[en] Neutron irradiation of reactor pressure vessel steels increases the T_k-values of transition temperature from ductile to brittle fracture. This effect is very important in emergency situations, when the water cooling injection in the reactor results in high thermal gradients. In such cases there is a risk from the appearance of a brittle fracture with catastrophic crack propagation speed at relatively low stresses. That is why the T_k-value determination is very important for the safe operation of the reactor systems. Some advanced experimental methods for T_k-testing and control have been discussed in the present article and the standards of different countries have been compared. The methods applying subsize specimens and welding-restored specimens have been reviewed. (author)

[en] Unit 3 and Unit 4 of NPP Kozloduy are second-generation WWER440/230 energy reactors. Unit 3 was in operation from 1980, Unit 4 - from 1982. No surveillance specimens for monitoring of RPV metal aging are available for both KNPP Units. The reactor pressure vessels rest lifetime assessment, Plant life management and Surveillance program development for the both units are presented and analyzed in this report. (author)

[en] In this paper the main mechanism for ageing of the primary circuit components, important to the installations' safety, are systematized. The mechanisms having commanding influence on the metal in the main components of primary circuit are identified. The factors influential to the ageing processes are defined. The methodology, program and approach for assessment of residual resource of the primary circuit components in charge of safety are presented. The shown results can be used in procedures for the ageing processes identification, organization of the metal monitoring, conclusions for its condition and decision making for management of the residual lifetime of the equipment

[en] We report on the observation of phase-sensitive Rayleigh backscattering from a single-mode fiber excited by light pulses obtained from a highly coherent single-frequency laser diode and an Erbium doped fiber amplifier (EDFA). The laser diode stability was tested using a highly imbalanced fiber optic Mach-Zehnder interferometer. The CW laser light was first modulated using fiber-optic EO modulator which formed 100ns to 500 ns pulses that correspond to 20 to 100 m of pulse length in the fiber. The backscattered light power is estimated to be about -51 dB lower than the launched power at the input and about N=500 averages are needed for a sensing length of L_s 40 km to be in the useful dynamic range.

[en] Activities made in the period 1989-1996 according to the Program for metal state monitoring of the Kozloduy NPP Unit 1 are described. Data on P and Cu content in the welded joint 4 are reported. Determination is made by wet chemical analysis of shavings taken out from the inner side of the wall, direct spectral analysis of the vessel itself and spectroscopy of the inner and outer side of 6 templates. The results obtained from 4 different study teams showed a good agreement. The real average P content is 0.046% and tends to diminish in depth. Microstructural investigation does not show any expressed inter-crystalline mechanism of brittle failure at low temperatures. The data on real P and Cu content, as well as the experimental values of the initial critical temperature of embrittlement (Tk_o), the residual part of temperature shift (Tk_r) and the re-embrittlement temperature after annealing at 475^o (Tk) allow to predict the change in Tk_o of the joint 4 during the next refueling cycles. The measured low value of Tk after 18-th refueling cycle is considerably lower than that forecasted by lateral re-embrittlement law. This means that the forecasting of Tk for the next cycles is made with big enough conservatisms, and that a second annealing of the vessel until 26-th cycle is not necessary. So according to the most conservative estimate, the Unit 1 can operate safely until the end of the 26-th refueling cycle. It is also concluded, that in terms of radiation degradation of the vessel metal the operation life time of the Unit 1 can reach and exceed the designed one. 2 tab., 7 ref

[en] Phosphorous content and in depth distribution in the NPP Kozloduy Unit 1 weld 4 metal were investigated. Wet chemical analysis was carried out on chips cut layer by layer at five levels reaching 7.1 mm distance from the inner RPV wall. The measured average concentration of P was 0.046% which is considerably lower than expected. No typical intercrystal fracture was found in 1 mm² specimen impact broken at -120 C under vacuum. Auger electron spectroscopy was applied in order to investigate the distribution of phosphorous on grain boundaries in the weld metal. Single regions of increased P content were found mainly on the periphery of secondary cracks or in zones of small size facets. In depth distribution of P in these regions was obtained by argon sputtering. The character of the in depth profile was more typical for the case of P-containing precipitations than of P segregation. The measured low P concentration and the character of P distribution could be regarded as less harmful to the mechanical properties, especially for the toughness of the weld metal, compared to the case of grain boundary segregation. (orig.)

[en] The changes of the annihilation parameters in quenched Fe-Ni samples were investigated as a function of different content of the alloying component (3, 10, 15, 18, 22, 27, 30 and 35 wt.% Ni). The measurements of positron lifetime τ₂, the S-parameter of Doppler broadening of annihilation gamma-line and the peak coincidence counts (PCC) of annihilation gamma-quanta were carried out. A fast-slow coincidence spectrometer and XP2230 photo multipliers with plastic scintillator NE111 as detectors were used for measurements of the positron lifetime. The Doppler broadened annihilation gamma-line was measured by pure Ge 1 cm³ detector (energy resolution ⁸⁵Sr 514 keV gamma-line). The longlived component τ₂ increased by 41 % between 22 and 27 wt % Ni. An increase in the S-parameter and PCC for the region between 22 and 24 wt % Ni was observed as well. A correlation between τ₂, S and PCC and the content of the residual austenite was established comparing these data with the change of the residual austenite as a function of the Ni composition obtained by other authors. The sharp increase of the annihilation parameters τ₂, S and PCC for contents of 22-24 wt % Ni can be attributed to annihilation of positrons in vacancy clusters and dislocations resulting from the twinned martensitic transformation in investigated binary Fe-Ni system

[en] During the normal operation of NPP in result of performed planned or emergency maintenance, some contaminated metal wastes are accumulated. A consortium was established to design, build and commission an installation for deactivation of these wastes. In this paper the used technology (combination of physical, chemical and electrochemical methods) is described in details and the technological radiation control and dosimetry control are also discussed

[en] Results from a novel method for in-situ analysis of the reactor pressure vessel (RPV) metal, developed by a joint team of Bulgarian researches and NPP staff, are reported. The elemental content of the weld 4 of WWER-440/230 reactor has been determined. The method consists in excitation of emission lines in the visible and the near and far UV region by spark and arc. The light is analyzed by high resolution diffraction grids and is detected by photomultipliers. During the analyses the SPECTROPPORT apparatus was used and placed in the Universal Maintenance Cabin (URK-3) fixed inside the RPV. Detailed description is given of basic requirements for the analyzer, effect of the contact between the spectrometer head and the analyzed surface on the element analysis, effect of gamma radiation, the system and equipment for spectral analyses and scrap sampling, the spectrometer calibration. The most important finding of the analysis is the reduction of the phosphorus concentration in the first 1-1.5 mm of the weld (from 0.100 til 0.052%). The measured values up to 2 mm depth are consistent with chemical analysis performed in two institutes. The data for the elemental composition of the weld and for the base metal practically coincide with the corresponding data for Unit 1 in Greifswald-1 reactor. Results from ultrasonic and strength analysis are also reported. All available data and calculations show that the Unit 1 can be operated within the safe margins

[en] The processes of metal embrittlement induced by neutron irradiation embrittlement (NIE) and neutron re-irradiation embrittlement (NRE) of the rector pressure vessel (RPV) are investigated. Radiation lifetime is calculated using two approaches for re-embrittlement: a conservative law and a lateral shift of the critical transition temperature curve after neutron irradiation. In order to prevent NIE the following measures have been taken at the Kozloduy NPP: loading of dummy elements into core periphery; heating the water for emergency core cooling to 55^o C; fast acting valves in the main steam piping, etc. The critical embrittlement temperature, the residual part of temperature shift and the radiation lifetime have been calculated for units 1 - 4 using the two approaches and updated information on P and Cu impurities content. It is concluded that if the lateral re-embrittlement law is adopted and the P content does not exceed 0.05%, all RPV should reach their design lifetime. The NIE in WWER-440/230 is related to C and P content in the weld 4 and is negligible for the Unit 4 in particular, which has low impurities content. In order to reach the design lifetime of the Units 1 - 3 it is necessary to install MSIV. A verification of chemical composition of the Unit 1 RPV weld 4 metal is recommended. 7 refs., 3 figs., 6 tabs