12.68. Pressurized-Water Reactors. A small break or crack in one of the primary coolant loops in a PWR would be most serious if it occurred in one of the reactor vessel inlet lines. The reactor would be tripped, but some water would escape and be largely flashed into steam in the con­tainment structure. In order to prevent damage to the fuel cladding, it would be necessary to keep the core covered with water while normal shutdown cooling proceeds. If the break were quite small, the water level could be maintained by operating the pumps in the intact loops at full capacity. Makeup water to compensate for that lost from the break would be provided by the CVCS. However, it should be recognized that should there be a problem in introducing makeup water, it would be necessary to lower the pressure to make use of the passive high-pressure ECCS sub­system.

12.69. For larger breaks in a PWR inlet line, the reactor vessel water

level might drop temporarily to an extent that would permit some fuel cladding damage by overheating. As the system pressure is lowered by the escaping water, the high-pressure injection subsystem of the ECCS would operate and keep the core covered. The reactor shutdown cooling system would then be used to decrease the temperature and pressure. Again, the pressure status is important. The high-pressure ECCS subsystem has a limited capacity. If the leak rate is too small to reduce the pressure to the range of the low-pressure ECCS subsystem and the high-pressure capacity is depleted, a cooling problem could occur. Some of these considerations became evident during the course of the Three Mile Island small-leak accident (§12.179).

12.70. A break in a PWR feedwater line, which is within the contain­ment, or in a steam line inside or outside the containment could result in the loss of all the water in that steam generator. The reactor would be tripped and the other steam generators would be able to provide sufficient cooling to permit safe shutdown. Since the radioactivity level in the steam from a PWR is very low, a break in the steam line outside the containment would not constitute a serious hazard.

12.71. Boiling-Water Reactors. If a relatively small break should occur in a feedwater or recirculation line, one of the auxiliary cooling systems, e. g., the reactor core isolation cooling system (§12.30), could maintain the water level in the reactor vessel. The reactor would be tripped and then be depressurized and cooled down in the normal manner. For larger breaks or those requiring the main steam-line isolation valves to be closed, water would be supplied from the suppression pool and from the condensate storage tanks. A break in a BWR main steam line would result in reactor trip and closure of the isolation valves to prevent escape of the radioactive steam from the containment. The core would be kept covered with water in the manner just described, and the system temperature and pressure would be decreased in the normal shutdown manner.