Nuclear plant

The race is on

The UK government has given the go-ahead for a new nuclear build, and the race is on for a slice of a very large pie.

The race is on

Since the turn of the century, six UK nuclear power plants have begun the long decommissioning process. A further six will end their lives within another ten years, leaving a mere  4,858MWe of power provided by nuclear facilities. It was perhaps inevitable that a new generation of plants would materialise, and the government confirmed its intentions on 10 January.

The country's existing nuclear set-up is spread across nine sites. These are comprised of old Magnox gas cooled reactors at Oldbury and Wylfa, the newer advanced gas cooled reactors (AGCR) at Hartlepool, Heysham, Hunterston, Dungeness, Hinkley Point and Torness, and a single pressurised water reactor (PWR) at Sizewell B.

Since the last UK AGCR plant - Torness 2 - entered service in 1989, however, nuclear reactor technology and efficiency has improved dramatically.

Four designs are on the table, all of which are generation III+ designs: the advanced CANDU reactor (ACR 1000) from Atomic Energy of Canada (AECL); the evolutionary power reactor (EPR) from EDF/Areva; the economic simplified boiling water reactor (ESBWR) from GE/Hitachi; and the AP1000, a PWR, from Westinghouse.

In an attempt to speed up the licensing process the four designs are currently going through generic design assessment (GDA), which will turn out three approved designs later this year. The GDA, run by the UK Nuclear Regulators (the Health and Safety Executive and the Environment Agency) was set up in response to a request from the government, and from a number of companies who had asked the government to review their designs.

Companies will be able to submit information on their reactor designs to the Nuclear Regulators, and these will be assessed in advance of any application to build a plant at a particular site within the UK.

This process will allow a rigorous and structured examination of detailed safety, security and environmental aspects of the design, and is likely to take around three-and-a-half years to complete. At the end of the assessment (and at key stages during it) the HSE will issue reports on its findings, confirming whether they judge that safety, security and environmental issues have been considered adequately.

GDA allows the government to liaise with potential designers and operators at the earliest stage - where regulators can have most influence and where lessons can be learned that may be applicable to other submitted designs. However, there is no guarantee that the GDA process will lead to a design being regarded as satisfactory, nor will it guarantee that a site licence will be granted in response to a site specific application based on that design.

AECL - ACR 1000

The Advanced CANDU Reactor (ACR) is a Generation III+ design - as are the other three entrants - and is an evolutionary development of existing CANDU (‘CANada Deuterium Uranium') reactors designed by AECL. It is a light-water-cooled reactor that incorporates features of both pressurised heavy water reactors and advanced pressurised water reactor technologies, while using a similar design concept to the steam generating heavy-water reactor.

The design uses slightly enriched uranium (SEU) fuel, light-water coolant, and a separate heavy-water moderator. The reactivity regulating and safety devices are located within the low-pressure moderator. The ACR also incorporates characteristics of the CANDU design, including on-power refuelling with the CANFLEX fuel system (CANDU FLEXible fuelling - an advanced fuel bundle design); a long prompt neutron lifetime; small reactivity hold up; two fast, totally independent, dedicated safety shutdown systems; and an emergency core cooling system.

The use of SEU fuel allows the reduction of coolant void reactivity coefficient to a small, negative value. The compact reactor core design reduces core size by half for the same power output over the older design. The current size for the ACR-1000 is approximately 1,200MWe, and it is planned to be in service by 2016.

The original CANDU reactor was developed in the late 1950s and 1960s by a partnership between AECL, the Hydro-Electric Power Commission of Ontario, Canadian General Electric, as well as several private industry participants.

The acronym ‘CANDU' is a reference to its deuterium-oxide (heavy water) moderator and its use of uranium fuel (originally, natural uranium). All current power reactors in Canada are of the CANDU type.

The CANDU reactor is conceptually similar to most light water reactors, although it differs in the details. Fission reactions in the nuclear reactor core heat a fluid, in this case heavy water. This coolant is kept under high pressure to raise its boiling point and avoid significant steam formation in the core. The hot heavy water generated in this primary cooling loop is passed into a heat exchanger heating light water in the less-pressurised secondary cooling loop. This water turns to steam and powers a conventional turbine with a generator attached to it. Any excess heat energy in the steam after flowing through the turbine is rejected into the environment in a variety of ways, most typically into a large body of cool water, such as a lake, river or ocean.

Recently-built CANDU plants, such as the Darlington Nuclear Generating Station near Toronto, Ontario, use a discharge-diffuser system, which limits the thermal effects in the environment to within natural variations.

EDF/Areva - EPR

The evolutionary power reactor (EPR or US-EPR for the United States-specific design) is a nuclear fission four-loop pressurised water reactor (PWR) design. It has been designed and developed mainly by Areva, EDF and Siemens. This reactor design was previously called the European pressurised reactor.

As of 2007, two EPR units were under construction, one each in Finland and France, and two units were planned as part of China's tenth economic plan, to start construction in 2009.

The main design objectives of the EPR design are increased safety and enhanced economic competitiveness through evolutionary improvements to previous PWR designs scaled up to an electrical power output of 1,600MWe.

The reactor can use five per cent enriched uranium oxide or mixed uranium plutonium oxide fuel. This reactor's core can be loaded with 100 per cent MOX (Mixed Oxide) fuel, whereas a typical PWR core can loaded with only about 33 per cent MOX fuel. The EPR is the evolutionary descendant of the Framatome N4 and Siemens Power Generation Division KONVOI reactors.

The EPR design has several active and passive protection measures against accidents: four independent emergency cooling systems, each capable of cooling down the reactor after shutdown; leak tight container around the reactor; an extra container and cooling area if a molten core manages to escape the reactor; and two-layer concrete wall with a total thickness of 2.6m, designed to withstand impact by airplanes.

The Olkiluoto 3 power plant in Finland, initially scheduled to go online in 2009, will be the first EPR reactor built. The construction will be a joint effort of French Areva and German Siemens AG through their common subsidiary Areva NP, for Finnish operator TVO. The power plant should have cost about €3.7bn, but has had several delays, both construction and safety related, and so completion is now expected in 2010-2011, well over budget.

For the UK, the fact that the teething problems of the EPR have been ironed out in this first build is good news and by the time that the second reactor - at Flamanville, France - is completed, the construction processes should be well proven.

First concrete was poured for the demonstration EPR reactor at the Flamanville Nuclear Power Plant on 6 December, 2007. This will be the third unit on the site and the second EPR ever constructed. Electrical output will be 1,600MW and it is projected to cost €3.3bn.

GE - ESBWR

The economic simplified boiling water reactor (ESBWR) is a passively safe generation III+ reactor that builds on the success of the advanced boiling water reactors (ABWR). Both are designs by General Electric, and are based on their boling water reactors (BWR) design. The ESBWR uses natural circulation with no recirculation pumps or their associated piping.

The passively safe characteristics are mainly based on isolation condensers, which are heat exchangers that take steam from the vessel or the containment system, condense the steam, transfer the heat to a water pool, and introduce the water into the vessel again.

This is also based on the gravity-driven cooling system (GDCS), which is comprised of pools above the vessel. When very low water level is detected in the reactor, the depressurisation system opens several
very large valves to reduce vessel pressure and finally to allow these GDCS pools to reflood the vessel.

The core is shorter than conventional BWR plants because of the smaller core flow (caused by the natural circulation). There are 1,132 bundles and the thermal power is 4,500MWth (1,550MWe). Below the vessel, there is a piping structure that allows for cooling of the core during a very severe accident. These pipes divide the molten core and cool it with water flowing through the piping.

The probability of radioactivity release to the atmosphere is several orders of magnitude lower than conventional nuclear power plants, and the building cost is 60-70 per cent of other light-water reactors.

The advanced boiling water reactor (ABWR) on which the ESBWR is based is a Generation III reactor based on the boiling water reactor. The standard ABWR plant design has a net output of about 1,350MWe.

Internal recirculation pumps inside the reactor pressure vessel (RPV) are a major improvement over previous GE reactor plant designs (BWR/6 and prior). These pumps are powered by wet-rotor motors with the housings connected to the bottom of the RPV and eliminating large diameter external recirculation pipes that are possible leakage paths. Construction costs are also reduced. The ten internal recirculation pumps are located at the bottom of the downcomer region (that is, between the core shroud and the inside surface of the RPV).

Even though BWRs can operate using only the available natural recirculation thermal pumping head without forced recirculation flow, forced flow is desirable in order to increase the available output from the reactor and as a convenient method to change the reactor output by changing the flow.

Westinghouse - AP 1000

The AP1000 is an advanced 1,117MWe to 1,154MWe PWR nuclear power plant that uses the forces of nature and simplicity of design to enhance plant safety and operations and reduce construction costs.

The Westinghouse AP1000 is a logical extension of its AP600 plant. Design studies have shown that a two-loop configuration could produce over 1,000MWe with minimal changes in the AP600 design. The primary purpose of developing the AP1000 was to retain the AP600 design objectives, design details and licensing basis, while optimising the power output, thereby reducing the resulting electric generation costs.

The passive safety systems are significantly simpler than the traditional PWR safety systems. They do not require the large network of safety support systems needed in typical nuclear plants, such as AC power, HVAC (heating, ventilation and air conditioning), cooling water systems and seismic buildings to house these components.

Simplification of plant systems, combined with increased plant operating margins, reduces the actions required by the operator. The AP1000 has 50 per cent fewer valves, 83 per cent less piping, 87 per cent less control cable, 35 per cent fewer pumps and 50 per cent less seismic building volume than a similarly sized conventional plant. These reductions in equipment and bulk quantities lead to major savings in plant costs and construction schedules.

The AP1000 Nuclear Steam Supply System (NSSS) plant configuration consists of two Delta-125 steam generators, each connected to the reactor pressure vessel by a single hot leg and two cold legs. There are four reactor coolant pumps that provide circulation of the reactor coolant for heat removal. A pressuriser is connected to one of the hot leg pipes to maintain sub cooling in the reactor coolant system (RCS).

The two-loop, 1,090MWe plant retains the same basic design of the AP600. Changes to the design to increase the electricity output have been minimised to allow the direct application of most of the existing design engineering already completed for the AP600. Examples of design features that remain unchanged include the nuclear island footprint and the core diameter.

Major component changes incorporated into the AP1000 design include a taller reactor vessel, larger steam generators (Delta-125), a larger pressuriser and slightly taller, canned reactor coolant pumps with higher reactor coolant flows. The designs for these reactor components are based on components that are used in operating PWRs or have been developed and tested for new PWRs. Performance of the passive safety features have been selectively increased, however, these changes have been accomplished with small changes to the AP600 plant design.

The AP1000 fuel design is based on the 17x17 XL (14 foot) design used successfully at plants in the United States and Europe. As with AP600, studies have shown that AP1000 can operate with a full core loading of MOX fuel.

Like the AP600, the AP1000 uses modularisation technique for construction, which allows many construction activities to proceed in parallel. This technique reduces the plant construction calendar time, which saves the IDC (Interest During Construction) cost and reduces the risks associated with plant financing.

The AP1000 has a site construction schedule of 36 months from first concrete to fuel loading.

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