# Long term trend of the cobalt activity in the primary coolant of Beznau NPP evaluation of the operational and shutdown chemistry

Hartmut Venz, Patrick Jolivet, Frédéric Dacquait

► To cite this version:

Hartmut Venz, Patrick Jolivet, Frédéric Dacquait. Long term trend of the cobalt activity in the primary coolant of Beznau NPP evaluation of the operational and shutdown chemistry. International Conference on Water Chemistry of Nuclear Reactor Systems, Apr 2002, Avignon, France. cea-04463852

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# Long term trend of the cobalt activity in the primary coolant of Beznau NPP evaluation of the operational and shutdown chemistry

Hartmut Venz, Patrick Jolivet, Frédéric Dacquait

► To cite this version:

Hartmut Venz, Patrick Jolivet, Frédéric Dacquait. Long term trend of the cobalt activity in the primary coolant of Beznau NPP evaluation of the operational and shutdown chemistry. International Conference on Water Chemistry of Nuclear Reactor Systems, Apr 2002, Avignon, France. cea-04463852

HAL Id: cea-04463852

https://cea.hal.science/cea-04463852v1

Submitted on 17 Feb 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
LONG TERM TREND OF THE COBALT ACTIVITY IN THE PRIMARY COOLANT OF
BEZNAU NPP EVALUATION OF THE OPERATIONAL AND SHUTDOWN CHEMISTRY

H. VENZ, Nordostschweizerische Kraftwerke, Kernkraftwerk Beznau, Switzerland

P. JOLIVET, FRAMATOME-ANP, Paris, France

F. DACQUAIT, CEA Cadarache, DEC /S3C /LTC, France

ABSTRACT

The nuclear power plant Beznau consists of two pressurized water reactors which went into operation in 1969 resp. in 1971. Numerous back-fittings were carried out like the replacements of the steam generators (SGR) realized at unit 1 in 1993 and at unit 2 in 1999. Since the SGR special attention has been paid to the behavior and inventory of cobalt nuclides and their contribution to the contamination level which is approximately 98% of the dose rate. A lot of different measures have been realized to decrease the dose rates. The results were checked by different investigations such as in situ-☐-spectrometry and analyses of the corrosion products in the coolant.

A shutdown chemistry was introduced in 1982 to stop the increasing trend of the dose rates, to have definite redox conditions, to remove the released activity, to realize a rapid degasification, to reach an activity in the pool water as low as possible and a high visibility of the water. During shutdown corrosion products are mobilized as a result of changes in power, temperature and water chemistry. This process is supported by the dosage of hydrogen peroxide. The peak of ⁵⁸Co is 10 to 15 GBq/m³ whereas about 4000 GBq ⁵⁸Co are released after oxygenation. To decrease the corrosion product sources, a replacement of fuel assemblies with grids made of Inconel by ones made of Zircaloy was realized step by step from 1989 to 1993. A second important measure was the SGR. The tube material (alloy 690 TT instead of alloy 600 MA) leads to a smaller release of cobalt due to both the lower corrosion rate and the lower cobalt content. The primary chemistry follows a modified mode of operation in reference to boron and lithium concentrations. A new specification of lithium was implemented after the analysis of previous cycles, especially the first extended one that took place at unit 2 in 1992/93 (21st cycle). The range of tolerance was decreased with the objective of a more constant pH₃₀₀ near 7.2. The upper limit for lithium was changed from 2.0 to 2.2 ppm. Investigations showed that the fine fraction of ⁶⁰Co (< 0.45 µm) contributes more than 90% of the total coolant activity of this isotope. In case of ⁵⁸Co the contributions of the fine and the crud fraction are in the same order of magnitude. Whereas the fine fraction activity decreases with increasing pH₃₀₀, the particulate one is less linked to pH₃₀₀ and even shows some increased release for operation around pH₃₀₀ = 7.2 which seems to be more pronounced for ⁵⁸Co than for ⁶⁰Co.

According to these conditions ⁶⁰Co contributes about 70% to the dose rates of the depositions and ⁵⁸Co about 30%. After implementation of extended cycles in both units, a study to check the introduction of a pH₃₀₀ of 7.4 and of an upper limit of Lithium of 2.5 ppm was performed with the objective to avoid a pH below 6.9 at the beginning of a cycle. This measure began with the new pH of 7.4 at the 26th cycle of unit 2 in 1999. One cycle later the upper limit of Lithium was increased to 2.5 ppm. At the same time the decision was made to return to annual cycles due to refueling. An outage with refueling and maintenance is carried out every second year. For that reason the 26th cycle of unit 2 was only 9 months. After the implementation of the new chemistry the total and the crud activity of ⁵⁸Co have been increased during the 27th cycle in comparison to previous ones. Furthermore the ⁵⁸Co-peak during shutdown was about 5 times higher than previously, and bad decontamination factors at the mixed bed filter due to the cobalt nuclides were observed during shutdown. The addition of a cation filter in line with the mixed bed led to the expected values. Later a high contamination by ⁵⁸Co appeared on the walls of the cavity which could not be removed by the normal means of decontamination. Both observations indicate that there was a high concentration of colloidal ⁵⁸Co. The dose rates in the rooms of the reactor building were two to three times higher than usually leading to a higher collective dose during the outage. The results of the in situ-☐-spectrometric measurements confirmed the dramatic increase of the ⁵⁸Co activity which contributed in average to 65% to the dose rate now.
LONG TERM TREND OF THE COBALT ACTIVITY IN THE PRIMARY COOLANT OF
BEZNAU NPP EVALUATION OF THE OPERATIONAL AND SHUTDOWN CHEMISTRY

H. VENZ, Nordostschweizerische Kraftwerke, Kernkraftwerk Beznau, Switzerland

P. JOLIVET, FRAMATOME-ANP, Paris, France

F. DACQUAIT, CEA Cadarache, DEC /S3C /LTC, France

ABSTRACT

The nuclear power plant Beznau consists of two pressurized water reactors which went into operation in 1969 resp. in 1971. Numerous back-fittings were carried out like the replacements of the steam generators (SGR) realized at unit 1 in 1993 and at unit 2 in 1999. Since the SGR special attention has been paid to the behavior and inventory of cobalt nuclides and their contribution to the contamination level which is approximately 98% of the dose rate. A lot of different measures have been realized to decrease the dose rates. The results were checked by different investigations such as in situ-☐-spectrometry and analyses of the corrosion products in the coolant.

A shutdown chemistry was introduced in 1982 to stop the increasing trend of the dose rates, to have definite redox conditions, to remove the released activity, to realize a rapid degasification, to reach an activity in the pool water as low as possible and a high visibility of the water. During shutdown corrosion products are mobilized as a result of changes in power, temperature and water chemistry. This process is supported by the dosage of hydrogen peroxide. The peak of ⁵⁸Co is 10 to 15 GBq/m³ whereas about 4000 GBq ⁵⁸Co are released after oxygenation. To decrease the corrosion product sources, a replacement of fuel assemblies with grids made of Inconel by ones made of Zircaloy was realized step by step from 1989 to 1993. A second important measure was the SGR. The tube material (alloy 690 TT instead of alloy 600 MA) leads to a smaller release of cobalt due to both the lower corrosion rate and the lower cobalt content. The primary chemistry follows a modified mode of operation in reference to boron and lithium concentrations. A new specification of lithium was implemented after the analysis of previous cycles, especially the first extended one that took place at unit 2 in 1992/93 (21st cycle). The range of tolerance was decreased with the objective of a more constant pH₃₀₀ near 7.2. The upper limit for lithium was changed from 2.0 to 2.2 ppm. Investigations showed that the fine fraction of ⁶⁰Co (< 0.45 µm) contributes more than 90% of the total coolant activity of this isotope. In case of ⁵⁸Co the contributions of the fine and the crud fraction are in the same order of magnitude. Whereas the fine fraction activity decreases with increasing pH₃₀₀, the particulate one is less linked to pH₃₀₀ and even shows some increased release for operation around pH₃₀₀ = 7.2 which seems to be more pronounced for ⁵⁸Co than for ⁶⁰Co.

According to these conditions ⁶⁰Co contributes about 70% to the dose rates of the depositions and ⁵⁸Co about 30%. After implementation of extended cycles in both units, a study to check the introduction of a pH₃₀₀ of 7.4 and of an upper limit of Lithium of 2.5 ppm was performed with the objective to avoid a pH below 6.9 at the beginning of a cycle. This measure began with the new pH of 7.4 at the 26th cycle of unit 2 in 1999. One cycle later the upper limit of Lithium was increased to 2.5 ppm. At the same time the decision was made to return to annual cycles due to refueling. An outage with refueling and maintenance is carried out every second year. For that reason the 26th cycle of unit 2 was only 9 months. After the implementation of the new chemistry the total and the crud activity of ⁵⁸Co have been increased during the 27th cycle in comparison to previous ones. Furthermore the ⁵⁸Co-peak during shutdown was about 5 times higher than previously, and bad decontamination factors at the mixed bed filter due to the cobalt nuclides were observed during shutdown. The addition of a cation filter in line with the mixed bed led to the expected values. Later a high contamination by ⁵⁸Co appeared on the walls of the cavity which could not be removed by the normal means of decontamination. Both observations indicate that there was a high concentration of colloidal ⁵⁸Co. The dose rates in the rooms of the reactor building were two to three times higher than usually leading to a higher collective dose during the outage. The results of the in situ-☐-spectrometric measurements confirmed the dramatic increase of the ⁵⁸Co activity which contributed in average to 65% to the dose rate now.
The usual effect of the SGR on the increase of the ⁵⁸Co activity cannot be avoided because the accommodation of the new 690 TT surfaces during the subsequent cycles can temporary lead to higher nickel release compared to "stabilized" older 600 alloy. Now the increase of cobalt nuclides seems to have been enhanced by additional and simultaneous conditions: higher nickel and ⁵⁸Co contents at the beginning of cycle after the short outage of cycle 26 and change in the primary pH₃₀₀ value from 7.2 to 7.4 accompanied by a higher upper lithium limit of 2.5 ppm. Especially that may enlarge the formation of metallic nickel which is the main source of coolant-born ⁵⁸Co. This may cause an increased crud release which also affects the reactor core zones where precipitation of Ni (m) increases the oxide film heterogeneity and instability of the layer as a whole. Solubility calculation results show that the driving forces for the deposition of Nickel species in the core are the lowest solubility limit and negative solubility coefficients. The rather high hydrogen content in the RCS at Beznau units imposes that metallic Nickel with a low solubility value (about 0.05 ppb) and a slightly negative coefficient is always present in the core. Moreover its solubility limit is lower for higher pH₃₀₀ values, that means between 7.0 (BOC) and 7.2 or 7.4 (target value). It is to note that the last change of the coolant chemistry does not meet the optimum operating conditions for the Beznau units. The increase of the ⁵⁸Co activity seemed to have been higher than normally expected after a SGR with alloy 690 TT tubing material. Therefore a decision was made to return to the former target pH₃₀₀ value of 7.2.

# 1. INTRODUCTION

The nuclear power plant Beznau consists of two pressurized water reactors. The primary system of the 2-loop-units was designed by Westinghouse, the secondary system by BBC (today Alstom).

Unit 1 went into operation in 1969, unit 2 in 1971. Regarding its age, Beznau has to be designated as an old plant. In fact one has to talk of two young, "state of the art" units taking into account the numerous back-fittings for which NOK spent approximately 1 billion Swiss francs. Important measures were the replacements of the steam generators (SGR) realized at unit 1 in 1993 and at unit 2 in 1999. The replacements became necessary because one year after commercial operation problems with the tubing of the steam generators began. The reasons for the replacement have been reported in [1].

The new steam generators with alloy 690 tubing have been made by FRAMATOME-ANP. Beznau's experiences from the radiological point of view have been reported in [2] and [3].

Since the SG replacements special attention has been paid to the behavior and inventory of cobalt nuclides and their contribution to the contamination level which is approximately 98% of the dose rate. A lot of different measures have been realized to decrease the dose rates. The results were checked by different investigations such as in situ-□-spectrometry and analyses of the corrosion products in the coolant.

# 2. MEASURES TO DECREASE THE DOSE RATES

## 2.1 Optimization of the shutdown chemistry

A shutdown chemistry was introduced in 1982 to influence the dose rate level which has reached its maximum in the early eighties (figures 1 and 2). An optimized version is used since 1988. The shutdown chemistry and our experiences have been published in [4]. The main objectives of the shutdown chemistry are to stop the increasing trend of the dose rates, to have definite redox conditions, to remove the released activity, to realize a rapid degasification, to reach an activity in the pool water as low as possible and a high visibility of the water.

During shutdown corrosion products are mobilized as a result of changes in power, temperature and water chemistry. This process is supported by the dosage of hydrogen peroxide.

The addition of hydrogen peroxide is implemented several times after cooling down to 120° C, a solid pressurizer and a purification rate at the highest (18 m³/h). During this period the target oxygen level is 3 - 5 ppm to reach and keep oxidizing conditions. A total amount of 30 - 35 l H₂O₂ (30%) is usually needed. The peak of ⁵⁸Co is 10 to 15 GBq/m³ whereas about 4000 GBq ⁵⁸Co are released after oxygenation. The temperature of the coolant is kept constant at 120°C during 12 hours and the released activity is removed using a fresh mixed bed filter. After 12 hours the temperature is decreased to 45°C and the purification is continued further 18 to 24 hours. The purification step can be finished if the following values with regard to corrosion products are met: ⁵⁸Co < 2 E 9 Bq/m³, ⁶⁰Co < 2 E 8 Bq/m³, Ni < 200 ppb, Dissolved Hydrogen (DH) < 0.5 ppm.

At the end of cycle 27 a significant higher spiking value during oxygenation of the primary coolant than for the previous cycles was recorded. The cobalt-58 spiking increased by a factor of about 5 compared to the previous cycles, especially to the cycles before the SGR. This higher-than-usual activity is visible at the evolution of
The usual effect of the SGR on the increase of the ⁵⁸Co activity cannot be avoided because the accommodation of the new 690 TT surfaces during the subsequent cycles can temporary lead to higher nickel release compared to "stabilized" older 600 alloy. Now the increase of cobalt nuclides seems to have been enhanced by additional and simultaneous conditions: higher nickel and ⁵⁸Co contents at the beginning of cycle after the short outage of cycle 26 and change in the primary pH₃₀₀ value from 7.2 to 7.4 accompanied by a higher upper lithium limit of 2.5 ppm. Especially that may enlarge the formation of metallic nickel which is the main source of coolant-born ⁵⁸Co. This may cause an increased crud release which also affects the reactor core zones where precipitation of Ni (m) increases the oxide film heterogeneity and instability of the layer as a whole. Solubility calculation results show that the driving forces for the deposition of Nickel species in the core are the lowest solubility limit and negative solubility coefficients. The rather high hydrogen content in the RCS at Beznau units imposes that metallic Nickel with a low solubility value (about 0.05 ppb) and a slightly negative coefficient is always present in the core. Moreover its solubility limit is lower for higher pH₃₀₀ values, that means between 7.0 (BOC) and 7.2 or 7.4 (target value). It is to note that the last change of the coolant chemistry does not meet the optimum operating conditions for the Beznau units. The increase of the ⁵⁸Co activity seemed to have been higher than normally expected after a SGR with alloy 690 TT tubing material. Therefore a decision was made to return to the former target pH₃₀₀ value of 7.2.

# 1. INTRODUCTION

The nuclear power plant Beznau consists of two pressurized water reactors. The primary system of the 2-loop-units was designed by Westinghouse, the secondary system by BBC (today Alstom).

Unit 1 went into operation in 1969, unit 2 in 1971. Regarding its age, Beznau has to be designated as an old plant. In fact one has to talk of two young, "state of the art" units taking into account the numerous back-fittings for which NOK spent approximately 1 billion Swiss francs. Important measures were the replacements of the steam generators (SGR) realized at unit 1 in 1993 and at unit 2 in 1999. The replacements became necessary because one year after commercial operation problems with the tubing of the steam generators began. The reasons for the replacement have been reported in [1].

The new steam generators with alloy 690 tubing have been made by FRAMATOME-ANP. Beznau's experiences from the radiological point of view have been reported in [2] and [3].

Since the SG replacements special attention has been paid to the behavior and inventory of cobalt nuclides and their contribution to the contamination level which is approximately 98% of the dose rate. A lot of different measures have been realized to decrease the dose rates. The results were checked by different investigations such as in situ-□-spectrometry and analyses of the corrosion products in the coolant.

# 2. MEASURES TO DECREASE THE DOSE RATES

## 2.1 Optimization of the shutdown chemistry

A shutdown chemistry was introduced in 1982 to influence the dose rate level which has reached its maximum in the early eighties (figures 1 and 2). An optimized version is used since 1988. The shutdown chemistry and our experiences have been published in [4]. The main objectives of the shutdown chemistry are to stop the increasing trend of the dose rates, to have definite redox conditions, to remove the released activity, to realize a rapid degasification, to reach an activity in the pool water as low as possible and a high visibility of the water.

During shutdown corrosion products are mobilized as a result of changes in power, temperature and water chemistry. This process is supported by the dosage of hydrogen peroxide.

The addition of hydrogen peroxide is implemented several times after cooling down to 120° C, a solid pressurizer and a purification rate at the highest (18 m³/h). During this period the target oxygen level is 3 - 5 ppm to reach and keep oxidizing conditions. A total amount of 30 - 35 l H₂O₂ (30%) is usually needed. The peak of ⁵⁸Co is 10 to 15 GBq/m³ whereas about 4000 GBq ⁵⁸Co are released after oxygenation. The temperature of the coolant is kept constant at 120°C during 12 hours and the released activity is removed using a fresh mixed bed filter. After 12 hours the temperature is decreased to 45°C and the purification is continued further 18 to 24 hours. The purification step can be finished if the following values with regard to corrosion products are met: ⁵⁸Co < 2 E 9 Bq/m³, ⁶⁰Co < 2 E 8 Bq/m³, Ni < 200 ppb, Dissolved Hydrogen (DH) < 0.5 ppm.

At the end of cycle 27 a significant higher spiking value during oxygenation of the primary coolant than for the previous cycles was recorded. The cobalt-58 spiking increased by a factor of about 5 compared to the previous cycles, especially to the cycles before the SGR. This higher-than-usual activity is visible at the evolution of
<figure><img src="image_2.png" /><figcaption>Figure 1: Dose rate at the loops of Beznau 2</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2: Dose rate in the SG Channel Heads of Unit 2</figcaption></figure>

$^{58}$Co during the cycle 27, compared to the previous cycles (see figure 9 in section 3.1). A total $^{58}$Co activity of around 20000 GBq has been released in the primary coolant mainly due to the H$_2$O$_2$ injection. The total released activity was about 5 times higher in comparison to previous cycles. That means a higher inventory of nickel was present in the reactor coolant system (RCS) and on the fuel elements during the cycle leading to a higher rate of activation in Cobalt-58. In the following sections, the history of the primary water chemistry is analyzed to explain those differences.

## 2.2 Decrease of corrosion product sources

### 2.2.1 Replacement of grids

The replacement of fuel assemblies with grids made of Inconel by ones made of Zircaloy contributed to lower dose rates at Beznau due to the significant decrease of the activated nickel and cobalt. The decreasing trend for the dose rates at the Beznau Steam Generator channel heads (figure 2) corresponds quite well with the replacement of the grids realized step by step from 1989 to 1993.
<figure><img src="image_2.png" /><figcaption>Figure 1: Dose rate at the loops of Beznau 2</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2: Dose rate in the SG Channel Heads of Unit 2</figcaption></figure>

$^{58}$Co during the cycle 27, compared to the previous cycles (see figure 9 in section 3.1). A total $^{58}$Co activity of around 20000 GBq has been released in the primary coolant mainly due to the H$_2$O$_2$ injection. The total released activity was about 5 times higher in comparison to previous cycles. That means a higher inventory of nickel was present in the reactor coolant system (RCS) and on the fuel elements during the cycle leading to a higher rate of activation in Cobalt-58. In the following sections, the history of the primary water chemistry is analyzed to explain those differences.

## 2.2 Decrease of corrosion product sources

### 2.2.1 Replacement of grids

The replacement of fuel assemblies with grids made of Inconel by ones made of Zircaloy contributed to lower dose rates at Beznau due to the significant decrease of the activated nickel and cobalt. The decreasing trend for the dose rates at the Beznau Steam Generator channel heads (figure 2) corresponds quite well with the replacement of the grids realized step by step from 1989 to 1993.
### 2.2.2 Replacement of steam generators

The steam generators of Unit 1 were replaced in 1993 and of Unit 2 in 1999. High significance has the replacement of the tubes made of alloy 600MA by such one made of alloy 690 TT. Concerning the activity build-up, the comparison between the cobalt amounts and the corrosion rates of the alloys (table 1) is of interest.

Table 1 shows that the cobalt content of the new steam generator tubes is about 80% lower according to the tube supplier's certificate. The chromium content in alloy 690 TT, which is twice the amount in alloy 600MA, is also supposed to reduce the corrosion rate of alloy 690 TT. With regard to laboratory investigations performed in France in primary conditions for T, P and chemistry [5], the corrosion rate of alloy 690 TT should come to about the half of the corrosion rate of alloy 600MA. Because of the relation between corrosion rate and activity build-up, the process of the incorporation of activated corrosion products in the oxide layer is expected to be significantly delayed or decreased with alloy 690 TT. Moreover, a smaller release of cobalt is expected due to both the lower corrosion rate and the lower cobalt content.

<table><caption>**Table 1:** Chemical composition of the steam generator tube materials</caption><thead><tr><td>Element</td><td>Alloy 600 MA percentage (%)</td><td>Alloy 690 TT percentage (%)</td></tr></thead><tbody><tr><td>Ni</td><td>≥ 73.70</td><td>≥ 58.00</td></tr><tr><td>Cr</td><td>≥ 14.54</td><td>≥ 28.00</td></tr><tr><td>Fe</td><td>≥ 8.24</td><td>≥ 7.00</td></tr><tr><td>Co</td><td>≤ 0.08</td><td>≤ 0.015</td></tr></tbody></table>

## 2.3 Improved reactor coolant chemistry

The primary chemistry follows a modified mode of operation in reference to boron and lithium concentrations. During the seventies and eighties the pH was not kept as constant as possible, and lithium was varied between 0.5 and 2.0 ppm depending on the boron concentration. A new specification of lithium was implemented after the analysis of previous cycles, especially the first extended one that took place at unit 2 in 1992/93 (21st cycle). The range of tolerance was decreased with the objective of a more constant pH₃₀₀ near 7.2. The upper limit for lithium was changed from 2.0 to 2.2 ppm in agreement with the fuel suppliers (figure 3). The experiences with this chemistry have been reported in [6] and [7].

After implementation of extended cycles in both units a study to check the introduction of a pH₃₀₀ of 7.4 and of an upper limit of Lithium of 2.5 ppm was performed with the objective to avoid a pH below 6.9 at the beginning of a cycle. This measure began with the new pH of 7.4 at the 26th cycle of unit 2 in 1999. One cycle later the upper limit of Lithium was increased to 2.5 ppm.

<figure><img src="image_4.png" /><figcaption>**Figure 3:** Lithium - boron concentrations in the primary coolant</figcaption></figure>
### 2.2.2 Replacement of steam generators

The steam generators of Unit 1 were replaced in 1993 and of Unit 2 in 1999. High significance has the replacement of the tubes made of alloy 600MA by such one made of alloy 690 TT. Concerning the activity build-up, the comparison between the cobalt amounts and the corrosion rates of the alloys (table 1) is of interest.

Table 1 shows that the cobalt content of the new steam generator tubes is about 80% lower according to the tube supplier's certificate. The chromium content in alloy 690 TT, which is twice the amount in alloy 600MA, is also supposed to reduce the corrosion rate of alloy 690 TT. With regard to laboratory investigations performed in France in primary conditions for T, P and chemistry [5], the corrosion rate of alloy 690 TT should come to about the half of the corrosion rate of alloy 600MA. Because of the relation between corrosion rate and activity build-up, the process of the incorporation of activated corrosion products in the oxide layer is expected to be significantly delayed or decreased with alloy 690 TT. Moreover, a smaller release of cobalt is expected due to both the lower corrosion rate and the lower cobalt content.

<table><caption>**Table 1:** Chemical composition of the steam generator tube materials</caption><thead><tr><td>Element</td><td>Alloy 600 MA percentage (%)</td><td>Alloy 690 TT percentage (%)</td></tr></thead><tbody><tr><td>Ni</td><td>≥ 73.70</td><td>≥ 58.00</td></tr><tr><td>Cr</td><td>≥ 14.54</td><td>≥ 28.00</td></tr><tr><td>Fe</td><td>≥ 8.24</td><td>≥ 7.00</td></tr><tr><td>Co</td><td>≤ 0.08</td><td>≤ 0.015</td></tr></tbody></table>

## 2.3 Improved reactor coolant chemistry

The primary chemistry follows a modified mode of operation in reference to boron and lithium concentrations. During the seventies and eighties the pH was not kept as constant as possible, and lithium was varied between 0.5 and 2.0 ppm depending on the boron concentration. A new specification of lithium was implemented after the analysis of previous cycles, especially the first extended one that took place at unit 2 in 1992/93 (21st cycle). The range of tolerance was decreased with the objective of a more constant pH₃₀₀ near 7.2. The upper limit for lithium was changed from 2.0 to 2.2 ppm in agreement with the fuel suppliers (figure 3). The experiences with this chemistry have been reported in [6] and [7].

After implementation of extended cycles in both units a study to check the introduction of a pH₃₀₀ of 7.4 and of an upper limit of Lithium of 2.5 ppm was performed with the objective to avoid a pH below 6.9 at the beginning of a cycle. This measure began with the new pH of 7.4 at the 26th cycle of unit 2 in 1999. One cycle later the upper limit of Lithium was increased to 2.5 ppm.

<figure><img src="image_4.png" /><figcaption>**Figure 3:** Lithium - boron concentrations in the primary coolant</figcaption></figure>
# 3. ACTIVITY IN THE COOLANT AND DEPOSITED ON INNER SURFACES

## 3.1 Coolant activity

An investigation program was carried out to study the results of the first implemented measures ($pH_{300}$ 7.2; Li 2.2 ppm) and to look for a further decrease of dose rates. The objective was to study the influence of the coolant chemistry on the coolant-born and the deposited activity. The evaluation was focussed on the activity of $^{58}$Co and $^{60}$Co because these activated corrosion products cause about 98% of the dose rate around the reactor system components.

Main points of the analytical program were the separation and determination of the concentration of iron, nickel, chromium and of the activity of all activated corrosion products in the crud (fraction > 0.45 µm) and in the filtrate (fine fraction < 0.45 µm). According to previous results [7] the total activity of both isotopes exhibits a decreasing trend with increasing $pH_{300}$. The target pH was 7.2 at that time.

* **Effect of $pH_{300}$ on corrosion product concentrations**

Figure 4 shows the plot of total coolant activity versus the calculated $pH_{300}$ for both isotopes. It is important to note that the fine fraction, especially that of $^{60}$Co, contributes more than 90% of the total coolant activity of this isotope. In the case of $^{58}$Co the contributions of the fine and the crud fraction are in the same order of magnitude (figures 5 and 6).

<figure><img src="image_5.png" /><figcaption>**Figure 4:** Dependence of total coolant activity of $^{58}$Co and $^{60}$Co on $pH_{300}$ at cycle 24</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 5:** Crud fraction activity (> 0.45 µm) as a function of $pH_{300}$</figcaption></figure>
# 3. ACTIVITY IN THE COOLANT AND DEPOSITED ON INNER SURFACES

## 3.1 Coolant activity

An investigation program was carried out to study the results of the first implemented measures ($pH_{300}$ 7.2; Li 2.2 ppm) and to look for a further decrease of dose rates. The objective was to study the influence of the coolant chemistry on the coolant-born and the deposited activity. The evaluation was focussed on the activity of $^{58}$Co and $^{60}$Co because these activated corrosion products cause about 98% of the dose rate around the reactor system components.

Main points of the analytical program were the separation and determination of the concentration of iron, nickel, chromium and of the activity of all activated corrosion products in the crud (fraction > 0.45 µm) and in the filtrate (fine fraction < 0.45 µm). According to previous results [7] the total activity of both isotopes exhibits a decreasing trend with increasing $pH_{300}$. The target pH was 7.2 at that time.

* **Effect of $pH_{300}$ on corrosion product concentrations**

Figure 4 shows the plot of total coolant activity versus the calculated $pH_{300}$ for both isotopes. It is important to note that the fine fraction, especially that of $^{60}$Co, contributes more than 90% of the total coolant activity of this isotope. In the case of $^{58}$Co the contributions of the fine and the crud fraction are in the same order of magnitude (figures 5 and 6).

<figure><img src="image_5.png" /><figcaption>**Figure 4:** Dependence of total coolant activity of $^{58}$Co and $^{60}$Co on $pH_{300}$ at cycle 24</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 5:** Crud fraction activity (> 0.45 µm) as a function of $pH_{300}$</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 6:** Dependence of coolant activity of fine fraction of <sup>58</sup>Co and <sup>60</sup>Co on pH<sub>300</sub></figcaption></figure>

Particulate fraction activity is less linked to pH<sub>300</sub> and even shows some increased release for operation around pH<sub>300</sub> = 7.2 which seems to be more pronounced for <sup>58</sup>Co than for <sup>60</sup>Co.

Unlike the crud, a marked relationship between the activity of the fine fraction (with a size of < 0.45 µm) and the pH<sub>300</sub> can be seen in figure 6.

* **Effect of Dissolved Hydrogen (DH) on corrosion product concentrations**

The crud activity (> 0.45 µm) for <sup>58</sup>Co and <sup>60</sup>Co versus dissolved hydrogen (DH) is shown in Figure 7. Despite the slight slope, especially for the <sup>58</sup>Co data, there is no clear dependence from the particle activity on DH.

Figure 8 represents the plot of <sup>58</sup>Co and <sup>60</sup>Co coolant activity associated with the fine fraction (< 0.45 µm). It is believed that <sup>60</sup>Co concentration is controlled by the behavior of the related ionic species. The evaluated impact of DH on the dissolution of mixed ferrite is insignificant for the range of concentrations shown. Therefore, it is reasonable to expect that the corresponding <sup>60</sup>Co-values are grouped around the line of the best fit which is rather parallel to the DH-coordinate.

A rather different picture is observed for <sup>58</sup>Co. While some of the recorded values match those of <sup>60</sup>Co which could be associated to the ionic form, the major part of data points are lying above the <sup>60</sup>Co-line showing a positive slope with increasing DH (solid line). That corresponds to the conclusion that the <sup>58</sup>Co-filtrate (< 0.45 µm) contains a significant quantity of fine particulate/colloid corrosion products (CP), as the amount of the latter is dependent on the level of DH.

<figure><img src="image_8.png" /><figcaption>**Figure 7:** Crud fraction activity (> 0.45 µm) versus Dissolved Hydrogen (DH)</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 6:** Dependence of coolant activity of fine fraction of <sup>58</sup>Co and <sup>60</sup>Co on pH<sub>300</sub></figcaption></figure>

Particulate fraction activity is less linked to pH<sub>300</sub> and even shows some increased release for operation around pH<sub>300</sub> = 7.2 which seems to be more pronounced for <sup>58</sup>Co than for <sup>60</sup>Co.

Unlike the crud, a marked relationship between the activity of the fine fraction (with a size of < 0.45 µm) and the pH<sub>300</sub> can be seen in figure 6.

* **Effect of Dissolved Hydrogen (DH) on corrosion product concentrations**

The crud activity (> 0.45 µm) for <sup>58</sup>Co and <sup>60</sup>Co versus dissolved hydrogen (DH) is shown in Figure 7. Despite the slight slope, especially for the <sup>58</sup>Co data, there is no clear dependence from the particle activity on DH.

Figure 8 represents the plot of <sup>58</sup>Co and <sup>60</sup>Co coolant activity associated with the fine fraction (< 0.45 µm). It is believed that <sup>60</sup>Co concentration is controlled by the behavior of the related ionic species. The evaluated impact of DH on the dissolution of mixed ferrite is insignificant for the range of concentrations shown. Therefore, it is reasonable to expect that the corresponding <sup>60</sup>Co-values are grouped around the line of the best fit which is rather parallel to the DH-coordinate.

A rather different picture is observed for <sup>58</sup>Co. While some of the recorded values match those of <sup>60</sup>Co which could be associated to the ionic form, the major part of data points are lying above the <sup>60</sup>Co-line showing a positive slope with increasing DH (solid line). That corresponds to the conclusion that the <sup>58</sup>Co-filtrate (< 0.45 µm) contains a significant quantity of fine particulate/colloid corrosion products (CP), as the amount of the latter is dependent on the level of DH.

<figure><img src="image_8.png" /><figcaption>**Figure 7:** Crud fraction activity (> 0.45 µm) versus Dissolved Hydrogen (DH)</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 8:** Dependence of the fine fraction activity of <sup>58</sup>Co and <sup>60</sup>Co on DH</figcaption></figure>

These observations showed an impact of water chemistry on coolant conditions by relationships between the level of coolant activity, the operated pH (T) and DH. The most significant effect was observed for the fine fraction which contributes the major part of the coolant-born activity for the main dose-contributing nuclides <sup>60</sup>Co and <sup>58</sup>Co. <sup>60</sup>Co was much less influenced by the change of coolant pH than <sup>58</sup>Co. It was calculated that there is a very good correlation of decreasing <sup>60</sup>Co-activity and the rate of boron removal by dilution with operation time. Whereas the crud activities of <sup>58</sup>Co and <sup>60</sup>Co and the fine fraction of <sup>60</sup>Co are rather independent from DH, the fine fraction of <sup>58</sup>Co shows a clear dependence. Therefore, pH (T) and DH should be considered as tools for joint control of the level of <sup>58</sup>Co in the coolant and on a smaller scale as a tool for influencing the <sup>60</sup>Co level.

* **Behavior of corrosion products after the implementation of a modified water chemistry**

Based on the conclusions mentioned above and on a study made by an external contractor the target pH<sub>300</sub> was increased to 7.4 at the 26th cycle, with the objective of a further suppression of corrosion products in the coolant and in the deposited activity. After getting the permission from all fuel suppliers the upper Lithium specification was extended to 2.5 ppm and used at the beginning of the 27th cycle. Those changes seemed to cause a different behavior of Ni and <sup>58</sup>Co than expected.

At the same time the decision was made to return to annual cycles due to refueling. An outage with refueling and maintenance is carried out every second year. For that reason the 26th cycle of unit 2 was only 9 months.

Figure 9 shows a higher concentration of the total <sup>58</sup>Co activity during the 27th cycle in comparison to previous ones. Furthermore the activity of the crud fraction has been increased during the last cycle (Figure 10) as well and became the dominant one in the second half of cycle 27th. As mentioned before the <sup>58</sup>Co-peak during shutdown was about 5 times higher than previously. Moreover, there were bad decontamination factors at the mixed bed filter due to the cobalt nuclides. The addition of a cation filter in line with the mixed bed led to the expected values. Later a high contamination by <sup>58</sup>Co appeared on the walls of the cavity which couldn't be removed by the normal means of decontamination. Both observations indicate that there was a high concentration of colloidal <sup>58</sup>Co. The dose rates in the rooms of the reactor building were two to three times higher than usually leading to a higher collective dose during the outage. The change of the radiological conditions was also observed by the in situ ☐-spectrometric measurements (see section 3.2).

## 3.2 Deposited activity

The SGR and the above-described variations in the primary chemistry were a great opportunity to investigate the impact on the deposited activity and thus on the dose rates. Therefore, the deposited activities have been measured by in-situ ☐ spectrometry on several locations of the primary loops with the EMECC system described in [8]. The comparative results for cycle 24 (1 cycle before SGR) and cycle 27 (2 cycles after SGR) of Unit 2 are given on figure 12 (average values on 2 loops). The new -respectively old- parts refer to the pieces of the primary piping that have been -respectively *not been*- replaced during the SGR. This information was checked, either visually (position of the welds) or with the help of the arrangement drawings of the loops.
<figure><img src="image_9.png" /><figcaption>**Figure 8:** Dependence of the fine fraction activity of <sup>58</sup>Co and <sup>60</sup>Co on DH</figcaption></figure>

These observations showed an impact of water chemistry on coolant conditions by relationships between the level of coolant activity, the operated pH (T) and DH. The most significant effect was observed for the fine fraction which contributes the major part of the coolant-born activity for the main dose-contributing nuclides <sup>60</sup>Co and <sup>58</sup>Co. <sup>60</sup>Co was much less influenced by the change of coolant pH than <sup>58</sup>Co. It was calculated that there is a very good correlation of decreasing <sup>60</sup>Co-activity and the rate of boron removal by dilution with operation time. Whereas the crud activities of <sup>58</sup>Co and <sup>60</sup>Co and the fine fraction of <sup>60</sup>Co are rather independent from DH, the fine fraction of <sup>58</sup>Co shows a clear dependence. Therefore, pH (T) and DH should be considered as tools for joint control of the level of <sup>58</sup>Co in the coolant and on a smaller scale as a tool for influencing the <sup>60</sup>Co level.

### ▪ Behavior of corrosion products after the implementation of a modified water chemistry

Based on the conclusions mentioned above and on a study made by an external contractor the target pH<sub>300</sub> was increased to 7.4 at the 26th cycle, with the objective of a further suppression of corrosion products in the coolant and in the deposited activity. After getting the permission from all fuel suppliers the upper Lithium specification was extended to 2.5 ppm and used at the beginning of the 27th cycle. Those changes seemed to cause a different behavior of Ni and <sup>58</sup>Co than expected.

At the same time the decision was made to return to annual cycles due to refueling. An outage with refueling and maintenance is carried out every second year. For that reason the 26th cycle of unit 2 was only 9 months.

Figure 9 shows a higher concentration of the total <sup>58</sup>Co activity during the 27th cycle in comparison to previous ones. Furthermore the activity of the crud fraction has been increased during the last cycle (Figure 10) as well and became the dominant one in the second half of cycle 27th. As mentioned before the <sup>58</sup>Co-peak during shutdown was about 5 times higher than previously. Moreover, there were bad decontamination factors at the mixed bed filter due to the cobalt nuclides. The addition of a cation filter in line with the mixed bed led to the expected values. Later a high contamination by <sup>58</sup>Co appeared on the walls of the cavity which couldn't be removed by the normal means of decontamination. Both observations indicate that there was a high concentration of colloidal <sup>58</sup>Co. The dose rates in the rooms of the reactor building were two to three times higher than usually leading to a higher collective dose during the outage. The change of the radiological conditions was also observed by the in situ ☐-spectrometric measurements (see section 3.2).

## 3.2 Deposited activity

The SGR and the above-described variations in the primary chemistry were a great opportunity to investigate the impact on the deposited activity and thus on the dose rates. Therefore, the deposited activities have been measured by in-situ ☐ spectrometry on several locations of the primary loops with the EMECC system described in [8]. The comparative results for cycle 24 (1 cycle before SGR) and cycle 27 (2 cycles after SGR) of Unit 2 are given on figure 12 (average values on 2 loops). The new -respectively old- parts refer to the pieces of the primary piping that have been -respectively *not been*- replaced during the SGR. This information was checked, either visually (position of the welds) or with the help of the arrangement drawings of the loops.
<figure><img src="image_10.png" /><figcaption>**Figure 9:** <sup>58</sup>Co RCS activity (GBq/m<sup>3</sup>) during cycles 24 to 27 of Unit 2</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 10:** Crud activity of <sup>58</sup>Co (GBq/m<sup>3</sup>) during cycles 24 to 27 of Unit 2</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 11:** <sup>60</sup>Co RCS activity (GBq/m<sup>3</sup>) during cycles 24 to 27 of Unit 2</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 9: $^{58}$Co RCS activity (GBq/m$^3$) during cycles 24 to 27 of Unit 2</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 10: Crud activity of $^{58}$Co (GBq/m$^3$) during cycles 24 to 27 of Unit 2</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Figure 11: $^{60}$Co RCS activity (GBq/m$^3$) during cycles 24 to 27 of Unit 2</figcaption></figure>
It can be seen that the ⁵⁸Co contamination level on the new part of the legs is near the one measured at the end of the cycle 24. On the contrary, the ⁵⁸Co deposited activities on the non-replaced parts of the legs have increased by a factor of about 7 to 8, which seems to be quite more important with regard to the usual trend after a SGR with alloy 690 tubing (discussion in section 4). This increase in the ⁵⁸Co contamination which is consistent with the higher activity concentration (section 3.1), clearly seems responsible for the measured higher dose rates.

Concerning the SG tubes, as generally noted for alloy 690, the hot side of the SG tubes is more contaminated than the cold side. Here, a factor of 2 is observed for ⁵⁸Co.

The same tendency is observed for the ⁶⁰Co deposited activities with an important discrepancy between the new and the old parts of the primary piping, particularly those of the closure legs. As for ⁵⁸Co the hot side of the SG tubes is more contaminated than the cold side by a factor of 3 for ⁶⁰Co, but the values remain low.

<figure><img src="image_13.png" /><figcaption>**Figure 12:** ⁵⁸Co and ⁶⁰Co deposited activity (GBq/m²) on Unit 2</figcaption></figure>

# 4. THEORETICAL CONSIDERATIONS AND DISCUSSION

## 4.1 Surface effect

Due to the same kind of observations for Beznau Unit 1 for several cycles since the SGR, it is likely that some differences in the surface characteristics between new parts and old parts of the primary piping explain the discrepancy observed for the contamination levels. Several studies have shown a clear effect of surface treatments on the diminution of both the release of corrosion products and the activity build-up. According to some investigations, the new parts have been finely grinded for both Units, normally with the same criteria as for the old parts.
It can be seen that the ⁵⁸Co contamination level on the new part of the legs is near the one measured at the end of the cycle 24. On the contrary, the ⁵⁸Co deposited activities on the non-replaced parts of the legs have increased by a factor of about 7 to 8, which seems to be quite more important with regard to the usual trend after a SGR with alloy 690 tubing (discussion in section 4). This increase in the ⁵⁸Co contamination which is consistent with the higher activity concentration (section 3.1), clearly seems responsible for the measured higher dose rates.

Concerning the SG tubes, as generally noted for alloy 690, the hot side of the SG tubes is more contaminated than the cold side. Here, a factor of 2 is observed for ⁵⁸Co.

The same tendency is observed for the ⁶⁰Co deposited activities with an important discrepancy between the new and the old parts of the primary piping, particularly those of the closure legs. As for ⁵⁸Co the hot side of the SG tubes is more contaminated than the cold side by a factor of 3 for ⁶⁰Co, but the values remain low.

<figure><img src="image_13.png" /><figcaption>**Figure 12:** ⁵⁸Co and ⁶⁰Co deposited activity (GBq/m²) on Unit 2</figcaption></figure>

# 4. THEORETICAL CONSIDERATIONS AND DISCUSSION

## 4.1 Surface effect

Due to the same kind of observations for Beznau Unit 1 for several cycles since the SGR, it is likely that some differences in the surface characteristics between new parts and old parts of the primary piping explain the discrepancy observed for the contamination levels. Several studies have shown a clear effect of surface treatments on the diminution of both the release of corrosion products and the activity build-up. According to some investigations, the new parts have been finely grinded for both Units, normally with the same criteria as for the old parts.
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>**Figure 13:** Solubility of iron and nickel species as a function of T, pH and DH</figcaption></figure>
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>**Figure 13:** Solubility of iron and nickel species as a function of T, pH and DH</figcaption></figure>
But depending on the technique (passivation, grinding, electropolishing, ...) and on temperature or chemistry conditions, factors from 2 to 10 are usually observed for the contamination of <sup>58</sup>Co and <sup>60</sup>Co between as-received and treated surfaces. Moreover, the activity build-up is also low for the newest 690 TT SGs manufactured by FRAMATOME-ANP compared to older manufacturing processes. The combination of these surface effects with chemistry changes and the usual impact of the SGR (see section 4.2) has probably led to a competition between those effects and to a transfer of activity towards the old parts of the pipes.

## 4.2 Effect of the SGR

The usual effect of the SGR on the increase of the <sup>58</sup>Co activity cannot be avoided because the accommodation of the new 690 TT surfaces during the subsequent cycles can temporary lead to higher nickel release compared to "stabilized" older 600 alloy. Nevertheless, this effect cannot be considered as the only cause, since it should have been already observed during cycle 26 (SGR + 1 year) with, as a consequence, almost as high dose rates as for cycle 27. Now the increase of cobalt nuclides seems to have been enhanced by additional and simultaneous conditions: higher nickel and <sup>58</sup>Co contents at the beginning of cycle after the short outage of cycle 26 and change in the primary pH<sub>300</sub> value from 7.2 to 7.4 accompanied by a higher upper lithium limit of 2.5 ppm.

## 4.3 pH and lithium values

As reported in [7], primary conditions with a pH<sub>300</sub> of about 7.2 and a sufficient level of DH indicate the system affinity to form (precipitate) metallic Ni on the system surfaces and reactor core as well. This may cause an increased crud release which also affects the reactor core zones where precipitation of Ni (m) increases the oxide film heterogeneity and instability of the layer as a whole.

Thus, the combined effect of pH<sub>T</sub> and DH influence the primary system where metallic Ni and a basic nickel ferrite coexist depending on the existing conditions. An increase of pH<sub>300</sub> up to 7.4 may enlarge the formation of metallic nickel which is the main source of coolant-born <sup>58</sup>Co. This confirms the trend which is already recognizable in figure 5. Moreover the conclusion is supported by figure 8 which shows that the <sup>58</sup>Co-filtrate (< 0.45 µm) comprises a significant amount of fine particulate /colloid corrosion products.

Solubility calculation results are gathered on figure 13 for different conditions of pH<sub>300</sub> (7.0, 7.2 and 7.4), for an upper Li limit at 2.5 ppm and DH values of about 3.5 and 1.8 ppm. It can be assumed that the driving forces for the deposition of Nickel species in the core are the lowest solubility limit and negative solubility coefficients. Then the rather high hydrogen content in the RCS at Beznau units imposes that metallic Nickel with a low solubility value (about 0.05 ppb) and a slightly negative coefficient is always present in the core. Moreover, its solubility limit is lower for higher pH<sub>300</sub> values, that means between 7.0 (BOC) and 7.2 or 7.4 (target value). The solubility of iron species (less important with regard to radiation protection) is linked to the nickel ferrite which exhibits a positive solubility coefficient for all pH<sub>300</sub> values. Figure 13 also shows that a lower hydrogen content (about 2 ppm) could avoid the deposition of metallic nickel.

## 5. CONCLUSION

In this study, the use of complementary investigation methods for the corrosion product behavior (coolant analysis – total, crud and fine particles – and in-situ γ spectrometry for deposits) was coupled with theoretical calculations and consideration of the experience feedback after a SGR. It has allowed performing a fine evaluation of the primary chemistry and of the radiological situation at the Beznau units after several changes in the primary chemistry. It has also led to useful recommendations to decrease the dose rates in the future cycles.

In particular, it was explained why the change of the coolant chemistry to pH<sub>300</sub> 7.4 was not an optimum operating condition for the Beznau units. There was a significant increase of the <sup>58</sup>Co activity with a lot of disadvantages with regard to the radiological conditions and which magnitude seemed to have been higher than normally expected after a SGR with alloy 690 TT tubing material.

Therefore, a decision was made to return to the former target pH<sub>300</sub> value of 7.2.
But depending on the technique (passivation, grinding, electropolishing, ...) and on temperature or chemistry conditions, factors from 2 to 10 are usually observed for the contamination of <sup>58</sup>Co and <sup>60</sup>Co between as-received and treated surfaces. Moreover, the activity build-up is also low for the newest 690 TT SGs manufactured by FRAMATOME-ANP compared to older manufacturing processes. The combination of these surface effects with chemistry changes and the usual impact of the SGR (see section 4.2) has probably led to a competition between those effects and to a transfer of activity towards the old parts of the pipes.

## 4.2 Effect of the SGR

The usual effect of the SGR on the increase of the <sup>58</sup>Co activity cannot be avoided because the accommodation of the new 690 TT surfaces during the subsequent cycles can temporary lead to higher nickel release compared to "stabilized" older 600 alloy. Nevertheless, this effect cannot be considered as the only cause, since it should have been already observed during cycle 26 (SGR + 1 year) with, as a consequence, almost as high dose rates as for cycle 27. Now the increase of cobalt nuclides seems to have been enhanced by additional and simultaneous conditions: higher nickel and <sup>58</sup>Co contents at the beginning of cycle after the short outage of cycle 26 and change in the primary pH<sub>300</sub> value from 7.2 to 7.4 accompanied by a higher upper lithium limit of 2.5 ppm.

## 4.3 pH and lithium values

As reported in [7], primary conditions with a pH<sub>300</sub> of about 7.2 and a sufficient level of DH indicate the system affinity to form (precipitate) metallic Ni on the system surfaces and reactor core as well. This may cause an increased crud release which also affects the reactor core zones where precipitation of Ni (m) increases the oxide film heterogeneity and instability of the layer as a whole.

Thus, the combined effect of pH<sub>T</sub> and DH influence the primary system where metallic Ni and a basic nickel ferrite coexist depending on the existing conditions. An increase of pH<sub>300</sub> up to 7.4 may enlarge the formation of metallic nickel which is the main source of coolant-born <sup>58</sup>Co. This confirms the trend which is already recognizable in figure 5. Moreover the conclusion is supported by figure 8 which shows that the <sup>58</sup>Co-filtrate (< 0.45 µm) comprises a significant amount of fine particulate /colloid corrosion products.

Solubility calculation results are gathered on figure 13 for different conditions of pH<sub>300</sub> (7.0, 7.2 and 7.4), for an upper Li limit at 2.5 ppm and DH values of about 3.5 and 1.8 ppm. It can be assumed that the driving forces for the deposition of Nickel species in the core are the lowest solubility limit and negative solubility coefficients. Then the rather high hydrogen content in the RCS at Beznau units imposes that metallic Nickel with a low solubility value (about 0.05 ppb) and a slightly negative coefficient is always present in the core. Moreover, its solubility limit is lower for higher pH<sub>300</sub> values, that means between 7.0 (BOC) and 7.2 or 7.4 (target value). The solubility of iron species (less important with regard to radiation protection) is linked to the nickel ferrite which exhibits a positive solubility coefficient for all pH<sub>300</sub> values. Figure 13 also shows that a lower hydrogen content (about 2 ppm) could avoid the deposition of metallic nickel.

## 5. CONCLUSION

In this study, the use of complementary investigation methods for the corrosion product behavior (coolant analysis – total, crud and fine particles – and in-situ γ spectrometry for deposits) was coupled with theoretical calculations and consideration of the experience feedback after a SGR. It has allowed performing a fine evaluation of the primary chemistry and of the radiological situation at the Beznau units after several changes in the primary chemistry. It has also led to useful recommendations to decrease the dose rates in the future cycles.

In particular, it was explained why the change of the coolant chemistry to pH<sub>300</sub> 7.4 was not an optimum operating condition for the Beznau units. There was a significant increase of the <sup>58</sup>Co activity with a lot of disadvantages with regard to the radiological conditions and which magnitude seemed to have been higher than normally expected after a SGR with alloy 690 TT tubing material.

Therefore, a decision was made to return to the former target pH<sub>300</sub> value of 7.2.
## Literature

[1] E. Sandona, Nucl. Eng. Int., Jan. 1991

[2] U. Weidmann, SG Replacement at Beznau 1, 15th Annual Radiation Exposure Management Seminar, 12.-15.10.1993, Pittsburgh, USA

[3] H. Venz et al., SG Replacement at Beznau NPP, 2nd ISOE Workshop on Occupational Dose, 04.-07.04.2000, Tarragona, Spain

[4] H. Venz, Shutdown Chemistry of the Primary Coolant of the Beznau NPP, WANO Chemistry Workshop, Lyon, France, Nov. 1999

[5] P. Berge, R. Riess, S-O-A of Non-Ferrous Heat Exchanger Tubing in PWR Environments Int. Conf. Interaction of Non-Iron Based Materials with Water and Steam, Piazenza, Italy, June 11-13, 1996

[6] H. Venz et al., Influence of Extended Cycles on Reactor Coolant Chemistry and Collective Dose at Nuclear Power Plant Beznau, Water Chemistry 7, BNES, Bournemouth Oct. 1996

[7] H. Venz et al., Effect of Primary Water Chemistry on Activity Transport at Unit 2 of Beznau PWR Plant 1998JAIF Int. Conf. on Water Chemistry in NPP, Kashiwazaki, Japan, October 13-16, 1998

[8] R. Eimecke, S. Anthoni Ensemble de Mesure et d'Etude de la Contamination des Circuits (EMECC), 7th Int. Conf. on Radiation Shielding, 12.-15. Sept. 1988, Bournemouth (UK)
## Literature

[1] E. Sandona, Nucl. Eng. Int., Jan. 1991

[2] U. Weidmann, SG Replacement at Beznau 1, 15th Annual Radiation Exposure Management Seminar, 12.-15.10.1993, Pittsburgh, USA

[3] H. Venz et al., SG Replacement at Beznau NPP, 2nd ISOE Workshop on Occupational Dose, 04.-07.04.2000, Tarragona, Spain

[4] H. Venz, Shutdown Chemistry of the Primary Coolant of the Beznau NPP, WANO Chemistry Workshop, Lyon, France, Nov. 1999

[5] P. Berge, R. Riess, S-O-A of Non-Ferrous Heat Exchanger Tubing in PWR Environments Int. Conf. Interaction of Non-Iron Based Materials with Water and Steam, Piazenza, Italy, June 11-13, 1996

[6] H. Venz et al., Influence of Extended Cycles on Reactor Coolant Chemistry and Collective Dose at Nuclear Power Plant Beznau, Water Chemistry 7, BNES, Bournemouth Oct. 1996

[7] H. Venz et al., Effect of Primary Water Chemistry on Activity Transport at Unit 2 of Beznau PWR Plant 1998JAIF Int. Conf. on Water Chemistry in NPP, Kashiwazaki, Japan, October 13-16, 1998

[8] R. Eimecke, S. Anthoni Ensemble de Mesure et d'Etude de la Contamination des Circuits (EMECC), 7th Int. Conf. on Radiation Shielding, 12.-15. Sept. 1988, Bournemouth (UK)