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Objectives

The goal of the SEARCH proposal is to group critical safety related research focussed on the coolability of the fuel with and without forced convection, the chemical behaviour of the heavy liquid metal reactor coolant and impurities, the interaction of fuel with the coolant and the behaviour of (volatile) radioisotopes in the coolant. Indeed, although information of this type is essential for the licencing process, it turns out that actual experimental data in this field are rather scarce or non-existent.

The scope of SEARCH encompasses basic studies including an experimental and numerical analyses of heat transfer in the fuel bundle, the determination of the impurities source terms, the investigation of migration, deposition and release of impurities and radioisotopes and finally the development of capture and purification techniques. In particular the objectives of the project are to:

  • Study the heat transfer of a wire spaced fuel bundle in forced and natural convection
  • Investigate the source term for impurities in the coolant including corrosion products, spallation and activation materials and water from a possible heat exchanger leakage.
  • Study the mass-transport of dissolved metals and oxides in the coolant.
  • Develop filtering techniques for the coolant in both loop and pool configuration.
  • Develop and validate an oxygen control system based on solid oxide particles including the influence of the cover gas, formation and dissolution kinetics and measurement techniques in a deep pool configuration.
  • Study coolant-fuel interaction and characterise reaction products with their influence on fuel.
  • Study the phase relations of the elements in MOX fuel and the coolant (Pb-U-O, Bi-U-O, Pb-Pu-O, Bi-Pu-O).
  • Investigate the dispersion of fuel through the reactor system in case of a core melt and in case of slow fuel release by computational fluid dynamics (CFD) and SIMMER simulations.
  • Identify and study the parameters that influence the evaporation, gas phase transport and deposition processes of radioisotopes from the coolant including the chemical composition of the evaporated compounds
  • Determine physicochemical parameters necessary for the prediction of vapour phase concentrations of radionuclides in an ADS and the development of gas phase filtering systems for their removal.
  • Use ab-initio quantum-mechanical calculations to calculate crucial properties of the evaporation and deposition processes of volatile radionuclide including the interaction with potential filter materials.

The main motivation for the selection of these specific topics is that they all are related to important operational and safety issues.

The coolability of the fuel is the most essential issue in the design and operation of a nuclear system. Both in forced convection and in natural convection which may occur after a loss of flow as happened in de Fukushima accident coolability must be guaranteed. Another type of failure of the cooling can happen because of an assembly or channel blockage. In this respect the chemistry control of the coolant of a heavy liquid metal cooled nuclear system in normal operation is very important. The chemistry control concept for heavy liquid metal coolants can be subdivided in two major aspects. The first aspect is the specific control of the oxygen content in the coolant. This is required to protect the structural materials in the reactor from corrosion on the one hand and to avoid excessive precipitation of lead- and bismuth oxides on the other hand. The second major aspect of chemistry control deals with the management of impurities in the coolant. It is clear that uncontrolled impurities can lead to depositions in the reactor causing reduced cooling or blockages with potential disastrous consequences. The main sources of impurities are lead oxide precipitation, corrosion products but also radioactive elements originating from spallation and neutron activation reactions.

The compatibility of the fuel and the coolant is a critical issue in the safety assessment of a nuclear system. Both low probability accident scenarios involving a (partial) core melt and the high probability event of a leaking fuel pin must be considered. The full analyses of these scenarios using appropriately validated codes like e.g. Simmer III require further experimental data on basic properties of the interactions between the materials involved in scenarios mentioned above.

For licencing preparation it is important to investigate the consequences of the reference severe accident scenarios. Although the selection of the reference accident for MYRRHA is not clear at this point, it will most likely involve damage to the fuel pins. In SEARCH two scenarios will be considered: slow, long term release of fuel corresponding to a set of leaking fuel pins and a scenario involving a large, short term release of fuel which corresponds to a partial or complete core melt. We will investigate the dispersion of molten fuel in the system by means of numerical simulations.

The main concern of the safety authorities is the prevention of risks to the general public, both in normal operation of a nuclear reactor as well as in accident scenarios. The most important risk is the escape of radioactive materials to the environment. This means that the release of these elements, both from fission in the fuel and from the spallation and activation reactions in the liquid metal of an ADS system must be investigated. Furthermore, possible methods to capture these elements in the covergas conditioning system and their kinetics and efficiency should be examined. In comparison to water cooled thermal reactors, a lead-bismuth cooled ADS system such as MYRRHA has, besides fission gasses and light activation products, the additional presence of heavy volatile elements such like Po and Hg. For this reason the main focus will be put on these elements. The release of Po into the environment in various accident scenarios involving damage to the confinement structure will be assessed using the experimental data gained in the project.

In all experiments LBE is selected as the primary coolant and not pure lead. The reason for this choice is twofold. Firstly, since MYRRHA will be the first HLM cooled nuclear system to be deployed in Europe as shown in the ESNII roadmap, SEARCH will put its emphasis on LBE as it is the coolant used in MYRRHA. Secondly, because LBE is chemically more complex than pure lead, experiments performed using LBE can more readily be extrapolated to pure lead at the same temperature range whereas the reverse is not the case. In this sense using LBE is more generic than using pure lead.

SEARCH will also include a work-package on education and training. The objective is to organise a number of workshops and schools with the objective to educate and train students and PhD's in topics relevant to the project.

SEARCH will be linked to existing FP7 projects related to Gen IV system development in general and to LFR/ADS systems in particular. These include CDT (Central Design Team), LEADER (Lead-cooled European Advanced Demonstration Reactor) and THINS (Thermal-Hydraulics of Innovative Nuclear Systems). SEARCH will partly build on results obtained in the FP6 project EUROTRANS (European Research Programme for the Transmutation of High Level Nuclear Waste in ADS).