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DEFENCE IN DEPTH
Defence in depth: facilities

EXAMPLE 5.2 LOSS OF SHUTDOWN COOLING

 

Now let's look at the same event but with the steam generators not available for cooling. 

  • The operators were alerted to the problem by alarms and they reduced the pressure and opened the valves.

how big?

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Fuel melting
- Level 5 or more

how close?

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  • Two lines of cooling
  • Common procedure to open two lines
  • Short time available

- One layer

Now, what would be the answer to the 'how close' question? (Obviously, the 'how severe' answer is still the same). Check your answer.

As temperatures would rise quickly, the procedure is now the limiting factor and there is only one layer.

So what is the basic rating?

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You can see from the table that this event would be rated at Level 3. If you want to read the full event, it is Example 46 on page 130 of the Manual.

Example 46. Loss of shutdown cooling due to increase in coolant pressure - Level 3

Event description

The plant design is the same as in Example 44, but the event occurred soon after shutdown when a rise in coolant pressure caused the isolating valves to close. The safety provisions are illustrated in Fig. 2.

Rating explanation

Criteria

Explanation

2. and 3. Actual consequences:

There were no actual consequences from the event.

6.2.1. Maximum potential
consequences:

The maximum potential consequences for an event
associated with a shutdown power reactor are
Levels 5-7.

6.2.2. Identification of number
of safety layers:

There now appear to be two safety layers as far as
hardware is concerned. However, both still rely on the
operating personnel to re-open the valves. The
reliability of the safety provisions is limited by the need
for action by operating personnel. Given the complexity
of the operation and the limited time available, it is
considered that there is only one effective safety layer
(i.e. an operating procedure requiring pressure
reduction and re-opening of the isolation valve).

6.2.3. Assessment of the basic
rating:

Based on Table 11, Level 3 is appropriate.

Overall rating:

Level 3.