s it true that a little pellet of uranium hexafluoride generates as much energy as a tonne of coal?
Yes, it’s roughly true. It’s true to within about a factor of 2.
But uranium hexafluoride, called hex for short, is not what is used in nuclear reactors.
Certainly it is not used in the most common light water and heavy water commercial power reactors. It is a solid at standard pressure and temperature, but at 56°C it sublimes into a very unpleasant gas, and it also reacts very violently with water, forming clouds of corrosive and extremely deadly HF. So hex is only used, and it must be used very carefully even then, during uranium enrichment. These days enrichment is almost universally done in ultracentrifuges, cascades of which gradually increase the abundance of U-235 in the product uranium hexafluoride, from the natural uranium abundance of U-235 present initially, which is about 0.7%, to the approximately 3-5% U-235 content that is usable in light water reactors.
After enrichment, the uranium is stripped chemically of the six fluorine atoms, and is usually made into uranyl nitrate, which is mixed with a base, such as ammonia, to form ammonium uranate, which is a solid. This solid is heated in air, calcinated, to form U₃O₈, an oxide of uranium. This oxide is then further heated to high temperature in a hydrogen/argon atmosphere to form UO₂, another oxide of uranium with a higher oxidation state.
UO₂ is a black solid and a semiconductor at standard temperatures and pressures, and it has a very high melting point, well above 2000°C. It is a ceramic and it is quite unreactive, so it is suitable for use as fuel in a reactor.
The UO₂ is finely powdered, mixed with an organic binder, and then pressed into small cylindrical pellets. The pellets are then heated to very high temperature, again in a hydrogen/argon atmosphere, to sinter the oxide powder, thus forming a solid pellet with very few pores in it. Fuel pellet manufacture is a complicated process. It is highly desirable to have as few pores as possible in the pellets, or else they will shrink inside the reactor.
Here’s a photo of such a pellet to give an idea of the scale.
These cylindrical fuel pellets are typically about 1 cm in diameter by 1 cm long, though that varies a bit. Larger pellets are used in BWRs than in PWRs I believe, and I’m sure that heavy water reactors like CANDU do this slightly differently too.
The fuel pellets typically mass about 10 grams - due to the high atomic weight of the uranium in comparison to the two oxygens, that mass is mostly coming from the uranium.
The pellets are formed into fuel rods, by stacking them in zircalloy tubes which act as cladding, and then bundles of the fuel rods are made into fuel assemblies, and the fuel assemblies are put into a reactor. The energy that is produced per pellet depends on how exactly the reactor is operated, so the original question is somewhat complicated to answer. But a typical achievable number, in modern reactors with the best fuel management, for what is called the burn up rate in the reactor is that 50,000 megawatt-days of power can be produced per ton of initial 3–5% enriched uranium in the fuel.
At that burnup fraction, just one such fuel pellet would produce energy that is equivalent to about 425 cubic meters of natural gas. At 37 megajoule per cubic meter of natural gas that is 15725 MJ, or 15.7 GJ.
One tonne of coal can be burned to release about 29 GJ.
So it’s a correct statement to within an order of magnitude, that one fuel pellet produces as much energy as a tonne of coal.
But that is deceptive. Better burnup can be achieved by better fuel management. It is ultimately the buildup of fission products in the fuel pellets that require they be removed from the reactor, and this happens long before all of the available energy has been extracted from the fuel.
In addition there is a large quantity of unused U-238 remaining, which, remember, is about 95–97% of the uranium in the fuel pellet initially.
That U-238 can be, and some of it is, bred by neutron absorption in the reactor, to heavier and also fissionable or fissile actinides, plutonium-239 and still heavier, which are partly but not completely fissioned while the pellet stays in the reactor. So chemical reprocessing of the fuel could be done to extract the fissile materials, mainly plutonium, and then one could make new fuel pellets of plutonium oxide or better, mixed oxide, and burn those up in turn, though not necessarily in exactly the same type of reactors operated in exactly the same way. There are some significant differences between plutonium and uranium as fuel, as it turns out.
Finally, fast breeder reactors could be built, so that pretty close to all of the initial uranium could be ultimately burned up, yielding a tenfold or more increase in the total energy released from such an initial fuel pellet.
This is why the term high level nuclear waste, often applied to spent fuel assemblies, is actually highly misleading. It is not waste in the sense that the millions of tons of coal ash that coal plants produce on a daily basis is waste. It still contains plenty of usable energy.
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