The CHTR has three independent and redundant passive heat removal systems to cater to different postulated accident conditions. These heat removal systems, which are individually capable of removing a neutronically-limited power of 200 kW(th) (200% of normal reactor power), may operate together or independently to prevent the temperature of the core and coolant from increasing beyond a set point. For a loss of load condition, when coolant circuit is intact, a system of six variable conducting sodium heat pipes dissipates heat to the atmosphere. A system of 12 carbon-carbon composite variable conducting heat pipes provided in the reactor core fills the need when coolant is lost. Another passive heat removal system involves the filling of two gas gaps, provided outside the reactor vessel, by a siphon action with molten metal to provide a conduction heat path from the reactor core to a heat sink outside the outer steel shell. Each of these three systems can be

classified as Category-B passive systems [X-3]. These are safety grade systems. A brief description of the gas gap filling system is provided below; its schematic view is shown in Fig. X-6.

The system consists of a reservoir located above the upper plenum and subdivided into compartments. Liquid metal is stored in the reservoir, which is fitted with siphon tubes and bulbs. One end of the siphon is dipped into the liquid metal and the other opens into the inner gas gap; multiple siphon tubes are employed. The bulb is located immediately downstream of the heat pipes and normally senses a temperature of 900°C. In a case of non-availability of the heat pipes, the coolant immediately senses a temperature of 1000°C. This would increase the pressure of the gas inside the bulb, cause the liquid metal to rise inside the siphon tube and ultimately, start the siphon. The liquid metal would then exit into the inner gas gap and also fill the outer air gap through holes in the inner gas gap wall. The gas inside the gas gap would be pushed into a gas tank. A connector between the liquid metal and the gas tank would handle the decrease in pressure caused by the fall in level of liquid metal in the reservoir, such that after some time, pressure in the reservoir and the gas gaps would be equalised.

The CHTR incorporates the following active systems, which are all non-safety-grade.

Passive shutdown — reset system:

In order to move the shut off rods to their position of suspension in electromagnets, CHTR employs a motorized and wire rope based active system. This is a backup system.

Passive gas gap heat removal — reset system:

In order to drain and move molten metal from the gas gaps to a reservoir, CHTR employs an electromagnetic pump based reset system. This is a backup system.

Defuelling and refuelling system:

After the operation of fuel up to a desired burnup, fuel tubes containing fuel compacts will be replaced by new fuel tubes carrying fresh fuel compacts. This replacement operation will be done using an active system. This is a backup system.