At present, measurements of CPS control rod worth are typically performed using the IKES method (inversed kinetics equation solution). The method is based on the assumption that a point kinetics model can be applied to a real reactor. The point kinetics reactor model can not be applied to measure large negative reactivities, introduction of which is connected with considerable deformation of the neutron field.
Considering the coarseness of the point kinetics reactor model at measuring large negative reactivities leading to the neutron field deformation, it is suggested to use the space-and-time mathematical model of the reactor with neutron detector introduced directly into the measurement scheme. It is assumed that the model and the corresponding code make it possible to solve both the steady-state eigenvalue problem and the kinetic space-and time problem, as well as model the detector response to the processes inside the reactor.
The measurement procedure comes down to the adaptation of the model to the measured detector signal. The model parameters connected with the measured reactivity are adjusted to provide the best agreement between the model response to control rod movement resulting in negative reactivity introduction and the alteration of the detector signal used during the measurements. As a result, the reactivities calculated using the adapted mathematical model comply with the measured data. This approach to measurements has been named model-oriented. Its advantages:
- ability to use models of any complexity without the need to state and solve inverse problems of neutron kinetics;
- ability to adjust the model to account for the data from various netronic characteristics measurements, which enhances the measurement reliability.
The suggested approach was checked against the results of measurements of neutronic characteristics of the first fuel charge of the Kalinin NPP Unit 3. During the tryout, a two-dimensional mathematical reactor model was used with one neutron group in terms of energy and six delayed neutron groups for the heightwise-infinite structure without a reflector, consisting of 163 assemblies of VVER-1000 with 61 CPS control rods, positioned in accordance with the first fuel loading pattern of the Kalinin NPP Unit 3 core. To calculate the detector response a model was used in the form of the sum of the neutron flux density in the periphery assemblies positioned opposite the detectors and contributing into the response with various "weight".
The model-oriented method of measuring large negative reactivities confirmed its viability as exemplified by the determination of protection system effectiveness even using comparatively simple yet adapted reactor model with a detector.
CPS control rods, reactor protection system, reactor kinetics equation, point kinetics model, neutron field deformation, negative reactivity, model-oriented method
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