How long does it take to have a safe nuclear fuel rod?

Answer 1

The amount of energy obtained from nuclear fuel is expressed as so many megawattdays per tonne, in a liquid water reactor (LWR) it is now around 60,000 MWd/tonne. So if a reactor power output is say 3000 MW (thermal), in one day it will use up 1/20 tonne of fuel, and in a year it will use 18.25 tonne. With reactors that can be refuelled during operation, or during short shutdown periods, the individual fuel assemblies can be changed as required giving flexibility, this applies to the advanced gas-cooled reactors (AGR's) built in the UK. With LWR's where the top of the pressure vessel has to be removed for fuelling, this is only possible during long outages (though they have become much shorter with practice), and so a large proportion of the fuel is changed each time. When a reactor has settled into a routine of regular fuelling outages, this would probably be at about two yearly intervals, and about 1/3 of the fuel changed each time, so any particular assembly could stay in the reactor for about six years on average.

Answer 2

• in a plutonium production reactor, several days to at most a couple weeks.
• in a power reactor, typically years.
• in a research reactor, depends on the research being done.

Answer 3

Well, all fuel rods are pretty much safe (although they emit small amounts of radiation on their own). It's the post-fission uranium/plutonium that is dangerous.