Fuel Costs and Future

Why nuclear fuel?

What's wrong with mined Uranium?

Mined Uranium is not very useful as it is. It is mixed with other kinds of rock, dirt and impurities. It is first necessary to extract the Uranium out of the rock. Once this is done, the Uranium still may not be useful in a nuclear power plant. This is because it has two naturally occurring isotopes: U235 and U238.

A very brief look at Fission

Fission is the process where an isotope is separated into two smaller particles, causing the release of energy which can (eventually) be used to boil water, drive a turbine, and produce electric power.

The energy released can be thought of as the energy required to hold the initial isotope together. So when the two smaller particles are produced, the energy holding them together is released from the system.

This process is useful IF the energy released is greater than the energy required to break up the isotope. This is obvious, because the incident 'breaking' particle needs to be accelerated using some energy in order to break up the initial isotope. If the released energy is lower than this accelerating energy, then the process was a waste of time. Also, if the energy released is greater than the energy required to break up the particle, then the energy released can be used to break up more particles, causing a cascading effect.

The big problem with mined Uranium, is that these fission cascades are only possible with U235, as U238 takes much more energy to break up than what is released when it does break up. So, when a particle comes in and breaks up a U235 isotope, the released energy may hit another U235 isotope and cause it to break up also, or it may hit a U238 isotope and nothing will happen. In order for the cascades to keep on going, there needs to be a certain percentage of U235 isotopes present. This percentage varies between power plants.

In natural mined Uranium, only about 0.3% is actually U235. For many power plants to cause enough cascades to produce energy, the U235 concentration needs to be increased to about 3 - 4%. This is a tricky (and long) process.

Costs and Efficiency

One of the major arguments used by opponents to nuclear power is that uranium fuel rods produce less power and cost more than coal or oil power. A typical fission reaction produces around 200 MeV of energy (1) so 1g of U-235 produces around 5 x 1023 MeV. By comparison 1 gallon of oil produces around 1021 MeV, so even if only 1% of fission events in a reactor produced electricity, this would still produce more energy than an oil power plant. Thus, so long as uranium is enriched to useable concentrations in a typical fuel rod, nuclear fuel is more energy efficient than oil (or coal) power.

According to the Uranium Information Centre, in 2003 the price (US) per kilowatt-hour for nuclear power generation was 3.73 cents, making it comparable with coal power at 3.27 c/kWh. Prices for power generation vary from country to country, with European nuclear power production being cheaper than for the US due to easier access to uranium. The breakdown of US costs required to produce 1kg of nuclear fuel is given in the following table:

In the future it is predicted that with further developments in nuclear fuel processing technology the cost for nuclear power generation will drop further, making it by 2010 cheaper than coal power in most countries (2).

Future Prospects

MOX is the combination of depleted uranium and weapons grade plutonium. It is being used in many European nations and will soon be implemented in some American plants.

In the enrichment process, uranium with a high concentration of U238 is enriched to increase the concentration of U235. Clearly if the amount of U235 in some uranium increases, then the concentration of U235 in some other uranium must decrease. This uranium is called 'depleted' uranium and is usually stored in containers in isolated regions because it is considered useless.

However, if extremely high concentration material like plutonium is mixed with this depleted, low concentration U235, then the result will be a medium concentration fuel called MOX. This can be used to fuel nuclear power plants and has two main benefits outlined above:

1. It decreases the worlds storage of weapons grade plutonium which can be used to make nuclear bombs.
2. It uses up the previously useless 'depleted' uranium.

References

• Krane, K. S. Introductory Nuclear Physics. John Wiley & Sons, 1988, p. 491.
• The Economics of Nuclear Power.