using the load following mode.

over the remaining lifetime of a nuclear power plant. These costs become higher as the depth and periodicity of load cycling increase. The plant owner/operator needs to know the effect of flexible operation on the plant equipment in the long term, in terms of overall reliability, in order to decide how to allocate and recover costs.

In addition, the nuclear power plant owner/operator needs to establish a formal way of predicting future costs so that system changes can be designed to minimize the total cost. Flexible operation will cause an increase in O&M costs, and the magnitude of those costs will depend on the extent of flexibility needs and also on the properties and programmes that exist for design and operation. As discussed in Section 5, some of these equipment and programme related effects include:

— Wear on components due to repeated use (e.g. CRDMs) and to vibrations and changes in temperature.

Operating lifetimes of SSCs may be affected in some cases, as frequent load changes translate into higher wear and tear of equipment.

— Load following can induce more frequent maintenance, while reducing the availability of power plants in terms of increased outage frequency/duration or increased risk of equipment degradation or failures, as well as additional monitoring and surveillance.

— Waste management changes may be considered in the O&M costs associated with flexible operation, such as control of liquid waste in PWRs, as they potentially result in additional operation costs.

It is important to note that plant level costs are unique to every unit. Several factors affect the cost of flexible operation at a plant, and differ from one site to another, including the following:

— Unit design and vintage of technology;

— Equipment design and manufacturer;

— Size (output capacity in megawatts);

— Economies of scale;

— Plant configuration;

FIG. 34. Example of relationships among capital improvements and forced outages for a 600 MW(e) thermal plant (courtesy of N. Kumar, Intertek, reproduced from Ref. [50]).

— I&C technology;

— Retrofit of equipment (operating procedures as well as capital improvements), for increased flexibility;

— Operation strategy and methods used to perform flexible operation (e.g. grey rods, pump speed or turbine bypasses);

— Past maintenance related activities;

— Age and condition of SSCs, including the effectiveness of ageing management programmes;

— Time between planned outages;

— Timing of major overhauls such as turbogenerator overhauls;

— O&M and capital expenditures with respect to forced outage rates;

— Past cycles and the characteristics of past cycles (e.g. number of planned or unplanned shutdowns (hot or cold), startups³⁷, power changes and extended low power operation);

— Strength of operator training and operating procedures.

Using plant specific and operational experiences, together with industry data for similar plants, the O&M costs can be estimated for each specific unit. This estimation utilizes unit composite damage accumulation models and actual and historical data on the component and system level, as well as experience collected by plant staff. There are several tools developed for analysis via statistical regression and benchmarking of these inputs, converting them to cost estimations applicable to each case or an industry average. Figure 35 presents one of the methods that is sampled from Ref. [50] and provides the methodology and elements of plant cycling costs accounting for changes to:

— System production costs (including fuel, radioactive waste and auxiliary power);

— Forced outage recovery costs;

— Maintenance and overhaul costs;

— Long term and short term efficiency costs;

— General engineering and management costs;

— Capital costs of cycling improvements;

— Regulatory/licensing (nuclear and environmental) costs.

The change in each category represents the increase/decrease in costs associated with load following. Costs associated with changes in production and system upgrades/analysis are relatively straightforward to estimate.

Costs that are very often difficult to quantify are associated with increased failure rates (owing to lower availability, increased EFOR and increased time of scheduled outages) and decreased component life (resulting in higher capital and O&M expenses).

While failure rate increases may not manifest themselves immediately on cycling, eventually, increases in component failure rates occur, resulting from components reaching end of life sooner, which causes higher plant forced outage rates. How soon these detrimental effects will occur will depend on the amount of cumulative damage present and the nature and frequency of load following. Figure 36 illustrates cumulative damage predictions — in terms of EFOR — based on the scenarios considered in Ref. [50]. As can be seen, early in the plant lifetime, the impacts manifest themselves later compared to faster manifestation as the plant ages.

6.2.1.3. Fuel costs

Operation at less than full load will have an impact on fuel costs. This is because fuel will likely be used in a non-optimal manner, and in some cases, fuel management may not be optimized in anticipation of the next several fuel cycles. Fuel costs may also be affected by decreases in the thermal efficiency of the power conversion (secondary) system, particularly by low turbine efficiency rates at low power operations.

In practice, it is difficult to quantify fuel cost impacts due only to load following, as they are difficult to distinguish from those that are attributable to other fuel cost drivers.

37 Typically, load following or transient reductions in power are much less damaging and operationally easier to achieve, so they are much less costly than shutdown and startup cycling.

6.2.1.4. Staff costs

Staff needs to be permanently available to adjust the load frequently, and, in some cases, unexpectedly, because ramping operations for nuclear plants are not always automated. Additionally, initial and continuing training (technical knowledge, and industrial, nuclear and radiological safety) of current and additional personnel, who will be needed for additional/revised monitoring, surveillance and maintenance and for more frequent or brisk plant system interventions, will be an additional cost to consider.

FIG. 35. Typical elements of methods to predict load following costs (courtesy of N. Kumar, Intertek).