13.54. Typical design specifications of an advanced CANDU type re­actor are given in Table 13.5, and the reactor core arrangement is shown in Fig. 13.12. The zircaloy pressure tubes containing the fuel rods are horizontal, thereby facilitating fuel handling (charge and discharge) while the reactor is operating at power. Each pressure tube contains 12 fuel bundles; in the older CANDU reactors there are 28 rods per bundle, but the number has been increased to 37 in the newer designs. The fuel rods, which are separated laterally by spacers, consist of natural uranium dioxide pellets clad with zircaloy. A number of vertical “booster” fuel rods con­taining slightly enriched uranium dioxide are not normally in the core; they are used only to override xenon poisoning if the reactor is restarted soon after an unexpected shutdown.

13.55. On-line refueling, i. e., without reactor shutdown, is conducted by means of two machines, one at each end of the core, operating in tandem. One machine inserts a fuel bundle in a pressure tube while the other machine removes the spent bundle displaced at the other end. Axial power flattening is achieved by limiting the number of bundles that are replaced at any time and by using bidirectional loading in adjacent chan­nels. Refueling strategy is determined on the basis of calculated channel fuel burnup and reactivity demands.