Question

why uranium mines can be closed if plutonium from present nuclear warheads worldwide can be dismantled and used as fission fuel for power reactors?

Answer 1

The main isotopes of plutonium are:

• Pu-238, (half-life 88 years, alpha decay to U-234, releasing 5.6 MeV)
• Pu-239, fissile (half-life 24,000 years, alpha decay to U-235)
• Pu-240, fertile (half-life 6,560 years, alpha decay to U-236)
• Pu-241, fissile (half-life 14.4 years, beta decay to Am-241)
• Pu-242, (half-life 374,000 years, alpha decay to U-238)
• Periodic tables show an atomic mass of 244 for plutonium, suggesting Pu-244 as the most stable isotope with the longest half-life – 82 million years. It is the only one found in trace quantities in nature, apparently cosmogenic in origin from the formation of the Earth. It is not very relevant to this paper. It alpha decays to U-240.

The decay heat of Pu-238 (0.57 W/g) enables its use as an electricity source in the radioisotope thermoelectric generators (RTGs) of some cardiac pacemakers, space satellites, navigation beacons, etc.

Over one-third of the energy produced in most nuclear power plants comes from plutonium. It is created in the reactor as a by-product. The most common plutonium isotope formed in a typical nuclear reactor is the fissile Pu-239, formed by neutron capture from U-238 (followed by beta decay), and which when fissioned yields much the same energy as the fission of U-235. Plutonium is formed in nuclear power reactors from uranium-238 by neutron capture. When operating, a typical 1000 MWe nuclear power reactor contains within its uranium fuel load several hundred kilograms of plutonium. Well over half of the plutonium created in the reactor core is 'burned' in situ and is responsible for about one-third of the total heat output of a light water reactor (LWR) and about 60% of the heat in a pressurized heavy water reactor (PHWR). Of the rest in the LWR, about one-third through neutron capture becomes Pu-240 (and Pu-241). In a fast reactor this proportion is much less. Approximately 1.15% of plutonium in the spent fuel removed from a commercial LWR power reactor (burn-up of 42 GWd/t) consists of about 53% Pu-239, 25% Pu-240, 15% Pu-241, 5% Pu-242 and 2% of Pu-238, which is the main source of heat and radioactivity.

Plutonium is the principal fuel in a fast neutron reactor, and in any reactor it is progressively bred from non-fissile U-238 that comprises over 99% of natural uranium. Plutonium is used in fast neutron reactors, where a much higher proportion of Pu-239 fissions and in fact all the plutonium isotopes fission, and so function as a fuel. As with uranium, the energy potential of plutonium is more fully realised in a fast reactor. All plutonium isotopes are fissionable with fast neutrons, though only two are fissile (with slow neutrons). For this reason, all are significant in a fast neutron reactor (FNR), but only one – Pu-239 – has a major role in a conventional light water power reactor. Each fission yields a little over 200 MeV, or about 82 TJ/kg.

Plutonium, both that routinely made in power reactors and that from dismantled nuclear weapons, is a valuable energy source when integrated into the nuclear fuel cycle. In a conventional nuclear reactor, one kilogram of Pu-239 can produce sufficient heat to generate nearly 8 million kilowatt-hours of electricity.

This is the reason why Uranium mines can be closed if all the plutonium from present nuclear warheads can be dismantled and used as fuel for nuclear power reactors