In the case of the P^^ and B^WR the refueling is off-load. It takes place ap­proximately once a year over a period of 4—6 weeks. Other maintenance work on the plant is scheduled to be done at the same time, which means that high load factors are still achievable with these reactors.

To carry out refueling in a P^^ or B^WR the system is partially drained to bring the liquid level to below the level of the flange that connects the main part of the vessel to the top part (referred to as the top head). All the control rods are fully inserted into the core and unlatched from their mechanisms (which pass through the head). The bolts attaching the top head to the vessel are then loos­ened, the cavity in which the reactor sits is flooded with water, and the head is removed. The upper structures in the reactor vessel are removed to expose the fuel, and handling operations are carried out under a significant depth of water in the reactor cavity (typically 5-10 m). This water is also circulated through a heat exchanger to provide liquid cooling for decay heat removal. Approxi­mately one-third of the total number of fuel elements are removed in any one operation, namely, about 5^0-0 elements out of the total inventory of 200.

As shown in Figure 7.4, in the case of the P^WR the fuel is passed into a transfer canal, in which it is transferred horizontally out of the reactor building and into a water-filled fuel storage pond.

The refueling route for the B^^ is similar to that illustrated in Figure 7.4, but with the additional complication that it is necessary to remove all of the devices above the core used to separate the steam from the steam-water mixture leav­ing the core (see Figure 4.27 for an illustration of the reactor structure). The B^^ fuel elements are somewhat smaller than those in the P^WR and therefore a correspondingly larger number of fuel movements must be made.

Once every 3 years it is common practice to remove all the fuel and the lower core structures and to carry out a thorough inspection of the pressure vessel from the inside surfaces. This provides a guarantee of the integrity of this vessel, which is essential to the safety of the system. The internal structures and fuel are then recharged into the vessel and the reactor restarted. Typically, this triennial inspection process might take up to 3 months.