Determination of high negative reactivities in the WWER reactors by ex-reactor sensors readings

The most important condition characterizing the dynamical properties of the core is the reactivity. At present various methods are used for its measurement by means of ex-reactor ion chambers (ERIC). However some methodical problems have still remained actual. This, in particular, relates to the interpretation of ERIC readings when measuring the emergency protection efficiency, including the case with one control rod stuck. The results of experiments and calculations indicate that in this case a long-term discrepancy in the reactivity values obtained by currents of different ERIC takes place. The discrepancy in the ERIC readings is due to the fact that in the full scram the point kinetics equation which is used in treatment of ion chamber currents does not quite adequately describe the behaviour of neutron fields, as the rates of attenuation of non-asymptotic and asymptotic components of neutron flux are simular. In the paper the rate of establishing asymptotic neutron distribution depending on the reactivity inserted is studied. The change in the inserted reactivity magnitude was reached by insertion of different numbers of control rods into the core. The influence of ERIC and stuck control rod positions relative to each other on the reactivity value determined by currents of the given ERIC is considered. (Authors)