Reactor concepts based on natural circulation

The power density of the reactor core is typically lower in a natural circulation reactor than in a forced circulation reactor, but the lower power density allows a longer continuous operation. The short heated length of the fuel and the low power density provide good thermal-hydraulic characteristics. With the 8 x g type fuel assembly selected, the power density is 34.2 kW/L the number of fuel assemblies is 708, and the equivalent core diameter is 4.65 m. The uranium enrichment of the refuelling batch, at equilibrium, is 3.6%, and the average fuel burn-up is 39 GWd/t for operation cycles of 23 months. 

Reactivity control is maintained by movement of control rods and by burnable poisons in the fuel. The reactor pressure vessel (RPV) is a vertical, cylindrical pressure vessel, with a removable top head, and head flanges, seals and bolting, and with venturi-shaped flow restrictors in the steam outlet nozzles. The RPV is 6 m in diameter, with a wall thickness of about 158 mm with cladding, and 24.5 m tall from the inside of the bottom head to the inside of the top head. The RPV height permits natural circulation driving forces to produce sufficient core coolant flow by increasing the internal flow path length. 

CAREM uses once-through straight tube steam generators. Twelve steam generators are arranged in an annular array inside the pressure vessel above the core. The primary side coolant flows through the inside of the tubes, and the secondary side water flows across the outside of the tubes. A shell and two tube plates form the barrier between primary and secondary circuits. 

The core is cooled by natural circulation in the range from full power operation to residual heat removal. There is a long riser on the core outlet to enhance the natural circulation capacity. The height of the riser is about 6 m. Even in case of interruption of natural circulation in the primary circuit due to a LOCA the residual heat of the core can be transmitted by steam condensed at the uncovered tube surface of the primary heat exchanger. 

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