Boron has a high neutron cross-section, it therefore absorbs neutrons and slows down the fission process. In a PWR you add it into the water as a way to modera

Boron has a high neutron cross-section, it therefore absorbs neutrons and slows down the fission process. In a PWR you add it into the water as a way to moderate the reaction, but in a BWR (like the reactor 1 in Fukushima Daiichi) you only add it as a last resort, since it is incredibly hard to clean out of the system.