# EDF strategy for the primary coolant pH

Jean-Luc Bretelle, Luc Guinard, Caroline Andrieu, Frédéric Dacquait, Francis Nordmann

► To cite this version:

Jean-Luc Bretelle, Luc Guinard, Caroline Andrieu, Frédéric Dacquait, Francis Nordmann. EDF strategy for the primary coolant pH. International conference on water chemistry in nuclear reactors systems, Oct 2004, San Francisco, United States. pp.309-318. cea-04464340

HAL Id: cea-04464340

https://cea.hal.science/cea-04464340

Submitted on 18 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
# EDF strategy for the primary coolant pH

Jean-Luc Bretelle, Luc Guinard, Caroline Andrieu, Frédéric Dacquait, Francis Nordmann

► To cite this version:

Jean-Luc Bretelle, Luc Guinard, Caroline Andrieu, Frédéric Dacquait, Francis Nordmann. EDF strategy for the primary coolant pH. International conference on water chemistry in nuclear reactors systems, Oct 2004, San Francisco, United States. pp.309-318. cea-04464340

HAL Id: cea-04464340

https://cea.hal.science/cea-04464340v1

Submitted on 18 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
EDF strategy for the primary coolant pH

J.L. BRETELLE*, L. GUINARD**, C.ANDRIEU**, F. DACQUAIT***, F. NORDMANN*

*Electricité de France – Energy Branch - CEIDRE
2 rue Ampère – 93206 Saint-Denis – France

Phone : 33 1 43 69 80 10 – Fax : 33 1 43 69 82 93

** EDF – SEPTEN, Nuclear Engineering Division – Basic Design Department,
12-14 avenue Dutrievoz – 69628 Villeurbanne Cedex – France*** CEA – Nuclear Energy Division

Contact: Jean-Luc.bretelle@edf.fr

Abstract

EDF has experienced different boron-Lithium coordination curves into the primary coolant of nuclear power plants. The applied chemistries have led to the target pH of 7.0 and 7.2 at 300°C for the middle of the fuel cycles. But different strategies have been tested for the beginning and the end of cycle giving a larger range of pH.

Since the load following implementation for EDF plants, it is difficult to keep a constant pH and it is more reasonable to
consider that it moves around the target value.

This paper also draws the influence of the pH evolution on the primary-secondary leaks for the oldest plants with 600 MA
steam generator tubes. Actually, the demonstrated effect of the primary coolant effect on crack oxide solubility explains the
level of the weak and stabilised leaks. This leads to propose a specific B/Li coordination to mitigate operation difficulties.
This analysis may help chemists to choose the suitable pH for different plant families according to the various objectives.

I. INTRODUCTION

The search of the pH best compromise for the
primary coolant of PWR is a true challenge. As a matter
of fact several goals have to be achieved:

* to limit corrosion degradation of the primary system material,

* to decrease the mass transport and ensure its management,

* to avoid abnormal flux deviation,

* to limit the wall ex-core contamination but get the well adapted distribution leading to minimize the maintenance dose rate,

* to take part in the solid, liquid and gaseous waste decrease.

Moreover, different parameters reinforce the
difficulty to determine “the” optimum operation pH. It is
important to take into account shutdown transients¹ ² ³ ⁴.
Indeed, the power operation pH influences the quantity of
corrosion products to manage during the shutdown and to
neglect it can alter the pH investigation at power.

To try to take advantage of national or international
feedback is another difficulty to get the “right” pH,
considering the different designs and practices. In
addition, the load follow implemented by EDF units
complicates analysis of 58 units with four principal basic
designs, performing different cycle durations.

In 2002, It was recalled that in the past one critical
point was to select the best pH₃₀₀°C by defining Li-B
curves to be applied ⁵. Improvements were mentioned.
This paper examines different French unit results that are
used to consolidate EDF strategy or to validate the one
considered for the near future.

The examination of non-exhaustive tendencies
connected to the pH performed in the primary coolant is
made. They deal with:

* the pH variation during cycles,

* the dose rate and the out of core contamination,

* the AOA occurrence,

* the primary-to-secondary leaks.

II - EDF STRATEGY EVOLUTION

II.A. *The mains phenomena to manage*

The conditioning choice, specially for pH, is linked to
the corrosion phenomena, the release of corrosion
products and the mass transport. As a result of the
searched compromise, the lithium content is able to
modify the corrosion sensibility of materials. Therefore,
the best adapted pH to the primary coolant of PWRs for
dose rate limitation is not always the same as what would
have been selected for the corrosion of the various
materials⁶⁷⁸.
EDF strategy for the primary coolant pH

J.L. BRETELLE*, L. GUINARD**, C.ANDRIEU**, F. DACQUAIT***, F. NORDMANN*

*Electricité de France – Energy Branch - CEIDRE
2 rue Ampère – 93206 Saint-Denis – France

Phone : 33 1 43 69 80 10 – Fax : 33 1 43 69 82 93

** EDF – SEPTEN, Nuclear Engineering Division – Basic Design Department,
12-14 avenue Dutrievoz – 69628 Villeurbanne Cedex – France*** CEA – Nuclear Energy Division

Contact: Jean-Luc.bretelle@edf.fr

Abstract

EDF has experienced different boron-Lithium coordination curves into the primary coolant of nuclear power plants. The applied chemistries have led to the target pH of 7.0 and 7.2 at 300°C for the middle of the fuel cycles. But different strategies have been tested for the beginning and the end of cycle giving a larger range of pH.

Since the load following implementation for EDF plants, it is difficult to keep a constant pH and it is more reasonable to
consider that it moves around the target value.

This paper also draws the influence of the pH evolution on the primary-secondary leaks for the oldest plants with 600 MA
steam generator tubes. Actually, the demonstrated effect of the primary coolant effect on crack oxide solubility explains the
level of the weak and stabilised leaks. This leads to propose a specific B/Li coordination to mitigate operation difficulties.
This analysis may help chemists to choose the suitable pH for different plant families according to the various objectives.

I. INTRODUCTION

The search of the pH best compromise for the
primary coolant of PWR is a true challenge. As a matter
of fact several goals have to be achieved:

* to limit corrosion degradation of the primary system material,

* to decrease the mass transport and ensure its management,

* to avoid abnormal flux deviation,

* to limit the wall ex-core contamination but get the well adapted distribution leading to minimize the maintenance dose rate,

* to take part in the solid, liquid and gaseous waste decrease.

Moreover, different parameters reinforce the
difficulty to determine “the” optimum operation pH. It is
important to take into account shutdown transients¹ ² ³ ⁴.
Indeed, the power operation pH influences the quantity of
corrosion products to manage during the shutdown and to
neglect it can alter the pH investigation at power.

To try to take advantage of national or international
feedback is another difficulty to get the “right” pH,
considering the different designs and practices. In
addition, the load follow implemented by EDF units
complicates analysis of 58 units with four principal basic
designs, performing different cycle durations.

In 2002, It was recalled that in the past one critical
point was to select the best pH<sub>300°C</sub> by defining Li-B
curves to be applied <sup>5</sup>. Improvements were mentioned.
This paper examines different French unit results that are
used to consolidate EDF strategy or to validate the one
considered for the near future.

The examination of non-exhaustive tendencies
connected to the pH performed in the primary coolant is
made. They deal with:

* the pH variation during cycles,

* the dose rate and the out of core contamination,

* the AOA occurrence,

* the primary-to-secondary leaks.

II - EDF STRATEGY EVOLUTION

II.A. *The mains phenomena to manage*

The conditioning choice, specially for pH, is linked to
the corrosion phenomena, the release of corrosion
products and the mass transport. As a result of the
searched compromise, the lithium content is able to
modify the corrosion sensibility of materials. Therefore,
the best adapted pH to the primary coolant of PWRs for
dose rate limitation is not always the same as what would
have been selected for the corrosion of the various
materials⁶⁷⁸.
The nuclear additional stake is to better control the personal dose-rate. This is got by the limitation of activated corrosion products on circuits wall close to workers performing maintenance. These radioactive deposits come from dissolution-precipitation phenomenon governed by pH temperature and thermal gradients.

These gradients depend on the basic and core designs. Dissolution and precipitation are continued due to the dynamic of the thermodynamic equilibrium. Their importance varies with kinetic phenomena. The different pH strategies for the coolant of the nuclear power plants results of the hope to limit the out of core radioactive wall deposits.

The target pH has to place the dissolution regime where it is increasing with temperature in the core area. At the same time this pH must impose the global solubility at the lower possible level. This leads to limit the life duration under flux of the corrosion products and their activation. Here it specially concerns the nickel, the chromium, the iron and their oxides as spinel and mixed forms.

## II.B. *The past EDF evolution strategy*

To get its own feedback in the case of the French design, EDF decided to implement a 7.2 pH₃₀₀°C for six

units on two plants with a coordinated boron-lithium until the end of cycles in 1987.

The pH have been admitted as a lever to decrease dose-rate, but in the meantime, the international community was convinced that the pH had to be settled between 6.9 and 7.4 with no real agreement on the target pH ⁹.

Indeed, the different code calculations, taking into account different transition metal oxides included spinel forms with bad-known behaviour at temperature (specially ferrite, we today know that chromite and nickel oxide are also important), diverge on the best pH to be implemented. The pHs are then announced within 7.3 and 7.4 but with different reference temperatures, most commonly at the average which are different according the units and at 300°C for the French plants. Actually, it is generally agreed that increasing the pH could lead to ensure the decrease of the release rates and the source term reduction of in-core deposits which can lead to improve the out-of-core dose-rate. Then, pH₃₀₀°C nearly 7.2 was announced like the next future target pH to reach. However, it was hard to compare with sufficient reliability the feedback of different companies. Operation practices, material modifications (Zircaloy grids) and the maintenance evolution could influence the diagnostic

This explains the rationale for EDF own experimentation.

<figure><img src="image_2.png" /><figcaption>Figure 1. Li and pH evolutions versus boron for the two different target pH₃₀₀°C of 7.0 and 7.2</figcaption></figure>
The nuclear additional stake is to better control the personal dose-rate. This is got by the limitation of activated corrosion products on circuits wall close to workers performing maintenance. These radioactive deposits come from dissolution-precipitation phenomenon governed by pH temperature and thermal gradients.

These gradients depend on the basic and core designs. Dissolution and precipitation are continued due to the dynamic of the thermodynamic equilibrium. Their importance varies with kinetic phenomena. The different pH strategies for the coolant of the nuclear power plants results of the hope to limit the out of core radioactive wall deposits.

The target pH has to place the dissolution regime where it is increasing with temperature in the core area. At the same time this pH must impose the global solubility at the lower possible level. This leads to limit the life duration under flux of the corrosion products and their activation. Here it specially concerns the nickel, the chromium, the iron and their oxides as spinel and mixed forms.

## II.B. *The past EDF evolution strategy*

To get its own feedback in the case of the French design, EDF decided to implement a 7.2 pH₃₀₀°C for six

units on two plants with a coordinated boron-lithium until the end of cycles in 1987.

The pH have been admitted as a lever to decrease dose-rate, but in the meantime, the international community was convinced that the pH had to be settled between 6.9 and 7.4 with no real agreement on the target pH ⁹.

Indeed, the different code calculations, taking into account different transition metal oxides included spinel forms with bad-known behaviour at temperature (specially ferrite, we today know that chromite and nickel oxide are also important), diverge on the best pH to be implemented. The pHs are then announced within 7.3 and 7.4 but with different reference temperatures, most commonly at the average which are different according the units and at 300°C for the French plants. Actually, it is generally agreed that increasing the pH could lead to ensure the decrease of the release rates and the source term reduction of in-core deposits which can lead to improve the out-of-core dose-rate. Then, pH₃₀₀°C nearly 7.2 was announced like the next future target pH to reach. However, it was hard to compare with sufficient reliability the feedback of different companies. Operation practices, material modifications (Zircaloy grids) and the maintenance evolution could influence the diagnostic

This explains the rationale for EDF own experimentation.

<figure><img src="image_2.png" /><figcaption>Figure 1. Li and pH evolutions versus boron for the two different target pH₃₀₀°C of 7.0 and 7.2</figcaption></figure>
<table><caption>Table 1. pH Evolution in EDF PWR units</caption><thead><tr><td>Year/period</td><td>pH - Li</td><td>Goal</td></tr></thead><tbody><tr><td>1977</td><td>0.2&lt;Li&lt;2.2</td><td>1</td></tr><tr><td>1978 - 80</td><td>pH &gt;6.9<br>0.6&lt;Li&lt;2.2</td><td>2</td></tr><tr><td>1980 - 97</td><td>pH 7.0<br>0.6&lt;Li&lt;2.2</td><td>3</td></tr><tr><td>Since 1997</td><td>pH 7.2 –<br>0.4&lt;Li&lt;2.2</td><td>4</td></tr></tbody></table>

1. pH₃₀₀°C sufficient to warrant the generalized corrosion and to limit the fuel cladding corrosion

2. Dose rate consideration – To avoid fuel deposits

3. Decreasing boron-lithium coordination with lithium constant at the end of the cycle to avoid low pH at the end of cycle and easier operation conditions (figure 1)¹⁰ ¹¹ ¹²

4. According to IAEA recommendation, code calculations, past experience with pH of 7.0 compared to 7.2, the international experience and, EDF decided to select the pH₃₀₀°C of 7.2 instead of pH₃₀₀°C of 7.0 as the target pH (Zone 1, figure 11)¹³ ¹⁴. The previous target band of lithium did not take into account minimisation of the dose rates. The new target of lithium value ± 0.1 mg/kg is associated with the reduction of dose-rates to help the operators to keep as more a stable pH as they can do (figure 1).

Effects on dose rate indicators of 100 cycles at pH₃₀₀°C 7.0, compared to 20 cycles at pH₃₀₀°C 7.2, did not induce significant benefits and at least less than the code calculation had estimated it. Meanwhile, no negative effects were detected. However, the PACTOLE code forecasted a potential positive influence of pH₃₀₀°C 7.2 with a limit of 7.4 to get effective positive effects, requiring a constant baseline¹⁵. MIT test loops gave also positive effects on dose-rate¹⁶.

Therefore, EDF began to implement pH₃₀₀°C 7.2 in 1997. This period corresponds exactly to the beginning of the long fuel cycles conversion performing also load follow for the 1300 MWe Plants. That leads EDF to examine a few parameters to assess the real effects of the new target pH.

### III - ASSESSMENT OF THE EDF ACTUAL STRATEGY

The will to keep a constant pH can seem illusive for EDF plants due to the load follow. Nevertheless, operators are asked to implement a more stringent coordinated chemistry considering the target pH band. Moreover, the extended cycles require higher boron contents at the beginning of cycles.

This is an additional reason to assess the new strategy compared to the previous one and the indicator evolutions like dose rate index, deposited activities, AOA occurrence.

#### III.A. *Impacts of load follow and Li deviation around the optimal target pH 7.2*

To evaluate the load follow impact, two simple indicators can be analysed. The first is the dose rate index and the second is the ⁵⁸Co oxygenation peak which roughly represents the Ni deposited mass on fuel cladding t during the cycle.

To simulate the pH variation EDF have elaborated a primary chemistry index. This index is the integrated area drawing by the pH, resulting of lithium evolution during the cycle. The reference index for a constant pH₃₀₀°C of 7.2 gives a value of zero.

The analysis deals with cycles implementing a pH₃₀₀°C of 7.0 and includes about 550 reactors cycles for the dose rate index and 320 for cobalt peaks just before the new coordinated pH 7.2 implementation. There has been a double interest, firstly to assess the correlation of dose rate with the operating pH and secondly to assess the need of the more stringent pH target band for pH₃₀₀°C, 7.2.

Figures 2 shows that no direct correlation exists between the dose rate index and the chemistry index. This is equivalent for the ⁵⁸Co peak and the chemistry index.

For the figure 2, the higher dose rate > 100 (= 1mSv/h) with a chemistry index in the average values represents the major part of the oldest plants (CP0). This shows that the improvement in more strictly following the coordinated chemistry does not lower the dose rate. About 75 % of the higher index (>10) concern the first and second cycles that means that the worst follow of the B-Li coordination does not lead necessary to the higher dose rate.

Those results confirm that the difficulties to keep constant the pH in the previous band of the coordinated chemistry were not the only parameter explaining the differences for dose rate and corrosion products release. The question could be also: is it reasonable and necessary to contract too much the target band of pH for the operators with load follow ?

#### III.B. *Dose rate evolution*

To carry out the impact of the change of pH, investigations on the evolution of dose rate are examined separately for 900 MWe and 1300 MWe taking into account the last two cycles with pH 7.0 and all the available results for the pH 7.2 of each plants studied.
<table><caption>Table 1. pH Evolution in EDF PWR units</caption><thead><tr><td>Year/period</td><td>pH - Li</td><td>Goal</td></tr></thead><tbody><tr><td>1977</td><td>0.2&lt;Li&lt;2.2</td><td>1</td></tr><tr><td>1978 - 80</td><td>pH &gt;6.9<br>0.6&lt;Li&lt;2.2</td><td>2</td></tr><tr><td>1980 - 97</td><td>pH 7.0<br>0.6&lt;Li&lt;2.2</td><td>3</td></tr><tr><td>Since 1997</td><td>pH 7.2 –<br>0.4&lt;Li&lt;2.2</td><td>4</td></tr></tbody></table>

1. pH₃₀₀°C sufficient to warrant the generalized corrosion and to limit the fuel cladding corrosion

2. Dose rate consideration – To avoid fuel deposits

3. Decreasing boron-lithium coordination with lithium constant at the end of the cycle to avoid low pH at the end of cycle and easier operation conditions (figure 1)¹⁰ ¹¹ ¹²

4. According to IAEA recommendation, code calculations, past experience with pH of 7.0 compared to 7.2, the international experience and, EDF decided to select the pH₃₀₀°C of 7.2 instead of pH₃₀₀°C of 7.0 as the target pH (Zone 1, figure 11)¹³ ¹⁴. The previous target band of lithium did not take into account minimisation of the dose rates. The new target of lithium value ± 0.1 mg/kg is associated with the reduction of dose-rates to help the operators to keep as more a stable pH as they can do (figure 1).

Effects on dose rate indicators of 100 cycles at pH₃₀₀°C 7.0, compared to 20 cycles at pH₃₀₀°C 7.2, did not induce significant benefits and at least less than the code calculation had estimated it. Meanwhile, no negative effects were detected. However, the PACTOLE code forecasted a potential positive influence of pH₃₀₀°C 7.2 with a limit of 7.4 to get effective positive effects, requiring a constant baseline¹⁵. MIT test loops gave also positive effects on dose-rate¹⁶.

Therefore, EDF began to implement pH₃₀₀°C 7.2 in 1997. This period corresponds exactly to the beginning of the long fuel cycles conversion performing also load follow for the 1300 MWe Plants. That leads EDF to examine a few parameters to assess the real effects of the new target pH.

### III - ASSESSMENT OF THE EDF ACTUAL STRATEGY

The will to keep a constant pH can seem illusive for EDF plants due to the load follow. Nevertheless, operators are asked to implement a more stringent coordinated chemistry considering the target pH band. Moreover, the extended cycles require higher boron contents at the beginning of cycles.

This is an additional reason to assess the new strategy compared to the previous one and the indicator evolutions like dose rate index, deposited activities, AOA occurrence.

#### III.A. *Impacts of load follow and Li deviation around the optimal target pH 7.2*

To evaluate the load follow impact, two simple indicators can be analysed. The first is the dose rate index and the second is the ⁵⁸Co oxygenation peak which roughly represents the Ni deposited mass on fuel cladding t during the cycle.

To simulate the pH variation EDF have elaborated a primary chemistry index. This index is the integrated area drawing by the pH, resulting of lithium evolution during the cycle. The reference index for a constant pH₃₀₀°C of 7.2 gives a value of zero.

The analysis deals with cycles implementing a pH₃₀₀°C of 7.0 and includes about 550 reactors cycles for the dose rate index and 320 for cobalt peaks just before the new coordinated pH 7.2 implementation. There has been a double interest, firstly to assess the correlation of dose rate with the operating pH and secondly to assess the need of the more stringent pH target band for pH₃₀₀°C, 7.2.

Figures 2 shows that no direct correlation exists between the dose rate index and the chemistry index. This is equivalent for the ⁵⁸Co peak and the chemistry index.

For the figure 2, the higher dose rate > 100 (= 1mSv/h) with a chemistry index in the average values represents the major part of the oldest plants (CP0). This shows that the improvement in more strictly following the coordinated chemistry does not lower the dose rate. About 75 % of the higher index (>10) concern the first and second cycles that means that the worst follow of the B-Li coordination does not lead necessary to the higher dose rate.

Those results confirm that the difficulties to keep constant the pH in the previous band of the coordinated chemistry were not the only parameter explaining the differences for dose rate and corrosion products release. The question could be also: is it reasonable and necessary to contract too much the target band of pH for the operators with load follow ?

#### III.B. *Dose rate evolution*

To carry out the impact of the change of pH, investigations on the evolution of dose rate are examined separately for 900 MWe and 1300 MWe taking into account the last two cycles with pH 7.0 and all the available results for the pH 7.2 of each plants studied.
Search of linear correlation - All units - ALL WHOLE CYCLES

<figure><img src="image_3.png" /><figcaption>Figure 2 : Chemistry index evolution versus dose rates in all the EDF units</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 3 : Insignificant dose rate evolution on 1300 MWe Plants after the switch of pH 300°C from 7.0 to 7.2</figcaption></figure>
Search of linear correlation - All units - ALL WHOLE CYCLES

<figure><img src="image_3.png" /><figcaption>Figure 2 : Chemistry index evolution versus dose rates in all the EDF units</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 3 : Insignificant dose rate evolution on 1300 MWe Plants after the switch of pH 300°C from 7.0 to 7.2</figcaption></figure>
Figure 3 shows that for the 1300 MWe, no real detectable effect of the evolution of the pH has been noted on the dose rate index. For the 900 Mwe plants, the conclusions are equivalent. However the dose rate index decreases with the improvement of standardized plant series.

<figure><img src="image_5.png" /><figcaption>Figure 4: Dose rate evolution of units with 690 SGR or original 690 SG since the implementation of pH 7.2</figcaption></figure>

This is relevant with CEA results Moreover, the index values tend to decrease with cycles specially for $CP0^{17}$. These conclusions explain the difficulty to discriminate the effect of $pH_{300°C}$ 7.2 in the general positive evolution of dose rate.

<figure><img src="image_6.png" /><figcaption>Figure 5. $^{58}$Co oxygenation peak evolution of units with 690 SGR (GR1-SLB1) or original Alloy 690 SG (CIV1-CHO2) since the implementation of pH 7.2.</figcaption></figure>

Figures 4 and 5 confirm the previous statement related to the new steam generators (SG) with Alloy 690 tubing either replaced or original SGs.

For these SG, we observe a decrease of the Co 58 at the oxygenation peak, and it does not seem that this result is linked to the pH switch respectively 4 cycles after the SGR for GR1 and 3 for SLB1. The pH effect is probably hidden by SGR; it is rather due to the specific behaviour of the SG release less and less important which is equivalent for SGR or SG of the newest plant.

A dose rate evolution is not always relevant with the corresponding Co 58 variations and with the change of pH for GR1 and SLB1.

In these cases, we can conclude that it is not easy to assess the role of the higher pH 7.2 towards to the others parameters, even if obviously there is no sign of any detrimental effect of this pH of 7.2.

### III.C. Deposited activity results

Aware of the difficulty to only use the dose rate which does not give not enough information on the origin and the repartition of radionuclides, others investigations are carried out with deposit activities performed by the CEA.

Almost 200 measurements campaign are analysed, regarding different parts of the plant: primary pipes and steam generator tube bundles.

As shown on figure 6, on the 900 MWe Plants (CP0,CP1,CP2), a target $pH_{300°C}$ of 7.2 (open symbols) seems to lead to a more important deposition of cobalt (for $^{58}$Co as well as $^{60}$Co) on the legs compared to a $pH_{300°C}$ 7.0 (full symbols). The SG deposits have an opposite tendency, showing a small advantage of pH 7.2 (figure 7).

On 1300 and N4 Plants, $pH_{300°C}$ 7.2 favours a less important deposition on the legs (Figure 8). The results are less clear on SG (Figure 9). Actually, the general average trends for each type of French plants (900 Mwe, 1300 Mwe, 1450 Mwe) clearly shows that the dose rates are globally decreasing $^{23}$.

For the steam generator deposit Co 58 activities, for the oldest plant, the average trend is not obvious but seems to be decreasing for the last few cycles (Figure 14).

These results emphasize that it is difficult to globally conclude on the pH effects. No real improvement appears and we can think that the pH is participating to the slow evolution specially if we pay attention that along the cycles the pH is not at the optimum value due to the increase of boron with the new fuel management and the limitation of the lithium. This point is treated later in this paper.

Meanwhile, others points are connecting with the pH, some concerning an improvement with the pH increase, that is normally true for the AOA. Others are concerning detrimental effects relatively to the operation conditions, that is the case for primary-secondary leaks.
Figure 3 shows that for the 1300 MWe, no real detectable effect of the evolution of the pH has been noted on the dose rate index. For the 900 Mwe plants, the conclusions are equivalent. However the dose rate index decreases with the improvement of standardized plant series.

<figure><img src="image_5.png" /><figcaption>Figure 4: Dose rate evolution of units with 690 SGR or original 690 SG since the implementation of pH 7.2</figcaption></figure>

This is relevant with CEA results Moreover, the index values tend to decrease with cycles specially for $CP0^{17}$. These conclusions explain the difficulty to discriminate the effect of $pH_{300°C}$ 7.2 in the general positive evolution of dose rate.

<figure><img src="image_6.png" /><figcaption>Figure 5. $^{58}$Co oxygenation peak evolution of units with 690 SGR (GR1-SLB1) or original Alloy 690 SG (CIV1-CHO2) since the implementation of pH 7.2.</figcaption></figure>

Figures 4 and 5 confirm the previous statement related to the new steam generators (SG) with Alloy 690 tubing either replaced or original SGs.

For these SG, we observe a decrease of the Co 58 at the oxygenation peak, and it does not seem that this result is linked to the pH switch respectively 4 cycles after the SGR for GR1 and 3 for SLB1. The pH effect is probably hidden by SGR; it is rather due to the specific behaviour of the SG release less and less important which is equivalent for SGR or SG of the newest plant.

A dose rate evolution is not always relevant with the corresponding Co 58 variations and with the change of pH for GR1 and SLB1.

In these cases, we can conclude that it is not easy to assess the role of the higher pH 7.2 towards to the others parameters, even if obviously there is no sign of any detrimental effect of this pH of 7.2.

### III.C. Deposited activity results

Aware of the difficulty to only use the dose rate which does not give not enough information on the origin and the repartition of radionuclides, others investigations are carried out with deposit activities performed by the CEA.

Almost 200 measurements campaign are analysed, regarding different parts of the plant: primary pipes and steam generator tube bundles.

As shown on figure 6, on the 900 MWe Plants (CP0,CP1,CP2), a target $pH_{300°C}$ of 7.2 (open symbols) seems to lead to a more important deposition of cobalt (for $^{58}$Co as well as $^{60}$Co) on the legs compared to a $pH_{300°C}$ 7.0 (full symbols). The SG deposits have an opposite tendency, showing a small advantage of pH 7.2 (figure 7).

On 1300 and N4 Plants, $pH_{300°C}$ 7.2 favours a less important deposition on the legs (Figure 8). The results are less clear on SG (Figure 9). Actually, the general average trends for each type of French plants (900 Mwe, 1300 Mwe, 1450 Mwe) clearly shows that the dose rates are globally decreasing $^{23}$.

For the steam generator deposit Co 58 activities, for the oldest plant, the average trend is not obvious but seems to be decreasing for the last few cycles (Figure 14).

These results emphasize that it is difficult to globally conclude on the pH effects. No real improvement appears and we can think that the pH is participating to the slow evolution specially if we pay attention that along the cycles the pH is not at the optimum value due to the increase of boron with the new fuel management and the limitation of the lithium. This point is treated later in this paper.

Meanwhile, others points are connecting with the pH, some concerning an improvement with the pH increase, that is normally true for the AOA. Others are concerning detrimental effects relatively to the operation conditions, that is the case for primary-secondary leaks.
<figure><img src="image_7.png" /><figcaption>Figures 6. Deposit activities on 900 MWe hot legs for various cycles with 2 different target pHs.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 7. Deposit activities on 900 MWe SG for various cycles with 2 different target pHs.</figcaption></figure>

## IV- EFFECT OF THE PH INCREASE

### IV.A. The role on AOA

The AOA route cause is not completely understood yet. The flux deviation are presumed to occur under sub-cooled nucleate conditions in addition to boron and corrosion products concentration orprecitation. In particular, the corrosion products depends on pH. Since 1988, about twenty abnormal flux deviation occurred on French plants ¹⁸. But since 1998, corresponding to the pH evolution on the French plants, the phenomenon only have concerned the 1300Mwe and the most of time begun early in the cycle.

<figure><img src="image_9.png" /><figcaption>Figure 8. Deposit activities on 1300 MWe hot legs for various cycles with 2 different target pHs.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 9. Deposit activities on 1300 MWe SG for various cycles with 2 different target pHs.</figcaption></figure>

At the same time, it is generally agreed on the need to keep pH 300°C over 6.9¹⁹ ²⁰ to limit the risk of AOA. One of the key chemistry points regarding this flux deviation is to ensure a sufficient pH at the beginning of the cycle.

For the extended fuel cycle of eighteen months, the boron content during start up favours a low pH, specially when it corresponds to the beginning of the zone 1 of French coordinated chemistry with a constant lithium of 2.2 mg/kg (Figure 19). Indeed, the switch of pH 300°C 7.0 to 7.2 does not modify the pH at the beginning of the cycle and that is one of the reason of the French “DUO” experimentation ²¹ (figure 20). The objective is to allow a pH of 7.0 then of 7.2 with a maximum concentration of 3.5 mg/kg of lithium at the beginning of the cycle in order to avoid flux anomaly due to a large amount of corrosion products.
<figure><img src="image_7.png" /><figcaption>Figures 6. Deposit activities on 900 MWe hot legs for various cycles with 2 different target pHs.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 7. Deposit activities on 900 MWe SG for various cycles with 2 different target pHs.</figcaption></figure>

## IV- EFFECT OF THE PH INCREASE

### IV.A. The role on AOA

The AOA route cause is not completely understood yet. The flux deviation are presumed to occur under sub-cooled nucleate conditions in addition to boron and corrosion products concentration orprecitation. In particular, the corrosion products depends on pH. Since 1988, about twenty abnormal flux deviation occurred on French plants ¹⁸. But since 1998, corresponding to the pH evolution on the French plants, the phenomenon only have concerned the 1300Mwe and the most of time begun early in the cycle.

<figure><img src="image_9.png" /><figcaption>Figure 8. Deposit activities on 1300 MWe hot legs for various cycles with 2 different target pHs.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 9. Deposit activities on 1300 MWe SG for various cycles with 2 different target pHs.</figcaption></figure>

At the same time, it is generally agreed on the need to keep pH 300°C over 6.9 ¹⁹ ²⁰ to limit the risk of AOA. One of the key chemistry points regarding this flux deviation is to ensure a sufficient pH at the beginning of the cycle.

For the extended fuel cycle of eighteen months, the boron content during start up favours a low pH, specially when it corresponds to the beginning of the zone 1 of French coordinated chemistry with a constant lithium of 2.2 mg/kg (Figure 19). Indeed, the switch of pH 300°C 7.0 to 7.2 does not modify the pH at the beginning of the cycle and that is one of the reason of the French “DUO” experimentation ²¹ (figure 20). The objective is to allow a pH of 7.0 then of 7.2 with a maximum concentration of 3.5 mg/kg of lithium at the beginning of the cycle in order to avoid flux anomaly due to a large amount of corrosion products.
Recently this French “modified chemistry” was not enough in Cattenom 2 to avoid a flux deviation. No reliable conclusion can be reached concerning the pH but it must be noted that no abnormal corrosion products has been reported before the anomaly detection. It can be thought that the flux deviation would have been more important without the modified chemistry.

### IV.B. *The role on primary-secondary leaks*

At the end of nineties, the physico-chemistry role for the primary-secondary leaks evolution, during the cycle for the old plants with 600MA steam generators tubes, has been very well understood. It deals in particular with the chemistry at crack inlet and specially with the pH. The governing phenomenon is linked to the oxide forms, of the primary coolant, transported to the cracks, which are able to precipitate or solubilize themselves in the cracks. The plugging or unplugging makes the cracks artificially varied and then the leak flow rate is modified. This phenomenon depends on the morphology of the cracks. These phenomena explain the well known behaviour of the leaks at the beginning or at the end of the cycle. Moreover, they sign the different values of the stabilized leaks during the cycles ²².

Thus, plants with 600 MA SG have seen an increase of their stabilized primary-secondary leaks after the implementation of the new coordinated chemistry (figure 10). These leaks can sometimes increase over the 10 l/h limit of steam generators for the more sensitive family. That can involve unavailability of the units.

The figure 10 also shows that the new coordinated pH 7.2 chemistry at the end of the cycle induces lower leaks than previously. This is due to the lower and more favourable pH at the end of the cycle compared to the coordinated chemistry pH 7.0 where the constant lithium at the end of the cycle led to a pH between 7.3 and 7.4 at 300°C.

Considering the advantages and the drawbacks of the new coordinated chemistry, pH 7.2, implemented in the French plants with primary-secondary leaks, EDF decided to relax the pH to avoid operators' difficulties. When the leak flow rate is stabilized or moves weakly with a simultaneous value of all the SG, it is possible to perform the new “Zone 1 bis”, keeping the pH within 6.9 and 7.2. It gives the possibility to maintain the leaks at a lower level without any risks for the safety (figure 11). This easy way to practise gives EDF the opportunity to increase plant availability and operation easiness without consequences on safety and dose rate.

### TRI 4 - pH Modicification in April 1998 (cycle 16)

<figure><img src="image_11.png" /><figcaption>Figure 10 : Leak flow rate evolutions after the pH switch.</figcaption></figure>
Recently this French “modified chemistry” was not enough in Cattenom 2 to avoid a flux deviation. No reliable conclusion can be reached concerning the pH but it must be noted that no abnormal corrosion products has been reported before the anomaly detection. It can be thought that the flux deviation would have been more important without the modified chemistry.

### IV.B. *The role on primary-secondary leaks*

At the end of nineties, the physico-chemistry role for the primary-secondary leaks evolution, during the cycle for the old plants with 600MA steam generators tubes, has been very well understood. It deals in particular with the chemistry at crack inlet and specially with the pH. The governing phenomenon is linked to the oxide forms, of the primary coolant, transported to the cracks, which are able to precipitate or solubilize themselves in the cracks. The plugging or unplugging makes the cracks artificially varied and then the leak flow rate is modified. This phenomenon depends on the morphology of the cracks. These phenomena explain the well known behaviour of the leaks at the beginning or at the end of the cycle. Moreover, they sign the different values of the stabilized leaks during the cycles ²².

Thus, plants with 600 MA SG have seen an increase of their stabilized primary-secondary leaks after the implementation of the new coordinated chemistry (figure 10). These leaks can sometimes increase over the 10 l/h limit of steam generators for the more sensitive family. That can involve unavailability of the units.

The figure 10 also shows that the new coordinated pH 7.2 chemistry at the end of the cycle induces lower leaks than previously. This is due to the lower and more favourable pH at the end of the cycle compared to the coordinated chemistry pH 7.0 where the constant lithium at the end of the cycle led to a pH between 7.3 and 7.4 at 300°C.

Considering the advantages and the drawbacks of the new coordinated chemistry, pH 7.2, implemented in the French plants with primary-secondary leaks, EDF decided to relax the pH to avoid operators' difficulties. When the leak flow rate is stabilized or moves weakly with a simultaneous value of all the SG, it is possible to perform the new “Zone 1 bis”, keeping the pH within 6.9 and 7.2. It gives the possibility to maintain the leaks at a lower level without any risks for the safety (figure 11). This easy way to practise gives EDF the opportunity to increase plant availability and operation easiness without consequences on safety and dose rate.

### TRI 4 - pH Modicification in April 1998 (cycle 16)

<figure><img src="image_11.png" /><figcaption>Figure 10 : Leak flow rate evolutions after the pH switch.</figcaption></figure>
Coordination BORE - LITHIUM

<figure><img src="image_12.png" /><figcaption>Figure 11 : EDF Guidelines for the coordinated B-Li Chemistry</figcaption></figure>

Knowing the ability of tritium to diffuse on the secondary coolant, its concentration has been also examined. A higher concentration of tritium can be observed in the secondary coolant corresponding to the higher level of leak flow rate. Obviously, there is not always a correlation between primary-to-secondary leak flow rate and tritium. It also depends on the unit operating mode.

Thus, the boron-lithium coordination pH 300°C 7.2 will mainly have detrimental effects on the primary-to-secondary leak flow rate and on associated unavailability and environmental impact.

V - DISCUSSIONS

Despite the evolution of EDF strategy, it is difficult to discriminate the real contribution of pH 7.2 from the others factors having also improved the general behaviour

of the French plants concerning dose rate. It should be also noted that the target pH band restriction does not seem a key point to ensure the effect of pH.

The pH range above and below the target pH can be adapted to reasonable operating requirements. However, many international data indicate that trying to achieve a pH 300°C of 7.2 is very close to the better solution.

We need a more reliable and precise code expectation taking into account the shutdown and all the new data concerning materials and oxides behaviour to define the optimum pH according to the encountered operation conditions. The improvements on the French code are in progress23.

The figure 1 shows that the optimum pH can only be followed during a short period. The target pH needs also . to be implemented as long as possible, especially at the beginning of the fuel cycles. The high duty cores and extended cycles lead to penalize the beginning of the cycles. Two major ways are possible to solve the problem.
Coordination BORE - LITHIUM

<figure><img src="image_12.png" /><figcaption>Figure 11 : EDF Guidelines for the coordinated B-Li Chemistry</figcaption></figure>

Knowing the ability of tritium to diffuse on the secondary coolant, its concentration has been also examined. A higher concentration of tritium can be observed in the secondary coolant corresponding to the higher level of leak flow rate. Obviously, there is not always a correlation between primary-to-secondary leak flow rate and tritium. It also depends on the unit operating mode.

Thus, the boron-lithium coordination pH 300°C 7.2 will mainly have detrimental effects on the primary-to-secondary leak flow rate and on associated unavailability and environmental impact.

V - DISCUSSIONS

Despite the evolution of EDF strategy, it is difficult to discriminate the real contribution of pH 7.2 from the others factors having also improved the general behaviour

of the French plants concerning dose rate. It should be also noted that the target pH band restriction does not seem a key point to ensure the effect of pH.

The pH range above and below the target pH can be adapted to reasonable operating requirements. However, many international data indicate that trying to achieve a pH 300°C of 7.2 is very close to the better solution.

We need a more reliable and precise code expectation taking into account the shutdown and all the new data concerning materials and oxides behaviour to define the optimum pH according to the encountered operation conditions. The improvements on the French code are in progress²³.

The figure 1 shows that the optimum pH can only be followed during a short period. The target pH needs also . to be implemented as long as possible, especially at the beginning of the fuel cycles. The high duty cores and extended cycles lead to penalize the beginning of the cycles. Two major ways are possible to solve the problem.
<figure><img src="image_13.png" /><figcaption>Figure 12 : French “Modified chemistry” (lower curve) and “High lithium” chemistry (upper curve)</figcaption></figure>

The first one is to increase lithium at the beginning of the cycle with a modified chemistry. The French “modified chemistry” choice is able to avoid a pH leading to a pH considered detrimental for the release of corrosion products. This modified chemistry should be soon implemented on all the 1300Mwe since no detrimental effects is expected on PWSCC with this coordinated chemistry reported on the lower curve of figure 12.

EDF is also evaluating the feasibility of experimenting “a high lithium chemistry” in order to reach as quickly as possible the real optimum pH 300°C 7.2 reported on the upper curve of figure 12. Such a chemistry would require a constant lithium concentration of 3.5 mg/kg during a few months. The international feedback indicates that a high and constant pH can be of some value for the term source control²⁴. Before applying such a chemistry, the lithium influence on corrosion of the various materials in presence must be carefully evaluated.

Indeed, for the fuel cladding, the studies show that no detrimental effect is expected, especially with Zirlo or M5. Nevertheless, for nickel base alloy (such as SG tubes or other components), the study is under progress. For the stainless steel, the situation is similar.

The second way to remedy the low pH at the beginning of the cycles is to decrease the overall boron concentration and therefore to use enriched boron 10.

The too expensive conversion for the French plants induced the EDF board not to implement it. However, now supplementary parameters perhaps would be worth to reconsider the situation.

Effectively, the use of EBA could led quickly to the target pH and solve several problems:

* the long term risk of PWSCC by decreasing the lithium content,

* the AOA occurrence for the high duty core,

* the release of wastes with boron in the new European context.

## VI - CONCLUSION

In the last 2000 Bournemouth BNES conference, it was underlined that different options in matter of primary chemistry were proposed ²⁵. Now, most of the PWR adapted chemistry for the primary coolant becomes closer and takes into account different parameters like dose rate, AOA, primary-to-secondary leaks, environmental consideration with boron, activity, operating contingencies and economical consideration.

Considering the French fleet with

* 58 units,

* four basic designs,

* performing extended long cycles or not with the very specific French load follow,

* three SG tubing (Alloy 600 MA, 600 TT, 690),

it is illusive to look for the standardized chemistry which will be able to show the same favourable results on all the plants.
<figure><img src="image_13.png" /><figcaption>Figure 12 : French “Modified chemistry” (lower curve) and “High lithium” chemistry (upper curve)</figcaption></figure>

The first one is to increase lithium at the beginning of the cycle with a modified chemistry. The French “modified chemistry” choice is able to avoid a pH leading to a pH considered detrimental for the release of corrosion products. This modified chemistry should be soon implemented on all the 1300Mwe since no detrimental effects is expected on PWSCC with this coordinated chemistry reported on the lower curve of figure 12.

EDF is also evaluating the feasibility of experimenting “a high lithium chemistry” in order to reach as quickly as possible the real optimum pH 300°C 7.2 reported on the upper curve of figure 12. Such a chemistry would require a constant lithium concentration of 3.5 mg/kg during a few months. The international feedback indicates that a high and constant pH can be of some value for the term source control²⁴. Before applying such a chemistry, the lithium influence on corrosion of the various materials in presence must be carefully evaluated.

Indeed, for the fuel cladding, the studies show that no detrimental effect is expected, especially with Zirlo or M5. Nevertheless, for nickel base alloy (such as SG tubes or other components), the study is under progress. For the stainless steel, the situation is similar.

The second way to remedy the low pH at the beginning of the cycles is to decrease the overall boron concentration and therefore to use enriched boron 10.

The too expensive conversion for the French plants induced the EDF board not to implement it. However, now supplementary parameters perhaps would be worth to reconsider the situation.

Effectively, the use of EBA could led quickly to the target pH and solve several problems:

* the long term risk of PWSCC by decreasing the lithium content,

* the AOA occurrence for the high duty core,

* the release of wastes with boron in the new European context.

## VI - CONCLUSION

In the last 2000 Bournemouth BNES conference, it was underlined that different options in matter of primary chemistry were proposed ²⁵. Now, most of the PWR adapted chemistry for the primary coolant becomes closer and takes into account different parameters like dose rate, AOA, primary-to-secondary leaks, environmental consideration with boron, activity, operating contingencies and economical consideration.

Considering the French fleet with

* 58 units,

* four basic designs,

* performing extended long cycles or not with the very specific French load follow,

* three SG tubing (Alloy 600 MA, 600 TT, 690),

it is illusive to look for the standardized chemistry which will be able to show the same favourable results on all the plants.
However, the search for the general best chemistry
have to give all the possibilities to find the more adapted
chemistry depending on the specific problem to mitigate.
This approach is new but was already clearly known <fnref n="26" />.

Then, a new adapted zone 1 bis in the coordinated pH
chemistry allowed a few plants to stay operating and save
unnecessary unavailability associated to primary to
secondary leaks without any safety risks.

The new EDF chemistry strategy for the primary
coolant will consist of the adaptation of the coordinated
chemistry to try to participate to the decrease of dose rate.

Now, the EDF strategy is to move on the “French
modified chemistry” with a Li concentration of 3.5
instead of 2.2 mg/kg at the beginning of the cycle. Then,
the “high lithium chemistry” will be evaluated. The
authorized range will be studied to facilitate the operation.
Nowadays, it would be of some use to reconsider, at least
for the new plants, the enriched boron 10 option,
particularly for new plants such as the EPR.

REFERENCES

<footnote n="1">¹ D. You et alii. Corrosion product transfer in PWR primary circuits during cold shutdowns – International Conference on Water Chemistry in Nuclear Reactor Systems – SFEN, Avignon, France, 22 - 26 April 2002.</footnote>

<footnote n="2">² Pressurized Water Reactors Primary Water Chemistry Guidelines: Vol 2, Revision 5, EPRI, Palo Alto, CA: 2003. 1002884.</footnote>

<footnote n="3">³ F. Dacquait et alii. Corrosion product transfer in French PWRs during shutdown - International Conference on Water Chemistry in Nuclear Reactor Systems - SFEN, Avignon, France, 22 - 26 April 2002.</footnote>

<footnote n="4">⁴ S. Anthoni. PWR Corrosion products behaviour during the shutdowns – International Conference on Water chemistry in Nuclear Reactor Systems - SFEN, Nice, France, 24-27 April 1994.</footnote>

<footnote n="5">⁵ F. Nordmann et alii. Overview of PWR chemistry options - International Conference on Water Chemistry in Nuclear Reactor Systems - SFEN, Avignon, France, 22 - 26 April 2002.</footnote>

<footnote n="6">⁶ P. Billot et alii. Role of lithium and boron on the corrosion of Zircaloy 4 under demanding PWR-type condition – Zirconium in the Nuclear Industry, tenth International Symposium. ASTM STP 1245, 1994.</footnote>

<footnote n="7">⁷ Pressurized Water Reactors Primary Water Chemistry Guidelines: Vol 1, Revision 5, EPRI, Palo Alto, CA: 2003. 1002884.</footnote>

<footnote n="8">⁸ PWSCC of Alloy 600 Materials in PWR Primary System Penetration, EPRI, CA: 1994, TR 103696.</footnote>

<footnote n="9">⁹ The role of coolant chemistry in PWR radiation field buildup. Research Project 825-2, 1985, EPRI, NP 4247.</footnote>

<footnote n="10">¹⁰ P. Beslu, G. Fréjaville. Mécanismes et models décrivant la création, le transport, et l'activation des produits de corrosion dans les REP. International Conference on Water Chemistry, AEIA, Vienne 1982.</footnote>

<footnote n="11">¹¹ Ph. Berge et alii. French experience on water chemistry control and corrosion products control of PWR reactors – International Conference on water chemistry, AEIA, Vienne 1982.</footnote>

<footnote n="12">¹² UNIPEDE Working Group, Primary of PWR : International Conference, Sorento – 30 May 1988.</footnote>

<footnote n="13">¹³ U. Staudt et alii. Comparison of French and German NPP water chemistry programs - International Conference on Water Chemistry in Nuclear Reactor Systems – SFEN, Avignon, France, 22 - 26 April 2002.</footnote>

<footnote n="14">¹⁴ A. Stutzmann. Spécifications chimiques des centrales REP. EDF Report D.5001/NRN/R84442 Ind 2, 1997.</footnote>

<footnote n="15">¹⁵ S. Anthoni et alii. Synthesis report on chemistry effect. CEA Report DTE/SCOS/LCC/90-107, 1990.</footnote>

<footnote n="16">¹⁶ H. Ocken. PWR primary coolant chemistry, the view from the USA – Nuclear Engineering international, 1992.</footnote>

<footnote n="17">¹⁷ F. Dacquait et alii. Status of French PWR primary System contamination – International Conference Water Chemistry of Nuclear Reactors Systems – EPRI, San Francisco, October 11-14, 2004.</footnote>

<footnote n="18">¹⁸ A. Tigeras et alii. EDF AOA experience : Chemical and thermal-hydraulic analysis – International Conference Water Chemistry of Nuclear Reactors Systems – EPRI, San Francisco, October 11-14, 2004.</footnote>

<footnote n="19">¹⁹ PWR Axial Offset Anomaly (AOA) Guidelines, Revision 1, EPRI, Palo Alto, CA: 2004. 1008102.</footnote>

<footnote n="20">²⁰ P.L. Frattini et alii. Corrosion product release from the corrosion of nickel base alloys in primary conditions – International Conference on Water chemistry in Nuclear Reactor Systems, BNES, Bournemouth, UK, Oct. 2000.</footnote>

<footnote n="21">²¹ L. Viricel et alii. Primary water chemistry optimisation for extended fuel cycle operation : Results of the DUO experimentation after three cycles - International Conference on Water Chemistry in Nuclear Reactor Systems - SFEN, Avignon, France, 22 - 26 April 2002.</footnote>

<footnote n="22">²² C. Hervouet et alii. Influence of chemistry on steam generator primary-to-secondary stabilized leak flow rate - International Conference on Water Chemistry in Nuclear Reactor Systems - SFEN, Avignon, France, 22 - 26 April 2002.</footnote>

<footnote n="23">²³ F. Nguyen et alii. First numerical simulations of contamination of PWR Primary Circuit by Activated Corrosion Products with the PACTOLE V3.0 Code - International Conference Water Chemistry of Nuclear Reactors Systems - EPRI, San Francisco, October 2004.</footnote>

<footnote n="24">²⁴ PWR Field Experience with Elevated, constant pH. EPRI Palo Alto, CA: Nov. 2001-1003153.</footnote>

<footnote n="25">²⁵ C.Wood : Developments in nuclear power plant chemistry - International Conference on Water chemistry in Nuclear Reactor Systems, BNES, Bournemouth, 2000.</footnote>

<footnote n="26">²⁶ F. Nordmann : Selection of the Right Chemistry with its Adequate Severity and Monitoring - International Conference Water Chemistry of Nuclear Reactors Systems – EPRI, San Francisco, October 11-14, 2004.</footnote>
However, the search for the general best chemistry have to give all the possibilities to find the more adapted chemistry depending on the specific problem to mitigate. This approach is new but was already clearly known<sup>26</sup>.

Then, a new adapted zone 1 bis in the coordinated pH chemistry allowed a few plants to stay operating and save unnecessary unavailability associated to primary to secondary leaks without any safety risks.

The new EDF chemistry strategy for the primary coolant will consist of the adaptation of the coordinated chemistry to try to participate to the decrease of dose rate.

Now, the EDF strategy is to move on the “French modified chemistry” with a Li concentration of 3.5 instead of 2.2 mg/kg at the beginning of the cycle. Then, the “high lithium chemistry” will be evaluated. The authorized range will be studied to facilitate the operation. Nowadays, it would be of some use to reconsider, at least for the new plants, the enriched boron 10 option, particularly for new plants such as the EPR.

## REFERENCES

<footnote n="1">¹ D. You et alii. Corrosion product transfer in PWR primary circuits during cold shutdowns – International Conference on Water Chemistry in Nuclear Reactor Systems – SFEN, Avignon, France, 22 - 26 April 2002.</footnote>

<footnote n="2">² Pressurized Water Reactors Primary Water Chemistry Guidelines: Vol 2, Revision 5, EPRI, Palo Alto, CA: 2003. 1002884.</footnote>

<footnote n="3">³ F. Dacquait et alii. Corrosion product transfer in French PWRs during shutdown - International Conference on Water Chemistry in Nuclear Reactor Systems – SFEN, Avignon, France, 22 - 26 April 2002.</footnote>

<footnote n="4">⁴ S. Anthoni. PWR Corrosion products behaviour during the shutdowns – International Conference on Water chemistry in Nuclear Reactor Systems - SFEN, Nice, France, 24-27 April 1994.</footnote>

<footnote n="5">⁵ F. Nordmann et alii. Overview of PWR chemistry options - International Conference on Water Chemistry in Nuclear Reactor Systems – SFEN, Avignon, France, 22 - 26 April 2002.</footnote>

<footnote n="6">⁶ P. Billot et alii. Role of lithium and boron on the corrosion of Zircaloy 4 under demanding PWR-type condition – Zirconium in the Nuclear Industry, tenth International Symposium. ASTM STP 1245, 1994.</footnote>

<footnote n="7">⁷ Pressurized Water Reactors Primary Water Chemistry Guidelines: Vol 1, Revision 5, EPRI, Palo Alto, CA: 2003. 1002884.</footnote>

<footnote n="8">⁸ PWSCC of Alloy 600 Materials in PWR Primary System Penetration, EPRI, CA: 1994, TR 103696.</footnote>

<footnote n="9">⁹ The role of coolant chemistry in PWR radiation field buildup. Research Project 825-2, 1985, EPRI, NP 4247.</footnote>

<footnote n="10">¹⁰ P. Beslu, G. Fréjaville. Mécanismes et models décrivant la création, le transport, et l'activation des produits de corrosion dans les REP. International Conference on Water Chemistry, AEIA, Vienne 1982.</footnote>

<footnote n="11">¹¹ Ph. Berge et alii. French experience on water chemistry control and corrosion products control of PWR reactors – International Conference on water chemistry, AEIA, Vienne 1982.</footnote>

<footnote n="12">¹² UNIPEDE Working Group, Primary of PWR : International Conference, Sorento – 30 May 1988.</footnote>

<footnote n="13">¹³ U. Staudt et alii. Comparison of French and German NPP water chemistry programs - International Conference on Water Chemistry in Nuclear Reactor Systems – SFEN, Avignon, France, 22 - 26 April 2002.</footnote>

<footnote n="14">¹⁴ A. Stutzmann. Spécifications chimiques des centrales REP. EDF Report D.5001/NRN/R84442 Ind 2, 1997.</footnote>

<footnote n="15">¹⁵ S. Anthoni et alii. Synthesis report on chemistry effect. CEA Report DTE/SCOS/LCC/90-107, 1990.</footnote>

<footnote n="16">¹⁶ H. Ocken. PWR primary coolant chemistry, the view from the USA – Nuclear Engineering international, 1992.</footnote>

<footnote n="17">¹⁷ F. Dacquait et alii. Status of French PWR primary System contamination – International Conference Water Chemistry of Nuclear Reactors Systems – EPRI, San Francisco, October 11-14, 2004.</footnote>

<footnote n="18">¹⁸ A. Tigeras et alii. EDF AOA experience : Chemical and thermal-hydraulic analysis – International Conference Water Chemistry of Nuclear Reactors Systems – EPRI, San Francisco, October 11-14, 2004.</footnote>

<footnote n="19">¹⁹ PWR Axial Offset Anomaly (AOA) Guidelines, Revision 1, EPRI, Palo Alto, CA: 2004. 1008102.</footnote>

<footnote n="20">²⁰ P.L. Frattini et alii. Corrosion product release from the corrosion of nickel base alloys in primary conditions – International Conference on Water chemistry in Nuclear Reactor Systems, BNES, Bournemouth, UK, Oct. 2000.</footnote>

<footnote n="21">²¹ L. Viricel et alii. Primary water chemistry optimisation for extended fuel cycle operation : Results of the DUO experimentation after three cycles - International Conference on Water Chemistry in Nuclear Reactor Systems – SFEN, Avignon, France, 22 - 26 April 2002.</footnote>

<footnote n="22">²² C. Hervouet et alii. Influence of chemistry on steam generator primary-to-secondary stabilized leak flow rate - International Conference on Water Chemistry in Nuclear Reactor Systems – SFEN, Avignon, France, 22 - 26 April 2002.</footnote>

<footnote n="23">²³ F. Nguyen et alii. First numerical simulations of contamination of PWR Primary Circuit by Activated Corrosion Products with the PACTOLE V3.0 Code - International Conference Water Chemistry of Nuclear Reactors Systems – EPRI, San Francisco, October 2004.</footnote>

<footnote n="24">²⁴ PWR Field Experience with Elevated, constant pH. EPRI Palo Alto, CA: Nov. 2001-1003153.</footnote>

<footnote n="25">²⁵ C.Wood : Developments in nuclear power plant chemistry - International Conference on Water chemistry in Nuclear Reactor Systems, BNES, Bournemouth, 2000.</footnote>

<footnote n="26">²⁶ F. Nordmann : Selection of the Right Chemistry with its Adequate Severity and Monitoring - International Conference Water Chemistry of Nuclear Reactors Systems – EPRI, San Francisco, October 11-14, 2004.</footnote>