The key to initiating and maintaining a self-sustaining chain reaction that can serve as a source of either neutrons (for research) or energy (for power applications) is to select an appropriate fissile material, or “nuclear fuel”. The most common fuel is uranium-235 which undergoes nuclear fission readily when bombarded with slow-moving neutrons, as described above. Unfortunately, uranium-235 makes up only 0.72 % of natural uranium, the rest being uranium-238 and uranium-234: in consequence, the probability that the neutrons from the fission of a uranium-235 atom will bump in to another uranium-235 atom (and cause it to fission) is quite low. To overcome this stumbling block, the nuclear reactors that are operational today use uranium fuel that is enriched in uranium-235, with a U-235 concentration of 1-90 %. (The notable exceptions to this are the Canadian CANDU power reactors, which have a specialized design that allows them to sustain a nuclear chain reaction using natural uranium.) Regardless of the U-235 enrichment level, the uranium fuel “meat” in reactors is usually in the chemical form of uranium (IV) oxide, UO2, due to the high thermal stability and low chemical reactivity of this compound.
Left: A nuclear fuel (uranium oxide) fuel pellet.
Right: Fuel pellets stacked end-to end to create rods, grouped into a ~0.5 m long fuel bundle.
Images courtesy of the Canadian Nuclear Association www.cna.org
The McMaster Nuclear Reactor was originally designed to operate using Highly Enriched Uranium (HEU, >20 % U-235) fuel as were all reactors of its generation. In the 1990s, the global nuclear community shifted away from the use of HEU as a reactor fuel due to concerns that stockpiles of this material at civilian installations were too-convenient targets for malfeasant factions seeking to create nuclear weapons. At this time, MNR elected to participate in the Reduced Enrichment for Research and Test Reactors program that was coordinated by the US Department of Energy and the International Atomic Energy Agency (IAEA), working toward the goal of securing the global supply of HEU by curbing its use at research reactors. Over a 10-year period, MNR converted to Low Enrichment Uranium (LEU, <20 % U-235) nuclear fuel with a full LEU core in place in April 2007, bringing MNR in line with current international guidelines for the non-proliferation of nuclear materials.
Left: Schematic of a nuclear fuel assembly composed of plate-type fuel.
Right: Director of Nuclear Operations Chris Heysel inspects a fuel assembly at MNR.
Rather than the familiar fuel “bundles” – actually numerous cylindrical pellets stacked end-to-end to form fuel “rods” that are grouped into “bundles” – that are used in many power reactors, MNR uses plate-style nuclear fuel. Each fuel plate consists of a mixture of sintered uranium silicide (U3Si2) and aluminum, with an outer coating (or “cladding”) of aluminum which prevents the uranium silicide and various fission products from dissolving into the reactor pool water and contaminating it. Fuel plates are stacked a fixed distance apart from one another inside an aluminum shell to form a “fuel assembly” (see image above). Several of these fuel assemblies are strategically arranged in the reactor pool to achieve a controlled, self-sustaining chain reaction, forming the core of the nuclear reactor.
The open pool of the MNR with a close-up of the reactor core inset.