[en] Embalse NPP is a 648 Mwe CANDU®-600 type pressurized heavy water reactor (PHWR). The primary heat transport system (PHTS) has two separated heat transport circuits, each of them with two steam generators (SGs) of the recirculating type. The SGs have Alloy 800 tubes and the PHTS piping is made of carbon steel (CS). The primary coolant is lithiated heavy water pHa_25^oC = 10.2-10.4, hydrogen content 3-10 cm³/kg D₂O. The Secondary Circuit (SC) has mixed metallurgy, i.e., copper alloys in the condenser tubing, Carbon Steel piping and Stainless Steel Alloys in some specific locations of the Steam/Water Cycle. In the Secondary Circuit ethanolamine is dosed for pH control and hydrazine for oxygen remosion. The process water and circulation water circuit (main condenser cooling water) source is fresh water from the Embalse Lake. As in other countries, bivalve larvae were transported to La Plata River, Parana River and then to other rivers and lakes located in the interior of the country. This caused serious inconveniences and concern not only in water intakes for drinking water production, but also in electric power production and manufacturing industries. During the observations and inspections that were carried out in different components of the Plant (pipes, heat exchangers), mollusks were found inside the process water and circulation water circuit. This phenomenon reached a peak in 2004 and 2005. Although experience has been taken from other countries, the products to be dosed must be the specific ones to be used in local organisms, local flora and fauna and environmental regulations. In order to study and control this problem, a research study was performed by experts from Buenos Aires Universities and a work plan was established, which consists in three stages: to construct a biobox, that is set in the circuit containing precolonized plates so as to obtain information about the efficiency and the conditions of use of chlorine and an alternative non-oxidizing toxic; to study the reproductive cycle through sampling with the aim of establishing which are the most sensitive periods to colonization and the formation of incrustations, and; to perform a growth monitoring through the installation of bioboxes in the letdown cooling water. Finally a strategy to dose the chlorine and the non-oxidizing toxic product with favorable results to date was selected. (author)

[en] Full text: The installing of helicoidal spacers in both moderator heat exchangers 3311 HX1 /HX2 is described. The operating experience data of both heat exchangers indicated a high level of vibrations due to the induced impacts by refrigeration water flux at secondary side. This vibrations level was reduced in order to avoid or to minimize the possibility of tubes damage resulting in a heavy water (with high isotopic level) leak to the environment. Vibrations analysis was done with flux distribution determination with the vein analysis of the HTRI (Heat Transfer Research Institute). This method finds the fluid vein fraction corresponding to the fractional velocities of different zones of heat exchangers The analysis was focused to the surface zones close to the tube U-bend and the by-pass partition by-pass because they are under the conditions with highest velocities of cross-flow The vibration induced phenomena is normally due to vortex, turbulence, instability, elastic flux. From results it concluded that the tubes of the by-pass lane, of U-bend and in the surface of the tube mallet are the most sensitive to the flux induced vibration. The tubes were structurally modified, trough additional supports installing with helicoidal spacers inserted in the tubes mallet. Practically there are not changes in the flux and the natural frequency of tube is augmented that is the mallet resistance to the flux induced vibration. The heat exchangers structurally modified are more resistant to tube vibration by at full rate flux and to 125% of full rate flux. After modifications vibrations tests were done in the heat exchangers without evidence of pulse signals or frequency variations due to impacts flux in the equipment. Conclusion is that the modification gives a proper supporting and avoids a damage of tubes mallet. An analysis about proper surveillance program for the heat exchangers is opened. (author)

[en] Full text: The work began replacing, in first place, the units with flux mapping detectors only, taking care not to replace detectors of the reactor regulation system and safety systems in order do not have an impairment on the safety systems in case of replacement program's interruption during the first steps of replacements. During the phase of connectors installing in the seventh vertical flux unit (7th VFD) the original Twinax cable was found with a lack of its original properties. The internal conductor insulation was stiffened. For that reason, the internal conductor insulation became very difficult to handle without possible cable damage. Immediately, it was decided to stop the detectors replacement because the general schedule of the outage must delayed by the additional unexpected task of cable replacement. Besides, properly replacement cable was not available at de plant. A series of insulation measurements were made on the detectors cables, including those which were not replaced. A periodical control of insulation resistance measures of detectors and cables was arranged to be carried out during normal operation, until the next outage. In the next Annual Outage the detectors replacement was completed and the PVC cables was replaced by KKK cables with EQ requirements At the present a decision making process must be realized taking into account the Long Term Operation of Plant. (author)

[en] Nuclear stations designed in the 70's and that began commercial operation in the early 80's have to deal with some conceptual design features including materials and chemistry control not entirely suited to cope with phenomena like flow accelerated corrosion (FAC) whose importance has appeared later and is today worldwide recognized. Although since 1984 Embalse has been carrying out a program of piping thickness inspections, it was mainly guided by experience but not on a basis of rationale. Also the lack of documentation in electronic form and equipment data sheets hardly made difficult the evaluation of the piping and components of the circuit

[en] The Argentine Republic is currently running two nuclear power plants: Atucha I (C.N.A. I, a PHWR designed by SIEMENS-KWU) and Embalse (C.N.E., a CANDU 6 PHWR designed by AECL Ltd.) operated by Nucleoelectrica Argentina ( N.A.S.A.), whereas the Argentine Atomic Energy Commission (C.N.E.A.), is among other activities responsible for research and development in the nuclear field, operates research reactors and carries out projects related to them. In particular, the Reactor Chemistry Section personnel (QR, currently part of the Chemistry Dept.) has been working on the field of reactor water chemistry for more than 25 years, on research and support to the N.P.P.s Chemistry and Processes Departments. The arising topics are related to the three main circuits of the station as well as to the many ancillary systems. Most of the on-going activities are based on concepts recently promoted in the literature³⁴ which emphasize a proactive attitude towards core plant components maintenance, like the SGs and their life management. In that sense, it is the purpose of the present work to communicate some data obtained from the primary circuit of C.N.E. while doing non-routine analysis of some chemical variables and characterization of the corrosion products. The results could add knowledge in regard to future interventions such as a chemical cleaning. Bearing that purpose on mind, and in agreement with the Engineering and Operations Departments of C.N.E. it has been agreed to carry-on with the aforementioned tasks. The primary side chemistry control consists only on lithium hydroxide addition and the pH_a is carefully tightened inside a narrow band range. Measurement of the composition of the circulating crud and the SGs deposits, removed in the recently performed mechanical cleaning, in terms of the two iron oxidation states, nickel and chromium, are informed. On the same samples, SEM observations-EDAX analysis and isotopes activity measurements have been done. The values of nickel-chromium concentrations found in the SGs deposits relative to total the iron are compared to the general corrosion data obtained from the on-line autoclave coupons placed in the SPTC, which are periodically inspected. (author)

[en] In particular PHWRs have a system devoted to the purification and upgrading of the collected heavy water leaks. The purification train is fed with different degradation ratios (D₂O/H₂O), activities and impurities. The water is distilled in a packed bed column filled with a mesh type packing. The mesh wire is made of a bronze substrate covered by copper oxides whose current composition has been determined by Moessbauer spectroscopy. With the purpose of minimizing the column stack corrosion, the water is pre-treated in a train consisting of an activated charcoal bed-strong cationic-anionic resin and a final polishing mixed bed resin. Ionic chemicals like acetic acid (whose provenance is suspected to come from the air treatment/D₂O recovery system where the regeneration is performed at high temperature) are detected by the conductivity and ion chromatography when they concentrate at the column bottom. Traces of oils are retained by the charcoal bed but some compounds extracted by the aqueous phase are suspected to be responsible for the resins fouling or precursors of potentially aggressive agents inside the distillation column. Those species have been detected and identified by gaseous chromatography-mass spectrometry (GC-MS). In the present work, the identification, evaluation of alternatives for the retention and results compared to the original products present in the water upgrading purification train have been summarized. (authors)

[en] Embalse NPP is a 648 Mwe CANDU®-600 type pressurized heavy water reactor (PHWR). The plant has been commissioned in 1983 and began commercial operation in 1984. The primary heat transport system (PHTS) has two separated heat transport circuits, each of them with two steam generators (SGs) of the recirculating type. The SGs have Alloy 800 tubes and the PHTS piping is made of carbon steel (CS). The primary coolant is lithiated heavy water pHa_25oC = 10.2-10.4, hydrogen content 3-10 cm³/kg D₂O. Secondary Circuit (SC) has mixed metallurgy, i.e., copper alloys in the condenser tubing, Carbon Steel piping and Stainless Steel Alloys in some specific locations of the Steam/Water Cycle. In the Secondary Circuit Embalse has always applied AVT chemistry, dosing Morpholine and Hydrazine for the corrosion control and Oxygen scavenging respectively. Dose rates have been gradually adapted considering the knowledge of phenomena like Flow Assisted Corrosion (FAC) and Pipe Wall Thinning Rates, SGs internal environment, etc. Therefore, a satisfactory reduction in both, corrosion product transport (CPT) and very low tubes failure rate attributive to chemistry have been demonstrated. However and given the existence of CS internals in the SGs and after an exhaustive evaluation, references search and consultations, Ethanol Amine (ETA) has been adopted after March 2007 Programmed Outage (PO), taking into account its properties and worldwide accumulated experience. It is considered that Embalse is the first CANDU® plant in applying ETA as Secondary Circuit pH conditioning agent. In that sense, transition from Morpholine to ETA Chemistry has been already published. On the primary circuit and considering the presence of carbon steel feeders a careful pH control based on Lithium concentration which in turn is maintained between the recommended limits is carried out. This is in agreement with the nowadays accepted relationship among FAC, temperature and magnetite solubility. Considering all the above mentioned modifications and taken into account that in some cases, i.e., in the Secondary Circuit, new operational ranges and limits based on self experience have been established, it has been decided to release a new Water Chemistry Design Manual Issue entirely updated. This has taken into account local experience and international standards as well. In the current paper experience of the three years using ETA as alkalinizing agent in the SC, primary circuit parameters and an overview of the Water Chemistry Manual and the current set of specifications are presented. (author)

[en] In order to improve water quality, and mitigate recurrent bio corrosion phenomena affecting the components of the Process Water System of the CNE, a combined water treatment adding a commercial biocide product, based on bromide, to the currently injected chlorine was proposed. The aim of this study was to evaluate the effect of the added biocide on the kinetics of biofilm formation and growth, which is the precursor process to microbiological corrosion, and on the corrosion rates of carbon steel of pipes, heat exchanger shells and other system devices. For this purpose, a test bench was designed and built, reproducing the flow conditions at certain parts of the system. This facility was installed in the filtration shed of the Water Plant of the CNE. The test bench consisted of two parallel chambers, I and II, each in turn divided into a section for determining biofilm growth and corrosion rates of carbon steel coupons and another one to measure the kinetics of biofilm growth on stainless steel coupons. Both chambers received lake water chlorinated for 15 minutes each day. The chamber II received also the biocide. The corrosion rate in carbon steel coupons was evaluated by weight loss and Linear Polarization Resistance (LPR) measurements. The kinetics of biofilm growth on carbon steel coupons was measured using disruptive methods followed by quantification of the protein and carbohydrate content as an estimation of total biomase. The following bacterial groups were quantified through the dilution-extinction method: total aerobic bacteria, acid-producing bacteria, total anaerobic bacteria, sulfate reducing bacteria and bacteria precipitating iron and manganese. On the stainless steel coupons, the percent of coverage was evaluated by epi fluorescence microscopy. The corrosion rate results obtained both by weight loss as by LPR, showed no significant differences between both chambers, with and without biocide. Regarding the kinetics of biofilm growth on carbon steel coupons, no significant differences between chambers were found. The analysis of the percentage of coverage on stainless steel coupons matches the results for carbon steel coupons. It was concluded that, under the current operating conditions, it is unlikely that the addition of a complementary biocide that acts by interaction with chlorine, could produce a beneficial effect of reduction and control of the biological material present in the water of the Process Water System of the CNE. It has been suggested to evaluate the possibility of revising the current chlorination scheme and improving its efficiency before implementing any complementary biocide addition scheme (author)