Base load vs Load Follow
Nuclear power plants may take many hours, if not days, to startup or to change their power output. Modern power plants can and do operate as load following power plants and alter their output to meet varying demands. But base load operation is the most economical and technically simple mode of operation. It is primarily due to the fact, they require a long period of time to heat up the nuclear steam supply system and the turbine-generator to operating temperature. From this point of view power plant generally are divided into two basic categories:
- Base Load Power Plant
- Load Following Power Plant
Base Load Power Plant
In general, nuclear power plants (NPPs) have been considered as base load sources of electricity as they rely on a technology with low variable costs and high fixed costs. This is the most economical and technically simple mode of operation. In this mode, power changes are limited to frequency regulation for grid stability purposes and shutdowns for safety purposes. Different plants and technologies may have differing capacities to power changes on demand. Base load power plants are generally run at close to maximum output (100% of rated power) continuously (apart from maintenance, refueling outages). During refueling, every 12 to 18 months, some of the fuel – usually one third or one quarter of the core – is replaced by a fresh fuel assemblies. After refueling the reactor is usually started up and operated again at nominal power.
Although most of nuclear power plants were designed as base load power plants, today, the utilities have had to implement or to improve the manoeuvrability capabilities of their NPPs in order to be able to adapt electricity supply to daily, seasonal or other variations in power demand. It is due to the share of nuclear power in the national electricity mix of some countries has become large, and due to the significant increase renewable energy sources.
Load Following Power Plant
As was written, nuclear power plants (NPPs) have been considered as base load sources of electricity as they rely on a technology with low variable costs and high fixed costs. However that situation is changing in several countries. Today, the main motivation for load following with nuclear power plants comes from the large-scale deployment of intermittent electricity sources (like wind and solar power). An intermittent energy source is any source of energy or electrical power that is not continuously available due to some factor outside direct control. For example, in the absence of an energy storage system, solar does not produce power at night or in bad weather and varies between summer and winter.
On the other hand, electrical energy must be generated at the same rate at which it is consumed. A sophisticated control system is required to ensure that the power generation very closely matches the demand. If the demand for power exceeds supply, the imbalance can cause power plants and transmission equipment to automatically disconnect and/or shut down to prevent damage. In the worst case, this may lead to a cascading series of shut downs and a major regional blackout. The challenge is not only technical. Due to the sudden influx of large amounts of wind power, German, Dutch, Czech and Austrian power markets have experienced several hours of negative electricity prices in recent years and many more hours with prices that were lower than the variable costs of nuclear power plants, which have the lowest variable costs among the large-scale established power sources. Moreover the load follow operation is economically very inefficient as nuclear power generation is composed almost entirely of fixed and sunk costs. Therefore, lowering the power output doesn’t significantly reduce generating costs and the plant.
To manage this problem, the utilities have had to implement or to improve the manoeuvrability capabilities of their NPPs in order to be able to adapt electricity supply to daily, seasonal or other variations in power demand. The power plant with this improved manoeuvrability is known as load follow power plant. In general, load following power plant is a power plant that adjusts its power output as demand for electricity fluctuates throughout the day. Load following plants are typically in-between base load and peaking power plants.
For example, according to the current version of the European Utilities Requirements (EUR) the nuclear power plant must be capable of daily load cycling operation between 50% and 100 % of its rated power, with a rate of change of electric output of 3-5% of rated power per minute.
From the reactor operation viewpoint, in the case of pressurised water reactors (PWRs), the reactor thermal power and the global power distribution (especially the AFD) are controlled by manipulating several factors which affect the core’s reactivity. In PWRs, these factors are especially:
- position of control rods,
- concentration of boric acid in the RCS
- core inlet temperature
A control rod is removed from or inserted into the reactor core in order to increase or decrease the reactivity of the reactor (increase or decrease the thermal power). For example, control rods insertion causes an addition of new absorbing material into the core and this causes a decrease in thermal power.
Boric acid is used to compensate an excess of reactivity of reactor core along the fuel burnup (long term reactivity control) as well as to compensate the negative reactivity from the power defect and xenon poisoning during power increase to nominal power.
Special attention requires the axial flux difference control. The axial flux difference, is defined as the difference in normalized flux signals (AFD) between the top and bottom halves of a two section excore neutron detector:
AFD or ΔI = Itop – Ibottom
The AFD is sensitive to many core related parameters such as control bank positions, core power level, axial burnup, axial xenon distribution and, to a lesser extent, reactor coolant temperature and boron concentrations.
During power maneuvers, the operating scheme used to control the axial power distribution involves maintaining the AFD within a tolerance band around a burnup dependent target, known as the target flux difference, to minimize the variation of the axial peaking factor and axial xenon distribution.
For plants operating with so called “constant axial offset control – CAOC”, the AFD limit involves a target band. This target band is, for example, +5% and -5% around the target value. Since the xenon concentration and distribution is flux and time dependent, the longer the core operates outside its target band, the greater the probability of initiating a xenon transient.
Load Follow without Boron Adjustment
Smaller load changes does not require boron adjustment. For a power decrease, without using boron addition to overcome the positive reactivity added by the power defect, the control rods must be inserted to maintain Tavg. From this point on with a continuing decrease in power, pressure inside the steam generator will increase adding negative reactivity.
Load Follow with Boron Adjustment
More extensive maneuvers usually require boron adjustment to compensate for xenon reactivity changes, thereby keeping the control rods within its axial flux distribution (AFD) target band. Boron concentration changes during plant operation are required whenever axial flux difference (AFD) would extend outside the allowable target band. As was written, this target band is, for example, +5% and -5% around the target value. Following the power level change made by the control rods, the plant operator or the control system makes boron concentration adjustments whenever the AFD approach the limits of the target band.
Grey control rods
Some nuclear power plants use so called “grey control rods” for load following. These plants have the capability to make power maneuvering between 30% and 100% of rated power, with a slope up to 5% of rated power per minute. They can respond very quickly to the grid requirements. In order to fulfill these requirements without introducing a large perturbation of the power distribution, special control rods have to be used. These control rods are called “grey” control rods. Grey control rods use a grey neutron absorber, which absorbs less neutrons than a “black” absorber. Consequently, they cause smaller depressions in the neutron flux and power in the vicinity of the rod. The rod control system automatically modulates the insertion of the axial offset (grey) control bank, which controls the axial power distribution.
In the case of the boiling water reactors (BWRs), the power regulation is performed by changing the coolant flow rate (using the recirculation pumps) and/or the control rods. No boron regulation is used in BWRs.