Review of the primary coolant chemistry at NPP Temelín and its impact on the fuel cladding

Transcription

1 Review of the primary coolant chemistry at NPP Temelín and its impact on the fuel cladding M. Mikloš, K. Vonková, J. Kysela Research Centre Řez Ltd, Řež, Czech Republic D. Ernst NPP Temelín, Reactor Physics Department, Temelín, Czech Republic Abstract One of the main goals in Research Centre Řež Ltd, in the department of Research Reactors is the study of the VVER, PWR and BWR primary coolant chemistry impact on the construct materials in the whole primary system. Reliability of nuclear fuel and radiation fields surrounding primary systems as well, are important aspects of overall nuclear reactor safety. With the use of research reactor LVR-15 and several experimental loops and devices Research Centre Řež is participating on the recommendations for all NPPs in Czech Republic (VVER-440 in Dukovany, VVER-1000 in Temelín) in the mentioned problematic. NPP Temelín is a first VVER-1000 reactor, where the Russian reactor design meets the American fuel design. Since 2009, Research Centre Řež is also participating on the post irradiation inspection program at NPP Temelín. Together with the previous fuel vendor Westinghouse Electric Company LLC, Research Centre Řež is performing independent support for the fuel inspection and repair. During these inspections large amount of data were measured and used as a feedback to the fuel vendor and to the fuel operator as well. Oxide measures at Zircaloy-4 cladding, which was used in Temelín water chemistry, showed excellent results and good applicability of Westinghouse fuel cladding. Paper describes excellent operational experiences with water chemistry regime at Temelín NPP proposed by Research Centre Řež as water chemistry guidelines, based on previous experiences from several corrosion experiments. 1 INTRODUCTION 1.1 VVER water chemistry VVER primary water chemistry is boric acid-potassium hydroxide based. Total alkalinity is given by the concentration of potassium, lithium and sodium. Different water chemistries are recommended for the VVER-440s and VVER-1000s to reflect their different operating temperatures. ph control is carried out in different ways at individual stations using a combination of potassium hydroxide addition and potassium removal using cation ion exchange resins. Also the boron-potassium coordination control during the cycle differs with respect to the use of standard or modified chemistry. Standard water chemistry was developed in late seventies and was designed to produce a nominally constant high temperature ph of ph 260 C 7.3 at 270 C [1]. ph was calculated according to the Meek method which is now known to give an error in high-temperature ph T determination nevertheless this type of boron/potassium control is still used at some VVER plants. Modified Water Chemistry was introduced in the beginning of nineties. This water chemistry ensures the constant ph T and

2 stable physical-chemical conditions during the whole reactor cycle which should reduce radiation fields. The choice of the optimal ph T was a result of plant data analysis as well as mathematical modeling. For VVER-440 units, the ph 300 is in the range of while was chosen as the optimum value for VVER Boron/potassium co-ordination for the standard and modified chemistries of VVER-440 and VVER-1000 units is given in Fig. 1. Typical high-temperature ph course during the reactor cycle for the standard and modified chemistries is shown in upper section of the picture. Fig. 1 Boron-potassium control in standard and modified water chemistry regimes Primary circuit materials undergo general corrosion by temperature and chemical stress environment. Products of corrosion release from the corroded surface and then they are transported through the entire primary circuit included core. Corrosion products become active by its deposition in the core and its re-deposition on the rest of primary circuit, especially piping and stem generators tubes, as well as on the fuel cladding. The activity build-up on primary piping is determined by the entire level of activated corrosion products in the coolant. 1.2 NPP Temelín NPP Temelín was built during the years In 1999 preparatory work for the commissioning of the first Unit has begun. During the years 2000 to 2002, hot tests and power activation took place at the Unit 1. In the middle of 2002, the first cycle at Unit 1 was started; Unit 2 was activated a year later. The installed output of NPP Temelín is 2x1000 MW; both reactors are of type VVER-1000, type V320. There are 4 identical loops with the high

3 temperature-high flow rate mechanical filter. The passivation and high temperature filtration during startup of both Units contributed to Temelín s very good radiation conditions. The reactor core contains 163 fuel assemblies, each with 312 fuel rods, and 61 regulating rods. Fuel rods are fuelled by UO 2, uranium dioxide enriched up to 4,95±0,05% of the fission isotope 235 U. Since the startup in 2002, the fuel for Temelín NPP was supplied by the Westinghouse Electric Company LLC, which also supplied the new instrumentation and control system. During eight years of operation, four fuel designs of VVantage-6 fuel assembly with Zircaloy-4 (ZIRLO TM since 2007) cladding were used. However, nuclear fuel contract with Westinghouse company was ended in 2010 (the last delivery at Unit 2) [2]. Following the tender for fuel supplier from 2004, new fuel vendor for the period was elected the Russian company TVEL, which already delivered new reload to the Unit 1 in April 2010 (163 FA) and in May 2011 at Unit 2 (163 FA). The new fuel assembly design TVSA-T with E110 cladding promises good operational practices. 2. EXPERIMENTAL BACKGROUND 2.1 Experimental loop Pressurized water reactor loop RVS-4 (Fig. 2) at light water research reactor LVR-15 (Fig. 3) in Research Centre Řež (CVR), Czech Republic was designed to perform a simulated high temperature, high pressure environment of VVER/PWR primary circuit with variable water chemistries for material behavior research in such stressed conditions [3][4]. LVR-15 design permits the usage of various diameters of irradiation channels. RVS-4 has active channel set straight inside the active zone of the research reactor so the effect of radiation and neutron flux on test samples can be monitored. Fig. 2 Experimental water loop RVS-4

4 Fig. 3 Research reactor LVR-15 The loop provides miniaturized, properly simulated NPP primary circuit. Basic thermohydraulic, chemical and radiation parameters of VVER/PWR primary circuit are well modeled. There is: core: active channel situated in reactor core with electrically heated fuel rod imitators, loop with steam-generator: pressure channel with field tube for flow division inside the channel, main circulation pump and primary piping, pressurizer and volume compensating system, make-up water preparation and dosing system (make-up water tank, required chemical ingredients, high pressure dosing pumps, hydrogen gas bubbling), primary coolant sampling system (isokinetic sampling, microfilter holder, micro-filter). The routine sampling includes corrosion products monitoring, activated corrosion products activity monitoring, water chemistry parameters levels (boric acid, ammonia, hydrogen, alkali etc.) and impurities measurement. Post irradiation inspection of the tested samples is carried out after each experiment, also the corrosion layers quality and thickness is evaluated. Due to relatively small volume of the loop it is most convenient for investigation of the effect of variable water regimes. 2.2 Water chemistry experiments and conclusions The radiation fields around the primary circuit of NPPs are formed by radionuclides created in the primary circuit during the Unit s operation. Among these radionuclides, besides fission

5 products are predominantly corrosion products from construction materials. One important factor having an influence on the amount of corrosion products of a facility already in operation is coolant chemistry. High temperature ph (ph 300 ) stability and optimum value is one of the most important factors that can influence the corrosion situation in the primary circuit. While the ph 300, is kept in the optimal range, the second thing for corrosion products affecting is the composition of the primary circuit coolant, such as hydrogen and ammonia level, ammonia presence overall, alkali type, zinc dosing etc. All contexts and activity mechanisms in the ammonia chemistry used under VVER conditions specifically related to issues of the behavior of radionuclide corrosion products with subsequent influence on the formation of radiation fields, the stability of protective (passivation) construction materials surface layers and deposition formation as a function of ammonia content in primary circuit coolant are not satisfactorily described. Neither operational nor experimental experiences with zinc addition technology in VVER reactor environments are also unavailable. Direct gaseous hydrogen dosing without the use of ammonia profitability in the VVER conditions is also not proved to satisfaction. Due to these facts an experimental project program proposal, Heightened Corrosion Product Formation Risk Reduction via Primary Circuit Chemistry Optimization was carried out in the CVR in the years 2003 to Goal of the experimental program was to gain the fundamental missing knowledge and data on the basis of comparative experiments in a specially constructed reactor water loop. Four experimental programs were set-up for water chemistry optimization comparison: standard VVER water chemistry direct hydrogen dosing without ammonia, standard VVER water chemistry with elevated ammonia level, zinc dosing to standard VVER water chemistry. All tested regimes showed their benefits towards to standard using water chemistry. Zinc addition had lowest volume activities peaks during shutdown. Hydrogen regime and higher ammonia showed both lower corrosion products release rate and higher ammonia also much lower surface activities on primary piping. Since the direct hydrogen dosing is rather complicated for VVER NPP operating usage, the best choice for corrosion products situation improvement seems to be the elevation of the ammonia level. During the years 2006 to 2009 another experiment was operated for proposing conditions and recommendations for NPP operation from the standpoint of decontamination procedure and subsequent operational strategy. The experimental program was aimed at investigating the influence of the presence of organic substances (TOC), concretely the influence of trace amounts of organic acids after decontamination, on the development and stability of passivation layers and deposits both on the surface of heating rods (fuel elements) and on interfaces with spacer grids (stainless steel Zr with 2.5% Nb) and on the inside surfaces of steam generator pipes. These experiments were part of the international technical cooperation project RER/0/076, held by IAEA. It was concluded that the practical methods to control crud formation are accurate control of primary water chemistry, careful decontamination and a surface layer passivation. However,

6 decontamination is only needed in the event of the refurbishment of any important component that requires a drastic decrease of dose rates to allow such maintenance activities. As mentioned above, RVS-4 loop is used to study the effect of environment on materials in the active zone of power reactors. Phenomena under study include corrosion, the influence of physical and radiation stresses on the rate of crack propagation, the interaction of fuel and coolant coverage, including cladding corrosion and the deposition of corrosion products on the surface of the fuel elements, further for the research of water chemistry of PWR and VVER reactors. Study of water chemistry regimes in CVR led to the preparation of the water chemistry guidelines for Czech NPPs, which are used at Dukovany NPP (VVER-440), as well as at Temelín NPP (VVER-1000). 3. OPERATIONAL EXPERIENCE 3.1 Corrosion situation in the primary circuit From the very beginning, Temelín NPP has been emphasizing thorough monitoring of the corrosion situation in its units. During the cycles a standard monitoring primary water chemistry parameters was performed, as well as monitoring of corrosion products concentrations and radioactivity in the primary coolant. Also, in-situ γ-spectrometry measurement was performed on the primary piping during the unit s shutdown. Thanks to an extensive set of measured values, the situation in the power station is very well depicted (Fig. 4). Corrosion products level is really low, so it is hard to determinate filter efficiency from concentration levels on inputs and outputs of the filtration system. 3.2 Fuel inspections Fig. 4 Temelín Unit 1 primary piping surface activities [5] Moreover periodical inspection and repair of fuel assemblies at Temelín NPP is performing as a Post Irradiation Inspection Program (PIIP). The main work is oriented for the inspection and repair of the fuel assemblies with use of Fuel Repair and Inspection Equipment (FRIE)

7 designed by Westinghouse Electric Company LLC. In addition to fuel repair, the PIIP is also oriented to study of leaking fuel rods and root causes of these leakers, too. Together with bowing and elongation of fuel rod measurements, oxide layers thickness measurements by eddy current method were performed. The oxide measurements were performed by the past fuel vendor Westinghouse during first four cycles at Unit 1 ( ). All measurements where executed on the first design of VVantage-6 fuel assemblies, where the Zircaloy-4 was used as a cladding material. The visual inspection was carried out too. The measurements were done on the peripheral rod as well as on the central rods, which were removed by special manipulator. Visual examinations as well as the measurements revealed very small amount of corrosion on the cladding surfaces, which was taken as a confirmation of compatibility of Zircaloy-4 alloy with the VVER-1000 water chemistry proposed by CVR. The maximum oxide thickness after 4 cycles was less than 40 µm. Fig. 5 Oxide measurement at Temelín NPP (Zircaloy-4) [6] In addition, since 2011 the fuel inspections and repairs are provided with present fuel vendor, Russian company TVEL with the participation of experts from RIAR and CVR. CONCLUSIONS Reliability of nuclear fuel and radiation fields surrounding primary systems are important aspects of overall nuclear reactor safety. Axial offset anomaly has occurred in a number of PWRs operating with extended fuel cycles and high boiling duty cores. The primary coolant chemistries used in VVER NPPs have similar basis to those used in PWRs and are designed primarily to minimize out-of-core radiation fields. That means to maintain the alkaline reducing condition in the primary circuit during the normal operation. In VVER NPPs the coolant chemistry differs from that in Western PWRs in that the alkali used is mainly potassium, and that ammonia or hydrazine is added to generate the hydrogen used to suppress radiolysis.

8 One of the main goals in CVR is the study of the impact of PWR and VVER primary coolant chemistry on construct materials in whole primary system. With the use of research reactor LVR-15 and experimental water loop RVS-4, presented in the paper is participating on the recommendations, as well as on the water chemistry guidelines for all NPPs in Czech Republic (Dukovany NPP (4xVVER-440), Temelín NPP (2xVVER-1000). The corrosion situation at Temelín NPP is very well depicted since startup, which was also proved by the oxide measurement at fuel assemblies VV6, where the Zircaloy-4 cladding was used. The maximum oxide layer after four cycles was less then 40 µm, which is an order of magnitude lower then is normally achieved on the western PWRs. Since the compatibility of Zircaloy-4 cladding with the VVER-1000 water chemistry was confirmed, there is no need to measure the oxide thickness at the fuel clad at Temelín NPP. In addition, since 2007 Westinghouse introduced new fuel assembly design, where the ZIRLO TM alloy was used, as well as since 2010 the new fuel assemblies TVSA-T from TVEL with E110 alloy are used. Both these alloys are well know as zirconium alloys with higher corrosion resistant as Zircaloy-4. REFERENCES [1] K. Vonkova, V. Svarc, J. Kysela (2010) Operation experience with elevated ammonia, 1-8. In NPC 2010 October 3-7, [2] V. Hlavinka, International Cooperation in Development and Supply of Nuclear Fuel for Czech NPPs / presentation, «ATOMEXPO 2009», 2009, p. 16. [3] Kysela, J., et al.: Overview of loop s facilities for in-core materials and water chemistry testing. In (ed.). JAIF International Conference on Water Chemistry in Nuclear Power Plants. Kashiwazaki, Japan, October [4] V. Svarc et al. Primary Water Chemistry of VVER Reactors: Comparison of Loop Experiments with Hydrogen, Ammonia and Zinc. International Conference on Nuclear water Chemistry in Reactor Systems, Jeju, Korea, October [5] K. Vonkova, J. Kysela, M. Martykan et al. (2008) Primary Water Chemistry and High Temperature Filtration System Experience at Temelín WWER-1000 NPP, In PowerPlant Chemistry 10 (12). [6] D. Ernst and L. Milisdorfer, 10 years of experience with Westinghouse fuel at NPP Temelín, VVER 2010, Prague: 2010, p. 30.