Abstract
The problem of determining the axial velocity profile from the in-core neutron noise measurements is revisited, with the purpose of developing an objective method for the determination of the void fraction. Until now it was assumed that in order to determine a realistic velocity profile which shows an inflection point and hence has to be at least a third order polynomial, one needs four transit times and hence five in-core detectors at various axial elevations. However, attempts to determine a fourth transit time by adding a TIP detector to the existing four LPRMs and cross-correlate it with any of the LPRMs were unsuccessful so far. In this paper we thus propose another approach, where the TIP detector is only used for the determination of the axial position of the onset of boiling. By this approach it is sufficient to use only three transit times. Moreover, with another parametrisation of the velocity profile, it is possible to reconstruct the velocity profile even without knowing the onset point of boiling, in which case the TIP is not needed. In the paper the principles are explained and the strategy is demonstrated by concrete examples.
Highlights
Determination of the local void content in BWRs from measurements with existing instrumentation has been a matter of interest for a long time
The method is primarily based on the recognition that it is necessary to know the local void velocity at the neutron detector positions
To find out the best analytical form of the axial velocity profile, a much wider data base should be available for each different reactor construction
Summary
Determination of the local void content in BWRs from measurements with existing instrumentation has been a matter of interest for a long time. There are many non-linear functions with an inflection point, which represent a much higher order non-linearity than third order, but which can be parametrised with only three parameters instead of four In this paper this latter approach will be developed and illustrated by examples
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