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Thermal resistance contributions of oxides growth on Incoloy 800 steam generator tubes
Nuclear Engineering and Design 219 (2003) 1 /10 www.elsevier.com/locate/ned
Thermal resistance contributions of oxides growth on Incoloy 800 steam generator tubes A.M. Iglesias *, M.A. del C. Raffo Calderon Unidad de Actividad Quı´mica, Comisio´n Nacional de Energı´a Ato´mica, Av. del Libertador 8250, 1429 Buenos Aires, Argentina Received 6 June 2001; received in revised form 25 October 2001; accepted 17 July 2002
Abstract The primary and secondary circuits of nuclear power plants have different water chemistries, corrosion product sources, temperatures and flow rates. All these parameters promote the growth or deposition of oxides of different compositions and morphologies on the surfaces of the steam generator (S.G.) tubes. This paper presents a methodology to evaluate the relative contributions to the total heat transfer resistance due to the different oxide layers. The values obtained from a sample of Alloy 800 tube at room temperature are similar to those encountered in the open literature at high temperatures. Important future planning guidelines for decontamination and/or chemical cleaning of S.G. units can be obtained from these results. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Fouling; Steam generator; Incoloy 800; Thermal resistance
1. Introduction Unlike a fossil-fired thermal station, pressurised heavy water reactors (PHWR) have two coolant circuits. Primary heat transport system (PHTS), whose function is to remove the fission heat produced in the reactor core, and a secondary circuit which collects the energy from the PHTS to produce the steam that drives the turbines. Both heat transport circuits have different water chemistries and corrosion product sources. They are subjected to different flows, temperatures and velocities. Under these circumstances it is usual the
* Corresponding author. Tel.: /54-11-6772-7143; fax: /5411-6772-7121 E-mail address: [email protected] (A.M. Iglesias).
growth and/or deposition of oxides of different compositions and morphologies on the surfaces of the steam generator (S.G.) tubes. The level of corrosion products in both circuits is kept as low as possible with careful water chemistry control. Some corrosion products which come from carbon steel pipes and tanks and condenser tube Cu-alloy are transported and deposited on the S.G. tube walls because of temperature changes or temperature gradients which can affect in a significant way the solubility of these products. In conjunction with other compounds produced by in-situ corrosion, these deposits originate the so-called ‘corrosion fouling’ phenomenon. The evaluation of the heat transfer as a function of the oxide type has been the object of some
0029-5493/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 0 2 ) 0 0 2 5 6 - X
2
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
research efforts and it is not yet totally well known and standardised. The possibility of predicting the corrosion fouling effects depends on an understanding of the processes associated with the growth and removal of the corrosion products. Somerscales pointed out in 1999 that there is clear experimental evidence that the products of a corrosion process can present significant resistance to the thermal heat transfer. One of the consequences of this process is a progressive degradation of the heat transfer capability of the involved equipment. In the case of nuclear power plants an increase in the reactor inlet temperature (primary side) can be measured due to the insufficient heat transfer to the secondary side in the S.G. tubes. This situation could produce a different operating state of primary circuit with a loose in the efficiency, or even more, it could produce a non planned shut down. So in these cases periodical cleaning must be carried out in order to restore the design values (Berge and Fiquet, 1993). Consequently, to plan the cleaning strategy it is important to know these points: the contribution to the thermal resistance of each layer deposited or growth on the S.G. tubes (Kelen and Arvesen, 1972); how each oxide layer contributes to the total heat transfer resistance; and what side (primary or secondary) would produce more benefits in the case of taking a decision about the necessary cleaning (it is important to take into account that the primary side is contaminated and its cleaning implies a lot and careful work). A 30-mm film oxide thickness of zirconium alloy (fuel rods) shows thermal conductivity values between 0.8 and 2 W m1 K 1. These values are important if we consider that Zircaloy thermal conductivity is 22 W m 1 K 1. (IAEA, 1998). According to a work by Mikk et al., 1972, who evaluated the oxide fouling resistance as a difference between two different conditions of a tube (with oxides and clean), this paper presents in a similar way a technique that allows at room temperature the evaluation of the different thermal heat transfer resistances. Our measurements were carried out with the sample immersed in water. This possibility to work with water allows the
oxides to be in contact with the same medium in which they appear during the normal operation of S.G. tubes.
2. Sample of Alloy 800 2.1. Nuclear reactor CANDU 600 In the case of nuclear reactor CANDU 600 both circuits, primary and secondary, have a large amount of carbon steel surface exposed to the fluid that produces an inventory of iron corrosion products, and these contribute together with their own corrosion to the corrosion fouling phenomenon (Warzee et al., 1967). During the 1994 annual planned shutdown of Embalse N.P.P., a small sample of the steam generator tube was taken out from the hot leg of the steam generator no. 4. This portion of tube was located between the tube sheet and the second tube support plate. Different sections of this tube have been used to characterise in-situ the growth or deposit of oxides on the internal and external walls of the tube, and one section was used to carry out the thermal resistance measurements for this work. The removed tube was operating under steam generator thermalhydraulic conditions ( / 309 8C) during seven effective full power years. The thermal barriers already observed in steam generator tubes consist, mainly, of four oxide layers: two on the internal wall of the tubes (primary side) and two on the external wall of the tubes (secondary side) (Lister, 1993). Different analyses of Alloy 800 have determined that the oxides consist of two layers (spinel form). The inner layer is enriched in Cr, /50% more than in base metal and less Ni concentration, and the external layer consists of crystals with a concentration less than 5% Cr, /30% Ni and Fe balance. (Alvarez et al., 1996; Stellwag, 1998). The inner layer has a highly compact structure that impedes the progression of the corrosion process. The outer layers, which are mainly formed by magnetite, as a result of water reductive conditions in both systems, are less compact and
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
have a significant contribution to the heat transfer thermal resistance. Figs. 1 and 2 show microscope scanning photographs of the oxides on the tube surface. Fig. 3 shows a sketch of the various oxide layers. Each layer is like an equivalent serial resistance that contributes to the total heat transfer thermal resistance. 2.2. Experimental set-up The methodology consists of generating a constant radial heat flux which goes through a 1-mm water thickness, internal oxide layers, a 1.4 mm metallic Alloy 800 thickness, external oxide layers and a 1-mm water thickness (Fig. 4). Therefore, a steady-state with a determined temperature gradient that depends on the power
3
applied, the intermediate layers and the temperature of the final heat reservoir are generated. The heating element is built in Nichrome wire coil (diameter 0.36 mm) and encapsulated in a glass tube (diameter 4 mm, long 100 mm). It acts as the constant heat source. The total electrical resistance of this wire is approximately 10 V and it is powered by a home-made constant dc power supply. Four glass encapsulated thermistors (SiemensMatsushita model M85) were selected as temperature sensors because of their small dimensions (diameter 1 mm, 1.5 mm long) and sensitivity. They were located 1 mm away from the internal an external wall of the sample tube respectively and in a midway from Incoloy 800 tube. Two brass pieces, one at the top and the other at the bottom, which act as a support frame, main-
Fig. 1. Microscope scanning photographs. Transversal view with the different types of oxides.
4
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
Fig. 2. Microscope scanning photographs. Top view of the different oxides.
Fig. 3. Sketch of oxide layers on Incoloy tube (not in scale).
tain the sample tube (10 cm long), the thermistors and the heater in a fixed rigid position (Fig. 4). Thin glass tubes (capillaries of 1.5 mm external diameter, 70 mm long) fix the thermistors in each
position (from the brass top piece) with epoxyresin. A stainless steel tube (diameter 30 mm, thickness 1 mm) is located in a concentric way with the components mentioned before. The stainless steel tube, that surrounds the set up (as shown in Fig. 4) is used to obtain a homogeneous heat transfer. The assembled set was immersed in distilled water inside of a 30 mm inner diameter glass tube and a length of 250 mm. The whole device is maintained within a constant temperature thermal bath (Fig. 5). The power range applied is between 0 and 10 Watts. These values were selected because this power range allows working with a mensurable temperature difference compatible with low powers. Therefore, the differences of measured temperatures considered between internal and external thermistors were about 1 /1.5 8C (including the water layer, all oxide films and Incoloy wall tube). These measurements were done each time that an oxide layer was removed by means of a cleaning procedure or decontamination process (this is
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
5
Fig. 4. Experimental design: Incoloy tube assembled with heater and thermistors for measurements (not in scale). (1) Stainless steel tube; (2) distilled water; (3) Incoloy tube; (4) thermistors; (5) heater.
carried out when the surface has radioactive deposits). An important requirement of this assembled device is that after each cleaning procedure all the elements are returned to the same relative position.
3. Calibration apparatus Before the installation in the measurement circuit, thermistors were calibrated in a thermostatic unit specially manufactured for this purpose (Fig. 6). It consists of an aluminium body with six holes for the insertion of the thermistors and a reference calibrated platinum resistance. A resistor wire heated the whole system and the temperature was controlled by a glass encapsu-
lated thermistor inserted in one of the aluminium body’s holes. During the calibration all the assembly (aluminium body and thermistors) was thermally insulated from the environment within a thermal dewar vessel. The thermistors electrical resistances provided the temperatures values for a given electrical applied power to the heating element. When the system has reached the steady state (which took / 10 h) the fluctuations of the temperature were 9/ 0.01 8C. After the calibration procedure each thermistor showed a resistance-temperature relation that responds to the following expression (Hoge, 1988) ( the coefficients were obtained from the best least square of equation)
6
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
3) The tube after removing the primary side outer layer, i.e. removing R 2 thermal resistance. 4) The tube after removing the secondary side inner layer, i.e. removing R 3 thermal resistance. 5) The tube after removing the primary side inner layer, i.e. removing R 4 thermal resistance.
Fig. 5. Complete equipment in the thermal jacket.
1=T A1 A2 ln Re A3 (ln Re )2 A4 (ln Re )3 where T is in Kelvin (K). Typical values for these coefficients are: A1, 1.0925 /103; A2, 7.99475/104; A3, / 1.07746 /104; A4, 9.31473 /10 6; correlation coefficients R2 /0.999998; and sensitivity, 0.01 V represents 0.001 8C.
4. Chemical cleaning and decontamination As pointed out in the introduction the tube showed four oxide layers, two on the internal wall (primary side) and two on the external wall (secondary side). In order to identify the oxide layer effects on the thermal resistances the measurements at the different applied powers were carried out in five different steps as follows: 1) The tube as received from the nuclear power plant. 2) The tube after removing the secondary side outer layer, i.e. removing R 1 thermal resistance.
The external wall outer layer and internal wall outer layer (mainly iron oxides) were removed by a chemical method based on an oxalic acid solution, 0.1 M, pH 3.5 adjusted with NH4OH, temperature /90 8C during 6 h (Blesa et al., 1987). The external wall inner layer and internal wall inner layer were removed by an electrochemical method with a citric acid 0.1 M, pH 3.5 adjusted with NH4OH, at room temperature, controlled by a potenciostat /galvanostat PAR model 273 in a galvanostatic mode, applying a current density of 10 mA/cm2. After each layer removal the tube was cleaned with distilled water and the attached solid particles were removed with the help of an ultrasonic bath. The suspension was filtered and the residual was analysed by EDAX in a Phillips scanning microscope (Table 1). The solutions originated in the chemical and electrochemical cleaning processes were also dried and the solids composition was determined by the same technique. The layer thicknesses were estimated by the weight loss, considering a density of 5.2 g/cm3 for the magnetite and chromium oxides. These results are also shown in Table 1.
5. Mathematical treatment Using the equation for heat transfer in cylindrical geometries (Gebhart, 1971) DoLp=Q 1=U1=(Ti To )
(1)
1/U can be expressed as the sum of the heat transfer resistances: 1=U R1R2R3R4RIncoloy Rwater
(2)
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
7
Fig. 6. Top view of thermostatic unit and a scheme of electronic circuit used for thermistors calibration.
where R 1 is the heat transfer resistance of the secondary side outer layer; R 2 is the heat transfer resistance of the primary side outer layer; R 3 is the
heat transfer resistance of the secondary side inner layer; R 4 is the heat transfer resistance of the primary side inner layer. RIncoloy is the heat
Table 1 Oxide layer thicknesses and compositions Oxides removing step
Chemical external cleaning Chemical internal decontam. Electrochem. external cleaning Electrochem. internal decontam.
Average thickness (mm)
25 7 0.9 0.7
Composition (%) Fe
Cr
Ni
Cu
Zn
Mn
Ca
P
75 83 3 9
0 1.77 58 65
0 8.26 12 19
10 0 25 0
8 0 2 0
2 4 0 4
5 0 0 2
0 0 0 1
8
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
transfer resistance of Incoloy 800 metal tube; and Rwater is the water heat transfer resistance for both sides of the Incoloy sample tube (it involves water natural convection phenomenon). With the measurements of P and DT done under different conditions (sample tube with all oxide layer and after each cleaning procedure) heat transfer resistances were found: (Area=P)I [R1R2R3R4RIncoloy Rwater ]1=DTI (Area=P)I [Rtot ]1=DTI
(3) (4)
Considering (area/P )I and DTI: measured data with all the oxide layers (Area =P)II [R2R3R4RIncoloy Rwater ]1=DTII (5) (Area=P)II [R?]1=DTII (6) Considering (area/P )II and DTII: measured data after the first chemical cleaning. Then from the Eqs. (4) and (6) taking into account the respective slopes it can be obtained, Rtot R?R1
(7)
In the same way after each cleaning procedure the other heat transfer resistances can be expressed by: Rj slope[(Area=P)j=(1=DT)j] slope[(Area=P)j 1=(1=DT)j 1]
Experimental values were represented in a graphic. Area/power as a function of 1/DT for each cleaning step (Fig. 7). The range of selected power to calculate the thermal resistances was 0/5 W, zone where the data A /P versus 1/DT shows a linear behaviour. Also a copper polished tube of similar dimensions was measured in the same way as the Incoloy tube to verify that the applied procedure was independent of the involved materials. The results are also shown in Fig. 7. Due to a problem in the regulation of the thermal bath it can be observed some data dispersion in some sets of measurements. This situation was detected with some delay and it was not possible to repeat the measurements because the oxide layer had been removed. Table 2 shows contributions to thermal resistance of each removed oxide layer. They can also be compared to the values found in open literature: 1.5 /10 3 /6.5 /10 4 m2 8C W 1 (Somerscales, 1999; Mikk et al., 1972).
(8)
where j /I,. . ., IV
6. Experimental procedure The heating element was electrically connected to a constant power supply. Current and voltage were measured with a Keithley multimeter (model 197 A) to obtain the applied power. Our measurements covered the range between 0 and 10 W. Temperature measurements of the glass-encapsulated thermistors were done with a Keithley voltmeter in a four-wire connection mode. The data were collected after a stabilisation period of about 12 h, when the temperature variations were lower than 0.01 8C, and a thermal steady state was assumed.
7. Results and conclusions Since we tried to keep the same device geometry at all moments for each of the measurements, we have supposed that for each given electrical power the natural convection phenomenon produced from the present water around the thermistors is the same in particular when the electrical power applied is low. It is important to emphasise that the accepted structure for an oxide, which is under hydrothermal conditions, is (Blesa et al., 1994)
So the measured thermal heat transfer resistances include all the associated phenomena with the hydratation of the oxides.
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
9
Fig. 7. Values of area/power vs. 1/DT for different conditions of the sample.
Table 2 Oxide layer thermal resistances Conditions
Equations correlation coeficient
Y/0.0082X/0.006, r2 /0.9921 [/ Tube without external oxide layer (secondary side) Y /0.0069X/0.0054, r2 /0.9715 [/ Tube without external oxide layer (primary side) Y /0.0063X/0.0056, r2 /0.9739 [/ Tube without internal oxide layer (secondary side) Y /0.006X/0.0052, r2 /0.9463 [/ Tube without internal oxide layer (primary side) Y /0.0058X/0.0054, r2 /0.9708 Incoloy polish tube Y/0.0058X/0.0056, r2 /0.9928 Copper sample tube Y/0.0044X/0.0044, r2 / 0.9977
Thermal resistance (calculated) (m2 8C W 1)
Tube with all oxide layers
Chemical external cleaning, R 1/0.0013
Chemical internal decontam., R 2/0.0006
Electrochem external cleaning, R 3/0.0003
Electrochem. internal decontam., R 4/0.0002
10
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
Along with this structure, in the case of iron oxides the presence of different water layers was observed: a first immobile layer physically absorbed and strongly attached to the hydroxilic layer; a second layer with certain mobility and subsequent ice-like formation layers (Mc Cafferty et al., 1970). The thicknesses of the primary and secondary side deposits are remarkably different, but its contribution to the heat transfer resistances is not so different. A four times value of oxide thickness only duplicates the heat transfer resistance value. This behaviour could be partially explained based on the presence of metallic copper in the secondary side deposits. Under the reductive conditions, which are present in the SG liquid phase, the copper, a product of brass condensers corrosion, appears as a deposit of metallic copper on the Incoloy tube oxides. All this process can happen according to the Pourbaix diagrams for high temperatures (Chen et al., 1983). Another significant point is that the inner oxide layers of both sides contribute with a non-negligible quantity to the total heat transfer resistance, this should be taken into account when the passivity procedure is designed. The R values obtained are not very different from the ones found in the bibliography for the same processes at high temperatures. Thus, this behaviour is similar to the one found for zirconium oxides, where thermal conductivity value found is kept nearly constant between room temperature and 650 8C (Garzarolli et al., 1982).
Appendix A: Nomenclature Do L P l Q Re Rj TI To
tube outside diameter (m) tube length (m) applied power (W) thermal conductivity (W m 1 8C 1) heat transfer (W) electric resistance (V) thermal resistance (m2 8C W1) measured inner temperature (8C) measured outer temperature (8C)
U
overall heat transfer coefficient (W m 2 8C 1)
References Alvarez, G., Olmedo, A., Villegas, M., 1996. Corrosion behaviour of Alloy 800 in high temperature aqueous solutions: long-term autoclave studies. J. Nuclear Mater. 229, 93 /101. Berge, Ph., Fiquet, J.M., 1993. The need for clean steam generators. Nucl. Energy 32 (2), 115 /120. Blesa, M.A, Marinovich, H.A., Baumgartner, E., et al., 1987. Mechanism of disolution of magnetite by oxalic acid-ferrous ion solution. Inorg. Chem. 26, 3713 /3717. Blesa, M.A., Morando, P.J., Regazzoni, A.E., 1994. Chemical Dissolution of Metal Oxides. CRC Press, Oxford. Chen, C.M., Aral, K., Theus, G.J., 1983. Computer-Calculated Potential pH-Diagrams to 300 8C Research Project, NP3137, Vol. 2, Electric Power Research Institute, California, USA. Garzarolli, F, Jung, W., Shoenfeld, H., 1982. Waterside Corrosion of Zircaloy Fuel Rods, NP-2789. Electric Power Research Institute, California, USA. Hoge, H.J., 1988. Useful procedure in least squares and tests of some equations thermistors. Rev. Sci. Instrum. 59 (6), 975 / 979. Gebhart, B., 1971. Heat Transfer. McGraw /Hill, Bombay New Delhi, pp. 460 /465. Kelen, T., Arvesen, J., 1972. Temperature increments from deposits on heat transfer surfaces. The thermal resistivity and thermal conductivity of deposits of magnetite, calcium hydroxy apatite, humus and copper oxides. AE-459/1972. Aktiebolaget Atomenergi, Studsvik. Lister, D.H., 1993. Activity transport and corrosion processes in PWR’s. Nucl. Energy 32 (2), 103 /114. Mc Cafferty, V., Pravdic, A.C., Zettlemayer, E., 1970. Dielectric behaviour of adsorbed water film on the Fe203 surface. Trans. Faraday Soc. 66, 1720 /1731. Mikk, I.R, Tiikma, T.B., Petrov, Yu.G., 1972. Determination of thermal resistance of an oxide film on heat transfer tubes in boilers. Teploenergetika 19 (8), 60 /61. Somerscales, E.F.C., 1999. Fundamentals of corrosion fouling. Br. Corros. J. 34 (2), 109 /124. Stellwag, B., 1998. The mechanism of oxide film formation on austenitic stainless steels in high temperature water. Corros. Sci., 40 (2/3), 337 /70. Warzee M., Sonnen C., Berge, P., 1967. The corrosion of carbon steels and stainless steels in pressurized water at high temperatures, EURAEC-1896, USAEC-Euratom, July. IAEA-TECDOC-996, Waterside corrosion of zirconium alloys in nuclear power plants, 1998.
Thermal resistance contributions of oxides growth on Incoloy 800 steam generator tubes A.M. Iglesias *, M.A. del C. Raffo Calderon Unidad de Actividad Quı´mica, Comisio´n Nacional de Energı´a Ato´mica, Av. del Libertador 8250, 1429 Buenos Aires, Argentina Received 6 June 2001; received in revised form 25 October 2001; accepted 17 July 2002
Abstract The primary and secondary circuits of nuclear power plants have different water chemistries, corrosion product sources, temperatures and flow rates. All these parameters promote the growth or deposition of oxides of different compositions and morphologies on the surfaces of the steam generator (S.G.) tubes. This paper presents a methodology to evaluate the relative contributions to the total heat transfer resistance due to the different oxide layers. The values obtained from a sample of Alloy 800 tube at room temperature are similar to those encountered in the open literature at high temperatures. Important future planning guidelines for decontamination and/or chemical cleaning of S.G. units can be obtained from these results. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Fouling; Steam generator; Incoloy 800; Thermal resistance
1. Introduction Unlike a fossil-fired thermal station, pressurised heavy water reactors (PHWR) have two coolant circuits. Primary heat transport system (PHTS), whose function is to remove the fission heat produced in the reactor core, and a secondary circuit which collects the energy from the PHTS to produce the steam that drives the turbines. Both heat transport circuits have different water chemistries and corrosion product sources. They are subjected to different flows, temperatures and velocities. Under these circumstances it is usual the
* Corresponding author. Tel.: /54-11-6772-7143; fax: /5411-6772-7121 E-mail address: [email protected] (A.M. Iglesias).
growth and/or deposition of oxides of different compositions and morphologies on the surfaces of the steam generator (S.G.) tubes. The level of corrosion products in both circuits is kept as low as possible with careful water chemistry control. Some corrosion products which come from carbon steel pipes and tanks and condenser tube Cu-alloy are transported and deposited on the S.G. tube walls because of temperature changes or temperature gradients which can affect in a significant way the solubility of these products. In conjunction with other compounds produced by in-situ corrosion, these deposits originate the so-called ‘corrosion fouling’ phenomenon. The evaluation of the heat transfer as a function of the oxide type has been the object of some
0029-5493/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 0 2 ) 0 0 2 5 6 - X
2
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
research efforts and it is not yet totally well known and standardised. The possibility of predicting the corrosion fouling effects depends on an understanding of the processes associated with the growth and removal of the corrosion products. Somerscales pointed out in 1999 that there is clear experimental evidence that the products of a corrosion process can present significant resistance to the thermal heat transfer. One of the consequences of this process is a progressive degradation of the heat transfer capability of the involved equipment. In the case of nuclear power plants an increase in the reactor inlet temperature (primary side) can be measured due to the insufficient heat transfer to the secondary side in the S.G. tubes. This situation could produce a different operating state of primary circuit with a loose in the efficiency, or even more, it could produce a non planned shut down. So in these cases periodical cleaning must be carried out in order to restore the design values (Berge and Fiquet, 1993). Consequently, to plan the cleaning strategy it is important to know these points: the contribution to the thermal resistance of each layer deposited or growth on the S.G. tubes (Kelen and Arvesen, 1972); how each oxide layer contributes to the total heat transfer resistance; and what side (primary or secondary) would produce more benefits in the case of taking a decision about the necessary cleaning (it is important to take into account that the primary side is contaminated and its cleaning implies a lot and careful work). A 30-mm film oxide thickness of zirconium alloy (fuel rods) shows thermal conductivity values between 0.8 and 2 W m1 K 1. These values are important if we consider that Zircaloy thermal conductivity is 22 W m 1 K 1. (IAEA, 1998). According to a work by Mikk et al., 1972, who evaluated the oxide fouling resistance as a difference between two different conditions of a tube (with oxides and clean), this paper presents in a similar way a technique that allows at room temperature the evaluation of the different thermal heat transfer resistances. Our measurements were carried out with the sample immersed in water. This possibility to work with water allows the
oxides to be in contact with the same medium in which they appear during the normal operation of S.G. tubes.
2. Sample of Alloy 800 2.1. Nuclear reactor CANDU 600 In the case of nuclear reactor CANDU 600 both circuits, primary and secondary, have a large amount of carbon steel surface exposed to the fluid that produces an inventory of iron corrosion products, and these contribute together with their own corrosion to the corrosion fouling phenomenon (Warzee et al., 1967). During the 1994 annual planned shutdown of Embalse N.P.P., a small sample of the steam generator tube was taken out from the hot leg of the steam generator no. 4. This portion of tube was located between the tube sheet and the second tube support plate. Different sections of this tube have been used to characterise in-situ the growth or deposit of oxides on the internal and external walls of the tube, and one section was used to carry out the thermal resistance measurements for this work. The removed tube was operating under steam generator thermalhydraulic conditions ( / 309 8C) during seven effective full power years. The thermal barriers already observed in steam generator tubes consist, mainly, of four oxide layers: two on the internal wall of the tubes (primary side) and two on the external wall of the tubes (secondary side) (Lister, 1993). Different analyses of Alloy 800 have determined that the oxides consist of two layers (spinel form). The inner layer is enriched in Cr, /50% more than in base metal and less Ni concentration, and the external layer consists of crystals with a concentration less than 5% Cr, /30% Ni and Fe balance. (Alvarez et al., 1996; Stellwag, 1998). The inner layer has a highly compact structure that impedes the progression of the corrosion process. The outer layers, which are mainly formed by magnetite, as a result of water reductive conditions in both systems, are less compact and
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
have a significant contribution to the heat transfer thermal resistance. Figs. 1 and 2 show microscope scanning photographs of the oxides on the tube surface. Fig. 3 shows a sketch of the various oxide layers. Each layer is like an equivalent serial resistance that contributes to the total heat transfer thermal resistance. 2.2. Experimental set-up The methodology consists of generating a constant radial heat flux which goes through a 1-mm water thickness, internal oxide layers, a 1.4 mm metallic Alloy 800 thickness, external oxide layers and a 1-mm water thickness (Fig. 4). Therefore, a steady-state with a determined temperature gradient that depends on the power
3
applied, the intermediate layers and the temperature of the final heat reservoir are generated. The heating element is built in Nichrome wire coil (diameter 0.36 mm) and encapsulated in a glass tube (diameter 4 mm, long 100 mm). It acts as the constant heat source. The total electrical resistance of this wire is approximately 10 V and it is powered by a home-made constant dc power supply. Four glass encapsulated thermistors (SiemensMatsushita model M85) were selected as temperature sensors because of their small dimensions (diameter 1 mm, 1.5 mm long) and sensitivity. They were located 1 mm away from the internal an external wall of the sample tube respectively and in a midway from Incoloy 800 tube. Two brass pieces, one at the top and the other at the bottom, which act as a support frame, main-
Fig. 1. Microscope scanning photographs. Transversal view with the different types of oxides.
4
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
Fig. 2. Microscope scanning photographs. Top view of the different oxides.
Fig. 3. Sketch of oxide layers on Incoloy tube (not in scale).
tain the sample tube (10 cm long), the thermistors and the heater in a fixed rigid position (Fig. 4). Thin glass tubes (capillaries of 1.5 mm external diameter, 70 mm long) fix the thermistors in each
position (from the brass top piece) with epoxyresin. A stainless steel tube (diameter 30 mm, thickness 1 mm) is located in a concentric way with the components mentioned before. The stainless steel tube, that surrounds the set up (as shown in Fig. 4) is used to obtain a homogeneous heat transfer. The assembled set was immersed in distilled water inside of a 30 mm inner diameter glass tube and a length of 250 mm. The whole device is maintained within a constant temperature thermal bath (Fig. 5). The power range applied is between 0 and 10 Watts. These values were selected because this power range allows working with a mensurable temperature difference compatible with low powers. Therefore, the differences of measured temperatures considered between internal and external thermistors were about 1 /1.5 8C (including the water layer, all oxide films and Incoloy wall tube). These measurements were done each time that an oxide layer was removed by means of a cleaning procedure or decontamination process (this is
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
5
Fig. 4. Experimental design: Incoloy tube assembled with heater and thermistors for measurements (not in scale). (1) Stainless steel tube; (2) distilled water; (3) Incoloy tube; (4) thermistors; (5) heater.
carried out when the surface has radioactive deposits). An important requirement of this assembled device is that after each cleaning procedure all the elements are returned to the same relative position.
3. Calibration apparatus Before the installation in the measurement circuit, thermistors were calibrated in a thermostatic unit specially manufactured for this purpose (Fig. 6). It consists of an aluminium body with six holes for the insertion of the thermistors and a reference calibrated platinum resistance. A resistor wire heated the whole system and the temperature was controlled by a glass encapsu-
lated thermistor inserted in one of the aluminium body’s holes. During the calibration all the assembly (aluminium body and thermistors) was thermally insulated from the environment within a thermal dewar vessel. The thermistors electrical resistances provided the temperatures values for a given electrical applied power to the heating element. When the system has reached the steady state (which took / 10 h) the fluctuations of the temperature were 9/ 0.01 8C. After the calibration procedure each thermistor showed a resistance-temperature relation that responds to the following expression (Hoge, 1988) ( the coefficients were obtained from the best least square of equation)
6
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3) The tube after removing the primary side outer layer, i.e. removing R 2 thermal resistance. 4) The tube after removing the secondary side inner layer, i.e. removing R 3 thermal resistance. 5) The tube after removing the primary side inner layer, i.e. removing R 4 thermal resistance.
Fig. 5. Complete equipment in the thermal jacket.
1=T A1 A2 ln Re A3 (ln Re )2 A4 (ln Re )3 where T is in Kelvin (K). Typical values for these coefficients are: A1, 1.0925 /103; A2, 7.99475/104; A3, / 1.07746 /104; A4, 9.31473 /10 6; correlation coefficients R2 /0.999998; and sensitivity, 0.01 V represents 0.001 8C.
4. Chemical cleaning and decontamination As pointed out in the introduction the tube showed four oxide layers, two on the internal wall (primary side) and two on the external wall (secondary side). In order to identify the oxide layer effects on the thermal resistances the measurements at the different applied powers were carried out in five different steps as follows: 1) The tube as received from the nuclear power plant. 2) The tube after removing the secondary side outer layer, i.e. removing R 1 thermal resistance.
The external wall outer layer and internal wall outer layer (mainly iron oxides) were removed by a chemical method based on an oxalic acid solution, 0.1 M, pH 3.5 adjusted with NH4OH, temperature /90 8C during 6 h (Blesa et al., 1987). The external wall inner layer and internal wall inner layer were removed by an electrochemical method with a citric acid 0.1 M, pH 3.5 adjusted with NH4OH, at room temperature, controlled by a potenciostat /galvanostat PAR model 273 in a galvanostatic mode, applying a current density of 10 mA/cm2. After each layer removal the tube was cleaned with distilled water and the attached solid particles were removed with the help of an ultrasonic bath. The suspension was filtered and the residual was analysed by EDAX in a Phillips scanning microscope (Table 1). The solutions originated in the chemical and electrochemical cleaning processes were also dried and the solids composition was determined by the same technique. The layer thicknesses were estimated by the weight loss, considering a density of 5.2 g/cm3 for the magnetite and chromium oxides. These results are also shown in Table 1.
5. Mathematical treatment Using the equation for heat transfer in cylindrical geometries (Gebhart, 1971) DoLp=Q 1=U1=(Ti To )
(1)
1/U can be expressed as the sum of the heat transfer resistances: 1=U R1R2R3R4RIncoloy Rwater
(2)
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
7
Fig. 6. Top view of thermostatic unit and a scheme of electronic circuit used for thermistors calibration.
where R 1 is the heat transfer resistance of the secondary side outer layer; R 2 is the heat transfer resistance of the primary side outer layer; R 3 is the
heat transfer resistance of the secondary side inner layer; R 4 is the heat transfer resistance of the primary side inner layer. RIncoloy is the heat
Table 1 Oxide layer thicknesses and compositions Oxides removing step
Chemical external cleaning Chemical internal decontam. Electrochem. external cleaning Electrochem. internal decontam.
Average thickness (mm)
25 7 0.9 0.7
Composition (%) Fe
Cr
Ni
Cu
Zn
Mn
Ca
P
75 83 3 9
0 1.77 58 65
0 8.26 12 19
10 0 25 0
8 0 2 0
2 4 0 4
5 0 0 2
0 0 0 1
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transfer resistance of Incoloy 800 metal tube; and Rwater is the water heat transfer resistance for both sides of the Incoloy sample tube (it involves water natural convection phenomenon). With the measurements of P and DT done under different conditions (sample tube with all oxide layer and after each cleaning procedure) heat transfer resistances were found: (Area=P)I [R1R2R3R4RIncoloy Rwater ]1=DTI (Area=P)I [Rtot ]1=DTI
(3) (4)
Considering (area/P )I and DTI: measured data with all the oxide layers (Area =P)II [R2R3R4RIncoloy Rwater ]1=DTII (5) (Area=P)II [R?]1=DTII (6) Considering (area/P )II and DTII: measured data after the first chemical cleaning. Then from the Eqs. (4) and (6) taking into account the respective slopes it can be obtained, Rtot R?R1
(7)
In the same way after each cleaning procedure the other heat transfer resistances can be expressed by: Rj slope[(Area=P)j=(1=DT)j] slope[(Area=P)j 1=(1=DT)j 1]
Experimental values were represented in a graphic. Area/power as a function of 1/DT for each cleaning step (Fig. 7). The range of selected power to calculate the thermal resistances was 0/5 W, zone where the data A /P versus 1/DT shows a linear behaviour. Also a copper polished tube of similar dimensions was measured in the same way as the Incoloy tube to verify that the applied procedure was independent of the involved materials. The results are also shown in Fig. 7. Due to a problem in the regulation of the thermal bath it can be observed some data dispersion in some sets of measurements. This situation was detected with some delay and it was not possible to repeat the measurements because the oxide layer had been removed. Table 2 shows contributions to thermal resistance of each removed oxide layer. They can also be compared to the values found in open literature: 1.5 /10 3 /6.5 /10 4 m2 8C W 1 (Somerscales, 1999; Mikk et al., 1972).
(8)
where j /I,. . ., IV
6. Experimental procedure The heating element was electrically connected to a constant power supply. Current and voltage were measured with a Keithley multimeter (model 197 A) to obtain the applied power. Our measurements covered the range between 0 and 10 W. Temperature measurements of the glass-encapsulated thermistors were done with a Keithley voltmeter in a four-wire connection mode. The data were collected after a stabilisation period of about 12 h, when the temperature variations were lower than 0.01 8C, and a thermal steady state was assumed.
7. Results and conclusions Since we tried to keep the same device geometry at all moments for each of the measurements, we have supposed that for each given electrical power the natural convection phenomenon produced from the present water around the thermistors is the same in particular when the electrical power applied is low. It is important to emphasise that the accepted structure for an oxide, which is under hydrothermal conditions, is (Blesa et al., 1994)
So the measured thermal heat transfer resistances include all the associated phenomena with the hydratation of the oxides.
A.M. Iglesias, M.A.d.C. Raffo Calderon / Nuclear Engineering and Design 219 (2003) 1 /10
9
Fig. 7. Values of area/power vs. 1/DT for different conditions of the sample.
Table 2 Oxide layer thermal resistances Conditions
Equations correlation coeficient
Y/0.0082X/0.006, r2 /0.9921 [/ Tube without external oxide layer (secondary side) Y /0.0069X/0.0054, r2 /0.9715 [/ Tube without external oxide layer (primary side) Y /0.0063X/0.0056, r2 /0.9739 [/ Tube without internal oxide layer (secondary side) Y /0.006X/0.0052, r2 /0.9463 [/ Tube without internal oxide layer (primary side) Y /0.0058X/0.0054, r2 /0.9708 Incoloy polish tube Y/0.0058X/0.0056, r2 /0.9928 Copper sample tube Y/0.0044X/0.0044, r2 / 0.9977
Thermal resistance (calculated) (m2 8C W 1)
Tube with all oxide layers
Chemical external cleaning, R 1/0.0013
Chemical internal decontam., R 2/0.0006
Electrochem external cleaning, R 3/0.0003
Electrochem. internal decontam., R 4/0.0002
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Along with this structure, in the case of iron oxides the presence of different water layers was observed: a first immobile layer physically absorbed and strongly attached to the hydroxilic layer; a second layer with certain mobility and subsequent ice-like formation layers (Mc Cafferty et al., 1970). The thicknesses of the primary and secondary side deposits are remarkably different, but its contribution to the heat transfer resistances is not so different. A four times value of oxide thickness only duplicates the heat transfer resistance value. This behaviour could be partially explained based on the presence of metallic copper in the secondary side deposits. Under the reductive conditions, which are present in the SG liquid phase, the copper, a product of brass condensers corrosion, appears as a deposit of metallic copper on the Incoloy tube oxides. All this process can happen according to the Pourbaix diagrams for high temperatures (Chen et al., 1983). Another significant point is that the inner oxide layers of both sides contribute with a non-negligible quantity to the total heat transfer resistance, this should be taken into account when the passivity procedure is designed. The R values obtained are not very different from the ones found in the bibliography for the same processes at high temperatures. Thus, this behaviour is similar to the one found for zirconium oxides, where thermal conductivity value found is kept nearly constant between room temperature and 650 8C (Garzarolli et al., 1982).
Appendix A: Nomenclature Do L P l Q Re Rj TI To
tube outside diameter (m) tube length (m) applied power (W) thermal conductivity (W m 1 8C 1) heat transfer (W) electric resistance (V) thermal resistance (m2 8C W1) measured inner temperature (8C) measured outer temperature (8C)
U
overall heat transfer coefficient (W m 2 8C 1)
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