In AHWR, natural circulation is used to remove heat from the reactor core under normal as well as shutdown conditions. Fig. III-2 shows the main heat transport (MHT) system and the passive decay heat removal system of AHWR. The two-phase steam water mixture generated in the core flows through the tail pipes to the steam drum, where steam gets separated from water. The separated water mixes with the subcooled feed water and flows down the downcomers to the reactor inlet header. From the header it flows back to the core through inlet feeders.

REACTOR BUILDING

GRAVITY DFHVEM

PASSIVE CWTAINWENT

CQOUNE SYSTEM

COOLING

WATER SEAL

TURBINE BUILDING

FIG. 111-1. Simplified flow sheet of AHWR.

FIG. III-2. MHT and decay heat removal system.

Larger density differences between hot and cold legs are possible to be achieved in two-phase flow systems compared to single-phase natural circulation flow systems. The absence of pumps not only reduces operating cost, but also eliminates all postulated transients and accidents involving failure of pumps and pump power supply.

Steady state flow prevails in a natural circulation loop when the driving buoyancy force is balanced by the retarding frictional forces. However, the driving force in a natural circulation system is much lower compared to a forced circulation system. With a low driving force, measures are needed to reduce the frictional losses. The methods adopted to reduce frictional losses include, elimination of mechanical separators in the steam drum and the use of large diameter piping. The larger pipes increase the amount of coolant needed in the primary system.

Elimination of mechanical separators makes the system dependent on natural gravity separation at the surface in the steam drum, which may increase carryover and carryunder. Carryover is the fraction of the liquid entrained by the steam, whereas carryunder is the fraction of vapour that is carried by the liquid flowing into the downcomer. Excessive carryover can damage the turbine blades due to erosion, whereas carryunder can significantly reduce the driving buoyancy force and hence the natural circulation flow rate. The steam drum size is chosen to keep carryunder and carryover within acceptable limits.

A rational start-up procedure of the AHWR has been worked out for low pressure and temperature conditions. For this, after the MHT is filled with water to a desired level in the steam drum, the MHT system is pressurized to an initial desired pressure by using steam generated from an external boiler. Subsequently, the control rods are partially withdrawn and coolant heating up continues at about 2% full power. Core boiling will start only after the steam drum pressure reaches 70 bar and the coolant temperature attains 285oC. The reactor power is increased gradually with controlled subcooling at the inlet of the reactor core until full power is reached.

Emergency core cooling system (ECCS) is designed to remove the core heat by passive means in case of a postulated loss of coolant accident (LOCA). In the event of rupture in the primary coolant pressure boundary, the cooling is initially achieved by a large flow of cold water from high pressure accumulators. Later, cooling of the core is achieved for three days by low pressure injection of cold water from gravity driven water pool (GDWP) located near the top of the reactor building. Fig. Ill-1 shows the emergency core cooling system.