The Callaway Energy Center owned by Ameren Missouri will shut down the nuclear reactor at the facility next week to start the plant's normal refueling process.
Some contract workers at the plant were injured Tuesday during what Ameren Missouri is describing as an "electrical flash" in a switchyard located about 300 yards from the nuclear reactor.
The shutdown of the plant was scheduled before Tuesday's accident. The injured workers were employed by another firm hired by Ameren working at the plant prior to the normal shutdown for refueling and maintenance.
Ameren Missouri announced Thursday a fourth person was injured in the incident while attempting to assist the three others impacted by the flash.
All of the injured workers are supplemental workers employed by Chicago Bridge & Iron.
Ameren Missouri officials report they are still investigating the incident.
"Out of respect for the privacy of those individuals and their families," the statement read, "Ameren Missouri isn't providing details of their status and recovery."
Ameren Missouri officials again stressed the electrical flash did not affect the nuclear reactor or its normal operation in any way.
The nuclear plant is scheduled to be shut down next week as part of the plant's normal refueling and maintenance schedule.
The plant has operated continuously since the last refueling in November of 2011.
Refueling and maintenance outages are scheduled every 18 months at the nuclear plant. They are scheduled to last for several weeks.
The Callaway Energy Center's nuclear reactor produces about 20 percent of the electricity supplied by Ameren Missouri's 1.2 million customers.
While out of service, the electricity generated by the nuclear plant will be replaced by other Ameren Missouri energy centers fueled by coal and natural gas.