Difference in the fuel #
The fuel in a nuclear reactor is not at all concentrated or clean enough. The content of uranium is not high enough to be explosive. Reactor-grade uranium contains about 3-5% U-235, while the rest is mostly U-238. For a nuclear bomb, 90% pure Uranium-235 is required, otherwise it will not be able to undergo fission fast enough to create a nuclear explosion [1].
Difference in the chain reaction #
Nuclear reactors and nuclear bombs are powered by the same reactions where atoms are split in a chain reaction to create the energy. But the way the chain reaction is controlled is very different.
An atomic bomb is designed with an uncontrolled chain reaction, so that enormous amounts of energy can be released in a very short time. The reactions happen quintillion times faster and it’s all over in a split second.
A nuclear reactor uses a controlled chain reaction where the same amount of energy is released slowly over a longer period of time (years to decades). It is controlled with negative feedback, so if something goes wrong, the reaction stops.
In the same way as the difference between a petrol car and a napalm bomb. The energy in the car’s internal combustion engine is released over a longer period of time and it is built completely differently. While a bomb releases it all in seconds. That doesn’t mean our petrol cars on the roads are little ticking napalm bombs.
What does it mean the reactor is critical #
But often hear claims that if a nuclear reactor is critical, it can get out of control. But that leads to a misunderstanding of the concept.
Critical simply describes a reactor state where a neutron from each split atom causes another fission so that the chain reaction is kept going. In other words, when the reactor is critical, it runs in steady state as it should.
A reactor accident is not a nuclear bomb #
A nuclear reactor cannot explode like a nuclear bomb (Nuclear explosion), it is against the laws of physics [2]. It was thus not a nuclear explosion that blew the lid off reactor-4 at Chernobyl, but a steam explosion (Hydrogen). The same applies to the accident at Fukushima. Such can occur (and does occur) at all coal, oil and gas thermal power plants.
At Chernobyl, things went wrong further because the reactor was designed with significant errors and at the same time the metre-thick containment building had been spared. Despite this, the Chernobyl plant’s 2 other reactor units in the building continued to produce electricity until the turn of the millennium when they were shut down for political reasons.
