CHaracterisation, Imaging and MaPping of fuel debris for safe retrieval (CHIMP)

This research is a joint UK and Japan effort to support ongoing clean-up operations at the Fukushima Daiichi Nuclear Power Plant (NPP). On March 11 2011, the tsunami that engulfed the Fukushima Daiichi NPP resulted in a loss of coolant accident and the partial meltdown of boiling water reactor Units 1 - 3. Immediately after the meltdown, seawater was injected into the high-temperature reactor cores for emergency cooling, however this was not sufficient and temperatures rose in excess of 2000C, causing pellets of UO2 nuclear fuel to melt and react with steam-oxidised zircaloy fuel cladding. The resulting material is highly hazardous and comprises a mixture of UO2 fuel, zirconium from the cladding and other melted reactor components, such as concrete and steel from the fuel pressure containment vessel. These materials are known as fuel debris and corium and are highly radioactive since they also incorporate fission products and minor actinides (e.g. plutonium) from the fission of the fuel.

Since the accident, water has been continuously injected to the reactor, which has successfully cooled the fuel debris and corium. In this so-called "stable cold shutdown condition", analysis of coolant water effluent has been shown to contain plutonium, which indicates that the fuel debris is being dissolved. This has prompted significant efforts to retrieve the fuel debris and corium from the reactors, but before any plans for retrieval can be made, and technologies selected to safely undertake the retrieval operations, it is necessary to understand: 1) the chemical and physical properties of the fuel debris and corium; and 2) where the fuel debris and corium reside within the reactor.

Because this material is not found in 'normal' nuclear decommissioning operations, and because it is not possible to enter the reactor to take samples, or to locate its position due to the extreme radiation field, other solutions are required. In this research project, we will make surrogate fuel debris and corium materials using UO2 doped with "cold" fission product analogues (e.g. stable isotopes), melted together with zirconium cladding and other reactor component materials (e.g. concrete and steel). In comparison with real fuel debris and corium, these will be safe to handle in the laboratory. By analysis of these materials, which will be the most realistic fuel debris and corium simulants created to date, we will build an understanding of their properties, which will support the choice of decommissioning methodologies (e.g. which type of robot and cutting tool) and how to handle the materials once they have been retrieved from the reactor.

These materials will also be used to validate two important techniques currently under development by our Japanese project partners to retrieve fuel debris and corium from the reactors. The first is an on-site remote 3D imaging technique, capable of identifying the type of radioactive elements in any given piece of fuel debris or corium within the reactor, without direct contact. It is proposed to test this method during our project at the Fukushima Daiichi NPP. The second is a statistical mapping model, which will use input from the characterisation of our simulant fuel debris and corium materials, combined with data from the gamma radiation emission from the fuel debris within the Fukushima Daiichi NPP, to make a predictive map of where the fuel debris and corium reside within the reactor. This model, our materials and the collaboration between UK and Japan partners in this project, will strongly support the planning and implementation of safe fuel debris retrieval operations, which are due to begin in 2021.

Through the generation of realistic simulant fuel debris and corium materials this research has the potential to underpin the decommissioning strategies for the Fukushima Daiichi NPP and also legacy nuclear sites in the UK Nuclear Decommissioning Authority estate that contain degraded and exotic fuels. Improved understanding of fuel debris and corium chemical and physical characteristics, and the alteration behaviour of these materials in water-logged conditions , will help to select optimal fuel retrieval technologies and methods. Development of remote 3D imaging techniques on-site at Fukushima, validated with our simulant materials, will support fast on-site screening of fuel debris and corium materials for nuclear material accountancy. Development of a predictive mapping tool will allow remote analysis of the location of fuel debris and corium within the reactor, and also of nuclear materials in hazardous UK legacy nuclear sites, supporting a reduction in the hazard, cost and timescale of decommissioning, enabling accelerated decommissioning of nuclear sites. This may have a secondary impact of enhancing public acceptance of civil nuclear energy generation and geological disposal of radioactive wastes at an important time prior to the launch of the geological disposal facility siting process.

The impacts will be realised through the primary beneficiaries, who include international organisations and academics undertaking decommissioning, or research in support of decommissioning, nuclear accident or legacy nuclear sites, where nuclear fuel is severely degraded, and also waste management organisations responsible for disposing severely degraded or exotic nuclear fuels. The impacts to these groups include:

-- New realistic fuel debris and corium simulant materials on which to safely investigate fuel degradation behaviour;
-- Scientific understanding and new remote imaging techniques to underpin decommissioning operations and disposal options for degraded spent fuels;
-- Publications in peer-reviewed journals, conference proceedings and lay-person articles (e.g. The Conversation);
-- Potential reduction of the estimated £18Bn cost of radioactive waste disposal in the UK, through reduction in the hazard, cost and timescale of decommissioning;
-- Improved public confidence in UK and Japan radioactive waste disposal strategies, through provision of underpinning scientific understanding and data;
-- Strengthened UK-Japan collaborative efforts in nuclear waste management and disposal.

The Pathways to Impact will focus on dissemination of knowledge generated in the research project; this will be through existing project websites (JUNO and the Nuclear Waste Repository), a final workshop with international end users (e.g. TEPCO, NDA etc.) and academics, and public engagement events intended to share our findings with the public.