Two competing designs of nuclear reactor, Areva’s EPR and Westinghouse’s AP1000, are coming under the scrutiny of the UK regulatory authorities in a generic design assessment due to conclude next June. When the assessment is complete, engineers will build the first new nuclear power plant this country has seen in more than 20 years.
Areva’s EPR
The bulk of the design work for Areva’s European Pressurised Reactor (EPR) was undertaken in the 1990s. The overriding engineering approach was to upgrade the most common European designs, the French N4 and the German Konvoi, in order to reduce the level of risk. The EPR’s primary system uses the same four-loop design used in the N4 and the Konvoi.
Symptomatic of this risk-averse approach, Francois Bouteille, Areva’s senior vice-president of safety and licensing for the EPR, believes the major improvements to the EPR over the older Gen 2 reactors are its safety features. He says: “Our first design objective with the EPR was to obtain a significant reduction of the core melt probability, and second a significant improvement of the reactor containment capability to take into account severe accidents – where you can have a core melt situation and it spreads within the containment.”
The containment area of the reactor was therefore designed so there is a “core catcher area”. In the event of a core meltdown, molten core escaping from the reactor vessel would be allowed to spread out and would be retained and cooled inside the reactor building.
The other major safety objective, Bouteille says, was to offer better protection against external hazards, especially an aircraft crashing into the nuclear site. The EPR has double containment with an external shell designed to protect against loads corresponding to the impact of a military aircraft or large commercial jet with full fuel loads. Incidents were modelled and analysed using CAD simulation software.
With safety the top priority, other improvements can be split into two categories: operational improvements and a commercially competitive environment. The two go together: if you make something more operationally effective, it usually makes the plant a better commercial proposition too.
Operational improvements in the design include a 10% reduction in the amount of waste, a 92% target availability and improved radiation protection for plant personnel. Commercially, the plant needed high-power output levels because other reactors in Areva’s portfolio, the Atmea1 and the Kerena, at 1,250MW and 1,100MW, address the mid-sized market. The 1,650MW-capacity EPR has a slightly higher output than its French predecessor, the N4, which had 1,500MW capacity.
However, a nuclear reactor cannot be treated like a boiler. You cannot just throw a few more rods of uranium into a reactor to get it running a bit hotter. “We focused our design to increase the power level of the plant by increasing its efficiency,” says Bouteille. “This efficiency increase is the main contributor to the improved economic and environmental footprint of the plant.”
The design achieves up to 37% electrical efficiency, the highest value claimed for water reactors, by running with a greater steam pressure and a higher temperature core, 4,500MWth. For comparison, the N4 reactor’s steam pressure is 73bar, whereas the EPR’s is 78bar. The EPR’s steam generator uses the latest equipment, such as an axial economiser to increase steam pressure, says Bouteille. The economiser increases heat transfer efficiency by directing all the cold feedwater into the cold part of the tube bundle in the steam generator, and around 90% of the hot recirculated water into the hot part.
Neutron reflector
There are several other features that increase efficiency by using fuel in a better way and reducing waste. The first is a capability to increase the discharge burn-up of the fuel. There is a trend throughout the nuclear industry, says Bouteille, to increase the fuel burn-up, but the EPR’s capability to extract higher amounts of residual heat is built into the design. This increased fuel burn-up is combined with an increase in reactor size and better thermal efficiency. This means that to produce the same amount of megawatt-hours you need less fuel. As a result the EPR uses 7-15% less uranium per produced MWh compared to Gen 2 reactors.
Another of the most innovative features, says Bouteille, is the reactor’s neutron reflector. This surrounds the core and is made of stainless steel rings on top of each other. The neutron reflector reduces the amount of neutrons escaping from the core, improving fuel utilisation and protecting the reactor pressure vessel from embrittlement and ensuring the 60-year design life of the EPR. “Altogether there is a 10% reduction of long-life waste per MWh,” says Bouteille. “In addition, reprocessing of the spent fuel entails a reduction of four to five times of the long-life waste.”
The EPR also has design features that allow engineers to perform predictive maintenance while the plant is in operation. The system architecture is arranged into “four trains”, designed so that one part of the system can be taken out of service for maintenance work to be carried out without compromising safety. The approach means that the EPR has an increased availability of at least 92%, says Bouteille. Its predecessor, the N4, achieves availability of only 80-85%.
It is of little doubt that the technical achievements, built on established and well-proven designs, are robust on paper. However, construction of the first wave of EPRs has not gone according to plan. The first EPR, Olkiluoto 3 in Finland, will be operational by the end of 2012, a staggering three years late and €1.7 billion over budget.
Key areas
Rob Davies, Areva’s vice-president of UK new build, says the whole company is focused on the cost of the EPR and its speed of construction. Constructability has a direct impact on the affordability of the plant, says Davies.
“We have captured 1,000 lessons learnt from our current four projects. In 2009 alone we captured 400 lessons, processed and fed back into current and future projects, and into our standard EPR platform,” he says.
“The key areas are to get the design right, to get the project management right, and to get the constructability right.”
Davies says that the lessons learnt from Olkiluoto 3 and Flamanville 3, the first and second EPR sites, have led to a fourfold reduction in the number of engineering hours required to build the EPR at the next site, Taishan 1 and 2 in China. “At Flamanville 3, EDF used a two-step pour for the basemat part of the foundation, while at Taishan our Chinese partners used a methodology that allowed a one-step pour, which meant a one-month reduction in the construction schedule,” he says.
Other areas that have been streamlined include fuel building and the design and manufacture of heavy components. For example, says Davies, the manufacture of the steam generators has been reduced by more than a year between Olkiluoto and Taishan.
All of these lessons lead Davies to predict that Olkiluoto 3 should be finished at the end of 2012, a construction time of 86 months, while Flamanville should take 71 months to build. According to Areva’s Chinese partner, China Guandong Nuclear Power Group, Taishan will take only 46 months to construct and commission.
“There is a classic learning curve,” says Davies. “The lessons we have learnt are allowing us to firm up our costs and construction schedule. We have a global supply chain that is tuned up and ready, methodologies which have been approved, and improved manufacturing techniques. All these factors give us confidence that the UK build will be a tight programme with a greater degree of certainty for our clients.”
Westinghouse’s AP1000
The AP1000 nuclear reactor Westinghouse hopes to build in the UK has been 30 years in the making. The 1,100MW design is already being constructed at sites in China, where the first of four new nuclear reactors is expected to begin supplying the grid in 2013. The reactor is also in an earlier phase of construction at sites owned by utilities in the US, the country’s first new nuclear reactors for decades.
The AP1000 is the big brother of a 600MW design, the AP600, which Westinghouse began to develop in the early 1980s but that was never built because it could not compete financially with natural gas-fired power stations.
The AP600 grew out of a project for the US military to develop a mobile, trailer-based 10MW nuclear reactor that would have been used to provide runway lighting at advance airbases in the Cold War.
Jim Winters, an expert on the AP1000 based at Westinghouse in Pennsylvania, says elements of that original project have been carried forward as part of the current design, which is the only passive safety reactor design to be built in the US that has the stamp of approval from regulators.
He says: “What we learnt from the trailer project is that you can remove a lot of the heat in general from an exposed steel pressure vessel. That created one of the germs of the passive safety system that we’ve included in the AP1000.”
The fact that the AP600 could not compete with natural gas meant that Westinghouse’s engineers needed to get more power out of the design while cutting costs. While they had to make the turbine and steam generators bigger, they decided not to increase the diameters of the reactor and containment vessels, and to keep the reactor’s auxiliary systems as similar as possible to the original design, although some of the pipework had to be lengthened and pumps below the steam generators had to be made larger.
The height of the reactor core and containment dome was also increased but the footprint of the design is essentially the same as that of its less powerful predecessor. The biggest change was to the turbine hall, which obviously had to accommodate a much larger design. The result is a reactor similar to the AP600 that delivers almost twice as much power.
The fact that the AP600 had already been licensed in 1999 speeded up the approval of the AP1000 in the US because of the many similarities between the two. “We had to get the capital cost as low as possible and still have a safe, reliable nuclear power plant, and that was our goal for both the AP600 and AP1000,” says Winters. “And of course, having achieved low capital costs, nuclear fuel is not as expensive or subject to as many market fluctuations as natural gas.”
The passive safety system of the AP1000 relies on natural forces including gravity, natural circulation and convection. It dispenses with the need for large levels of redundant systems to keep the reactor running safely in the event of, say, a power failure.“
The design idea for the AP1000 was that we would reduce all probability of failure by designing a plant that doesn’t rely on back-up systems at all,” Winters explains.
“In our case, in the event of a failure of AC current, we have no emergency diesel systems. We don’t have the redundant systems of other plants because we don’t need them. Gravity always works.
”In the event of a power failure, Westinghouse says the AP1000 would shut down and remain cool even with no operator intervention. In the event of the core overheating, the plant is designed to drain the high-capacity (500,000-gallon) in-containment refuelling water storage tank – essentially a giant heat sink – which is located “significantly vertically higher” than the reactor core, into the reactor vessel. Water heated by the core would flow up to the heat sink, boil and then condense on the inner containment structure and run back down into the tank.
“We say that our safety systems are dependent on only two things: Newton’s law of universal gravitation and the Zeroth law of thermodynamics, which states that energy flows from hot places to cold ones,” says Winters.
This same water pins the reactor cavity. If the core is not cooled by the design’s passive systems, the water in the cavity is meant to provide cooling on the outside of the reactor vessel to prevent reactor vessel failure and the subsequent spilling of molten core debris into containment, eliminating the possibility of damage to the containment dome and subsequent release of radiation into the environment.
Natural disasters
The AP1000 has also been carefully designed to cope with accidents or natural disasters outside the plant. These include earthquakes, for which various sites in the US were seismically surveyed to ensure the reactor could withstand such an event. The plant’s heating, ventilation and air-conditioning systems were also designed to allow it to cope with a freezing winter in, say, Minnesota as well as scorching summers in the southern US (where Westinghouse has signed contracts with, for example, Georgia Power). The velocity with which objects caught up in a 300mph tornado might strike the outside of the reactor was also considered.
Winters believes the parameters in terms of natural disasters or freak weather conditions that Westinghouse has considered in the US make the design suitable for build in the UK and Europe.
It has been considered prudent to locate the reactor sufficiently far enough away from other industrial plant where severe accidents could occur. “In the US we impose a certain distance away from those external forces that could come from a catastrophe at a neighbouring industrial site,” Winters explains.
In terms of terrorist strikes, post-9/11 the regulators have been keen to ensure that new nuclear reactors could withstand, say, an aircraft crash. This includes both the impact of an aircraft fuselage and hard, dense projectiles such as engines. Strictures in both the US and Europe state that the plant’s containment dome must not be damaged by such an event. “If we protect containment to the extent that it can’t be penetrated, which is where our safety systems are, then you protect the core,” says Winters. The plant’s outer structure relies on a “sandwich” steel plate-concrete-steel plate construction that would allow the energy from an object striking the AP1000 to dissipate across the structure. An analogy, Winters suggests, would be the way in which a Kevlar jacket deforms to prevent a bullet passing through it.
The AP1000 creates somewhat less high-level radioactive waste than current reactor designs, but the difference is not enough for it to be a major selling point. If the reactor is built in the UK, the resins that are used to clean water used in the plant will be allowed to get hotter than they would in the US, where there is more land available for burying intermediate-level waste. Long-term storage of spent fuel remains a problem for any country operating nuclear plant.
“What are our advantages?” Winters asks. “We have passive safety systems that don’t rely on AC current. We have simplified the plant and we have a whole lot less equipment that can fail – therefore our reliability is higher. We don’t necessarily overlay redundant equipment to mitigate a design-basis loss of equipment; we eliminate the need for the equipment altogether, if possible. So the design is simpler and there is less maintenance required overall.”
He acknowledges that the regulatory environment in the UK is different to the US, but feels that, in terms of AP1000, the concerns are less over safety and more to do with a detailed probe into how the plant actually works. “They want to know: ‘just how do you do that?’” he says. “I am certain we will get through the generic design assessment.”