Nuclear Power - fuel rods

In order to be a nuclear fuel isotope the nucleus has to be capable of fission and form part of a critical mass of such nuclei. Of all the potential nuclear fuels found in nature, only Uranium-235 is suitable, but any naturally occurring sample of uranium would not have enough U-235 within it to form a critical mass. It must therefore be 'enriched' to increases the concentration of Uranium-235.

Natural uranium is made up of:

Isotope	Percentage Composition	Half Life	Radioactive decay particles
U-238	99.2742%	7.038×108 years	and
U-235	0.7204%	7.038×108 years	and
U-234	0.0054%	245,500 years	and

	Because of the relatively short half life of the U-234 compared to the other two isotopes about half of the activity of a naturally occurring uranium sample is made up of counts from the decay of that isotope. That is true even though it makes up the tiniest proportion of the uranium isotopes.
	Enriched Uranium: Because U-235 is used for fission in nuclear reactors and nuclear weapons, natural uranium is enriched in U-235 by separating the isotopes out by mass to produce a sample that has about 2% to 4% U-235.
	Weapons grade enrichment results in about 90% U-235 ! This type of Uranium has to be carefully monitored and stored so that a critical mass is not reached until required.

Why is the fuel made up of rods rather than being a solid single piece of fuel?

To maximise efficiency the moderator needs to be interspersed between the fuel. Neutrons need to pass through a moderator to slow them (in order to cause further fissions or prevent U-238 absorbing them) - this can be inserted between fuel rods. If a neutron passes out of the fuel rod it is unlikely to re-enter it, but there is a possibility that it may enter another one. It also makes it easier to replace the fuel in stages.

Why Spent Fuel Rods are More Dangerous than Unused Fuel Rods

Fission fragments are in general neutron heavy and therefore radioactive or unstable emitting beta and gamma radiation Some fission fragments have short half-lives or high activities whereas the Uranium has a long half life and a relatively low activity compared to the fission products. Therefore the activity of spent fuel rods is much higher than ones that have not been inserted in a reactor.

Why, after a period of use, the fuel rods in a nuclear reactor become less effective for power production

Power production depends upon the number of fissions in unit time. After a period of time the amount of (fissionable) uranium (235) in the fuel decreases because they have undergone fission. Two fission fragments are produced for each completed fission reaction and they can absorb neutrons preventing further fission.

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