# Optimized high temperature oxidation and cleaning at BUGEY 3

Gilles Ranchoux, Matthieu Wintergerst, Martin Bachet, Jean-Daniel Duron, Serge Blond, Frédéric Dacquait

► To cite this version:

Gilles Ranchoux, Matthieu Wintergerst, Martin Bachet, Jean-Daniel Duron, Serge Blond, et al. Optimized high temperature oxidation and cleaning at BUGEY 3. NPC2012 - Nuclear Plant Chemistry Conference - International Conference on Water Chemistry of Nuclear Reactors Systems, SFEN, Sep 2012, Paris, France. cea-04475797

HAL Id: cea-04475797

https://cea.hal.science/cea-04475797

Submitted on 23 Feb 2024

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# Optimized high temperature oxidation and cleaning at BUGEY 3

Gilles Ranchoux, Matthieu Wintergerst, Martin Bachet, Jean-Daniel Duron, Serge Blond, Frédéric Dacquait

► To cite this version:

Gilles Ranchoux, Matthieu Wintergerst, Martin Bachet, Jean-Daniel Duron, Serge Blond, et al. Optimized high temperature oxidation and cleaning at BUGEY 3. NPC2012 - Nuclear Plant Chemistry Conference - International Conference on Water Chemistry of Nuclear Reactors Systems, SFEN, Sep 2012, Paris, France. cea-04475797

HAL Id: cea-04475797

https://cea.hal.science/cea-04475797v1

Submitted on 23 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
OPTIMIZED HIGH TEMPERATURE OXIDATION AND CLEANING AT BUGEY 3

Gilles Ranchoux (EDF/SEPTEN), France (gilles.ranchoux@edf.fr)

Matthieu Wintergerst (EDF/CEIDRE)

Martin Bachet, Stéphanie Leclercq (EDF/R&D/MMC)

Jean-Daniel Duron, Jean-Pierre Meunier (EDF/CNPE/BUGEY)

Serge Blond (EDF/DPN/EM)

Frédéric Dacquait (CEA/DEN)

ABSTRACT

As a part of the EDF Source Term Reduction project, an experimental procedure was carried out at BUGEY 3 further to the steam generator replacement. This innovative procedure consists in theory in two complementary phases /1/:

* Phase 1: a SG tubes optimized oxidation performed during pre-critical hot functional tests (basic and reducing chemistry) aims to generate an as protective as possible inner oxide layer allowing to reduce the later nickel release,

* Phase 2: a cleaning procedure of the primary circuit performed under acid and reducing chemical conditioning at 170°C intends to dissolve and eliminate the outer oxide layer by a simultaneous purification.

The objective of such a procedure is to reduce corrosion products inventory (mainly nickel) generated by the first SG tube oxidation during hot functional tests and first operation months by carrying out an appropriate cleaning procedure.

Gains were expected not only on RCS and auxiliary systems contamination, dose rates and thus collective dose but also on next outages duration.

The objective of this paper is to describe the process implementation at BUGEY 3: effective procedure put in place, monitoring program (chemistry and dose rate measurements, EMECC campaign) and firsts results.
OPTIMIZED HIGH TEMPERATURE OXIDATION AND CLEANING AT BUGEY 3

Gilles Ranchoux (EDF/SEPTEN), France (gilles.ranchoux@edf.fr)

Matthieu Wintergerst (EDF/CEIDRE)

Martin Bachet, Stéphanie Leclercq (EDF/R&D/MMC)

Jean-Daniel Duron, Jean-Pierre Meunier (EDF/CNPE/BUGEY)

Serge Blond (EDF/DPN/EM)

Frédéric Dacquait (CEA/DEN)

ABSTRACT

As a part of the EDF Source Term Reduction project, an experimental procedure was carried out at BUGEY 3 further to the steam generator replacement. This innovative procedure consists in theory in two complementary phases /1/:

* Phase 1: a SG tubes optimized oxidation performed during pre-critical hot functional tests (basic and reducing chemistry) aims to generate an as protective as possible inner oxide layer allowing to reduce the later nickel release,

* Phase 2: a cleaning procedure of the primary circuit performed under acid and reducing chemical conditioning at 170°C intends to dissolve and eliminate the outer oxide layer by a simultaneous purification.

The objective of such a procedure is to reduce corrosion products inventory (mainly nickel) generated by the first SG tube oxidation during hot functional tests and first operation months by carrying out an appropriate cleaning procedure.

Gains were expected not only on RCS and auxiliary systems contamination, dose rates and thus collective dose but also on next outages duration.

The objective of this paper is to describe the process implementation at BUGEY 3: effective procedure put in place, monitoring program (chemistry and dose rate measurements, EMECC campaign) and firsts results.
# 1. INTRODUCTION

The reduction of collective dose is an important objective for all nuclear operators. This consists not only in improving the reactor shutdown organization (time spent in control area, biological shielding, ...) but also in improving the radiological state of the unit and the efficiency of the source term reduction operations.

Significant efforts are continuously performed to contribute to source term and contamination reduction in primary and auxiliary circuits. Actions put in place at EDF concerns at the same time preventive ways (operational procedures optimization, purification media optimization) /1/ and curative ones (decontamination).

One of the most investigated way to reduce source term prevention is the first Steam Generator (SG) tubes oxidation for a new built unit or after Steam Generator Replacement (SGR) for units already in operation. As a part of the EDF Source Term Reduction project, an innovative procedure was proposed by EDF/R&D department in 2010 thanks to the accumulated knowledge on metal release and dissolution mechanism of Ni-based alloy SG tubes /2/. This procedure was carried out at Bugey 3 further to the SGR. Due to an induced low nickel inventory and thus to a low ⁵⁸Co activity, gains were expected not only on next outages duration (shorter purification time before the last RCP shutdown) but also mainly on RCS and auxiliary systems contamination, dose rates and thus collective dose.

The objective of this paper is to describe the process implementation at BUGEY 3 : effective procedure put in place, monitoring program (chemistry and dose rate measurements, EMECC campaign) and first results.

# 2. PROCEDURE DESCRIPTION

## 2.1. Theoretical reminder /2/

When they are first exposed to the primary coolant at high temperatures, nickel-based alloys get covered by an inner oxide layer (rich in chromium) that strongly slows down their ulterior generalized corrosion and an outer oxide layer providing a significant amount of nickel available for dissolution. For Thermal Treated 690 alloy equipping recently built steam generator, this phenomenon can become extremly low after several cycles, which makes the nickel source term in the primary circuit really small and by consequence little ⁵⁸Co is generated by activation of natural nickel.

Many studies aimed at determining the best chemistry to get a very protective passive layer and it has long been recognized that an important mass of nickel is oxidized during its formation. Oxidized nickel is not immediately released to water because at high temperture, nickel oxide, hydroxide and ferrite have very low solubility and precipitate, giving the well known chromium-poor external oxide. However, the release of nickel is not inhibited, but only delayed. It is therefore desirable to clean nickel out of the primary circuit, but despite a few trials, especially during Sizewell B commissioning, this cleaning has probably never been really efficient.

The new idea developed by EDF R&D department is that the cleaning of the primary circuit (mainly SG surfaces) can be achieved by an acid and reducing chemistry at a temperature close to 170°C, which would enable the dissolution of nickel containing oxides, while preserving the chromium rich passive layer on primary circuit surfaces. This temperature has been chosen as the result of a compromise between thermodynamic (nickel solubility decreases when temperature increases) and kinetics (dissolution kinetic increases with temperature). Before this cleaning step, a pre-oxidation of the tubes is necessary, and should lead to a stable and as protective as possible passive layer, while minimizing the mass of nickel made available for later release.

The main benefit associated with this cleaning would be to avoid elevated ⁵⁸Co oxygenation peaks activities following new steam generators operation, and to obtain quickly a plant with both low release rates and low nickel inventory. Other effects will probably be a reduction in dose rates, no more need for prolonged purification steps during shutdown and a generally cleaner reactor.

The optimum conditions for dissolving nickel after the pre-oxidation step should focus on the different possible nickel-bearing phases, which are predicted to be slightly different just after commissioning new steam generator to what is usually encountered during PWR shutdowns. Based on the general knowledge on the behavior of corrosion products, a compromise should be made between solubility, which is higher at lower temperature and dissolution kinetics which show the opposite trend. Acidic pH is favorable for the dissolution of all phases, and hydrogen is required for the dissolution of nickel ferrites. Hydrogen also provide protection against the dissolution of the chromium oxide layer responsible for the slow corrosion rate.
# 1. INTRODUCTION

The reduction of collective dose is an important objective for all nuclear operators. This consists not only in improving the reactor shutdown organization (time spent in control area, biological shielding, ...) but also in improving the radiological state of the unit and the efficiency of the source term reduction operations.

Significant efforts are continuously performed to contribute to source term and contamination reduction in primary and auxiliary circuits. Actions put in place at EDF concerns at the same time preventive ways (operational procedures optimization, purification media optimization) /1/ and curative ones (decontamination).

One of the most investigated way to reduce source term prevention is the first Steam Generator (SG) tubes oxidation for a new built unit or after Steam Generator Replacement (SGR) for units already in operation. As a part of the EDF Source Term Reduction project, an innovative procedure was proposed by EDF/R&D department in 2010 thanks to the accumulated knowledge on metal release and dissolution mechanism of Ni-based alloy SG tubes /2/. This procedure was carried out at Bugey 3 further to the SGR. Due to an induced low nickel inventory and thus to a low ⁵⁸Co activity, gains were expected not only on next outages duration (shorter purification time before the last RCP shutdown) but also mainly on RCS and auxiliary systems contamination, dose rates and thus collective dose.

The objective of this paper is to describe the process implementation at BUGEY 3 : effective procedure put in place, monitoring program (chemistry and dose rate measurements, EMECC campaign) and first results.

# 2. PROCEDURE DESCRIPTION

## 2.1. Theoretical reminder /2/

When they are first exposed to the primary coolant at high temperatures, nickel-based alloys get covered by an inner oxide layer (rich in chromium) that strongly slows down their ulterior generalized corrosion and an outer oxide layer providing a significant amount of nickel available for dissolution. For Thermal Treated 690 alloy equipping recently built steam generator, this phenomenon can become extremly low after several cycles, which makes the nickel source term in the primary circuit really small and by consequence little ⁵⁸Co is generated by activation of natural nickel.

Many studies aimed at determining the best chemistry to get a very protective passive layer and it has long been recognized that an important mass of nickel is oxidized during its formation. Oxidized nickel is not immediately released to water because at high temperture, nickel oxide, hydroxide and ferrite have very low solubility and precipitate, giving the well known chromium-poor external oxide. However, the release of nickel is not inhibited, but only delayed. It is therefore desirable to clean nickel out of the primary circuit, but despite a few trials, especially during Sizewell B commissioning, this cleaning has probably never been really efficient.

The new idea developed by EDF R&D department is that the cleaning of the primary circuit (mainly SG surfaces) can be achieved by an acid and reducing chemistry at a temperature close to 170°C, which would enable the dissolution of nickel containing oxides, while preserving the chromium rich passive layer on primary circuit surfaces. This temperature has been chosen as the result of a compromise between thermodynamic (nickel solubility decreases when temperature increases) and kinetics (dissolution kinetic increases with temperature). Before this cleaning step, a pre-oxidation of the tubes is necessary, and should lead to a stable and as protective as possible passive layer, while minimizing the mass of nickel made available for later release.

The main benefit associated with this cleaning would be to avoid elevated ⁵⁸Co oxygenation peaks activities following new steam generators operation, and to obtain quickly a plant with both low release rates and low nickel inventory. Other effects will probably be a reduction in dose rates, no more need for prolonged purification steps during shutdown and a generally cleaner reactor.

The optimum conditions for dissolving nickel after the pre-oxidation step should focus on the different possible nickel-bearing phases, which are predicted to be slightly different just after commisioning new steam generator to what is usually encountered during PWR shutdowns. Based on the general knowledge on the behavior of corrosion products, a compromise should be made between solubility, which is higher at lower temperature and dissolution kinetics which show the opposite trend. Acidic pH is favorable for the dissolution of all phases, and hydrogen is required for the dissolution of nickel ferrites. Hydrogen also provide protection against the dissolution of the chromium oxide layer responsible for the slow corrosion rate.
<figure><img src="image_2.png" /><figcaption>Figure 1 : Schematic representation of the oxide grown in high temperature primary water on tubes made of alloy 690TT /2/</figcaption></figure>

## 2.2. Procedure description

As written before, this innovative procedure consists in theory in two complementary phases (figure 2):

* **Phase 1** : a SG tubes optimized oxidation performed during pre-critical hot functional tests (basic and reducing chemistry) during 300 hours aimed to generate an as protective as possible inner oxide layer,

* **Phase 2** : a cleaning procedure of the primary circuit performed under acid and reducing chemical conditioning at 170°C intending to dissolve and eliminate the outer oxide layer by a simultaneous purification (200 hours at the highest possible purification rate).

<figure><img src="image_3.png" /><figcaption>Figure 2 : Description of the theoretical procedure</figcaption></figure>

Several operational issues led to adapt the theoretical procedure to the plant schedule. First of all, the restart of the plant after SGR (thus, optimized oxidation during hot functional tests) was planned in december 2010. For questions of electricity supply during winter, it has not been possible to perform the second phase immidiaity after the first one. The first optimized oxidation has been limited to the hot functional tests length (136 h) and continued during operation. The cleaning phase had to be shifted 7.5 months later during summer 2011.

The consequences of such a shift were estimated to be negligible provided that the second phase ended about 8 month before the following outage. As a matter of fact, this time is enough to allow a significant radioactive decay of the ⁵⁸Co (half life : 70.8 d) generated between the first oxidation and the cleaning phase. Moreover, it is likely
<figure><img src="image_2.png" /><figcaption>Figure 1 : Schematic representation of the oxide grown in high temperature primary water on tubes made of alloy 690TT /2/</figcaption></figure>

## 2.2. Procedure description

As written before, this innovative procedure consists in theory in two complementary phases (figure 2):

* **Phase 1** : a SG tubes optimized oxidation performed during pre-critical hot functional tests (basic and reducing chemistry) during 300 hours aimed to generate an as protective as possible inner oxide layer,

* **Phase 2** : a cleaning procedure of the primary circuit performed under acid and reducing chemical conditioning at 170°C intending to dissolve and eliminate the outer oxide layer by a simultaneous purification (200 hours at the highest possible purification rate).

<figure><img src="image_3.png" /><figcaption>Figure 2 : Description of the theoretical procedure</figcaption></figure>

Several operational issues led to adapt the theoretical procedure to the plant schedule. First of all, the restart of the plant after SGR (thus, optimized oxidation during hot functional tests) was planned in december 2010. For questions of electricity supply during winter, it has not been possible to perform the second phase immediaity after the first one. The first optimized oxidation has been limited to the hot functional tests length (136 h) and continued during operation. The cleaning phase had to be shifted 7.5 months later during summer 2011.

The consequences of such a shift were estimated to be negligible provided that the second phase ended about 8 month before the following outage. As a matter of fact, this time is enough to allow a significant radioactive decay of the ⁵⁸Co (half life : 70.8 d) generated between the first oxidation and the cleaning phase. Moreover, it is likely
that the supplementary oxidation time during operation (more than 7 months) leads to a more stable protective inner oxide layer than for the theoretical procedure.

Furthermore, a high purification rate is required during the cleaning phase in order to assure a fast nickel elimination. For safety reasons (mainly RHRS thermal fatigue), it has not been possible to stay in a RHRS cooling state for 200 hours taking advantage of the same purification rate as during oxygenation (27 t/h). A steam generator cooling state has been preferred but imposed to stay with a primary pressure about 80 bars. At that pressure, the minimal temperature acceptable for safety reasons is about 190°C, temperature which is slightly higher than the ideal temperature of 170°C. However, considering available nickel solubility data, it has been estimated that the efficiency loss caused by a 20°C deviation from the ideal temperature would be offset by a high purification rate (≈ 20 t/h) and a higher dissolution kinetic.

The schematic diagram of the effective procedure is shown on figure 2.

<figure><img src="image_4.png" /><figcaption>Figure 3: Description of the effective operational procedure</figcaption></figure>

Moreover, a decisional flow chart has been elaborated in order to stop the procedure if efficiency criteria (mainly nickel concentration) were not reached. The minimal nickel concentration above which a significant efficiency of the procedure could be expected has been set to 1 ppm.

Expected gains on outage collective dose have been estimated to about 30% from hypothesis based on the more updated international theoretical and experimental feedback concerning on one hand nickel solubility and dissolution kinetic and on the other hand oxide layer structure characterization :

* 20% are attributed to nickel oxide and/or nickel hydroxide elimination "directly" at the root on steam generator tubes;

* 10% are attributed to nickel ferrite generated on the tube walls on steam generator but also on the other parts of the primary circuit (hot, cold and crossover legs).

A lower $^{58}$Co oxygenation peak activity is also expected due to a significant decrease of the nickel mass potentially activable under neutronic flux. However, availability gains due to a shorter purification time between oxygenation and the reach of the last RCP shutdown criteria ($^{58}$Co activity = 50 GBq/t) are low with regards to an incompressible time below which it is not possible to avoid some obligatory simultaneous works during the outage.

This procedure, as proposed above, had never been experimented as by EDF or by any other operators. However, although innovative, this does not use new chemical conditioning likely to cause damages on the primary circuit (stress corrosion cracking, fuel damage, axial offset anomaly). In fact, chemical specifications are totally respected (boron, lithium and hydrogen concentration) for both phases. Moreover, this procedure is composed by several steps which have all been rolled out independently by EDF or at an international level.
that the supplementary oxidation time during operation (more than 7 months) leads to a more stable protective inner oxide layer than for the theoretical procedure.

Furthermore, a high purification rate is required during the cleaning phase in order to assure a fast nickel elimination. For safety reasons (mainly RHRS thermal fatigue), it has not been possible to stay in a RHRS cooling state for 200 hours taking advantage of the same purification rate as during oxygenation (27 t/h). A steam generator cooling state has been prefered but imposed to stay with a primary pressure about 80 bars. At that pressure, the minimal temperature acceptable for safety reasons is about 190°C, temperature which is slightly higher than the ideal temperature of 170°C. However, considering available nickel solubility data, it has been estimated that the efficiency loss caused by a 20°C deviation from the ideal temperature would be offset by a high purification rate (≈ 20 t/h) and a higher dissolution kinetic.

The schematic diagram of the effective procedure is shown on figure 2.

<figure><img src="image_4.png" /><figcaption>Figure 3 : Description of the effective operational procedure</figcaption></figure>

Moreover, a decisional flow chart has been elaborated in order to stop the procedure if efficiency criteria (mainly nickel concentration) were not reached. The minimal nickel concentration above which a significant efficiency of the procedure could be expected has been set to 1 ppm.

Expected gains on outage collective dose have been estimated to about 30% from hypothesis based on the more updated international theoretical and experimental feedback concerning on one hand nickel solubility and dissolution kinetic and on the other hand oxide layer structure caracterization :

* 20% are attributed to nickel oxide and/or nickel hydroxide elimination "directly" at the root on steam generator tubes ;

* 10% are attributed to nickel ferrite generated on the tube walls on steam generator but also on the other parts of the primary circuit (hot, cold and crossover legs).

A lower ⁵⁸Co oxygenation peak activity is also expected due to a significant decrease of the nickel mass potentially activable under neutronic flux. However, availibility gains due to a shorter purification time between oxygenation and the reach of the last RCP shutdown criteria (⁵⁸Co activity = 50 GBq/t) are low with regards to an incompressible time below which it is not possible to avoid some obligatory simultaneous works during the outage.

This procedure, as proposed above, had never been experimented as by EDF or by any other operators. However, although innovative, this does not use new chemical conditioning likely to cause damages on the primary circuit (stress corrosion cracking, fuel damage, axial offset anomaly). In fact, chemical specifications are totally respected (boron, lithium and hydrogen concentration) for both phases. Moreover, this procedure is composed by several steps which have all been rolled out independently by EDF or at an international level.
## 4. RESULTS OF THE MONITORING PROGRAM

In addition to the current monitoring program, specific supplementary measurements (nickel and iron concentration, dose rates and gamma spectrometry) were carried out especially during the second acid and reducing cleaning phase. This section presents the different results and gives a preliminary analysis.

Figure 4 shows the variation of the main operational parameters during this specific outage (boron, lithium and hydrogen concentration, temperature, purification rate, ⁵⁸Co volume activity and nickel concentration) for the cleaning phase.

As shown in that graph, the nickel concentration did not reach the criterion value of 1 ppm above which a significant efficiency would have been expected. Thus, it has been decided to stop the cleaning phase after only 36 hours (compared to initial 200 hours) with respect to the operational flow chart.

<figure><img src="image_5.png" /><figcaption>Figure 4 : Operational parameters during the cleaning procedure outage</figcaption></figure>

### 4.1. Metal concentration measurements

Figure 5 shows the variation of nickel and iron concentrations in the primary coolant measured in real time.

During the acid and reducing cleaning, one can observe that the nickel concentration increased up to 190 ppb consequently to the lithium concentration decrease and stabilised at about 130 ppb after boron dilution (necessary to reach the hot shutdown boron concentration in preparation of a quick restart). Approximately 150 g of nickel have been removed from the primary circuit which is not much compared to the expected 5 kg.

With regard to these results, two phenomena have to be explained: the nickel concentration variation and the low nickel concentration level.

Considering nickel variation, the concentration follows calculated solubility trends as a function of pH. One can observe:
## 4. RESULTS OF THE MONITORING PROGRAM

In addition to the current monitoring program, specific supplementary measurements (nickel and iron concentration, dose rates and gamma spectrometry) were carried out especially during the second acid and reducing cleaning phase. This section presents the different results and gives a preliminary analysis.

Figure 4 shows the variation of the main operational parameters during this specific outage (boron, lithium and hydrogen concentration, temperature, purification rate, ⁵⁸Co volume activity and nickel concentration) for the cleaning phase.

As shown in that graph, the nickel concentration did not reach the criterion value of 1 ppm above which a significant efficiency would have been expected. Thus, it has been decided to stop the cleaning phase after only 36 hours (compared to initial 200 hours) with respect to the operational flow chart.

<figure><img src="image_5.png" /><figcaption>Figure 4 : Operational parameters during the cleaning procedure outage</figcaption></figure>

### 4.1. Metal concentration measurements

Figure 5 shows the variation of nickel and iron concentrations in the primary coolant measured in real time.

During the acid and reducing cleaning, one can observe that the nickel concentration increased up to 190 ppb consequently to the lithium concentration decrease and stabilised at about 130 ppb after boron dilution (necessary to reach the hot shutdown boron concentration in preparation of a quick restart). Approximately 150 g of nickel have been removed from the primary circuit which is not much compared to the expected 5 kg.

With regard to these results, two phenomena have to be explained: the nickel concentration variation and the low nickel concentration level.

Considering nickel variation, the concentration follows calculated solubility trends as a function of pH. One can observe:
* an increase when the lithium concentration decreases at constant boron concentration,

* a quick decrease when the boron concentration decreases,

* a stabilization when the lithium, boron and hydrogen concentration are stable.

Considering the nickel concentration level, the concentration measured during the cleaning phase stays lower than the value expected by EDF. Two complementary studies are in progress to better understand this unexpected behaviour: the chemical species present in the primary circuit on one hand, and the effective cleaning temperature upper than the temperature for which a feedback was available on the other hand.

The procedure has been performed at a temperature between 190°C and 195°C instead of the ideal temperature of 170°C for optimizing the purification rate. Solubilities are known to decrease when the temperature grows from 170°C to 190°C. Nevertheless, this aspect had been taken into account during the procedure design studies and such a solubility decrease remained acceptable as regards the expected gain on the purification rate, dissolution kinetic and the fact that the cleaning procedure acted on the nickel contained into oxides $NiFe_2O_4$, $NiO$ and $Ni(OH)_2$.

With regards to the information from chemical analysis, one can point out that the species present in the primary coolant seems to be mainly under a nickel ferrite form ($NiFe_2O_4$) instead of a more soluble nickel oxide ($NiO$). In fact, iron concentration is approximately twice as high as nickel concentration which is consistent with the nickel ferrite stoichiometric ratio. A better balanced ratio between both chemical species was expected considering the usual nickel excess compared to iron when steam generator tubes are made of 690 alloy.

<figure><img src="image_6.png" /><figcaption>Figure 5: Ni and Fe concentration during the cleaning phase</figcaption></figure>

In conclusion, a temperature between 190°C and 195°C seems to be not adapted to an efficient cleaning procedure under acid and reducing conditioning, although this is probably not the only reason of the observed low dissolution rate. Furthermore, specific nickel measurements have been made during the following outage RHRS connection phase (at about 170°C) and will allow to determine if the procedure would be more efficient at the ideal temperature. The origin of that phenomenon is not explained at the moment but is still under review by EDF and its partners.
* an increase when the lithium concentration decreases at constant boron concentration,

* a quick decrease when the boron concentration decreases,

* a stabilization when the lithium, boron and hydrogen concentration are stable.

Considering the nickel concentration level, the concentration measured during the cleaning phase stays lower than the value expected by EDF. Two complementary studies are in progress to better understand this unexpected behaviour: the chemical species present in the primary circuit on one hand, and the effective cleaning temperature upper than the temperature for which a feedback was available on the other hand.

The procedure has been performed at a temperature between 190°C and 195°C instead of the ideal temperature of 170°C for optimizing the purification rate. Solubilities are known to decrease when the temperature grows from 170°C to 190°C. Nevertheless, this aspect had been taken into account during the procedure design studies and such a solubility decrease remained acceptable as regards the expected gain on the purification rate, dissolution kinetic and the fact that the cleaning procedure acted on the nickel contained into oxides $NiFe_2O_4$, $NiO$ and $Ni(OH)_2$.

With regards to the information from chemical analysis, one can point out that the species present in the primary coolant seems to be mainly under a nickel ferrite form ($NiFe_2O_4$) instead of a more soluble nickel oxide ($NiO$). In fact, iron concentration is approximately twice as high as nickel concentration which is consistent with the nickel ferrite stoichiometric ratio. A better balanced ratio between both chemical species was expected considering the usual nickel excess compared to iron when steam generator tubes are made of 690 alloy.

<figure><img src="image_6.png" /><figcaption>Figure 5: Ni and Fe concentration during the cleaning phase</figcaption></figure>

In conclusion, a temperature between 190°C and 195°C seems to be not adapted to an efficient cleaning procedure under acid and reducing conditioning, although this is probably not the only reason of the observed low dissolution rate. Furthermore, specific nickel measurements have been made during the following outage RHRS connection phase (at about 170°C) and will allow to determine if the procedure would be more efficient at the ideal temperature. The origin of that phenomenon is not explained at the moment but is still under review by EDF and its partners.
## 4.2. Deposited activities (EMECC campaign)

In addition to chemical and volume activity measurements, EMECC gamma spectrometry measurements have been performed to determine the impact of the procedure on the surface activity on wall pipes and on steam generator tubes.

EMECC campaigns (performed by CEA for EDF needs) have been commissioned for more than 30 years on French fleet units in order to better characterize contamination mechanisms. At the same time, EDF has also commissioned and financed EMECC campaigns on foreign units (Doel, Sizewell, Trillo during the 4 last years /4/) with the contribution of several European operators in order to compare different good international practices design impact.

<figure><img src="image_7.png" /><figcaption>Figure 6: EMECC device</figcaption></figure>

That device /3/ allows non destructive surface activity by gamma spectrometry measurements (HP germanium detector cooled by liquid nitrogen) based on a photon flux measurement converted in activity with the help of a pre-computed transfer function (MERCURE code /5/). A very large range of geometries can be measured as primary circuit (cold and hot side steam generator tube bundle, hot, cold and cross-over legs), CVCS (letdown line, regenerative heat exchangers), RHRS (heat exchanger, piping), ... The volume activity contribution can be removed in order to focus only on the deposited activity.

Figure 7 and 8 shows the evolution of the ⁵⁸Co and ⁶⁰Co surface activities deposited on hot legs, crossover legs and steam generator tubes (cold side) and normalized to 1 at the beginning of the cleaning phase. One can observe a slight decrease trend for ⁵⁸Co surface activity but not significant because lower than the measurement uncertainty. Moreover the same trend is not observed for ⁶⁰Co surface activity.

These results confirm that this specific cleaning procedure did not allow removing contamination from out-of-flux surfaces (wall pipes and steam generator tubes surface) and are consistent with the low removal of activated corrosion products inferred from coolant sampling.

<figure><img src="image_8.png" /><figcaption>Figure 7: Evolution of the ⁵⁸Co surface activity on legs and SG tubes during the cleaning phase</figcaption></figure>
## 4.2. Deposited activities (EMECC campaign)

In addition to chemical and volume activity measurements, EMECC gamma spectrometry measurements have been performed to determine the impact of the procedure on the surface activity on wall pipes and on steam generator tubes.

EMECC campaigns (performed by CEA for EDF needs) have been commissioned for more than 30 years on French fleet units in order to better characterize contamination mechanisms. At the same time, EDF has also commissioned and financed EMECC campaigns on foreign units (Doel, Sizewell, Trillo during the 4 last years /4/) with the contribution of several European operators in order to compare different good international practices design impact.

<figure><img src="image_7.png" /><figcaption>Figure 6: EMECC device</figcaption></figure>

That device /3/ allows non destructive surface activity by gamma spectrometry measurements (HP germanium detector cooled by liquid nitrogen) based on a photon flux measurement converted in activity with the help of a pre-computed transfer function (MERCURE code /5/). A very large range of geometries can be measured as primary circuit (cold and hot side steam generator tube bundle, hot, cold and cross-over legs), CVCS (letdown line, regenerative heat exchangers), RHRS (heat exchanger, piping), ... The volume activity contribution can be removed in order to focus only on the deposited activity.

Figure 7 and 8 shows the evolution of the ⁵⁸Co and ⁶⁰Co surface activities deposited on hot legs, crossover legs and steam generator tubes (cold side) and normalized to 1 at the beginning of the cleaning phase. One can observe a slight decrease trend for ⁵⁸Co surface activity but not significant because lower than the measurement uncertainty. Moreover the same trend is not observed for ⁶⁰Co surface activity.

These results confirm that this specific cleaning procedure did not allow removing contamination from out-of-flux surfaces (wall pipes and steam generator tubes surface) and are consistent with the low removal of activated corrosion products inferred from coolant sampling.

<figure><img src="image_8.png" /><figcaption>Figure 7: Evolution of the ⁵⁸Co surface activity on legs and SG tubes during the cleaning phase</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 8: Evolution of the <sup>60</sup>Co surface activity on legs and SG tubes during the cleaning phase</figcaption></figure>

Table 1 gives the <sup>58</sup>Co and <sup>60</sup>Co deposited activities on hot legs, crossover legs and steam generator tubes.

<table><caption>Table 1: <sup>58</sup>Co and <sup>60</sup>Co deposited activities on hot legs, crossover legs and steam generator tubes</caption><thead><tr><th>Circuit component</th><th><sup>58</sup>Co</th><th><sup>60</sup>Co</th></tr></thead><tbody><tr><td>Hot Leg</td><td>2 GBq/m<sup>2</sup></td><td>0.8 GBq/m<sup>2</sup></td></tr><tr><td>Crossover leg</td><td>6 GBq/m<sup>2</sup></td><td>0.4 GBq/m<sup>2</sup></td></tr><tr><td>Steam Generator (cold side)</td><td>0.8 GBq/m<sup>2</sup></td><td>0.01 GBq/m<sup>2</sup></td></tr></tbody></table>

The comparison of <sup>58</sup>Co and <sup>60</sup>Co deposited activities in Bugey 3 RCS with all the EMECC data relating to units with SG replacement shows that:

* Bugey 3 <sup>58</sup>Co contamination level is coherent with the typical average value on crossover legs and steam generator tubes but slightly lower on hot legs,

* Bugey 3 <sup>60</sup>Co contamination level is lower in all RCS parts.

Based on the conclusion that the cleaning phase (acid and reducing conditioning at 190 °C) has not been efficient to dissolve nickel-based oxides, the similar values of <sup>58</sup>Co contamination, observed at Bugey 3 compared to other units, can lead to confirm that the optimized first oxidation phase (basic and reducing conditioning at 285 °C) has not led to a significant <sup>58</sup>Co contamination reduction.

Moreover, the low <sup>60</sup>Co contamination level can be explained both by a long SGR outage (about 20 months) during which the <sup>60</sup>Co activity has decreased naturally by 20% and by a probably low contamination level on legs before SGR (no EMECC campaign before SGR). In other words, apart from any chemical consideration, this very long outage led to the complete reset of the <sup>58</sup>Co deposited activity and to a significant decrease of <sup>60</sup>Co deposited activity. A 20 month long storage of the fuel assemblies before refueling may have also led to a low dissolution of the oxide rod deposit.

### 4.3. Dose rates measurements

In order to monitor the eventual impact of the cleaning phase on contamination levels, two shielded dose rate measurement sets have also been performed in parallel all along the duration procedure on hot legs, crossover legs and steam generator tubes. As for surface activities, even if a slight decrease trend could be assumed in the
<figure><img src="image_9.png" /><figcaption>Figure 8: Evolution of the <sup>60</sup>Co surface activity on legs and SG tubes during the cleaning phase</figcaption></figure>

Table 1 gives the <sup>58</sup>Co and <sup>60</sup>Co deposited activities on hot legs, crossover legs and steam generator tubes.

<table><caption>Table 1: <sup>58</sup>Co and <sup>60</sup>Co deposited activities on hot legs, crossover legs and steam generator tubes</caption><thead><tr><th>Circuit component</th><th><sup>58</sup>Co</th><th><sup>60</sup>Co</th></tr></thead><tbody><tr><td>Hot Leg</td><td>2 GBq/m<sup>2</sup></td><td>0.8 GBq/m<sup>2</sup></td></tr><tr><td>Crossover leg</td><td>6 GBq/m<sup>2</sup></td><td>0.4 GBq/m<sup>2</sup></td></tr><tr><td>Steam Generator (cold side)</td><td>0.8 GBq/m<sup>2</sup></td><td>0.01 GBq/m<sup>2</sup></td></tr></tbody></table>

The comparison of <sup>58</sup>Co and <sup>60</sup>Co deposited activities in Bugey 3 RCS with all the EMECC data relating to units with SG replacement shows that:

* Bugey 3 <sup>58</sup>Co contamination level is coherent with the typical average value on crossover legs and steam generator tubes but slightly lower on hot legs,

* Bugey 3 <sup>60</sup>Co contamination level is lower in all RCS parts.

Based on the conclusion that the cleaning phase (acid and reducing conditioning at 190 °C) has not been efficient to dissolve nickel-based oxides, the similar values of <sup>58</sup>Co contamination, observed at Bugey 3 compared to other units, can lead to confirm that the optimized first oxidation phase (basic and reducing conditioning at 285 °C) has not led to a significant <sup>58</sup>Co contamination reduction.

Moreover, the low <sup>60</sup>Co contamination level can be explained both by a long SGR outage (about 20 months) during which the <sup>60</sup>Co activity has decreased naturally by 20% and by a probably low contamination level on legs before SGR (no EMECC campaign before SGR). In other words, apart from any chemical consideration, this very long outage led to the complete reset of the <sup>58</sup>Co deposited activity and to a significant decrease of <sup>60</sup>Co deposited activity. A 20 month long storage of the fuel assemblies before refueling may have also led to a low dissolution of the oxide rod deposit.

### 4.3. Dose rates measurements

In order to monitor the eventual impact of the cleaning phase on contamination levels, two shielded dose rate measurement sets have also been performed in parallel all along the duration procedure on hot legs, crossover legs and steam generator tubes. As for surface activities, even if a slight decrease trend could be assumed in the
second set, it is not possible to observe a clear impact of the acid and reducing chemistry, the values remaining below measurement uncertainties.

<figure><img src="image_10.png" /><figcaption>Figure 8: Normalized dose rate measured during the cleaning procedure</figcaption></figure>

4. CONCLUSION

After the steam generator replacement in 2010 at Bugey 3 NPP, an experimental SG tubes optimized first oxidation procedure during hot functional tests followed by a cleaning RCS phase have been rolled out. This ideal procedure consists in two successive phases:

* the "optimized first oxidation" phase consists in an optimization of the chemical conditioning during the reactor start-up under basic and reducing conditioning at hot temperature,

* the "cleaning" phase consists in a acid and reducing conditioning at about 170°C during a dedicated outage.

With regards to operational aspects (safety and reactor control), the procedure had to be realised in two time separated phases.

In theory, such a procedure aims to reduce the inventory in corrosion products (specifically nickel) with expected gains on circuit contamination of about 30%.

A specific monitoring program (metal concentration measurements, volume and deposited activity by gamma spectrometry and dose rates) has been performed to estimate in real time the procedure efficiency. Thus, the nickel concentration variation, lower than the defined criterion of 1 ppm, led to stop the procedure after only 36 hours instead of the initially planned 200 hours. The contamination characterization by gamma spectrometry (EMECC campaign) and the dose rate monitoring have not allowed identifying a positive impact of the procedure on surface contamination.

The numerous data obtained during the experimentation have to be investigated more in depth in order to explain the observed phenomena and to understand why this chemical treatment has not led to the expected gains as much as for nickel elimination as for contamination and dose rate reduction. Research and development actions are already in progress:

* firstly, to better understand the impact of this procedure in terms of nickel release rate (BOREAL loop experiments /5/),

* secondly, to reduce the uncertainties on nickel, nickel oxide and nickel ferrite solubilities at medium or hot temperature,

* finally, to acquire new data about dissolution kinetics of nickel, nickel oxide and nickel ferrite.

The final objective is to decide if such a procedure can be viable in operation if some adjustments are proposed to improve the treatment efficiency with a view to an eventual deployment for future SG replacements of the EDF fleet and new-built nuclear power plants start-up.
second set, it is not possible to observe a clear impact of the acid and reducing chemistry, the values remaining below measurement uncertainties.

<figure><img src="image_10.png" /><figcaption>Figure 8: Normalized dose rate measured during the cleaning procedure</figcaption></figure>

4. CONCLUSION

After the steam generator replacement in 2010 at Bugey 3 NPP, an experimental SG tubes optimized first oxidation procedure during hot functional tests followed by a cleaning RCS phase have been rolled out. This ideal procedure consists in two successive phases:

* the "optimized first oxidation" phase consists in an optimization of the chemical conditioning during the reactor start-up under basic and reducing conditioning at hot temperature,

* the "cleaning" phase consists in a acid and reducing conditioning at about 170°C during a dedicated outage.

With regards to operational aspects (safety and reactor control), the procedure had to be realised in two time separated phases.

In theory, such a procedure aims to reduce the inventory in corrosion products (specifically nickel) with expected gains on circuit contamination of about 30%.

A specific monitoring program (metal concentration measurements, volume and deposited activity by gamma spectrometry and dose rates) has been performed to estimate in real time the procedure efficiency. Thus, the nickel concentration variation, lower than the defined criterion of 1 ppm, led to stop the procedure after only 36 hours instead of the initially planned 200 hours. The contamination characterization by gamma spectrometry (EMECC campaign) and the dose rate monitoring have not allowed identifying a positive impact of the procedure on surface contamination.

The numerous data obtained during the experimentation have to be investigated more in depth in order to explain the observed phenomena and to understand why this chemical treatment has not led to the expected gains as much as for nickel elimination as for contamination and dose rate reduction. Research and development actions are already in progress:

* firstly, to better understand the impact of this procedure in terms of nickel release rate (BOREAL loop experiments /5/),

* secondly, to reduce the uncertainties on nickel, nickel oxide and nickel ferrite solubilities at medium or hot temperature,

* finally, to acquire new data about dissolution kinetics of nickel, nickel oxide and nickel ferrite.

The final objective is to decide if such a procedure can be viable in operation if some adjustments are proposed to improve the treatment efficiency with a view to an eventual deployment for future SG replacements of the EDF fleet and new-built nuclear power plants start-up.
Beyond these technical questions not solved yet, this experimentation highlights the EDF capacity to bring on innovative procedures via its R&D department and to implement them to an industrial and operational level. Finally, even if the results have not been as good as expected, the wide mobilization of EDF teams and of its partners on that subject demonstrates the EDF will to improve radiological state of its facilities and as a consequence, the working conditions of its staff and its external contractors.

## 5. REFERENCES

1. G. Ranchoux and al., EDF Source Term Reduction Project – Main Outcomes and Further Developments, NPC Conference, Paris, 2012

2. M. Bachet and al., Cleaning the primary circuit after the first high temperature oxidation of steam generator tubing: why and how to do it, NPC Conference, Quebec, 2010

3. R. Eimecke and al., Ensemble de Mesures et d'Etudes de la Contamination des Circuits, International Conference on Radiation Shielding, Bornemouth (UK), 1988

4. L. Guinard and al., Characterization of contamination : French analysis of international EMECC campaigns, NPC Conference, Paris 2012

5. C. Suteau and al., Improvement of MERCURE-6's General Formalism for Calculating Gamma-Ray Buildup Factors in Multilayer Shields, Nuclear Science and Engineering 147, p. 43-55 (2004)

6. S. Leclercq and al., Steam generator Tubes Pre-oxidation and Cleaning – Effect on Release and Oxides Formation, NPC Conference, Paris, 2012
Beyond these technical questions not solved yet, this experimentation highlights the EDF capacity to bring on innovative procedures via its R&D department and to implement them to an industrial and operational level. Finally, even if the results have not been as good as expected, the wide mobilization of EDF teams and of its partners on that subject demonstrates the EDF will to improve radiological state of its facilities and as a consequence, the working conditions of its staff and its external contractors.

## 5. REFERENCES

1. G. Ranchoux and al., EDF Source Term Reduction Project – Main Outcomes and Further Developments, NPC Conference, Paris, 2012

2. M. Bachet and al., Cleaning the primary circuit after the first high temperature oxidation of steam generator tubing: why and how to do it, NPC Conference, Quebec, 2010

3. R. Eimecke and al., Ensemble de Mesures et d'Etudes de la Contamination des Circuits, International Conference on Radiation Shielding, Bornemouth (UK), 1988

4. L. Guinard and al., Characterization of contamination : French analysis of international EMECC campaigns, NPC Conference, Paris 2012

5. C. Suteau and al., Improvement of MERCURE-6's General Formalism for Calculating Gamma-Ray Buildup Factors in Multilayer Shields, Nuclear Science and Engineering 147, p. 43-55 (2004)

6. S. Leclercq and al., Steam generator Tubes Pre-oxidation and Cleaning – Effect on Release and Oxides Formation, NPC Conference, Paris, 2012