In-core vs. out-of-core measuring technique to monitor the neutron flux during the refuelling period at the VVER 440 type reactor units
The main role of a refuelling monitor system is to measure neutron flux in an already stopped reactor of deeply sub critical state, during the refuelling procedure. If the magnitude of neutron flux or its period time exceeds a predetermined threshold value, the system would call the refuelling staff to interrupt the activity under way. The warning signals appear in the main control room and in the cabin of the refuelling equipment as well as on the remote control console of the refuelling machine.
The original in-core refuelling monitoring system (SKP) at Paks Nuclear Power Plant used three (originally six) detector assemblies equipped with fission chambers, put into the core (at its peripheral points) after the removal of the reactor-cover and right before the beginning of refuelling. The refuelling could not be started until the installation, adjustment/calibration of the measuring chain was finished, increasing thereby the down-time of the reactor.
The out-of-core refuelling monitoring system uses detectors of proportional counter types, arranged in out-of-core positions. The sensitivity of the used proportional counter type is 20-times higher (typically 10 cps/nv) than the sensitivity of the fission chamber (typically 0.5 cps/nv), used by the former in-core system, to compensate for the decreased neutron flux in the out-of core detector positions. Detectors are permanently assembled, but their vertical positions are movable relative to the zone to avoid unnecessary exposure during normal reactor operation.
Refuelling time has become shorter by one / one and a half day owing to the operation of the out-of-core measuring technique.
The new refuelling monitoring system consists of four main structural units:
Main characteristics of the new system are:
Main Technical Data