Research Reactors

For decades, research reactors have been providing a source of neutrons for a wide variety of scientific purposes in dozens of countries around the world.

Research reactors comprise a wide range of different reactor types, which are generally not used for power generation. The primary use of research reactors is to provide a neutron source for research and other applications. They are small relative to power reactors whose primary function is to produce electricity. Their power is designated in megawatts and their output ranges up to 100 MWth , compared with 3000 MWth (i.e. 1000  MWe ) for a typical power reactor.

Research reactors are simpler than power reactors and operate at lower temperatures. They need far less fuel, and far less fission products build up as the fuel is used.

On the other hand, their fuel requires more highly enriched uranium (HEU), typically up to 20% U-235, although some of them use up to 93% U-235 (HEU).

Research reactors also have a very high power density in the core, which requires special design features. Like power reactors, the core needs cooling, and usually a moderator is required to slow down the neutrons to enhance fission. Research reactors also need a reflector to reduce neutron loss from the core and to sustain the chain reaction.

Many of the world’s nuclear reactors are used for research and training, materials testing, production of radioisotopes for medicine and industry.

These research reactors are much smaller than power reactors or those propelling ships, and many are on university campuses.

In fact the total power of the world’s 274 research reactors is little over 3000 MWth .

Looking into a reactor pool
There is a much wider array of designs in use for research reactors than for power reactors and they also have different operating modes, producing energy which may be steady or pulsed.

A common design is the pool type reactor, where the core is a cluster of fuel elements sitting in a large pool of water. Between the fuel elements are control rods and empty channels for experiments. In one particular design (Material Testing Reactor) a fuel element comprises several curved aluminum-clad fuel plates in a vertical box. The water moderates and cools the reactor, and graphite or beryllium is generally used for the reflector, although other materials may also be used. Apertures to access the neutron

beams are set in the wall of the pool. Tank type research reactors are similar, except that cooling is more active.

The TRIGA reactor is another common design. This kind of reactor can safely be pulsed to very high power levels (e.g. 25,000 MW) for fractions of a second.

Other designs are moderated by heavy water or graphite. A few are fast reactors, which require no moderator and can use a mixture of uranium and plutonium as fuel. Homogenous type reactors have a core comprising a solution of uranium salts as a liquid, contained in a tank.