One of the distinguishing features of the RBMK-type reactor is its online refueling capability. This complicated operation is accomplished by an especially designed fuel loading and unloading machine. Besides changing fuel bundles without shutting down the reactor, the refueling machine can also be used to inspect a fuel channel by a special fuel-assembly- shaped calibration instrument, seal off a fuel channel with a standard or an emergency sealing plug, and to correct certain emergency conditions.
On line reloading operations at nominal power must ensure that the coolant flow to the fuel assembly being changed is not interrupted. The refueling machine is capable of changing up to five fuel assemblies per day when the reactor is in operation, and up to 20 when the reactor is shut down.
The principal components of the refueling machine are: the refueling machine transport mechanism (1), a container (3) which serves as a biological shield, two replaceable caskets (4) (one mounted in the machine the other kept in the repair area), a metal frame (9). the positioning mechanisms (8), and control equipment.
The refueling machine is moved through the refueling reactor hall by means of the transport mechanism. A bridge-type crane (2), consisting of 21 m long beams, moves on transverse tracks located in the upper section of the refueling hall with 39.6 m of travel distance. On the bridge, 11 m above the hall floor, a carriage (1)transports the refueling machine equipment along the other axis. The travel distance of the carriage is 125 m. The bridge and the carriage can be moved at two speeds, 9.75 and 1.2 m/min. The slow speed is used for final, accurate positioning. During this operating phase the bridge and the carriage move in 1 mm increments.
The metal frame (9) above the carriage houses the equipment supplying the refueling machine with process water, feedwater, pressurized air, inspection and measurement instrument and control-related equipment.
The container (3) is a steel cylinder, assembled from six sections. Its inside diameter is 0.77 m and the wail thickness s 0.5 m. The lower part of the container incorporates removable biological shield (7) used to screen the gap which appears between the bottom of the container and the hall floor during refueling operations. The operator's booth and access platforms are located on the outside of the container, the inside provides space for the movable fuel casket (4).
The standpipe is used in performing the connection to the fuel channel. When the refueling machine is positioned over the required fuel channel, the standpipe is lowered so that it encloses the top part of the fuel channel. The joint is sealed by means of an inflatable rubber gasket. A special key located inside the standpipe is used for activating the fuel channel seal plug when sealing or unsealing the channel.
Two methods are used for positioning the refueling machine so that it coincides with the fuel channel coordinates:
In case steam escapes from the fuel channel, and it is impossible to use either the optical or the television system to position the machine accurately over the fuel channel, a contact positioning system is utilized.