Costs of nuclear fuel, burn-up, and fuel economics: A nice link.

One way to reduce the already low environmental impact of nuclear energy and to improve the already excellent economics of most nuclear plants, is to increase burn up, which is a measure of the amount of energy produced per unit of mass.

A unit frequently used in nuclear engineering is the megawatt-day/ton. Not long ago, typical burn-ups were below 40,000 MW-day/ton, but they have been steadily increasing and now are typically close to 50,000 MW-day/ton. This has important implications on the amount of spent fuel that must be processed, and the amount of uranium that must be mined or otherwise made available.

Here is a nice, albeit slightly technical link on the subject, breaking down the cost of nuclear fuel: http://www.nea.fr/html/ndd/reports/efc/efca09.pdf

Megawatts per day, per kilogram of elemental Uranium (in any form, e.g., Uranium Oxide) -- yes? no?

I'm pretty sure that's what it is, but I'm not quite certain.

--p!

Megawatt-days/Metric ton. Sometimes it is written as MWd/MTHM. Megawatt-day/Metric Ton of Heavy Metal. This means it is for the uranium content of the material used in the fuel, generally UO₂, which has a very high melting point, 2880^oC. Note that it doesn't make all that much difference, since UO₂ is (depending on the enrichment) about 88% uranium.

Most nuclear engineering texts use metric tons, but in some places you see it written as MWd/kgU as you have written it. The number then is different by a factor of 1000, of course.

Not all fuels have the same chemical composition. This is why one refers to the heavy metal content and not the actual mass of the fuel form.

For instance, one of the drawbacks associated with uranium oxide is that it does not have as high a thermal conductivity as one would like. For this reason other forms including metallic forms, carbides, and various alloys (usually of the high melting metal zirconium which has a low neutron capture cross section) have been used or explored. A fuel with a better thermal conductivity and a higher melting point is thoria, thorium oxide. Thorium oxide, which is itself a nuclear fuel that can produce breeding in thermal reactors is one of the highest melting substances known. In fact one of the non-nuclear uses of thoria has been to manufacture refractory ceramics that can tolerate the most extreme industrial and laboratory temperatures without melting. For the long term, should humanity survive global climate change, I favor the thorium fuel cycle, with the resulting U-233 diluted with depleted or natural uranium. This will allow for the simultaneous use of uranium and thorium resources.

The use of plutonium metal as a reactor fuel is highly problematic. Plutonium metal has the most solid phases of any element, i.e. the most allotropes. (An allotrope is an alternate form of an element, like diamond and graphite for carbon, or white, red and yellow phosphorus.) These phases have transition temperatures very near reactor operating conditions, and each of the phases has an alternate density. This can lead to expansion and contraction of the fuel, which is not a good thing. Therefore very few reactors have operated that have metallic plutonium as a fuel. Those that have, have not been particularly successful.

The phase properties of plutonium can be exploited to complicate weapons design. To my knowledge, all plutonium based nuclear weapons use the metal. If one places an isotope of plutonium in the commercial plutonium that generates considerable heat - plutonium-238 is ideal for this process as is plutonium-241 - the weapons designer will need to include some form of cooling in order to prevent phase changes that might cause the weapon to fail or fizzle. (These isotopes can also damage the chemical explosives shortening the usuable lifetime of the weapons.) In theory, the denaturing of plutonium can never prevent weapons diversion - nuclear weapons can never be disinvented - but, on the other hand, it can greatly increase the barriers to diversion and the associated costs. (Denaturing also decreases the possible yields of such weapons.) I believe that plutonium can and should be used in industry, and that the risk of diversion is acceptably small - it has in fact, never happened that commercial fuel has been so diverted - however I also believe that everything that can be done to further minimize this risk - especially denaturing - should also be done. For this reason I believe it is very important to use multiple plutonium recycles to complicate isotopic content, to develop pyroprocessing methods that can separate plutonium at high temperatures, as is the case when the fuel is newly removed from the reactor, and to transmute neptunium-237 to plutonium-238 as a regular part of the fuel cycle. The transmutation of neptunium has an added benefit inasmuch as neptunium is associated with the bulk of the long term radiotoxicity of spent fuel.