DCNS in France is currently developing an underwater nuclear reactor concept with a rated power of 160 MWe, the Flexblue reactor, based on proven PWR technology. The entire plant is designed to fit within a hull of 14 m diameter and 145 m length, placed horizontally on or near the ocean seabed at a depth between 30-100 m. This work examines the most viable nuclear technology options for future underwater designs that would meet high safety standards as well as good economic potential, for construction in the 2030-40 time frame. A survey of 13 nuclear technologies was performed and assessed against the following priorities and constraints:
- Achieve safety (neutronic reactivity control, decay heat removal and radioactivity containment) by passive means, for an indefinite period following shutdown.
- Fit in a compact volume:
- The hull size is limited to 15 m diameter due to manufacturing concerns.
- The hull vertical height is limited to 20 m since the reactor needs to be deployable under between 100 m (328 ft) and 30.5 m (100 ft) of water.
- Operate with a long period between refueling (more than 5 years)
- The fuel U235 enrichment must remain below 19.75%.
- Maximize compactness of the nuclear system, thus maximize Plant Power Density
- The power output is restricted to 160 MWe
- Maximize power cycle thermodynamic efficiency
- Have a compact power conversion cycle (turbine-generator power plant)
- Provide high dual-use resistance: Dual-use resistance includes weapons proliferation resistance and unsuitability to military applications (propulsion).
- Rely on technology maturity level to enable deployment by 2030-40.
Among the reactor concepts considered, the sodium fast reactor was eliminated due to incompatibility of sodium with water. The gas fast reactors (He and SCO2 cooled designs) were eliminated due to lack of obvious ways to achieve a fully passive safe design. Four concepts (supercritical water, molten salt fuel, salt cooled and gas-cooled high temperature thermal reactors) were eliminated due to an inability to achieve greater than 5 year refueling intervals while achieving satisfactory economic operation by the 2030-2040 time frame. The CANDU design was eliminated due to requiring a larger hull size than the design constraint. The 5 concepts that remained viable according to the adopted design priority and constraints were: the PWR, the BWR, the Superheated Water Reactor (SWR), the Lead Bismuth Fast reactor (LBFR) and the Organic Cooled Reactor (OCR). For the BWR, the Toshiba LSBWR and for the LBFR, the Russian SVBR-100 were chosen as reference designs that can be used without further development, while additional investigations were performed for the other three concepts.
The advanced PWR concept investigated in this study was based on the currently popular integral PWR (iPWR) for Small and Modular Reactor (SMR) designs, in which the core, steam generators and pressurizer are in the same vessel. The main goal of the investigation of this configuration was to attempt to achieve a more compact hull size, primarily by reducing the volume needed to accommodate the peak containment pressure resultant from a large break loss of coolant accident. In addition, in order to meet the 20 m limit on the vertical height of the plant, compact heat exchangers were employed within the iPWR. This led to an iPWR design having a primary containment size 30% more compact than the Flexblue design while meeting the desired safety priority. The iPWR potential for both 5 year and 9 year refueling frequency, and the associated fuel cycle cost, were also assessed through consideration of different fuel geometry, cladding material, and neutronic reactivity management techniques.
Advanced Nuclear Power Program