'Meltdowns' Making Safer Reactors: CEA's PLINIUS Platform and Finland's VTT

[See also "Alexander Borovoi: Chernobyl Explorer"]

The mention of the word meltdown conjures our deepest fears about nuclear power: an out-of-control reactor core collapsing into molten radioactive "lava" that consumes metal, reactor housings, concrete foundations - even, as cautionary tales warn, the ground beneath the reactor itself. While the "China Syndrome" is thankfully just a frightening fiction, complete or partial meltdowns have occurred in several severe accidents, notably Three Mile Island, Chernobyl, and most recently at three of Fukushima Daiichi's reactors. Recent European studies have even suggested that the likelihood of a severe accident is much higher than previously thought:

"Based on the operating hours of all civil nuclear reactors and the number of nuclear meltdowns that have occurred, scientists at the Max Planck Institute for Chemistry in Mainz and the Cyprus Institute have calculated that such events may occur once every 10 to 20 years (based on the current number of reactors) - some 200 times more often than estimated in the past."

Clearly, one important goal must be designing reactors that resist losing structural integrity in the event of an accident or meltdown, that can reliably withstand molten corium's extreme temperatures without breaching containment. Our scientific understanding has been limited in the past by the difficulty of studying meltdowns during actual emergencies, and in recreating their volcanic conditions in a controlled environment. This is one of the reasons why the innovative prototypic core melt research being performed at facilities like CEA's PLINIUS Platform, and the VTT Technical Research Centre of Finland, is crucial in developing new building materials for safer nuclear plants, as well as learning to prevent meltdowns and minimizing environmental exposure should one occur.

PLINIUS is an experimental platform for the study of severe accidents using prototypic corium, laboratory-created high-temperature molten mixtures containing depleted uranium oxides that recreate the "melt" of theoretical "meltdowns".

Its four facilities study different aspects of corium formation and behavior: VULCANO is a "50-100 kg corium melting facility," where oxides and metals are combined in a rotating cylindrical furnace and melted using induction heating, transferred plasma arc technology, or through exothermic redox reactions ("uranium thermite"). The "melt" is then poured into a test section of concrete, ceramic, or other material to study flow patterns, chemistry, and resulting material structure. COLIMA is a smaller, enclosed induction-heated unit that allows study of corium/gas interaction and aerosol formation, while KROTOS is used to observe and measure the behavior of small (~5kg) quantities of corium when dropped into water. As PLINIUS' site understatedly describes, "energetic steam explosions can be triggered and studied":

Heat transfer between the hot molten core with the colder volatile water [in KROTOS] is so intense and rapid that the timescale for heat transfer is shorter than the timescale for pressure relief, leading to the formation of a shock wave. This shock wave is intensified as a result of further mixing and energy transfer as it travels through the mixture.

The smallest of PLINIUS' facilities, VITI, (image above is a tiny molten corium droplet in the VITI crucible) specializes in tests using corium samples of less than 100 grams, mainly used for thermophysical/thermochemical property analysis, or controlled-atmosphere material interaction tests.

While Finland's Technical Research Center (VTT) conducts international applied industrial research and analysis in a variety of fields, its nuclear reactor safety program has also developed advanced simulated reactor environmental modeling (APROS) and reactor aging studies, as well as joint corium melt research with CEA's PLINIUS. Ongoing research into the science behind meltdowns, such as the work being done at PLINIUS and VTT, will help create safer, more accident-proof nuclear reactors and containment structures for our world's endlessly growing need for cleaner energy.

CEA PLINUS Platform Research Links:

• The first PLINIUS-LACOMECO Workshop on Transnational Access to Large Severe Accident Research Infrastructures, held in Aix en Provence, October 26, 2010
• Joint PLINIUS VULCANO and VTT HECLA research: "Current European Experiments on 2D Molten Core Concrete Interaction: HECLA and VULCANO," Christophe Journeau, et al., (from International Conference on Advances in Nuclear Power Plants, ICAPP 2008. Anaheim, CA)
• "Research at the CEA in the field of safety in 2nd and 3rd generation light water reactors," P. Billot, Comptes Rendus Physique, Volume 13, Issue 4, May 2012, Pages 340–351, Science of nuclear safety post-Fukushima [PDF]
• PLINIUS Home
• "TRANSNATIONAL ACCESS TO THE PLINIUS PROTOTYPIC CORIUM EXPERIMENTAL PLATFORM," C. Journeau et al., (2003)
• "Corium Behaviour Research at CEA Cadarache: The PLINIUS prototypic corium experimental platform," P. Piluso, et al., (2002)
• "Two EU-funded tests in VULCANO to assess the effects of concrete nature on its ablation by molten corium," C. Journeau et al., (2010)

VTT Research Links:

• "Finland: VTT-led project aims to reduce disaster impact"
• VTT and Nuclear Energy Research, presentation by Rauno Rintamaa, Vice President, VTT Business Solutions at the Workshop for the Presentation of French offering in the field of nuclear energy as part of low carbon energies, Finlandia House, Helsinki, November 28, 2010 [PDF]
• VTT Nuclear Reactor Safety Analysis
• SAFIR2010 Program, March 11, Final Seminar of the Finnish National Research Programme on Reactor Safety
• "Molten Core Concrete/Interactions in Nuclear Accidents: Theory and Design of an Experimental Facility," T. Sevón, (ESPOO 2005), VTT Research Notes 2311 [PDF][Helsinki University of Technology dissertation]
• "HECLA experiments on interaction between metallic melt and hematite-containing concrete," by T. Sevón, Nuclear Engineering and Design, Volume 240, Issue 10, October 2010, Pages 3586–3593, 4th International Topical Meeting on High Temperature Reactor Technology (HTR 2008), with Regular Papers
• Serpent: a three-dimensional continuous-energy Monte Carlo reactor physics burnup calculation code from VTT