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The Joint Research Centre (JRC) is the European Commission's science and knowledge service which employs scientists to carry out research in order to provide independent scientific advice and support to EU policy.
The JRC and the Japanese Atomic Energy Agency (JAEA) have successfully demonstrated the principle of an innovative technique analysing particle-like debris of melted nuclear fuel. The demonstration experiment was carried out during a scientific workshop held on 4 and 5 March at JRC premises in Geel, Belgium. It brought together also specialists from the International Atomic Energy Agency (IAEA), the US Department of Energy, the European Commission’s Directorate-General for Energy and other European and international bodies.
The Neutron Resonance Densitometry (NRD) method has been jointly developed by the JRC and JAEA and is a fruit of intense collaboration since 2012. It accurately quantifies the amount of uranium and plutonium in complex materials such as debris of melted nuclear fuel from the damaged Fukushima Daiichi reactor cores. So far, accurately measuring fissile components in these materials has been a challenging exercise for nuclear experts.
The new method will be a valuable contribution to international safeguards inspection, which ensures that the analysed nuclear material is not diverted to non-peaceful use. The Neutron Resonance Densitometry (NRD) technique could potentially also be of interest in nuclear security, in decommissioning and for the characterisation of spent fuel.
The method uses neutrons to identify and quantify fissile materials. When neutrons interact with matter their interaction probability shows important variations as a function of their energy. These variations, called resonances, are characteristic for the nuclear materials in the sample. Accordingly, the resonances can be used as a 'fingerprint' to characterise the composition of the sample.
NRD relies on the principles of Neutron Resonance Transmission Analysis (NRTA) and Neutron Resonance Capture Analysis (NRCA), which are two methods that have been developed at the JRC’s facility GELINA (GEel LINear Accelerator). JAEA had contacted the JRC for assistance in the development of the new method, thanks to its expertise in the field.
Quantifying the amount of uranium and plutonium in particles of melted fuel has always been a very challenging task, due to the lack of an accurate quantification technique. This sort of analysis falls under the international nuclear safeguards control system, designed to deter nuclear proliferation and to address undeclared nuclear activities, ensuring that fissile nuclear material (as uranium and plutonium) is well accounted. The safeguards system includes regular inspections of civil nuclear facilities and related audits of the movement of nuclear materials.
Complex materials like particle debris (having variable grain size), may be produced as a result of a severe nuclear accident. For example, small nuclear particles can be formed in a steam explosion when high temperature melted fuels drop in water of a damaged reactor. Small particles are also created during the decommission process of melted reactor cores, when lump debris is broken into smaller fragments.