Economic Simplified Boiling Water Reactor - Passive Safety System

Passive Safety System

The passive safety systems in an ESBWR operate without using any pumps, which creates increased design safety, integrity, and reliability, while simultaneously reducing overall reactor cost. It also uses natural flow for coolant circulation within the reactor pressure vessel (RPV); this results in fewer systems to maintain. There are no circulation pumps or associated piping, power supplies, heat exchangers, instrumentation, or controls needed for these systems.

ESBWR's passive safety systems include a combination of three systems that allow for the efficient transfer of decay heat (created from nuclear decay) from the reactor to pools of water outside of containment – the Isolation Condenser System, the Gravity Driven Cooling System, and the Passive Containment Cooling System. These systems utilize natural circulation based on simple laws of physics to transfer the decay heat outside of containment while maintaining water levels inside the reactor, keeping the nuclear fuel submerged in water and adequately cooled.

In events where the reactor coolant pressure boundary remains intact, the Isolation Condenser System (ICS) is used to remove decay heat from the reactor and transfer it outside of containment. The ICS system is a closed loop system that connects the reactor pressure vessel to a heat exchanger located in the upper elevation of the reactor building. Steam leaves the reactor through the ICS piping and travels to the ICS heat exchangers which are submerged in a large pool. The steam is condensed in the heat exchangers and the heavier condensate then flows back down to the reactor to complete the cooling loop. Reactor coolant is cycled through this flow path to provide continuous cooling and to add water to the reactor core.

In cases where the reactor coolant pressure boundary does not remain intact and water inventory in the core is being lost, the Passive Containment Cooling System (PCCS) and Gravity Driven Cooling System (GDCS) work in concert to maintain the water level in the core and remove decay heat from the reactor by transferring it outside of containment.

If the water level inside the reactor pressure vessel drops to a predetermined level, due to the loss of water inventory, the reactor is depressurized and the GDCS is initiated. It consists of large pools of water inside containment located above the reactor that are connected to the reactor pressure vessel. When the GDCS system is initiated, gravity forces water to flow from the pools into the reactor. The pools are sized to provide sufficient amounts of water to maintain the water at a level above the top of the nuclear fuel. After the reactor has been depressurized, the decay heat is transferred to containment as water inside the reactor boils and exits the reactor pressure vessel into containment in the form of steam.

The PCCS system consists of a set of heat exchangers located in the upper portion of the reactor building. The steam from the reactor rises through containment to the PCCS heat exchangers where the steam is condensed. The condensate then drains from the PCCS heat exchangers back to the GDCS pools where it completes the cycle and drains back to the reactor pressure vessel.

Both the ICS and PCCS heat exchangers are submerged in a pool of water large enough to provide 72 hours of reactor decay heat removal capability. The pool is vented to the atmosphere and is located outside of containment. The combination of these features allows the pool to be refilled easily with low pressure water sources and pre-piped connections.

The reactor core is shorter than in conventional BWR plants to reduce the pressure drop over the fuel, thereby enabling natural circulation. There are 1,132 fuel rod bundles and the thermal power is 4,500 MWth in the standardized SBWR. The nominal summertime output is rated at 1,575-1,600 MWe, yielding an overall plant Carnot efficiency of approximately 35%.

In case of an accident, the ESBWR can remain in a safe, stable state for 72 hours without any operator action or even electrical power. Below the vessel, there is a piping structure that allows for cooling of the core during any very severe accident. These pipes facilitate cooling above and below the molten core with water. GEH's Probabilistic Risk Analysis indicates that a core damage event would occur no more often than once in 59 million years.