The two MW generated by a Combined Heat and Power (CHP) module hardly seem worthy of mention compared to the output of a nuclear power plant. But since nuclear power stations in Germany started to be taken off the grid as a result of the political decision to opt out of nuclear power, modular CHP plants have become increasingly important. And that means inside the nuclear plants themselves – as the example of Unterweser illustrates. The facility has not delivered power for a good three and a half years now, but is being served by a CHP module from MTU Onsite Energy that provides it with heat and electricity.
Leaving the heavily trafficked motorway between Bremen and Bremerhaven, visitors crossing the Weser on their way to the NPP are suddenly confronted with the proverbial flatlands on the far side of the river. Narrow roads lead across fields where cows graze alongside the occasional wind turbine. On the dykes there are sheep that scrutinize the passing cars with interest. Right in the middle of this idyllic scene, not far from the hamlet of Kleinensiel and only a stone’s throw from the lower reaches of the Weser, the concrete dome of a nuclear reactor looms skywards. The Unterweser nuclear power plant was the region’s largest electrical power provider for a good 30 years. But within days of events in Fukushima, it turned from a large-scale producer of power to a major consumer.
“All those years we were able to identify positively with our jobs and saw ourselves as performing a useful purpose by generating electrical power on a large scale for vital use,” recounts E.ON engineer Dominic Ransby half an hour later on the power plant premises following a thorough check by security staff. Those days were abruptly brought to an end in March 2011 when the German government took the decision to immediately shut down the country’s oldest nuclear power stations. Since then, the Unterweser plant has been in “continuous zero output mode”. The word “decommissioning” does not exist in E.ON vocabulary at present. The company is legally contesting the withdrawal of its operating license, for which it received no compensation. So decommissioning work cannot begin until the courts have reached their verdict.
Now only silence
In the generator block shortly afterwards, Dominic Ransby opens the door to the gigantic turbine room. “In the past you would have been blasted by the heat, the smell of hot oil and the noise,” he recounts. “Now there is only silence.” The high-pressure turbine and the three lowpressure turbines once steam-powered by the heat arising from the nuclear fission process next door now stand idle.
Ransby says he does not get emotional at the sight of the disused installation. “We cherished and cared for it for years, but ultimately, as an engineer, it makes no difference to me whether I am putting something together or taking it apart.” He is fully aware, however, that some of his colleagues see things differently and quite definitely have their problems with the present situation. Obviously, they talk about the future: “A lot of the people here are worried about what will happen next.” Of the 400 employees working at the Unterweser facility at the beginning of 2011, only 200 or so remain, and not even all of them will be able to stay for the entire duration of the decommissioning work. It is expected to take a good ten years once it gets underway. Aged 50, Ransby himself is unlikely to need to look for a new job at this stage in his career – in all probability he will stay until the end and then retire.
CHP module delivers 2 MW
Nevertheless, he has had to find himself new work to do in the everyday activities that remain. “The changes after the shutdown have demanded a degree of flexibility from all of us,” he says. So it suited him well that his colleague Dr Uwe Werner, a former shift manager like himself and as such responsible for the operation of the nuclear plant, started planning for the construction of a modular CHP plant – a project in which he could get involved. “We still have quite a big power requirement,” explains Werner, “which is for things like pumps, fuel-rod cooling, and the normal building infrastructure.” In total, the present internal energy demand is around 3.5 MW of electrical power and – depending on the outside temperature – up to 4 MW of heat. Since the beginning of 2014, a good proportion of that has been delivered by a CHP module from MTU Onsite Energy. Based on a Series 4000 gas engine, it generates just under 2,000 kW of electrical power and 2,200 kW of heat.
The CHP plant is housed in a shipping container standing on a solid foundation in the outer part of the grounds directly between the generator house and the Weser dyke. There it produces just under 2 MW of electrical power and 2.2 MW of heat, achieving an efficiency of roughly 86%. “That saves us money because we only have to draw part of our electrical power requirement from the grid and can cover virtually all of our heat demand ourselves,” outlines Ransby.
Auxiliary boiler steps in when it gets cold
It is only in the coldest few weeks of the year that the CHP module cannot entirely cover demands. When that happens, one of the two auxiliary boilers comes into action. In the past, the oil-fired 12.7 MW-rated boilers were there to bridge the brief downtimes that occurred about once a year when the fuel rods were replaced. To further reduce costs and also lessen the impact on the environment, one of them is currently being converted from oil to gas and simultaneously rerated to an output of 4.5 MW. That is only possible because a gas supply line was laid to the facility when the CHP module was being installed. Another reason why the auxiliary boilers are needed is because, unlike the CHP plant, they produce steam. “And we need that for our water treatment,” Ransby explains. Water treatment in a nuclear power plant refers to evaporating water that is contaminated because it has been use to cool, for example, the fuel rods. As soon as a certain quantity has accumulated, the boilers are started up temporarily for that purpose.
Ransby and Werner are happy to have found a new and interesting project to take care of in the shape of the Combined Heat and Power module. “People at nuclear power stations understand all about water plumbing systems and combustion plants, so it is no mystery to us,” observes Werner. “It’s a nice job that we enjoy.” And the reliability of the CHP module is outstanding according to Ransby. From the beginning, the two were in no doubt that MTU Onsite Energy would be on the short list of potential suppliers for a CHP plant. After all, the emergency generator sets installed years before were still in perfect working order and had been supplied by MTU. In all, there are seven of them spread across the site – four just for the safety-related systems (everything that is safety-related exists multiply), two “emergency backup gensets” (in case the emergency diesels should fail), and one for nonsafety-related systems.
If everything goes according to plan, the CHP module will play a key role in supplying power to the nuclear plant for at least another ten years – until all that remains of the former NPP is a green field where it once stood. “As soon as decommissioning starts, we will again need more electrical power, simply because there will be more equipment in use,” illuminates Ransby. “It will be an entirely different project from pulling down a conventional power plant. Everything has to be individually dismantled and, where necessary, decontaminated – every single nut and bolt is separately logged,” he said, explaining that ultimately, radioactive contamination is just pollution that can be cleaned up like any other kind of dirt. That means that the vast majority of the material from the Unterweser nuclear facility will be ultimately recycled. “Only a small part is seriously radioactive and has to be taken to the nuclear waste repository. Of the total decommissioning volume of 300,000 tonnes, it represents a very small percentage.”
Until then, the used fuel rods are being stored at the Unterweser site under maximum security. The older ones are stored in sealed petalite containers in a temporary repository opened in 2007; the more recent ones are still in the reactor building. The reactor building is the last stop on our tour of the facility. Before entering the dome, we have to don safety suits and shoes, while a dosimeter constantly measures the radiation exposure level. We are then taken through an air lock into the inner chamber and finally to the spent fuel pool, which at first sight looks hardly any different from a small swimming pool. On closer examination, however, you can clearly see the fuel rods at the bottom being cooled by the boracic acid-infused water. Not far away, a large frame stands on the floor that can be opened underneath. Directly below it is the reactor pressure vessel. Dominic Ransby looks across and smiles. In the past, he was often the last person to check the gigantic bolts after the fuel rods had been changed. “That’s something not many people can claim,” he says thoughtfully, “that they were the last to stand on the reactor cover.”
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