At some point before the outbreak, the power plant was built to withstand terrorists and had enough supplies to keep the staff alive for months. It had two pressurized water reactors.
After things started getting bad, Grace, an operations manager at the plant, called in all of the shifts to keep the plant running. When things got worse, she called in all of their families to join them. As the world came to an end and the cities fell, people no longer required the power the plant provided, but Grace chose not to evacuate. Instead, Grace turned the plant into a safe-zone for survivors, taking advantage of the supplies and the plant's sturdy construction which would keep out the dead.
However, the survivors lacked the infrastructure to keep the plant going the right way and the cooling system in one of the reactors failed. The reactor melted down, killing everyone inside the plant and irradiating the surrounding area. Grace returned to the plant in an attempt to find survivors and spent days looking but couldn't find anyone. In the process, Grace was exposed to a lethal dose of radiation despite taking precautions.
A year later, Grace had established roadblocks and fences warning of high radiation levels to keep people out of the area around the plant that would be lethal to the living. The survivors who had perished during the meltdown became radioactive walkers, dangerous to anyone living who encountered them. Grace worked to hunt down the radioactive walkers and bury them in some caverns near the plant where their remains couldn't contaminate anyone living.
After capturing and forming an alliance with Al, Isabelle explains that while on a supply run with her partner Beckett, they saw the plant and the aftermath of the meltdown. Seeing that and the radioactive walkers drove Beckett insane with the fear that he would end up like them, forcing Isabelle to kill him.