Revisiting Nuclear Power : Part 3 : Can it be weaponised ?

Following on from my previous articles on how nuclear power works, and why we need to rethink the dangers posed by it, it’s time to talk about the other safety-related concerns that are often raised in the debate about the viability of nuclear power. Can a nuclear power station explode like a nuclear bomb ? What happens if a nuclear power station finds itself in the theatre of military conflict, as is currently happening in Ukraine ?

To deal with the first part, we need to talk a little about how a nuclear bomb works. Of course, the low level details of how to build and detonate a nuclear weapon remain closely-guarded secrets held only by a relatively small number of the larger countries around the world. Building a nuclear weapon is both complex and difficult to hide, to the extent that it can only practically be done by a state. But the broad strokes of how nuclear weapons work are generally understood.

There are obviously fundamental differences between a bomb and a power station. In an explosion the power is released immediately. If you set a match (figuratively speaking) to a tanker full of natural gas, all of the gas will burn pretty much immediately and release a huge amount of energy – basically it will be an incendiary bomb. To avoid this, gas power stations are designed to release small amounts of the gas at a time into the combustion chamber within the station’s boiler. Accidents of this kind can and do happen however – they don’t always make the news, but there is history of fuel storage depots producing devastating explosions.

So, what is the nuclear equivalent of setting a match to a tanker of gas ? Happily, there isn’t one.

Recall that nuclear fuel consists of uranium. There are two naturally occurring forms, or isotopes of uranium – uranium-235 (which is rare, but fissionable in the right conditions) and uranium-238 (abundant, but non fissionable). Refined uranium is about 0.7% uranium-235 and 99.3% uranium-238. With these proportions, the uranium-238 will absorb most of the neutrons released when a uranium-235 nucleus fissions. In the presence of other substances that absorb neutrons – such as water – enough neutrons can be lost that the chain reaction will cease.

In a nuclear reactor, this problem can be solved in two ways. The first is to ensure that there is no water in the reactor, which requires moderating and cooling it with something else. The early British reactors were moderated by graphite and cooled with carbon dioxide gas. The Canadian CANDU design had a really interesting approach, which was to cool the reactor with heavy water. This is a form of water where the hydrogen nucleus already has an existing neutron, which makes it much less likely to absorb neutrons released by the reaction.

The second way to solve the problem is to process the uranium to increase the proportion of uranium-235 and reduce the proportion of uranium-238 – this process is called enrichment. With slightly enriched fuel in place, fewer neutrons are lost during the reaction, allowing the chain reaction to continue. This means that you can use water as a moderator – even though it absorbs some of the neutrons, it doesn’t absorb enough of them to stop the reaction.

The earlier designs sought to avoid enrichment as it was expensive. However, as time moved on, enriched fuel became cheaper and reactors capable of using unenriched fuel became relatively expensive to build. The world has now standardized on reactor designs cooled and moderated by regular water.

The overall story here is that nuclear reactors are intentionally set up to burn the fuel slowly. A fuel rod in a modern pressurized water reactor, enriched to around 3-4%, provides energy over a five year period. The need for moderation in the presence of uranium-238 means that the reaction is fundamentally slow – the neutrons have to bounce off the moderator and move more slowly in order to be captured by the uranium-235 and fission. In a modern nuclear reactor, if the energy released in the reaction is too high, the water turns to steam and the reaction stops.

In a bomb, you want the reaction to be fast – within tiny fractions of a second. For this to happen, you need to maximise the number of fissions that take place before the force of the explosion causes the reaction to cease due to the disintegration of the material. That rules out the use of the moderator, which fundamentally serves to slow down the physical processes during fission.

The early nuclear bomb designers identified a few solutions to this. Since a small proportion of uranium-235 fissions happen with fast neutrons, a simple but expensive approach is to enrich the uranium up to 90% uranium-235. Enough uranium-235 nuclei will fission to give you a pretty big explosion, much to the peril of the wretched citizens of Hiroshima, where this type of bomb was deployed. However, the expense of the enrichment process coupled with the low yield (the vast bulk of the uranium-235 is simply wasted) meant that this approach was superseded.

The other way is to use a different bomb material which fissions easily without a moderator and avoids the need for a lot of enrichment. That material is the man-made element plutonium, which is created artificially within nuclear reactors. When a uranium-238 nucleus absorbs a neutron, becoming uranium-239, it will decay within a few days in a series of steps leading to a plutonium-239 nucleus. All nuclear reactors produce, and burn, plutonium during their normal operation. Bomb-making reactors need to be designed to allow the nuclear fuel to be easily removed for processing to get at the plutonium. If the fuel is left in the reactor, the plutonium can itself absorb a further neutron to become plutonium-240, which is unstable. For this reason, early British and Soviet nuclear power reactors, such as the Magnox and RBMK, were designed to be refuellable while operating, which allowed the fuel to be easily extracted and processed to remove the plutonium without disrupting the power station’s operation. This is why reactors capable of online refuelling are considered to be a nuclear proliferation risk.

 

Plutonium sample (Wikipedia)

The weapons I have described so far were state of the art as of the late 1940s. Soon after, hydrogen bombs came along which combined fission and fusion processes to build much higher yielding devices. Where the Hiroshima bomb yielded 15 kilotons, and the Nagasaki bomb (which was plutonium-based) 21 kilotons, the Ivy Mike hydrogen bomb a few years later yielded 10,400 kilotons. In 1961, the Soviet Union demonstrated the world’s largest atomic explosion which yielded an estimated 58,000 kilotons – nearly 4000 times more powerful than the Hiroshima bomb.

From this, it can be clearly seen that it is physically impossible for a nuclear reactor to explode like a nuclear bomb. A nuclear weapon is a carefully tuned device consisting of very specific materials produced for weapons purposes. Despite what is sometimes seen in science fiction movies, there is no way that a device based on nuclear fission or fusion can be converted either accidentally or intentionally into a bomb.

The prime safety consideration with any nuclear power station is preventing a meltdown and the release of radioactive materials into the environment. This is what happened at Chernobyl and to a lesser extent at Fukushima. In past articles, I covered why the Chernobyl reactor melted down and how this kind of failure is more or less impossible at the most common power stations due to the extra safety features and containment.

The ongoing conflict in Ukraine has given rise to questions around would could happen in the event that a belligerent state would intentionally damage a nuclear reactor. Claims that a nuclear explosion could be triggered are, as explained above, in the realms of wild exaggeration; however, an actor in a conflict could move to intentionally trigger the release of radioactive material.  The containment structures around modern nuclear plants are strong enough to withstand plane crashes and even heavy weapons assault – but only up to a point; a sustained assault could breach containment. A nuclear meltdown could occur if the backup power to a station was intentionally disrupted, triggering a core breach. This would of course be a serious incident but the damage would be limited to the reactor vessel and the surrounding buildings. Ironically, both the Russians and the Ukrainians have reasons to make Western powers afraid of a nuclear scorched earth policy. However, for the Russians, destroying one of these reactors would create little military advantage for them while running the risk of hardening Western opinion further as well as endangering their own troops.

As the renewed debate about the role of nuclear power continues to gather traction, I think it’s important that it is understood that a nuclear power station cannot be converted into a bomb. The focus of nuclear safety considerations lies in protecting the plant from a loss of coolant scenario and properly securing the waste materials. Beyond this, Western foreign policy and diplomacy must continue to focus on identifying and eliminating nuclear weapons proliferation to ensure that no nuclear bomb is ever used in a military conflict again.

 

 

 

 

 

 

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