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Fission Process

Core Description

Reactor Systems

Core Description

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 two to four positions are filled with either a solid aluminum "dummy" element or an In-Core Experiment.

A boron-stainless steel shim blade is positioned on each side of the hexagonal core, making a total of six blades, any one of which is capable of shutting down the reactor. These blades are connected to electromagnets which are capable of dropping the blades into the core, shutting down the reactor, in less than one second. Any one of twenty-two different safety sensors (as well as manually by the reactor operator) can shut off power to these magnets, shutting down the reactor.

The regulating rod, a cadmium-aluminum rod, is used for fine control of the reactor. This rod is moved to make slight changes in reactor power and to maintain power exactly constant. The MITR is also licensed to use digital control technology to control reactor power.

Water coolant flows down through the entrance channels and then up through the fuel elements to cool the reactor as well as to provide moderation for reactor operation.

Below are some photos of the reactor core:


This is a view looking down into the reactor core tank. The core housing (empty) is visible in the center, while the fuel elements are visible in the fuel storage ring around the core. 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).


Fuel Elements

MITR-II fuel consists of fifteen fuel plates in a rhomboid-shaped element. Each fuel plate consists of fuel sandwiched between sides of aluminum cladding, which are finned to increase the heat transfer surface area. The uranium fuel is in a uranium-aluminum matrix called cermet.


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 so as 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.


Insertion of an in-core experiment. The moving of fuel is performed similarly, using special handling tools.