1 tonne = 1 GWye

The basic number: 1 tonne of fissionable metal = 1GWye

A basic claim made for all of the molten salt reactors discussed on this site is that they are able to produce 1GWye out of 1 (metric) tonne of fissionable metal (U235, U233 or Pu239). One GWye means one gigawattyear of electrical power.

The primary energy of course is thermal: the tonne of fissionable metal delivers about 2,6GWy of thermal energy. The amount of electricy produced depends on the type of turbine that is used for the conversion. The usual Rankine turbine has an efficiency of about 33% and your tonne will produce about 0,85GWye. The Brayton turbine proposed for the LFTR is expected to have an efficiency of 45-55% and your tonne will produce 1,1 to 1,4 GWye.

In all of 2015, Amsterdam used 0,6 GWye of electric power. That means that Amsterdam’s year supply of fuel for electricity could have been transported in a pickup-van.

If you have a hard time believing these numbers, you’re not the only one. However, that doesn’t mean they are inaccurate. Charles Barton, a respected blogger on the subject, had been asked by David MacKay to provide evidence for the above claim. Barton turned to Yoon Il Chang. I took the liberty of taking this long quote from Barton’s site, because there is no way to improve on the words of Barton and Yoon.

[Charles Barton:] I could not find an online discussion of the fuel efficiency of fast reactors, so I did what any sensible person would do, i asked one of the world’s leading experts on Fast Reactors, Yoon Il Chang, for help. Dr. Chang graciously responded to my request:

[Yoon Il Chang (my underling-GZ):] Dear Charles,
I am not sure if there is an on-line document, but it is a simple, straightforward calculation.
Fissioning of 1 gram produces 1 MWD energy. (This is derived from 1 gm equals Avogadro number 6×10**23 divided by 235 atoms, 1 atom fissioning produces 210 MeV energy. 1 MeV is equivalent to 1.6×10**-13 watt-sec. If you convert in proper units, you reach 1 gm = 1 MWD.)
1,000 MWe plant is equivalent to 2700 MWth if you assume 37% net thermal efficiency.
2700 MWth x 365 days/yr x 1 gm/MWD = 0.9855 tonnes ~ 1T.
Since the reactor does not operate 100%, the net fissioning will be somewhat less than 1 T/GWye.
LWRs have a lower thermal efficiency (~33%), so their consumption will be somewhat greater.
But as a rounded number, I tend to use 1 T equals 1 GWye, regardless of reactor types, actual capacity factors, etc.
In the LWR, the uranium resource utilization is far less than 1%. (About 85-90% discarded in enrichment tails, of the 10-15% loaded in the reactor only 3-5% is fissioned, and therefore >99% is discarded as waste.)
In fast reactors, all uranium, including depleted uranium and used uranium and actinides in spent fuel can be fissioned through continuous recycling. In theory, more than a factor of 100 improvement in uranium resource utilization. In practice, some will be lost as processing wastes and a factor of 60 or 70 is assigned taking this into consideration. The LWR figure is more like 0.6-0.8% (higher number with recycle). Therefore, a factor of 100 is more representative ratio even if very conservative loss factors are assumed.
I hope this helps.

[Charles Barton:] I hope that Dr. Chang’s Statement will provide a satisfactory response to Dr MacKay’s request for more information.

Here, please find the full discussion on the site of Charles Barton.

How much energy per tonne? – the calculate-it-yourself-way

In facebook discussions, I’ve seen many requests for references to online articles or pages that prove the claim above. I haven’t been able to locate any such online resource. The reason for this is that this is basically textbook knowledge: a matter of finding the right tables and running the numbers.

For those of you who like chemistry, the calculation that underlies these claims is really not that difficult. All you have to do is multiply the energy per ‘event’ (= one fission) with Avogadro’s number (the number of atoms in two grammes of H2) and then devide it by the atomic weight of the element that fissions (U or Pu – they roughly produce the same amount of energy per ‘event’: 200MeV

First, use this unit converter to change the MeV’s into Watt/seconds and find the value of 1,9e+13W/s.

If we fill in the values, we get the following:

1,9e+13W/s (=energy per event) *  6,022E+23 (= Avogadro’s number) / 233 (=atomic weight of U233) and find:
= 1,9e+13W/s (use unitconvertert again and find:)
= 0,0226 GWh = the energy of one gramme of U233.
= 22600kWh
One tonne of U233 then produces 1.000.000 * 0,0226 = 22600GWh = 2,58GWyt (meaning 2,58 Gigawattyear of thermal energy.

In short: one tonne equals 2,58GWyt. Hence our ‘rule of thumb’ that states that 1 tonne equals 1 GWye.

Due to efficiency differences, you’ll need a bit more in light water reactors, and a bit less in molten salt reactors.

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