Transient Reactor Test Facility (TREAT)

Fuel and Core Description

The TREAT fuel assemblies are approximately 9 feet long, and 4 inches square in cross section. The fuel is a graphite uranium mixture, with 1 part uranium to 10,000 parts graphite. The active portion of the fuel assembly is about 48 inches, with a graphite reflector of about 24 inches above and below the active portion. The active portion of the fuel assemblies are encased with Zircaloy. There is also a graphite axial reflector, composed of two parts. The first part of the axial reflector consists of movable assemblies similar to the fuel assemblies, but containing only graphite and no fuel. The second part of the axial reflector consists of permanent blocks of graphite, approximately 24 inches thick, stacked outside the core cavity. This permanent reflector was reclaimed from Chicago Pile-1, the world's first nuclear reactor. The core may be loaded to a size of 5 feet by 5 feet (nominal) up to 6 feet by 6 feet (maximum), depending on the needs of the experiment.

As described above, the fuel is composed of a mixture of graphite and uranium. The uranium is in the form of uranium oxide particles that are approximately 20 microns in size, and are in direct contact with the graphite moderator. The graphite, in addition to being the neutron moderator, also acts as a large thermal heat sink. The time lag of the heat transfer is on the order of 1 millisecond, much faster than the heat transfer to a liquid coolant flowing past fuel assemblies. Also, when the graphite is heated it creates a sizable negative moderator temperature coefficient. These characteristics allow TREAT to produce large ‘self-limited’ transients, which are limited by the fuel negative moderator coefficient without control rod movement.

Experimental Capabilities

TREAT is capable of a wide range of operations and test conditions. TREAT can operate at a steady state power of 100 kW, produce short transients of up to 19 GW, or produce shaped transients controlled by the TREAT automatic reactor control system and the Control Rods. A test assembly can be inserted in the center of the core. The test assembly is a self-contained vehicle, that can contain fuel or materials for a variety of reactor types. These test assemblies, also referred to as test vehicles or test loops, can simulate the conditions of a light water reactor, heavy water reactor, liquid metal fast breeder reactor, or a gas cooled reactor.

Hodoscope

TREAT has a fast-neutron hodoscope that collimates and detects fast fission neutrons emitted by the experiment fuel sample.

The TREAT hodoscope consists of a front collimator, a rear collimator, a bank of detectors, electronics to interface to the detectors, and a data acquisition system. The collimator has 10 columns with 36 rows, which are aligned to an array (or arrays) of 360 detectors. The hodoscope provides time and spatial resolution of fuel motion during transients and in-place measurement of fuel distribution before, during, and after an experiment. One array of detectors consists of Hornyak Button detectors. The Hornyak Button is a fast neutron detector that consists of a film of ZnS applied to lucite, which together form the 'button'. The button is attached to a photomultiplier tube. This detector shows good efficiency at detecting fast neutrons in a background of thermal neutrons and gamma radiation.

Neutron Radiography Facility

TREAT has a neutron radiography facility on the west face of the reactor. This allows non-destructive examination of experiment test assembly (or other materials) up to 15 feet in length. TREAT can operate at steady state power levels of up to 100 kW to product neutrons for the radiography facility.REAT

Control Rod Drive Systems

TREAT has three banks of control rod drive mechanisms, the control/shutdown rods, the compensation/shutdown rods, and the transient rods. There are 4 drive mechanisms in each group. The drive mechanisms are below the reactor, and raise the control rods out of the reactor to increase reactivity. These banks of control rods are arranged in two rings. The inner ring contains four drive mechanisms, the compensation/shutdown rods, with one control rod for each drive mechanism. The outer ring has four control/shutdown drive mechanisms and four transient rod drive mechanisms. The control/shutdown and transient rod drive mechanisms have two control rods for each drive mechanism. All the control rods contain B₄C poison sections. The compensation/shutdown and control/shutdown drive mechanisms are mechanical lead screw driven, and have use pneumatic pressure to assist the scram function. The four transient rod drives are hydraulically actuated, and are controlled by the ARCS to control transients. These transient rods move up to 170 in/sec over a total travel of 40 inches (i.e. full 40 inch stroke in about 0.24 seconds).