Nuclear fission fuels

There are a large number of fuels used in the various types of nuclear reactors. The most common fuel is uranium, whereby the naturally abundant U-238 has been partially enriched to more fissionable U-235.

The nuclear power that has been used since the 1950s utilises the fission of uranium. The large uranium atoms, a mix of the 235 and 238 isotopes, is used as fuel. A mass of uranium fuel is allowed to enter a controlled chain reaction, and uranium atoms decay into smaller atoms, releasing neutrons and heat. The heat is created by some of the mass during the decay process being converted to energy, according to Einstein’s E = mc2, where E is the energy released, m is the mass deficit between the initial mass of uranium and the final products, and c is the speed of light, 3.0 × 108 m/s.

RGPu = reactor grade plutonium; WGPu = weapons-grade plutonium; MOX = Mixed Oxide Fuel

FuelHalf-Life /MySource
U-2330.159Thorium
U-235704enriched natural uranium ore, from 0.720% to 3-4%
U-238446899.27% of natural uranium
Pu-2390.0241weapons decommissioning, neutron bombardment of U-238

The energy released by one uranium-235 atom decaying is: E = mc2 = 3.24 × 10-11 J (83.14 TJ/kg)

The half-life of U-235 is 704 My. The half-life of U-238 is 4.468 billion years. This difference explains why U-238 has a natural abundance of 99.27%. U-235 has halved its quantity at least 8 times since its formation in the supernova which formed the solar system, 6 billion years ago. The quantity of U-238 has halved only once.

Uranium can be caused to decay at higher than natural rates in nuclear reactors. This releases huge amounts of heat which can be used to generate electricity. Uranium does not release pollution of the type fossil fuels do.

Uranium-238 fission:

10n + 23892U

23992U + γ

β-

23993Np

β-

23994Pu

Uranium-235 fission:

10n

+ 23592U → 23692U → 14054Xe

+ 9438Sr

+ 210n

Control rods

A nuclear reactor controls the rate of fission by two means: the moderator and the control rods. Control rods are made of a neutron-absorbing material, such as boron or cadmium. They can be inserted or removed from a nuclear reactor core to vary the degree the neutrons generated by the chain reaction of the nuclear fuel are absorbed. If fewer rods are present, more neutrons will collide with more uranium fuel atoms, causing the chain reaction to increase.

By inserting all the rods fully, the reactor can be shut down entirely. If too many of the rods were removed, the reactor could go critical, a condition at which the chain reaction is at a dangerous level.

One atom of U-233 releases 197.9 MeV (3.171 × 10−11 J), or 19.09 TJ/mol = 81.95 TJ/kg, of energy during fission. The half-life of U-233 is 160,000 years, and alpha decays to Th-229. Th-233 has a half-life of 22 minutes, and decays into Pa-233 (half-life = 27 days), which beta decays to U-233 (half-life = 160,000 years). U-233 usually fissions, releasing an alpha particle to form Th-229, when impacted by a neutron, but a portion of it keeps the neutron and becomes U-234. The overall capture-fission ratio is lower for U-233 than for U-235 and Pu-239, which means the chain reaction requires a higher neutron density to reach sustainable levels.