Reactor Dosimetry: Accurate Determination and Benchmarking of Radiation Field Parameters, relevant for Pressure Vessel Monitoring (REDOS)

Radiation embrittlement of reactor pressure vessel (RPV) steels is a degradation mechanism that requires crucial consideration for the safe operation of aged nuclear power plants. Plant life management needs a reliable estimation of radiation field parameters, including their uncertainty, to avoid the use of conservative approaches.
The particular objectives of the REDOS project are the improvements of the RPV monitoring, the improvement of the neutron-gamma calculation methodologies through the LR-0 engineering benchmarks for VVER-1000 and VVER-440, and the accurate determination of radiation field parameters in the vicinity and over the thickness of the RPV.
The REDOS project started in November 2001 and has a duration of three years. At its mid term stage in the project, all the required NPP experimental data have been collected. A review of the available experimental data of Kozloduy RPV unit 1 has been performed and the attenuation coefficients through the vessel wall have been estimated. The collection of the available revised data for VVER-440 Mock-up have been completed. Furthermore, the existing data for the VVER-1000 Mock-up have been reviewed and the preparatory work for subsequent measurement and experimental data analysis was carried out.

Measurements of the space-energy distribution of the mixed neutron-gamma field in the VVER-1000 model have been performed over the RPV simulator. The photon and neutron spectra have being measured simultaneously with the stilbene scintillator and the multiparameter technique, and the thermal neutron flux has been measured in several points with a He-3 miniature chamber. The most important improvements in the experimental techniques used are the multiparameter spectrometer and a new low noise precise monitoring system in the LR-0 reactor, developed for this type of measurements. The calculational models of the benchmark exercise were prepared by the project participants using their respective codes.