Nuclear Fuel Shortage?

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This article concerning a nuclear fuel shortage, on MIT’s website, leaves me shaking my head and wonder what they up to. There has to be an agenda behind this besides a genuine concern over having enough fuel for reactors because if we really had a fuel shortage we wouldn’t be creating a national waste repository that is intended to store nuclear waste for 20,000 years.

Some background is necessary regarding the entire nuclear fuel cycle. We mine natural uranium ore, the uranium once extracted from that ore contains proximately .72% U-235, and 99.27% U-238, and .0055% U-234.

In a uranium fueled nuclear fission reactor, the isotope U-235 is the fuel because it can be caused to fission, that is break into lighter elements, by absorbing a slow (thermal) neutron, and in turn also give off neutrons when that happens, which are then absorbed by another U-235 atom, continuing the chain. The resulting daughter elements are slightly lighter than the original uranium element and that mass difference is converted into energy.

U-238 isn’t completely inert however. It absorbs neutrons becoming U-239, decays to Np-239 through beta emission (emits an electron, one of the neutrons becomes a proton), and then Np-239 decays into Pu-239, again through beta decay. Pu-239, like U-235 is fissionable with thermal neutrons.

Thus in a reactor fueled with U-235 and U-238, a percentage of the U-238 atoms will absorb neutrons and become Pu-239 adding to the fissionable fuel inventory in the reactor.

However, notice that U-238 does not emit neutrons during it’s conversion to Pu-239. A light-water moderated reactor can not sustain a nuclear chain reaction with a natural isotopic mix. It is possible to sustain a nuclear fission chain reaction using natural uranium if graphite or heavy water is used as a moderator. Heavy water is expensive in the quantities required and graphite is a fire hazard as was demonstrated in Chernobyl.

To get around this problem uranium is enriched, isotopes are separated using gas diffusion, centrifugal, or laser isotopic enrichment methods so that fuel containing 2.5%-5% U-235 and 95-97.5% U-238 results.

When natural uranium is enriched, typically 15% of the natural uranium becomes enriched uranium at 3.5% U-235, and 85% becomes deplented uranium with .3% U-235. So only 29.2% of the U-235 present in the original ore ever gets into the reactor and about 17% of the U-238. More than 80% of the energy potential of the ore has been eliminated before it ever makes it into the reactor.

During the reactors operation, approximately 1% of the U-238 is turned into Pu-239 and a percentage of that is then fissioned along with U-235 fuel. But at some point the ratio of fissionable isotopes drops too far and waste products that absorb neutrons but do not fission and supply neutrons increases to the point where a chain reaction can no longer be sustained and the reactor must be refueled.

At that point when the spent fuel is removed from the reactor and placed in waste ponds, only 4-5% of the U-238 present in the original fuel (1% or so of what was in the original ore) was used and much of the Pu-239 bred from that is still unused as well as a percentage of the original U-235. Between than .75% and 1% of the natural uranium’s energy potential has been utilized in a one-pass fuel cycle.

MIT is saying we have a fuel shortage because we are presently mining uranium at a rate of only about 65% of the rate that we’re using it in reactors. In the United States, reactors are only using it in a 1-pass fuel cycle and thus more than 99% of the energy potential of that fuel is being wasted.

In addition to that we have huge stockpiles of Pu-239 that have accumulated as part of the weapons programs in the United States and Russia and this also could be used to fuel reactors. It is more problematic however because it is relatively easy to build a bomb from Pu-239, but not from reactor grade uranium which is only enriched to a maximum of around 5% and more typically 2.5-3.5%.

The problem that shuts down the reaction isn’t just a lack of fissionable U-235 and Pu-239, it is the build-up of other actinides, elements heavier than uranium, which absorb thermal neutrons but do not fission and provide additional neutrons as well as some fission products that are neutron absorbers which poison the reaction by absorbing too many neutrons. Fission products also create mechanical problems.

If we reprocess the spent fuel, remove the U-235 and Pu-239, and re-use those in MOX fuel (mixed metal oxides, U-235 and Pu-239), and do this to the greatest extend that we can, we can extract around 2% of the uraniums fuel potential.

Just moving from a single pass to a multi-pass fuel cycle would reduce the amount of uranium that we would need to mine to less than that which is currently being mined. There really isn’t a shortage of uranium, it’s just less expensive to mine fresh ore than to reprocess waste.

We should reprocess, because by doing so not only do we reduce the amount of uranium that needs to be mined, but we reduce the long-lived radioactive wastes by a similar amount.

However, there is a way that we can extract 60-70% of the uraniums fuel potential (with current isotopic separation efficiency) and eliminate most of the long-lived radioactive waste.

In the spent fuel there are two radioactive components. Actinides which includes uranium and all elements that are heavier, and fission products, the daughter products of the fission products. The fission products are highly unstable and decade to stable isotopes in a relatively short time frame. By the time 300 years have passed, they will have decayed to the point where there radioactivity is no higher than the natural ore that was originally mined and are considered safe for disposal at that point.

It is the actinides that are problematic because they have much longer half-lives and it will require in excess of 20,000 years for them to decay to the point where radioactivity is no greater than the original ore.

Of all the actinides produced, only the isotopes U-235 (present in the original uranium fuel), U-233 (bred from thorium, and Pu-239 and Pu-241 are fissionable with thermal neutrons. But all of the actinides are fissionable with fast neutrons. Burning these requires the use of fast-flux nuclear reactors. Early fast-flux reactors had some inherent safety issues because reaction rates could vary rapidly.

However, generation IV reactors have been developed which have inherent stability features. For example, fuel designs that have negative temperature – reactivity coefficients such that as the temperature increases the reaction rate automatically decreases. There are numerous schemes for accomplishing this.

In general fast-flux reactors use either a liquid metal (typically sodium or lead), a liquid salt, or a gas coolant operating at greater temperatures than a boiling water or pressurized water reactor would operate at. This higher temperature translates into higher efficiency. Also, the liquid metal cooled reactors operate at close to atmospheric pressure so there are less plumbing problems.

By using these reactors, 60-70% of the energy content of the uranium ore can be extracted. This can be improved further as separation efficiency improves. Right now however, we have an excess of weapons plutonium stockpiled that can be blended with natural uranium rather than enriching it, and that is providing for a portion of our fuel needs and is the reason we do not have to mine more.

The cost of uranium is a trivial portion of the total cost of producing electricity. If it were to increase by 100x, it would still only account for approximately .7ยข/KWh and a 100x cost increase would increase the supply tremendously.

A uranium shortage doesn’t exist and won’t limit nuclear power production. Intelligent use of uranium could reduce the fuel consumption to less than 2% of that which is presently required to generate a given amount of electricity. Thorium can be used as a fuel and there is 3x as much thorium in the Earth’s crust as there is uranium, and what’s more uranium produces more long-lived actinides than does thorium for the amount of energy produced.

What is the motivation behind MIT’s announcement of an impending uranium shortage?

Category: Future

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