A lasting solution? Nuclear safety Q&A with Christophe Poinssot

Materials World magazine
3 Jul 2011
"Question mark" image courtesy of Chris Baker

Public opinion of nuclear power has taken a battering in the aftermath of the earthquake in Sendai, Japan, which disrupted activity at the Fukushima nuclear power reactor. This has led to European countries reassessing whether energy generation should come from another source. Professor Christophe Poinssot from the French Atomic Energy Commission gives his view on this controversial area.

Q: What is your background in the nuclear industry?

After a PhD in Material Sciences and Earth Sciences, I have worked for the French Atomic Energy Commission (CEA) since 1998. In 2003, I took charge, successively, of the research project on spent nuclear fuel evolution, and a team working on radionuclides behaviour in all types of environment at CEA Saclay (Paris). In 2008, I moved to CEA Marcoule, Bagnols-sur-Cèze (Southern France), becoming Deputy Head of the RadioChemistry and Processes Department – an R&D department working on current and future recycling processes of nuclear fuels. I am also Professor of Nuclear Chemistry at the French National Institute of Nuclear Science and Technology (INSTN), and I co-chair the executive committee of the ACTINET-I3 actinide chemistry European network.

Q: Why is nuclear power reported as being the best option to replace fossil fuels?

One of the crucial global problems in the coming years is to find a new energy model that will have a limited impact on the environment. We have to manage –

  • The significant increase in global energy consumption, due to both rising population, and fast and significant economic development of Asian countries such as India and China.
  • The need to decrease greenhouse gas (GHG) emissions to limit climate change.

The only way to meet this requirement is to decrease our reliance on fossil fuels and to promote low-carbon energy. Low-carbon energies are either renewable (wind power, photovoltaic, biomass, hydroenergy), or nuclear energy. Nuclear energy is the only one that is able to produce base electricity 24hr/day, whatever the weather, at a predictable and reasonable cost, and with a low impact on the environment, as far as safety is ensured.

Therefore, nuclear energy is anticipated to grow in the next decades, in what is being called the ‘nuclear renaissance’. However, nuclear energy is only part of the answer, and renewable energies also have to play more significant roles. 

Q: What went wrong in the high profile disasters of Three Mile Island, Chernobyl and Fukushima, and can nuclear power ever be considered safe?

Three main nuclear accidents have occurred at reactors since the launch of civilian nuclear energy in the 1950s. Three Mile Island (USA) in 1979, rated as five on the International Nuclear Event scale (INES), Chernobyl (the former USSR) in 1986, and Fukushima (Japan), in 2011, rated as seven on the scale.

Three Mile Island (TMI) was caused by an incident on the primary circuit and a misunderstanding of the reactor’s state by the operators, leading to the core’s partial fusion. However, this accident had almost no significant impact on the environment.

Chernobyl was mainly caused by a voluntary bypass of all the safety procedures and systems in order to test a new cooling procedure. It led to complete thermal explosion of the reactor core, and the release of all radionuclides, which then dispersed throughout Europe. There is a large debate on the exact impact of this accident, with total number of deaths ranging from the expert view of 4,000 (World Health Organisation) to the very pessimistic figure of 600,000 (Greenpeace).

The Fukushima accident was caused by a rare event – the simultaneous loss of all power sources (including the safety diesel) and the cooling systems, due to tsunami flooding. It led to a melting of the fuels in the reactor cores, as well as hydrogen external explosion following voluntary decompression of the reactor vessel.

Although it is too early to definitively assess the environmental and health impact of the accident, no deaths have been reported, and radiation levels encountered by the onsite workers are within the regulations’ limitations (<250mSv/y), except for four of them at that date.

On average, the global impact of nuclear power remains much lower than other energy sources. For instance, Chinese coal mining already leads to several thousands of deaths per year, whereas coal combustion releases a significant amount of uranium in the atmosphere.

Q: How can nuclear plants be made safer to prevent these types of disasters?

The first improvement is to increase indepth safety reactor design. After each accident, useful information is gained and used to upgrade existing and future reactors (the experience return).

Three Mile Island demonstrated the need to manage partial core melting and hydrogen accumulation without environmental impact. This was used to design the third generation reactors, in particular the European Pressurised Reactor (EPR). It is designed to withstand complete fuel core melting by containing the subsequent corium in a core catcher without any leakage. It is also designed to withstand hydrogen accumulation without explosion, due to the presence of hydrogen mitigators within the core vessel. The core vessel is even designed to withstand a hydrogen explosion (hydrogen is produced by the zircaloy cladding corrosion).

Numerous other examples could be given to show how experience of past accidents is accounted for, and how safety can still be improved in existing reactors.

However, the third generation reactors are just beginning deployment, with the first plants under construction in Olkiluoto (Finland), Flammanville (France) and Tianshan (China). In addition, the results of a recent call for tenders have shown that some countries are not ready to pay for these safety improvements and prefer to commission second generation reactors, at required safety levels. This may change after the events of 11 March in Japan.

Finally, safety culture is the cornerstone of any approach, and still needs to be further developed. In particular, developing and sharing international safety standards is important, as well as the existence of common nuclear task forces that could be deployed at any nuclear accident.

Q: How can nuclear waste be safely disposed of over time, as repository solutions are limited?

Nuclear waste has been studied since the early days of nuclear energy, and, as early as 1957, the American National Academy of Science claimed it should be disposed deep underground in a geological repository (salt rock at that time). Since then, many studies have been performed and have shown –

  • Development of relevant conditioning matrices, particularly for high level nuclear waste.
  • Identification and selection of relevant sites with appropriate long-term confinement properties.
  • Development of repository designs.

France has been a leader in this field, and some key issues must be retained. Managing the waste issue using a purely technical approach failed, leading to low public opinion. This conflicting situation was solved by simultaneously managing the societal and technical approaches. It led the French Parliament voting for two successive Waste Management Acts that allowed the scientific community to study all the potential options between 1991 and 2006. Based on the previous results, they decided in 2006 on the best option – the geological repository – and agreed on implementation planning, leading to a provisional opening date of 2025.

Furthermore, France chose to reprocess spent nuclear fuel, which allows it to recover energy matter (uranium and plutonium), but also confines the most radiotoxic radionuclides (fission products and minor actinides) in a stable matrice nuclear glass – the lifetime of which is in the range of a million years.

Q: Can spent fuel be recycled?

Spent nuclear fuel still contains 96% of uranium and plutonium, which can be used to produce electricity by fission reactions. In terms of sustainability, considering spent nuclear fuels as a waste is not a sensible option. Therefore, France and other countries, including the UK and Japan, have decided to reprocess spent nuclear fuel. It means dissolving the material to separate the actual energetic material, uranium and plutonium, from other elements such as fission products and minor actinides, which can be considered as ultimate waste. The core of the process – the separation step – is based on liquid/liquid extraction using organic molecules that display a high affinity towards uranium and plutonium.

The current process, called PUREX, is based on tri-butyl-phosphate, which is able to bond tetravalent (plutonium) and hexavalent (uranium) cations. This approach is implemented in continuous flows in liquid/liquid contactors, and several steps are needed to reach the required separation and purification levels.

Beyond the separation steps, numerous processes are necessary to recycle the extracted molecules, manage safely the gaseous radionuclides, treat and decontaminate the contaminated effluents, and safely condition the ultimate waste. A final conversion step has to be implemented to convert plutonium and uranium nitric acid solutions to plutonium and uranium oxides. The Pu-oxides are used to produce mixed oxide fuel (MOX), composed of (U,Pu)O2 and used as fuel in Pressurised water reactors (PWRs). This process has demonstrated its efficiency over recent decades, with more than 50,000 tonnes heavy metal (tHM) of spent fuel reprocessed with success in the La Hague and Sellafield plants.

Near-future improvements will come from the co-managing of uranium and plutonium in a so-called COEX process to produce mixed (U,Pu)O2 oxides and increase the intrinsic proliferation resistance of the process. At mid-term, ensuring the multirecycling of plutonium and the use of natural uranium in fast neutron reactors will be important to drastically increase nuclear sustainability (one tonne per year of natural uranium will be sufficient to produce one gigawatt electrical per year, instead of 160t with the current systems).

Additional improvements could come from separating and recycling minor actinides, which are the main contributors to the waste’s long-term radiotoxicity and heatpower. Recycling minor actinides could mean reducing the lifetime of nuclear waste by a few hundred years, making it easier to implement socially accepted repositories for ultimate waste.

Q: Protests from the public have made Germany reassess its decision to maintain nuclear reactors past their shutdown dates. What can be done for the public to accept it as a viable energy solution?

Nuclear energy has a bad image. It is linked partly to the implicit references to military origins and the Hiroshima and Nagasaki explosions. It is also linked to the increasing complexity of science, whereas it is decreasingly studied in secondary schools, and to the lack of education on radioactivity. This leads to the current situation where it is difficult to have a rational debate or explanations.

Q: What can be done?

I think it is necessary to teach some basics of radioactivity and nuclear energy quite early in secondary school to allow people to understand some of what is going on. Also, promoting conferences and unbiased debates where they could learn about the pros and cons of nuclear energy.

More generally, society is not used to assessing and comparing long-term risks and lives in a ‘no-risk’ world, which is removed from reality. Correcting this situation will take time, but is necessary. In particular, most of the crucial issues for humankind are long-term problems, quite far from the timeframe of politics. They require a modification of the decision-making process within society.

It is much easier to close nuclear power plants in the short term, but it is only hiding more significant issues, such as the need for mitigating climate change, and for changing energy production and consumption modes, which all require taking unpopular short-term decisions for longer-term common benefits.

Further information

French Alternatives Energy and Nuclear Energy Commission, CEA MARCOULE, Nuclear Energy Division, DEN/DRCP/DIR – Bât.400 F-30207, Bagnols sur Cèze, Cedex, France. Tel: +33 (0) 466 79 66 18. Email: christophe.poinssot@cea.fr

From next September, Christophe will be a visiting Professor at the University of Sheffield, UK