Czech Republic
Information
Technical Details
Design Philosophy
Energy Well is a Fluoride High temperature micro-Reactor of 20 MW thermal power. It has a seven-year long fuel cycle, a low power density and high focus on passive safety and simplicity. It is designed to be transportable with fresh or spent fuel so that no refuelling on site is required.
Target application
Energy Well is designed for operation in remote and populated areas and focusing on production of electricity, heat and hydrogen as a means of energy storage. The purpose is to provide a clean stable energy source to be implemented alongside the large-scale nuclear power reactors, heating plants and the renewable sources of energy.
Power plant concept
The power plant includes three cooling circuits. Liquid fluoride salts are used as a heat transfer medium (FLiBe, NaBF4) in the primary and secondary circuits. Carbon dioxide in a supercritical state (sCO2) is used in the tertiary circuit. The tertiary circuit uses an optimized Ericsson-Brayton cycle configuration to transform heat into electricity. The conversion cycle with the sCO2 was chosen due to its higher cycle efficiency and small dimensions of the components comparing to a conventional steam cycle. The primary circuit removes the heat generated in the core of the reactor, while the secondary circuit separates the active primary and the highpressure tertiary circuit while ensuring the heat transfer.
Major Technical Parameters
Fluoride High Temperature reactor
pool type
Molten Salt
FLiBe
Forced (mechanical pumps)
Atmospheric pressure
650 / 700
TRISO
Control rods
Reactor core and fuel
The reactor core design includes 19 hexagonal fuel assemblies with TRISO fuel. TRISO stands for TRi-structural ISOtropic particle fuel. Each TRISO particle is made up of a uranium, carbon and oxygen fuel kernel. The kernel is encapsulated by three layers of carbon- and ceramic-based materials that prevent the release of radioactive fission products. TRISO fuels are structurally more resistant to neutron irradiation, corrosion, oxidation and high temperatures (the factors that most impact fuel performance) than traditional reactor fuels. Each particle acts as its own containment system thanks to its triple-coated layers. This allows them to retain fission products under all reactor conditions. TRISO particles cannot melt in a reactor and can withstand extreme temperatures that are well beyond the threshold of current nuclear fuels.
Primary cooling circuit
Energy Well is a pool type reactor with molten salt FLiBe as the primary coolant. The primary cooling circuit consists of the following main parts:
- Reactor core with top mounted control rods,
- Reactor vessel,
- Graphite reflector,
- Core supporting plate,
- Flow skirting,
- Six primary heat exchangers molten salt/molten salt,
- Two reactor vessel top flanges.
The core support plate and the graphite top reflector have holes to allow the molten FLiBe to flow in each fuel assembly. The molten salt flows upward through the core and then enters the primary heat exchangers located on the periphery of the reactor vessel above the core. In the heat exchangers the heat generated in the reactor core is transferred to the secondary molten salt circuit. The main circulation pumps placed above each heat exchanger push the molten salt downward in the heat exchangers. The flow skirt direct the molten salt downward to the bottom of the reactor vessel.
The whole primary circuit is placed inside a transport container that allows the reactor to be shipped on road or rails with fresh fuel or spent fuel. On the top of the transport container are located, the main pumps motors, the control rods actuators, secondary circuit connecting pipes and other pipes for tritium removal or primary coolant purification. An additional shielding is foreseen on the top of the transport container to allow access when the reactor is shut down. The space between the transport container and the reactor vessel shall be filled with helium at a pressure slightly above 1 bar. Helium is also foreseen as a cover gas above the FLiBe molten salt level in the reactor vessel.
Secondary cooling circuit
The secondary circuit physically separates the primary circuit from the tertiary circuit and creates a pressure barrier in case of leaks in the salt/salt exchanger between the primary and the secondary circuit. The secondary circuit includes three main components:
- Heat exchanger salt/salt,
- Heat exchanger salt/sCO2,
- Circulation pump.
Heat exchangers create the interface of circuits, and the pump ensures the required mass flow of secondary salt NaBF4 to recover the thermal power out of the primary circuit. The secondary circuit must be equipped with auxiliary systems to ensure a reliable and safe operation. Auxiliary systems include molten salt refilling, salt purification system, expansion volume to scope with molten salt volume change due to temperature.
Tertiary circuit
The tertiary circuit intended for conversion of heat to electric energy with the sCO2 as a working fluid was selected as the sCO2 technology was identified as the most compatible with this kind of small modular reactor. The main benefit is the higher thermodynamic efficiency of the cycle at a relatively high temperature level. This is mainly caused by the compression close to the critical point (7.38 MPa, 30.98°C), or even in the liquid phase. As a result, the requirements of the compressor are lower, and the intercooling is not required. High pressure in the circuit and relatively high density of the liquid also significantly reduce the size of the individual components.
Based on preliminary studies, the recompression Brayton cycle with heat regeneration was selected for the Energy Well system as reasonable compromise between the complexity and efficiency of the cycle. After the heat exchangers at the low-pressure side, part of the flow is recompressed to the higher pressure and it then flows in the by-pass of the cooler. This significantly reduces the heat capacity of the low-pressure flow in the low temperature heat exchanger and the regeneration process continues. This lowers the heat recovery power in the cooler, but it increases the regeneration efficiency making the cycle more efficient.
The circuit is composed of the compressor with the turbine at oneshaft layout, recompressor, lowtemperature regenerative heat exchanger (LTR HX), hightemperature regenerative heat exchanger (HTR HX), secondary heat exchanger where heat from the secondary circuit is transferred to the tertiary circuit, power turbine with generator and the heat sink.
Safety features
The design of Energy Well reactor has a high focus on passive safety and simplicity. The main safety features of the reactor are:
- Atmospheric pressure in primary and secondary circuit,
- Low power density,
- Underground location of the primary circuit,
- Natural circulation in primary circuit in case of loss of flow accident,
- Passive decay heat removal from the reactor to the ground,
- Reduction of core reactivity with increase of temperature,
- High temperature resistant fuel,
- Multiple barriers containment system (reactor vessel, transport container, pit, reactor building)
- Protection from external events (earthquake, flood, airplane crash…)
- Protection from internal events (air ingress, fire…)
Containment system
The TRISO fuel envelope is the first barrier to prevent the spread of fission products. Depending on the design, the fuel assembly could be considered as a barrier since the TRISO fuel is enclosed in a thick graphite matrix. The reactor vessel, the transport container, the pit (together with the maintenance room shielding ceiling) and the reactor building are additional barriers of the containment system.
During transport of the reactor with fresh/spent fuel, the transport container of the reactor is protected by shock absorbers to ensure its integrity during accident scenarios.
Reactivity control
The primary safety system ensuring reactivity control are the Y-shaped control rods located in the middle of fuel assemblies with a set SCRAM signal. SCRAM is activated when the neutron flux or other technological parameters in the core are increased. During normal operation, control rods are kept in operational position by magnets that are power fed. In case of deviation from normal operation parameters, the Limiting System (LS) regulates the reactor power. In case of loss of power, the magnets are not power fed, and the rods are lowered in the core by gravity. The second independent safety system is a ‘capsule with an absorber’, which is located in the primary circuit. The capsule melts after reaching a threshold temperature and the absorber (NaBF4) is released in the active zone. This is a passive safety system.
Logistic and transport
One of the key advantages of the Energy Well concept is the transportability to places, where electricity and/or heat production is needed. Thanks to the unique transport container design and small dimensions of the primary cooling circuit, it is possible to ship the core as one assembly (with fresh or spent fuel) and the primary cooling circuit. The secondary and tertiary circuit are designed to fit in ISO containers. On-site assembly operations are thus minimized to gain time and reduce cost. At the end of the fuel cycle (7 years), the first reactor module remains in its pit, for 3 years, to cool and reach the specifications needed for transport back to the production site.
Another reactor module with fresh fuel is installed in a second pit and connected to the secondary cooling circuit. After the first module cools down, it is sent back to factory for refuelling. This allows a continuous supply of either electricity or heat. The dose rates limits for transport container with spent fuel are < 2 mSv/h on the container surface and < 0.1 mSv/h at a 2 m distance from container surface. After disconnection of the reactor primary circuit in the container from the secondary cooling circuit, natural circulation of primary molten salts occurs allowing a uniform distribution of the decay heat within the reactor vessel.
Plant layout arrangement
The primary, secondary and tertiary circuit are located in a common steel building. The three circuits are located in separate room with separate transfer doors. In the primary circuit room are located two concrete pits. In the first pit is located the operating reactor while the second pit is used to cool the reactor with spent fuel before its shipment for refueling. A trolley on rails is used to transfer the reactor in its container between the reactor room and outside the building. A mobile crane, temporary placed outside the building is used for lifting operations through openings in the building ceiling. In the reactor room, an overhead crane is used to handle shielding lids and the upper components of the reactor (pump motors, control rods actuator…).
