CHTR incorporates a passive power regulation system (PPRS). This system operates on the principle of an increase in gas pressure with temperature, thereby pressurizing and forcing a column of molten metal with floating absorbing material into the core. This introduces negative reactivity in the core. Depending on the sensed temperature rise, the system would stabilize at a particular value of reactivity insertion. PPRS operation was analyzed using an in-house developed computer code. This passive system can be classified as a category-B passive system [X-3]. It is a safety grade system. A brief description of the system is provided below.

The passive power regulation system consists of 18 different passive power regulation units (PPRU), each of which is centrally housed in the 18 beryllia reflector blocks. Schematic view of a PPRU is shown in Fig. X-4.

The PPRU has a tube-in-tube design. The outer tube is a control tube and the inner tube is the driver tube. The driver tube also serves as a guide to the absorber. The boron carbide (B4C) based absorber is an annular structure; it is housed in the annular space between the control and driver tubes. There is liquid lead-bismuth in these tubes, and the two tubes are in fluidic communication via orifices at the bottom of the driver tube. Free liquid surfaces are maintained in both of the tubes. The volume above the liquid is filled with helium. The

© Gas Header © Guide Tube © Control Tube © Absorber Rod © Driving Liquid

—©

FIG. X-4. Schematic view of PPRS.

absorber floats on the lead-bismuth. A gas header is provided at the top of the driver tube; it is located in the upper plenum, submerged in the coolant. This system operates on the principle of a change in gas pressure with temperature and, therefore, is a category-B passive system [X-3].

When the reactor is critical, the PPRS absorber is located at particular insertion in the core. At this steady state, the gas in the header will be at equilibrium with the coolant temperature in the upper plenum. Any deviation from this equilibrium state will cause the gas to either pressurize or depressurize the driver tube, due to a respective increase or decrease in temperature. As the control and driver tubes are in fluidic communication, this pressure change will be communicated to the control tube. The net result will be a change in liquid lead-bismuth levels in both tubes. Since the absorber is riding on the free liquid surface in the annular space between the control and driver tubes, it will also be pushed in or pulled out with pressurization or depressurization, respectively, thereby changing the reactivity. This system is capable of shutting down the reactor.