Figure 7.2.5 Transients of the event, partial loss of reactor coolant flow (ABWR)

3. Accidents

1. Assumption of Events

Postulated events which have a small possibility of occurrence compared with that of "anticipated operational occurrences" but which have significant effects on the plant and on the public in the vicinity of the plant must be analyzed as "accidents." For BWRs, the following twelve events in four categories are assumed.

(l)Loss of the reactor coolant or significant change in core cooling conditions

1. Reactor coolant loss

2. Reactor coolant flow loss

3. Reactor coolant pump locked rotor (except for ABWRs)

2. Abnormal reactivity insertion or rapid change in reactor power

1. Control rod drop

2. Abnormal release of radioactive materials to the environment

1. Radioactive gaseous waste processing facility failure

2. Main steam line break

3. Fuel assembly drop

4. Reactor coolant loss

5. Control rod drop

2. Abnormal change in reactor containment internal pressure, atmosphere etc.

1. Reactor coolant loss

2. Flammable gases generation

3. Dynamic load generation

The criteria for the "accidents" are as shown in Table 7.1.1. Accidents are events that lead to core damage or a release of radioactive materials outside of the facility, differing from the "anticipated operational occurrences". Therefore, the criteria are applied for phenomena assuming such conditions (for example, core cooling capability or effective dose of the public in the vicinity of the plant)

As examples of safety evaluation on "accidents", the "loss of reactor coolant" and "main steam line break" for a BWR-5 plant (1,100 MW) are discussed here. In addition, these "accidents" are used for the evaluation of "major accidents" and "hypothetical accidents" performed for evaluation of siting of a nuclear reactor facility.

2. Loss of Reactor Coolant (in case of a Large Break)

If a large break of the recirculation piping occurs, the water level outside the core shroud (water level in the downcomer) goes down rapidly. Since a loss of external power is assumed to occur at the same time for the safety evaluation, boiling transition occurs as the core flow decreases rapidly. During this event, the fuel cladding temperature increases temporally (Figure 7.3.1 (2)(a)). Moreover, when the water level outside the core shroud reaches the jet pump nozzle (Figure 7.3.1 (3)), the core flow decreases further, and the core is exposed

NSRA, Japan

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