EPR (nuclear reactor)

The EPR is a third generation pressurised water reactor design. It has been designed and developed mainly by Framatome (part of Areva between 2001 and 2017) and Électricité de France (EDF) in France, and Siemens in Germany. In Europe this reactor design was called European Pressurised Reactor, and the internationalised name was Evolutionary Power Reactor, but it is now simply named EPR.

The first operational EPR unit was China's Taishan 1, which started commercial operation in December 2018. Taishan 2 started commercial operation in September 2019. European units have been so far plagued with prolonged construction delays and substantial cost overruns. The first EPR unit to start construction, at Olkiluoto in Finland, achieved criticality in December 2021. It is expected to begin commercial operation after February 2023, a delay of thirteen years. The second EPR unit to start construction, at Flamanville in France, is also facing a decade-long delay (to 2024). Two units at Hinkley Point in the United Kingdom received final approval in September 2016; the first unit is expected to begin operating in 2027.

EDF has acknowledged severe difficulties in building the EPR design. In September 2015, EDF stated that the design of a "New Model" EPR was being worked on, which will be easier and cheaper to build.

Design

First EPR design

The main design objectives of the third generation EPR design are increased safety while providing enhanced economic competitiveness through improvements to previous pressurised water reactor designs scaled up to an electrical power output of around 1650  MWe (net) with thermal power of 4500 MW. The reactor can use 5% enriched uranium oxide fuel, reprocessed uranium fuel or 100%  mixed uranium plutonium oxide fuel, clad in Areva's M5 variant of zirconium alloy. The EPR is the evolutionary descendant of the Framatome N4 and Siemens Power Generation Division "" reactors. Siemens ceased its nuclear activities in 2011.
The EPR was designed to use uranium more efficiently than older Generation II reactors, using approximately 17% less uranium per kilowatt-hour of electricity generated than these older reactor technologies.

The design had gone through a number of iterations. The 1994 conceptual design had a power output of 1450 MWe, the same as the Framatome N4, but using Siemens Konvoi derived instrumentation and including a new core catcher safety system. In 1995, there was concern over excessive cost per MW, and output was increased to 1800 MWe in the 1997 design, though this was subsequently reduced to 1650 MWe in the final certified design.

The EPR design has several active and passive protection measures against accidents:
*Four independent emergency cooling systems, each providing the required cooling of the decay heat that continues for 1 to 3 years after the reactor's initial shutdown (i.e., 300% redundancy)
*Leak-tight containment around the reactor
*An extra container and cooling area if a molten core manages to escape the reactor (see containment building and core catcher)
*Two-layer concrete wall with a total thickness of 2.6 m, designed to withstand impact by aeroplanes and internal overpressure

The EPR has a design maximum core damage frequency of 6.1 × 10⁻⁷ per station per year.

EPR 2 design

, EDF acknowledged the difficulties it was having building the EPR design, with its head of production and engineering, Hervé Machenaud, saying EDF had lost its dominant international position in design and construction of nuclear power stations. Machenaud indicated EDF was considering designing two new lower powered reactors, one with output of 1500 MWe and the other 1000 MWe. Machenaud stated there would be a period of reflection on the best way to improve the EPR design to lower its price and incorporate post-Fukushima safety improvements.

In September 2015, EDF's chief executive Jean-Bernard Lévy stated that the design of a "New Model" EPR, or "EPR 2", was being worked on, which will be easier to build, to be ready for orders from about 2020, describing it in 2016 as "a reactor offering the same characteristics as today’s EPR but it will be cheaper to build with optimised construction times and costs".

In 2016, EDF planned to build two new model EPR reactors in France by 2030 to prepare for renewing its fleet of older reactors. However, following financial difficulties at Areva and its merger with EDF, French Ecology Minister Nicolas Hulot said in January 2018, "for now uilding a new model EPRis neither a priority or a plan. Right now the priority is to develop renewable energy and to reduce the share of nuclear." The industry-government plan for 2019–2022 included work on "a new version of the EPR".

In July 2019, the French nuclear safety authority ASN issued an opinion on the safety of an outline EPR new model (EPR 2) design. It found that general safety was on the whole satisfactory, though identifying areas for further examination. The most notable simplification is a single layer containment building with a liner as opposed to the EPR's double layer with a liner. ASN highlighted that the EPR design basis assumption that primary and secondary cooling circuit piping could not fail may no longer be appropriate for the simplified EPR 2, and requires additional safety demonstrations. Another simplification is that, unlike the first EPR design, the EPR 2 design does not allow access to the reactor building for maintenance during reactor operation, which simplifies the design of the reactor building.

In 2020, French Energy Minister Élisabeth Borne announced the French government would not decide on the construction of any new reactors until the much delayed Flamanville 3 started operation after 2022. EDF had estimated building six EPR 2 nuclear reactors would cost at least €46 billion. A Court of Audit report concluded that EDF is no longer able to finance EPR 2 construction on its own, so financing and profitability issues need to be resolved.
The audit office requires that EDF ensure the financing and profitability of EPR 2 before constructing any in France.

In January 2022, junior environment minister Bérangère Abba said that plans for new EPR 2 reactors, to be operational between 2035 and 2037, should be submitted around 2023. The decision was accelerated by the impact of 2021 global energy crisis.

Operational plants

Olkiluoto 3 (Finland)

Construction of the Olkiluoto 3 power station in Finland began in August 2005. The station will have an electrical power output of 1600  MWe (net). The construction was a joint effort of French Areva and German Siemens AG through their common subsidiary Areva NP, for Finnish operator TVO. Siemens ceased nuclear activities in 2011. Initial cost estimates were about €3.7 billion, but the project has since seen several severe cost increases and delays, with latest published cost estimates (from 2012) of more than €8 billion. The station was initially scheduled to go online in 2009.

In May 2006, construction delays of about one year were announced, following quality control problems across the construction. In part, the delays were due to the lack of oversight of subcontractors inexperienced in nuclear construction. The delays led to disappointing financial results for Areva. It blamed delays on the Finnish approach to approving technical documentation and designs.

In December 2006, TVO announced construction was about 18 months behind schedule so completion was now expected 2010–11, and there were reports that Areva was preparing to take a €500 million charge on its accounts for the delay.

At the end of June 2007, it was reported that Säteilyturvakeskus (STUK), the Finnish Radiation and Nuclear Safety Authority, had found a number of safety-related design and manufacturing 'deficiencies'. In August 2007, a further construction delay of up to a year was reported associated with construction problems in reinforcing the reactor building to withstand an aeroplane crash, and the timely supply of adequate documentation to the Finnish authorities.

In September 2007, TVO reported the construction delay as "at least two years" and costs more than 25% over budget. Cost estimates by analysts for the overrun range up to €1.5 billion.

A further delay was announced in October 2008, making the total delay three years, giving an expected online date of 2012. The parties are in arbitration to resolve a dispute over responsibility for the delays and final cost overruns.

As of May 2009, the station was at least three and a half years behind schedule and more than 50 percent over-budget. Areva and the utility involved "are in bitter dispute over who will bear the cost overruns and there is a real risk now that the utility will default". In August 2009, Areva announced €550 million additional provisions for the build, taking station costs to €5.3 billion, and wiped out interim operating profits for the first half-year of 2009.

The dome of the containment structure was topped out in September 2009. 90% of procurement, 80% of engineering works and 73% of civil works were completed.

In June 2010, Areva announced €400 million of further provisions, taking the cost overrun to €2.7 billion. The timescale slipped from June 2012 to the end of 2012. In December 2011, TVO announced a further delay to August 2014. As of July 2012, the station was scheduled to start electricity production no earlier than 2015, a schedule slippage of at least six years. In December 2012 Areva's Chief Executive estimated costs to €8 billion.

In September 2014, Areva announced that operations would start in 2018. In October 2017, the date was pushed back to the spring of 2019. During testing between 2018 and 2021, multiple further delays were announced, of around three years in total.

Olkiluoto 3 achieved first criticality in December 2021. Grid connection took place in March 2022, In May 2022, foreign material was found in the turbine steam reheater, and the plant was shut down for about three months of repair work. Regular production had been expected to begin in December 2022, after a test production phase. On 28 October 2022, it was announced cracks of a few centimetres had been found in all four of the feedwater pump impellers. The cause of the cracks was yet to be determined, and it was unclear how the commissioning schedule would be affected. The feedwater pumps are larger than in other nuclear reactors.

Taishan 1 and 2 (China)

In 2006, Areva took part in the first bidding process for the construction of four new nuclear reactors in China, together with Toshiba-owned Westinghouse and Russian Atomstroyexport. However Areva lost this bid in favour of Westinghouse's AP1000 reactors, in part because of Areva's refusal to transfer the expertise and knowledge to China.

Subsequently, Areva managed to win a deal in February 2007, worth about €8 billion ($10.5 billion) for two EPRs located in Taishan, Guangdong Province in southern China, in spite of sticking to its previous conditions. The General Contractor and Operator is the China General Nuclear Power Group (CGN).

The construction of the first reactor at Taishan started officially on 18 November 2009, and the second on 15 April 2010. Construction of each unit was then planned to take 46 months, significantly faster and cheaper than the first two EPRs in Finland and France.

The reactor pressure vessel of the first reactor was installed in June 2012, and the second in November 2014. The first pressure vessel had been imported from Mitsubishi Heavy Industries in Japan, and steam generators from Areva in France. The second pressure vessel and associated steam generators had been made in China, by Dongfang Electric and Shanghai Electric.

In 2014, construction was reported to be running over two years late, mainly due to key component delays and project management issues.

Cold function tests were performed on Taishan 1 in February 2016, with start up expected in the first half of 2017. Taishan 2 was scheduled to start up later that year.
However, commissioning dates were put back six months in February 2017, with commercial operation expected in the second half of 2017 and the first half of 2018.

In December 2017, Hong Kong media reported that a component had cracked during testing, needing to be replaced.
In January 2018, commissioning was rescheduled again, with commercial operation expected in 2018 and 2019.

In June 2018, Taishan 1 achieved criticality for the first time. On June 29, 2018, Taishan 1 was connected to the grid. It entered commercial operation in December 2018. Taishan 2 reached these milestones in May 2019 June 2019 and September 2019, respectively.

The Taishan project is led by Taishan Nuclear Power Joint Venture Co. (TNPJVC), a joint venture founded by CGN (51% ownership stake), EDF (30%), and Chinese utility Guangdong Energy Group (19%), also known as Yuedian.

Companies involved in supplying equipment to Taishan Unit 1 include Framatome, which manufactured the steam generators and pressurizer in France, and China’s Dongfang Electric Corp. (DEC), which manufactured the Arabelle turbine in the engine room. That turbine was designed and licensed by General Electric. Other equipment suppliers for Unit 1 include Mitsubishi (reactor vessel); Škoda, a Czech company (core internals); and France’s Jeumont Electric, which along with DEC provided primary pumps.

In April 2020, Framatome signed a long-term service contract with TNPJVC to support operations of the two EPRs. This contract covers nuclear plant outage and maintenance work, including spare parts supply and engineering services for eight years.

In June 2021, higher than expected concentrations of radioactive gases were detected in the primary circuit of unit 1. This was later attributed to faulty fuel cladding. The reactor was taken offline in July 2021 and restarted in August 2022.

Plants under construction

Flamanville 3 (France)

First concrete was poured for the demonstration EPR reactor at the Flamanville Nuclear Power Plant on 6 December 2007.
As the name implies, this will be the third nuclear reactor on the Flamanville site, and the second instance of an EPR being built. Electrical output will be 1630 MWe (net). The project was planned to involve around €3.3 billion of capital expenditure from EDF.

From 19 October 2005 to 18 February 2006, the project was submitted to a national public debate. On 4 May 2006, the decision was made by EDF's Board of Directors to continue with the construction. Between 15 June and 31 July 2006, the unit underwent a public enquiry, which rendered a "favourable opinion" on the project. That summer, site preparation works began.

In December 2007, construction of the unit itself began. This was expected to last 54 months, with commissioning planned for 2012.

In April 2008, the French nuclear safety authority ('' Autorité de sûreté nucléaire'', ASN) reported that a quarter of the welds inspected in the secondary containment steel liner are not in accordance with norms, and that cracks have been found in the concrete base. EDF stated that progress was being made on these issues, which were raised very early in construction; however, on 21 May, ASN ordered a suspension of concrete pouring on the site. A month later, concreting work resumed after ASN accepted EDF's corrective action plan, which included external oversight checks.

In May 2009, Stephen Thomas reported that after 18 months of construction, and after a series of quality control problems, the project is "more than 20 percent over budget and EDF is struggling to keep it on schedule".

In August 2010, the regulator, ASN, reported further welding problems on the secondary containment steel liner. The same month, EDF announced that costs had increased 50% to €5 billion, and commissioning was delayed by about two years to 2014.

In July 2011, EDF announced that the estimated costs had escalated to €6 billion, and that completion of construction was delayed to 2016.

In December 2012, EDF announced that the estimated costs had escalated to €8.5 billion. Also in December 2012, the Italian power company Enel announced it was relinquishing its 12.5% stake in the project, and five future EPRs, so would be reimbursed its project stake of €613 million, plus interest.

In November 2014, EDF announced that completion of construction was delayed to 2017, due to delays in component delivery by Areva.

In April 2015, Areva informed the French nuclear regulator ASN that anomalies had been detected in the reactor vessel steel, causing "lower than expected mechanical toughness values". Further tests are underway. In July 2015 The Daily Telegraph reported that Areva had been aware of this problem since 2006. In June 2015, multiple faults in cooling system safety valves were discovered by ASN. In September 2015, EDF announced that the estimated costs had escalated to €10.5 billion, and the start-up of the reactor was delayed to the fourth quarter of 2018.

In April 2016, ASN announced that additional weak spots had been found in the reactor steel, and Areva and EDF responded that new tests would be conducted, though construction work would continue.

In February 2017, the Financial Times stated the project was six years late, and €7.2 billion over budget, while renewed delays in the construction of the EPR-reactors at Taishan Nuclear Power Plant prompted EDF to state that Flamanville 3 remains on schedule to start operations by the end of 2018, assuming it receives regulatory approval. In June 2017, the French regulator issued a provisional ruling that Flamanville 3 is safe to start.

The discovery of quality deviations in the welding led to a further revision of the schedule in July 2018. Fuel loading was delayed until the end of 2019, and the cost estimate was increased from €10.5 billion to €10.9 billion.

In June 2019, nuclear regulator ASN determined that eight welds in steam transfer pipes passing through the two wall containment, that EDF had hoped to repair after startup, must be repaired before the reactor is commissioned. By then, estimated costs were €11 billion.Pierre Breteau
''Visualisez en graphiques les multiples dérapages de l’EPR de Flamanville''.
Le Monde, update 13 Jan 2022

In October 2019, EDF announced that because of this issue costs would increase to €12.4 billion and that fuel loading would be delayed until the end of 2022. Pierre Moscovici, president of the Court of Audit, gave a statement on 9 July 2020 concerning the release of the report on the delay costs of the Flamanville 3. The report of the Court of Audit revealed that the costs could reach €19.1 billion instead of €12.4 billion when taking into account the additional charges due to the delay in construction.

In January 2022, it was announced that more time was needed for the repair of faulty welds and the solving of other issues. In December 2022, EDF announced a further delay of at least six months with an estimated cost increase of €500 million due to more work to establish a new process for the stress relieving heat treatment of some welds close to sensitive equipment. Fuel loading is now forecast for early 2024. Estimated total costs increased to €13.2 billion.

Hinkley Point C (United Kingdom)

Hinkley Point C is a nuclear power station under construction with two EPR reactors and an electrical output of 3,200 MWe in Somerset, England.

The EPR underwent Generic Design Assessment by the Office for Nuclear Regulation, along with the Westinghouse AP1000. Interim Design Acceptance Confirmations were postponed until lessons from the Fukushima Daiichi nuclear disaster had been taken into account. EDF bought British Energy in 2009. EDF planned to build 4 new EPRs, subject to electricity pricing agreement with the government. Areva has signed a strategic partnership with Rolls-Royce to support the build of EPRs. On 19 March 2013, the Development Consent Order granting planning permission for Hinkley Point C was given, but negotiations with the UK government about electricity pricing, and project financing with private investors, still needed to be concluded.

On 21 October 2013, EDF Energy announced that an agreement had been reached regarding the nuclear stations to be built on the site of Hinkley Point C. EDF Group and the UK Government agreed on the key commercial terms of the investment contract. The final investment decision was conditional on completion of the remaining key steps, including the agreement of the European Commission.

On 8 October 2014, the European Commission announced their agreement, with 16 out of 28 commissioners agreeing with the go ahead of the construction. On 21 September 2015, the British government announced it would provide a £2 billion support package for Hinkley Point C as Britain's first nuclear power station in 20 years.

On 21 October 2015, during Chinese president Xi Jinping's state visit to the United Kingdom, EDF and CGN signed an investment agreement for the £18 billion (€21.3 billion) project to build two reactors at Hinkley Point.

In June 2016, EDF managers told Members of Parliament that the Hinkley Point C proposal should be postponed, until it has "solved a litany of problems", including EDF's "soaring debts". On 28 July 2016, after the resignation of a board member, the EDF board approved the final investment decision for the project. However Greg Clark, the new Secretary of State for Business, Energy and Industrial Strategy in the new government of Theresa May, then announced that the government would not sign the contract over the next few days as expected, but delay the contract to autumn to "consider carefully all the component parts of this project". Final government approval was given in September 2016.

In July 2017, following an internal review, EDF announced revised estimates for the scheme, which included at least £1.5 billion of additional costs and up to 15 months of additional programme, leading to updated total cost estimates of £19.6–20.3 billion. In September 2019 cost estimates were revised again, to be between £21.5 and £22.5 billion, and again in January 2021 to £2223 billion, with an expected operational start date of June 2026.

In May 2022 cost estimates were revised again, to be between £25 and £26 billion (€29.630.7 billion). The plant is now estimated to start generating electricity in June 2027.

Possible future power stations

France

In July 2008, the French President announced a second EPR would be built in France due to high oil and gas prices. Penly was chosen as the site in 2009, with construction planned to start in 2012. However, in 2011, following the Fukushima Daiichi nuclear disaster, EDF postponed public consultations. In February 2013, the Minister of Industrial Renewal Arnaud Montebourg stated that the plans for a new EPR reactor at Penly had been cancelled, citing the capacity for electricity production and massive investments in renewable energy along with his confidence in the EPR as a competitive project in foreign countries.

Plans to build new reactors in France were later revived. Penly and Gravelines are among the candidates for the installation of a pair of EPR reactors.

In October 2019, newspaper ''Le Monde'' reported that the French government had sent EDF a "mission letter" in which it asked the company to prepare to build a total of six EPR reactors across three sites in the next 15 years. A government decision on the construction of new reactors was not expected until after 2022. EDF has submitted a proposal to build six EPR 2s for around €50 billion. In February 2022, French president Emmanuel Macron announced that France would in fact build six new EPR 2 reactors, the first to be commissioned by 2035, and with an option for eight more.

India

In February 2009, the Nuclear Power Corporation of India (NPCIL) signed a memorandum of understanding with Areva to set up two EPR reactors at Jaitapur in Maharashtra. This was followed by a framework agreement in December 2010.

In January 2016, during French president François Hollande's state visit to India a joint statement with Indian Prime Minister Narendra Modi was issued. According to the statement the two leaders "have agreed on a roadmap of cooperation to speed up discussions on the Jaitapur project".

In March 2018, an Industrial Way Forward Agreement between EDF and NPCIL was signed, with an objective of producing a tender for six reactors.

In April 2021, EDF submitted to NPCIL an offer to develop six EPR reactors at the Jaitapur site, with a combined installed capacity of 9.6 GWe.

United Kingdom

Two EPR units at Sizewell, Suffolk are in the planning phase. In May 2020, EDF Energy applied for a development consent order. If the project goes ahead electricity production is expected to start in 2031 at the earliest.

Two further EPR units have been proposed for construction at the Moorside site near Sellafield, Cumbria, as part of a future clean energy hub that would also incorporate modular reactors, renewable energy generation, hydrogen production and battery storage technologies.

Czech Republic

In October 2012, Czech utility company ČEZ announced that Areva was eliminated from a tender for the construction of two reactors for Temelín nuclear plant. Areva failed to comply with legal requirements of the tender. In April 2014, ČEZ cancelled the tender, because of low power prices and the government's refusal to support a minimum guaranteed energy price.

In June 2021, the Czech Ministry of Industry and Trade invited EDF, along with Westinghouse and Korea Hydro & Nuclear Power to participate in a pre-qualification round for a new unit at the Dukovany Nuclear Power Station. EDF is proposing a 1200 MWe version of the EPR for the project.

Unsuccessful proposals

Canada

EPR was considered for the two (possible expansion to four) reactor addition to the Darlington Nuclear Generating Station in Ontario, Canada. However, the official bids had to include all contingencies, and Areva failed to enter a final bid meeting these requirements. The project was ultimately abandoned when the only bid, made by Canada's AECL, came in at well over $10/Wp.

EPR was briefly considered for an installation in New Brunswick, replacing or supplanting that province's single CANDU 6. These plans lasted only from June 2010 until an election two months later, when the plan immediately disappeared from further study.

Finland

In 2010, the Finnish parliament decided to allow two new reactors. Both TVO and Fennovoima were considering the EPR.
In December 2013, Fennovoima confirmed it had selected a Russian AES-2006 VVER pressurised water reactor in preference to the EPR. In May 2022, after significant delays in the design and licensing phase of the project and in light of the 2022 Russian invasion of Ukraine, Fennovoima cancelled the contract with Rosatom to build the power plant.

Italy

On 24 February 2009, Italy and France agreed to study the feasibility of building 4 new nuclear power stations in Italy. Following this, on 3 August 2009, EDF and Enel established a joint venture, Sviluppo Nucleare Italia, to study the feasibility of building at least four EPRs.

However, in the 2011 referendum, soon after the Fukushima nuclear disaster, Italians voted to repeal the new regulations permitting nuclear power in Italy. Abrogation of laws is put in effect when at least 50%+1 electors make a valid vote and a majority of these voters are in favour of abrogation. In this referendum, there was a 55% valid voter turnout and 94% voted to abrogate the new regulations.

United Arab Emirates

In March 2008, French president Nicolas Sarkozy reached an agreement with the UAE cabinet that "outlines a cooperation framework for the assessment and possible use of nuclear energy for peaceful ends". This agreement was not a contract for EPR construction by any of the French nuclear companies, Total S.A., Suez or Areva.

In May 2009, US President Barack Obama signed a similar agreement with the UAE. Contracts for reactors were not given, nor was there any guarantee made that US companies would receive them.

In December 2009, the United Arab Emirates declined both the American and French bids and awarded a contract for construction of four non-EPR stations (APR-1400) to a South Korean group including Korea Electric Power Corporation, Hyundai Engineering and Construction, Samsung and Doosan Heavy Industries.

After losing this order, Areva considered whether it should reintroduce the marketing of a smaller and simpler second-generation reactor design alongside the EPR, for countries that are new to nuclear power. As of 2011 Areva and Mitsubishi Heavy Industries offer a smaller 1100 MWe ATMEA1 Generation III PWR.

United States

The US-EPR, the version of the EPR submitted to the U.S. regulator, is one of the competitors for the next generation of nuclear stations in the United States, along with the AP1000 and the ESBWR. In February 2015, Areva asked to suspend the Design Certification Application Review process at the U.S. Nuclear Regulatory Commission (NRC). It had been under review there with expectation to submit an application for final design approval and standard design certification since 14 December 2007. UniStar, Amarillo Power, PPL Corp and AmerenUE announced plans to file a Combined Construction and Operating License application in 2008 for the US-EPR at its Callaway station. UniStar filed a partial application in July 2007 for a proposed third unit at the Calvert Cliffs Nuclear Power Plant in Maryland. However, both proposals were subsequently cancelled.

In April 2009, Missouri legislators balked at preconstruction rate increases, prompting AmerenUE to suspend plans for its reactor. In July 2010, Constellation Energy Group cut spending on UniStar for the Calvert Cliffs Nuclear Power Station because of uncertainties for a loan guarantee from the U.S. Department of Energy, and subsequently pulled out of the project. In October 2008, Areva announced that it would partner with US defense firm Northrop Grumman to establish a $380 million facility to construct modules and assemblies for the EPR and US-EPR reactors at Northrop Grumman's Newport News Shipyard in Virginia.
The project was suspended indefinitely in May 2011.

Poland

In October 2021, EDF made an offer to Poland to build four or six EPR reactors across two to three sites. The combined installed capacity of the reactors would be either 6.6 or 9.9 GWe.

In October 2022, Poland selected Westinghouse's AP1000 design, with construction of the first three-unit plant expected to start in 2026.

See also

* Economics of new nuclear power plants
* Nuclear power by country
Other Generation III+ designs:
* US-APWR
*VVER-TOI
* ACR

References

{{Nuclear power in France

Pressurized water reactors