Outage type code

The outage type is coded by a three-character code describing several circumstances of outage occurrence. It can provide answers to questions like: Could the plant management prevent the outage? Was the outage planned? Was the unit disconnected from the grid? Was the unit power reduced in a controlled manner or was the outage due a reactor scram? Wasn't it just an extension of a planned outage?

The type coding should include one of the characters showed in Table1.

TABLE 1. TYPE CODE, FIRST CHARACTER Code Description

P Planned outage due to causes under the plant management control (internal) U Unplanned outage due to causes under the plant management control (internal) X Outage due to causes beyond the plant management control (external)

The outage is considered planned (P), if it is scheduled at least four weeks in advance (generally at the time when the annual overhaul, refuelling or maintenance programme is established), and if the beginning of the unavailability period can be largely controlled and deferred by plant management. An outage is considered unplanned (U), if it has not been scheduled at least four weeks in advance. The “external” (X) outages may be also considered planned or unplanned. Although this aspect is not explicitly coded, adding the third character (see below) to the "external" outage code will imply the unplanned “external” outage.

For historical continuity reasons, the first character of the code combines two "levels"

of outage type code. It simultaneously identifies, whether the outage could or could not be controlled by the plant management. At the same time it codifies, if the outage was planned or unplanned. For the "internal" outages, the planned and unplanned outages are explicitly coded, as they are more important for the coding purpose.

The codes P and U thus imply an outage due to causes under plant management control.

Although the “external” outages, which are coded only X, may be also considered planned or unplanned, this aspect is not explicitly coded for this type of outage. However, adding the third character (see below) to the "external" outage code X would imply an unplanned

“external” outage.

In general, any change in the planned outage start date is considered unplanned, unless it is announced at least four weeks in advance. If the start date is anticipated, the outage is considered unplanned until the originally scheduled start date. If the start date is postponed, the outage is still considered planned until the originally scheduled completion date. The unplanned portions of planned outages should be coded as separate outages.

Any extension of the planned outage beyond the original completion date is considered unplanned, unless it is announced at least four weeks in advance. The unplanned extension of outages is codified separately using a third character. The planned extensions of outages are considered a part of the planned outages and are not coded separately.

For example, if a unit is shut down due to an equipment failure shortly before the refueling outage, so that it cannot be restarted until the outage beginning, the plant management may decide to start the outage earlier. In such a case, the portion of the outage until the originally planned refueling start date should be reported as a separate unplanned outage caused by an equipment failure, while the rest of the outage would be coded as planned for annual maintenance and refueling. No outage extension would be reported provided the planned refueling outage was finished within the originally planned completion date.

TABLE 2. TYPE CODE, SECOND CHARACTER Code Description

F Full outage

P Partial outage

The second character identifies whether some energy was still delivered by the unit during the outage. As the coding refers to outage unit power, the outage is considered full (F) whenever the unit is disconnected from the transmission grid (the main generator output breaker is opened) and from all other energy consumers (the off-site steam/hot water lines are isolated). In some cases, the outage may be considered full, even if the reactor remains at power. Table 2 presents codes for the second character.

An outage is considered full (F) if the actual unit output power has been reduced to zero percent (unit disconnected from all off-site power supply lines). An outage is considered partial (P) if the actual unit output power is lower than its reference value, but is greater than zero percent.

TABLE 3. TYPE CODE, THIRD CHARACTER (FOR UNPLANNED OUTAGES ONLY) Code Description

1 Controlled shutdown or load reduction that could be deferred but had to be performed earlier than four weeks after the cause occurred or before the next refueling outage, whatever comes first

2 Controlled shutdown or load reduction that had to be performed in the next 24 hours after the cause occurred

3 Outage extension

4 Reactor scram, automatic 5 Reactor scram, manual.

The third character codification (Table 3) describes special circumstances of the outage.

It recognises, if the outage was achieved by controlled power reduction at normal operating rate or if it was due to an unplanned reactor scram. It also identifies extension of planned outage (X). Any reactor protection function reducing reactor power is considered controlled power reduction, unless it fully inserts into the core all the control elements (rods) designed to scram the reactor.

The third character should be assigned also to outages due to causes beyond plant management control ("external" outages), which can be considered unplanned (e.g. the causes coded J, M, N, R, T and U in the table provided below).

Using the above characters, the outage type code can have one of the following forms (planned scrams and planned extension of outages are not considered):

PF or PP

UF1-5 or UP1-3

XF or XP

XF1-5 or XP1-3