Page 246 - Dynamics and Control of Nuclear Reactors

APPENDIX B Advanced reactors 247

• Accumulators. Accumulators provide flow to the reactor coolant systems when
the pressure drops to medium levels. They are tanks that are pressurized with
nitrogen to 700 psig.
• In-Core Refueling Water Storage Tank (IRWST). The IRWST is a large tank
containing borated water (590,000gal) and is open to the interior of the
containment building. Water flows by gravity from the IRWST into the reactor
coolant system after system depressurization. The Automatic Depressurization
System reduces system pressure by releasing steam into the IRWST or directly
into the containment building. Water spray on the outside of the steel containment
vessel cools and condenses in-containment steam. The condensate returns to the
IRWST. The IRWST also serves as the heat sink for long-term cooling. A heat
exchanger located in the IRWST receives hot fluid from the reactor coolant
system and returns cooler water by natural circulation. If the IRWST water boils,
the steam is condensed by the containment cooling system and the condensate
returns to the IRWST.

B.5.2 Boiling water reactors
Here, we consider the Economic Safe Boiling Water Reactor (ESBWR), a Genera-
+
tion III design [8, 9]. The ESBWR employs passive heat transfer in the reactor cool-
ing system and in the safety system.
The reactor coolant flows through the core by natural circulation, thereby elim-
inating coolant pumps as used in the Advanced Boiling Water Reactor (ABWR) and
most of the earlier BWRs. Two design modifications enhance natural circulation.
The fuel region is shorter than in earlier BWRs, thereby reducing the core pressure
drop. A chimney above the core region further enhances natural circulation. The
ESBWR design eliminates coolant pumps and associated piping, thereby simplifying
construction, reducing cost and enhancing safety.
The safety system has four main components: The Isolation Condenser System
(ICS), the Gravity Driven Cooling System (GDCS), the Passive Containment Cool-
ing System (PCCS) and The Standby Liquid Control System (SLCS). Each employs
a reservoir containing water and no electrical power or operator action is required for
operation.
The ICS operates when the reactor coolant system remains intact and at elevated
pressure. The ICS is connected to a heat exchanger located in a water reservoir
located above the reactor. Heat transfer in the heat exchanger condenses steam
released from the reactor cooling system and the resulting liquid water returns to
the reactor cooling system by natural circulation.
The GDCS transfers water by gravity from a large pool above the reactor after
depression by the Automatic Depressurization System that automatically releases
steam to the Suppression Pool below the reactor.
The PCCS uses a heat exchanger to transfer heat from the containment volume to
water in a pool above the reactor.
The SLCS injects borated water into the reactor.