At the most fundamental level, no difference. They both employ chain-reacting nuclear fission to release energy.
In reactors, though, the chain reaction is controlled. In bombs, it is uncontrolled. Reactors are controlled bombs, you could say. And bombs are uncontrolled reactors. Of course there’s a little more to it than that.
With reactors you want to get only a regulated amount of nuclear fission. In bombs, you want to get as much fission as possible as quickly as you can.
With reactors, if you’ve acquired enough fissile fuel and located the pieces of it at the right distance from each other, a chain reaction of fission will begin in the fuel. The chain reaction will be started by the neutrons that are always being released by the atoms of the fissile fuel—uranium, plutonium, or thorium, all very heavy elements, and all can serve as the fuel. These neutrons shoot off and “fish” other atoms of the fissile fuel. This fission releases more very fast-flying neutrons, somewhat more than two new neutrons per fission, on average. Because the number of neutrons being released is more than one, the number of fissions rises exponentially--that is, not at a steady rate but at a quickly increasing rate. If not controlled.
When the chain reaction begins, we say the reactor has “gone critical.”
In a reactor, the chain reaction can be controlled by drawing out and pushing in rods that have been threaded among the fuel elements, rods that have been clad in elements, like cadmium, that absorb neutrons. That’s how it was done in the very first reactor ever to go critical, Pile-1. Pile-1 had been constructed in 1942 under the stands at a football field at the University of Chicago by the great physicist Enrico Fermi. Pile-1 went critical on December 2, a year after Pearl Harbor, in the first ever human-caused nuclear chain reaction on earth.
You can also control a chain reaction by removing or separating fuel elements if that’s an option. You wouldn’t have been able to do that with the Pile-1. It’s a good thing for the University of Chicago that the control rods worked to control the reaction. If the chain reaction hadn’t been controlled, the heat generated by the fission would have built up, quickly, until the graphite housing for the fissile fuel burned away and the fuel began to “melt down.” It wouldn’t have exploded but soon, under the stands at the football field, there would have been a pile of very highly radioactive material that would have been very dangerous for a very long time. Likely no more University of Chicago. Maybe more no more in the rest of Chicago.
Remember, producing a controlled chain reaction had never been done before.
After a while, heat is generated in the fuel elements not just by nuclear fission but by the decay of the radioactive fission products that are created in the elements by the fission. To control what is happening in a reactor, you need not just a way of controlling the nuclear fission but also a way of cooling off the fuel to keep it from being overheated by this radioactive decay.
For bombs, you will want to enrich the uranium fissile fuel far more than is necessary for fuel elements in a reactor. Enriching it means increasing the amount of U-235 in it. You do that so you won’t need a boatload of fuel for the bomb and it can be delivered in something other than a boat. Like a B-29. You then take this highly enriched fuel in an amount that is not yet enough to go critical—not yet a “critical mass”--and either slap pieces of it together or compress it quickly. If you can do that fast enough, the fissile fuel will become not just critical, but supercritical. A supercritical mass of fissile fuel will produce so many generations of fission in such a small amount of time— like two millionths of a second--that you’ll get a very big release of energy in a very small time, which is what an explosion is.
At the center of a nuclear bomb, an unimaginable heat is generated--temperatures higher than in the center of the sun--and large amounts of radioactive fission products are created. After two millionths of a second the energy being produced by the chain reaction in the supercritical mass of fuel will push the fuel apart to the point that it isn’t critical anymore. The chain reaction stops. The energy being released by the fission products doesn’t stop.
With a bomb, you might not be worried about the radioactive fission products. Unless you have been attacked with one, of course. Or when bombs are being tested in your own country. With reactors, those long-lasting radioactive fission products are always a problem. What are you going to do with them?
Once you’ve acquired enough highly enriched fissile fuel, nuclear bombs are, not exactly cheap to make but less expensive than many weapons. Reactors are never cheap, not because of the nuclear machinery at their core, but because of all that has to be done to prevent a release of radioactive fission products. A release could happen in several ways--in a leak or a steam explosion or a meltdown.
For reactors, your control systems must be strong and redundant. You have to build cooling systems that you have good reason to think won’t fail in a big storm or for any reason. And you must build “containment structures” around the nuclear core to contain radioactive fission products that might escape from the core. All those things are expensive.
We now know from experience that no matter how strong you make them, the “containment structures” won’t always contain what you want contained. Explosions can breach them. A melted down core—called a “corium”—still very hot because of the decay of the radioactive fission products, can liquefy concrete until it flows like lava.
By now, accidents and meltdowns at nuclear reactors have happened in a number of countries. Not a huge number so far, like thirty or forty. The big one was in the Soviet Union at Chernobyl in 1988. The contamination there—with cesium-137, a radioisotope that has a half-life of about thirty years—covered an area of 10,800 square miles. A Zone of Exclusion of about a thousand square miles is still in place. Nobody can live there.
But let’s say you have a nuclear reactor and all goes well and you have operated it safely for six years or so and now it’s time to replace the fuel assemblies. Those assemblies are now brimming with radioactive fission products that, even though the fission has stopped, are releasing a lot of heat and radioactivity. What are you supposed to do with those “spent” fuel elements?
To cool the assemblies and keep them from melting down, they are often placed in 40-foot deep concrete-lined pools of water at the site of the nuclear reactor. As the shorter-lived radioactive products decay, the assemblies do cool down but they remain highly radioactive from fission products that take longer to decay. Like cesium-137 and strontium-90, both of which have half-lives of about thirty years. The plutonium in the spent elements has a half-life of 24,000 years. It will be dangerous for ten times longer than that. Some of the fission products in spent fuel elements have even longer half-lives.
When they have cooled down enough, the assemblies may be placed in big “dry casks” and stored near the reactor.
The assemblies are “high-level radioactive waste.” Ten years after removal, they will be producing a surface dose rate of 10,000 rems an hour, according to our Nuclear Regulatory Commission. A fatal dose of radiation is 800 rems. You can do the arithmetic. If you were standing unprotected next to one of these assemblies, you would get a fatal dose of radiation in under five minutes.
How much high-level radioactive waste is there in the world now? In 2010, 250,000 tons of high-level waste was being stored at reactor sites. About 12,000 metric tons of high-level waste is added the total every year, by one estimate.
The production of energy, even by humans from food, produces waste. High-level radioactive waste poses a special challenge.
What are we supposed to do finally with this waste? Where might it be permanently and safely stored? We don’t have a good answer to that question yet. Not even close. Some think it would be best to leave it where it is, in the pools by the reactors, and in dry casks, out where we can see it. But we might end up burying it down deep in the earth somewhere, maybe in the drill holes from old oil wells. We would hope it stays there. We wouldn’t want it to get into the water or something.
Here’s a thought experiment: What kind of warning sign could you put at a waste burial site that you could count on being readable 24,000 years from now?
Here’s a recent report from the Associated Press on the meltdown at the Fukishima Power Plant ten years ago and the plant’s condition today. With pictures.
Coming: What was the hardest part about making the first bomb? How was the uranium enriched? How is uranium enriched today? What is the “other” fissile fuel and how is it produced? How much fissile fuel is there now in the world?
Next: A listing of items now in the Archive and coming attractions
Hey, Todd. I responded to your email on this one though didn't offer the treatise the questions deserves. One big thing here is that while solar does go off at night, it doesn't produce waste.
How does nuclear energy compare with solar energy: cost, danger, efficiency - wise?