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Periodic Report Summary 1 - SEARCH (Safe ExploitAtion Related CHemistry for HLM reactors)

Project Context and Objectives:

During the previous decades scientists and engineers have been working on the development of fourth generation nuclear systems to contribute to the improvement of the safety and sustainability of nuclear energy. One type that is being investigated is reactors cooled with a heavy liquid metal for which MYRRHA, being a Lead-Bismuth cooled accelerator driven system, will act as the European Technology Pilot Plant (ETPP). SEARCH is a collaborative project that supports the development of MYRRHA by investigating the safe behaviour of the fuel and the coolant in the reactor. In previous EC framework programmes, design work and research for the development of heavy liquid metal cooled nuclear systems has been performed. SEARCH intends to take the work a step further by using the results of the previous efforts to study the safe operation of the design. In particular we will look at the coolability of the fuel pins in all circumstances, the behaviour of the coolant, the compatibility of the fuel with the liquid metal and the dispersion of the fuel in the reactor in accident cases and finally the possible release of radioactive elements. All these aspects need to be thoroughly studied to ensure the safe operation of the proposed reactors.
The work in SEARCH has been split into several technical work packages. Firstly, the project will investigate the coolability of a fuel bundle in all operational circumstances by measuring the heat transfer to the coolant. Emphasis is put on a wire spaced fuel geometry as has been employed in previous fast reactors and as will be used in MYRRHA. Numerical simulations will support the interpretation of the results. Secondly, the control of the oxygen content and the management of impurities in the melt will be studied to ensure that no blockage due to poor coolant chemistry control takes place. The next step in the safety assessment of a nuclear system is the compatibility of the fuel with the coolant should a fuel clad failure occur. The full analyses of clad failure with computer codes require experimental data on "basic" properties of the interactions between the materials involved. For that we will study the chemical behaviour of a mixture of fuel and coolant to determine the energy release, solubility and the formation of new chemical compounds. After severe clad failure the fuel pellets could disperse in the coolant. With computer simulations we will look at the migration of the fuel and the possibilities of fuel accumulation. The prevention of risks to the general public will be studied by looking into the escape of volatile radioactive elements like Po and Hg into the environment. Methods to capture these elements in the covergas and their evaporation from the coolant will be studied. This work will also addressed by quantum-mechanical calculations. Particular attention will be paid to dissemination of the results to which a separated work package is dedicated. Besides publication to the scientific community via scientific journals and conferences also direct dissemination to the research community is envisaged by employing the excellent connections between the SEARCH project and other related European framework projects. In addition two workshops and a training school will be organised.

Project Results:

The main results are summarized below:

To study the coolability of the MYRRHA wire spaced fuel assembly in flowing LBE, two experiments covering forced and natural circulation are foreseen at KIT and ENEA. For both, the pre-test analyses of the rod bundles were done using CFD or CGCFD methods at KIT and NRG. Results provided good information to the experimentalists for the instrumentation and design. For both set-ups this phase has now been completed and construction in ongoing. The experimental campaigns are expected for October-November 2013 in line with the project scheduling.

The design and construction for experimental set-ups for filtering tests and mass transport experiments has been done. The source terms for impurity content, activity and decay heat were calculated for the LBE pool in the worst case scenario. Quite a large number of new elements are generated during irradiation while of impurities initially present, irradiation only significantly affects Ag that transforms into Cd. Radioactivity and decay heat release are on a mid-term scale dominated by Po-210. Long-term activity and decay heat is determined by Bi-208 and Bi-210m. The source term for non-radioactive impurities coming from steel corrosion has been estimated. However, experimental data in relevant conditions is scarce and data extrapolation is needed to determine the maximum expected amount of corrosion products.

An experimental approach for the interaction experiments between liquid metal and fuel has been agreed upon within the project and steel containers for the experiment were designed, made and distributed the partners by ITU. A literature review of the existing data for the Pb-U-O and Bi-U-O systems was done. Experiments started by studying the reactions between PbO and UO2. The use of steel capsules at the start leads to reduction of the PbO. New capsules in copper were made and used for experiments with UO2 and PbO and Bi2O3. Interpretation is pending. Shipping of BN MOX from SCK•CEN to Chalmers was organised.

Four complementary models for fuel dispersion studies are constructed including a 2D and 3D SIMMER coarse grid model, a 3D single phase stationary and two-phase transient CFD model. Each is suited to investigate a different type of fuel release in terms of space and time distribution. First tests show that conjugate heat transfer is needed to capture some particular peripheral flow and thermal patterns. Moreover, porosity of the fuel particles has more influence than its specific weight and the swirl component of the primary pumps flow has an important effect on the cold plenum flow.

WP6: A significant set of experiments on Po and Hg evaporation and capture have been done. For Po evaporation at high temperatures a well-established correlation from literature was confirmed. Po evaporation below ca. 500 °C is complex and depends on composition of the cover gas and the oxygen content of the LBE. The vapour pressure of Po and Hg over ppm solutions in liquid and solid LBE have been determined. Ab-initio calculations predict Po will not dissolve spontaneously in solid LBE, as it is energetically more favourable to form Po-containing alloys. Solutions enthalpies of Te and Po in LBE and other Pb- and or Bi-based alloys are linearly correlated. Knowing one is sufficient to predict the other.

The first SEARCH International Workshop entitled “Innovative Nuclear Reactors cooled by Heavy Liquid Metals” has been held in Pisa (Italy) on April 17-20, 2012. It attracted 140 participants from 15 countries. Venue and time period for the 2nd SEARCH workshop has been set for the first two weeks of October 2014 at the Karlsruhe Institute of Technology (KIT). The event is organized in collaboration with the MAXSIMA project. The Training Course on “Fluid Mechanics and Chemistry for Safety issues in HLM reactors” will be organized at the VKI on 25 - 29 November 2013. The programme is finalized and lecturers identified.

Potential Impact:

Each of the topics of research mentioned above have a clearly defined outcome with a direct impact on the safety assessment of MYRRHA. The heat transfer between the fuel pin and the coolant directly determines the requirements for the coolant flow rate and allows one to calculate the maximal temperature that will occur on the fuel clad. In this way it can be demonstrated that a sufficient safety margin is kept. The work-package on coolant chemistry control will provide methods to control the oxygen content and impurities in the liquid metal coolant. The results will also put conditions on the sizing of the purification system thus ensuring that the coolant can be kept in the right condition. The work packages on fuel coolant interaction and fuel dispersion will aid in the analyses of an accident scenario involving severe clad failure. The goal here is to make sure that in this case the fuel will remain within the confinement of the reactor vessel. Finally, the work package on release and capture or radioisotopes will yield methods to capture volatile radioisotopes in the cover-gas. By measuring the evaporation rates of these elements from the liquid metal and the efficiency of the capture systems the project will determine the required filtering installation as well as contribute to the assessment of the possible release of these elements to the environment in accident scenarios.

With regard to nuclear energy our society is facing some kind of dilemma. In a world where climate change is challenging our lives and that of our children, the search for efficient low-carbon energy is of high priority. Nuclear energy can play a role in this energy mix. On the other hand concerns are raised with regard to the complete safety of nuclear energy and the problem of nuclear waste. It is clear that future development in nuclear energy must address both issues. Generation IV systems, in particular the heavy liquid metal cooled types show promising features in this direction by combining improved passive safety with a better fuel sustainability by using fuel more efficiently while producing less waste. However, it is obvious that the claimed advantages in terms of safety need to be demonstrated before any step towards the deployment of these reactors can be taken. By investigating the safe behaviour of the fuel and the coolant in MYRRHA in its role as the European Technology Pilot Plant (ETPP) for the heavy liquid metal cooled reactor the SEARCH project directly addresses this issue.

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