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Evaluation and Viability of Liquid Fuel Fast Reactor System

Final Report Summary - EVOL (Evaluation and Viability of Liquid Fuel Fast Reactor System)

Executive Summary:
An innovative molten salt reactor concept, the MSFR (Molten Salt Fast Reactor) is developed by CNRS (France) since 2004. Based on the particularity of using a liquid fuel, this concept is derived from the American molten salt reactors (included the demonstrator MSRE) developed in the 1960s.
The major drawbacks of these designs were (1) a short lifetime of the graphite blocks, (2) a reactor fuelled with 233U, not a natural fissile isotope, (3) a salt constituted of a high chemical toxic element: BeF2, and (4) a fuel reprocessing flux of 4000 liters per day required reaching a high breeding gain. However, this concept is retained by the Generation IV initiative, taking advantages of using a liquid fuel which allows more manageable on-line core control and reprocessing, fuel cycle flexibility (U or Th) and minimization of radiotoxic nuclear wastes.
With MSFR, MSR concept has been revisited by removing graphite and BeF2. The neutron spectrum is fast and the reprocessing rate strongly reduced down to 40 liters per day to get a positive breeding gain. The reactor could be started with 233U or with a Pu and minor actinides (MA) mixture from PWR spent fuel. The MA consumption with burn-up demonstrates the burner capability of MSFR.
The objective of this project is to propose a pre-conceptual design of MSFR in 2013 given the best system configuration issued from physical (neutronic and thermal-hydraulic), safety, chemical and material studies, for the reactor core, the reprocessing unit and the wastes conditioning.
In this way, demonstration that MSFR can satisfy the goals of Gen IV, in terms of sustainability (Th breeder), non proliferation (integrated fuel cycle, multi-recycling of actinides), resources (close U/Th fuel cycle, no uranium enrichment), safety (no reactivity reserve, strongly negative feedback coefficient) and waste management (actinide burner) will be done.
The success of this project was strongly depending on the selected consortium. Indeed, the concept based on the use of a liquid fuel requires a multidisciplinary skill in the following domains: neutronic, reactor physic, safety analysis, thermal-hydraulic, physico-chemical analysis, chemical processing, materials and engineering. One particularity of this project is to be a collaborative project with ROSATOM. The two parallel projects EVOL (EURATOM) and MARS (ROSATOM) lead to have strong exchanges with the Russian researcher especially about corrosion and materials tests and for the physical properties of the fuel salt. This collaboration leads to the possibility of using the Russian facilities (molten salt loops) and that is a great advantage to reach the objectives of the EVOL project.
The main objectives of the project have been achieved:
- An innovative design was proposed by the consortium of physicists in charge of this task. Optimization of the design was based on safety analysis combined with thermal-hydraulic studies and neutronic calculations. Previous studies highlighted the necessity to actively circulate and cool the blanket salt. In the present design option, the blanket salt flows in a circuit external to the salt container. This arrangement ensures a very good neutron economy because most of the neutrons escaping from the reactor core are captured in the surrounding blanket. Moreover, the blanket salt follows a path which improves the cooling of the main metallic structures of the fuel circuit.
- The no-availability of 233U requires starting MSFR with another fissile material, 235U and/or 239Pu (or transuranic elements). Two options for the fuel composition have been proposed that meet requirement of neutronic, melting temperature, physico-chemical properties (viscosity, density, solubility of elements...) redox potential (required for material compatibility), fuel reprocessing, economy. The two options retained are: LiF-ThF4-UF4-PuF3 (78.6-12.9-3.5-5 mol%) and LiF-ThF4-UF4-(TRU)F3 (77.5-6.6-12.3-3.6). The two options require an enrichment of 238U into 235U respectively equal to 20 and 13%.
- The fuel reprocessing scheme has been achieved and the chemical database required for efficiency calculations has been established in the frame of the project. New steps have been proposed for the reprocessing leading to recover the composition of all the phases involving in the process. The behavior of all the fission products in the reprocessing has been evaluated.
- Three heats of NiWCr alloys have been manufactured in the project. Based on their study, the optimized composition for MSFR is (in wt%):W 21.7 - Cr 7.5 - Mn <0.02 - Al 0.22 - Zr 350 (ppm) - Ti 0.22. The corrosion studies have evidenced the role of oxygen and of the ratio U(IV)/U(III) to decrease the corrosion reactions. Dynamic studies have been performed through ROSATOM/EURATOM collaboration.

Project Context and Objectives:
The Molten Salt Fast Reactor (MSFR) is an innovative concept. Based on the use of a liquid fuel it is the only one concept of Generation IV to operate with the Th/U fuel cycle. Both breeder and burner concept, the MSFR presents a fast neutron spectrum and the fuel reprocessing is included in the concept. The particularity of this concept requires a multidisciplinary consortium to achieve the objectives of the project. Physicists, chemists and material experts have been gathered to work on this concept.
The consortium was also extended to Russian researchers through collaboration between ROSATOM and EURATOM and the two parallel projects EVOL and MARS (Minor Actinides Recycling in molten Salt - ROSATOM project). Through this collaboration, experiments were performed in molten salts loop in Russia to test metallic samples in several conditions.

The project was structured into five work-packages. The objectives and the main results obtained in the frame of the WP dedicated to R&D are described below.

WP2: PRE-CONCEPTUAL DESIGN AND SAFETY
This WP will address the optimisation of the system configuration to result in a demonstrator pre-conceptual design by the end of the project. That will be done using thermal-hydraulic coupled with neutronic calculations and sensitivity studies. A preliminary analysis based on IAEA guidelines and GIF methodology document will be addressed. The approach for assessing MSFR safety by taking into account today's regulations and requirements based on the Defense-in-Depth principles will be established.
This WP is divided into three tasks as described below:

Task 2.1: Thermal-hydraulic calculations of primary circuit
- Thermal hydraulic calculations to optimize design parameters to obtain the most stable salt flux.
- Thermal-hydraulic and neutronic coupling to optimize core geometry and design components (reflector, heat exchangers, pumps).
- Sensitivity studies
- Calculation of irradiation damage of structural materials.
- Energy conversion system studies.

Task 2.2: Safety
- Specific analysis of events which might occur due to non-usual properties of molten salt reactor in operation which do not benefit of the large LWR operational experience.
- Preliminary analysis based on IAEA guidelines and GIF methodology documents
- Establishment of safety approach based on the Defense-in-Depth principles.
- Assessment of potential risks (source term, residual heat) associated with the use of molten salt as fuel (delayed neutron precursors mobility, criticality) with specific design and operating conditions. - Transient analyses and development or adaptation of dedicated tools will be done.

Task 2.3: Demonstrator design
This task will address the recommendations for a demonstrator design (dimensioning of the core, the primary circuit and heat exchangers). The technical specifications of heat exchangers will be given. The first objective of this project is to provide a very accurate design of the reactor with dimensions of primary circuit (pipes and tank), description of pumps, recommendations for pump material, gates, junctions.

Neutronic and thermal-hydraulic benchmark was done at the beginning of the project. These studies lead to propose an innovative pre-conceptual design for MSFR with a spherical shape. The present design option has been achieved adopting a “combined” optimization approach, which considers the strongly-coupled behavior of the reactor neutronics and thermal fluid dynamics, according to the following main optimization criteria:
- Ensure good neutron economy and breeding performance
- Avoid hot spots at walls and reduce the overall temperature of structural materials
- Improve the effectiveness of natural circulation under pump blockage accidents
- Keep the design as simple as possible
- Enable easy replacement of the main components of the fuel loop
The geometry of the core is a sphere. The cold fuel salt is injected at the top of the spherical core. The cold flow goes down staying close to the wall, keeping it at a low temperature, and then rises again and is extracted from the top after being heated from the fission reactions. The salt flows in a vertical riser and enters in the modular pump-HEX blocks.
Coupled neutronics-CFD simulations show that recirculation vortices appear inside the core but are likely to be located far from structural materials.
Previous studies highlighted the necessity to actively circulate and cool the blanket salt. In the present design option, the blanket salt flows in a circuit external to the salt container. This arrangement ensures a very good neutron economy because most of the neutrons escaping from the reactor core are captured in the surrounding blanket. Moreover, the blanket salt follows a path which improves the cooling of the main metallic structures of the fuel circuit.

The studies performed in the frame of the WP2 leads to define a safety approach dedicated to molten salt reactor, that means considering that the fuel is liquid and flows in the reactor. A "cold plug" at the bottom of the reactor vessel has been designed for passive safety issue. This leads to drainage of the core when it fuses (in case of electrical shut down or strong temperature increase).
A transient analysis was realized and the main conclusions are:
- 3D ULOF calculation results show that with the actual design, the risk of salt solidification in the intermediate heat exchanger appears. This may happen very fast, but mainly due to the non-optimized heat exchanger design. With optimized heat-exchangers and/or other improvements in the MSFR design one may try to prevent a rapid salt solidification. This is a possible subject for future investigations.
- It must be stated here that the performed analyses, do not go deeply into the domain of severe accident scenarios. They do, however, determine which initiators and further additional failures could/maybe lead to severe accident conditions and whether there is a time available to perform corrective actions by e.g. an operator, i.e. whether an accident prevention and management strategy is feasible.
- The results of the different transient analyses demonstrate that the MSFR plant as proposed by EVOL project is a robust reactor with inherent safety features and a high safety potential. Further développent of calculation models, accumulation of safety-relevant experimental results, design optimizations and additional safety analyses, in particular for hypothetical severe accidents, would provide a deeper understanding of MSFR safety issues.

WP3: FUEL CHEMISTRY AND REPROCESSING
This WP will address the issue of the exact composition and the properties of the fuel salt. Starting from the LiF-ThF4 binary system, the composition will be optimized with respect to margin to freezing, actinide solubility and physical properties (melting point, density, viscosity, heat capacity and thermal conductivity). As fissile components UF4 and PuF3 will be considered. Experimental studies will form a significant part of this work, and will be supplemented with theoretical calculations (first principles, molecular dynamics) and modeling. The fuel reprocessing will also be addressed in this WP and each step will be examined by experimental measurements.
A simulation code, based on an analytical analysis, will be developed to optimize the process by determining the experimental parameters required for the extraction and back-extraction steps. The last issue of this WP is the control (and its stabilization to a constant and low value) of the fuel salt redox potential which increases with the fission reaction and is responsible of the corrosion of structural materials.
This WP is divided into the following three tasks:

Task 3.1: Fuel salt parameters determination
The properties of the fuel salt will be determined using molecular dynamical simulations: physico-chemical properties (activity coefficients and solubility), heat-transport properties (density, heat capacity, viscosity, thermal expansion and thermal conductivity) and phase diagrams of the systems (CALPHAD calculations).

Task 3.2: PuF3 solubility determination – Selection of the MSFR fuel salt composition
A main issue of this project is the PuF3 solubility. To determine this quantity, synthesis of this compound will be necessary. Comparison between calculated and experimental measurements (EURATOM and ROSATOM) will give an accurate value of PuF3 solubility. The composition of MSFR fuel salt could be proposed which will be an input for thermal-hydraulic, neutronic, safety and reprocessing calculations.

Task 3.3: Fuel salt processing
Based on the analytical analysis of the extraction and back-extraction steps involved in the reference reprocessing scheme, a code will be developed to simulate the mass flows of each element as function of the extraction experimental parameters. In this way the processing scheme and the experimental parameters will be optimized. The efficiency of helium bubbling for removing noble metals will be addressed in this task by analyzing the results obtained using a forced convective molten salt loop currently under construction in France (CNRS).

Task 3.4: Fuel salt redox control
The fuel salt redox potential increases when the fission reaction occurs. This potential is controlled by the ratio UF4/UF3. This task will address the measurement and the control of the fuel salt potential by adding a reducing reagent in the fuel salt when the potential becomes too high.

One of the main results obtained in this WP is the composition of the fuel salt. The no-availability of 233U requires starting MSFR with another fissile material, 235U and/or 239Pu (or transuranic elements). Two options for the fuel composition have been proposed that meet requirement of neutronic, melting temperature, physico-chemical properties (viscosity, density, solubility of elements...) redox potential (required for material compatibility), fuel reprocessing, economy. The two options retained are: LiF-ThF4-UF4-PuF3 (78.6-12.9-3.5-5 mol%) and LiF-ThF4-UF4-(TRU)F3 (77.5-6.6-12.3-3.6). The two options require an enrichment of 238U into 235U respectively equal to 20 and 13%.

For these two options, physico-chemical parameters (viscosity, density, melting point, solubilities, heat capacity and vaporization point) have been determined combined experimental measurements and molecular simulation. As experiments are the always basis for CALPHAD-type modeling and are also needed for verification of the developed thermodynamic database, several calorimetric equipments are installed at JRC-ITU providing novel data. With this objective, PuF3 was synthesized by ITU and the results were compared to ROSATOM results. Activity coefficients of LiF, ThF4, UF4 and PuF3 were calculated.

The fuel reprocessing scheme has been achieved and the chemical database required for efficiency calculations has been established in the frame of the project. New steps have been proposed for the reprocessing leading to recover the composition of all the phases involving in the process. The behavior of all the fission products in the reprocessing scheme has been evaluated.

The influence of the redox properties on the corrosion of Ni-based alloys has been studied in the frame of the project. It was clearly demonstrated that the ratio [UF4]/[UF3] is related to the corrosion process. When the ratio increases, the corrosion increases. It was also shown that metallic uranium addition decreases rapidly the redox potential by the reaction (3UF4 + U  4UF3) and limits the corrosion. Therefore, the ability of control the redox potential of the salt was demonstrated.

WP4: STRUCTURAL MATERIALS
This WP will focus on the optimization of the materials for the reactor primary system components. AD will provide all the partners with Ni-W alloys produced at a semi-industrial scale. The mechanical properties of the materials will be studied as well as their corrosion at high temperature. The primary circuit will be studied with emphasis on the effects of corrosion by the selected molten salt(s) and the radiation effects of neutron irradiation. Since the material corrosion depends strongly on the redox conditions of the salt, the redox control via the salt chemistry must be addressed in detail in WP3. The study of the mechanical properties of these alloys will be addressed also.
This WP is divided in three tasks as described below:

Task 4.1: Production and characterization of alloys
This task is dedicated to the development of improved Ni-W-Cr alloys for primary (reference fuel salt) circuits.
This work will include the definition of the manufacturing route and associated parameters processing and experimental characterization of their microstructure. The material processed in this task will be used as the reference material of the work-package.

Task 4.2: Mechanical properties of Ni-W-Cr alloys
Mechanical properties of Ni-W-Cr will be examined in this task. The mechanical properties to be investigated are secondary creep behavior and tensile properties in the temperature range of interest for MSFR (700-900°C). It will be complemented by toughness measurements at low temperature (up to 200°C).

Task 4.3: Corrosion behavior of alloys in the fuel salt
Tests in static conditions will be addressed in this task to study the electrochemical behaviour of NiWCr alloys in LiF-ThF4 molten salt under controlled redox potential of the fuel salt. Corrosion tests in dynamical conditions will be realized in the framework of the MARS project
In the framework of the bilateral EURATOM/ROSATOM project:
The experiments performed in dynamic conditions and the sample analysis will be realized in Russia in the framework of the MARS project using the natural convective loops implemented in Russia in the framework of the ISCT#1606 project. The samples studied by Russian researchers will be provided by the EVOL structure.

Three heats of NiWCr alloys have been manufactured in the project. Forging conditions and processing have been defined to provide alloys of high quality. It has now been clearly established that a ternary composition by itself will not be sufficient to provide a good material. It has therefore been shown that microalloying is essential and a candidate material needs to have some deoxidizer such as Zr or Al. It was also concluded that in order to add Zr, some Ti may be necessary at the processing level. Based on this study, the optimized composition for MSFR is (in wt%):W 21.7 - Cr 7.5 - Mn <0.02 - Al 0.22 - Zr 350 (ppm) - Ti 0.22.
The corrosion studies have evidenced the role of oxygen and of the ratio U(IV)/U(III) to decrease the corrosion reactions. Dynamic studies have been performed through ROSATOM/EURATOM collaboration and have evidenced the role of [UF4]/[UF3] ratio to decrease the corrosion by the molten salt and also by the oxidizing elements such as Te.


Project Results:
This document included figures and tables is given in the attached file.
Potential Impact:
1. A common European Molten Salt Reactor concept for GENIV
After a variety of molten salt burner and breeder reactor concepts have been proposed by European institutions in the last decades, the EVOL proposal addresses a common concept supported by all partners involved in the European MSR research. It is an innovative non-moderated system that is capable of breeding 233U from 232Th and that can operate also as a burner for plutonium and minor actinides. The proposed work will be a major European contribution to the GENIV initiative and specifically to the MSR activity, in which France, USA and EURATOM are the partners.


2. Thorium as a nuclear fuel
The MSR is one of the few reactor systems that are very well suited for operation with the thorium fuel cycle, and is the only system in the GENIV initiative addressing this issue. Moreover, this innovative concept includes an integrated fuel processing which results in a closed MSR fuel cycle. A thorium-based fuel cycle has several interesting advantages, such as the large available thorium resources and the low transuranium actinide cycle inventory, that are consistent with the sustainability goals. The proposed work will demonstrate the potential of the MSFR as a sustainable energy system and open the possibility for large scale thorium utilisation.


3. Partitioning and Transmutation
The EVOL project will also address the issue of transmutation of the plutonium and minor actinides in the MSFR system. This will give an alternative option for P&T compared to the solid fuel / aqueous processing route that is currently prevailing.
We aim at designing a critical thorium-based MSR that is able to burn plutonium and minor actinides produced in the current reactors, and consequently will convert them into 233U while minimizing the production of long lived waste. This will allow the closure of the current LWR fuel cycle by employing MSFR systems started with transuranic elements in the Th/Pu fuel cycle, as an alternative to fast neutron reactors operated in the U/Pu fuel cycle.


4. An improved understanding of liquid salt properties
The approach proposed in this work to study the properties of the molten salts – a combination of atomistic calculations, local structure studies and bulk property determinations – will help to advance the scientific knowledge on liquid salts in general. Bridging the scales from the atomistic level to the bulk will help to will reduce uncertainties and allow the assessment and accurate prediction of basic material properties of liquid salts. Such knowledge is of specific interest for the molten salt reactor research, but has a more general relevance to energy technology in fields in which liquid salts for are used industrial processes.

The dissemination activities during the project were realized under workshops manifestations (3 workshop organized in the project period) and a winter school opened for all students (and free for the students of the partners).
This project leads to the publications of 62 papers and proceedings, 85 oral communications in international conferences.
14 PhD and 14 Master of Sciences were involved in the project.
List of Websites:
http://www.li2c.upmc.fr/
Contact: Christian SIMON: christian.simon@upmc.fr