Refueling the reactor is one of the most critical evolutions that takes place during a nuclear plant outage. It is a complex process requiring a crane, referred to as a manipulator, to move the nuclear fuel assemblies individually underwater out of the reactor vessel core and to the reactor building upender. The upender turns the fuel assembly from vertical to horizontal (basically laying it down on its side). The fuel transfer system then takes the fuel assembly through a transfer tube into the spent fuel pool in an adjacent building.
In the spent fuel pool, the fuel assembly is again turned vertical with another upender so that it can be placed in the spent fuel rack. The crane in that building then moves the fuel assembly to a specified cell in the spent fuel pool where it remains to decay-off and cool down
The process must take place underwater because of the extremely high radiation levels produced by the fuel assemblies and associated irradiated components. While the water protects personnel from dangerous radiation exposure, it also makes access to the fuel transfer system and upender very difficult. If there is a problem with the system, refueling operations must be halted until repairs can be made. Refueling delays are very expensive for a nuclear plant, so it is vital that repairs be made as quickly as possible.
Recently, UESI was asked to assist in such a repair. To perform the repair, divers were required to enter the water in the vicinity of the reactor and spent fuel storage pool. To make the repair, it was necessary for the divers to remove a damaged component on the upender and reinstall it using a specialized underwater welding procedure. During the dives, divers were closely monitored by radiation protection personnel to ensure that their radiation exposure remained as low as possible. Radiation protection personnel used computer-based telemetry to track the diver’s dose real-time. Because water is an excellent shield, divers received less than 100 millirem over the course of the work. The work was completed without incident in one shift further reducing total accumulated project dose rate for all personnel involved. Using divers kept the total exposure as low as reasonably achievable and resulted in significant cost savings by reducing expensive downtime.