Why is spent nuclear fuel more dangerous than unused fuel?

Answer 1

In order to be nuclear fuel in an conventional nuclear plant, the isotopes have to be capable of fission and have a critical mass. Of the potential fuels found in nature, only 235U has this capacity, and there is not enough of it in uranium ore to provide for fuel of a simple reactor without enrichment.

The half lives of two uranium isotopes most commonly found in nature are roughly 703,800,000, for 235U, which is 0.720% of what is found, and 4,468,900,000 years for 238U, which is 99.274%. The percentage of 235U is increased to about 4% or 5% for fuel in power plants. This means that even though uranium can have critical mass, it is not especially dangerously radioactive.

Critical mass is achievable because the uranium can produce a chain reaction in which the fission of one atom causes the fission of one or more other atoms. This happens because uranium occasionally undergo spontaneous fission instead of its normal alpha decay, producing neutrons in the process, and the neutrons can cause other atoms to undergo fission. If there is a sufficiently abundant supply of atoms capable of fission, the fission event produces on average more than one other fission event. As this continues, the speed of fission increases and a chain reaction follows, going on until the fuel runs out.

While the original uranium is not especially radioactive, the products of fission are. They cannot support fission, because they do not have sufficient mass, but their half lives are mostly very, very short. Each daughter atom of fission has a high probability of multiple decays during the first seconds of its existence. The isotopes with half lives of seconds or less are mostly gone by the time the fuel rod is removed from the reactor, but they are followed by isotopes with half lives ranging from days to years.

The decay of short term fission products happens so rapidly that even in the absence of fission, the rods need special cooling for several years. As the decay continues, the half lives of the remaining isotopes get longer, until the atoms of isotopes with short half lives are mostly gone, at which point the rods can be removed to longer term storage.

Medium term fission products have half lives of 10 to 90 years, making them very radioactive. Nuclear waste needs centuries of storage just because of these. And long term fission fragments have half lives that range from 211 to 80,000,000 years.

It has been calculated that the time it takes for spent fuel to decay to the level of radioactivity of naturally occurring uranium ore is about 6,000,000 years.