A nuclear reactor is a device used to initiate and control a sustained nuclear chain reaction. This reaction produces heat, which, in power plants, would be used to convert water into steam. The steam would then be used to turn a turbine connected to a generator to produce electricity. Emmett Brown used a nuclear reactor in his first version of the DeLorean time machine to generate the momentary 1.21 gigawatts of power flowing through the flux capacitor to enable temporal displacement.
In a nuclear reactor, fuel cells undergoing nuclear fission are cooled with a nuclear coolant to extract their heat. This coolant is usually water, but gas, liquid metal, or molten salt are common alternatives. As the time travel process generates a lot of heat in the reactor, it was equipped with rear louvers, which would rapidly expel a cloud of gas, presumably hot steam carrying heat away from the car, as the time machine underwent a venting process shortly after a trip through time was completed.
Marcus Irving used an unlicensed nuclear reactor as part of his Yugo time machine. When he dismantled the Yugo, Doc hoped that he would return the reactor, as not everybody could be trusted with one.
By 2015, the Mr. Fusion home energy reactor could provide nuclear power for individual homes in a compact unit which housed a tiny nuclear fusion reactor. Doc adapted one of these devices to replace the nuclear fission reactor in the DeLorean time machine.
In 2045, a nuclear holocaust is caused by Griff Tannen. He uploaded a virus to the popular social network created by GriffTech, ThingMeme. It was supposed to flash the word butthead on every object on Earth, but instead, the virus causes a short circuit in the Mr. Fusion network, causing a nuclear explosion in all one hundred million of the tiny nuclear reactors present in every unit. Doc traveled back in time to prevent the invention of the Mr. Fusion, which prevented the 2045 nuclear holocaust from occurring.
