Science & Mathematics
Posted in: Science & Mathematics.
Tagged: fuel rod · moderators · neutron · nuclear reaction · poisons · reactor · Uranium Fuel
Left alone, the rate of decay of uranium is constant and doesn’t change. But this is not what happens in a nuclear reactor, since you can control the rate of reaction by inserting/retracting fuel rods and moderator (neutron-absorber) rods. So I agree with you when you say the "how long" depends on the rate of energy production (power output, i.e. energy divided by time).
Given that enriched uranium for energy production has about 3% of U-235, that means that a "CANDU" fuel bundle (for example) weighing 20 kilograms and having some 40 individual rods has about 3% times 20 kg divided by 40 rods = 15 grams of U-235. The yield just for U-235 to U-238 conversion (disregarding other channels) is about 83 TeraJoule per kilogram, therefore each rod should yield about 1.2 TeraJoule, which is enough to power a 60 watt lightbulb for some 600 years. This is a very rough estimate, of course.
<QUOTE>how long would a fuel rod last if the reactor was run at maximum?</QUOTE>
We can’t just turn the dial up on a nuclear power station, because the sudden heat would increase the vapor pressure like a cooking pan, and the burst "pan" would spread out the fuel into the atmosphere were it would disperse and deposit elsewhere. See Three Mile Island and Chernobyl.
Given the world’s largest nuclear power plant, which is in Japan, which can produce up to about 8 Gigawatt from its 7 reacts, it means it should consume U-235 at about 8 GW divided by 83 TJ/kg = 0.1 miligram of U-235 per second. That means that a single rod (not to be confused with a bundle of rods) would last 150 seconds (about 2.5 minutes) — if all other rods of all the other 7 reactors were "switched off", that is!…
Mine usually lasts about 20minutes, although I have been known to go a few hours if I think of Margaret Thatcher on a cold day.
I think fuel rods last about two years in power plants.
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