A review of nuclear power within the United Kingdom
Nuclear power currently provides about 16.67% of the United Kingdom's (UK's) electrical generation. Several coal-fired power plants are scheduled to be closed over the next decade or so. To meet the UK's carbon dioxide reduction emission targets of 80% by 2050, nuclear generation is set to play a large role in this particular strategy.
Author: Richard Poole
This article highlights some of the key technical issues associated with the connection of large modern nuclear power plants (NPPs) to the National Electricity Transmission Systems (NETS). First a brief summary of the evolution of NPPs is given. The importance of the electricity grid to the NPP and vice-versa is highlighted. The need for reliable and secure electrical supplies to power the safety systems of the NPP is of vital importance. Following the lessons learned from the post Fukushima Daiichi nuclear accident, a review of the newly proposed safety systems associated with the new generation of nuclear power reactor design is presented. These will possess multiple levels of redundant ‘passive’ safety systems to cater for the loss of all power supplies to the NPP and attempt to keep the reactor core cooled.
Nuclear power currently provides about 16.67% of the United Kingdom's (UK's) electrical generation. Several coal-fired power plants are scheduled to be closed over the next decade or so. To meet the UK's carbon dioxide reduction emission targets of 80% by 2050, nuclear generation is set to play a large role in this particular strategy.
Nuclear power plants (NPPs) are prone to a number of disturbances. These include equipment failures, perturbations associated with the electricity grid, human error, weather related issues and the operation of protection and control systems. Such disturbances can increase the stress on mechanical and electrical equipment, upset the operation of reactor cooling pumps and motors and disturb the interaction between the NPP and the electricity grid.
The above disturbances can cause the operation of protection systems. The protection systems can trip the nuclear power reactor (scram) and in some cases initiate an emergency shut-down of the respective control systems. The power output of the nuclear reactor can be disrupted and this can have an impact on the system frequency within the ac grid. If the ac grid is dependent on the supply from the NPP before the contingency the equilibrium and power balance within the network can be affected.
In respect of past nuclear disasters such as Chernobyl in 1996 and more recently Fukushima Daiichi, the consensus of this potentially explosive technology has been negative in the eyes of the public. However, if the National Electricity Transmission System (NETS) is to meet the large future demand, nuclear is an inevitable and feasible option .
Much larger generator units up to 1600 MWe in power rating will need to be connected into the NETS. The existing transmission system is already being stretched and more infrastructures will need to be built to accommodate such large NPP connections. The new NPPs will need to provide ancillary services such as frequency response and voltage support to the NETS when required. This increases the power cost and complexity of the design, which in turn causes project timescales to increase.
It is expected that in the future several countries will decide to build small modular reactors instead of larger units (1000 MWe or more) for safety reasons such as capital investment, construction time, among others. This tendency could be adopted by the UK in the near future.
What is a nuclear power reactor?
A nuclear power reactor is a system that contains and controls sustained nuclear chain reactions. Fuel made of heavy atoms that split when they absorb neutrons is placed into the reactor vessel along with a small neutron source. The neutrons start a chain reaction where each atom that splits releases more neutrons that cause the atoms to split. Each time an atom splits, it releases large amounts of energy in the form of heat. The heat is carried out of the reactor via a coolant, which most of the time is just plain water. The coolant heats up, produces steam and enters a turbine to spin a generator shaft which produces electricity [ 1 ]. This process is otherwise known as fission power.
The fusion reactor is expected to prevail in the market by 2050. Fusion powers the Sun and stars as hydrogen atoms fuse together to form helium, and matter is converted into energy. Hydrogen, heated to very high temperatures changes from gas to plasma, in which the negatively charged electrons are separated from the positively charged atom nuclei (ions).
Normally, fusion is not possible because the strongly repulsive electrostatic forces between the positively charged nuclei and prevents them from getting close enough together to collide and for fusion to occur. If the conditions are such that the nuclei can overcome the electrostatic forces to the extent that they come within a very close range of each other, then the attractive nuclear force (which binds protons and neutrons together in atomic nuclei) between the nuclei will outweigh the repulsive (electrostatic) force, allowing the nuclei to fuse together. Such conditions can occur, when the temperature increases, causing the ions to move faster and eventually speeds high enough to bring the ions together. The nuclei can then fuse, causing a release of energy .
Fusion reactors have been getting a lot of press recently because they offer some major advantages over other power sources. They will use abundant sources of fuel, they will not leak radiation above normal background levels and they will produce less radioactive waste fission reactors.
Pressurised water reactor
In Fig 1, the most common type of nuclear power reactor in industry is the pressurised water reactor (PWR), which uses light water as coolant. The primary cooling water is kept at very high pressure so it does not boil.
Fig 1: PWR
Water goes through a heat exchanger, transferring heat to a secondary coolant loop, which then spins a turbine. These use oxide fuel pellets stacked in zirconium tubes although thorium or plutonium fuel can also be used. PWRs use dioxide uranium as a fuel which is different to that used by the pressurised heavy water reactor (PHWR).
Pressurised heavy water reactor
A PHWR is a nuclear power reactor, commonly using enriched natural uranium as its fuel. Heavy water (deuterium oxide) is used as a coolant and moderator. The heavy water coolant is kept under pressure, allowing it to be reheated to higher temperatures without boiling.
Although heavy water is significantly more expensive than ordinary light water, it creates greatly enhanced neutron economy, allowing the reactor to operate without fuel-enrichment facilities. This lowers the additional expense created by the heavy water and enhances the ability of the reactor to make use of alternate fuel cycles.
Boiling water reactor
In Fig 2 , the second most common is the boiling water reactor (BWR) which is similar to the PWR in many ways. One of the main differences is a single coolant loop compared with two in the PWR.
Fig 2: BWR
These types of reactors were originally designed by Allis-Chalmers and General Electric (GE). The first GE United States commercial plant was at Humboldt Bay (near Eureka) in California. Other suppliers of the BWR design world-wide have included Kraftwerk Union and Hitachi .
An alternate version known as the advanced BWR contains more sophisticated controls and monitoring functions compared with the latter. This is currently being offered by GE Hitachi Nuclear energy (GEH) and Toshiba .
High-temperature gas cooled reactors
This is a specific gas-cooled reactor (GCR) that uses helium as fuel and the nuclear material is included in pellets. The correct classification of this type of nuclear power reactor is GCR. In general, this type of nuclear power reactor is cooled by carbon dioxide or helium. All GCRs are located in the UK. China is developing the so-called ‘high-temperature GCR’ and Russia and the USA are developing the so-called ‘gas turbine modular helium reactor (GT-MHR)’.
According to the International Atomic Energy Agency (IAEA) and Alternative Energies and Atomic Energy Commission, Centre d'etudes de Cadaruche (CEA), there were 18 operating GCRs cooled by carbon dioxide plus two test reactors cooled by helium worldwide in 2007. All of these units are located in the UK with a net capacity of 9034 MWe. The load factor of this type of reactor in 2011 was 68.2% (fifth place). In China, work continues on safety tests and design improvements for the 10 MWth high-temperature GCR (HTR-10), and plans are in place for the design and construction of the first power reactor prototype high temperature reactor-Pebble Module (HTR-PM).
The Russian federation and the USA continue research and development on a 284 MWe GT-MHR for plutonium burning. France has an active research and development programme on both thermal as well as fast gas reactor concepts and, in the USA, efforts by the Department of Energy to continue on the qualification of advanced gas reactor (AGR) in the 50 MWth range are planned for operation around 2017 in France. Graphite-moderated gas-cooled nuclear power reactors, formerly operated in France and still operated in the UK, are not built any more in spite of some advantages that this type of reactor has [ 2 ].
Connection of an NPP to the UK electricity grid
The introduction and development of nuclear power is a major undertaking within any country. To accommodate such large power plants as these, the country concerned has to develop the infrastructure required to construct and operate an NPP securely, safely and technically in a sound manner [ 2 ].
According to the IAEA safety fundamentals, the following ten safety principles should be followed by all governments that have decided to introduce a nuclear power programme:
- The prime responsibility for safety must rest with the person or organisation responsible for facilities and activities that give rise to radiation risks.
- An effective legal and governmental framework for safety, including an independent regulatory authority, must be established and sustained.
- Effective leadership and management for safety must be established and sustained in organisations concerned with, and facilities and activities that give rise to, radiation risks.
- Facilities and activities that give rise to radiation risks must yield an overall benefit.
- Protection must be optimised to provide the highest level of safety that can reasonably be achieved.
- Measures for controlling radiation risks must ensure that no individual bears an acceptable risk of harm.
- People and the environment, present and future, must be protected against radiation risks.
- All practical efforts must be made to prevent and mitigate nuclear or radiation accidents.
- Arrangements must be made for emergency preparedness and response for nuclear or radiation incidents.
- Protective actions to reduce existing or unregulated radiation risks must be justified and optimised. Most of the advanced nuclear power reactor designs available today are evolutionary improvements on previous designs. This situation has the benefit of maintaining proven design features and thus minimising technological risks improving, at the same time, some important features and improving.
Some important features of current nuclear power reactor designs on the basis of the lessons learned on past nuclear accidents and the experience gained in the construction of hundreds of nuclear power reactors in different countries. These evolutionary designs generally require little further research and development confirmatory testing.
Examples of commonly utilised elements of evolutionary design for improved economics are:
- Simplified reactor designs.
- Increased reactor power.
- Shortening the construction schedule, reducing the financial charges that accrue without countervailing revenue.
- Standardisation and construction in series spreading fixed costs over several units.
- Multiple unit construction at a single site.
- Self-reliance and local participation.
NPPs are unique and powerful generators compared with other types of plants such as coal fired or combined cycle gas turbine. As well as generators, NPPs are also customers. They supply large amounts of energy to the NETS, but equally rely on it to provide power for crucial safety systems, especially during emergency situations. The safe start-up and shut-down of such plants requires a stable and secure supply from the NETS, this often referred to as off-site power [ 2 ].
The NETS principal function is to transport electricity from the generating plants to the load centre (customers). A reliable and well-maintained/balanced system is crucial for bringing NPPs online as well as operating them cost-effectively during emergency situation.
The cooling system of the NPP is a vital safety system which is responsible for keeping the nuclear fuel cool after a reactor has been shut-down. On-site back up supplies in the form of batteries, diesel generators or gas turbines are present. The fewer the instabilities and interruptions there are in the NPP connections, the more reliable the supply of power is to the consumer.
Electric grid vulnerability
The NETS must maintain a precise frequency of current where only a small imprecision can cause disturbances or interruption in the supply or energy. Even a well-balanced system can be subject to events that have the potential to cause large-scale disturbances and even system collapse. A small shift in power flows caused by a sudden increase or decrease of electricity generation or load can trip circuit breakers which in turn send large power flows to neighbouring lines. This can trigger a chain reaction of failures which could have disastrous consequences.
In developed countries the transmission systems are normally designed and operated with a contingency margin. They are operated so that no single fault on the system can lead to problems such as abnormal frequency, disconnection or demand or abnormal voltage. If this margin is not maintained, or multiple faults happen close together in time, major failures can occur [ 2 ].
Interfacing NPPs with the NETS
Electric transmission systems and NPPs are captivating engineering achievements in their own right. However, when they come together in a highly controlled and distributed network, further complexity is created. This complexity is down to several factors which include the following:
- The sheer size and connectivity of the transmission system.
- The safety requirements imposed on the NPPs.
NPPs are normally operated in base load (steady-state operation at full power) and less frequently in load following mode. When NPPs are not generating, they still need electricity to support maintenance work, operate other equipment, keep the plant ready to start and very importantly operate critical safety systems.
In NPPs, the source of energy (the nuclear chain reaction) can be turned off in a few seconds. However, significant heat is still generated from the long-term decay of highly radioactive fission products. The residual heat has to be removed from the reactor core indefinitely in order to prevent overheating of the reactor fuel and its consequent damage. The reactor cooling systems must therefore be powered by a long-term stable source of electricity. Sufficient and reliable power is needed to maintain conditions in the coolant systems and containment to run vital safety related instrumentation, control, monitoring and surveillance systems [ 2 ].
The reliability of off-site-power is usually secured by two or more physically independent circuits to the NPP to minimise the likelihood of simultaneous failure. The transmission system is an important factor in NPP site selection. This must take into account the plant's position within the network as well as its proximity to centres of electricity demand and a cooling source.
Influence of NETS disturbances on NPPs
A ‘load rejection’ is a sudden rejection in the electric power demanded. Such a reduction could be the sudden opening of a connection with another part of the system that carries a substantial load. An NPP is usually designed to withstand load rejections up to a certain limit without tripping the reactor (50% or more). An NPP's ability to cope with a load rejection depends on how fast the reactor power can be increased back to the original level when the fault is cleared. Load rejections of up to 50% are accommodated by a combination of several actions [ 2 ]:
- Rapidly running back the steam turbine to the lower demand level, diverting the excess steam from the turbine to the main steam condenser unit or atmosphere if permitted.
- Reducing reactor power via insertion of control rods without tripping (scram) the reactor.
Degraded grid voltage or frequency may occur when an imbalance is created between generation and load. Isolating the section of transmission with the NPP from the rest of the system (islanding) may reduce the load on the NPP. However, correct islanding will help prevent the NPP from tripping because of the lower frequency, but may in turn further aggravate the power imbalance on the rest of the system.
A plant trip including reactor shut-down should be considered as a last resort. During a trip, the plant will be subjected to varying changes in power, pressure and temperature, which shortens the lifetime of the plant. Any change in system frequency will have an effect on the NPPs operation by changing the speed of the NPP's turbo generator and the speed of the cooling pumps that circulate coolants through the reactor core. The speed of the reactor's main coolant pumps is directly proportional to the frequency of the electrical power supply. If the frequency of the power drops far enough, pumps will slow, leading to inadequate core cooling and the corresponding reactor/reactors will trip accordingly [ 2 ].
Motors in the NPP may also trip due to rising currents and resulting overheating caused by the frequency reduction. The performance of ac motors is directly affected by the voltage and frequency of their respective supplies. If system voltages are not sufficient, motors cannot develop sufficient motor torque to start. In abnormal conditions, safety systems are required to trip the reactor and turbine, separating the plant electrical systems from the degraded conditions of the grid.
Any loss of off-site power (LOOP) is usually caused by external events beyond the NPP's switchyard. Examples may include transmission line faults and weather effects like lightning strikes, ice storms and hurricanes. A LOOP interrupts supplies to all in-plant loads such as pumps and motors, and to the NPPs safety systems. Diesel generators may not be reliable as off-site power due to them failing to start or run 1% of the time .
Influence of NPP disturbances on the NETS
If an NPP trips unexpectedly, the result can be a significant mismatch between generation and load on the system. Unless additional power sources are quickly connected, this can degrade the grid's voltage and frequency and thus off-site power supply to the NPP.
The system operator's response over time to the sudden loss of the NPP can be modelled by computer simulations. However, in some scenarios involving poorly connected or controlled transmission systems, the sudden shut-down of a large NPP or any other large generation plant elsewhere on the system might result in severe degradation of the system voltage and frequency. Similarly, when an NPP is sited on a well maintained, but small isolated grid of limited generating capacity, the sudden loss of its generation may lead to the same outcome .
Review of UK NPP operating experience
Under a condition attached to the nuclear site licence, periodic safety reviews (PSRs) are required to be carried out by the licencee of an NPP. They provide the opportunity to undertake a comprehensive study of the safety of the plant, taking into account aspects such as its operational history, ageing factors which could lead to deterioration in safety and the advances in safety standards since the time of construction or the previous review.
A programme of plant improvements was put into place for some of the UK's older AGRs; this was known as the AGR Safety Review and Enhancement Programme (ASREP). The key issues addressed were diverse shut-down, improved fire-protection, increased diversity of post-trip cooling and provision of an alternative indication centre. The early consideration of these issues was of great benefit to the PSRs of the older AGRs. The exception to the above was both Heysham 2 and Torness NPPs as these newest stations have been designed against more modern standards which made provision for the issues identified in the ASREP programme .
The initiating events for Torness 2 NPP are shown in Table 1:
|NITIATING EVENT||DATE OF OCCURRENCE||CONSEQUENCE/ACTION|
|strong winds deposited salt on national grid 400 kV insulators||December 1997 (Heysham 2 NPP)||insulator flashover resulted in the disconnection of ac circuits from the ac grid to the NPP|
|strong winds deposited salt on national grid 400 kV insulators||December 1998 (Heysham 2 NPP)||insulator flashover resulted in the disconnection of ac circuits from the ac grid to the NPP|
|leak of treated water from the pressure vessel Water cooling (PVWC) system||March 1997 (Heysham 2 NPP)||temporary modification to the PVWC system to increase the flow rates. Leak repaired using a sealing compound|
|during periodic shut-down one of four reactor vessel relief valves operated below the required||June 1992 (Heysham 2 NPP)||none, valve reset correctly on the discovery of the incident|
|failed fuel pin detected in a fuel assembly||July 1992(Heysham 2 NPP)||none, a specific justification was presented which supported the discharge of the fuel assembly|
|reactor seawater system pump discharge valve failed to open||May 1993(Heysham 2 NPP)||damage to some valves due to erosion of the wear and tear of the shafts|
|four of the eight gas circulators showed anomalous behaviour. IGVs, shaft vibration appeared||November 1992–June 1994 (Heysham 2 and Torness NPPs)||fall in motor current and circulator pressure differential. Four circulators exchanged in June 1994. Monthly monitoring has confirmed that anomalous behaviour has not returned since June 1994|
|main coolant gas bypasses plant isolation valves failing to operate||1993 (Torness NPP only)||recent maintenance and testing indicated improvements. Simple operator action implemented to free the locked spindle, allowing valve to close|
Table 1: Initiating events for Heysham 2 and Torness NPPs
An event occurred in December 1997 were all four grid supplies to Heysham 2 were lost in high winds. Salt from the sea was deposited on the National Grid Company's 400 kV substation which resulted in flashovers on the insulators. Grid supplies were re-established, but shortly afterwards another disruption on the grid caused a further loss of all four supplies and then only one station supply was re-established. The same event occurred in December 1998, although the nuclear power station's automatic diverse standby supplies worked well, investigations were carried out to see what could be done to minimise such disturbances in the future  or .
Between the dates of November 1992–June 1994, four of the eight gas circulators at Heysham 2 showed anomalous behaviour. In a certain position of the inlet guide vanes (IGVs), shaft vibration appeared, coupled with a fall in motor current and circulator pressure differential. The four circulators were exchanged during the nuclear power reactor outage in June 1994. A year after a further gas circulator (number 10) started to exhibit the same behaviour and was also exchanged. The root cause of the behaviour was judged to be a deposit built up on the impeller side plate inlet radius. The 16 gas circulators currently installed at Heysham 2 are demonstrating acceptable behaviour [ 5 ].
For the same events between the dates of November 1992–June 1994 at Torness NPP increased vibration levels coupled with a drop in motor current were observed on one circulator. However, as these IGV settings were above those used for full nuclear power reactor load, this behaviour was not considered to be important for normal operation. For depressurisation fault conditions, the effect of anomalous gas circulator behaviour has been addressed in the safety case.
Moreover, at Torness NPP there have been several reported events associated with the main coolant gas bypass plant isolation valves failing to operate on demand. Maintenance and testing indicates that reliability has improved, but for any similar failures in the future, a simple operator recovery action has been implemented. This will free the locked spindle allowing the valve to be closed. No such failures have been reported [ 5 ].
Future nuclear power reactor design
Following the Fukushima Daiichi incidents specifications concerning advanced safety systems for new reactor designs are being enforced. Strategies to ensure a continuous supply of power to the reactor safety systems is maintained if electricity from the grid fails are being advocated. Despite multiple back up supplies on-site, additional passive safety systems that do not need power supplies are being assessed by the Office for Nuclear Regulation. New nuclear power reactor designs undergo two assessment stages prior to final construction. These are the generic design assessment (GDA) and site specific assessment (SSA). There are three reactor designs undergoing the GDA. All of the designs use uranium as the fuel and water as the moderator and coolant. The new designs will have an operating life of 60 years or more.
As part of the SSA, all future designs proposed for the UK will use both active and passive safety systems.
Active safety systems
Active safety systems rely on back up power controlled by human or machines. An example of this would include the automatic insertion of the control rods in to the reactor to slow down a nuclear reaction and prevent overheating of the reactor core. This still requires power to perform this function [ 6 ].
Passive safety systems
Passive safety systems take advantage of natural processes such as gravity or pressure. An example would involve flooding of the reactor core by a gravity based water system if pressure inside the reactor increases exponentially. Pressure valves detect the increase and open to allow the extra water to flow into the reactor core and provide instant cooling. These systems can maintain correct operation for up to 72 h if power is lost. The reactors should have redundant and segregated safety systems so the failure of one system does not affect the other [ 6 ].
Future reactive power control systems
Controlling reactor power and distribution automatically are fundamental functions of the control system. The positions of reactor core control rods are varied to regulate the distribution of the reactive power. This control system will help future NPPs to respond to the following load change transients:
- Ramp increases and decreases of 5% per minute.
- Step load changes of ±10%.
- Power ramps from 100 to 50% in 2 h.
- About 10% power change at 2% per minute to provide grid frequency response.
The above capabilities can be achieved without a reactor trip or steam dump actuation [ 7 ].
Key essential safety systems of future nuclear power reactors
Some new nuclear power reactors will be designed to sustain a 100% load rejection or a turbine trip from full power. No reactor trip, operation of steam safety valves or pressuriser actuation is required. The steam dump control system, in conjunction with other systems helps to accommodate abnormal load rejections and helps to reduce the transients imposed on the reactor cooling systems [ 7 ].
The load rejection or steam load rejection steam dump controller (LRSDC) helps to prevent a large increase in reactor coolant temperature following a large, sudden load decrease. Following a reactor trip, the LRSDC is bypassed and the plant trip steam dump controller becomes active [ 7 ].
The rapid power reduction system (RPRS) helps to decrease the output power to a level within the capability of the steam dump system (>50% reduction at maximum rate). On the detection of a large and rapid power reduction a pre-selected number of control rods allow a 50% reduction to be achieved [ 7 ].
Emergency core-cooling system
The emergency core-cooling system (ECCS) is a set of interrelated safety systems which are designed to protect the fuel within the reactor pressure vessel and prevent overheating.
High pressure systems are designed to protect the reactor core by injecting large amounts of water to prevent the fuel from being uncovered by a decrease in the water level. High pressure cooling systems consists of pumps that have sufficient pressure to inject coolant into the reactor vessel and provide automatic injection when the level drops below the threshold. This system is normally the first line of defence for a reactor as it can be used while the whole of the vessel is still pressurised [ 7 ].
Depressurisation systems are designed to maintain the reactor core pressure within safety limits. If the reactor water level cannot be maintained with high-pressure coolant systems alone, a depressurisation system will reduce the reactor core pressure to a level in which the low pressure coolant systems can be used. This system consists of a series of valves which open to vent steam under the surface of a large pool of liquid water in pressurised containment vessels or other types of similar structures. The actuation depressurises the vessel and allows lower coolant injection systems to operate. Some systems are automatic and can be inhibited as well as operated manually [ 7 ].
Low pressure systems are designed to function after the depressurisation systems function. These are supplied by multiple, redundant power sources. The water level is maintained. The low pressure coolant injection system injects coolant into the reactor vessel once it has been depressurised. In some NPPs this system is a mode of a residual heat removal system and is not generally a stand-alone system.
A reactor core spray system uses special spray nozzles within the reactor pressure vessel to spray water directly onto the fuel rods. This helps to suppress steam generation [ 8 ].
The containment spray system consists of a series of pumps that spray coolant into the primary containment structure. Here the steam is condensed into liquid to prevent over-pressure [ 9 ].
Key lessons learned
The key considerations and assessment philosophies for new nuclear power reactor designs in the UK is crucial. Multiple levels of passive safety systems will be provided, for the latest generation of reactor designs following the lessons learned from the events which occurred in Fukushima Daiichi NPP [ 10 ]. Owing to the larger ratings of such generator units and ancillary services required from them, the impacts that these could have on the NETS in case of a reactor scram needs to be carefully considered.
This article has presented an overview of NPPs and current reactors employed in practice. The reliance of the NPP on the grid and vice-versa is unique. Some of the key technical challenges associated with such connections have been covered. In the event of loss of on-site power, secure and reliable supplies of off-site power are required to power the essential safety systems and coolant pumps to keep the reactor from overheating. A brief history of NPP events, together with their causes has been presented within the UK. Initiating events including equipment failures and grid perturbations have accounted for the majority of disturbances to the NPPs [ 11 ] or [ 12 ].
The author gratefully acknowledges the co-operation and support of National Grid Electricity Transmission Ltd. and Hinckley point B and Size B NPPs. Particular thanks go to the University of Hertfordshire for their support in the preparation and construction of this article.
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