Conference Paper - International Nuclear Information System ...

Conference Paper

Atomic Energy ofCanada Limited

Chalk River, OntarioCanada K0J 1J0

Énergie Atomique duCanada Limitée

Chalk River (Ontario)Canada K0J 1J0

Chi Lisheng - Research Chemist

Prepared byRédigé par

Turner Carl - Senior Scientist

Semmler Jaleh - Sr Chemist

Reviewed byVérifié par

Angell Peter - Manager -Component Life TechnologyBranch

Approved byApprouvé par

OXYGEN CONSUMPTION BYHYDRAZINE IN LONGSAMPLE LINES

COMPANY WIDE

CW-127410-CONF-001

Revision 0

2012/07/20UNRESTRICTED

2012/07/20ILLIMITÉ

UNRESTRICTED - 1 - CW-127410-CONF-001 Rev. 0

OXYGEN CONSUMPTION BY HYDRAZINE IN LONG SAMPLE LINES

Lisheng Chi (Atomic Energy of Canada Limited), Canada (chil@aecl.ca) Carl W. Turner (Atomic Energy of Canada Limited), Canada (turnerc@aecl.ca)

ABSTRACT In nuclear power plants secondary side system dissolved oxygen concentration is a strictly controlled chemistry parameter intended to minimize corrosion and fouling of steam cycle components. Low dissolved oxygen concentration is maintained by a combination of mechanical deaeration and chemical reaction. The dissolved oxygen concentration in feedwater is monitored by sampling systems to ensure it remains within station specification during operation. The sample lines in a nuclear power plant’s sampling system can be from 5 to nearly 200 meters in length, resulting in sample residence times between the take-off point to the analyzer from a few seconds to several minutes, depending on the flow rate and the length of the sample line. For many chemical parameters the residence time is of no concern. For measurements of dissolved oxygen and hydrazine in the secondary coolant, however, for residence times longer than one minute, it is uncertain whether the sample is representative of conditions in the secondary coolant, especially for samples taken from locations where the temperature is well over 100ºC. To address this concern, a series of tests were conducted under both warm-up and power operation conditions, respectively, to investigate the effect of temperature, residence time, sample line length, surface area, hydrazine-to-oxygen ratio, and the concentrations of dissolved oxygen and hydrazine on the consumption of oxygen by hydrazine. The test results revealed that dissolved oxygen measurements in CANDU plants are underestimated to various degrees, depending on the sampling system operating conditions. Two distinct types of behaviours are observed for the oxygen removal rate: 1) the percentage removal of dissolved oxygen is invariant with time during the tests, and increases with increasing residence time in the test section, when the reaction between hydrazine and oxygen is better described by a homogenous reaction mechanism, and 2) the percentage oxygen removal decreases with time, tending towards an asymptotic value when the reaction between hydrazine and oxygen is better described by a heterogeneous reaction mechanism. The rate constants are calculated based on two different reaction mechanisms.

1. INTRODUCTION

In the secondary side system of nuclear power plants (NPPs), dissolved oxygen concentration has been a strictly controlled chemistry parameter because the transport of oxygen to the steam generator (SG) via the feedwater system increases the electrochemical corrosion potential (ECP) and crack growth rate of the SG tubes [1]. For example, when hydrogenated water was oxygenated to contain 2,000 µg/kg dissolved oxygen, the ECP of cold-worked Alloy 600 SG tube increased from -550 mV (vs SHE) to 200 mV (vs SHE), leading to an increase in crack growth rate from 3 × 10

-8 mm/s to 2 × 10

-7 mm/s [2]. Even relatively small increases in dissolved oxygen

have been reported to result in a significant increase in the ECP of SG tubes; the ECP of Alloy 600 at 220ºC was reported to increase by 220 mV when the dissolved oxygen concentration in the condensate increased from 7 µg/kg to 12 µg/kg at a hydrazine concentration of 30 µg/kg [3]. As a result, NPP operators limit air leakage to the condensate, add hydrazine to the feedwater, and employ a direct-contact steam-heating deaerator, in an effort to maintain dissolved oxygen at low concentrations in the SGs during start up and subsequent operation. On the other hand, when oxygen concentration is reduced to less than 1 µg/kg, flow accelerated corrosion (FAC) may become significant. Experimentally, it has been shown that the FAC rate was significantly increased when the dissolved oxygen concentration was less than 0.6 µg/kg [4]. The oxygen concentration at various locations of the secondary side system is continuously monitored to ensure it remains within the station specifications during operation. Figure 1 shows a typical sampling configuration for feedwater dissolved oxygen in a NPP. The purpose of the feedwater sampling system is to provide cooled and depressurized feedwater to the oxygen analyser and grab sampling. It is shown that a temperature

gradient (T/L) exists between the take-off point and the sample cooler. There is uncertainty regarding how much oxygen is consumed by hydrazine in the sample line. In CANDU®

1 plants, two locations for which accurate

dissolved oxygen measurements are required are: 1) the outlet of the deaerator storage tank during warm-up, and 2) the outlet of the high pressure (HP) heater during power operation. Nominal temperatures at the outlet of the deaerator storage tank and HP heater are 120 and 180ºC, respectively. Both these temperatures are high enough to result in measurable reaction between hydrazine and oxygen in the sample line before reaching the cooler [5]. As a result, the concentration of dissolved oxygen in the feedwater will likely be underestimated to some degree, depending on the length of the sample line, the flow rate, the concentrations of hydrazine and oxygen, the temperature and, possibly, the surface area of the sample line. Therefore, plant operators need to know by how much the dissolved oxygen concentration is underestimated by the dissolved oxygen measurements so that oxygen control can be modified accordingly to mitigate the risk of SG tube degradation. This paper presents the test results obtained using 304 stainless steel (SS) sample lines under the feedwater sample line conditions simulating the deaerator storage tank outlet during warm-up and the HP heater outlet during power operation. A discussion of the parameters likely affecting the rate of the reaction between oxygen and hydrazine in the feedwater is provided.

1 CANDU

® is a registered trademark of Atomic Energy of Canada Limited.

Feedwater

Sample

Cooler

Oxygen

Analyzer

Sample Line

<60oCL

Figure 1 Schematic of a Typical Feedwater Sample Line Configuration for Oxygen Sampling in NPPs

2. EXPERIMENTAL All the tests were performed in a high temperature recirculating (HTR) loop (Figure 2). The loop consists of chemical injection system, main pump, heater, test section, sampling system and heat exchangers. The loop is constructed of 304 SS and has a design temperature of 310ºC and a design pressure of 10.3 MPa. The loop flow rate is controlled using a variable speed centrifugal pump and was measured using an orifice plate with a hole diameter of 3 mm. The loop chemistry is controlled using a feed-and-bleed system. For this investigation, the feed from the make-up tank was injected at the inlet to the test section, whereas the bleed was normally taken from upstream of the pump and switched to either upstream or downstream of the test section during sampling to determine changes in the concentrations of dissolved oxygen and hydrazine. The test section consisted of multiple 6.1 m lengths of 304 SS sample line tubing with 0.95 cm outer diameter and 0.12 cm wall thickness connected in series. The entire 54.9 m test section was conditioned at 120ºC with feedwater with a specified concentration of dissolved oxygen during commissioning of the loop. The residence time of water in the test section ranged from 19 to 242 seconds, depending on the length of the test section and the loop flow rate. Prior to the start of each test, the concentrations of hydrazine and oxygen in the loop were each less than a few parts per billion (i.e., µg/kg). The test solutions were prepared using deionized (DI) water in a 200 L make-up tank. Morpholine was used to set the pH of the test solution at 9.5 at 25°C during the tests. The concentrated (64 to 65 (wt/wt)%) hydrazine monohydrate was directly added to the tank to prepare the desired test solution. The solution in the make-up tank was continuously purged using an Ar-O2 mixture to obtain a concentration that, with dilution, would achieve the target value of dissolved oxygen concentration at the inlet to the loop test section. The tests were performed with two lengths (12.2 and 54.9 m) at three flow rates (0.5, 1.0, and 1.35 kg/min) and at temperatures representative of the outlet of the deaerator storage tank during warm-up (e.g., 120ºC) and the outlet of the HP heater during power operation (e.g., 180ºC).

Section

Loop Heat Exchangers

Heater

Takeoff

Takeoff Point

Orifice Plate

Injection System

Make-up

Injection

On-Line AnalyzerHeat Exchanger

Grab Sample

and Drain

<30 oC

Sample System

Figure 2 Schematic Diagram of the HTR Loop

Once the loop had reached steady state with respect to the desired temperature and loop mass flow rate, the test solution was injected into the loop with continuous feed and bleed for chemistry control. Throughout this paper, the flow rate stated for each test is the loop flow rate. The residence time in the test section was calculated using the test section flow rate, which is the sum of the loop flow rate plus the injection flow rate. Each test was performed for two to seven hours. The loop was sampled at the inlet and outlet of the test section at regular intervals during each test while the loop chemistry was tending towards steady state in the long tests (greater than 4 hours), thus providing data on the extent of the reaction between oxygen and hydrazine on a single pass through the test section over a range of hydrazine and oxygen concentrations during each test. The residence time in the loop sample line between the take-off point and Orbisphere is less than one second; therefore, the oxygen loss in the sample line in the loop can be ignored, compared to loss that may occur in the test section. Figure 3 shows some of the loop operating parameters along with dissolved oxygen concentration monitored for a typical test. The loop flow rate and the temperature measured at the inlet and outlet of the test section were very steady (the changes were no more than 1.5%) throughout the test. The temperature at the inlet to the test section is taken as the test temperature. The dissolved oxygen concentration in the loop was measured through the bleed upstream of the main pump and increased steadily throughout each test. The periodic “jumps” observed in the dissolved oxygen concentration (Figure 3) were due to switching the oxygen analyser (Figure 2) to the inlet and the outlet of the test section.

-0.1 0.9 1.9 2.9 3.9 4.9 5.9 6.9

re (ºC

Elapsed Time (h)

Test Section Outlet Temperature Test Section Inlet TemperatureDissolved [O2] in loop and at Test Section Inlet_Outlet Dissolved [O2] in the Make-up TankHTR Loop Mass Flow Rate

Figure 3 Monitoring of the Loop Operation Conditions and Dissolved Oxygen

Grab samples for hydrazine analysis were taken downstream of the oxygen analyser, which ensured that: 1) taking the grab samples did not affect the oxygen measurement, and 2) the concentrations of hydrazine and oxygen were measured at the same time. The concentrations of hydrazine and oxygen in the loop varied with time during each test. The concentrations of both hydrazine and dissolved oxygen at the outlet were corrected to the inlet sampling time to account for the 5 to 8 minutes lapsed between collecting the inlet and outlet samples. Grab samples were collected every hour at the inlet and the outlet of the test section to measure hydrazine concentrations while concentrations of dissolved oxygen at the inlet and the outlet were measured by on-line instrumentation. Hydrazine concentration was measured using the CHEMetrics V-2000 photometer with a detection limit of 1 µg/kg and an accuracy of ± 10%. The performance of the oxygen Orbisphere, with a membrane without a protection cap, was determined in DI water at various flow rates and found to provide high precision (± 1%) at flow rates lower than those recommended (180 mL/min) by the manufacturer.

3. RESULTS 3.1. Dissolved Oxygen Removed under the Simulated Feedwater Sample Line Conditions at the Deaerator Storage Tank Outlet during Warm-up Figure 4 presents the concentrations of hydrazine and dissolved oxygen at the inlet and the outlet of the 54.9 m sample line test section as a function of time during a test with a [N2H4]/[O2] ratio of 1.5 in the make-up tank conducted at 120ºC, representative of sample line conditions at the deaerator storage tank outlet during warm-up. The pH25ºC of the test section ranged from 9.45 to 9.55 during the test. It is calculated that the significant fraction (i.e., > 70%) of hydrazine is ionized under the test conditions (i.e., at temperatures ranging from 120 to 180ºC). The concentration of hydrazine at the inlet of the test section increased steadily with time during the test. The concentration is expected to reach an asymptotic value within about 6 hours, consistent with a feed-and-bleed rate of 125 mL/min and an estimated loop volume of 10 L. In contrast, the concentration of dissolved oxygen remained relatively invariant with time. The concentrations of both hydrazine and dissolved oxygen at the outlet were lower than those at the inlet, indicating that some of the oxygen had reacted with hydrazine in the sample line test section. Figure 4 also shows that the amount of oxygen that was removed by reaction with hydrazine in the test section decreased with time. This result suggests that the percentage removal of dissolved oxygen may be decreasing as a result of increasing concentrations of hydrazine and dissolved oxygen. When the [N2H4]/[O2] ratio in the make-up tank was 1.125, the concentration of dissolved oxygen at the inlet of the test section increased faster than that of hydrazine for this test, which caused the ratio of hydrazine to oxygen to decrease with time as observed in Figure 4 (right).

0 50 100 150 200 250 300 350 400 450

Time (minutes)

[N2H4]in

[N2H4]out

[O2]in

[O2]out

[N2H4]in/[O2]in

0 50 100 150 200 250 300 350 400 450

Time (minutes)

[N2H4]in

[N2H4]out

[O2]in

[O2]out

[N2H4]in/[O2]in

Figure 4 Variation in the Concentrations of Hydrazine and Dissolved Oxygen at the Inlet and the Outlet of the 54.9 m Sample Line Test Section and Their Ratios at the Inlet during the Tests with the [N2H4]/[O2] Ratio

of 1.5 (Left) and 1.125 (Right) in the Make-up Tank

Figure 5 and Figure 6 present the effects of temperature and flow rate on oxygen removed, respectively, for tests conducted under deaerator outlet conditions. For these tests, the hydrazine-to-oxygen ratio at the inlet to the test section ranged between approximately 5 and 10. Analysis of the experimental data (not shown in the Figures) indicates that the percentage removal of dissolved oxygen decreases with increasing concentrations of both hydrazine and dissolved oxygen when hydrazine is present in excess. The rate at which the percentage removal of dissolved oxygen decreases is initially relatively consistent from one test to another, and generally reaches an asymptotic value within 5 to 6 hours. Therefore, it is reasonable to use the dissolved oxygen removal measured in the last sample of each test for evaluating how much dissolved oxygen is underestimated in a feedwater sample line. At temperatures less than 120ºC, flow rates between 0.5 and 1.35 kg/min, and concentrations of hydrazine between 2.30 and 2.77 mg/kg and dissolved oxygen between 0.299 and 0.587 mg/kg, dissolved oxygen removal in the 54.9 m sample line test section was less than 30%. When a 12.2 m sample line test section was used, dissolved oxygen removal was reduced to less than 10% at concentrations of hydrazine between 3.26 and 3.76 mg/kg and oxygen between 0.329 and 0.576 mg/kg.

0 50 100 150 200 250 300 350 400 450

Time (minutes)

0 50 100 150 200 250 300 350 400 450

Time (minutes)

0.5 kg/min

1.0 kg/min

1.35 kg/min

Figure 5 Dissolved Oxygen Removed in the 54.9 m Sample Line Test Section at Various Temperatures (Left) and at Various Flow Rates (Right) under the Feedwater Sample Line Conditions at the

Deaerator Storage Tank Outlet during Warm-up

0 50 100 150 200 250 300 350 400 450

Time (minutes)

0 50 100 150 200 250 300 350 400 450

Time (minutes)

0.5 kg/min

0.8 kg/min

1.0 kg/min

1.35 kg/min

Figure 6 Dissolved Oxygen Removed in the 12.2 m Sample Line Test Section at Various Temperatures (Left) and at Various Flow Rates (Right) under the Feedwater Sample Line Conditions at the

Deaerator Storage Tank Outlet during Warm-up

3.2. Dissolved Oxygen Removed under the Simulated Feedwater Sample Line Conditions at the High Pressure Heater Outlet during Power Operation Figure 7 shows the variation in concentrations of both hydrazine and oxygen at the inlet and the outlet of the sample line test section and the ratio of hydrazine to oxygen at the inlet for a test conducted at 180ºC under the HP heater outlet feedwater sample line conditions during power operation. The hydrazine concentration increased steadily during the test whereas the dissolved oxygen concentration changed very little throughout the test. The percentage removal of dissolved oxygen in the test section ranged from 73 to 75%, and did not decrease with increasing concentration of hydrazine, as it did for the tests performed under deaerator storage tank outlet conditions.

0 50 100 150 200 250 300

Time (minutes)

[N2H4]in

[N2H4]out

[O2]in

[O2]out

Figure 7 Concentrations of Hydrazine and Dissolved Oxygen at the Inlet and Outlet of the 54.9 m Sample Line Test Section and the Ratio of Hydrazine to Oxygen at the Inlet during the Test conducted at 180ºC and

at 1 kg/min

Figure 8 and Figure 9 present the effects of temperature and flow rate on oxygen removal under HP heater outlet power conditions for 54.9 and 12.2 m test sections, respectively. Figure 8 shows that for the 54.9 m test section, the percentage removal of dissolved oxygen increased with increasing test temperature (left) and with decreasing flow rate (right). In addition, the percentage oxygen removal did not decrease with time during the tests conducted

at 180C. At 150ºC, however, the percentage oxygen removal decreased with time, as was observed for tests conducted under deaerator outlet conditions. For the 12.2 m test section, the percentage removal of dissolved oxygen decreased with time with the exception of the test performed at 180ºC and a flow rate of 1.35 kg/min. The

decline in the percentage removal of dissolved oxygen was less, however, than for corresponding tests under deaerator storage tank outlet conditions during warm-up. With the exception of the test performed at 0.5 kg/min and 180ºC (Figure 9, right), the trends in percentage oxygen removal with temperature and flow rate are the same with both the 12.2 m and 54.9 m test sections, At temperatures between 150 and 180ºC and flow rates between 0.5 and 1.35 kg/min, dissolved oxygen removal was 49 to 89% in the 54.9 m sample line test section at concentrations of hydrazine between 0.27 and 0.31 mg/kg and oxygen between 0.017 and 0.047 mg/kg and was less than 31% in the 12.2 m sample line test section at concentrations of hydrazine between 0.36 and 0.46 mg/kg and oxygen between 0.018 and 0.047 mg/kg.

0 50 100 150 200 250 300

Time (minutes)

0 50 100 150 200 250 300

Time (minutes)

0.5 kg/min

1.0 kg/min

1.35 kg/min

Figure 8 Dissolved Oxygen Removed in the 54.9 m Sample Line Test Section with Time at Various Temperatures at 1 kg/min (Left) and at Various Flow Rates at 180ºC (Right)

0 50 100 150 200 250 300 350 400 450

Time (minutes)

0 50 100 150 200 250 300 350 400 450

Time (minutes)

0.5 kg/min

1.0 kg/min

1.35 kg/min

Figure 9 Dissolved Oxygen Removed in the 12.2 m Sample Line Test Section with Time at Various Temperatures at 1 kg/min (Left) and at Various Flow Rates at 180ºC (Right)

4. DISCUSSION This investigation of oxygen removal by reaction with hydrazine under sample line conditions at the outlet of the deaerator storage tank on a CANDU reactor under warm-up conditions and at the outlet of the HP heater during power operation has identified several useful trends and insights for operators at nuclear power stations who may be concerned about how representative their water samples are for measurements of dissolved oxygen. Two distinct types of behaviours were observed for the oxygen removal rate. One type of behaviour (called Type I in this discussion) is exemplified by measurements made at 180ºC for tests using the 54.9 m test section. In Type I behaviour, the percentage removal of dissolved oxygen is invariant with time during the test, and increases with increasing temperature and decreasing flow rate, the latter corresponding to increasing residence time in the test section. The other type of behaviour, called Type II behaviour in this paper, is exemplified by tests done under deaerator storage tank outlet conditions. Type II behaviour is typified by increasing oxygen removal with increasing

temperature and a decreasing value of the percentage oxygen removal versus time, tending towards an asymptotic value with a time constant that appears to coincide with the time constant for the loop reaching steady-state chemistry conditions. In other words, the decrease in the percentage removal of dissolved oxygen by reaction with hydrazine in the test section correlates with the increase in the value of the inlet concentration of hydrazine to the test section. Type I behaviour was only observed for tests performed at 180ºC in the 54.9 m test section, whereas Type II behaviour was observed for tests done in the 54.9 m test section for hydrazine concentrations ≥ 1 mg/kg and/or temperatures ≤ 150ºC, and in all tests performed with the 12.2 m test section. It is suggested here that Type I behaviour is observed when the reaction between hydrazine and oxygen is dominated by a homogenous reaction mechanism, while Type II behaviour is observed when the reaction between hydrazine and oxygen is dominated by a heterogeneous reaction mechanism. Both the homogenous and heterogeneous reactions are thermally activated, so the rate of reaction by either reaction mechanism will increase with increasing temperature, as observed. The extent of reaction by the homogenous mechanism, however, should increase with increasing residence time in the test section and be independent of surface area, while the extent of the heterogeneous reaction should increase with increasing surface area and be independent of the residence time. The decrease in percentage removal of oxygen with increasing concentration of hydrazine for the Type II reaction is consistent with the reaction taking place on a surface, i.e., a heterogeneous reaction, as follows (Equation 1, [7]).

𝑵𝟐𝑯𝟒 + 𝑶𝟐 → [𝑵𝟐𝑯𝟒]𝒂𝒅𝒔 + [𝑶𝟐]𝒂𝒅𝒔 → 𝑵𝟐 + 𝟐𝑯𝟐𝑶 Equation 1

Being a polar molecule, hydrazine will likely have a higher affinity than oxygen for adsorption onto the surface of the test section. At sufficiently high concentrations of hydrazine, hydrazine may take up the majority of the available reaction sites on the surface, leaving relatively few sites for adsorption of dissolved oxygen molecules. This will be especially so when hydrazine is present in excess compared to dissolved oxygen, as it was for the test illustrated in Figure 4 (left) where the ratio of hydrazine to dissolved oxygen at the inlet to the test section ranged from 5.7 to 7.5. When the situation is reversed, and the concentration of dissolved oxygen is in excess compared to hydrazine at the inlet to the test section, the percentage removal of oxygen through the test section does not decrease with time, as illustrated by the data shown in Figure 4 (right) where the ratio of hydrazine to dissolved oxygen at the inlet is less than 1 for most of the test. The continuity equation (Equation 2) is used to model the removal of oxygen by reaction with hydrazine in a sample line at constant velocity to develop the equations for calculation of rate constant.

Equation 2

where U is fluid velocity. Under the steady state, the time rate of change of dissolved oxygen concentration in a unit volume is zero; therefore, one-dimensional divergence of the flux of oxygen entering and leaving the unit (i.e., UdC/dx) is equal to the oxygen removed by chemical reaction. For a homogeneous reaction mechanism, the Dalgaard equation [6] can be used to calculate the rate of oxygen removal per unit volume. Substituting the Dalgaard equation into the continuity equation, using U = dx/dt, and rearranging the equation obtains

Equation 3

Intergrating Equation 3 between t = 0, [O2]in to tresidence, [O2]out gives Equation 4 for calculating rate constant of a homogeneous reaction.

Equation 4

For a heterogeneous reaction catalyzed by the wall surface of the sample line, it is proposed that the oxygen removal rate is proportional to the rate at which the wall surface area (A) is swept by the flowing

solution (i.e., U*π*D). Because hydrazine was in excess in the tests conducted, i.e., [N2H4]/[O2] 4.7, its concentration did not change significantly in the test section during a given test and, therefore, was assumed as a first approximation not to affect the reaction kinetics. Assuming that the reaction order for oxygen is one, the heterogeneous reaction rate at the wall is expressed in Equation 5.

Equation 5

where πDU is the rate at which the wall surface is being swept by a fluid flowing at constant velocity U along a tube of diameter D and [O2]wall is concentration of dissolved oxygen at the wall. Because the hydrazine-oxygen reaction is rather low and the fluid is turbulent under the conditions investigated, it is reasonable to assume that [O2]wall ≈ [O2]bulk. Combining the continuity equation and Equation 5 and integrating the equation, between x = 0, [O2]in and x = l, [O2]out, and π*D*l = A, obtains Equation 6 for calculating rate constant of a heterogeneous reaction.

Equation 6

Therefore, for the homogenous reaction, ln([O2]out/[O2]in) should vary linearly with residence time in the test section, while for the heterogeneous reaction ln([O2]out/[O2]in) should vary linearly with the surface area. Table 1 shows rate constants calculated from both homogenous and heterogeneous reaction kinetics for tests showing Type I behaviour, i.e., those tests performed at 180ºC in the 54.9 m test section. Table 2 shows rate constants calculated in a similar way for tests showing Type II behaviour at hydrazine concentrations between 0.27 and 3.49 mg/kg. All but one of the tests showing Type II behaviour were done using the 12.2 m test section. Rate constants for tests showing Type II behaviour were calculated only for measurements made after the concentrations of hydrazine and oxygen had effectively reached steady state. As shown by the rate constants listed in Table 1, the reaction between hydrazine and oxygen at 180ºC for tests performed using the 54.9 m test section is better represented by the rate law for a homogenous reaction than for a heterogeneous reaction.

Table 1 Rate Constants Calculated for Homogenous and Heterogeneous Reaction Kinetics for Tests Displaying Type I Behaviour

Surface Area (m

Residence Time (min)

[N2H4]in (mg/kg)

[N2H4]out (mg/kg)

[O2]in (mg/kg)

[O2]out (mg/kg)

khomo (kg

1/2•mg

-1/2•min

khetero (m

180 1.22 3.12 0.28 0.25 0.044 0.005 1.35 1.84

180 1.22 1.72 0.22 0.21 0.023 0.006 1.70 1.12

180 1.22 1.30 0.30 0.29 0.017 0.007 1.24 0.70

Table 2 shows that for tests showing Type II behaviour, the reaction between hydrazine and dissolved oxygen is much better described by the rate law for a heterogeneous reaction than for a homogenous reaction. The heterogeneous rate constants listed in Table 2 are well described by an Arrhenius plot with an activation energy of 39.3 kJ/mole, as shown in Figure 10, which is higher than the activation energy reported for a homogeneous reaction mechanism represented by the Dalgaard equation [6].

Table 2 Rate Constants Calculated for Homogenous and Heterogeneous Reaction Kinetics for Tests Displaying Type II Behaviour

Surface Area (m2)

Residence Time (min)

[N2H4]in (mg/kg)

[N2H4]out (mg/kg)

[O2]in (mg/kg)

[O2]out (mg/kg)

khomo (kg1/2•mg-1/2•min-1)

khetero (m-2)

120 0.27 0.40 3.49 3.44 0.378 0.357 0.074 0.21

120 0.27 0.31 3.40 3.37 0.329 0.314 0.081 0.17

150 0.27 0.39 0.46 0.45 0.028 0.024 0.55 0.57

150 1.22 1.76 0.27 0.25 0.027 0.014 0.72 0.54

165 0.27 0.39 0.42 0.41 0.026 0.022 0.66 0.62

180 0.27 0.29 0.36 0.36 0.018 0.014 1.42 0.93

2.10E-03 2.20E-03 2.30E-03 2.40E-03 2.50E-03 2.60E-03

1/T(K-1)

Figure 10 Arrhenius Plot of the Rate Constants Listed in Table 2 for the Heterogeneous Reaction Kinetics

5. CONCLUSIONS A series of tests was performed in a recirculating loop with continuous feed and bleed to measure the removal of dissolved oxygen by hydrazine under sample line conditions at a CANDU nuclear power plant. The amount of dissolved oxygen removed by hydrazine under two representative feedwater sample line conditions at 120 and 180ºC in CANDU power plants was determined. The test results show that dissolved oxygen measurements in CANDU plants are underestimated to varying degrees, depending on the temperature, residence time in the sample line, concentrations of hydrazine and dissolved oxygen, and surface area. Based on the test results obtained, the following conclusions are drawn: 1. Two distinct types of behaviour are observed for the oxygen removal rate. For the tests conducted in

the 54.9 m test section at temperatures of greater than 150ºC and hydrazine concentrations of less than 0.3 mg/kg, the percentage removal of dissolved oxygen was invariant with time during these tests, and increased with increasing the residence time in the test section. The hydrazine-oxygen reaction in these tests is better described by a homogeneous reaction mechanism. For the tests conducted under deaerator storage tank outlet conditions with hydrazine concentrations of greater than 1 mg/kg and conducted under HP heater outlet conditions in the 12.2 m test section, the percentage oxygen removal decreased with increasing hydrazine concentration. The hydrazine-oxygen reaction in these tests is better described by a heterogeneous reaction mechanism.

2. Under warm-up conditions, less than 10% of the dissolved oxygen is expected to be lost in a 12 m sample line

for temperatures less than 120ºC, flow rates between 0.5 and 1.35 kg/min, oxygen concentrations

between 300 and 600 µg/kg and hydrazine to oxygen ratios of 6 to 10. Under similar conditions, loss of

dissolved oxygen increases to less than 30% for a sample line length of 55 m.

3. During power operation, up to 31% of the dissolved oxygen can be lost due to reaction with hydrazine in

a 12 m sample line at temperatures in excess of 150ºC, flow rates between 0.5 and 1.35 kg/min, oxygen

concentrations between 17 and 47 µg/kg and hydrazine to oxygen ratios of 10 to 20. Under similar conditions, the loss of oxygen increases to between 50 and 90% for a sample line length of 55 m.

6. ACKNOWLEDGEMENTS This work was funded by CANDU Owners Group (COG). The authors would like to thank J. Semmler, G. Burton and S. Klimas for valuable discussions and comments on this paper. The authors also acknowledge H. Searle and J. Qian for performing all the tests.

7. REFERENCES

[1] E. Pierson and C. Laire, “The Influence of Copper on the SCC of Alloy 600 and Alloy 690 Steam Generator Tubes,” Proceedings of Fontevraud IV, 381, 1998 September.

[2] P.L. Andresen and J. Hickling, “Effect of Dissolved H2, B/Li/pH, and Zinc on PWSCC of Alloy 600”, Materials Reliability Program: Proceedings of the 2005 International PWSCC of Alloy 600 Conference and Exhibit Show (MRP-154), Electric Power Research Institute, Palo Alto, CA, AREVA, Lynchburg, VA, Westinghouse. Madison, PA, Structural Integrity Associates, Centennial, CO, and Welding Services, Inc.,Norcross, GA: 2005, 1012089.

[3] W.R. Kassen, “Electrochemical Potential Monitoring in the PWR Secondary Cycle of St. Lucie 2”, Electric Power Research Institute Report TR-104951, 1995 March.

[4] “Effect of Redox Conditions on Flow Accelerated Corrosion: Influence of Hydrazine and Oxygen”, Interim Report, EPRI Report 1002768, 2002.

[5] N.L. Dickinson, D.N. Felgar, and E.A. Pirsh, “An Experimental Investigation of Hydrazine-Oxygen Reaction Rates in Boiler Feedwater,” Proceedings of the American Power Conference, 1957 March 27 to 29.

[6] S.B. Dalgaard and M.O. Sanford, “Review of the Hydrazine/Oxygen Reaction Kinetics,” Materials Performance, 21, 32-38 (1982).

[7] R. Woodward, “Hydrazine for Boiler Feedwater Treatment”, Power 100, 80-82 (1956).

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