High-temperature reactors passive heat removal systems

Several inherent and passive safety features are incorporated in compact high temperature reactor. Due to negative temperature coefficient of reactivity, the power of the reactor comes down without necessitating any external control in case of increase in core temperature. The reactor also adopts passive systems like removal of core heat by natural circulation of liquid metal coolant in the main heat transport circuit, passive regulation and shut down systems. The reactor is also able to remove heat passively by way of conduction in the reactor block and by radiation and natural convection from the outer surface of the reactor during loss of heat sink. The paper deals with the details of passive systems incorporated in the AHWR and CHTR and the analysis performed for these systems. 

The size of most passively safe high temperature reactors is limited by the design characteristics of the passive decay heat removal systems to about 600 MW(th) with power outputs of 200 to 300 MW(e). The AHTR does not have this technological size limitation (see section XXVI-1.6.3). As a consequence, it can be built as a medium (600 MW(e)) or as a large reactor. This offers economics of scale. 

Nuclear heat from the reactor core is removed passively by a lead-bismuth eutectic alloy coolant [XXIX-4], which flows due to natural circulation between the bottom and top plenums, upward through the fuel tubes and returning through the downcomer tubes. On top of the upper plenum, the reactor has multi-layer heat utilization vessels to provide an interface to systems for high temperature heat applications. A set of sodium heat pipes is in the upper plenum of the reactor to passively transfer heat from the upper plenum to the heat utilization vessels with a minimum drop of temperature. Another set of heat pipes transfers heat from the upper plenum to the atmospheric air in the case of a postulated accident. To shut down the reactor, a set of seven shut-off rods has been provided, which fall by gravity in the central seven coolant channels. Appropriate instmmentation like neutron detectors, fission/ ion chambers, various sensors and auxiliary systems such as a cover gas system, purification systems, active interventions etc. are being incorporated in the design as necessary. 

In the highly unlikely event that the IHTS becomes completely unavailable, the safety-related RVACS will passively remove decay heat from the reactor vessel. As the temperature of the reactor sodium and reactor vessel automatically rise, the radiant heat transfer across the argon gap to the contairunent vessel increases to accommodate the heat load. With the increase in containment vessel temperature, the heat transfer from the containment vessel to the atmospheric air surrounding the containment vessel increases. The RVACS system... 

The LFR system provides for ambient pressure single-phase primary coolant natural circulation heat transport and removal of core power under all operational and postulated accident conditions. The high boiling temperature of the Pb coolant enables heat transport by natural circulation of the primary coolant at significantly higher temperatures than with traditional liquid metal cooled reactors. External natural convection driven passive air-cooling of the guard/containment vessel is always in effect and removes power at decay heat levels. 

---