A closed system is one in which there is no free exchange of elements in a material with the outside environment.Because these elements (and their isotopes) all have different half-lives, elements with longer half-lives will be present in greater quantities than those with shorter half-lives.The uranium within these items is radioactive and should be treated with care.Uranium's most stable isotope, uranium-238, has a half-life of about 4,468,000,000 years.
Thorium-233 has a half-life of about 22 minutes and decays into protactinium-233 through beta decay.
Although it does not occur naturally, uranium-233 is also a fissionable material that can be used as a fuel in nuclear reactors.
To produce uranium-233, atoms of thorium-232 are exposed to neutrons.
Protactinium-233 has a half-life of about 27 days and decays into uranium-233, also through beta decay.
If completely fissioned, one pound (0.45 kilograms) of uranium-233 will provide the same amount of energy as burning 1,500 tons (1,350,000 kilograms) of coal.