From just after midnight to 3:00am on September 27th, 1944, Hanford's B-Reactor began its work as a self-perpetualizing fission reaction. Scientists who had been working towards this moment, were excited about the flawless execution. They had hoped previously that nothing would go awry because once the pile began producing plutonium, there was no safe way to correct errors since all machinery would have massive amounts of radiation.

For three hours the reactor ran perfectly; after 3:00am, however, the pile began losing power. By the next evening, the pile had lost all power. The next day it began again at the same power and slowly decreased yet again. Finally the scientists realized that the by-product xenon, was absorbing the neutrons needed to keep the reactor running. When the reactor reached such a low power that it ceased working, the xenon had the chance to decay, which then allowed more neutrons to be available for the chain reaction, kick starting the reactor to work again in a negative feedback loop. Luckily, the contractor DuPont had anticipated possible error, and had installed more tubing than was necessary to run the reactor. By using these extra tubes for a higher power level, the reactor's process was able to overpower its by-product, xenon, and work indefinitely without fail.

References

How the Reactor Worked