Characterization of gamma flux in VVER-1000 internals

Abstract Accurate prediction of gamma flux in reactor internals is essential for structural assessments studies. This paper presents gamma spectrum measurements in a VVER-1000 full-scale mock-up at the LR-0 reactor, using a stilbene scintillator connected to the NGA-01 spectrometer. Deconvolution of the gamma spectrum was performed using Maximum Likelihood Estimation with a validated response matrix. Measurements were conducted in the reactor baffle and in front of the reactor pressure vessel (RPV) and compared with MCNP6.2 simulations using ENDF/B-VIII.0 and JEFF-3.3 libraries. Good agreement was observed in the baffle region below 5 MeV for ENDF/B-VIII.0. However, both libraries underpredicted the measured spectrum in front of the RPV, particularly at higher energies. These results indicate persisting deficiencies in gamma production data for key reactor materials and emphasize the need for continued validation