The Wikipedia page is a lot more detailed than I could hope to be, presumably you just turn off the power to the particle accelerator. Existing safety measures

The Wikipedia page is a lot more detailed than I could hope to be, presumably you just turn off the power to the particle accelerator. Existing safety measures rely on some degree of integrity in the reactor at the time they are triggered. If it gets too damaged before the control rods are inserted it may not be possible to insert them. https://en.m.wikipedia.org/wiki/Subcritical_reactor#:~:text=A%20subcritical%20reactor%20is%20a,neutrons%20from%20an%20outside%20source https://en.m.wikipedia.org/wiki/Subcritical_reactor#:~:text=....