# Primary water chemistry optimization for extended fuel cycle operation: results of the “DUO experimentation” after three cycles

Laure Viricel, Caroline Andrieu, Jean-Claude Segura, Alain Rocher, Joël Thomazet, Marie-Hélène Clinard, Frédéric Dacquait

► To cite this version:

Laure Viricel, Caroline Andrieu, Jean-Claude Segura, Alain Rocher, Joël Thomazet, et al.. Primary water chemistry optimization for extended fuel cycle operation: results of the “DUO experimentation” after three cycles. International Conference on Water Chemistry of Nuclear Reactor Systems, Apr 2002, Avignon, France. cea-04463862

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# Primary water chemistry optimization for extended fuel cycle operation: results of the “DUO experimentation” after three cycles

Laure Viricel, Caroline Andrieu, Jean-Claude Segura, Alain Rocher, Joël Thomazet, Marie-Hélène Clinard, Frédéric Dacquait

## ► To cite this version:

Laure Viricel, Caroline Andrieu, Jean-Claude Segura, Alain Rocher, Joël Thomazet, et al.. Primary water chemistry optimization for extended fuel cycle operation: results of the “DUO experimentation” after three cycles. International Conference on Water Chemistry of Nuclear Reactor Systems, Apr 2002, Avignon, France. cea-04463862

HAL Id: cea-04463862

https://cea.hal.science/cea-04463862v1

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
PRIMARY WATER CHEMISTRY OPTIMIZATION FOR EXTENDED FUEL CYCLE
OPERATION : RESULTS OF THE « DUO EXPERIMENTATION » AFTER THREE CYCLES

L. VIRICEL, ELECTRICITE DE FRANCE, GROUPE DES LABORATOIRES, FRANCE

C. ANDRIEU, J.C. SEGURA, ELECTRICITE DE FRANCE, SEPTEN, FRANCE

A. ROCHER, ELECTRICITE DE FRANCE, FTC, GROUPE PREVENTION DES RISQUES, FRANCE

J. THOMAZET, M.H. CLINARD, FRAMATOME ANP, FRANCE

F. DACQUAIT, COMMISSARIAT A L'ENERGIE ATOMIQUE, FRANCE

SUMMARY

The primary coolant conditioning in French nuclear power plants is essentially based on the boron-lithium coordinated chemistry, with a target pH of 7.2 at 300°C and a maximum lithium concentration of 2.2 mg/kg.

In 1996, EDF 1300 MWe units began operating 18-month fuel cycles, increasing boron concentrations at the beginning of the cycles. Since today the maximum lithium concentration in normal operation is 2.2 mg/kg, extended cycle operation results in a decrease in the pH at the beginning of the cycles, which may possibly lead to deposits in RCS, and particularly on the fuel cladding, and increased dose rates. It has to be noted that today, the fuel assemblies maximum burnup is set at 52 GWd/tU.

One solution is to adjust the pH by increasing the lithium content at the beginning of the cycles, which is easy to implement and does not require any modification on the units. Hence, EDF is testing a "modified" chemistry regime in the « Duo experimentation » during 4 fuel cycles, with a maximum authorized lithium content of 3.5 mg/kg at the beginning of the cycles in the Cattenom 2 pilot unit. The Golfech 1 reference unit implements a standard boron-lithium coordination pH₃₀₀ 7.2. The major goal of the experimentation is to assess the impact of elevated lithium concentrations at the beginning of the cycles on fuel cladding oxide behavior, mass transport and dose rates. This paper presents the results of the first three cycles of the Duo experimentation.

Oxide thickness measurements results show that operating with EDF modified chemistry, hence lithium contents up to 3.5 mg/kg at the beginning of the cycles, is encouraging. No influence of elevated lithium concentrations was observed on oxide thickness up to a burnup of about 50 GWd/tU, which is slightly lower than the maximum burnup a fuel assembly is allowed to achieve ; moreover, advanced fuel cladding alloys show a good behavior.

Concerning contamination and dose rates, the ⁵⁸Co surface activities measured in Golfech 1 primary system decreased from the last 12-month cycle to the first 18-month cycle, whereas the ⁶⁰Co surface activity increased after the first two conversion cycles, probably due to an abnormal cobalt input during a conversion cycle. In Cattenom 2, the ⁵⁸Co and ⁶⁰Co surface activities in the steam generators increased from the last 12-month cycle to the third conversion cycle, whereas they remain overall stable or even decrease in the crossover legs and in the hot legs. Overall, the Duo experimentation results are comparable to French plants usual behavior.

While any beneficial impact of a constant pH of 7.2 at 300°C throughout the cycle on dose rates is not proved given the French plants results, the easy implementation of increased lithium concentrations at the beginning of the cycles may help face issues due to new French fuel management strategies, leading to fuel assemblies maximum burnups of 52 GWd/tU.

As the Duo fuel assemblies were re-loaded for a fourth cycle in Cattenom 2, burnups of about 55 to 60 GWd/tU should be achieved. The same surveillance program as during the three previous cycles is applied ; hence, it should provide extensive data, especially on the fuel cladding behavior up to 60 GWd/tU and on contamination when a modified chemistry regime is implemented.

Today, given the pH improvement obtained by operating EDF modified chemistry with increased lithium concentrations up to 3.5 mg/kg at the beginning of the fuel cycles, generalizing this chemistry regime to the French plants is under consideration.
PRIMARY WATER CHEMISTRY OPTIMIZATION FOR EXTENDED FUEL CYCLE
OPERATION : RESULTS OF THE « DUO EXPERIMENTATION » AFTER THREE CYCLES

L. VIRICEL, ELECTRICITE DE FRANCE, GROUPE DES LABORATOIRES, FRANCE

C. ANDRIEU, J.C. SEGURA, ELECTRICITE DE FRANCE, SEPTEN, FRANCE

A. ROCHER, ELECTRICITE DE FRANCE, FTC, GROUPE PREVENTION DES RISQUES, FRANCE

J. THOMAZET, M.H. CLINARD, FRAMATOME ANP, FRANCE

F. DACQUAIT, COMMISSARIAT A L'ENERGIE ATOMIQUE, FRANCE

SUMMARY

The primary coolant conditioning in French nuclear power plants is essentially based on the boron-lithium coordinated chemistry, with a target pH of 7.2 at 300°C and a maximum lithium concentration of 2.2 mg/kg.

In 1996, EDF 1300 MWe units began operating 18-month fuel cycles, increasing boron concentrations at the beginning of the cycles. Since today the maximum lithium concentration in normal operation is 2.2 mg/kg, extended cycle operation results in a decrease in the pH at the beginning of the cycles, which may possibly lead to deposits in RCS, and particularly on the fuel cladding, and increased dose rates. It has to be noted that today, the fuel assemblies maximum burnup is set at 52 GWd/tU.

One solution is to adjust the pH by increasing the lithium content at the beginning of the cycles, which is easy to implement and does not require any modification on the units. Hence, EDF is testing a "modified" chemistry regime in the « Duo experimentation » during 4 fuel cycles, with a maximum authorized lithium content of 3.5 mg/kg at the beginning of the cycles in the Cattenom 2 pilot unit. The Golfech 1 reference unit implements a standard boron-lithium coordination pH₃₀₀ 7.2. The major goal of the experimentation is to assess the impact of elevated lithium concentrations at the beginning of the cycles on fuel cladding oxide behavior, mass transport and dose rates. This paper presents the results of the first three cycles of the Duo experimentation.

Oxide thickness measurements results show that operating with EDF modified chemistry, hence lithium contents up to 3.5 mg/kg at the beginning of the cycles, is encouraging. No influence of elevated lithium concentrations was observed on oxide thickness up to a burnup of about 50 GWd/tU, which is slightly lower than the maximum burnup a fuel assembly is allowed to achieve ; moreover, advanced fuel cladding alloys show a good behavior.

Concerning contamination and dose rates, the ⁵⁸Co surface activities measured in Golfech 1 primary system decreased from the last 12-month cycle to the first 18-month cycle, whereas the ⁶⁰Co surface activity increased after the first two conversion cycles, probably due to an abnormal cobalt input during a conversion cycle. In Cattenom 2, the ⁵⁸Co and ⁶⁰Co surface activities in the steam generators increased from the last 12-month cycle to the third conversion cycle, whereas they remain overall stable or even decrease in the crossover legs and in the hot legs. Overall, the Duo experimentation results are comparable to French plants usual behavior.

While any beneficial impact of a constant pH of 7.2 at 300°C throughout the cycle on dose rates is not proved given the French plants results, the easy implementation of increased lithium concentrations at the beginning of the cycles may help face issues due to new French fuel management strategies, leading to fuel assemblies maximum burnups of 52 GWd/tU.

As the Duo fuel assemblies were re-loaded for a fourth cycle in Cattenom 2, burnups of about 55 to 60 GWd/tU should be achieved. The same surveillance program as during the three previous cycles is applied ; hence, it should provide extensive data, especially on the fuel cladding behavior up to 60 GWd/tU and on contamination when a modified chemistry regime is implemented.

Today, given the pH improvement obtained by operating EDF modified chemistry with increased lithium concentrations up to 3.5 mg/kg at the beginning of the fuel cycles, generalizing this chemistry regime to the French plants is under consideration.
# INTRODUCTION

The primary coolant conditioning in French nuclear power plants is essentially based on the boron-lithium coordinated chemistry, with a pH target value of 7.2 at 300°C and a maximum lithium concentration of 2.2 mg/kg [1]. Some years ago, the target pH value was changed from 7.0 to 7.2 to decrease dose rates [2].

In 1996, EDF 1300 MWe units began operating 18-month extended fuel cycles, increasing fuel uranium oxide enrichment from 3.2 to 4 %. This change induced :

- a significant increase in the boron concentration at the beginning of the fuel cycles (progressively for the first two conversion cycles);

- a decrease of the pH at the beginning of the cycles, since today the maximum lithium concentration in normal operation is 2.2 mg/kg.

However, the twenty 1300 MWe units operating extended cycles are not the only EDF units concerned by higher boron concentrations at the beginning of the cycles. Other French nuclear reactors can be impacted if new core designs are implemented (Mixed OXide, ...).

It is well known that operating at pH₃₀₀ below 6.9 for a significant period of time makes it difficult to keep corrosion products as low as possible, possibly leading to :

- deposits in RCS, and particularly on the fuel cladding, which may participate in axial offset anomalies occurrence [3]-[4];

- increase in dose rates.

One solution is to adjust the pH by increasing the lithium concentration at the beginning of the cycles. This solution is easy to implement and does not require any modification on the units. A new maximum lithium concentration, consistent with materials behavior and especially with fuel cladding, has to be determined.

EDF has decided to test a "modified" chemistry regime in the « Duo experimentation » during 4 fuel cycles, one major goal being assessing of the corrosion behavior of several fuel cladding materials.

This paper deals with the results of the controls and expertise performed during the first three cycles of the Duo experimentation.

# DUO EXPERIMENTATION FEATURES

This experimentation deals with the specific surveillance, from the first conversion cycle to the second 18-month cycle, of two 1300 MWe units operating extended fuel cycles. The reference unit and the pilot unit for this experimentation were chosen to be Golfech 1 and Cattenom 2 respectively. They had to have similar design (steam generator tubes bundle in 600 TT alloy, fuel grids assemblies, no hot spots, no flux anomalies, ...).

Golfech 1 implements a classic boron-lithium coordination pH₃₀₀ 7.2 (Figure 1). Cattenom 2 operates EDF modified chemistry at the beginning of the cycles, with a maximum authorized lithium concentration of 3.5 mg/kg (Figure 2).

The major goal of the experimentation is to assess the impact of elevated lithium concentrations at the beginning of the cycles on fuel cladding oxide behavior, mass transport and dose rates.
## INTRODUCTION

The primary coolant conditioning in French nuclear power plants is essentially based on the boron-lithium coordinated chemistry, with a pH target value of 7.2 at 300°C and a maximum lithium concentration of 2.2 mg/kg [1]. Some years ago, the target pH value was changed from 7.0 to 7.2 to decrease dose rates [2].

In 1996, EDF 1300 MWe units began operating 18-month extended fuel cycles, increasing fuel uranium oxide enrichment from 3.2 to 4 %. This change induced :

- a significant increase in the boron concentration at the beginning of the fuel cycles (progressively for the first two conversion cycles);

- a decrease of the pH at the beginning of the cycles, since today the maximum lithium concentration in normal operation is 2.2 mg/kg.

However, the twenty 1300 MWe units operating extended cycles are not the only EDF units concerned by higher boron concentrations at the beginning of the cycles. Other French nuclear reactors can be impacted if new core designs are implemented (Mixed OXide, ...).

It is well known that operating at pH₃₀₀ below 6.9 for a significant period of time makes it difficult to keep corrosion products as low as possible, possibly leading to :

- deposits in RCS, and particularly on the fuel cladding, which may participate in axial offset anomalies occurrence [3]-[4];

- increase in dose rates.

One solution is to adjust the pH by increasing the lithium concentration at the beginning of the cycles. This solution is easy to implement and does not require any modification on the units. A new maximum lithium concentration, consistent with materials behavior and especially with fuel cladding, has to be determined.

EDF has decided to test a "modified" chemistry regime in the « Duo experimentation » during 4 fuel cycles, one major goal being assessing of the corrosion behavior of several fuel cladding materials.

This paper deals with the results of the controls and expertise performed during the first three cycles of the Duo experimentation.

## DUO EXPERIMENTATION FEATURES

This experimentation deals with the specific surveillance, from the first conversion cycle to the second 18-month cycle, of two 1300 MWe units operating extended fuel cycles. The reference unit and the pilot unit for this experimentation were chosen to be Golfech 1 and Cattenom 2 respectively. They had to have similar design (steam generator tubes bundle in 600 TT alloy, fuel grids assemblies, no hot spots, no flux anomalies, ...).

Golfech 1 implements a classic boron-lithium coordination pH₃₀₀ 7.2 (Figure 1). Cattenom 2 operates EDF modified chemistry at the beginning of the cycles, with a maximum authorized lithium concentration of 3.5 mg/kg (Figure 2).

The major goal of the experimentation is to assess the impact of elevated lithium concentrations at the beginning of the cycles on fuel cladding oxide behavior, mass transport and dose rates.
<figure><img src="image_2.png" /><figcaption>Figure 1 : Classic B-Li coordinated chemistry pH₃₀₀ 7.2 applied in Golfech 1</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2 : EDF B-Li modified chemistry applied in Cattenom 2</figcaption></figure>
<figure><img src="image_2.png" /><figcaption>Figure 1 : Classic B-Li coordinated chemistry pH₃₀₀ 7.2 applied in Golfech 1</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2 : EDF B-Li modified chemistry applied in Cattenom 2</figcaption></figure>
# SURVEILLANCE PROGRAM OF THE DUO EXPERIMENTATION

A specific surveillance program has been implemented during the four cycles of the Duo experimentation (Table 1). During each cycle, the primary coolant surveillance deals with its chemical and radiochemical features. During shutdown transients and outages, it concerns primary coolant activity, surface activity and fuel oxide thickness measurements.

These controls aim at assessing the performance of an extended cycles modified chemistry when elevated initial boron concentrations are used compared to a classic chemistry, which may lead to operate, at the beginning of the cycles, at a less favorable pH for fuel crud deposits and dose rates.

<table><caption>Table 1: Surveyed parameters</caption><thead><tr><th></th><th>Field</th><th>Surveillance</th></tr></thead><tbody><tr><td rowspan="5">During operation</td><td>Chemistry</td><td>B, Li, H<sub>2</sub><br>Ca, Mg, Al, SiO<sub>2</sub><br>Cr, Ni, Co<br>CVCS filters<br>O<sub>2</sub> in boron and water makeup tanks<br>Volumes of boron and water makeups</td></tr><tr><td>Radiochemistry</td><td>Gammaspectrometry</td></tr><tr><td>During shutdown transient</td><td>Radiochemistry</td><td>Primary coolant activity</td></tr><tr><td rowspan="3">During outage</td><td>Contamination</td><td>Radioactive corrosion products on crossover legs,<br>SG tubes bundle, hot legs</td></tr><tr><td>Fuel</td><td>Televisual inspection, oxide thickness</td></tr><tr><td>SG tubing</td><td>Regular non destructive examinations</td></tr></tbody></table>

# DUO EXPERIMENTATION : RESULTS AFTER THREE CYCLES

## Chemistry

During the first three cycles of the Duo experimentation in Golfech 1 and Cattenom 2, despite load following, the boron-lithium coordinated chemistry regime matched the specifications (Figures 3 and 4). Data out of the normal operating area are essentially due to shutdowns, physical testing, rapid load variations and makeups for tritium dilution in RCS. No specific pollution was observed during these cycles in Golfech 1 nor in Cattenom 2.
# SURVEILLANCE PROGRAM OF THE DUO EXPERIMENTATION

A specific surveillance program has been implemented during the four cycles of the Duo experimentation (Table 1). During each cycle, the primary coolant surveillance deals with its chemical and radiochemical features. During shutdown transients and outages, it concerns primary coolant activity, surface activity and fuel oxide thickness measurements.

These controls aim at assessing the performance of an extended cycles modified chemistry when elevated initial boron concentrations are used compared to a classic chemistry, which may lead to operate, at the beginning of the cycles, at a less favorable pH for fuel crud deposits and dose rates.

<table><caption>Table 1: Surveyed parameters</caption><thead><tr><th></th><th>Field</th><th>Surveillance</th></tr></thead><tbody><tr><td rowspan="5">During operation</td><td>Chemistry</td><td>B, Li, H<sub>2</sub><br>Ca, Mg, Al, SiO<sub>2</sub><br>Cr, Ni, Co<br>CVCS filters<br>O<sub>2</sub> in boron and water makeup tanks<br>Volumes of boron and water makeups</td></tr><tr><td>Radiochemistry</td><td>Gammaspectrometry</td></tr><tr><td>During shutdown transient</td><td>Radiochemistry</td><td>Primary coolant activity</td></tr><tr><td rowspan="3">During outage</td><td>Contamination</td><td>Radioactive corrosion products on crossover legs,<br>SG tubes bundle, hot legs</td></tr><tr><td>Fuel</td><td>Televisual inspection, oxide thickness</td></tr><tr><td>SG tubing</td><td>Regular non destructive examinations</td></tr></tbody></table>

## DUO EXPERIMENTATION : RESULTS AFTER THREE CYCLES

### Chemistry

During the first three cycles of the Duo experimentation in Golfech 1 and Cattenom 2, despite load following, the boron-lithium coordinated chemistry regime matched the specifications (Figures 3 and 4). Data out of the normal operating area are essentially due to shutdowns, physical testing, rapid load variations and makeups for tritium dilution in RCS. No specific pollution was observed during these cycles in Golfech 1 nor in Cattenom 2.
<figure><img src="image_4.png" /><figcaption>Figure 3 : Golfech 1, cycle 8, 3<sup>rd</sup> cycle of the Duo Experimentation : B-Li diagram</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4 : Cattenom 2, cycle 10, 3<sup>rd</sup> cycle of the Duo Experimentation : B-Li diagram</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3 : Golfech 1, cycle 8, 3<sup>rd</sup> cycle of the Duo Experimentation : B-Li diagram</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4 : Cattenom 2, cycle 10, 3<sup>rd</sup> cycle of the Duo Experimentation : B-Li diagram</figcaption></figure>
The initial boron concentrations at hot zero power and full power are reported in table 2.

<table><caption>Table 2 : Boron at hot zero power and full power</caption><thead><tr><th rowspan="2"></th><th colspan="3">GOLFECH 1</th><th colspan="3">CATTENOM 2</th></tr><tr><th>Cycle #</th><th>B at hot zero power (mg/kg)</th><th>B at full power (mg/kg)</th><th>Cycle #</th><th>B at hot zero power (mg/kg)</th><th>B at full power (mg/kg)</th></tr></thead><tbody><tr><td>Last 12-month cycle</td><td>5</td><td></td><td>970</td><td>7</td><td></td><td>1020</td></tr><tr><td>1<sup>st</sup> conversion cycle</td><td>6</td><td>~ 1450</td><td>1020</td><td>8</td><td>~ 1480</td><td>1020</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>7</td><td>~ 1450</td><td>1010</td><td>9</td><td>~ 1580</td><td>1100</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>8</td><td>~ 1630</td><td>1220</td><td>10</td><td>~ 1650</td><td>1163</td></tr></tbody></table>

The Cattenom 2 pilot unit maintained a lithium concentration between 3.5 and 2.2 mg/kg for about three months during its third conversion cycle; operation with a lithium content between 3.5 and 2.5 mg/kg lasted about one month (Table 3).

<table><caption>Table 3 : Operating time with lithium concentrations greater than 2.2 mg/kg in Cattenom 2</caption><thead><tr><th></th><th>Cycle #</th><th>Operating time with lithium between 3.5 and 2.5 mg/kg</th><th>Operating time with lithium between 2.5 and 2.2 mg/kg</th></tr></thead><tbody><tr><td>Last 12-month cycle</td><td>7</td><td>0</td><td>0</td></tr><tr><td>1<sup>st</sup> conversion cycle</td><td>8</td><td>~ 1 week</td><td>~ 2 weeks</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>9</td><td>~ 2 weeks</td><td>~ 7 weeks</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>10</td><td>~ 4 weeks</td><td>~ 8 weeks</td></tr></tbody></table>

In the Golfech 1 reference unit, pH was around 6.8 at 300°C at the beginning of the first and second conversion cycles, and 2 to 3 days were needed to reach 6.9. At the beginning of the first 18-month cycle (third conversion cycle), pH was around 6.7 and about 10 days were needed to reach 6.9; overall, it took about 42 weeks to reach a pH of 7.2 at 300°C. In Cattenom 2, pH at the beginning of the cycles was always above 6.9 at 300°C, and 32 weeks were needed during the third conversion cycle to reach a pH of 7.2 at 300°C. The modified chemistry in Cattenom 2 allows a better control of the pH just before criticality than does the standard chemistry regime in Golfech 1.

Feed-and-bleeds due to load following may result in primary water chemistry transient changes because of system equilibrium disrupting, dilution, oxygen ingress, etc ... Particularly, water and boron makeups may generate oxygen ingress in RCS which may impact the coolant-materials interactions. Table 4 summarizes the makeup volumes in RCS in Golfech 1 and Cattenom 2. The greater makeup volumes in Golfech 1 compared to Cattenom 2 can be explained by more load following in Golfech. Similarly, the variations in the volumes used during the different cycles on both units is explained by different load variations.

<table><caption>Table 4 : Golfech 1 and Cattenom 2 makeups to RCS</caption><thead><tr><th rowspan="2"></th><th colspan="3">Makeup water</th><th colspan="2">Boron makeup water</th></tr><tr><th>Cycle #</th><th>Volume (m<sup>3</sup>)</th><th>Mean dissolved O<sub>2</sub> (µg/kg)</th><th>Volume (m<sup>3</sup>)</th><th>Mean dissolved O<sub>2</sub> (µg/kg)</th></tr></thead><tbody><tr><td colspan="6"><strong>Golfech 1</strong></td></tr><tr><td>1<sup>st</sup> conversion cycle</td><td>6</td><td>12304</td><td>37</td><td>827</td><td>1700</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>7</td><td>9242</td><td>49</td><td>714</td><td>1777</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>8</td><td>14398</td><td>57</td><td>1381</td><td>1923</td></tr><tr><td colspan="6"><strong>Cattenom 2</strong></td></tr><tr><td>1<sup>st</sup> conversion cycle</td><td>8</td><td>9600</td><td>45</td><td>467</td><td>2200</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>9</td><td>8528</td><td>51</td><td>444</td><td>1523</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>10</td><td>9963</td><td>54</td><td>694</td><td>2502</td></tr></tbody></table>
The initial boron concentrations at hot zero power and full power are reported in table 2.

<table><caption>Table 2 : Boron at hot zero power and full power</caption><thead><tr><th rowspan="2"></th><th colspan="3">GOLFECH 1</th><th colspan="3">CATTENOM 2</th></tr><tr><th>Cycle #</th><th>B at hot zero power (mg/kg)</th><th>B at full power (mg/kg)</th><th>Cycle #</th><th>B at hot zero power (mg/kg)</th><th>B at full power (mg/kg)</th></tr></thead><tbody><tr><td>Last 12-month cycle</td><td>5</td><td></td><td>970</td><td>7</td><td></td><td>1020</td></tr><tr><td>1<sup>st</sup> conversion cycle</td><td>6</td><td>~ 1450</td><td>1020</td><td>8</td><td>~ 1480</td><td>1020</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>7</td><td>~ 1450</td><td>1010</td><td>9</td><td>~ 1580</td><td>1100</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>8</td><td>~ 1630</td><td>1220</td><td>10</td><td>~ 1650</td><td>1163</td></tr></tbody></table>

The Cattenom 2 pilot unit maintained a lithium concentration between 3.5 and 2.2 mg/kg for about three months during its third conversion cycle; operation with a lithium content between 3.5 and 2.5 mg/kg lasted about one month (Table 3).

<table><caption>Table 3 : Operating time with lithium concentrations greater than 2.2 mg/kg in Cattenom 2</caption><thead><tr><th></th><th>Cycle #</th><th>Operating time with lithium between 3.5 and 2.5 mg/kg</th><th>Operating time with lithium between 2.5 and 2.2 mg/kg</th></tr></thead><tbody><tr><td>Last 12-month cycle</td><td>7</td><td>0</td><td>0</td></tr><tr><td>1<sup>st</sup> conversion cycle</td><td>8</td><td>~ 1 week</td><td>~ 2 weeks</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>9</td><td>~ 2 weeks</td><td>~ 7 weeks</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>10</td><td>~ 4 weeks</td><td>~ 8 weeks</td></tr></tbody></table>

In the Golfech 1 reference unit, pH was around 6.8 at 300°C at the beginning of the first and second conversion cycles, and 2 to 3 days were needed to reach 6.9. At the beginning of the first 18-month cycle (third conversion cycle), pH was around 6.7 and about 10 days were needed to reach 6.9; overall, it took about 42 weeks to reach a pH of 7.2 at 300°C. In Cattenom 2, pH at the beginning of the cycles was always above 6.9 at 300°C, and 32 weeks were needed during the third conversion cycle to reach a pH of 7.2 at 300°C. The modified chemistry in Cattenom 2 allows a better control of the pH just before criticality than does the standard chemistry regime in Golfech 1.

Feed-and-bleeds due to load following may result in primary water chemistry transient changes because of system equilibrium disrupting, dilution, oxygen ingress, etc ... Particularly, water and boron makeups may generate oxygen ingress in RCS which may impact the coolant-materials interactions. Table 4 summarizes the makeup volumes in RCS in Golfech 1 and Cattenom 2. The greater makeup volumes in Golfech 1 compared to Cattenom 2 can be explained by more load following in Golfech. Similarly, the variations in the volumes used during the different cycles on both units is explained by different load variations.

<table><caption>Table 4 : Golfech 1 and Cattenom 2 makeups to RCS</caption><thead><tr><th rowspan="2"></th><th colspan="3">Makeup water</th><th colspan="2">Boron makeup water</th></tr><tr><th>Cycle #</th><th>Volume (m³)</th><th>Mean dissolved O₂ (µg/kg)</th><th>Volume (m³)</th><th>Mean dissolved O₂ (µg/kg)</th></tr></thead><tbody><tr><td colspan="6"><strong>Golfech 1</strong></td></tr><tr><td>1<sup>st</sup> conversion cycle</td><td>6</td><td>12304</td><td>37</td><td>827</td><td>1700</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>7</td><td>9242</td><td>49</td><td>714</td><td>1777</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>8</td><td>14398</td><td>57</td><td>1381</td><td>1923</td></tr><tr><td colspan="6"><strong>Cattenom 2</strong></td></tr><tr><td>1<sup>st</sup> conversion cycle</td><td>8</td><td>9600</td><td>45</td><td>467</td><td>2200</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>9</td><td>8528</td><td>51</td><td>444</td><td>1523</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>10</td><td>9963</td><td>54</td><td>694</td><td>2502</td></tr></tbody></table>
In table 5 are reported the results of corrosion products measurements performed in Golfech 1 and Cattenom 2 during power increase at the beginning of the Duo experimentation cycles or during power decrease before refueling shutdown. These results agree with the values typically observed in French plants. In Golfech 1, the concentration in soluble nickel is higher at the beginning of the second conversion cycle and of the first real 18-month cycle than it was for the first conversion cycle. For both units, no significant differences can be observed at the end of the cycles, from the second to the third conversion cycle, except for a slight increase in the chromium content in Golfech 1 and a significant decrease at 0 % power in Cattenom 2.

<table><thead><tr><th>Unit</th><th>Cycle</th><th>Cycle #</th><th>% Power</th><th>Ni (ng/kg)</th><th>Co (ng/kg)</th><th>Cr (ng/kg)</th></tr></thead><tbody><tr><td colspan="7" style="text-align: center; font-weight: bold;">Power increase at beginning of cycles</td></tr><tr><td rowspan="3">GOL 1</td><td>1<sup>st</sup> conversion cycle</td><td>6</td><td>100</td><td>350</td><td>6</td><td>1 600</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>7</td><td>100</td><td>3 170</td><td>10</td><td>1 190</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>8</td><td>100</td><td>2 300</td><td>&lt; 20</td><td>500</td></tr><tr><td rowspan="3">CAT 2</td><td>1<sup>st</sup> conversion cycle</td><td>8</td><td>8<br>100</td><td>11 000<br>260</td><td>&lt; 4<br>&lt; 4</td><td>60<br>230</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>9</td><td>8<br>100</td><td>1 355<br>1 012</td><td>22<br>&lt; 4</td><td>117<br>NA</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>10</td><td>8<br>100</td><td>4 780<br>660</td><td>20<br>6</td><td>180<br>200</td></tr><tr><td colspan="7" style="text-align: center; font-weight: bold;">Power decrease at end of cycles</td></tr><tr><td rowspan="3">GOL 1</td><td rowspan="3">2<sup>nd</sup> conversion cycle</td><td rowspan="3">7</td><td>90</td><td>392</td><td>&lt; 10</td><td>166</td></tr><tr><td>70</td><td>215</td><td>&lt; 4</td><td>190</td></tr><tr><td>0 (290°C)</td><td>20 000</td><td>NA</td><td>800</td></tr><tr><td rowspan="3">3<sup>rd</sup> conversion cycle</td><td rowspan="3">8</td><td>80</td><td>2 630</td><td>&lt; 100</td><td>1 290</td></tr><tr><td>0 (296°C)</td><td>16 400</td><td>&lt; 100</td><td>1 320</td></tr><tr><td>0 (175°C)</td><td>730 000</td><td>4 000</td><td>1 100</td></tr><tr><td rowspan="3">CAT 2</td><td rowspan="3">2<sup>nd</sup> conversion cycle</td><td rowspan="3">9</td><td>0 (120°C)</td><td>960 000</td><td>6 500</td><td>1 400</td></tr><tr><td>75</td><td>890</td><td>&lt; 4</td><td>690</td></tr><tr><td>0 (286°C)</td><td>610</td><td>NA</td><td>1 150</td></tr><tr><td rowspan="3">3<sup>rd</sup> conversion cycle</td><td rowspan="3">10</td><td>0 (175°C)</td><td>&gt; 200 000</td><td>NA</td><td>3 210</td></tr><tr><td>0 (120°C)</td><td>1 584 000</td><td>NA</td><td>6 880</td></tr><tr><td>80</td><td>440</td><td>&lt; 10</td><td>1 070</td></tr><tr><td rowspan="3">CAT 2</td><td rowspan="3">3<sup>rd</sup> conversion cycle</td><td rowspan="3">10</td><td>0 (290°C)</td><td>370</td><td>&lt; 10</td><td>320</td></tr><tr><td>0 (175°C)</td><td>1 060 000</td><td>720</td><td>970</td></tr><tr><td>0 (120°C)</td><td>1 080 000</td><td>870</td><td>460</td></tr></tbody></table>

NA = Not Available

<caption>Table 5 : Soluble corrosion products in Golfech 1 and Cattenom 2</caption>
In table 5 are reported the results of corrosion products measurements performed in Golfech 1 and Cattenom 2 during power increase at the beginning of the Duo experimentation cycles or during power decrease before refueling shutdown. These results agree with the values typically observed in French plants. In Golfech 1, the concentration in soluble nickel is higher at the beginning of the second conversion cycle and of the first real 18-month cycle than it was for the first conversion cycle. For both units, no significant differences can be observed at the end of the cycles, from the second to the third conversion cycle, except for a slight increase in the chromium content in Golfech 1 and a significant decrease at 0 % power in Cattenom 2.

<table><thead><tr><th>Unit</th><th>Cycle</th><th>Cycle #</th><th>% Power</th><th>Ni (ng/kg)</th><th>Co (ng/kg)</th><th>Cr (ng/kg)</th></tr></thead><tbody><tr><td colspan="7" style="text-align: center; font-weight: bold;">Power increase at beginning of cycles</td></tr><tr><td rowspan="3">GOL 1</td><td>1<sup>st</sup> conversion cycle</td><td>6</td><td>100</td><td>350</td><td>6</td><td>1 600</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>7</td><td>100</td><td>3 170</td><td>10</td><td>1 190</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>8</td><td>100</td><td>2 300</td><td>&lt; 20</td><td>500</td></tr><tr><td rowspan="3">CAT 2</td><td>1<sup>st</sup> conversion cycle</td><td>8</td><td>8<br>100</td><td>11 000<br>260</td><td>&lt; 4<br>&lt; 4</td><td>60<br>230</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>9</td><td>8<br>100</td><td>1 355<br>1 012</td><td>22<br>&lt; 4</td><td>117<br>NA</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>10</td><td>8<br>100</td><td>4 780<br>660</td><td>20<br>6</td><td>180<br>200</td></tr><tr><td colspan="7" style="text-align: center; font-weight: bold;">Power decrease at end of cycles</td></tr><tr><td rowspan="4">GOL 1</td><td rowspan="3">2<sup>nd</sup> conversion cycle</td><td rowspan="3">7</td><td>90</td><td>392</td><td>&lt; 10</td><td>166</td></tr><tr><td>70</td><td>215</td><td>&lt; 4</td><td>190</td></tr><tr><td>0 (290°C)</td><td>20 000</td><td>NA</td><td>800</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>8</td><td>80<br>0 (296°C)<br>0 (175°C)<br>0 (120°C)</td><td>2 630<br>16 400<br>730 000<br>960 000</td><td>&lt; 100<br>&lt; 100<br>4 000<br>6 500</td><td>1 290<br>1 320<br>1 100<br>1 400</td></tr><tr><td rowspan="4">CAT 2</td><td rowspan="3">2<sup>nd</sup> conversion cycle</td><td rowspan="3">9</td><td>75<br>0 (286°C)<br>0 (175°C)<br>0 (120°C)</td><td>890<br>610<br>&gt; 200 000<br>1 584 000</td><td>&lt; 4<br>NA<br>NA<br>NA</td><td>690<br>1 150<br>3 210<br>6 880</td></tr><tr><td>80<br>0 (290°C)<br>0 (175°C)<br>0 (120°C)</td><td>440<br>370<br>1 060 000<br>1 080 000</td><td>&lt; 10<br>&lt; 10<br>720<br>870</td><td>1 070<br>320<br>970<br>460</td></tr><tr><td></td><td></td><td></td><td></td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>10</td><td></td><td></td><td></td><td></td></tr></tbody></table>

NA = Not Available

<caption>Table 5 : Soluble corrosion products in Golfech 1 and Cattenom 2</caption>
## Radiochemistry

Spectrogammametry was used to measure cobalt 58, cobalt 60 and chromium 51 in RCS during the DUO experimentation in Golfech 1 and Cattenom 2. Activities are overall below:

- 45 MBq/t for <sup>58</sup>Co in Golfech and 40 MBq/t in Cattenom;

- 5 MBq/t for <sup>60</sup>Co in both units;

- 40 MBq/t for <sup>51</sup>Cr in Golfech and 25 MBq/t in Cattenom.

Some higher activities could sometimes be observed, most of them related to significant makeups or load reductions to 0 % power.

The activities measured during shutdowns are presented in table 6. In Golfech 1, the cobalt 58 peak activity was similar for the second conversion cycle and for the first real 18-month cycle; it was somewhat smaller than previously observed. In Cattenom 2, an increase was noted at the end of the first conversion cycle compared to the last 12-month cycle; then, it decreased continuously until the third conversion cycle. In Cattenom 2, the total released activity increased during the first two conversion cycles, but at the end of the third one, it decreased and reached the initial level before extended cycle operation. So today, no significant effect of the cycles length is observed on the released activity. Moreover, the two units behave similarly, despite the differences in the chemistry regime implemented for the last three cycles: they have shown an overall decreasing trend in the <sup>58</sup>Co peak activity for several years.

<table><thead><tr><th rowspan="2"></th><th colspan="3">Golfech 1</th><th colspan="3">Cattenom 2</th></tr><tr><th>Cycle #</th><th><sup>58</sup>Co peak activity (GBq/t)</th><th>Total released activity (GBq)</th><th>Cycle #</th><th><sup>58</sup>Co peak activity (GBq/t)</th><th>Total released activity (GBq)</th></tr></thead><tbody><tr><th>Last 12-month cycle</th><td>5</td><td>160</td><td>100 000</td><td>7</td><td>165</td><td>100 000</td></tr><tr><th>1<sup>st</sup> conversion cycle</th><td>6</td><td>170</td><td>60 000</td><td>8</td><td>207</td><td>134 000</td></tr><tr><th>2<sup>nd</sup> conversion cycle</th><td>7</td><td>105</td><td>70 000</td><td>9</td><td>197</td><td>135 000</td></tr><tr><th>3<sup>rd</sup> conversion cycle</th><td>8</td><td>100 + 33 <sup>(1)</sup></td><td></td><td>10</td><td>180</td><td>100 000</td></tr></tbody></table>

(1) : A cold shutdown with oxygenation occurred three months before refuelling shutdown; the <sup>58</sup>Co peak activity was 100 GBq/t for this shutdown and 33 GBq/t for the refuelling shutdown

<caption>Table 6 : Shutdown activities</caption>

## Deposits

Deposited activities were measured on steam generators (SG) tube bundle, hot legs (HL) and crossover legs (CL). Cobalt 58 and cobalt 60 representing more than 80 % of the dose rates around the primary system, only their activities are reported in tables 7 and 8 for Golfech 1 and Cattenom 2 respectively. The accuracy is about 25 % for <sup>58</sup>Co measurements and 15 % for <sup>60</sup>Co measurements. Moreover, dose rates were measured around steam generators and legs (table 9).

<table><caption>Table 7 : Surface activity in Golfech 1</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Cycle #</th><th colspan="2">SG (GBq/m<sup>2</sup>)</th><th colspan="2">HL (GBq/m<sup>2</sup>)</th><th colspan="2">CL (GBq/m<sup>2</sup>)</th></tr><tr><th><sup>58</sup>Co</th><th><sup>60</sup>Co</th><th><sup>58</sup>Co</th><th><sup>60</sup>Co</th><th><sup>58</sup>Co</th><th><sup>60</sup>Co</th></tr></thead><tbody><tr><th>Last 12-month cycle</th><td>5</td><td>2.41</td><td>0.30</td><td>1.81</td><td>0.71</td><td>4.31</td><td>0.64</td></tr><tr><th>1<sup>st</sup> conversion cycle</th><td>6</td><td>2.36</td><td>0.52</td><td>1.49</td><td>0.89</td><td>3.69</td><td>0.89</td></tr><tr><th>2<sup>nd</sup> conversion cycle</th><td>7</td><td>2.42</td><td>0.61</td><td>1.21</td><td>2.03</td><td>3.53</td><td>1.02</td></tr><tr><th>3<sup>rd</sup> conversion cycle</th><td>8</td><td>1.72</td><td>0.60</td><td>1.07</td><td>1.46</td><td>3.12</td><td>0.99</td></tr></tbody></table>

<table><caption>Table 8 : Surface activity in Cattenom 2</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Cycle #</th><th colspan="2">SG (GBq/m<sup>2</sup>)</th><th colspan="2">HL (GBq/m<sup>2</sup>)</th><th colspan="2">CL (GBq/m<sup>2</sup>)</th></tr><tr><th><sup>58</sup>Co</th><th><sup>60</sup>Co</th><th><sup>58</sup>Co</th><th><sup>60</sup>Co</th><th><sup>58</sup>Co</th><th><sup>60</sup>Co</th></tr></thead><tbody><tr><th>Last 12-month cycle</th><td>7</td><td>2.06</td><td>0.53</td><td>2.35</td><td>1.49</td><td>1.72</td><td>0.73</td></tr><tr><th>1<sup>st</sup> conversion cycle</th><td>8</td><td>2.23</td><td>0.57</td><td>1.50</td><td>1.62</td><td>1.52</td><td>0.68</td></tr><tr><th>2<sup>nd</sup> conversion cycle</th><td>9</td><td>3.20</td><td>0.59</td><td>1.41</td><td>1.04</td><td>1.77</td><td>0.63</td></tr><tr><th>3<sup>rd</sup> conversion cycle</th><td>10</td><td>2.64</td><td>0.61</td><td>1.06</td><td>1.50</td><td>1.46</td><td>0.57</td></tr></tbody></table>
## Radiochemistry

Spectrogammametry was used to measure cobalt 58, cobalt 60 and chromium 51 in RCS during the DUO experimentation in Golfech 1 and Cattenom 2. Activities are overall below:

- 45 MBq/t for ⁵⁸Co in Golfech and 40 MBq/t in Cattenom;

- 5 MBq/t for ⁶⁰Co in both units;

- 40 MBq/t for ⁵¹Cr in Golfech and 25 MBq/t in Cattenom.

Some higher activities could sometimes be observed, most of them related to significant makeups or load reductions to 0 % power.

The activities measured during shutdowns are presented in table 6. In Golfech 1, the cobalt 58 peak activity was similar for the second conversion cycle and for the first real 18-month cycle; it was somewhat smaller than previously observed. In Cattenom 2, an increase was noted at the end of the first conversion cycle compared to the last 12-month cycle; then, it decreased continuously until the third conversion cycle. In Cattenom 2, the total released activity increased during the first two conversion cycles, but at the end of the third one, it decreased and reached the initial level before extended cycle operation. So today, no significant effect of the cycles length is observed on the released activity. Moreover, the two units behave similarly, despite the differences in the chemistry regime implemented for the last three cycles: they have shown an overall decreasing trend in the ⁵⁸Co peak activity for several years.

<table><thead><tr><th rowspan="2"></th><th colspan="3">Golfech 1</th><th colspan="3">Cattenom 2</th></tr><tr><th>Cycle #</th><th>⁵⁸Co peak activity (GBq/t)</th><th>Total released activity (GBq)</th><th>Cycle #</th><th>⁵⁸Co peak activity (GBq/t)</th><th>Total released activity (GBq)</th></tr></thead><tbody><tr><th>Last 12-month cycle</th><td>5</td><td>160</td><td>100 000</td><td>7</td><td>165</td><td>100 000</td></tr><tr><th>1<sup>st</sup> conversion cycle</th><td>6</td><td>170</td><td>60 000</td><td>8</td><td>207</td><td>134 000</td></tr><tr><th>2<sup>nd</sup> conversion cycle</th><td>7</td><td>105</td><td>70 000</td><td>9</td><td>197</td><td>135 000</td></tr><tr><th>3<sup>rd</sup> conversion cycle</th><td>8</td><td>100 + 33 <sup>(1)</sup></td><td></td><td>10</td><td>180</td><td>100 000</td></tr></tbody></table>

(1) : A cold shutdown with oxygenation occurred three months before refuelling shutdown; the ⁵⁸Co peak activity was 100 GBq/t for this shutdown and 33 GBq/t for the refuelling shutdown

<caption>Table 6 : Shutdown activities</caption>

## Deposits

Deposited activities were measured on steam generators (SG) tube bundle, hot legs (HL) and crossover legs (CL). Cobalt 58 and cobalt 60 representing more than 80 % of the dose rates around the primary system, only their activities are reported in tables 7 and 8 for Golfech 1 and Cattenom 2 respectively. The accuracy is about 25 % for ⁵⁸Co measurements and 15 % for ⁶⁰Co measurements. Moreover, dose rates were measured around steam generators and legs (table 9).

<table><caption>Table 7 : Surface activity in Golfech 1</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Cycle #</th><th colspan="2">SG (GBq/m²)</th><th colspan="2">HL (GBq/m²)</th><th colspan="2">CL (GBq/m²)</th></tr><tr><th>⁵⁸Co</th><th>⁶⁰Co</th><th>⁵⁸Co</th><th>⁶⁰Co</th><th>⁵⁸Co</th><th>⁶⁰Co</th></tr></thead><tbody><tr><th>Last 12-month cycle</th><td>5</td><td>2.41</td><td>0.30</td><td>1.81</td><td>0.71</td><td>4.31</td><td>0.64</td></tr><tr><th>1<sup>st</sup> conversion cycle</th><td>6</td><td>2.36</td><td>0.52</td><td>1.49</td><td>0.89</td><td>3.69</td><td>0.89</td></tr><tr><th>2<sup>nd</sup> conversion cycle</th><td>7</td><td>2.42</td><td>0.61</td><td>1.21</td><td>2.03</td><td>3.53</td><td>1.02</td></tr><tr><th>3<sup>rd</sup> conversion cycle</th><td>8</td><td>1.72</td><td>0.60</td><td>1.07</td><td>1.46</td><td>3.12</td><td>0.99</td></tr></tbody></table>

<table><caption>Table 8 : Surface activity in Cattenom 2</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Cycle #</th><th colspan="2">SG (GBq/m²)</th><th colspan="2">HL (GBq/m²)</th><th colspan="2">CL (GBq/m²)</th></tr><tr><th>⁵⁸Co</th><th>⁶⁰Co</th><th>⁵⁸Co</th><th>⁶⁰Co</th><th>⁵⁸Co</th><th>⁶⁰Co</th></tr></thead><tbody><tr><th>Last 12-month cycle</th><td>7</td><td>2.06</td><td>0.53</td><td>2.35</td><td>1.49</td><td>1.72</td><td>0.73</td></tr><tr><th>1<sup>st</sup> conversion cycle</th><td>8</td><td>2.23</td><td>0.57</td><td>1.50</td><td>1.62</td><td>1.52</td><td>0.68</td></tr><tr><th>2<sup>nd</sup> conversion cycle</th><td>9</td><td>3.20</td><td>0.59</td><td>1.41</td><td>1.04</td><td>1.77</td><td>0.63</td></tr><tr><th>3<sup>rd</sup> conversion cycle</th><td>10</td><td>2.64</td><td>0.61</td><td>1.06</td><td>1.50</td><td>1.46</td><td>0.57</td></tr></tbody></table>
<table><caption>Table 9 : Dose rates</caption><thead><tr><th rowspan="2"></th><th colspan="3">Golfech 1 (mSv/h)</th><th colspan="3">Cattenom 2 (mSv/h)</th></tr><tr><th>Cycle #</th><th>SG</th><th>Legs</th><th>Cycle #</th><th>SG</th><th>Legs</th></tr></thead><tbody><tr><td>Last 12-month cycle</td><td>5</td><td>0.11</td><td>0.28</td><td>7</td><td>0.15</td><td>0.27</td></tr><tr><td>1<sup>st</sup> conversion cycle</td><td>6</td><td>0.14</td><td>0.40</td><td>8</td><td>0.12</td><td>0.22</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>7</td><td>0.13</td><td>0.26</td><td>9</td><td>0.15</td><td>0.29</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>8</td><td>0.13</td><td>0.21</td><td>10</td><td>0.15</td><td>0.31</td></tr></tbody></table>

In Golfech 1, the <sup>58</sup>Co surface activity in the steam generators was constant from the last 12-month cycles to the second conversion cycle, and then decreased at the end of the first 18-month cycle ; the <sup>58</sup>Co surface activity in the hot legs and crossover legs decreased constantly from the last 12-month cycle to the first 18-month cycle. The <sup>60</sup>Co surface activity results increased from the last 12-month cycle to the second conversion cycle, probably because of an abnormal input of cobalt (i.e. stellites) [5] ; then, it stabilized at the end of the first 18-month cycle.

In Cattenom 2, the <sup>58</sup>Co surface activity in the steam generators increased significantly from the first to the second conversion cycle, then decreased at the end of the third conversion cycle. From the last 12-month cycle to the third conversion cycle, the <sup>58</sup>Co surface activity decreased constantly in the hot legs and remained stable in the crossover legs. The <sup>60</sup>Co surface activity increased slightly from the last 12-month cycle to the third conversion cycle in the steam generators and decreased slightly in the crossover legs ; it remained overall stable in the hot legs when comparing the last 12-month cycle to the third conversion cycle. Concerning dose rates, the values of legs are impacted by high surrounding dose rates : 50 % to 70 % of the dose rates in the primary coolant pipes come from the auxiliary system.

Hence, compared to the previous cycles, the results of the third conversion cycle show that :

- in Golfech 1, the <sup>58</sup>Co surface activity in the primary system decreased from the last 12-month cycle to the first 18-month cycle, whereas the <sup>60</sup>Co surface activity increased after the first two conversion cycles, probably due to an abnormal cobalt input ;

- in Cattenom 2, the <sup>58</sup>Co and <sup>60</sup>Co surface activities in the steam generators increased from the last 12-month cycle to the third conversion cycle, whereas they remain overall stable or even decrease in the crossover legs and in the hot legs.

Overall, the cobalt 58 and cobalt 60 surface activities measured in the Golfech 1 and Cattenom 2 primary systems are comparable to French plants usual behavior.

## Fuel assemblies

In order to assess the impact of the reactor coolant chemistry on the fuel cladding corrosion, oxide thickness measurements were planned on characterized fuel assemblies loaded in the Golfech 1 and Cattenom 2 reactors.

Four characterized fuel assemblies were loaded in Golfech 1 and Cattenom 2. The fuel rods claddings are made of Zircaloy-4, alloy M4 or alloy M5. Each alloy comes from the same tube batch. The chemical composition of the alloys is reported in table 10.

<table><caption>Table 10 : Chemical composition of fuel rods cladding alloys</caption><thead><tr><th>Alloy</th><th>Sn (%)</th><th>Nb (%)</th><th>Fe (%)</th><th>Cr (%)</th><th>V (%)</th><th>O (%)</th></tr></thead><tbody><tr><td>Zircaloy-4</td><td>1.3</td><td></td><td>0.2</td><td>0.1</td><td></td><td>0.12</td></tr><tr><td>M4</td><td>0.5</td><td></td><td>0.6</td><td></td><td>0.4</td><td>0.12</td></tr><tr><td>M5</td><td></td><td>1</td><td></td><td></td><td></td><td>0.12</td></tr></tbody></table>

The fuel rods were loaded into eight assemblies so as to distribute uniformly all the rods cladding batches into each assembly. Their locations in the assemblies are shown in figure 5. Concerning the advanced alloys, the location of the corresponding rods were chosen in order to be able to measure zirconia layer thickness along four axes 90° apart. However, some of them were also loaded in peripheral location in order to run in-depth visual examinations.
<table><caption>Table 9 : Dose rates</caption><thead><tr><th rowspan="2"></th><th colspan="3">Golfech 1 (mSv/h)</th><th colspan="3">Cattenom 2 (mSv/h)</th></tr><tr><th>Cycle #</th><th>SG</th><th>Legs</th><th>Cycle #</th><th>SG</th><th>Legs</th></tr></thead><tbody><tr><td>Last 12-month cycle</td><td>5</td><td>0.11</td><td>0.28</td><td>7</td><td>0.15</td><td>0.27</td></tr><tr><td>1<sup>st</sup> conversion cycle</td><td>6</td><td>0.14</td><td>0.40</td><td>8</td><td>0.12</td><td>0.22</td></tr><tr><td>2<sup>nd</sup> conversion cycle</td><td>7</td><td>0.13</td><td>0.26</td><td>9</td><td>0.15</td><td>0.29</td></tr><tr><td>3<sup>rd</sup> conversion cycle</td><td>8</td><td>0.13</td><td>0.21</td><td>10</td><td>0.15</td><td>0.31</td></tr></tbody></table>

In Golfech 1, the ⁵⁸Co surface activity in the steam generators was constant from the last 12-month cycles to the second conversion cycle, and then decreased at the end of the first 18-month cycle; the ⁵⁸Co surface activity in the hot legs and crossover legs decreased constantly from the last 12-month cycle to the first 18-month cycle. The ⁶⁰Co surface activity results increased from the last 12-month cycle to the second conversion cycle, probably because of an abnormal input of cobalt (i.e. stellites) [5]; then, it stabilized at the end of the first 18-month cycle.

In Cattenom 2, the ⁵⁸Co surface activity in the steam generators increased significantly from the first to the second conversion cycle, then decreased at the end of the third conversion cycle. From the last 12-month cycle to the third conversion cycle, the ⁵⁸Co surface activity decreased constantly in the hot legs and remained stable in the crossover legs. The ⁶⁰Co surface activity increased slightly from the last 12-month cycle to the third conversion cycle in the steam generators and decreased slightly in the crossover legs; it remained overall stable in the hot legs when comparing the last 12-month cycle to the third conversion cycle. Concerning dose rates, the values of legs are impacted by high surrounding dose rates: 50 % to 70 % of the dose rates in the primary coolant pipes come from the auxiliary system.

Hence, compared to the previous cycles, the results of the third conversion cycle show that:

- in Golfech 1, the ⁵⁸Co surface activity in the primary system decreased from the last 12-month cycle to the first 18-month cycle, whereas the ⁶⁰Co surface activity increased after the first two conversion cycles, probably due to an abnormal cobalt input;

- in Cattenom 2, the ⁵⁸Co and ⁶⁰Co surface activities in the steam generators increased from the last 12-month cycle to the third conversion cycle, whereas they remain overall stable or even decrease in the crossover legs and in the hot legs.

Overall, the cobalt 58 and cobalt 60 surface activities measured in the Golfech 1 and Cattenom 2 primary systems are comparable to French plants usual behavior.

## Fuel assemblies

In order to assess the impact of the reactor coolant chemistry on the fuel cladding corrosion, oxide thickness measurements were planned on characterized fuel assemblies loaded in the Golfech 1 and Cattenom 2 reactors.

Four characterized fuel assemblies were loaded in Golfech 1 and Cattenom 2. The fuel rods claddings are made of Zircaloy-4, alloy M4 or alloy M5. Each alloy comes from the same tube batch. The chemical composition of the alloys is reported in table 10.

<table><caption>Table 10 : Chemical composition of fuel rods cladding alloys</caption><thead><tr><th>Alloy</th><th>Sn (%)</th><th>Nb (%)</th><th>Fe (%)</th><th>Cr (%)</th><th>V (%)</th><th>O (%)</th></tr></thead><tbody><tr><td>Zircaloy-4</td><td>1.3</td><td></td><td>0.2</td><td>0.1</td><td></td><td>0.12</td></tr><tr><td>M4</td><td>0.5</td><td></td><td>0.6</td><td></td><td>0.4</td><td>0.12</td></tr><tr><td>M5</td><td></td><td>1</td><td></td><td></td><td></td><td>0.12</td></tr></tbody></table>

The fuel rods were loaded into eight assemblies so as to distribute uniformly all the rods cladding batches into each assembly. Their locations in the assemblies are shown in figure 5. Concerning the advanced alloys, the location of the corresponding rods were chosen in order to be able to measure zirconia layer thickness along four axes 90° apart. However, some of them were also loaded in peripheral location in order to run in-depth visual examinations.
<figure><img src="image_6.png" /></figure>

**Legend :**

**black squares** : rods with cladding other than Zircaloy 4

**empty circles** : guide tubes

<caption>Figure 5 : Characterized assemblies location map</caption>

The zirconia layer thickness measurements were performed using the blade device developed by FRAMATOME ANP, based on Eddy Current measurements [6]. During each refueling outage of the DUO experimentation, the device is installed on a criss-cross measurement table, allowing blade insertion into each row of rods at a chosen measurement altitude. All analyzed rods, particularly those with claddings made of advanced alloys, are therefore measured during blade insertion and extraction from all four assembly faces leading to more than 2000 measurements collected per campaign. Statistical process using the « Lagos » and « Zircone » software programs is used for this analysis. The measurement accuracy is estimated to be ± 5 µm.

During the first three cycles of the DUO experimentation, neither in core axial offset anomalies nor abnormal cruds on assemblies had to be reported. Hence, only oxide thickness measurements were carried out on fuel rods.

Concerning Zircaloy-4, the maximum oxide thicknesses measured at Golfech 1 and Cattenom 2 at the end of each cycle are plotted on figure 6. Up to a burnup of about 45 GWd/tU, no difference was observed between the maximum oxide thickness measured in Golfech 1 and in Cattenom 2. Since higher burnups were achieved in Cattenom 2 than in Golfech 1 on the characterized assemblies, it was not possible to compare directly the behavior of the Zircaloy-4 fuel cladding in these two units. However, burnups similar to Cattenom 2 were achieved in the Paluel 4 reactor, were the same chemistry regime as the one in Golfech was implemented. Comparing the maximum fuel oxide thickness measured in Cattenom 2 and Paluel 4, no significant differences could be observed for burnups up to about 50 GWd/tU.

Moreover, the results obtained on the advanced alloys M4 and M5 in Cattenom 2 and Golfech 1 are within the scattering observed on these alloys in French reactors (900 and 1300 MWe) implementing a 2.2 mg/kg maximum lithium chemistry regime (figures 7 and 8 for M5 and M4 alloys respectively).

The modified lithium chemistry does not seem to have an obvious impact on the corrosion behavior of fuel cladding after three fuel cycles, and for average fuel rod burnups up to about 50 GWd/tU.
<figure><img src="image_6.png" /><figcaption>Figure 5 : Characterized assemblies location map</figcaption></figure>

**Legend** :

**black squares** : rods with cladding other than Zircaloy 4

**empty circles** : guide tubes

The zirconia layer thickness measurements were performed using the blade device developed by FRAMATOME ANP, based on Eddy Current measurements [6]. During each refueling outage of the DUO experimentation, the device is installed on a criss-cross measurement table, allowing blade insertion into each row of rods at a chosen measurement altitude. All analyzed rods, particularly those with claddings made of advanced alloys, are therefore measured during blade insertion and extraction from all four assembly faces leading to more than 2000 measurements collected per campaign. Statistical process using the « Lagos » and « Zircone » software programs is used for this analysis. The measurement accuracy is estimated to be ± 5 µm.

During the first three cycles of the DUO experimentation, neither in core axial offset anomalies nor abnormal cruds on assemblies had to be reported. Hence, only oxide thickness measurements were carried out on fuel rods.

Concerning Zircaloy-4, the maximum oxide thicknesses measured at Golfech 1 and Cattenom 2 at the end of each cycle are plotted on figure 6. Up to a burnup of about 45 GWd/tU, no difference was observed between the maximum oxide thickness measured in Golfech 1 and in Cattenom 2. Since higher burnups were achieved in Cattenom 2 than in Golfech 1 on the characterized assemblies, it was not possible to compare directly the behavior of the Zircaloy-4 fuel cladding in these two units. However, burnups similar to Cattenom 2 were achieved in the Paluel 4 reactor, were the same chemistry regime as the one in Golfech was implemented. Comparing the maximum fuel oxide thickness measured in Cattenom 2 and Paluel 4, no significant differences could be observed for burnups up to about 50 GWd/tU.

Moreover, the results obtained on the advanced alloys M4 and M5 in Cattenom 2 and Golfech 1 are within the scattering observed on these alloys in French reactors (900 and 1300 MWe) implementing a 2.2 mg/kg maximum lithium chemistry regime (figures 7 and 8 for M5 and M4 alloys respectively).

The modified lithium chemistry does not seem to have an obvious impact on the corrosion behavior of fuel cladding after three fuel cycles, and for average fuel rod burnups up to about 50 GWd/tU.
<figure><img src="image_7.png" /><figcaption>Figure 6 : Zircaloy-4 external corrosion experience</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 7 : M5 alloy external corrosion experience</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 6 : Zircaloy-4 external corrosion experience</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 7 : M5 alloy external corrosion experience</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 8 : M4 alloy external corrosion experience</figcaption></figure>

DISCUSSION

The results of the DUO experimentation surveillance program after three fuel cycles show a good behavior of the fuel cladding in Cattenom 2, the pilot unit implementing EDF modified chemistry with a maximum authorized lithium concentration of 3.5 mg/kg at the beginning of the extended cycles.

Concerning contamination and dose rates, the ⁵⁸Co surface activities measured in Golfech 1 primary system decreased from the last 12-month cycle to the first 18-month cycle, whereas the ⁶⁰Co surface activity increased after the first two conversion cycles, probably due to an abnormal cobalt input during a conversion cycle. In Cattenom 2, the ⁵⁸Co and ⁶⁰Co surface activities in the steam generators increased from the last 12-month cycle to the third conversion cycle, whereas they remain overall stable or even decrease in the crossover legs and in the hot legs. Overall, the DUO experimentation results are comparable to French plants usual behavior.

Moreover, non destructive steam generator tubing examinations proved that no abnormal corrosion due to elevated lithium concentrations occurred on 600 TT alloy, since the beginning of extended cycle operation in Cattenom 2.

From these results, it appears possible to improve the pH at the beginning of extended fuel cycles, beneficial to a better behavior of the RCS. It can be noticed that boron concentrations up to 1800 mg/kg at the beginning of fuel cycles can be managed with EDF modified chemistry, ensuring pH₃₀₀ ≥ 6.9.

Studying operating with a constant lithium concentration of 3.5 mg/kg until pH₃₀₀ 7.2 is reached could also be of interest in order to reach the target pH₃₀₀ 7.2 as soon as possible, but has not been tested in this DUO Experimentation.
<figure><img src="image_9.png" /><figcaption>Figure 8 : M4 alloy external corrosion experience</figcaption></figure>

DISCUSSION

The results of the DUO experimentation surveillance program after three fuel cycles show a good behavior of the fuel cladding in Cattenom 2, the pilot unit implementing EDF modified chemistry with a maximum authorized lithium concentration of 3.5 mg/kg at the beginning of the extended cycles.

Concerning contamination and dose rates, the ⁵⁸Co surface activities measured in Golfech 1 primary system decreased from the last 12-month cycle to the first 18-month cycle, whereas the ⁶⁰Co surface activity increased after the first two conversion cycles, probably due to an abnormal cobalt input during a conversion cycle. In Cattenom 2, the ⁵⁸Co and ⁶⁰Co surface activities in the steam generators increased from the last 12-month cycle to the third conversion cycle, whereas they remain overall stable or even decrease in the crossover legs and in the hot legs. Overall, the DUO experimentation results are comparable to French plants usual behavior.

Moreover, non destructive steam generator tubing examinations proved that no abnormal corrosion due to elevated lithium concentrations occurred on 600 TT alloy, since the beginning of extended cycle operation in Cattenom 2.

From these results, it appears possible to improve the pH at the beginning of extended fuel cycles, beneficial to a better behavior of the RCS. It can be noticed that boron concentrations up to 1800 mg/kg at the beginning of fuel cycles can be managed with EDF modified chemistry, ensuring pH₃₀₀ ≥ 6.9.

Studying operating with a constant lithium concentration of 3.5 mg/kg until pH₃₀₀ 7.2 is reached could also be of interest in order to reach the target pH₃₀₀ 7.2 as soon as possible, but has not been tested in this DUO Experimentation.
## CONCLUSION

Oxide thickness measurements results show that operating with EDF modified chemistry, hence lithium concentrations up to 3.5 mg/kg at the beginning of the cycles, is encouraging. No influence of elevated lithium concentrations was observed on oxide thickness up to a burnup of about 50 GWd/tU, which is slightly lower than the maximum burnup a fuel assembly is allowed to achieve; moreover, advanced fuel cladding alloys show a good behavior.

Concerning contamination and dose rates, the ⁵⁸Co surface activities measured in Golfech 1 primary system decreased from the last 12-month cycle to the first 18-month cycle, whereas the ⁶⁰Co surface activity increased after the first two conversion cycles, probably due to an abnormal cobalt input during a conversion cycle. In Cattenom 2, the ⁵⁸Co and ⁶⁰Co surface activities in the steam generators increased from the last 12-month cycle to the third conversion cycle, whereas they remain overall stable or even decrease in the crossover legs and in the hot legs. Overall, the DUO experimentation results are comparable to French plants usual behavior.

While any beneficial impact of a constant pH of 7.2 at 300°C throughout the cycle on dose rates is not proved given the French plants results, the easy implementation of increased lithium concentrations at the beginning of the cycles may help face issues due to French 18-month extended cycles management, leading to burnups of about 50 GWd/tU.

As going to very high burnups is under investigation, the DUO experimentation has been extended to a fourth cycle. The characterized fuel assemblies were re-loaded for this fourth cycle in Cattenom 2, and burnups of about 55 to 60 GWd/tU should be achieved at the end of this cycle. The same surveillance program as during the three previous cycles is applied; hence, it should provide extensive data, especially on the fuel cladding behavior up to 60 GWd/tU and on contamination when a modified chemistry regime is implemented.

However, new French fuel management strategies could lead EDF to examine again the possible use of enriched boron 10. According to studies carried out in the Robust Fuel Program, the impact of boron on flux anomalies will also have to be taken into account when defining EDF strategy on using enriched boron 10.

Today, given the pH improvement obtained by operating EDF modified chemistry with increased lithium concentrations up to 3.5 mg/kg at the beginning of the fuel cycles, generalizing this chemistry regime to the French plants is under consideration.

## REFERENCES

[1] F. Nordmann, Primary and secondary water chemistry of French PWR units, Fourth International Seminar on Primary and Secondary Side Chemistry of NPP, Balatonfüred, Hungary, September 28-October 2, 1999

[2] S. Anthoni, Ph. Ridoux, O. Menet, C. Weber, Effects of pH of primary coolant on PWR contamination, Water Chemistry of nuclear reactor system 6, BNES, London, 1992

[3] J.C. Robin, J. Godlewski, A. Giordano, S. Anthoni, P. Billot, H. Amanrich, J.L. Bretelle, Ph. Ridoux, M.C. Thiry, J. Thomazet, C. Brun, Modified lithium chemistry for extended fuel cycles, International Conference of Water Chemistry of Nuclear Reactor Systems, 1996, Bournemouth, UK

[4] PWR primary water chemistry, Volume 1, Revision 4, EPRI report TR-105714-V1R4, March 1999

[5] J.L. Bretelle, J. Bourgoin, L. Viricel, Ph. Ridoux, A. Miquet, A. Rocher, J. Thomazet, Y. Musante, C. Brun, F. Dacquait, Extended fuel cycle : results of the first two cycles of the « DUO Experimentation », International Conference of Water Chemistry of Nuclear Reactor Systems, 2000, Bournemouth, UK

[6] J. Thomazet, J. Pigelet, Y. Musante, In-reactor fuel cladding external corrosion measurement process and results, IAEA Technical Committee meeting, Hluboka nad Vltavou, Czech Republic, September 28-October 2, 1998
## CONCLUSION

Oxide thickness measurements results show that operating with EDF modified chemistry, hence lithium concentrations up to 3.5 mg/kg at the beginning of the cycles, is encouraging. No influence of elevated lithium concentrations was observed on oxide thickness up to a burnup of about 50 GWd/tU, which is slightly lower than the maximum burnup a fuel assembly is allowed to achieve; moreover, advanced fuel cladding alloys show a good behavior.

Concerning contamination and dose rates, the ⁵⁸Co surface activities measured in Golfech 1 primary system decreased from the last 12-month cycle to the first 18-month cycle, whereas the ⁶⁰Co surface activity increased after the first two conversion cycles, probably due to an abnormal cobalt input during a conversion cycle. In Cattenom 2, the ⁵⁸Co and ⁶⁰Co surface activities in the steam generators increased from the last 12-month cycle to the third conversion cycle, whereas they remain overall stable or even decrease in the crossover legs and in the hot legs. Overall, the DUO experimentation results are comparable to French plants usual behavior.

While any beneficial impact of a constant pH of 7.2 at 300°C throughout the cycle on dose rates is not proved given the French plants results, the easy implementation of increased lithium concentrations at the beginning of the cycles may help face issues due to French 18-month extended cycles management, leading to burnups of about 50 GWd/tU.

As going to very high burnups is under investigation, the DUO experimentation has been extended to a fourth cycle. The characterized fuel assemblies were re-loaded for this fourth cycle in Cattenom 2, and burnups of about 55 to 60 GWd/tU should be achieved at the end of this cycle. The same surveillance program as during the three previous cycles is applied; hence, it should provide extensive data, especially on the fuel cladding behavior up to 60 GWd/tU and on contamination when a modified chemistry regime is implemented.

However, new French fuel management strategies could lead EDF to examine again the possible use of enriched boron 10. According to studies carried out in the Robust Fuel Program, the impact of boron on flux anomalies will also have to be taken into account when defining EDF strategy on using enriched boron 10.

Today, given the pH improvement obtained by operating EDF modified chemistry with increased lithium concentrations up to 3.5 mg/kg at the beginning of the fuel cycles, generalizing this chemistry regime to the French plants is under consideration.

## REFERENCES

[1] F. Nordmann, Primary and secondary water chemistry of French PWR units, Fourth International Seminar on Primary and Secondary Side Chemistry of NPP, Balatonfüred, Hungary, September 28-October 2, 1999

[2] S. Anthoni, Ph. Ridoux, O. Menet, C. Weber, Effects of pH of primary coolant on PWR contamination, Water Chemistry of nuclear reactor system 6, BNES, London, 1992

[3] J.C. Robin, J. Godlewski, A. Giordano, S. Anthoni, P. Billot, H. Amanrich, J.L. Bretelle, Ph. Ridoux, M.C. Thiry, J. Thomazet, C. Brun, Modified lithium chemistry for extended fuel cycles, International Conference of Water Chemistry of Nuclear Reactor Systems, 1996, Bournemouth, UK

[4] PWR primary water chemistry, Volume 1, Revision 4, EPRI report TR-105714-V1R4, March 1999

[5] J.L. Bretelle, J. Bourgoin, L. Viricel, Ph. Ridoux, A. Miquet, A. Rocher, J. Thomazet, Y. Musante, C. Brun, F. Dacquait, Extended fuel cycle : results of the first two cycles of the « DUO Experimentation », International Conference of Water Chemistry of Nuclear Reactor Systems, 2000, Bournemouth, UK

[6] J. Thomazet, J. Pigelet, Y. Musante, In-reactor fuel cladding external corrosion measurement process and results, IAEA Technical Committee meeting, Hluboka nad Vltavou, Czech Republic, September 28-October 2, 1998