discovered_01_2015

RESEARCH// THE HZDR RESEARCH MAGAZINE WWW.HZDR.DE 26 27 sodium – with the fatal result that this causes particularly intense reactions. This is why, many years ago, Germany and the UK decided to abandon breeder technology, shortly after, or even before launching their facilities. In France, on the other hand, ‘ASTRID’ is a serious project, slated to be the first Generation IV reactor. It could be launched as early as 2025. Sodium fires did, however, break out at its predecessor project Phénix. ‘Nuclear science has learned from these incidents and the facilities have been revamped accordingly,’ the HZDR scientist says. This included automatic fire extinguishers and retention systems as well as double-walled pipes for the sodium. A sodium leak at Japanese reactor Monju in 1995 demonstrated another critical requirement: ‘It is vital that the operating team is extremely well trained and able to react to an incident immediately. And of course, the population must be fully informed,’ Merk explains. Safety first Light water reactors and fast reactors both have a high level of inherent safety. That means that the facility stabilizes itself in a physical sense. For example, the power production of a light water reactor will drop in the event of a loss of water and pressure. When fuel temperature rises, both light water reactor and fast breeder fuel will capture more neutrons without triggering a fission reaction. As soon as a fast breeder heats up, it expands – which also throttles its performance. Bruno Merk has been pondering a way to increase reactor core stability with the objective of capturing a larger number of neutrons in the fuel at rising temperatures. In order to achieve this, more neutrons must be safely decelerated. While water does decelerate neutrons, it isn’t safe to use in a fast breeder due to the highly reactive sodium. The desired deceleration effect can, however, be achieved by binding hydrogen in a compound with a metallic substance such as yttrium and using this material in the spacers between the many individual fuel rods in the core of the reactor. In this process, it is important to finely disperse the yttrium hydride in the fuel assemblies. This idea has already been patented. ‘German nuclear scientists are driven by the same kinds of issues as our international colleagues,’ Merk says. ‘We want to find out which methods achieve the highest safety levels.’ Another goal is to further optimize the burning of fuel in fast reactors in order to minimize nuclear waste. Other scientific fields also contribute to the safety of sodium- cooled reactors. In Germany, intensive research is being conducted on the topic of liquid metals, as they are also used in many other industries and are of increasing importance in future technologies such as innovative batteries or solar power plants. Liquid metal very effectively stores or conducts large amounts of energy or heat, with a heat conductivity that beats water by a factor of up to 100. New measuring methods, which are also being developed at the Helmholtz Center in Dresden, make it possible to fully monitor liquid metal flows. Recent years have seen a significant improvement in the operational safety of liquid metal technologies. ‘At the end of the day, research will always just open up possibilities, point the way, and trigger informed debates,’ Bruno Merk believes, now more than ever. Society must decide if it wants to use a technology, carefully weighing its risks and dangers. By 2022, Germany will have abandoned electricity production from nuclear power plants. Yet the country needs experts like Bruno Merk and his colleagues at HZDR and the Helmholtz Centers in Jülich and Karlsruhe to help ensure the safety of reactors that are being built just a few miles beyond our borders in Belgium, France, the Czech Republic or Switzerland. PUBLICATION: B. Merk et al.: ‘Progress in reliability of fast reactor operation and new trends to increased inherent safety’, in Applied Energy 2015 (DOI: 10.1016/j.apenergy.2015.02.023) _Institute of Resource Ecology at HZDR Dr. Bruno Merk b.merk@hzdr.de CONTACT TEST: View of the BN-800 control room at the first stage of criticality. Photo: Rosenergoatom

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