Bhabha Atomic Research Centre (BARC),
India

X — 1. DESCRIPTION OF THE CHTR CONCEPT

The Compact High Temperature Reactor (CHTR) is a lead-bismuth cooled beryllium oxide moderated reactor, designed to operate mainly with 233U-Th fuel. The concept of this reactor, which is initially being developed to generate about 100 kW(th), has a core lifetime of 15 years and incorporates several advanced passive safety features to enable its operation as a compact power pack in remote areas not connected to the electrical grid. The reactor, being designed to operate at 1000°C, would also facilitate demonstration of technologies for high temperature process heat applications, such as hydrogen production by splitting of water. The CHTR concept is described in detail in [X-1].

The CHTR core consists of 19 prismatic beryllium oxide (BeO) moderator blocks. These moderator blocks have graphite fuel tubes located centrally. Each fuel tube carries fuel inside 12 equidistant longitudinal bores. The fuel tube also serves as a coolant channel. CHTR fuel is based on tri-isotropic (TRISO) coated particle fuel. Coated particles are mixed with graphite powder as a matrix material and shaped into cylindrical fuel compacts. Fuel bores of each of the 19 fuel tubes are packed with fuel compacts. Eighteen blocks of beryllium oxide reflector surround the moderator blocks. Centrally, these blocks accommodate the passive power regulation system. Graphite reflector blocks surround these beryllium oxide reflector blocks. Cross-sectional layout of the reactor core is shown in Fig. X-1 below.

The core and the reflector part of the reactor are contained in a metallic shell resistant to corrosion against Pb-Bi eutectic alloy coolant, and suitable for high temperature applications. Top and bottom closure plates made of similar material close this reactor shell. Above the top cover plate and below the bottom cover plate, coolant plenums are provided. These plenums have flow guiding blocks made of graphite and have passages for coolant flow to increase the velocity of coolant between fuel tubes and down-comer tubes. Two gas gaps surround the reactor shell and act as insulators during normal reactor operation, reducing heat loss in the radial direction. A finned outer steel shell is provided, which is surrounded by a heat sink. Nuclear heat from the reactor core is removed passively by Pb-Bi eutectic alloy coolant, which flows due to natural circulation between the bottom and the top plenums; upward through fuel tubes, and returning downward through down-comer tubes. Heat utilization vessels are located on top of the upper plenum, providing an interface to systems for high temperature heat applications. A set of sodium heat pipes is provided in the upper plenum of the reactor for passive transfer of heat from the upper plenum to the heat utilization vessels. Three passive systems are provided to remove heat in the case of postulated accident conditions. One of the systems has a set of heat pipes to transfer heat from the upper plenum to the atmosphere in the case of a postulated accident. Another passive system is intended to fill gas gaps with molten metal in the case of an abnormal rise in coolant outlet temperature, so as to facilitate conduction flow of reactor heat to the outside heat sink. To shut down the reactor, a set of seven shut off rods is included, which fall driven by gravity into the central seven coolant channels. Major design and operating parameters of the CHTR are shown in Table X-1.

CHTR component layout is shown in Fig. X-2.

CHTR fuel consists of 233UC2, ThC2, and small amounts of gadolinium as burnable poison (provided only in central fuel tube). Thorium and burnable poisons make the fuel temperature coefficient negative, thus making the reactor inherently safe. The fuel is in the form of fuel compacts made up of TRISO coated particle fuel embedded in graphite matrix. This type of fuel can withstand temperatures up to 1600°C [X-1, X-2]. A typical CHTR fuel bed consists of a prismatic BeO moderator block with a centrally located graphite fuel tube carrying the fuel compacts. Schematics of a fuel particle, a fuel compact, and a single fuel bed are shown in Fig. X-3.

Outer Steel Shell Gas Gaps High Conductivity shells Inner Shell

Graphite Reflector Downcomer Tubes

BeO Reflector Reactor Regulating System

BeO Moderator

Fuel Tube Fuel

FIG. X-1. Cross-sectional layout of CHTR core.

TABLE X-1. MAJOR DESIGN AND OPERATING PARAMETERS OF CHTR [X-1]

Attributes	Design parameters
Reactor power	100 kW(th)
Core configuration	Vertical, prismatic block type
Fuel	233UC2+ ThC2 based TRISO coated fuel particles shaped into fuel compacts
Fuel enrichment by 233U	33.75 weight %
Refuelling interval	15 effective full power years
Fuel burnup	и 68 000 MW-day/t of heavy metal
Moderator	BeO
Reflector	Partly BeO, and partly graphite
Coolant	Molten Pb-Bi eutectic alloy (44.5% Pb and 55.5% Bi)
Mode of core heat removal	Natural circulation of coolant
Coolant flow rate through core	6.7 kg/s
Coolant inlet temperature	900°C
Coolant outlet temperature	1000°C
Loop height	1.4 m (actual length of the fuel tube)
Core diameter	1.27 m (including radial reflectors)
Core height	1.0 m (Height of the fuelled part and axial reflectors)
Primary shutdown system	18 floating annular B4C elements in the passive power regulation system
Secondary shutdown system	7 mechanical shut off rods

Shutdown System

HUSI Vessels Heat Pipes

Gas Gap Filling System

Upper Plenum

Downcomer Tubes

Fuel Tube Coolant

BeO Moderator

BeO Reflector

Graphite Reflector

Inner Shell

Gas Gaps

High Conductivity shells

Outer Steel Shell

Downcomer Tubes

Lower Plenum

Passive Power Regulation System

X — 2. PASSIVE SAFETY DESIGN FEATURES OF THE CHTR

The inherent and passive safety features falling under category A defined in IAEA-TECDOC-626 [X-3] are the following:

• A strong negative Doppler coefficient of the fuel for any operating condition, resulting in a reduction of reactor power in the case of fuel temperature rise during any postulated accident scenario;

• High thermal inertia of the all ceramic core and low core power density, resulting in very slow temperature rise of reactor core components as well as fuel during a condition when all heat sinks are lost;

• A large margin between normal operating temperature of the fuel (around 1100°C) and the allowable limit of TRISO coated particle fuels (1600°C), intended to retain fission products and gases and resulting in their negligible release during normal operating conditions. This also provides a ‘healthy’ margin of around 500°C to take care of any unwanted global or local power excursions;

FIG. X-3. Schematic of TRISO coated particle fuel, fuel compact and a single fuel bed.

• A negative moderator temperature coefficient results in lowering of reactor power in the case of an increase in moderator temperature due to any postulated accident condition;

• Due to the use of a lead-bismuth alloy based coolant having a very high boiling point (1670°C), there is a very large thermal margin to Pb-Bi boiling, the normal operating temperature being 1000°C. This eliminates the possibility of heat exchange crisis and increases the reliability of heat removal from the core. The coolant operates at low pressure, there is no over pressurization and no chance of reactor thermal explosion due to coolant overheating;

• The high temperature Pb-Bi coolant, which is maintained in an inert gas atmosphere, is itself chemically inert. Even in the eventuality of accidental contact with air or water, it does not react violently and does not cause any explosions or fires;

• Due to the above atmospheric melting point of 123°C, even in the case of a primary system leakage, coolant solidifies and prevents further leakage;

• There is small thermal energy stored in the coolant, which is available for release in the event of a leak or accident;

• Very low coolant pressure allows for the use of a graphite/carbon based coolant channel having a low neutron absorption cross-section, thus improving the neutronics of the reactor;

• Low induced long lived gamma activity of the coolant, such that in the case of leakage the coolant retains iodine and other radio-nuclides;

• For Pb-Bi coolant, the reactivity effects (void, power, temperature, etc.) are negative; thus reducing reactor power in the case of any inadvertent power or temperature increase.

The passive safety systems falling under Categories B, C, D defined in IAEA-TECDOC-626 [X-3] are described below.