13.60. Containment of the fuel bundles within individual pressure tubes localizes the consequences of possible fuel cladding failures and contributes to overall safety. Further, the combination of natural uranium fuel and on-line refueling results in a fuel burnup only about one fourth that of an LWR. Hence, the fission-product inventory is much lower.

13.61. In addition to the normal shutdown system, a backup system is provided for emergency shutdown. In earlier CANDU designs, the mod­erator could be dumped in an emergency. In the larger cores of recent design, injection of gadolinium nitrate solution into the moderator is preferred.

13.61. An emergency core-cooling system is available in the event of a loss-of-coolant accident. However, the separate moderator system provides a substantial heat sink should the coolant circuit be breached. Since there are many coolant inlet and outlet lines of relatively small diameter, com­pared with those in a LWR, the probability of a rapid depressurization accident is greatly reduced. A containment structure, consisting of a prestressed-concrete enclosure containing the nuclear steam supply system, combined with a water dousing arrangement, provides a pressure suppres­sion feature should there be a break in a coolant manifold.