Regional Basic Professional Training Course in Korea

❙ 493 ❙
8-A. Deterministic Accident Analysis (Non‐LOCA)
Among the events listed above, the first six are AOOs and the last is an accident. A CEA
ejection accident may be initiated by the loss of integrity of the control rod (CR) drive
housing with rapid expulsion of a control assembly from the core due to the differential
pressure between primary coolant and the containment. This event leads to a rapid
reactivity insertion and power excursion, thus called as reactivity insertion accident
(RIA). It is similar to a small break LOCA in break size, but indicates transient
characteristics completely different from the LOCA. The major concern of the accident is
power excursion induced by a rapid reactivity insertion, and as aforementioned, it is not
treated as the LOCA.
The safety aspects, threatening the barriers against radioactive material release, are as
follows:
(a) Rapid reactor power increase resulting in a fuel temperature rise and in a reduction
of DNBR; hence a reduction of heat removal and potential for consequential fuel
rod damage and radioactivity release.
(b) Primary coolant pressure increase as a consequence of power excursion as well as
turbine trip. This depends on the actual break size, on whether the pressure
increase will in fact occur and also on whether there will be a need for actuation of
the ECCS due to a loss of primary coolant.
(c) Containment pressure and differential pressures increase, leading to pressure
loading of the containment walls; owing to the smaller break size, this aspect is
usually much less important.
(d) Radiological consequences due to a loss of primary coolant, potentially also due
to a loss of cladding integrity or fuel disintegration.
Figure 8‐5 shows one example predicted fuel and clad temperatures trends versus time for
a control element assembly ejection.