SMR-Book_2016-FLEXBLUE (2)

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Flexblue (DCNS, France) 
 
 
1. Introduction 
Flexblue is a transportable and immersed nuclear power system (TINPS), with an output 
capacity of 160 MW(e). The system is designed to deliver electricity using a Transportable & 
Immersed Nuclear Unit (TINU), later called “module” that is anchored with a positive 
floatability on the sea bed, between 40 m and a maximum depth of 100 m. Each module has 
the length of 146-meters and 14-meter diameter. Additional modules can be installed as 
demand increases. The nuclear units are positioned close to the shore and must be within the 
territorial waters. The reactor section is sealed in this configuration. A module encompasses a 
reactor (PWR with fuel enrichment classically less than 5%) and a turbine generating the 
electricity: it is manufactured in an assembly-line and assembled in a shipyard. System 
control and connection to the power grid are grouped together in a land-based facility called 
the “Shore Control Center” (SCC). They are connected to the immersed modules (or the farm 
of modules) via underwater buried cables. Fuel unloading and reloading and major 
maintenance operations are performed on a coastal base called the “Service Factory” 
(Supporting Site), which may be located at significant distances from the Operating Site (in 
the same country or not). Figure 1 shows reactor system configuration. 
 
Figure 1: Reactor system configuration of Flexblue (Reproduced courtesy of DCNS) 
 
2. Target Applications 
Flexblue is designed to supply electricity to coastal grids. Figure 2 shows the life cycle and 
arrangement of Flexblue that comprises two sites for Operating Site and Service Factory. The 
production cycle duration is equal to 3 years (in the Operating Site) and the theoretical 
availability over the 60 years of life-time is equal to 91%. After each cycle, the module is 
transported back to the Service Factory for major maintenance and refueling. Every ten years, 
a detailed inspection is also performed on this site, according to the international 
requirements. The Service Factory can be used and mutualized for many modules, whatever 
their owner and Operating Site origin. 
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Figure 2: Flexblue Life Cycle and Arrangement (Reproduced courtesy of DCNS) 
 
The following scenario is retained for the purpose of this document: these two installations 
are each located on the territory of two distinct countries. The life-cycle steps follow as 
shown in Figure 2; steps 4 – 5 – 6 – 7 – 8 are performed for the 60 years of the nuclear unit 
life-time: 
1 – The module is assembled within the shipyard. Preliminary testing is performed without 
fuel. 
2 – The module is transported to the Service Factory without any fuel. 
3 – First fuelling and module trials are performed in the Service Factory. The first criticality 
is committed at that time. 
4 – Module transportation with fresh fuel is carried out toward the Operating Site, throughout 
the international sea. 
5 – Nautical operations are performed to position and to anchor the module on the sea-bed. 
6 – The module delivers power to the grid via the Shore Control Centre. Minor maintenance 
is performed for the production operations in the Operating Site. 
7 – Module transportation with spent fuel is carried out toward the Service Factory, 
throughout the international sea. 
8 – It corresponds to the module’s outage. Irradiated fuel is put within a devoted spent fuel 
pool in the Service Factory site and refuelling is carried out. Waste is removed from the 
module and major maintenance operations are performed. 
 
By design (except when testing), the modules reactivity is authorized only when it is 
anchored and connected to the grid. Under all other circumstances, the reactor is shut down 
and criticality made impossible. 
Country of 
Origin 
Assembly Line 
& Shipyard 
Without Fuel Module 
Transportation 
Fuelled Module 
Transportation 
Service Factory 
Operating Site 
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3. Development Milestones 
2012 – 2016 The Project gathers international financial and operator stakeholders. 
 
4. General Design Description 
Design Philosophy 
Flexblue design adopts proven technology that draw on DCNS’s experience in nuclear 
propulsion and submarine power plants. The reactor design coupled with the concept of 
mooring it 100 m below the sea surface is envisaged to ease some of the safety issues of land-
based nuclear power plants. The reactor is perceived to be resistant to natural disasters such 
as earthquakes, tsunamis and floods. The reactor will have the advantages of the ocean as a 
built-in coolant and ultimate heat sink. 
Nuclear Steam Supply System 
The module encompasses a 2-loop PWR with two horizontal steam generators. The Nuclear 
Steam Supply System (NSSS) operation diagram is presented below, in relation with the 
diverse configurations of the module. 
 
Figure 3: Reactor Operation Diagram of the module’s NSSS 
(Reproduced courtesy of DCNS) 
 
Reactor Core 
The reference core is made of 77 classical 17x17 fuel assemblies with an active length of 
2.15 m. Enrichment is kept below 5% and reactivity is controlled without soluble boron. 
This latter characteristic reduces the generation of radioactive wastes and simplifies the 
chemical control system. Flexblue power production cycle lasts 38 months. At the end of 
a production cycle, the module is taken back to its support facility. The reactor is then 
refuelled and periodic maintenance is carried out. Major overhauls of the modules are 
scheduled every 10 years. 
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MAJOR TECHNICAL PARAMETERS 
 
 
 
 
Parameter Value 
Technology developer DCNS 
Country of origin France 
Reactor type PWR 
Electrical capacity (MW(e)) 160 
Thermal capacity (MW(th)) 530 
Expected capacity Factor (%) 91 
Design life (years) 60 
Plant footprint (m2) N/A for submerged modules 
Coolant/moderator Light water 
Primary circulation Forced circulation 
System pressure (MPa) 15.5 
Core inlet/exit temperatures (oC) 288/318 
Main reactivity control mechanism Control rods and solid burnable poison 
RPV height (m) 7.65 
RPV diameter (m) 3.84 
Module weight (metric ton) 25000 
Configuration of reactor coolant system Loop type 
Power conversion process Indirect Rankine Cycle 
Fuel type/assembly array UO2 and Zircaloy cladding/17x17 rods in square assembly 
Fuel assembly active length (m) 2.15 
Number of fuel assemblies 77 
Fuel enrichment (%) 4.95 
Fuel burnup (GWd/ton) 38 
Fuel cycle (months) 38 
Cogeneration capability N/A 
Approach to engineered safety systems Passive 
Number of safety trains 2 hydraulic trains and 2 x 3 I&C channels 
Refuelling outage (days) 30 
Distinguishing features Transportable NPP, Submerged operation. 
Modules per plant Up to 6 per onshore main control room 
Target construction duration (months) 36 
Seismic design Not comparable – subsea conditions 
Predicted core damage frequency (per reactor year)coolant system (RCS) is designed to ensure adequate 
cooling of reactor core under all operational states, and during and following all postulated 
off normal conditions. Sea water provides a permanent cooling of the reactor core and acts as 
the ultimate heat sink. 
5. Safety Features 
The aim of the design is to make impossible significant radioactive releasing on the 
Operating Site when the nuclear unit is anchored on the sub-marine site. The nuclear unit 
submersion during the electricity-production phase represents a clear advantage. The safety 
level of the module is greater compared to most of land-based facilities, small or large: 
• Passive systems associated with the infinite heat sink which the sea provides; 
• Inherent protection against numerous external hazards above the sea (natural or 
anthropic) due to the immersion and the water column (aircraft impacts, typhoons and 
other meteorological effects, etc.); 
• Reduced intensity of tsunami and earthquake under the sea, in comparison with Land 
Based Reactors. 
Furthermore, in case of implausible severe accident leading to core meltdown, the defense in 
depth is based on the In-Vessel Corium retention (I-VCR). The reactor residual thermal 
power (the heat decay) is smaller than any large reactors. It allows considering the external 
vessel cooling is sufficient to avoid any vessel rupture. 
Engineered Safety System Approach and Configuration 
The reactor integrates diversity in safety systems and segregates the operating stations to 
achieve a high level of safety. Water offers a natural protection against many of the possible 
external events and guarantees a permanently available heat sink, and an additional barrier to 
fission products in the case of an accident. The submerged Flexblue reactor has a minimal 
environmental footprint. Only the systems required for residual heat removal, control and 
monitoring are required in navigation conditions. At the end of its life, the power unit is 
transported back to a dismantling facility, which results in a quick, easy and full recovery of 
the natural site. The use of passive safety systems brings the reactor to a safe and stable state 
without external intervention for an indefinite period of time. In particular, the positive 
buoyancy of the submerged unit allows extremely efficient de-correlation from the seabed in 
case of earthquakes. Furthermore, at the depth the unit is fixed, tsunami effects are not 
critical. Still, even in postulated extreme situations like large early release of radioactivity in 
the water, atmospheric release would be so reduced that it practically excludes any quick 
health impact on populations: water quality would have to be watched over, but no 
evacuation of population would be required. If a safety issue developed with the reactor, it 
could be brought to the surface and taken to a shipyard for repair. It could be refuelled in the 
same way, and at end of life it would be repatriated to the shipyards for decommissioning. 
The emergency power sources (batteries) are sized to ensure the autonomy of the modules up 
to 15 days without any external power. The safety of the plant is improved considerably with 
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four (4) operating areas in accident conditions: the main control room, the main remote 
shutdown station, the local control room and the local remote shutdown station. 
Decay Heat Removal System 
Decay heat removal is performed by four cooling loops, each one able to remove 50% of 
decay heat: 
• Two primary chains are connected to the primary circuit: each one includes a heat 
exchanger immersed in a large safety water tank which is cooled by the ocean through 
the metallic hull. 
• Two secondary chains are connected to the secondary circuit: each one includes an 
emergency condenser directly immersed in seawater. 
With the infinite heat sink – seawater – and to the elevation difference of the heat sink with 
respect to the heat sources, the four chains operate passively by natural circulation. The long-
term safe state of the reactor is a shutdown state where continuous cooling of the reactor core 
is achieved by natural circulation. 
Emergency Core Cooling System 
Protection against loss-of-coolant accidents is ensured by a passive safety injection system 
including direct vessel injection lines, core makeup tanks, accumulators and large safety 
tanks. All these injection sources inject passively into the vessel In addition, a two-train 
automatic depressurization system is connected to the pressurizer. Once these systems have 
actuated, a passive recirculation path is in place. Decay heat is removed through the metallic 
hull (the containment). In case of an accident, active systems designed for normal/shutdown 
core cooling or for controlling coolant inventory are used if alternating-current power is 
available. If not, the passive safety systems described earlier are actuated automatically when 
emergency set points are reached. In all accident scenarios, a safe shutdown state is achieved 
and maintained for an indefinite period of time without the need for operator action. 
Emergency battery power is only required for the opening valves and communication (data 
transfer from the module to the remote Control Room). The fourteen (14) days of 
autonomous monitoring ability can be extended by reloading the batteries. An emergency 
communications system works via radio links in case of loss of cables. Also, an emergency 
ultimate system can be run via an acoustic link, when losing both the submarine cables and 
the microwave link. This enables to communicate with a ship/submarine and transmits the 
module state and enables communications. 
Containment System 
The reactor containment is bounded by the reactor sector: hull on the sides and reactor sector 
walls on the front and on the back. A large share of the metal containment walls are therefore 
in direct contact with seawater, which provides very efficient cooling without the need for 
containment spray or cooling heat exchanger. The containment system is also designed to 
sustain severe accident with core meltdown, although the safety features are designed to 
avoid core damage. In this case, the mitigation strategy consists in in-vessel corium retention 
assisted by passive ex-vessel core cooling. 
 
6. Plant safety and Operational Performances 
The reactor is based on the PWR technology and significantly benefits from a large operating 
experience in commercial power plants as well as in the naval environment. The concept 
adopts a tele-operation which is a great multiple stakes issue that contributes to the 
optimization of the life cycle cost, accident response and management, and ensure a good 
availability factor. The module is accessible via a submarine vehicle that connects to access 
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hatches, so that maintenance, inspection and operation can be performed on-board while on 
the seafloor. The refueling takes place approximately every 3 years. The module is removed 
and transported back to a coastal facility, which hosts the spent fuel pool. Major overhaul 
occurs every 10 years, i.e., every three fuel-cycles. 
7. Instrumentation and Control Systems 
The I&C and communications systems are based on different technologies, following the 
safety levels in conformity with the IEC standards. Several possible solutions to allocate the 
classes of functions to the controllers are considered with different technological and 
functional diversification schemes. I&C system is robust and incorporates experience from 
naval industry, especially from nuclear submarines. The I&C system adopts redundancies of 
trains, different levels of defence especially on communication systems, technological 
diversification and the principle of reactor unit I&C autonomy. The I&C system includes a 
diversified operating station (DOS) enabling commands of safety functions in case of 
technological failure on the nominal station. This diversified system works diversifiedsolutions from the instrumentation to the man-machine interface (MMI). The main principles 
of I&C systems are based on module autonomy towards the shore control facility. The 
module autonomy is ensured by the fact that: all the systems supporting the safety functions 
are in the module; the reactor can be operated from two different places in the module during 
accident conditions; the transition from the initial state to a safe shutdown state when losing 
submarine cables is automatic; Local emergency power source. The concept also includes the 
possibility of multi-units operations from a single control room in order to run a cluster of 
modules. This aims to reduce the operating crew size considerably. Flexblue employs 
redundant main and auxiliary submarine cables that transport electricity as well as 
information between the modules and the onshore control centre. 
 
8. Plant Arrangement 
The module is composed of a turbine & alternator section, a reactor section, an aft section 
and a fore section. These two latter sections accommodate: emergency batteries, a secondary 
control room, process auxiliaries, I&C control panels, spares, living areas for a crew and 
emergency rescue devices. Several units can operate on the same site and hence share the 
same support systems. 
9. Design and Licensing status 
Presentations of the concept have been made to the French safety authority. Technical 
discussions have been initiated with the French technical safety authority. Preliminary 
conclusions have been drawn: these ones lead going ahead the project. 
 
 
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