|Fig. 1: Energy of various nuclear weapons in the US nuclear arsenal. (See also Table 1. Source: N. Martelaro after Kristensen and Norris. )|
Though nuclear war has not been overbearing threat since the end of the Cold War, there are still fairly large stockpiles of nuclear weapons across the world. As of 2015, the US still has approximately 2000 deployed warheads, 2000 reserve warheads, and 2000 waiting to be dismantled.  These stockpiles present a clear danger to humanity. However, it is possible to convert these weapons of mass destruction into usable energy for civilian usage. This idea was actually implemented during the 1990's and early 2000's in a program called "Megatons to Megawatts."  It presents an alternative use of nuclear fuels that has the potential to curb weapons proliferation.  The premise of this program was that the US should buy decommissioned Russian nuclear warheads as a fuel source for civilian nuclear reactors. This is done through a process of down-blending the highly enriched uranium (90% enrichment) with natural uranium to create low enrichment uranium (5% enrichment) usable in nuclear reactors.  The plan worked well for many years until the agreement between Russian and the US deteriorated. Overall, it was reported that about 20,000 bombs (~500 tons of HEU) were decommissioned and turned into nuclear fuel.  The previous references give a nice overview of the Megatons to Megawatts program and the process for down-blending weapons grade HEU to civilian power ready LEU. [2,3,5] The purpose of this article is to begin to answer the questions:
How much energy can we get from nuclear weapons?
What can one do with this amount energy?
The energy contained in bombs is listed in tonnes of TNT. This is an energy measurment and is equal to 4.2 × 109 J. For our calculations, we will assume that all the energy in a bomb can be converted to into usable heat energy. We will then assume that about 2/3 of this heat energy is turned into electricity in the plant's steam turbines. With this energy, we can then calculate how long it might power a city. In this case, we use Los Angeles, CA, USA as an example city. The 5-county Los Angeles metropolitan area uses on average 1.4 × 1010 W.
As short example, we convert the energy of the Fat Man bomb dropped on Nagasaki. Fat Man was rated at 20 kT (kilotonne TNT). Converting this energy to joules, we obtain
The corresponding amount of electric energy is 2/3 of this heat energy, or 5.5 × 1013 J. This amount of energy could power the Los Angeles metropolitan area for
While this may not seem that long, it is interesting to consider that a city uses the same amount of energy in two hours as it would take to destroy it. We can also consider the cost of this energy as another benchmark of the value in the bomb. Nuclear energy costs (estimated for 2022 and without government subsidies) about 10 cents per kilowatt hour to produce, giving 
With these energy conversions we can estimate how much energy and how useful the energy may be for the current US nuclear deployment. There are currently about 2000 nuclear weapons deployed in the US arsenal, spread across various types weapons. Table 1 shows the total amount of energy that each class of nuclear weapon would yield and the total amount of time this energy could power Los Angeles. 
|Table 1: Energy estimate of the current US nuclear arsenal. |
We see that the currently deployed US nuclear aresenal has a combined total of roughly 5.27 × 1018 J of energy. Converted at 66% efficiency to electricity, this gives 3.84 ×1018 J - enough to power Los Angeles for roughly 7.8 years. It should be noted that this calculation only includes the currently deployed arsenal. There are still another ~2000 warheads kept as spares and another ~2000 awaiting disarmament.  Thus, this estimate is most likely a lower bound on the total amount of energy that can be reclaimed from the US nuclear arsenal.
© Nikolas Martelaro. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 H. M. Kristensen and R. S. Norris, "US Nuclear Forces, 2015," Bull. At. Sci. 71, 107 (2015).
 C. DiOrio, "The Megatons to Megawatts Program," Physics 241, Stanford University, Winter 2016.
 B. Madres, "Market Justifiable Nuclear Energy," Physics 241, Stanford University, Winter 2011.
 T. Strobel, "Demilitarization of Nuclear Weapons," Physics 241, Stanford University, Winter 2016.
 G. Brumfiel, "Megatons To Megawatts: Russian Warheads Fuel U.S. Power Plants," NPR, 11 Dec 13.
 "Annual Energy Outlook 2016," U.S. Energy Information Administration, DOE/EIA-0383(2016), August 2016.