# Corrosion product transfer in French PWRs during shutdown

Frédéric Dacquait, Caroline Andrieu, Martial Berger, Jean-Luc Bretelle, Alain Rocher

► To cite this version:

Frédéric Dacquait, Caroline Andrieu, Martial Berger, Jean-Luc Bretelle, Alain Rocher. Corrosion product transfer in French PWRs during shutdown. International conference on water chemistry in nuclear reactors systems, SFEN, paper 113, 2002. cea-04463311

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# Corrosion product transfer in French PWRs during shutdown

Frédéric Dacquait, Caroline Andrieu, Martial Berger, Jean-Luc Bretelle, Alain Rocher

► To cite this version:

Frédéric Dacquait, Caroline Andrieu, Martial Berger, Jean-Luc Bretelle, Alain Rocher. Corrosion product transfer in French PWRs during shutdown. International conference on water chemistry in nuclear reactors systems, SFEN, paper 113, 2002. cea-04463311

HAL Id: cea-04463311

https://cea.hal.science/cea-04463311v1

Submitted on 16 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
CORROSION PRODUCT TRANSFER IN FRENCH PWRs DURING SHUTDOWN

F. DACQUAIT, COMMISSARIAT A L'ENERGIE ATOMIQUE, CEA / DEN, FRANCE

C. ANDRIEU & M. BERGER, ELECTRICITE DE FRANCE, EDF / SEPTEN, FRANCE

J-L BRETELLE, ELECTRICITE DE FRANCE, EDF / GDL, FRANCE

A. ROCHER, ELECTRICITE DE FRANCE, EDF / GPR, FRANCE

ABSTRACT

Physicochemical conditions significantly vary during cold shutdown in French PWRs: power and temperature decrease, chemical conditions are modified (pH decrease and change from reducing to oxidizing conditions). These changes involve corrosion product releases in the primary water (dissolving and crud bursts) which can be seen by a considerable corrosion product increase, from 2 to 3 orders of magnitude, in the activity concentrations (about a hundred GBq/t of ⁵⁸Co, about one GBq/t of ⁶⁰Co...) and in the chemical species quantity (several kilograms of nickel, several hundred grams of chromium...).

The ⁵⁸Co peak activities which are lower than the French PWR average are generally due to delayed or incomplete oxygenations or unscheduled shutdowns with oxygenation during operating cycles. An unscheduled shutdown with oxygenation in the last six months of an operating cycle must reduce the ⁵⁸Co activity peak during refueling shutdown, more especially as the unscheduled shutdown occurs near the end of the cycle. Thus, when the shutdown happens in the three months prior to the end of the cycle, it is very likely that the ⁵⁸Co peak during the refueling shutdown will be two times lower than the expected peak.

The large quantity of corrosion products released during shutdown mainly come from the dissolution of the in-core deposits due to primary system oxygenation, in particular the dissolution of nickel metal or nickel oxide, leading to the dissolution of the ⁵⁸Co derived from the nickel sites in the crud. Thus, the total ⁵⁸Co activity released during oxygenation is equal to, or even higher than, the out-of-core total surface activity. Therefore, the cold shutdown procedure must prevent the transfer of this considerable activity from the in-core to the out-of-core surfaces. Deposited activity measurements carried out in French PWR primary circuits have never shown any visible contamination reduction during cold shutdown. On the contrary, recontamination can occur because of a temperature plateau or an oxygenation at a temperature over 80°C.

That is the reason why:

* Primary circuit oxygenation at 80°C,

* Rapid temperature decrease and

* Improvements on primary fluid purification,

during cold shutdowns, have, up to this day, been the main points in the EDF strategy to reach the required dosimetric goals (ALARA), all the while optimizing unit outage schedules (availability).

This paper thus proposes a description of corrosion product behaviour during French PWR shutdowns in order to optimise cold shutdown procedures.

F. Dacquait, C. Andrieu, M. Berger, J.L. Bretelle, A. Rocher A, "Corrosion product transfer in French PWRs during shutdown", Proceedings of the International conference on water chemistry in nuclear reactors systems, 22-26/04/2002, Avignon (France), paper 113
CORROSION PRODUCT TRANSFER IN FRENCH PWRs DURING SHUTDOWN

F. DACQUAIT, COMMISSARIAT A L'ENERGIE ATOMIQUE, CEA / DEN, FRANCE

C. ANDRIEU & M. BERGER, ELECTRICITE DE FRANCE, EDF / SEPTEN, FRANCE

J-L BRETELLE, ELECTRICITE DE FRANCE, EDF / GDL, FRANCE

A. ROCHER, ELECTRICITE DE FRANCE, EDF / GPR, FRANCE

ABSTRACT

Physicochemical conditions significantly vary during cold shutdown in French PWRs: power and temperature decrease, chemical conditions are modified (pH decrease and change from reducing to oxidizing conditions). These changes involve corrosion product releases in the primary water (dissolving and crud bursts) which can be seen by a considerable corrosion product increase, from 2 to 3 orders of magnitude, in the activity concentrations (about a hundred GBq/t of ⁵⁸Co, about one GBq/t of ⁶⁰Co...) and in the chemical species quantity (several kilograms of nickel, several hundred grams of chromium...).

The ⁵⁸Co peak activities which are lower than the French PWR average are generally due to delayed or incomplete oxygenations or unscheduled shutdowns with oxygenation during operating cycles. An unscheduled shutdown with oxygenation in the last six months of an operating cycle must reduce the ⁵⁸Co activity peak during refueling shutdown, more especially as the unscheduled shutdown occurs near the end of the cycle. Thus, when the shutdown happens in the three months prior to the end of the cycle, it is very likely that the ⁵⁸Co peak during the refueling shutdown will be two times lower than the expected peak.

The large quantity of corrosion products released during shutdown mainly come from the dissolution of the in-core deposits due to primary system oxygenation, in particular the dissolution of nickel metal or nickel oxide, leading to the dissolution of the ⁵⁸Co derived from the nickel sites in the crud. Thus, the total ⁵⁸Co activity released during oxygenation is equal to, or even higher than, the out-of-core total surface activity. Therefore, the cold shutdown procedure must prevent the transfer of this considerable activity from the in-core to the out-of-core surfaces. Deposited activity measurements carried out in French PWR primary circuits have never shown any visible contamination reduction during cold shutdown. On the contrary, recontamination can occur because of a temperature plateau or an oxygenation at a temperature over 80°C.

That is the reason why:

* Primary circuit oxygenation at 80°C,

* Rapid temperature decrease and

* Improvements on primary fluid purification,

during cold shutdowns, have, up to this day, been the main points in the EDF strategy to reach the required dosimetric goals (ALARA), all the while optimizing unit outage schedules (availability).

This paper thus proposes a description of corrosion product behaviour during French PWR shutdowns in order to optimise cold shutdown procedures.

F. Dacquait, C. Andrieu, M. Berger, J.L. Bretelle, A. Rocher A, "Corrosion product transfer in French PWRs during shutdown", Proceedings of the International conference on water chemistry in nuclear reactors systems, 22-26/04/2002, Avignon (France), paper 113
# INTRODUCTION

Physicochemical conditions significantly vary during cold shutdown in French PWRs: power and temperature drops, chemical condition changes of the primary coolant. These changes lead to corrosion product releases in the primary coolant (dissolving and crud bursts), which can be seen by considerable corrosion product increase in the radiochemical and chemical species.

This is why the cold shutdown procedure must meet the following objectives:

* Minimize corrosion product release when this does not constitute an out-of-core contamination decrease,

* Lower primary coolant activity as quickly as possible,

* Avoid contaminating the primary circuit,

* Facilitate fuel-handling operations while maintaining reactor cavity water transparency and reducing the surface dose rate during core unloading.

To meet these objectives, studies have been carried out in France to optimise the shutdown procedure for many years. These studies are founded on the chemical and radiochemical analysis of primary coolant samplings and on the measurements of the activities deposited on the circuits during cold shutdowns.

# COLD SHUTDOWN DESCRIPTION

The physicochemical parameter variations during a cold shutdown on a French 1300 MWe PWR (Reactor A) are described in Figure 1.

<figure><img src="image_2.png" /><figcaption>Figure 1: Cold shutdown – Parameter variation</figcaption></figure>

After an $H_2$ decrease to a content lower than 3 cc/kg STP, in order to oxygenate the primary circuit, 15 litres (900 MWe PWR) or 23 litres (1300 MWe PWR) of hydrogen peroxide (30%) are injected and the vapor space of the CVCS volume control tank is scavenged. The hydrogen peroxide injection is carried out when the primary coolant temperature is at 80°C, if possible with all the primary coolant pumps in operation.

Because of the oxygenation, corrosion product activity concentrations significantly increase in the primary circuit, especially the $^{58}$Co volume activity.
## INTRODUCTION

Physicochemical conditions significantly vary during cold shutdown in French PWRs: power and temperature drops, chemical condition changes of the primary coolant. These changes lead to corrosion product releases in the primary coolant (dissolving and crud bursts), which can be seen by considerable corrosion product increase in the radiochemical and chemical species.

This is why the cold shutdown procedure must meet the following objectives:

* Minimize corrosion product release when this does not constitute an out-of-core contamination decrease,

* Lower primary coolant activity as quickly as possible,

* Avoid contaminating the primary circuit,

* Facilitate fuel-handling operations while maintaining reactor cavity water transparency and reducing the surface dose rate during core unloading.

To meet these objectives, studies have been carried out in France to optimise the shutdown procedure for many years. These studies are founded on the chemical and radiochemical analysis of primary coolant samplings and on the measurements of the activities deposited on the circuits during cold shutdowns.

## COLD SHUTDOWN DESCRIPTION

The physicochemical parameter variations during a cold shutdown on a French 1300 MWe PWR (Reactor A) are described in Figure 1.

<figure><img src="image_2.png" /><figcaption>Figure 1: Cold shutdown – Parameter variation</figcaption></figure>

After an $H_2$ decrease to a content lower than 3 cc/kg STP, in order to oxygenate the primary circuit, 15 litres (900 MWe PWR) or 23 litres (1300 MWe PWR) of hydrogen peroxide (30%) are injected and the vapor space of the CVCS volume control tank is scavenged. The hydrogen peroxide injection is carried out when the primary coolant temperature is at 80°C, if possible with all the primary coolant pumps in operation.

Because of the oxygenation, corrosion product activity concentrations significantly increase in the primary circuit, especially the $^{58}$Co volume activity.
## ACTIVITY AND CHEMICAL ELEMENT CONCENTRATIONS

To assess corrosion product behaviour, primary coolant samplings are made during PWR cold shutdown. During the reactor A shutdown described in the previous paragraph, 250 to 1000 ml samplings were filtered through 0.45 µm millipore filters and ion exchange papers. The volume activities of the corrosion products in solution and suspension were measured by gamma spectrometry (Figure 2 and Figure 3), the chemical concentrations by X-ray fluorescence (Figure 4). Thanks to these measurements, the total released nickel mass, the ⁵⁸Co total released activity and the ⁵⁸Co specific activity during the shutdown can be evaluated (Figure 5).

<figure><img src="image_3.png" /><figcaption>Figure 2 : Volume activities</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 3 : Dissolved radionuclide proportion</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4 : Chemical element concentration</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 5 : ⁵⁸Co total release and specific activity</figcaption></figure>
ACTIVITY AND CHEMICAL ELEMENT CONCENTRATIONS

To assess corrosion product behaviour, primary coolant samplings are made during PWR cold shutdown. During the reactor A shutdown described in the previous paragraph, 250 to 1000 ml samplings were filtered through 0.45 µm millipore filters and ion exchange papers. The volume activities of the corrosion products in solution and suspension were measured by gamma spectrometry (Figure 2 and Figure 3), the chemical concentrations by X-ray fluorescence (Figure 4). Thanks to these measurements, the total released nickel mass, the ⁵⁸Co total released activity and the ⁵⁸Co specific activity during the shutdown can be evaluated (Figure 5).

<figure><img src="image_3.png" /><figcaption>Figure 2 : Volume activities</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 3 : Dissolved radionuclide proportion</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4 : Chemical element concentration</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 5 : ⁵⁸Co total release and specific activity</figcaption></figure>
Moreover, during another 1300 MWe PWR (Reactor B) shutdown, particle size analyses were determined by 1 litre samplings through a succession of millipore filters with decreasing meshes: 10 µm, 5 µm, 1.2 µm and 0.45 µm (□ Figure 6).

<figure><img src="image_7.png" /><figcaption>Figure 6 : Radionuclide particle size distribution before and during shutdown</figcaption></figure>

Three main phases lead to corrosion product transfers [1, 2]:

* Power reduction,

* Primary coolant boration and cooling,

* Primary circuit oxygenation.

Power reduction is characterized by a rapid increase in volume activity. However, this increase is slight, about 100 GBq of $^{58}$Co, versus the total activity released during shutdown. This activity mainly consists of particles, part of which deposits very rapidly. The radionuclide particle size is between 5 and 10 µm for reactor B before shutdown, while it is mostly greater than 10 µm during hot shutdown.
Moreover, during another 1300 MWe PWR (Reactor B) shutdown, particle size analyses were determined by 1 litre samplings through a succession of millipore filters with decreasing meshes: 10 µm, 5 µm, 1.2 µm and 0.45 µm (□ Figure 6).

<figure><img src="image_7.png" /><figcaption>Figure 6 : Radionuclide particle size distribution before and during shutdown</figcaption></figure>

Three main phases lead to corrosion product transfers [1, 2]:

* Power reduction,

* Primary coolant boration and cooling,

* Primary circuit oxygenation.

Power reduction is characterized by a rapid increase in volume activity. However, this increase is slight, about 100 GBq of $^{58}$Co, versus the total activity released during shutdown. This activity mainly consists of particles, part of which deposits very rapidly. The radionuclide particle size is between 5 and 10 µm for reactor B before shutdown, while it is mostly greater than 10 µm during hot shutdown.
The ⁵⁸Co/⁶⁰Co activity ratio, about 20 (this varies between 2 and 6 for out-of-core deposits), and the ⁵⁸Co specific activity, about 30 GBq/g (this is about 45 GBq/g at in-core equilibrium), show that these particles originate from an in-core deposit detachment, due to thermal and mechanical stress variations during control rod insertion.

The primary coolant boration and temperature decrease (from 290 to 180°C and then from 180 to 80°C) result in a fast increase in the activity and chemical concentration mainly by dissolution. This corrosion product dissolution is related to the solubility increase due to temperature and pH decreases.

The ⁵⁸Co/⁶⁰Co ratio, about 20, and the ⁵⁸Co specific activity, less than 10 GBq/g, indicate that this dissolution affects in-core and out-of-core deposits. The activities and masses released in the primary coolant come from deposit decomposition (nickel ferrite). During the temperature plateau at 180°C, the similar variation of the nickel and iron concentrations and the low ⁵⁸Co specific activity (less than 5) show that this nickel ferrite decomposition especially takes place on out-of-core surfaces.

As the soluble radionuclide ratio decreases after the dissolution due to the falling temperature or pH, part of these radionuclides must precipitate. For reactor B during the temperature plateau, approximately 25% of the particulate activity is in the 5-10 µm range and about 75% greater than 10 µm. During the temperature decrease from 180 to 80°C, half of the particulate activity is equally distributed between the 1.2-5 µm and 5-10 µm ranges, the other half being greater than 10 µm.

The ⁵⁸Co activity concerned during this phase is about several thousands of GBq, a low activity compared to the total activity released during shutdown.

Primary circuit oxygenation causes a large dissolving of nickel (a concentration of about 8 ppm), and, to a lesser extent, of cobalt. On the other hand, iron precipitates to form particles (Fe²⁺ oxidized to Fe³⁺), the ⁵⁹Fe activity being mainly in particulate form. Chromium, in particulate form under reducing conditions, dissolves in oxidizing conditions. The particle size distribution is not greatly modified by oxygenation.

The high ⁵⁸Co/⁶⁰Co ratio (> 200) and the ⁵⁸Co specific activity (approximately 25 GBq/g) show that an in-core nickel deposit dissolves. It is an in-core nickel metal or nickel oxide deposit that is stable in reducing conditions at high temperature and that dissolves in oxidizing conditions at low temperature. This is confirmed by an experiment in the SOZIE loop [3]. The nickel dissolution involves the dissolution of ⁵⁸Co, derived from the nickel sites in the crud (⁵⁸Ni (n,p) ⁵⁸Co). A ⁵⁸Co activity peak then appears within the half-hour following hydrogen peroxide injection. The total ⁵⁸Co activity released during oxygenation is considerable, about 150000 GBq for reactor A. The total nickel mass released during this shutdown is about 5.7 kg, including 5 kg during oxygenation.

Oxygenation thus creates high activity peaks. The available statistical data on ⁵⁸Co and ⁶⁰Co peak values in French PWRs are gathered in Table 1.

<table><caption>Table 1 : Peaks during refueling shutdown</caption><thead><tr><th rowspan="2">PWR</th><th rowspan="2">Data</th><th colspan="2">⁵⁸Co<br>(GBq/t)</th><th>⁶⁰Co<br>(GBq/t)</th></tr><tr><th>Average<br>peak</th><th>Standard<br>deviation</th><th>Average<br>peak</th></tr></thead><tbody><tr><td rowspan="2">900 MWe</td><td>All available data</td><td>110</td><td>55</td><td></td></tr><tr><td>Year 2000</td><td>103</td><td>47</td><td>0.7</td></tr><tr><td rowspan="2">1300 MWe</td><td>All available data</td><td>180</td><td>80</td><td></td></tr><tr><td>Year 2000</td><td>165</td><td>64</td><td>1.0</td></tr></tbody></table>

The high ⁵⁸Co peak values in 900 MWe PWRs are generally due to Steam Generator Replacements (SGR) that cause a ⁵⁸Co released activity increase which lasts during several shutdowns after the SGR cycle [4].

For the 1300 MWe PWRs, the higher peaks occurred during the first cycles, probably because of a larger release of Steam Generator (SG) tubing at the beginning of life.

Note that the SG tubing material in the French PWRs is alloy 600 for the oldest or alloy 690 for the most recent.

The low peak values are generally due to delayed, incomplete oxygenations or unscheduled shutdowns with oxygenation in the course of cycle. The unscheduled shutdowns allow nickel and ⁵⁸Co deposited on fuel to be solubilized and purified. These deposits do not have time to reform before the refueling shutdown.

Figure 7 shows the ratio between peaks during refueling shutdowns after an unscheduled shutdown with oxygenation and the expected peaks if there had not been any unscheduled shutdown, versus operating time after an unscheduled shutdown with oxygenation.
The ⁵⁸Co/⁶⁰Co activity ratio, about 20 (this varies between 2 and 6 for out-of-core deposits), and the ⁵⁸Co specific activity, about 30 GBq/g (this is about 45 GBq/g at in-core equilibrium), show that these particles originate from an in-core deposit detachment, due to thermal and mechanical stress variations during control rod insertion.

The primary coolant boration and temperature decrease (from 290 to 180°C and then from 180 to 80°C) result in a fast increase in the activity and chemical concentration mainly by dissolution. This corrosion product dissolution is related to the solubility increase due to temperature and pH decreases.

The ⁵⁸Co/⁶⁰Co ratio, about 20, and the ⁵⁸Co specific activity, less than 10 GBq/g, indicate that this dissolution affects in-core and out-of-core deposits. The activities and masses released in the primary coolant come from deposit decomposition (nickel ferrite). During the temperature plateau at 180°C, the similar variation of the nickel and iron concentrations and the low ⁵⁸Co specific activity (less than 5) show that this nickel ferrite decomposition especially takes place on out-of-core surfaces.

As the soluble radionuclide ratio decreases after the dissolution due to the falling temperature or pH, part of these radionuclides must precipitate. For reactor B during the temperature plateau, approximately 25% of the particulate activity is in the 5-10 µm range and about 75% greater than 10 µm. During the temperature decrease from 180 to 80°C, half of the particulate activity is equally distributed between the 1.2-5 µm and 5-10 µm ranges, the other half being greater than 10 µm.

The ⁵⁸Co activity concerned during this phase is about several thousands of GBq, a low activity compared to the total activity released during shutdown.

Primary circuit oxygenation causes a large dissolving of nickel (a concentration of about 8 ppm), and, to a lesser extent, of cobalt. On the other hand, iron precipitates to form particles (Fe²⁺ oxidized to Fe³⁺), the ⁵⁹Fe activity being mainly in particulate form. Chromium, in particulate form under reducing conditions, dissolves in oxidizing conditions. The particle size distribution is not greatly modified by oxygenation.

The high ⁵⁸Co/⁶⁰Co ratio (> 200) and the ⁵⁸Co specific activity (approximately 25 GBq/g) show that an in-core nickel deposit dissolves. It is an in-core nickel metal or nickel oxide deposit that is stable in reducing conditions at high temperature and that dissolves in oxidizing conditions at low temperature. This is confirmed by an experiment in the SOZIE loop [3]. The nickel dissolution involves the dissolution of ⁵⁸Co, derived from the nickel sites in the crud (⁵⁸Ni (n,p) ⁵⁸Co). A ⁵⁸Co activity peak then appears within the half-hour following hydrogen peroxide injection. The total ⁵⁸Co activity released during oxygenation is considerable, about 150000 GBq for reactor A. The total nickel mass released during this shutdown is about 5.7 kg, including 5 kg during oxygenation.

Oxygenation thus creates high activity peaks. The available statistical data on ⁵⁸Co and ⁶⁰Co peak values in French PWRs are gathered in Table 1.

<table><caption>Table 1 : Peaks during refueling shutdown</caption><thead><tr><th rowspan="2">PWR</th><th rowspan="2">Data</th><th colspan="2">⁵⁸Co<br>(GBq/t)</th><th>⁶⁰Co<br>(GBq/t)</th></tr><tr><th>Average<br>peak</th><th>Standard<br>deviation</th><th>Average<br>peak</th></tr></thead><tbody><tr><td rowspan="2">900 MWe</td><td>All available data</td><td>110</td><td>55</td><td></td></tr><tr><td>Year 2000</td><td>103</td><td>47</td><td>0.7</td></tr><tr><td rowspan="2">1300 MWe</td><td>All available data</td><td>180</td><td>80</td><td></td></tr><tr><td>Year 2000</td><td>165</td><td>64</td><td>1.0</td></tr></tbody></table>

The high ⁵⁸Co peak values in 900 MWe PWRs are generally due to Steam Generator Replacements (SGR) that cause a ⁵⁸Co released activity increase which lasts during several shutdowns after the SGR cycle [4].

For the 1300 MWe PWRs, the higher peaks occurred during the first cycles, probably because of a larger release of Steam Generator (SG) tubing at the beginning of life.

Note that the SG tubing material in the French PWRs is alloy 600 for the oldest or alloy 690 for the most recent.

The low peak values are generally due to delayed, incomplete oxygenations or unscheduled shutdowns with oxygenation in the course of cycle. The unscheduled shutdowns allow nickel and ⁵⁸Co deposited on fuel to be solubilized and purified. These deposits do not have time to reform before the refueling shutdown.

Figure 7 shows the ratio between peaks during refueling shutdowns after an unscheduled shutdown with oxygenation and the expected peaks if there had not been any unscheduled shutdown, versus operating time after an unscheduled shutdown with oxygenation.
It appears that an unscheduled shutdown with oxygenation of about 6 months before the refueling shutdown allows the ⁵⁸Co volume activity released during the refueling shutdown at the end of the cycle to be reduced, particularly as this unscheduled shutdown occurs near the end of the cycle. Thus, when the shutdown occurs three months before the end of the cycle, it is extremely likely that the peak during the refueling shutdown will be reduced by at least half compared to the expected peak.

However, before the increase in the primary circuit temperature during an unscheduled shutdown, the corrosion product volume activities must be sufficiently low (EDF criteria: ⁵⁸Co < 7 GBq/t and γtotal < 14 GBq/t) to prevent their precipitation, due to temperature and pH increase, from causing their deposition on out-of-core surfaces. This risk of primary circuit recontamination would lead to a dose rate increase during the refueling shutdown at the end of the cycle.

<figure><img src="image_8.png" /><figcaption>Figure 7: Impact of unscheduled shutdowns with oxygenation</figcaption></figure>

## PRIMARY COOLANT PURIFICATION

The previous paragraph showed that, during a cold shutdown, a great quantity of corrosion products was transported by primary coolant. This huge quantity of corrosion products is trapped in the filters and demineralizers of CVCS as shown in Figure 8. The CVCS Decontamination Factor (DF = upstream filtration activity / downstream filtration activity), given during a 1300 MWe PWR shutdown, shows that purification efficiency is high for radioactive corrosion products: from 100 to 10⁶. It would seem that the higher the decontamination factor, the more significant the upstream filtration activity. But this is perhaps also due to the particular behaviour of the ion exchange resins with respect to radioelement retention.
It appears that an unscheduled shutdown with oxygenation of about 6 months before the refueling shutdown allows the ⁵⁸Co volume activity released during the refueling shutdown at the end of the cycle to be reduced, particularly as this unscheduled shutdown occurs near the end of the cycle. Thus, when the shutdown occurs three months before the end of the cycle, it is extremely likely that the peak during the refueling shutdown will be reduced by at least half compared to the expected peak.

However, before the increase in the primary circuit temperature during an unscheduled shutdown, the corrosion product volume activities must be sufficiently low (EDF criteria: ⁵⁸Co < 7 GBq/t and γtotal < 14 GBq/t) to prevent their precipitation, due to temperature and pH increase, from causing their deposition on out-of-core surfaces. This risk of primary circuit recontamination would lead to a dose rate increase during the refueling shutdown at the end of the cycle.

<figure><img src="image_8.png" /><figcaption>Figure 7: Impact of unscheduled shutdowns with oxygenation</figcaption></figure>

## PRIMARY COOLANT PURIFICATION

The previous paragraph showed that, during a cold shutdown, a great quantity of corrosion products was transported by primary coolant. This huge quantity of corrosion products is trapped in the filters and demineralizers of CVCS as shown in Figure 8. The CVCS Decontamination Factor (DF = upstream filtration activity / downstream filtration activity), given during a 1300 MWe PWR shutdown, shows that purification efficiency is high for radioactive corrosion products: from 100 to 10⁶. It would seem that the higher the decontamination factor, the more significant the upstream filtration activity. But this is perhaps also due to the particular behaviour of the ion exchange resins with respect to radioelement retention.
<figure><img src="image_9.png" /><figcaption>Figure 8 : Purification efficiency</figcaption></figure>

Thus, the activities and the masses of the “traditional” corrosion products are trapped by the prefilter and by the ion exchange resins of the CVCS. On the other hand, other undesirable radioelements such as ¹²²Sb and ¹²⁴Sb, as well as ¹¹⁰mAg, are not systematically properly trapped by filters and demineralizers and these can significantly add to dosimetry [5].

The activity criteria currently specified for corrosion products at EDF during cold shutdowns in French PWRs are defined in Table 2.

<table><caption>Table 2 : Activity criteria</caption><thead><tr><th>Hold points</th><th>Activity criteria relating to corrosion products during purification by CVCS</th></tr></thead><tbody><tr><td>Primary pump shutdown</td><td><sup>58</sup>Co &lt; 50 GBq/t and γ<sub>total</sub> &lt; 100 GBq/t</td></tr><tr><td>Reactor cavity filling</td><td><sup>58</sup>Co &lt; 2 GBq/t and γ<sub>total</sub> &lt; 4 GBq/t</td></tr></tbody></table>

In order to quickly meet these goals by means of purification, while taking into account specific problems (presence of ¹²²Sb, ¹²⁴Sb and ¹¹⁰mAg), two complementary research projects are under development and design engineering at EDF: increase the purification flow rate and optimise the purification supports [6].

Volume activity $A_t$ (at moment $t$) in the primary circuit is :

$$A_t = A_0 \cdot \exp \left[ -\frac{Q \cdot t}{M} \left( 1 - \frac{1}{DF} \right) \right]$$

with

* $A_0$: initial volume activity

* Q : CVCS letdown flow rate

* M : total water mass to be purified (including CVCS volume)

The decontamination factors being higher than 100, this expression is simplified in:

$$A_t = A_0 \cdot \exp \left[ -\frac{Q \cdot t}{M} \right]$$

Thus, the purification flow rate increase allows an increase in the CVCS purification constant to be obtained ($Q/M$). One can discount a significant savings of time in obtaining the primary pump shutdown criteria (availability) or, for the same scheduled purification time, in obtaining the prescribed activity criteria (ALARA) below. Moreover, the increase in the CVCS purification flow rate helps meet the reactor cavity filling criteria more quickly.
<figure><img src="image_9.png" /><figcaption>Figure 8 : Purification efficiency</figcaption></figure>

Thus, the activities and the masses of the “traditional” corrosion products are trapped by the prefilter and by the ion exchange resins of the CVCS. On the other hand, other undesirable radioelements such as ¹²²Sb and ¹²⁴Sb, as well as ¹¹⁰mAg, are not systematically properly trapped by filters and demineralizers and these can significantly add to dosimetry [5].

The activity criteria currently specified for corrosion products at EDF during cold shutdowns in French PWRs are defined in Table 2.

<table><caption>Table 2 : Activity criteria</caption><thead><tr><th>Hold points</th><th>Activity criteria relating to corrosion products during purification by CVCS</th></tr></thead><tbody><tr><td>Primary pump shutdown</td><td><sup>58</sup>Co &lt; 50 GBq/t and γ<sub>total</sub> &lt; 100 GBq/t</td></tr><tr><td>Reactor cavity filling</td><td><sup>58</sup>Co &lt; 2 GBq/t and γ<sub>total</sub> &lt; 4 GBq/t</td></tr></tbody></table>

In order to quickly meet these goals by means of purification, while taking into account specific problems (presence of ¹²²Sb, ¹²⁴Sb and ¹¹⁰mAg), two complementary research projects are under development and design engineering at EDF: increase the purification flow rate and optimise the purification supports [6].

Volume activity $A_t$ (at moment $t$) in the primary circuit is :

$$A_t = A_0 \cdot \exp \left[ -\frac{Q \cdot t}{M} \left(1 - \frac{1}{DF}\right) \right]$$

with

* $A_0$ : initial volume activity

* Q : CVCS letdown flow rate

* M : total water mass to be purified (including CVCS volume)

The decontamination factors being higher than 100, this expression is simplified in:

$$A_t = A_0 \cdot \exp \left[ -\frac{Q \cdot t}{M} \right]$$

Thus, the purification flow rate increase allows an increase in the CVCS purification constant to be obtained ($Q/M$). One can discount a significant savings of time in obtaining the primary pump shutdown criteria (availability) or, for the same scheduled purification time, in obtaining the prescribed activity criteria (ALARA) below. Moreover, the increase in the CVCS purification flow rate helps meet the reactor cavity filling criteria more quickly.
Consequently, the increased CVCS letdown flow rates retained by EDF are:

<table><thead><tr><th colspan="2">Standardized plants</th><th>Letdown flow rate Q (t/h)</th></tr></thead><tbody><tr><td>CPY</td><td>900 MWe</td><td>40 (in two prototype units)</td></tr><tr><td>P4 and P'4</td><td>1300 MWe</td><td>47</td></tr><tr><td>N4</td><td>1450 MWe</td><td>50 (studies in progress to increase to 70 t/h)</td></tr></tbody></table>

## SURFACE ACTIVITIES

In order to evaluate the cold shutdown impact on contamination, the activities deposited in the PWR primary circuit have been measured for many years by means of the EMECC gamma spectrometry system [7]. These measurements are carried out before and after oxygenation, mainly on hot legs and SG tube bundles. Figure 9 shows the variation in the ⁵⁸Co and ⁶⁰Co activities deposited in the hot legs and SG tubing, measured before and after oxygenation. Those variations of less than 20% are not significant because of the measurement uncertainty.

<figure><img src="image_10.png" /><figcaption>Figure 9 : Surface activity variation during oxygenation</figcaption></figure>

On approximately 80 analysed shutdowns, a visible reduction in the ⁵⁸Co and ⁶⁰Co activities deposited on the primary circuit during oxygenation has never been observed. However, a strong increase in ⁵⁸Co deposited activity can occur on out-of-core surfaces if the effective oxygenation temperature of the primary circuit is higher than 80°C (□ Figure 10) [1]. Recontamination then occurs very rapidly (less than one hour) and at the time of the activity peak. On the contrary, the oxygenation temperature does not seem to have an impact on ⁶⁰Co contamination (□ Figure 9). This is due to the fact that the ⁶⁰Co released quantity, suitable to be deposited on the out-of-flux surfaces, is probably not significant enough to be measured. Let us note that at identical oxygenation temperatures, ⁵⁸Co SG recontamination, when significant, is roughly the same as in the procedures using hydrogen peroxide or CVCS venting (□ Figure 10).

<figure><img src="image_11.png" /><figcaption>Figure 10 : Impact of the effective oxygenation temperature</figcaption></figure>
Consequently, the increased CVCS letdown flow rates retained by EDF are:

<table><thead><tr><th colspan="2">Standardized plants</th><th>Letdown flow rate Q (t/h)</th></tr></thead><tbody><tr><td>CPY</td><td>900 MWe</td><td>40 (in two prototype units)</td></tr><tr><td>P4 and P'4</td><td>1300 MWe</td><td>47</td></tr><tr><td>N4</td><td>1450 MWe</td><td>50 (studies in progress to increase to 70 t/h)</td></tr></tbody></table>

## SURFACE ACTIVITIES

In order to evaluate the cold shutdown impact on contamination, the activities deposited in the PWR primary circuit have been measured for many years by means of the EMECC gamma spectrometry system [7]. These measurements are carried out before and after oxygenation, mainly on hot legs and SG tube bundles. Figure 9 shows the variation in the ⁵⁸Co and ⁶⁰Co activities deposited in the hot legs and SG tubing, measured before and after oxygenation. Those variations of less than 20% are not significant because of the measurement uncertainty.

<figure><img src="image_10.png" /><figcaption>Figure 9 : Surface activity variation during oxygenation</figcaption></figure>

On approximately 80 analysed shutdowns, a visible reduction in the ⁵⁸Co and ⁶⁰Co activities deposited on the primary circuit during oxygenation has never been observed. However, a strong increase in ⁵⁸Co deposited activity can occur on out-of-core surfaces if the effective oxygenation temperature of the primary circuit is higher than 80°C (□ Figure 10) [1]. Recontamination then occurs very rapidly (less than one hour) and at the time of the activity peak. On the contrary, the oxygenation temperature does not seem to have an impact on ⁶⁰Co contamination (□ Figure 9). This is due to the fact that the ⁶⁰Co released quantity, suitable to be deposited on the out-of-flux surfaces, is probably not significant enough to be measured. Let us note that at identical oxygenation temperatures, ⁵⁸Co SG recontamination, when significant, is roughly the same as in the procedures using hydrogen peroxide or CVCS venting (□ Figure 10).

<figure><img src="image_11.png" /><figcaption>Figure 10 : Impact of the effective oxygenation temperature</figcaption></figure>
The increases in the ⁵⁸Co activities deposited on the hot legs and the SG tubes during oxygenation presented in Figure 9 are due to an effective oxygenation temperature higher than 80°C, in particular for the most significant variation rates. However, these increases can also be due to prolonged temperature plateaux.

Thus Figure 11 shows the evolution of the ⁵⁸Co activity deposited on a SG tube bundle during a cold shutdown characterized by a temperature plateau at 120°C during 5 days. Although the volume activity during this plateau is not very significant (between 15 and 20 GBq/t), the SG ⁵⁸Co contamination increases appreciably during the plateau at 120°C. Note that since part of the ⁵⁸Co activity is released during the temperature plateau, the activity peak due to oxygenation at 80°C is not very high (33 GBq/t). However, the total released activity is equivalent to what it would have been without a temperature plateau, but with a more significant peak at the time of oxygenation. Finally, let us note that the H₂O₂ injection temperature being 80°C, oxygenation did not lead to primary circuit recontamination.

<figure><img src="image_12.png" /><figcaption>Figure 11: Impact of a temperature plateau</figcaption></figure>

## DISCUSSION

During a cold shutdown in an EDF PWR, a significant transfer of corrosion products mainly occurs at the time of primary circuit oxygenation. A nickel peak (several ppm) and a ⁵⁸Co peak (a hundred GBq/t) then appear half an hour after the H₂O₂ injection.

During reactor A shutdown (□ § Cold shutdown description), about 95% of the total ⁵⁸Co released is due to oxygenation. This activity is equal to, or even higher than, the total activity deposited on the out-of-core primary surfaces. Indeed, the ⁵⁸Co total activity released during the shutdown is about 150000 GBq, whereas the ⁵⁸Co out-of-core total surface activity is about 75000 GBq before and after oxygenation, that is to say two times less. This confirms that the activity released during oxygenation comes from in-core deposits and not out-of-core deposits. This activity is due to the dissolution of nickel oxide or metal nickel in-core deposits in an oxidizing medium, leading to the dissolution of the ⁵⁸Co derived from the nickel sites in the crud. Although the released activity is primarily in soluble form, crud bursts occur during cold shutdown, and these particle sizes can be relatively significant: larger than 10 µm.

Gamma spectrometry measurements carried out by the CEA have never shown a visible reduction in the ⁵⁸Co and ⁶⁰Co contamination on the out-of-core primary surfaces. On the other hand, a significant ⁵⁸Co recontamination of the primary circuit can occur if the effective temperature of the primary circuit oxygenation is higher than 80°C. Moreover, ⁵⁸Co recontamination is probably accompanied by a cobalt and nickel re-deposition. These elements are redistributed during the following cycle and can be activated in core, thus creating an additional source of radioactive corrosion products.

In the paragraph "Activity and chemical element concentrations", it was stated that the nickel ferrite deposits were to decompose during the temperature plateau at 180°C. But this dissolution of the out-of-core deposits is negligible, since it represents less than 1% of the out-of-core total surface activity. Moreover, the measurements of the activities deposited on the primary circuit have never shown a reduction in the ⁵⁸Co and ⁶⁰Co contamination of the out-of-core surfaces during this phase.

However, as nickel ferrites are decomposed by dissolved hydrogen, the hydrogen concentration is perhaps not high enough during French PWR shutdowns to be able to observe a reduction of the out-of-core surface
The increases in the ⁵⁸Co activities deposited on the hot legs and the SG tubes during oxygenation presented in Figure 9 are due to an effective oxygenation temperature higher than 80°C, in particular for the most significant variation rates. However, these increases can also be due to prolonged temperature plateaux.

Thus Figure 11 shows the evolution of the ⁵⁸Co activity deposited on a SG tube bundle during a cold shutdown characterized by a temperature plateau at 120°C during 5 days. Although the volume activity during this plateau is not very significant (between 15 and 20 GBq/t), the SG ⁵⁸Co contamination increases appreciably during the plateau at 120°C. Note that since part of the ⁵⁸Co activity is released during the temperature plateau, the activity peak due to oxygenation at 80°C is not very high (33 GBq/t). However, the total released activity is equivalent to what it would have been without a temperature plateau, but with a more significant peak at the time of oxygenation. Finally, let us note that the H₂O₂ injection temperature being 80°C, oxygenation did not lead to primary circuit recontamination.

<figure><img src="image_12.png" /><figcaption>Figure 11: Impact of a temperature plateau</figcaption></figure>

## DISCUSSION

During a cold shutdown in an EDF PWR, a significant transfer of corrosion products mainly occurs at the time of primary circuit oxygenation. A nickel peak (several ppm) and a ⁵⁸Co peak (a hundred GBq/t) then appear half an hour after the H₂O₂ injection.

During reactor A shutdown (□ § Cold shutdown description), about 95% of the total ⁵⁸Co released is due to oxygenation. This activity is equal to, or even higher than, the total activity deposited on the out-of-core primary surfaces. Indeed, the ⁵⁸Co total activity released during the shutdown is about 150000 GBq, whereas the ⁵⁸Co out-of-core total surface activity is about 75000 GBq before and after oxygenation, that is to say two times less. This confirms that the activity released during oxygenation comes from in-core deposits and not out-of-core deposits. This activity is due to the dissolution of nickel oxide or metal nickel in-core deposits in an oxidizing medium, leading to the dissolution of the ⁵⁸Co derived from the nickel sites in the crud. Although the released activity is primarily in soluble form, crud bursts occur during cold shutdown, and these particle sizes can be relatively significant: larger than 10 µm.

Gamma spectrometry measurements carried out by the CEA have never shown a visible reduction in the ⁵⁸Co and ⁶⁰Co contamination on the out-of-core primary surfaces. On the other hand, a significant ⁵⁸Co recontamination of the primary circuit can occur if the effective temperature of the primary circuit oxygenation is higher than 80°C. Moreover, ⁵⁸Co recontamination is probably accompanied by a cobalt and nickel re-deposition. These elements are redistributed during the following cycle and can be activated in core, thus creating an additional source of radioactive corrosion products.

In the paragraph "Activity and chemical element concentrations", it was stated that the nickel ferrite deposits were to decompose during the temperature plateau at 180°C. But this dissolution of the out-of-core deposits is negligible, since it represents less than 1% of the out-of-core total surface activity. Moreover, the measurements of the activities deposited on the primary circuit have never shown a reduction in the ⁵⁸Co and ⁶⁰Co contamination of the out-of-core surfaces during this phase.

However, as nickel ferrites are decomposed by dissolved hydrogen, the hydrogen concentration is perhaps not high enough during French PWR shutdowns to be able to observe a reduction of the out-of-core surface
activities ([H₂] < 3 cc/kg). According to [8], the dissolved hydrogen content should be higher than 10 cc/kg and the temperature should range between 120 and 150°C during at least 16 hours, in order to decompose nickel ferrites. However, the reaction kinetics is not known and a temperature plateau can lead to out-of-core surface recontamination (Figure 11). The hypothetical benefit is thus likely to be masked, all the more so as the acid-reducing conditions can also decompose the in-core nickel ferrites and thus recontaminate out-of-flux surfaces. In order to evaluate its impact, an acid-reducing chemistry will be applied and monitored during a French PWR shutdown this year.

In France, the current procedure for the cold shutdown prescribed by EDF does not aim at decontaminating the out-of-core surfaces, but, at, as soon as possible, reaching the criteria of the reactor cavity filling by avoiding recontaminating the primary circuit out-of-core surfaces. That is why:

* The primary coolant temperature is reduced as quickly as possible,

* The oxygenation temperature of the primary circuit is 80°C,

* The high flow rate purification is being evaluated so as to be applied to EDF PWRs. The corrosion products are effectively trapped on the CVCS filters and demineralizers, except for specific pollutions (antimony and silver) for which studies on the optimisation of the purification media are in progress.

Although penalizing, among other things, to availability, an unscheduled shutdown with oxygenation can have a beneficial effect on the refueling shutdown at the end of the cycle. Indeed, an unscheduled shutdown with oxygenation in the last six months of a cycle allows the ⁵⁸Co activity peak to be decreased during the refueling shutdown. As for purification, an unscheduled shutdown makes it possible to more quickly reach the criteria of reactor cavity filling or, for the same purification duration, to obtain activity values below the criteria.
This is why EDF recommends oxygenating the primary circuit if the unscheduled shutdown lasts more than 3 days. Moreover, it allows the reduction of out-of-core contamination, since part of the in-core corrosion product deposits is removed and trapped by CVCS filtration. Let us recall that it is advisable to purify the primary circuit sufficiently to avoid recontaminating the out-of-flux surfaces, because of the temperature and pH increase at the end of the unscheduled shutdown (EDF criteria: ⁵⁸Co < 7 GBq/t and γtotal < 14 GBq/t). These criteria are to be respected all the more so as, since the unscheduled shutdown occurs near the end of the cycle, the radionuclides have less time to decrease.

## CONCLUSION

Cold shutdown in French PWRs leads to considerable corrosion product increase in the primary coolant (2 to 3 orders of magnitude). This is highlighted by high activity peaks (about a hundred GBq/t of ⁵⁸Co, about one GBq/t of ⁶⁰Co) and high chemical concentrations (several ppm of nickel...). This abrupt increase is due to the changes in the thermal and chemical conditions of the primary circuit: temperature and pH decrease, change from oxidizing to reducing conditions.

This significant quantity of corrosion products primarily comes from the dissolution of in-core deposits during primary circuit oxygenation, in particular, the dissolution of metal nickel or nickel oxide, involving the ⁵⁸Co formed from nickel. Thus, the total ⁵⁸Co activity released during oxygenation is equal to, or even higher than, the total activity deposited out-of-core. It is thus advisable that the cold shutdown procedure be adapted to avoid transferring this considerable activity from in-core surfaces to out-of-flux surfaces, particularly as the measurements of primary circuit surface activities in French PWRs have never shown a visible reduction in contamination during cold shutdown. On the contrary, recontamination can occur during temperature plateaux or oxygenations at a temperature higher than 80°C.

That is why:

* Primary circuit oxygenation at 80°C,

* Rapid temperature decrease and

* Improvements on primary fluid purification,

during cold shutdowns, have, up to this day, been the main points in the EDF strategy to reach the required dosimetric goals (ALARA), all the while optimizing unit outage schedules (availability).
activities ([H₂] < 3 cc/kg). According to [8], the dissolved hydrogen content should be higher than 10 cc/kg and the temperature should range between 120 and 150°C during at least 16 hours, in order to decompose nickel ferrites. However, the reaction kinetics is not known and a temperature plateau can lead to out-of-core surface recontamination (Figure 11). The hypothetical benefit is thus likely to be masked, all the more so as the acid-reducing conditions can also decompose the in-core nickel ferrites and thus recontaminate out-of-flux surfaces. In order to evaluate its impact, an acid-reducing chemistry will be applied and monitored during a French PWR shutdown this year.

In France, the current procedure for the cold shutdown prescribed by EDF does not aim at decontaminating the out-of-core surfaces, but, at, as soon as possible, reaching the criteria of the reactor cavity filling by avoiding recontaminating the primary circuit out-of-core surfaces. That is why:

* The primary coolant temperature is reduced as quickly as possible,

* The oxygenation temperature of the primary circuit is 80°C,

* The high flow rate purification is being evaluated so as to be applied to EDF PWRs. The corrosion products are effectively trapped on the CVCS filters and demineralizers, except for specific pollutions (antimony and silver) for which studies on the optimisation of the purification media are in progress.

Although penalizing, among other things, to availability, an unscheduled shutdown with oxygenation can have a beneficial effect on the refueling shutdown at the end of the cycle. Indeed, an unscheduled shutdown with oxygenation in the last six months of a cycle allows the ⁵⁸Co activity peak to be decreased during the refueling shutdown. As for purification, an unscheduled shutdown makes it possible to more quickly reach the criteria of reactor cavity filling or, for the same purification duration, to obtain activity values below the criteria.
This is why EDF recommends oxygenating the primary circuit if the unscheduled shutdown lasts more than 3 days. Moreover, it allows the reduction of out-of-core contamination, since part of the in-core corrosion product deposits is removed and trapped by CVCS filtration. Let us recall that it is advisable to purify the primary circuit sufficiently to avoid recontaminating the out-of-flux surfaces, because of the temperature and pH increase at the end of the unscheduled shutdown (EDF criteria: ⁵⁸Co < 7 GBq/t and γtotal < 14 GBq/t). These criteria are to be respected all the more so as, since the unscheduled shutdown occurs near the end of the cycle, the radionuclides have less time to decrease.

## CONCLUSION

Cold shutdown in French PWRs leads to considerable corrosion product increase in the primary coolant (2 to 3 orders of magnitude). This is highlighted by high activity peaks (about a hundred GBq/t of ⁵⁸Co, about one GBq/t of ⁶⁰Co) and high chemical concentrations (several ppm of nickel...). This abrupt increase is due to the changes in the thermal and chemical conditions of the primary circuit: temperature and pH decrease, change from oxidizing to reducing conditions.

This significant quantity of corrosion products primarily comes from the dissolution of in-core deposits during primary circuit oxygenation, in particular, the dissolution of metal nickel or nickel oxide, involving the ⁵⁸Co formed from nickel. Thus, the total ⁵⁸Co activity released during oxygenation is equal to, or even higher than, the total activity deposited out-of-core. It is thus advisable that the cold shutdown procedure be adapted to avoid transferring this considerable activity from in-core surfaces to out-of-flux surfaces, particularly as the measurements of primary circuit surface activities in French PWRs have never shown a visible reduction in contamination during cold shutdown. On the contrary, recontamination can occur during temperature plateaux or oxygenations at a temperature higher than 80°C.

That is why:

* Primary circuit oxygenation at 80°C,

* Rapid temperature decrease and

* Improvements on primary fluid purification,

during cold shutdowns, have, up to this day, been the main points in the EDF strategy to reach the required dosimetric goals (ALARA), all the while optimizing unit outage schedules (availability).
REFERENCES

[1] S. Anthoni, P. Ridoux, C. Caramel – *Behaviour of the corrosion products during PWR shutdowns*, Chemistry in Water Reactors: operating experience & new developments, Nice (France), 1994

[2] S. Anthoni, P. Ridoux, C. Chevalier – *Evaluation of corrosion products in a pressurised water reactor during shutdown*, Water Chemistry of Nuclear Reactor systems 5, BNES, London (GB), 1989

[3] D. You, E. Blanchard, D. Féron, C. Masson – *Corrosion product transfer in PWR primary circuits during cold shutdowns*, this conference

[4] F. Dacquait, S. Anthoni, P. Ridoux, C. Brun, N. Engler – *Impact of Steam Generator Replacement on PWR primary circuit contamination by radioactive corrosion products*, BNES, Bournemouth (UK), 2000

[5] J.L. Bretelle, A. Rocher, M. Berger, F. Dacquait, R. Rosset – *Study of various chemical species behaviour for contamination risk*, this conference

[6] M. Berger, J. Cohen, A. Rocher – *Policy and Good Practices in Electricité de France PWRs, for Primary Coolant Purification during Refuelling Cold Shutdown*, WANO Workshop: Low Level Waste and Liquid Separation Technology in Nuclear Applications, Varbergs (S), 2001.

[7] R. Eimecke, S. Anthoni – *Ensemble de Mesure et d'Etude de la Contamination des Circuits (EMECC)*, 7th Int. Conf. On Radiation Shielding, Bournemouth (UK), 1988

[8] PWR Primary Water Chemistry Guidelines: Volume 2, Revision 4, EPRI, Palo Alto, CA:1999. TR-105714-V2R4
REFERENCES

[1] S. Anthoni, P. Ridoux, C. Caramel – *Behaviour of the corrosion products during PWR shutdowns*, Chemistry in Water Reactors: operating experience & new developments, Nice (France), 1994

[2] S. Anthoni, P. Ridoux, C. Chevalier – *Evaluation of corrosion products in a pressurised water reactor during shutdown*, Water Chemistry of Nuclear Reactor systems 5, BNES, London (GB), 1989

[3] D. You, E. Blanchard, D. Féron, C. Masson – *Corrosion product transfer in PWR primary circuits during cold shutdowns*, this conference

[4] F. Dacquait, S. Anthoni, P. Ridoux, C. Brun, N. Engler – *Impact of Steam Generator Replacement on PWR primary circuit contamination by radioactive corrosion products*, BNES, Bournemouth (UK), 2000

[5] J.L. Bretelle, A. Rocher, M. Berger, F. Dacquait, R. Rosset – *Study of various chemical species behaviour for contamination risk*, this conference

[6] M. Berger, J. Cohen, A. Rocher – *Policy and Good Practices in Electricité de France PWRs, for Primary Coolant Purification during Refuelling Cold Shutdown*, WANO Workshop: Low Level Waste and Liquid Separation Technology in Nuclear Applications, Varbergs (S), 2001.

[7] R. Eimecke, S. Anthoni – *Ensemble de Mesure et d'Etude de la Contamination des Circuits (EMECC)*, 7th Int. Conf. On Radiation Shielding, Bournemouth (UK), 1988

[8] PWR Primary Water Chemistry Guidelines: Volume 2, Revision 4, EPRI, Palo Alto, CA:1999. TR-105714-V2R4