Regional Basic Professional Training Course in Korea

❙ 489 ❙
8-A. Deterministic Accident Analysis (Non‐LOCA)
Among the events listed above, the first six events are AOOs and the last, assuming the
double ended break of the feed water line, is an accident. The main characteristics of feed
line breaks are summarized:
Feed water system pipe breaks differ from steam line breaks due to the fact that the
water outflow leads to a rapid decrease of the affected SG secondary side water
level. Thus, the secondary side heat removal capability is reduced, while cooling,
due to energy outflow, is not so high. However, depending upon the size of the
break and the plant operating conditions at the time of the break, a RCS cooldown
prevails by the excess energy. In any cases, the effect on RCS cooldown is bounded
by steam line breaks and a RCS heatup is the major concern of this feed water
system pipe breaks.
For a small break of the feedline, normal plant control systems are capable of
maintaining nominal or near nominal operating conditions. An intermediate size
breaks are lower bounded by those sizes in which normal plant control systems are
unable to maintain nominal plant conditions and upper bounded by those sizes in
which plant protective functions do not occur within approximately few minutes
following the initiation of accident.
The system response to double ended feedline rupture is characterized by a rapid
decrease in SG water level in at least one SG, and its safety aspects, threatening the
barriers against radioactive material release, are as follows:
(a) RCS overheating, with expansion and pressurization of the RCS coolant due to
reduction or loss of secondary side heat removal.
(b) Threatening of core heat removal and fuel rod overheating in the longer term
phase.
(c) Release of radioactivity into the containment. The pressurizer relief or safety
valves may stay opened, and the RCS coolant is released into the containment in