12.78. Loss-of-coolant accidents fall into several categories depending on the size of the postulated break in the primary coolant circuit. Smaller breaks, which could lead to minor fuel cladding damage at worst, were considered earlier. For the design basis accident, however, a “guillotine” (or double-ended) break is postulated in one of the cold legs of a PWR or in one of the recirculation pump intake lines of a BWR (§12.73). As a result of such a break, the primary system pressure would drop and almost all the reactor water would be expelled into the containment. The drop in pressure resulting from such a loss-of-coolant accident (LOCA) would actuate the protection system and the reactor would be tripped. The fission chain reaction in the core would thus be terminated, as it would in any case because of the loss of coolant (moderator). Nevertheless, heat would continue to be released at a high rate from the sources mentioned in §12.28. The various ECCS subsystems must then provide sufficient cooling in time to minimize overheating and fuel cladding damage. The steam flow limiters and isolation valves, inside and outside the containment vessel, would close automatically to prevent the spread of possibly contaminated steam.

12.79. The thermal-hydraulic (and other) phenomena are very complex, and studies are being made to provide a better understanding for computer modeling (§12.126 et seq.). These phenomena are somewhat different for PWRs and BWRs and will be outlined separately.