CHAPTER 3: INSTRUMENT AVAILABILITY

Observed parameters only

The severe accident guidelines should be initiated, throttled (if applicable) and terminated on the basis of observed plant parameters only. Parameters which are only available through complex calculations (for example, the peak cladding temperature) are not suitable for execution of SAMG.

The parameters needed are derived from direct measurements or through inferring parameter values from other observations. Where no observations are available, so-called 'Computational Aids' can be developed which give guidance on needed but missing parameters (see below).

In situations when these are also not available, plant staff should estimate the value of relevant parameters from their knowledge of plant behaviour. For example, in a severe accident that was initiated by a station black-out and where the RCS pressure signal has been lost, and actions to depressurize RCS were initiated in EOP domain, it can be assumed that the RCS is at high pressure, and thus the guidelines would be initiated accounting for high RCS pressure even though no RCS pressure indication (in this case: high RCS pressure measurement) is available. Alternatively, if no reading of an important parameter and no Computational Aid is available, it is advised to assume a pessimistic value in assessing plant status. For example, if no steam generator level indication is available it may (or even should) be assumed that the steam generator is empty.

Qualification of instruments

For the key plant parameters, it should be known to what level the associated instruments will give reliable readings, also in a degraded environment (elevated pressure, temperature, humidity, radiation). This can be beyond their formal qualification range. Where prime readings cannot be used, alternate measurements should be sought. For example, the pressure measurement in the RHR loop can give a reliable indication if the primary measurement is unavailable (and the RHR is connected). It is recommended to use various measurements for one parameter to reduce the risk of incorrect measurements.

Instrument deviations

Where possible, potential deviations from instrument readings should be investigated and documented in the SAMG. For example, the steam generator level reading will deviate if the containment pressure increases. The SAMG should contain mechanism to correct the observed values.

Trends can be more important than point values. So, where point values are no longer reliable, trends may still be.

Instruments for non-dedicated use

Instruments that have been designed for a certain parameter can sometimes be used for other parameters, relevant for SAMG. For example, the current in the Average Power Range Monitor (APRM) can give an indication of the progress of core damage.

Computational Aids

Pre-calculated graphs can be used to determine parameters which otherwise are not available. For example, the amount of water needed to remove decay heat can be calculated from the time after shutdown and be kept in a graph, which then can be simply read by plant staff in order to determine the amount of water needed to remove the decay heat. Another example can be used in the situation if no H2 measurement is available. The amount of H2 can be estimated based on the assumed fraction of the total mass of Zr cladding that was oxidised (25% RPV failure, 50% best estimate value for many cases, 75% if water has been added and additional H2 generated).

See SRS 32, Appendix III. Read more →