The design modification package for implementation of EVND

As a subcontractor of Westinghouse, SIPRO provided the design modification package for implementation of Ex-Vessel Neutron Dosimetry (EVND) system for Krško NPP. The EVND is an additional fast neutrons measurement system, which upgrades the reactor vessel material surveillance program.

Basic problem considered is the embrittlement of material due to exposure to fast neutrons. Neutron that strikes the nucleus imparts energy to that nucleus and degrade the metal structure before stopping. Dislocations and defects lead to embrittlement. The phenomenon affects material Nil Ductility Temperature (NDT) causing brittle fracture even at higher temperatures. EVND system is arranged external to the reactor vessel in the annulus gap towards the biological shield, allowing ease of dosimetry removal and replacement. Radiometric sensors are employed at discrete locations to characterize the neutron energy spectrum variations axially and azimuthally. Once the internal surveillance capsules have been removed the EVND based neutron transport calculations will allow the projection of embrittlement gradients through the reactor vessel. Stainless steel gradient chains are used for locating the dosimeters to complete the mapping between the discrete locations chosen for spectrum determinations. Modification is implemented in outage 2009.


Figure 1 - Embrittlement versus Ductility


Figure 2 - EVND system layout


Project Manager: Mladen Debeljak, B.Sc.M.E