Improved safety for Europe’s nuclear power plants
Combustible gases are a major safety risk in nuclear power plants. During a severe accident, these gases could ignite and explode, potentially leading to the release of radioactive materials. “After the Three Mile Island(opens in new window) and the Fukushima Daiichi(opens in new window) accidents, huge efforts were made both in the research and the nuclear industry communities to address the potential threats caused by hydrogen combustion in the containment building,” explains Gonzalo Jimenez, associate professor at the Technical University of Madrid (UPM). “Such an event can endanger the integrity of the last physical barriers of the plant.” In the EU-funded AMHYCO(opens in new window) project, researchers used experiments and simulations to boost our knowledge of the combustion threat from mixtures of hydrogen (H2) and carbon monoxide (CO), which could take place in the late phase of a severe accident. AMHYCO aimed to update existing severe accident management guidelines (SAMGs), which provide operators with guidance on such events. The project considered practical methods to reduce the threat of combustible gases, which could also help bolster safety elsewhere. “The risk of combustion, especially hydrogen deflagrations or even detonations, does not only affect nuclear power plants, but many other industries, from chemical factories to hydrogen transport and storage facilities,” says Jimenez, AMHYCO project coordinator.
Improving severe accident management guidelines
The AMHYCO team sought to improve operator actions by increasing our knowledge of H2/CO mixture combustion levels, and to provide recommendations for containment cooling systems operations under severe accident conditions. “This avoids increasing the risk of combustion while, at the same time, keeping the cooling systems limiting the pressure and temperature rise in the containment building,” adds Jimenez. The team also created recommendations based on a set of long-term simulations regarding safety system operation thresholds, including venting time frames, and the repositioning of monitoring sensors and passive autocatalytic recombiners (PARs), which remove H2 from the containment building during an accident.
Expanding the experimental combustion database
There were several key outcomes from the project. The first was the creation of a new PAR correlation, the AMHYCO PAR correlation(opens in new window), which substantially improves on the previous one for late-phase severe accident conditions. The second major outcome was to broaden the experimental combustion database to include H2/CO mixtures. “This has improved the prediction of flammability limits, especially during the ex-vessel accident phase, where in-containment atmospheric conditions are less explored,” notes Jimenez. The team also created a new methodology to improve simulations of severe accidents in the containment building, and a series of recommendations to SAMGs for safety operations and mitigation systems. “These will probably have a noticeable impact on how nuclear power plants address combustion management in the near future,” remarks Jimenez.
Spreading new knowledge to boost nuclear safety
Although the project has ended, there will be many follow-up events in the coming months. These include three PhD students defending their theses, several journal and conference papers soon to be published in academic journals, and the release of all non-restricted project deliverables to the general public. Major SAMG developers, nuclear power plant owners and regulatory bodies in several countries will also all be informed about the recommendations to improve SAMGs. “The AMHYCO community will be ready to apply to future European Commission calls, to continue working to make nuclear energy safer against combustion threats in severe accidents,” says Jimenez.
