Above: The Chooz nuclear power plant was the first reactor to use the new axial economiser steam generator design Photo credit: Raimond Spekking/CC BY-SA 4.0 (via Wikimedia Commons)


a horizontal pre-heater, but due to the fact that it was horizontal it generated water hammer.” In contrast, Framatome’s economiser sends the


recirculated water in a vertical plane parallel to the steam tubes. This allows pre-heating without getting any water hammer or vibration in the pipes. Merlin explains the key challenge of water hammer and the benefits of designing it out of the steam generator: “It’s very important because you have always a thermal stratification and water hammer decreases the life expectancy of the steam generator tubes. There is no cross flow, so no potential tube vibration and also increased water level stability in the steam generator. This economiser contributes to an increased lifespan over a normal steam generator because there is no water hammer”. He also highlights several additional benefits of the design: “There is a dedicated auxiliary feed water system to improve the pressure shell design. It also increases operating flexibility because there is a bigger separation of steam and water.” Merlin also argues that the design helps to minimise


maintenance costs too: “It also facilitates better sludge management and has a higher permeability of the tube support plate to reduce the clogging risk and a foreign object catcher is included by default on this type of design,” he says.


Summarising the benefits of the new design he notes: “In the end you get an increase in outlet pressure and power for a given heat exchange surface so you can get less tubes for the same power and thus a more compact design or get a potential upgrade by keeping the same number of tubes and the same surface, but much higher surface heat exchange.” Merlin continues: “The first effect is to increase the


thermal transfer in the steam generator but also you get more cooling capacity and because it creates increased margins for safety analysis it is possible to increase the core power. In the event of an accident and even during normal functioning given this there is a higher margin to increase power from the core, so the fact that you have this double effect gives you the possibility to have this larger power capacity.” To realise this opportunity in an existing facility some additional requirements and upgrades are needed to other elements of the balance of plant. “To transform the operating electricity output you need turbine investment, but it’s definitely the cornerstone of any such very large improvements on the nuclear steam supply system,” says Merlin


The economiser also takes advantage of materials development that have taken place over the decades, as Merlin points out: “Beyond the economiser on this state-of- the-art steam generator architecture we use 690 stainless steel thermal treated tubing for the internals”.


Steam generator replacement at Ringhals 4 Although the enhanced economiser design has been installed in the four units at Chooz and Civaux as well as the EPRs at Taishan in China, Flamanville in France, Olkiluoto 3 in Finland, and at Hinkley Point C in the UK, this design has also been deployed as an upgrade at the Ringhals 4 plant in Sweden. Ringhals, located some 60 km south of Göteborg on the


Värö Peninsula, is host to three PWRs (units 2, 3 and 4) and one boiling water reactor (BWR). All units were constructed between 1969 and 1985. Ringhals is owned by Ringhals AB, which in turn is owned by Vattenfall (70.4%) and EOn (29.6%).


The Ringhals 4 unit started operations in 1982 and


features a WH-3 loop steam system with W-D3 steam generators. In 2007 Ringhals AB applied to the regulator to increase the plant’s thermal output from 2783 MWth to 3300 MWth. A prerequisite for the uprate was the replacement of the unit’s three steam generators. These were replaced during the summer of 2011 as part of an uprating and life extension project. In January 2007, following a series of studies carried


out by Framatome – at the time AREVA – the nuclear technology company was awarded a contract for life extension of the unit and a simultaneous uprate in capacity with the replacement of the three steam generators and the pressuriser. In another project in 2007 replacement of the low-pressure steam turbines led to a 40 MWe power increase. Nuclear power plants typically replace steam


generators periodically and it is a process that is also often considered when existing plants are modernised as part of a life extension programme. Merlin, explaining the overall strategy behind the design, says: “In the original Westinghouse philosophy the steam generators are smaller. To keep everything in the containment building without having a very large steam generator and four loops we had to find a solution to increase the thermal transfer”. Merlin notes the strategy behind the adoption of the new steam generator design: “Generally when you do steam generator replacements, you try to get a steam generator which has the same volumes as the previous one you are replacing.


www.neimagazine.com | January 2025 | 19


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