Nuclear Thought Experiment

by katyhuff

I present the following thought experiment.

What would happen at the nuclear reactors of the world if all humans simultaneously disappeared? 

Generally speaking, most nuclear reactors would kindof gently keep running for a while and then will either scram and shut down quickly or gently cease to be critical and shut down a little more slowly. The question for me, is what happens first:

I. The core will eventually run out of reactivity:

A typical power reactor doesn’t require a human to physically do much at all, and (if the grid is behaving appropriately and there are no earthquakes or volcanos shaking the pool) it just plods along until it gradually consumes all of the reactivity available in its current configuration. That said, there is a reactor operator whom is a human responsible for adding reactivity to the reactor. This person is therefore required to physically turn a knob periodically while the reactor slowly consumes its fuel over the course of its 18 month cycle. Without the reactor operator there to bring the control rods out of the pool in increments over the course of reactor operation, the reactor will cease to be critical within a few days or a month, depending on the reactor configuration and the age of the fuel. In this scenario, the reactor is cooled at the same rate as ever, and slowly loses reactivity over the course of weeks or months until it is no longer critical. It will cease to sustain its reaction at that point and with the help of its typical cooling system, will be cool within a few weeks.

II. Chaotic grid behavior could cause a scram:

A scram is when the emergency shutdown sequence in a reactor is set in motion. By design, power reactors faced with any signs of danger ( increased heat, radiation, seismic activity, loss of power, etc. ) shut down effortlessly, automatically ramping up their alternate power supply to support active cooling systems that are automatically brought online. Once this happens, the reactor should be cool within a week or two.

In the no humans scenario, power loss is the likely culprit for tripping a scram. The likelihood of this depends on the other power sources on the grid, largely. Solar, wind, and gas are fairly automated, as I understand it, but what about coal? And what about the grid itself?  In the case of coal, its clear to me that without daily trainloads of coal (driven by humans) refueling them, these coal plants will cease to function normally very quickly. In the case of the grid itself, I have to admit I don’t know enough about how our domestic grid works to know exactly how automated it is, but presumably there’s some human in the chain necessary for power to keep going to the right places. Without that human, we’ll certainly eventually scram a lot of reactors.

III. What else could happen? Well, I can think of a few things, but let’s not, since they’ll all basically lead to the resolutions outlined above.