A passive concrete cooling system is designed to protect the concrete structure of the reactor in a high temperature zone (volume V1). A schematic of the passive concrete cooling system is shown in Fig. VI-8. Cooling is achieved by the circulation of a coolant from the GDWP in natural convection mode through cooling pipes located between the concrete structure and the insulation panel surrounding the MHT system hot piping. Heat loss from the high temperature MHT piping is reduced by the insulation panel. Heat transferred through the insulation panel is removed in a natural convection mode by GDWP water through pipes fixed on a corrugated plate on the outer surface of the insulation panel. This passive design maintains the concrete temperature at below 55°C. It also eliminates the need for high capacity blowers and prevents consequences that otherwise may result from equipment or power supply failures which might lead to a temperature increase in the concrete structure.

The AHWR incorporates two independent fast acting wired (sensors, signal carriers and actuators) shutdown systems, which could be categorized as category D passive systems [VI-2]; they are:

• Shutdown system-1 (SDS-1), based on mechanical shut-off rods with boron carbide absorbers in 40 lattice positions. In case of a signal requiring rector trip, shut-off rods fall under gravity into the core in less than two seconds to achieve required reactivity worth;

• Shutdown system-2 (SDS-2), based on liquid poison injection into the moderator. On a trip signal, a quick opening valve located between the helium gas tank and the poison tank opens, letting high pressure helium gas communicate with the poison tank. As a result, the liquid poison is driven out from the poison tank into the moderator by helium gas pressure.

The AHWR incorporates no dedicated active safety systems. As was already mentioned above, when both the IC and the main condenser are unavailable, decay heat can be removed in an active mode, using MHT purification coolers.

The passive systems are safety grade.

Some major highlights of passive safety design features in the MARS, structured in accordance with the

various levels of defence in depth [VI-3, VI-4], are described below.

Level 1: Prevention of abnormal operation and failure

(a) Elimination of the hazard of loss of coolant flow:

• Heat removal from the core under both normal full power operating conditions and shutdown conditions is performed by natural convection of the coolant; this eliminates the hazard of a loss of coolant flow;

(b) Reduction of the extent of overpower transient:

• Slightly negative void coefficient of reactivity;

• Low core power density;

• Negative fuel temperature coefficient of reactivity;

• Low excess reactivity.

Level 2: Control of abnormal operation and detection of failure

• An increased reliability of the control system achieved with the use of high reliability digital control using advanced information technology;

• Increased operator reliability achieved with the use of advanced displays and diagnostics using artificial intelligence and expert systems;

• Large coolant inventory in the main coolant system.

Level 3: Control of accidents within the design basis

• Increased reliability of the emergency core cooling system, achieved through passive injection of cooling water (initially from an accumulator and later from the overhead GDWP) directly into a fuel cluster through four independent parallel trains;

• Increased reliability of a shutdown, achieved by providing two independent shutdown systems, one comprising the mechanical shut off rods and the other employing injection of a liquid poison into the low pressure moderator. Each of the systems is capable of shutting down the reactor independently. Further enhanced reliability of the shutdown is achieved by providing an additional passive shutdown device operated by steam pressure for the injection of a poison in the case of a extremely low probability failure of both the mechanical shut-off rods and the liquid poison shutdown system;

• Increased reliability of decay heat removal, achieved through a passive decay heat removal system, which transfers decay heat to the GDWP by natural convection;

• Large inventory of water inside the containment (about 6000 m3 of water in the GDWP) provides prolonged core cooling, meeting the requirement of an increased grace period.

Level 4: Control of severe plant conditions, including prevention of accident progression and mitigation of consequences of severe accidents [51]

• Double containment;

• Passive containment isolation;

• Vapour suppression in GDWP;

• Passive containment cooling.