The CHTR incorporates a passive shutdown system. Under normal operation, this system has a set of seven shut off rods made of tungsten and held above the reactor core by individual electro-magnets, with their magnetic holding power energized by a set of low power batteries. These shut off rods are passively released under abnormal conditions when the temperature of the coolant or core goes up. These shut off rods fall into the central bore of the fuel tubes provided for coolant flow. This is a fail safe system; in case of a loss of battery power, the shut off rods would fall and shut down the reactor. This passive system can be classified as a Category-D passive system [X-3]. It is a safety grade system.

Passive core heat removal under normal operation

During normal operation of the reactor, core heat is removed by natural circulation of lead-bismuth eutectic alloy coolant. This passive system can be classified as a Category-B passive system. It is a safety grade system. A brief description of it is given below.

The reactor operates at 100 kW(th) and the lead-bismuth eutectic alloy coolant flowing in the main heat transport system by natural circulation removes heat generated in the fuel. Lead-bismuth eutectic alloy has a high boiling point (1670°C) at atmospheric pressure. This facilitates a low pressure primary system, which is a safety feature of liquid metal cooled reactors. The main coolant circulating loop comprises fuel tubes, down­comers and top and bottom plenums. A simplified view of the system discussed is shown in Fig. X-5. The fuel transfers energy to the coolant flowing upward inside the fuel tubes due to natural circulation. At 900°C, the

coolant enters the fuel tube in the lower plenum and takes the reactor heat; at 1000°C it is delivered to the upper plenum. The active heat generation length in the reactor is 700 mm. The buoyancy head developed in the coolant loop is adequate to maintain the required flow rate for normal power levels. A computer code, based on the law of conservation of momentum, was developed for this analysis.

Passive transfer of heat to the secondary system

A set of 12 high temperature sodium heat pipes passively transfer heat from upper plenum of the reactor to a set of heat utilization vessels, which are kept directly above the upper plenum. This system can be classified as a Category-B passive system [X-3]. It is a safety grade system.