The core consists of 27 positions, most of which are filled with fuel elements, such as the one shown in position C-9. The remaining three positions are filled with either a solid aluminum "dummy" element or an In-Core Experiment.
High boron, stainless steel shim blades are positioned on each side of the hexagonal core, each one of these six blades is capable of shutting down the reactor. The blades are connected to electromagnets which, when de-powered drop the blades into the core, shutting down the reactor in less than one second. Twenty-two different safety sensors (as well as a manually operated button in the control room) can shut off power to these magnets, and thus shut down the reactor.
The reactor also uses a cadmium-aluminum rod for fine control of the reactor. This rod is moved to make slight changes in reactor power and to maintain the reactor at a constant power level. The MITR is licensed to use digital control technology to control reactor power.
THE MITR core is cooled by ordinary or ‘light’ water which down the outside of the core tank and then up through the fuel elements; this water also slows or ‘moderates’ the neutrons emitted when the uranium-235 fuel fissions. The core is surrounded by a ‘heavy’ water tank and by high-purity graphite which also moderate the neutrons.
Below are some photos of the reactor core:
This is a view looking down into the reactor core tank. The core itself is visible in the center, while some used fuel elements are visible in the fuel storage ring around the core. Three position is the core are filled with unfueled or ‘dummy’ elements. The drives for each of the reactor control blades extend to connect to the blades (located on each of the six sides of the core). The electromagnets used to "scram" the reactor can be seen in the upper areas of the photo.
The blue glow of Cerenkov radiation can be seen emanating from the reactor core (as well as from some of the fuel in the storage ring). The dark components on the left of the picture are the parts of Emergency Core Coolant System (ECCS).
MITR-II fuel elements consist of fifteen fuel plates in a rhomboid-shaped element. Each fuel plate consists of uranium-aluminum fuel sandwiched between sides of aluminum cladding, which cladding is finned to increase the heat transfer surface area.
Refueling of the reactor occurs 3 to 4 times per year, depending on the utilization of the reactor. A ‘refueling’ can be as simple as replacing two or three fuel elements with new fuel or a complete rearrangement of the core, including flipping fuel elements, The rearrangement of elements in the core is done to even the amount of uranium used along an element. A typical fuel element will remain in various positions in the core for about three years.
This view of the core shows experiments positioned in the center or ‘A-ring’ of the core. An empty experimental dummy element is at the top of the picture in the core’s ‘B-ring’.