# Status of primary system Contamination in French PWRs

Frédéric Dacquait, Luc Guinard, Caroline Andrieu, Jean-Luc Bretelle, Florence Bardet, Alain Rocher, Christian Brun

► To cite this version:

Frédéric Dacquait, Luc Guinard, Caroline Andrieu, Jean-Luc Bretelle, Florence Bardet, et al. Status of primary system Contamination in French PWRs. International Conference on Water Chemistry of Nuclear Reactor Systems, Oct 2004, San Francisco, United States. pp.332-341. cea-04463913

HAL Id: cea-04463913

https://cea.hal.science/cea-04463913

Submitted on 17 Feb 2024

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

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

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

Distributed under a Creative Commons Attribution 4.0 International License
Status of Primary System Contamination in French PWRs

F. Dacquait<fnref n="1" />, L. Guinard<fnref n="2" />, C. Andrieu<fnref n="2" />, J.L. Bretelle<fnref n="3" />, F. Bardet<fnref n="3" />, A. Rocher<fnref n="4" />, C. Brun<fnref n="5" />

**Abstract ---** The reduction of collective dose is an important objective for all nuclear plant operators. Since the beginning of French PWR commissioning, the French strategy has been to study the impact of design and operating parameters. With the codes and the test loops, the third support to these studies is the measurement campaigns of contamination in PWRs.

Since 1971, the deposited activities in PWR primary systems have been measured by gamma spectrometry, deposited activities providing more information than dose rates. These gamma spectrometry measurements with the EMECC system represent about 250 measurement campaigns in France and abroad. This paper presents the status of primary system contamination in French PWRs and some of the main lessons learnt from the EMECC campaign analyses.

The primary system ⁶⁰Co contamination in French PWRs has a tendency to increase for several cycles and then to be constant or even to decrease, whereas ⁵⁸Co contamination tends to decrease on primary coolant piping and to slightly increase on SG tubing. Thanks to the improvements of French PWR design and operation (pH₃₀₀°C of 7.2, Zry spacer grids, 690TT Steam generator tubing with a low cobalt content...), the dose rate indices tend to decrease with PWR generations and operating cycles.

I. INTRODUCTION

Deposits of activated corrosion products on primary
circuit surfaces generate a radiation field that is the
greatest contributor (85%) to the dose absorbed by
nuclear power plant staff during shutdown and
maintenance operations in PWRs<fnref n="1" />.

Moreover, in specific conditions, deposits on the fuel
elements (crud) can lead to axial offset anomalies<fnref n="2" />.
Considerable crud can also decrease the thermal transfer
between the primary coolant and the fuel cladding,
increase the pressure drop in the core and therefore
decrease the fluid mass flow. These phenomena result in a
temperature rise that could affect cladding oxidation and
integrity<fnref n="3" />.

Therefore the understanding of the mechanisms that
control corrosion product transfer is of great importance.
For about 30 years, the French strategy has been to study
the impact of PWR design and operating parameters on
corrosion product transfer. With the PACTOLE code<fnref n="4" />,<fnref n="5" />
and the test loops, the third support to these studies is the
measurement campaigns of contamination in PWRs.

These campaigns allow to validate the contamination
code, to study the impact of PWR operating and design
parameters and to make diagnoses of circuit
contamination in order to propose solutions.

Thus, since 1971, about 250 contamination
measurement campaigns have been carried out in French
and foreign PWRs (58 different plant units). More
particularly, from the first cycle of the first French 900
MWe PWR (FESSENHEIM unit 1) to the last French
1450MWe PWR (CIVAUX unit 2), i.e. about 190
campaigns in 44 plant units, the deposited activities in
French PWR primary systems have been measured thanks
to the gamma spectrometry EMECC device.

II. EMECC CAMPAIGNS

At each refueling shutdown of French PWRs, EDF
measures the dose rates around the primary piping and
calculates an average dose rate: the dose rate indexª. This
index allows the comparison of contamination levels
between PWRs and the monitoring of the contamination
level of a PWR for several cycles.

Fig. 1 presents the average dose rate indices by
standardized plant series: the first 900 MWe PWR
generation called CP0 (6 plants), the next 900 MWe PWR
generation called CPY (28 plants), the 1300 MWe PWR
generation called PQY (20 plants) and the last French

<footnote n="1">¹CEA, Centre de Cadarache, DEN, 13108 Saint Paul Lez Durance, France, frederic.dacquait@cea.fr</footnote>

<footnote n="2">²EDF, SEPTEN, 12-14 rue Dutrievoz, 69628 Villeurbanne, France, luc.guinard@edf.fr, caroline.andrieu@edf.fr</footnote>

<footnote n="3">³EDF, GDL, 2 rue Ampère, 93206 Saint Denis, France, jean-luc.bretelle@edf.fr, florence.bardet@edf.fr</footnote>

<footnote n="4">⁴EDF, CAPE, 1 place Pleyel, 93282 Saint Denis, France, alain.rocher@edf.fr</footnote>

<footnote n="5">⁵Framatome-ANP, Tour Areva, 92080 Paris La Défense, France, christian.brun@framatome-anp.com</footnote>

<footnote>ª Average of dose rate measurements on contact with the thermal insulation of the hot legs, crossover legs and cold legs.</footnote>
Status of Primary System Contamination in French PWRs

F. Dacquait<fnref n="1" />, L. Guinard<fnref n="2" />, C. Andrieu<fnref n="2" />, J.L. Bretelle<fnref n="3" />, F. Bardet<fnref n="3" />, A. Rocher<fnref n="4" />, C. Brun<fnref n="5" />

**Abstract ---** The reduction of collective dose is an important objective for all nuclear plant operators. Since the beginning of French PWR commissioning, the French strategy has been to study the impact of design and operating parameters. With the codes and the test loops, the third support to these studies is the measurement campaigns of contamination in PWRs.

Since 1971, the deposited activities in PWR primary systems have been measured by gamma spectrometry, deposited activities providing more information than dose rates. These gamma spectrometry measurements with the EMECC system represent about 250 measurement campaigns in France and abroad. This paper presents the status of primary system contamination in French PWRs and some of the main lessons learnt from the EMECC campaign analyses.

The primary system ⁶⁰Co contamination in French PWRs has a tendency to increase for several cycles and then to be constant or even to decrease, whereas ⁵⁸Co contamination tends to decrease on primary coolant piping and to slightly increase on SG tubing. Thanks to the improvements of French PWR design and operation (pH₃₀₀°C of 7.2, Zry spacer grids, 690TT Steam generator tubing with a low cobalt content...), the dose rate indices tend to decrease with PWR generations and operating cycles.

I. INTRODUCTION

Deposits of activated corrosion products on primary
circuit surfaces generate a radiation field that is the
greatest contributor (85%) to the dose absorbed by
nuclear power plant staff during shutdown and
maintenance operations in PWRs<fnref n="1" />.

Moreover, in specific conditions, deposits on the fuel
elements (crud) can lead to axial offset anomalies<fnref n="2" />.
Considerable crud can also decrease the thermal transfer
between the primary coolant and the fuel cladding,
increase the pressure drop in the core and therefore
decrease the fluid mass flow. These phenomena result in a
temperature rise that could affect cladding oxidation and
integrity<fnref n="3" />.

Therefore the understanding of the mechanisms that
control corrosion product transfer is of great importance.
For about 30 years, the French strategy has been to study
the impact of PWR design and operating parameters on
corrosion product transfer. With the PACTOLE code<fnref n="4" />,<fnref n="5" />
and the test loops, the third support to these studies is the
measurement campaigns of contamination in PWRs.

These campaigns allow to validate the contamination
code, to study the impact of PWR operating and design
parameters and to make diagnoses of circuit
contamination in order to propose solutions.

Thus, since 1971, about 250 contamination
measurement campaigns have been carried out in French
and foreign PWRs (58 different plant units). More
particularly, from the first cycle of the first French 900
MWe PWR (FESSENHEIM unit 1) to the last French
1450MWe PWR (CIVAUX unit 2), i.e. about 190
campaigns in 44 plant units, the deposited activities in
French PWR primary systems have been measured thanks
to the gamma spectrometry EMECC device.

II. EMECC CAMPAIGNS

At each refueling shutdown of French PWRs, EDF
measures the dose rates around the primary piping and
calculates an average dose rate: the dose rate indexª. This
index allows the comparison of contamination levels
between PWRs and the monitoring of the contamination
level of a PWR for several cycles.

Fig. 1 presents the average dose rate indices by
standardized plant series: the first 900 MWe PWR
generation called CP0 (6 plants), the next 900 MWe PWR
generation called CPY (28 plants), the 1300 MWe PWR
generation called PQY (20 plants) and the last French

<footnote n="1">¹CEA, Centre de Cadarache, DEN, 13108 Saint Paul Lez Durance, France, frederic.dacquait@cea.fr</footnote>

<footnote n="2">²EDF, SEPTEN, 12-14 rue Dutrievoz, 69628 Villeurbanne, France, luc.guinard@edf.fr, caroline.andrieu@edf.fr</footnote>

<footnote n="3">³EDF, GDL, 2 rue Ampère, 93206 Saint Denis, France, jean-luc.bretelle@edf.fr, florence.bardet@edf.fr</footnote>

<footnote n="4">⁴EDF, CAPE, 1 place Pleyel, 93282 Saint Denis, France, alain.rocher@edf.fr</footnote>

<footnote n="5">⁵Framatome-ANP, Tour Areva, 92080 Paris La Défense, France, christian.brun@framatome-anp.com</footnote>

<footnote>ª Average of dose rate measurements on contact with the thermal insulation of the hot legs, crossover legs and cold legs.</footnote>
operating generation, the 1450 MWe PWR, called N4 (4 plants).

It is to be noted that there is a diminution of dose rate index with the improvement of standardized plant series. The indices tend to decrease with cycles, especially for CP0 PWRs of which the index was high for about 12 cycles and reached the CPY PWR index at the 17th cycle.

<figure><img src="image_1.png" /><figcaption>Fig. 1. Average dose rate indices of French PWRs</figcaption></figure>

But these measurements give no information on the origin of these dose rates. Moreover, the dose rates at a specific point depend on radionuclide (gamma energies), activities (volume and/or surface activities), component characteristics (density, thickness) and ambient dose rates. And only dose rate variations above 30% are significant. Therefore, the CEA has developed a measurement device by gamma spectrometry: the EMECC system⁶.

Thanks to this system, the deposited activities are measured in the primary circuit of several PWRs over one year during their refueling shutdown. Typical measurements are carried out at the primary coolant pipes (hot, crossover and cold legs) and at the Steam Generator (SG) tube bundle of the primary system loops. The determination of the SG tubing activities is very important because these represent the largest primary surfaceᵇ and out-of-flux activities. Some parts of other circuits are also measured, e.g. heat exchangers and pipes of the Chemical and Volume Control System (CVCS) or the Residual Heat Removal System (RHRS).

The EMECC measurements allow the quantification and identification of detectable radionuclides. They are more accurate than the dose rate measurements (absolute uncertainties of about 20%). They are carried out at the very beginning of PWR shutdowns to study the impact of the operating cycle on contamination and during the cold shutdown procedure to study its impact on contamination. Badly controlled, the cold shutdown procedure can lead to

a great increase in circuit contamination⁷ and thus mask the impact of the operating cycle.

### III. FRENCH PWR CONTAMINATION

All detectable gamma-emitting nuclides have been monitored. However, two radioactive isotopes, ⁵⁸Co and ⁶⁰Co, contribute by over 90% on average to the dose rates around French primary systems (see para. IV). That is why we will focus on these two radionuclides.

These come from the activation of stable nickel and cobalt isotopes:

$$ ^{58}\text{Ni} + n \rightarrow ^{58}\text{Co} + p \quad (^{58}\text{Co radioactive half-life: 71 days}) $$

$$ ^{59}\text{Co} + n \rightarrow ^{60}\text{Co} + \gamma \quad (^{60}\text{Co radioactive half-life: 5.3 years}) $$

The figures presenting the contamination status of French PWRs (Fig. 2 to Fig. 4 and Fig. 10 to Fig. 12) show the ranges of ⁵⁸Co and ⁶⁰Co deposited activities versus cycles in the primary systems for the different standardized plant series. For each type of PWR, over 95% of the measured deposited activities are between the “Min” and the “Max” curves. These data represent 146 campaigns in 900 MWe PWRs (CP0 and CPY), 37 campaigns in 1300 MWe PWRs (PQY) and 7 campaigns in 1450 MWe PWRs (N4).

#### III.A. ⁵⁸Co Contamination

Fig. 2 to Fig. 4 present the ⁵⁸Co deposited activities, respectively, on the hot legs, on the cold legs (or crossover legs) and on the steam generator tubing of French PWRs.

<figure><img src="image_2.png" /><figcaption>Fig. 2. ⁵⁸Co deposited activities on the hot legs of French PWRs</figcaption></figure>

<footnote>ᵇ About 64% for a 900 MWe PWR and 72% for a 1300 MWe PWR</footnote>
operating generation, the 1450 MWe PWR, called N4 (4 plants).

It is to be noted that there is a diminution of dose rate index with the improvement of standardized plant series. The indices tend to decrease with cycles, especially for CP0 PWRs of which the index was high for about 12 cycles and reached the CPY PWR index at the 17th cycle.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Average dose rate indices of French PWRs</figcaption></figure>

But these measurements give no information on the origin of these dose rates. Moreover, the dose rates at a specific point depend on radionuclide (gamma energies), activities (volume and/or surface activities), component characteristics (density, thickness) and ambient dose rates. And only dose rate variations above 30% are significant. Therefore, the CEA has developed a measurement device by gamma spectrometry: the EMECC system⁶.

Thanks to this system, the deposited activities are measured in the primary circuit of several PWRs over one year during their refueling shutdown. Typical measurements are carried out at the primary coolant pipes (hot, crossover and cold legs) and at the Steam Generator (SG) tube bundle of the primary system loops. The determination of the SG tubing activities is very important because these represent the largest primary surfaceᵇ and out-of-flux activities. Some parts of other circuits are also measured, e.g. heat exchangers and pipes of the Chemical and Volume Control System (CVCS) or the Residual Heat Removal System (RHRS).

The EMECC measurements allow the quantification and identification of detectable radionuclides. They are more accurate than the dose rate measurements (absolute uncertainties of about 20%). They are carried out at the very beginning of PWR shutdowns to study the impact of the operating cycle on contamination and during the cold shutdown procedure to study its impact on contamination. Badly controlled, the cold shutdown procedure can lead to

a great increase in circuit contamination⁷ and thus mask the impact of the operating cycle.

### III. FRENCH PWR CONTAMINATION

All detectable gamma-emitting nuclides have been monitored. However, two radioactive isotopes, ⁵⁸Co and ⁶⁰Co, contribute by over 90% on average to the dose rates around French primary systems (see para. IV). That is why we will focus on these two radionuclides.

These come from the activation of stable nickel and cobalt isotopes:

$$ ^{58}\text{Ni} + n \rightarrow ^{58}\text{Co} + p \quad (^{58}\text{Co radioactive half-life: 71 days}) $$

$$ ^{59}\text{Co} + n \rightarrow ^{60}\text{Co} + \gamma \quad (^{60}\text{Co radioactive half-life: 5.3 years}) $$

The figures presenting the contamination status of French PWRs (Fig. 2 to Fig. 4 and Fig. 10 to Fig. 12) show the ranges of ⁵⁸Co and ⁶⁰Co deposited activities versus cycles in the primary systems for the different standardized plant series. For each type of PWR, over 95% of the measured deposited activities are between the “Min” and the “Max” curves. These data represent 146 campaigns in 900 MWe PWRs (CP0 and CPY), 37 campaigns in 1300 MWe PWRs (PQY) and 7 campaigns in 1450 MWe PWRs (N4).

#### III.A. ⁵⁸Co Contamination

Fig. 2 to Fig. 4 present the ⁵⁸Co deposited activities, respectively, on the hot legs, on the cold legs (or crossover legs) and on the steam generator tubing of French PWRs.

<figure><img src="image_3.png" /><figcaption>Fig. 2. ⁵⁸Co deposited activities on the hot legs of French PWRs</figcaption></figure>

<footnote>ᵇ About 64% for a 900 MWe PWR and 72% for a 1300 MWe PWR</footnote>
<figure><img src="image_3.png" /><figcaption>Fig. 3. <sup>58</sup>Co deposited activities on the cold legs of French PWRs</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 4. <sup>58</sup>Co deposited activities on the steam generator tubing of French PWRs</figcaption></figure>

It is to be noted that the <sup>58</sup>Co deposited activity scatter tends to decrease with the PWR type, like the <sup>58</sup>Co contamination dispersion of the primary coolant pipes with cycles. Overall, there is a tendency of the <sup>58</sup>Co contamination to decrease with cycles on primary coolant pipes and to slightly increase on steam generator tubing.

The <sup>58</sup>Co deposited activities were very high on the primary coolant pipes of some PWRs for the first cycles, up to 40 GBq/m² for some 900 MWe PWRs. The last PWR generation (N4) presents the lowest <sup>58</sup>Co contamination levels.

The large <sup>58</sup>Co deposited activities on the primary coolant pipes for the first cycles are most likely due to the higher Nickel release of SG tubes during the formation of the inner protective oxide layer.

After a Steam Generator Replacement (SGR), a similar variation is observed⁸. After an increase, the <sup>58</sup>Co deposited activities on the primary circuit decrease. Note that SGRs involve the enlargement of the <sup>58</sup>Co contamination range in 900MWe PWRs. And, some of their values correspond to the 5% of measurements out of

the range limits, in particular, the very low <sup>60</sup>Co contamination of SG tubing after an SGR⁸.

For the oldest PWRs, the contamination during the first cycles is intensified by a poorly controlled chemistry. Indeed, during the first cycles of these PWRs, the pH₃₀₀°C was below 6.9 for several dozen days per cycle. Then, at the beginning of the 1980s, the Boron-Lithium coordinated chemistry at pH₃₀₀°C 7.0 must have led to a contamination reduction. In the 1990s, even if the new target pH₃₀₀°C value of 7.2 did not show an immediate effect⁹, this specification has been probably beneficial in the long term. Today, the pH₃₀₀°C target value is 7.2 for all 58 French units with aimed at H₂ concentrations between 25 and 35 ml/kg¹⁰.

The contamination decrease is also due to the replacement of alloy 718 spacer grids by Zircaloy grids¹. In the past, the grid assemblies were in Inconel 718. As the Nickel content of this alloy is high (about 52%), this alloy was a significant potential source of Nickel under neutron flux, therefore of <sup>58</sup>Co. Progressively in the 1990s, the Inconel spacer grids were replaced. Today, all the fuel assemblies are equipped with Zircaloy grids.

The Steam Generator tubing material, and especially their manufacturing process, partly explain the dispersion of the <sup>58</sup>Co deposited activities. The French PWR Steam Generators are equipped with tubing made of alloy 600 Mill-Annealed (MA) for the oldest, 600 Thermally-Treated (TT) or 690 TT for the most recent. Three manufacturers (called A, B and C) have supplied French SG tubing.

The evolution of the total mean <sup>58</sup>Co deposited activity on the alloy 600MA SG tubes is presented in Fig. 5 and on the primary coolant pipes in Fig. 6. We have distinguished the 900 MWe PWRs equipped with at least one SG with tubes manufactured by A (X<sup>c</sup> SG A) from the PWRs with no SG tubes manufactured by A (0 SG A).

It appears that the PWRs equipped with X SG tube bundles manufactured by A have a total <sup>58</sup>Co deposited activity on the SG tubes higher than the PWRs with no SG tubes manufactured by A (manufacturers B or C). But, on the contrary, they have a lower total <sup>58</sup>Co deposited activity on the primary coolant pipes and therefore a generally lower dose rate index¹¹.

<footnote>c X = 1 to 3</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 3. <sup>58</sup>Co deposited activities on the cold legs of French PWRs</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. <sup>58</sup>Co deposited activities on the steam generator tubing of French PWRs</figcaption></figure>

It is to be noted that the <sup>58</sup>Co deposited activity scatter tends to decrease with the PWR type, like the <sup>58</sup>Co contamination dispersion of the primary coolant pipes with cycles. Overall, there is a tendency of the <sup>58</sup>Co contamination to decrease with cycles on primary coolant pipes and to slightly increase on steam generator tubing.

The <sup>58</sup>Co deposited activities were very high on the primary coolant pipes of some PWRs for the first cycles, up to 40 GBq/m² for some 900 MWe PWRs. The last PWR generation (N4) presents the lowest <sup>58</sup>Co contamination levels.

The large <sup>58</sup>Co deposited activities on the primary coolant pipes for the first cycles are most likely due to the higher Nickel release of SG tubes during the formation of the inner protective oxide layer.

After a Steam Generator Replacement (SGR), a similar variation is observed⁸. After an increase, the <sup>58</sup>Co deposited activities on the primary circuit decrease. Note that SGRs involve the enlargement of the <sup>58</sup>Co contamination range in 900MWe PWRs. And, some of their values correspond to the 5% of measurements out of

the range limits, in particular, the very low <sup>60</sup>Co contamination of SG tubing after an SGR⁸.

For the oldest PWRs, the contamination during the first cycles is intensified by a poorly controlled chemistry. Indeed, during the first cycles of these PWRs, the pH₃₀₀°C was below 6.9 for several dozen days per cycle. Then, at the beginning of the 1980s, the Boron-Lithium coordinated chemistry at pH₃₀₀°C 7.0 must have led to a contamination reduction. In the 1990s, even if the new target pH₃₀₀°C value of 7.2 did not show an immediate effect⁹, this specification has been probably beneficial in the long term. Today, the pH₃₀₀°C target value is 7.2 for all 58 French units with aimed at H₂ concentrations between 25 and 35 ml/kg¹⁰.

The contamination decrease is also due to the replacement of alloy 718 spacer grids by Zircaloy grids¹. In the past, the grid assemblies were in Inconel 718. As the Nickel content of this alloy is high (about 52%), this alloy was a significant potential source of Nickel under neutron flux, therefore of <sup>58</sup>Co. Progressively in the 1990s, the Inconel spacer grids were replaced. Today, all the fuel assemblies are equipped with Zircaloy grids.

The Steam Generator tubing material, and especially their manufacturing process, partly explain the dispersion of the <sup>58</sup>Co deposited activities. The French PWR Steam Generators are equipped with tubing made of alloy 600 Mill-Annealed (MA) for the oldest, 600 Thermally-Treated (TT) or 690 TT for the most recent. Three manufacturers (called A, B and C) have supplied French SG tubing.

The evolution of the total mean <sup>58</sup>Co deposited activity on the alloy 600MA SG tubes is presented in Fig. 5 and on the primary coolant pipes in Fig. 6. We have distinguished the 900 MWe PWRs equipped with at least one SG with tubes manufactured by A (X<sup>c</sup> SG A) from the PWRs with no SG tubes manufactured by A (0 SG A).

It appears that the PWRs equipped with X SG tube bundles manufactured by A have a total <sup>58</sup>Co deposited activity on the SG tubes higher than the PWRs with no SG tubes manufactured by A (manufacturers B or C). But, on the contrary, they have a lower total <sup>58</sup>Co deposited activity on the primary coolant pipes and therefore a generally lower dose rate index¹¹.

<footnote>c X = 1 to 3</footnote>
<figure><img src="image_5.png" /><figcaption>Fig. 5. Total mean $^{58}$Co deposited activity on SG tubes</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 6. Total mean $^{58}$Co deposited activity on primary coolant pipes</figcaption></figure>

The alloy 600MA SG tubes manufactured by A release and trap more corrosion products which results in a lower contamination of the primary coolant pipes. This is due to the tube manufacturing process: pilgering or drawing with or without sandblasting. 600MA SG tubes manufactured by A were pilgered and sandblasted, whereas by B these were drawn without sandblasting and by C pilgered without sandblasting.

In Fig. 7, a correlation can be found between the number of SG tube bundles manufactured by A in a PWR and the $^{58}$Co contamination mean ratio SGs/Primary pipes. However, the ratios of PWRs with at least one SG tube bundle manufactured by A are similar.

<figure><img src="image_7.png" /><figcaption>Fig. 7. $^{58}$Co contamination mean ratio SGs/Pipes</figcaption></figure>

The SG tube manufacturing process is indisputably responsible for the difference in contamination. Indeed, in PWRs equipped with different SG tube bundles (same alloy but different suppliers), which thus undergo the same conditions (chemistry, thermohydraulics), the difference in contamination between the SGs can be significant, up to 5. However, this difference decreases with cycles, manufacturers and heat treatment. So, as can be seen in Fig. 8, the $^{58}$Co contamination mean ratio between alloy 600MA tubes manufactured by A and tubes manufactured by B in a same PWR is around 5 at the first cycle and around 2 after about 10 cycles. This ratio has been about 2 since the first cycle if we compare manufacturer A and C, and it is less than 2 for alloy 600TT whether it is manufacturer B or C.

<figure><img src="image_8.png" /><figcaption>Fig. 8. Mean ratio of SG $^{58}$Co contamination</figcaption></figure>

This difference in contamination is also seen in Fig. 9, which presents the average $^{58}$Co contamination of French SGs normalized to the mean contamination of 600MA SG tubes manufactured by A. On average, the heat treatment of alloy 600 manufactured by A results in a reduction of about 2, like 600MA tubes manufactured by C, and the use of alloy 690TT manufactured by A allows a reduction of about 3, like 600MA tubes manufactured by B.
<figure><img src="image_6.png" /><figcaption>Fig. 5. Total mean $^{58}$Co deposited activity on SG tubes</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6. Total mean $^{58}$Co deposited activity on primary coolant pipes</figcaption></figure>

The alloy 600MA SG tubes manufactured by A release and trap more corrosion products which results in a lower contamination of the primary coolant pipes. This is due to the tube manufacturing process: pilgering or drawing with or without sandblasting. 600MA SG tubes manufactured by A were pilgered and sandblasted, whereas by B these were drawn without sandblasting and by C pilgered without sandblasting.

In Fig. 7, a correlation can be found between the number of SG tube bundles manufactured by A in a PWR and the $^{58}$Co contamination mean ratio SGs/Primary pipes. However, the ratios of PWRs with at least one SG tube bundle manufactured by A are similar.

<figure><img src="image_8.png" /><figcaption>Fig. 7. $^{58}$Co contamination mean ratio SGs/Pipes</figcaption></figure>

The SG tube manufacturing process is indisputably responsible for the difference in contamination. Indeed, in PWRs equipped with different SG tube bundles (same alloy but different suppliers), which thus undergo the same conditions (chemistry, thermohydraulics), the difference in contamination between the SGs can be significant, up to 5. However, this difference decreases with cycles, manufacturers and heat treatment. So, as can be seen in Fig. 8, the $^{58}$Co contamination mean ratio between alloy 600MA tubes manufactured by A and tubes manufactured by B in a same PWR is around 5 at the first cycle and around 2 after about 10 cycles. This ratio has been about 2 since the first cycle if we compare manufacturer A and C, and it is less than 2 for alloy 600TT whether it is manufacturer B or C.

<figure><img src="image_9.png" /><figcaption>Fig. 8. Mean ratio of SG $^{58}$Co contamination</figcaption></figure>

This difference in contamination is also seen in Fig. 9, which presents the average $^{58}$Co contamination of French SGs normalized to the mean contamination of 600MA SG tubes manufactured by A. On average, the heat treatment of alloy 600 manufactured by A results in a reduction of about 2, like 600MA tubes manufactured by C, and the use of alloy 690TT manufactured by A allows a reduction of about 3, like 600MA tubes manufactured by B.
<figure><img src="image_9.png" /><figcaption>Fig. 9. <sup>58</sup>Co normalized mean contamination of French SGs</figcaption></figure>

It should be noted that the difference in contamination between the SG tubes supplied by different manufacturers is also observed whatever the radionuclide considered, including <sup>60</sup>Co.

### III.B. <sup>60</sup>Co Contamination

Fig. 10 to Fig. 12 present the <sup>60</sup>Co deposited activities, respectively, on the hot legs, on the cold legs (or crossover legs) and on the steam generator tubes of French PWRs.

<figure><img src="image_10.png" /><figcaption>Fig. 10. <sup>60</sup>Co deposited activities on the hot legs of French PWRs</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 11. <sup>60</sup>Co deposited activities on the cold legs of French PWRs</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 12. <sup>60</sup>Co deposited activities on the steam generator tubing of French PWRs</figcaption></figure>

The <sup>60</sup>Co contamination range narrows with PWR types, like the <sup>60</sup>Co contamination range of primary coolant pipes with cycles. The evolution of the <sup>60</sup>Co contamination is more marked on the SG tubes than on the primary coolant pipes. On average, the <sup>60</sup>Co deposited activities tend to slightly increase on the primary pipes for 3 to 5 cycles whereas they increase more significantly on the SG tubes for 5 to 10 cycles. Then, they tend to be constant or even to decrease.

The <sup>60</sup>Co deposited activities have been very high, up to about 12 GBq/m², on the primary coolant pipes of some CP0 plants for several cycles (symbol circle). The N4 plants present the lowest <sup>60</sup>Co contamination levels and the maximum values of N4 plants are equivalent to the minimum values of the other PWR generations.

For many years, there has been a controversy about the main cobalt source in PWR circuits: SG tubing? Stellites? In core materials? Stainless steel piping? The difficulty in identifying the main Co source comes from the fact that the cobalt mass transferred in the primary
<figure><img src="image_10.png" /><figcaption>Fig. 9. <sup>58</sup>Co normalized mean contamination of French SGs</figcaption></figure>

It should be noted that the difference in contamination between the SG tubes supplied by different manufacturers is also observed whatever the radionuclide considered, including <sup>60</sup>Co.

### III.B. <sup>60</sup>Co Contamination

Fig. 10 to Fig. 12 present the <sup>60</sup>Co deposited activities, respectively, on the hot legs, on the cold legs (or crossover legs) and on the steam generator tubes of French PWRs.

<figure><img src="image_11.png" /><figcaption>Fig. 10. <sup>60</sup>Co deposited activities on the hot legs of French PWRs</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 11. <sup>60</sup>Co deposited activities on the cold legs of French PWRs</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 12. <sup>60</sup>Co deposited activities on the steam generator tubing of French PWRs</figcaption></figure>

The <sup>60</sup>Co contamination range narrows with PWR types, like the <sup>60</sup>Co contamination range of primary coolant pipes with cycles. The evolution of the <sup>60</sup>Co contamination is more marked on the SG tubes than on the primary coolant pipes. On average, the <sup>60</sup>Co deposited activities tend to slightly increase on the primary pipes for 3 to 5 cycles whereas they increase more significantly on the SG tubes for 5 to 10 cycles. Then, they tend to be constant or even to decrease.

The <sup>60</sup>Co deposited activities have been very high, up to about 12 GBq/m², on the primary coolant pipes of some CP0 plants for several cycles (symbol circle). The N4 plants present the lowest <sup>60</sup>Co contamination levels and the maximum values of N4 plants are equivalent to the minimum values of the other PWR generations.

For many years, there has been a controversy about the main cobalt source in PWR circuits: SG tubing? Stellites? In core materials? Stainless steel piping? The difficulty in identifying the main Co source comes from the fact that the cobalt mass transferred in the primary
system is insignificant (≈100 g after about 10 cycles⁴)
whereas ⁶⁰Co is the major contributor to the dose rates
around the primary system (see para. IV).

Actually, the sources are multiple and the main one
depends on the level of the others! To illustrate this
statement, let us consider Fig. 13. This figure presents the
total ⁶⁰Co contamination of out-of-core primary surfaces
as a function of the SG tubing Co content.

<figure><img src="image_13.png" /><figcaption>Fig. 13. Impact of SG tube Co content</figcaption></figure>

It seems that there is a correlation between the SG
tubing Co content and the ⁶⁰Co contamination of the
primary system. Up to a certain point, the lower the Co
content, the lower the ⁶⁰Co contamination.

However some PWRs present a high ⁶⁰Co
contamination whereas the Co content of their SG tubing
is low, especially the PWR with the lowest Co content
(108 ppm) and one of the highest ⁶⁰Co contamination
(0,43 GBq/m²). It is clear that for this PWR, the main
cobalt source is not the SG tubing. It could be Stellites or
Inconel spacer grids.

The Stellite release due to corrosion is low¹², but in
case of abnormal wear, it can be the major source. This
low release is confirmed by the ⁶⁰Co contamination
evolution after an SGR by SG tubes with a lower Co
content. The ⁶⁰Co deposited activities increase on the SG
tubes by a lower rate after an SGR than before one, and
even decrease on the primary coolant pipes⁸. If Stellites
were the main Co source in this case, the increase rate
would have been the same after the SGR as before the
latter, on the SG tubes and the primary coolant pipes.

Another identified cobalt main source is Inconel
spacer grids¹, which directly release activated corrosion
products as they are under neutron flux. For instance, the
high ⁶⁰Co contamination level in BIBLIS B is due to the
use of nickel-plated fuel spacers with increased cobalt
content¹³. This could also explain the significant ⁶⁰Co
deposited activities on the primary coolant pipes in some
CP0 PWRs (see Fig. 10 and Fig. 11).

In one of the CP0 PWRs indicated in Fig. 10 and Fig.
11, once the abnormal cobalt source was removed
(replacement of fuel spacer grids by Zircaloy grids from
cycle 11) or exhausted, the ⁶⁰Co contamination decreased
significantly by a factor of between 2 and 4 over roughly
10 years (between cycles 12 and 20) on the primary
coolant pipes and this, without Zinc injection. It seems
that the ⁶⁰Co build-up coming from the remaining sources
is low or even negligible compared to the former ⁶⁰Co
contamination on the primary coolant pipes.

Nevertheless, as the decrease rate seems to slow
down, EDF has scheduled Zinc addition in this CP0 PWR
to confirm or infirm its beneficial effect on the ⁶⁰Co
contamination observed abroad¹⁴, ¹⁵.

IV. DOSE RATES

By using deposited activities and component
characteristics (diameter, thickness, density), the
radionuclide contributions to the dose rates around the
French primary systems have been calculated thanks to
the MERCURE code¹⁶. The contribution is determined on
contact with the thermal insulations of the primary pipes
and steam generators.

The average value of the total ⁵⁸Co and ⁶⁰Co
contribution is 93% with a standard deviation of 4%. The
minimum value is 75%. The other main radionuclides
contributing to the dose rates are ⁵⁴Mn, ⁵⁹Fe, ⁶⁵Zn, ⁹⁵Zr,
⁹⁵Nb, ¹¹⁰mAg, ¹²²Sb and ¹²⁴Sb. Generally, their
contribution is 2% or less. In rare cases, the contributions
around primary systems can reach about 5% for ⁶⁵Zn,
⁹⁵Zr, ⁹⁵Nb and ¹²²Sb, 10% for ⁵⁴Mn and ⁵⁹Fe, 15% for
¹¹⁰mAg and 20% for ¹²⁴Sb. ⁵¹Cr is also present in French
PWRs, but its contribution to the dose rates around the
primary system is negligible thanks to its low radiation
energy (320 keV).

⁶⁵Zn comes from the activation of Zinc (⁶⁴Zn). Remember that there is no Zinc injection in French PWRs yet. It would originate from boric acid and water makeup tanks of the Reactor Boron and Water Makeup System. The presence of zinc in some French primary systems (up to 3.5 ppb in a PWR) does not seem to result in a lower contamination in these PWRs, it can be due to too low zinc concentrations.

⁹⁵Zr comes from activation of ⁹⁴Zr, ⁹⁵Nb being a daughter product of ⁹⁵Zr. ⁹⁵Zr can be a fission product or be generated by Zircaloy clad spalling.

⁵⁴Mn and ⁵⁹Fe are iron tracers.

¹¹⁰mAg, an activation product of Silver, comes from Ag-In-Cd control rod perforation or from silver coated seals¹⁷.

<footnote>d Several dozens of kg for iron and nickel</footnote>

<footnote>e ⁶⁰Co decay leads to a contamination decrease of about 4 over 10 years.</footnote>
system is insignificant (≈100 g after about 10 cycles⁴)
whereas ⁶⁰Co is the major contributor to the dose rates
around the primary system (see para. IV).

Actually, the sources are multiple and the main one
depends on the level of the others! To illustrate this
statement, let us consider Fig. 13. This figure presents the
total ⁶⁰Co contamination of out-of-core primary surfaces
as a function of the SG tubing Co content.

<figure><img src="image_14.png" /><figcaption>Fig. 13. Impact of SG tube Co content</figcaption></figure>

It seems that there is a correlation between the SG
tubing Co content and the ⁶⁰Co contamination of the
primary system. Up to a certain point, the lower the Co
content, the lower the ⁶⁰Co contamination.

However some PWRs present a high ⁶⁰Co
contamination whereas the Co content of their SG tubing
is low, especially the PWR with the lowest Co content
(108 ppm) and one of the highest ⁶⁰Co contamination
(0,43 GBq/m²). It is clear that for this PWR, the main
cobalt source is not the SG tubing. It could be Stellites or
Inconel spacer grids.

The Stellite release due to corrosion is low¹², but in
case of abnormal wear, it can be the major source. This
low release is confirmed by the ⁶⁰Co contamination
evolution after an SGR by SG tubes with a lower Co
content. The ⁶⁰Co deposited activities increase on the SG
tubes by a lower rate after an SGR than before one, and
even decrease on the primary coolant pipes⁸. If Stellites
were the main Co source in this case, the increase rate
would have been the same after the SGR as before the
latter, on the SG tubes and the primary coolant pipes.

Another identified cobalt main source is Inconel
spacer grids¹, which directly release activated corrosion
products as they are under neutron flux. For instance, the
high ⁶⁰Co contamination level in BIBLIS B is due to the
use of nickel-plated fuel spacers with increased cobalt
content¹³. This could also explain the significant ⁶⁰Co
deposited activities on the primary coolant pipes in some
CP0 PWRs (see Fig. 10 and Fig. 11).

In one of the CP0 PWRs indicated in Fig. 10 and Fig.
11, once the abnormal cobalt source was removed
(replacement of fuel spacer grids by Zircaloy grids from
cycle 11) or exhausted, the ⁶⁰Co contamination decreased
significantly by a factor of between 2 and 4 over roughly
10 years (between cycles 12 and 20) on the primary
coolant pipes and this, without Zinc injection. It seems
that the ⁶⁰Co build-up coming from the remaining sources
is low or even negligible compared to the former ⁶⁰Co
contamination on the primary coolant pipes.

Nevertheless, as the decrease rate seems to slow
down, EDF has scheduled Zinc addition in this CP0 PWR
to confirm or infirm its beneficial effect on the ⁶⁰Co
contamination observed abroad¹⁴, ¹⁵.

IV. DOSE RATES

By using deposited activities and component
characteristics (diameter, thickness, density), the
radionuclide contributions to the dose rates around the
French primary systems have been calculated thanks to
the MERCURE code¹⁶. The contribution is determined on
contact with the thermal insulations of the primary pipes
and steam generators.

The average value of the total ⁵⁸Co and ⁶⁰Co
contribution is 93% with a standard deviation of 4%. The
minimum value is 75%. The other main radionuclides
contributing to the dose rates are ⁵⁴Mn, ⁵⁹Fe, ⁶⁵Zn, ⁹⁵Zr,
⁹⁵Nb, ¹¹⁰mAg, ¹²²Sb and ¹²⁴Sb. Generally, their
contribution is 2% or less. In rare cases, the contributions
around primary systems can reach about 5% for ⁶⁵Zn,
⁹⁵Zr, ⁹⁵Nb and ¹²²Sb, 10% for ⁵⁴Mn and ⁵⁹Fe, 15% for
¹¹⁰mAg and 20% for ¹²⁴Sb. ⁵¹Cr is also present in French
PWRs, but its contribution to the dose rates around the
primary system is negligible thanks to its low radiation
energy (320 keV).

⁶⁵Zn comes from the activation of Zinc (⁶⁴Zn). Remember that there is no Zinc injection in French PWRs yet. It would originate from boric acid and water makeup tanks of the Reactor Boron and Water Makeup System. The presence of zinc in some French primary systems (up to 3.5 ppb in a PWR) does not seem to result in a lower contamination in these PWRs, it can be due to too low zinc concentrations.

⁹⁵Zr comes from activation of ⁹⁴Zr, ⁹⁵Nb being a
daughter product of ⁹⁵Zr. ⁹⁵Zr can be a fission product or
be generated by Zircaloy clad spalling.

⁵⁴Mn and ⁵⁹Fe are iron tracers.

¹¹⁰mAg, an activation product of Silver, comes from
Ag-In-Cd control rod perforation or from silver coated
seals¹⁷.

<footnote>d Several dozens of kg for iron and nickel</footnote>

<footnote>e ⁶⁰Co decay leads to a contamination decrease of about 4
over 10 years.</footnote>
$^{124}$Sb, an activation product of Antimony, originates from Be-Sb source rod failure or from pump bearing of the Boron Recycle System$^{17}$.

The average contribution of $^{58}$Co and $^{60}$Co versus cycles in French PWRs is shown in Fig. 14

<figure><img src="image_14.png" /><figcaption>Fig. 14. Average $^{58}$Co and $^{60}$Co contributions to the dose rates around primary loops</figcaption></figure>

We have distinguished the CP0 PWRs from the other PWRs. Indeed, the CP0 PWRs are characterized by a higher $^{60}$Co contribution to the dose rates. This high contribution is due to high $^{60}$Co activities (see para. III.B). At the end of the first cycle, the $^{58}$Co and $^{60}$Co contribution is practically fifty-fifty, and then $^{60}$Co becomes predominant and reaches about 80% (15% for $^{58}$Co) starting around the 10$^{th}$ cycle.

For the other PWRs, the average $^{58}$Co contribution remains predominant until the third cycle. Progressively, its contribution decreases giving way to $^{60}$Co. Stability is reached after around 10 cycles, the contributions are then about 30% for $^{58}$Co and about 65% for $^{60}$Co.

These evolutions are due to high $^{58}$Co activities during the first cycles, which decrease, and to the $^{60}$Co activity rise during about 10 cycles (see para. III). $^{60}$Co is the main contributor to the dose rate around primary components after several cycles because of its radioactive half-life (accumulation) and its radiation energy (two photons with an average energy of 1.25 MeV against one photon at 0.81 MeV for $^{58}$Co).

It should be noted that the cycles after an SGR have not been taken into account because SGRs significantly modify $^{58}$Co and $^{60}$Co contributions during several cycles$^8$.

V. DISCUSSION

In para. III, the primary system $^{60}$Co contamination in French PWRs was shown to have a tendency to increase

for several cycles and then to be constant or even to decrease, whereas $^{58}$Co contamination tends to decrease on primary coolant piping and to slightly increase on SG tubing. These are average evolutions, as a PWR can have more or less different variations. Consider the contamination evolution of a 900 MWe PWR (CPY).

Fig. 15 and Fig. 16 present the $^{58}$Co and $^{60}$Co deposited activities in the primary system of an old French 900 MWe PWR from cycle 2 to cycle 17. The pH$_{300°C}$ target was 7.0 for the 17 cycles, the pH$_{300°C}$ was below 6.9 for several dozens of days during cycles 1 and 2.

<figure><img src="image_15.png" /><figcaption>Fig. 15: $^{58}$Co deposited activities in a 900 MWe PWR</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 16: $^{60}$Co deposited activities in a 900 MWe PWR</figcaption></figure>

$^{58}$Co contamination increased until cycle 4 and then decreased on the primary coolant pipes. After some variations, the overall SG tubing $^{58}$Co contamination slightly increased.

$^{60}$Co contamination increases for 5 cycles and then tends to decreases.

These variations are consistent with the average ones. The decrease of the $^{58}$Co average contamination on the primary coolant pipes for the first cycles is due to some PWRs whose $^{58}$Co contamination was very high at cycle 1. As already pointed out (see para. III.A), there is the same variation after an SGR. This is probably due to a
$^{124}$Sb, an activation product of Antimony, originates from Be-Sb source rod failure or from pump bearing of the Boron Recycle System$^{17}$.

The average contribution of $^{58}$Co and $^{60}$Co versus cycles in French PWRs is shown in Fig. 14

<figure><img src="image_15.png" /><figcaption>Fig. 14. Average $^{58}$Co and $^{60}$Co contributions to the dose rates around primary loops</figcaption></figure>

We have distinguished the CP0 PWRs from the other PWRs. Indeed, the CP0 PWRs are characterized by a higher $^{60}$Co contribution to the dose rates. This high contribution is due to high $^{60}$Co activities (see para. III.B). At the end of the first cycle, the $^{58}$Co and $^{60}$Co contribution is practically fifty-fifty, and then $^{60}$Co becomes predominant and reaches about 80% (15% for $^{58}$Co) starting around the 10$^{th}$ cycle.

For the other PWRs, the average $^{58}$Co contribution remains predominant until the third cycle. Progressively, its contribution decreases giving way to $^{60}$Co. Stability is reached after around 10 cycles, the contributions are then about 30% for $^{58}$Co and about 65% for $^{60}$Co.

These evolutions are due to high $^{58}$Co activities during the first cycles, which decrease, and to the $^{60}$Co activity rise during about 10 cycles (see para. III). $^{60}$Co is the main contributor to the dose rate around primary components after several cycles because of its radioactive half-life (accumulation) and its radiation energy (two photons with an average energy of 1.25 MeV against one photon at 0.81 MeV for $^{58}$Co).

It should be noted that the cycles after an SGR have not been taken into account because SGRs significantly modify $^{58}$Co and $^{60}$Co contributions during several cycles$^8$.

V. DISCUSSION

In para. III, the primary system $^{60}$Co contamination in French PWRs was shown to have a tendency to increase

for several cycles and then to be constant or even to decrease, whereas $^{58}$Co contamination tends to decrease on primary coolant piping and to slightly increase on SG tubing. These are average evolutions, as a PWR can have more or less different variations. Consider the contamination evolution of a 900 MWe PWR (CPY).

Fig. 15 and Fig. 16 present the $^{58}$Co and $^{60}$Co deposited activities in the primary system of an old French 900 MWe PWR from cycle 2 to cycle 17. The pH$_{300°C}$ target was 7.0 for the 17 cycles, the pH$_{300°C}$ was below 6.9 for several dozens of days during cycles 1 and 2.

<figure><img src="image_16.png" /><figcaption>Fig. 15: $^{58}$Co deposited activities in a 900 MWe PWR</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 16: $^{60}$Co deposited activities in a 900 MWe PWR</figcaption></figure>

$^{58}$Co contamination increased until cycle 4 and then decreased on the primary coolant pipes. After some variations, the overall SG tubing $^{58}$Co contamination slightly increased.

$^{60}$Co contamination increases for 5 cycles and then tends to decreases.

These variations are consistent with the average ones. The decrease of the $^{58}$Co average contamination on the primary coolant pipes for the first cycles is due to some PWRs whose $^{58}$Co contamination was very high at cycle 1. As already pointed out (see para. III.A), there is the same variation after an SGR. This is probably due to a
higher Ni release of SG tubing during the formation of the passive layer. The first cycles, and especially the first startup of a PWR seem to be a major concern because they can induce effects lasting several cycles. A passivation procedure of SG tubing must be an important step during PWR commissioning.

Thanks to the improvement of the SG tubing manufactured by A (alloy 600TT and then 690TT), the 690TT SG tubing has a similar contamination level to the 600MA SG tubing manufactured by B (see Fig. 9) while the contamination level of the primary coolant pipes stays low. This has contributed to reducing the contamination in the primary system of French 1300 and 1450 MWe PWRs.

The effect of SG tubes manufactured by B or C (low deposits on the SG tubes but high on the primary coolant pipes) explains why the ⁶⁰Co deposited activities on the SG tubes in some CP0 PWRs are not as high as the ones on the primary coolant pipes (see Fig. 10 to Fig. 12). Moreover this effect has accentuated the high level of ⁶⁰Co deposited activities on the primary coolant pipes. This contamination in these CP0 PWRs could be due to the abnormal wear of Stellites. But generally, it results in the formation of hot spots¹⁸, i.e. ⁶⁰Co particles generating considerable dose rates. However, the high ⁶⁰Co contamination in CP0 PWRs was probably due to the use of Inconel spacer grids with a large Co content.

In Fig. 15 and Fig. 16, we observe a diminution of contamination from cycle 5 for ⁵⁸Co and cycle 6 for ⁶⁰Co, whereas the replacement of Inconel spacer grids by Zircaloy spacer grids began at cycle 5. It is very likely that these decreases were the consequence of the Inconel spacer grid removal, at least partly. The delay of one cycle for ⁶⁰Co is probably due to the longer ⁶⁰Co radioactive half-life.

The dose rate index of the considered PWR versus cycles is described in Fig. 17. This figure compared to Fig. 15 and Fig. 16 shows the difficulty in understanding the contamination variations. For example, the primary coolant pipe deposited activities increased between cycles 2 and 3 whereas the dose rate index decreased, or the primary system contamination decreased between cycles 12 and 17, whereas the dose rate index increased. But let us remember, on the one hand, the dose rate variations lower than 30% between two cycles are insignificant† and, on the other hand, the dose rate measurements greatly depend on the ambient dose rate¹¹. For instance, there is a factor of up to 4, or even more, between the measured dose rate and the dose rate coming from a hot leg. Actually, instead of measuring the dose rate coming from a hot leg, it is the dose rate coming from auxiliary piping that is measured, especially coming from the temperature

sensor bypass piping¹¹, a little pipe with a thickness of about 1 cm against about 7 cm for a hot leg. That is why these little pipes have been removed in the N4 plants. To mitigate ambient dose rates, the CEA has also developed a shielded dose rate measurement device.

<figure><img src="image_17.png" /><figcaption>Fig. 17 : Dose rate index of a 900 MWe PWR</figcaption></figure>

The average contamination ratios ⁵⁸Co/⁶⁰Co of the French PWRs are described in Fig. 18. The evolutions of the ⁵⁸Co and ⁶⁰Co deposited activities on the primary loops of French PWRs lead to a decrease of the average contamination ratios ⁵⁸Co/⁶⁰Co. We have distinguished CP0 PWRs from other PWRs. Indeed, CP0 PWRs present a lower ratio because of their higher ⁶⁰Co deposited activities. Standard deviations (1 σ) are indicated in the figure.

The ratios decrease for about the first 6 cycles. The mean value of the CP0 ratio is 6 at the first cycle and drops to about 1.1 and, for the other PWRs, it is 16 at the beginning and decreases down to about 2.8.

<figure><img src="image_18.png" /><figcaption>Fig. 18. Average contamination ratio ⁵⁸Co/⁶⁰Co in French PWRs</figcaption></figure>

Lastly, when reconsidering the average dose rates in French PWRs (see Fig. 1), we can now give some explanations about their variations.

<footnote>† The dose rate variation during several cycles is significant, particularly the decrease between cycles 1 and 17.</footnote>
higher Ni release of SG tubing during the formation of the passive layer. The first cycles, and especially the first startup of a PWR seem to be a major concern because they can induce effects lasting several cycles. A passivation procedure of SG tubing must be an important step during PWR commissioning.

Thanks to the improvement of the SG tubing manufactured by A (alloy 600TT and then 690TT), the 690TT SG tubing has a similar contamination level to the 600MA SG tubing manufactured by B (see Fig. 9) while the contamination level of the primary coolant pipes stays low. This has contributed to reducing the contamination in the primary system of French 1300 and 1450 MWe PWRs.

The effect of SG tubes manufactured by B or C (low deposits on the SG tubes but high on the primary coolant pipes) explains why the ⁶⁰Co deposited activities on the SG tubes in some CP0 PWRs are not as high as the ones on the primary coolant pipes (see Fig. 10 to Fig. 12). Moreover this effect has accentuated the high level of ⁶⁰Co deposited activities on the primary coolant pipes. This contamination in these CP0 PWRs could be due to the abnormal wear of Stellites. But generally, it results in the formation of hot spots¹⁸, i.e. ⁶⁰Co particles generating considerable dose rates. However, the high ⁶⁰Co contamination in CP0 PWRs was probably due to the use of Inconel spacer grids with a large Co content.

In Fig. 15 and Fig. 16, we observe a diminution of contamination from cycle 5 for ⁵⁸Co and cycle 6 for ⁶⁰Co, whereas the replacement of Inconel spacer grids by Zircaloy spacer grids began at cycle 5. It is very likely that these decreases were the consequence of the Inconel spacer grid removal, at least partly. The delay of one cycle for ⁶⁰Co is probably due to the longer ⁶⁰Co radioactive half-life.

The dose rate index of the considered PWR versus cycles is described in Fig. 17. This figure compared to Fig. 15 and Fig. 16 shows the difficulty in understanding the contamination variations. For example, the primary coolant pipe deposited activities increased between cycles 2 and 3 whereas the dose rate index decreased, or the primary system contamination decreased between cycles 12 and 17, whereas the dose rate index increased. But let us remember, on the one hand, the dose rate variations lower than 30% between two cycles are insignificant† and, on the other hand, the dose rate measurements greatly depend on the ambient dose rate¹¹. For instance, there is a factor of up to 4, or even more, between the measured dose rate and the dose rate coming from a hot leg. Actually, instead of measuring the dose rate coming from a hot leg, it is the dose rate coming from auxiliary piping that is measured, especially coming from the temperature

sensor bypass piping¹¹, a little pipe with a thickness of about 1 cm against about 7 cm for a hot leg. That is why these little pipes have been removed in the N4 plants. To mitigate ambient dose rates, the CEA has also developed a shielded dose rate measurement device.

<figure><img src="image_18.png" /><figcaption>Fig. 17 : Dose rate index of a 900 MWe PWR</figcaption></figure>

The average contamination ratios ⁵⁸Co/⁶⁰Co of the French PWRs are described in Fig. 18. The evolutions of the ⁵⁸Co and ⁶⁰Co deposited activities on the primary loops of French PWRs lead to a decrease of the average contamination ratios ⁵⁸Co/⁶⁰Co. We have distinguished CP0 PWRs from other PWRs. Indeed, CP0 PWRs present a lower ratio because of their higher ⁶⁰Co deposited activities. Standard deviations (1 σ) are indicated in the figure.

The ratios decrease for about the first 6 cycles. The mean value of the CP0 ratio is 6 at the first cycle and drops to about 1.1 and, for the other PWRs, it is 16 at the beginning and decreases down to about 2.8.

<figure><img src="image_19.png" /><figcaption>Fig. 18. Average contamination ratio ⁵⁸Co/⁶⁰Co in French PWRs</figcaption></figure>

Lastly, when reconsidering the average dose rates in French PWRs (see Fig. 1), we can now give some explanations about their variations.

<footnote>† The dose rate variation during several cycles is significant, particularly the decrease between cycles 1 and 17.</footnote>
The high mean dose rate index of CP0 PWRs is due to the great ⁶⁰Co deposited activities on the primary coolant pipes. Remember that the dose rate index characterizes the dose rates around the primary pipes. Once the cobalt source has been removed (replacement of Inconel spacer grids by Zircaloy grids) or exhausted at about cycle 10, the dose rate index dropped down to the CPY index.

The quasi stability of the CPY index for about ten cycles is due to the ⁵⁸Co contamination decrease counter-balanced by the relatively high ⁶⁰Co contamination. Then, thanks to the reduction of the average ⁵⁸Co and ⁶⁰Co contaminations after about 10 cycles, the 900 MWe PWR indices decrease. The contamination reduction is probably due to the release of SG tubing being lower than the release during the first cycles, to the replacement of the alloy 718 spacer grids by Zircaloy grids and to the improvement of chemistry (coordinated bore-lithium pH₃₀₀°C 7.0 and then 7.2). The SGRs contribute to contamination reduction, especially for ⁶⁰Co deposited activities. Their favourable effect on the ⁵⁸Co contamination is not generally immediate.

The lower dose rate indices of 1300 MWe PWRs are probably due to the improvement in SG tubing material (alloy 600TT and then 690TT), to the chemistry and grid replacement.

The 1450 MWe PWRs have benefited from the improvements of prior generations as early on as the first operating cycle: pH₃₀₀°C of 7.2, Zry spacer grids, 690TT SG tubing with a low cobalt content, reduction by about 20% of Stellite valves, removal of the temperature sensor bypass piping.

As seen in para. IV, ⁶⁰Co is the greatest contributor to the dose rates around the primary systems after several cycles, between 65 and 80%, against 15 and 30% for ⁵⁸Co. Even if we try to act on both radionuclides, in order to further reduce the dose rates around the primary systems, it would be better to act on ⁶⁰Co contamination rather than on ⁵⁸Co contamination, the potential profit being greater. Zinc injection could be a good candidate. Two experiments are planned on two CP0 PWRs and should provide answers.

## VI. CONCLUSIONS

To characterize primary system contamination in French PWRs, EDF, like foreign power utilities, uses dose rate indices.

However, behind constant values of dose rate index, there can be great variations in contamination that counter-balance each other, thus masking the phenomena. In particular, that is why the French strategy has been to develop a deposited activity measurement system (EMECC) under any conditions (all types of components with or without volume activities) and use it for more than 30 years in France and abroad.

Thus, this system has contributed to the studies of PWR design and operating parameters in order to reduce the dose rates around PWR primary systems. And, thanks to the improvements of French PWR design and operation, the dose rate indices tend to decrease with PWR generations and operating cycles.

## REFERENCES

1. S. ANTHONI and P. RIDOUX, "Origin of Corrosion Products in PWR Primary Circuits", Eurocorr '96, Nice 24-26 September 1996, Session IX Nuclear Corrosion and Protection, OR1, Nice (1996).

2. P.L. FRATTINI, J. BLOCK, S. CHAUFFRIAT, J. SAWICKI and J. RIDDLE, "Axial Offset Anomaly: Coupling PWR Primary Chemistry with Core Design", Proceedings of the International Conference of Water Chemistry of Nuclear Reactor systems 8, Bournemouth 22-26 October 2000, Vol. 1, p. 24-33, BNES, London (2000).

3. "Waterside Corrosion of Zirconium Alloys in Nuclear Power Plants", IAEA-TECDOC-996, p. 225-235, IAEA, Vienna (1998).

4. P. BESLU, G. FRÉJAVILLE and A. LALET, "A Computer Code PACTOLE to Predict Activation and Transport of Corrosion Products in a PWR", Proceedings of the International Conference of Water Chemistry of Nuclear Reactors Systems 1, Bournemouth 24-27 October 1977, p. 195-201, BNES, London (1978).

5. F. NGUYEN, H. MARTEAU, F. DACQUAIT, N. PEROT, G. RANCHOUX, L. GUINARD and A. LONG, "First Numerical Simulations of Contamination of the PWR Primary Circuit by Activated Corrosion Products with the PACTOLE V3.0 Code", this conference, (2004).

6. R. EIMECKE and S. ANTHONI, "Ensemble de Mesures et d'Etude de la Contamination des Circuits", Proceedings of the 7th International Conference on Radiation Shielding, Bournemouth 12-16 September 1988, Vol. 1, p.103-113, UKAEA & OECD, Winfrith (1988).

7. F. DACQUAIT, C. ANDRIEU, M. BERGER, J.L. BRETELLE and A. ROCHER, "Corrosion Product Transfer in French PWRs during Shutdown", Proc. of the International Conference on Water Chemistry in Nuclear Reactors Systems, Avignon 22-26 April 2002, paper 113, SFEN, Paris (2002).

8. F. DACQUAIT, S. ANTHONI, P. RIDOUX, C. BRUN and N. ENGLER, "Impact of Steam Generator Replacement on PWR Primary Circuit Contamination by Radioactive Corrosion products", Proceedings of the International Conference of Water Chemistry of Nuclear Reactor systems 8,
The high mean dose rate index of CP0 PWRs is due to the great ⁶⁰Co deposited activities on the primary coolant pipes. Remember that the dose rate index characterizes the dose rates around the primary pipes. Once the cobalt source has been removed (replacement of Inconel spacer grids by Zircaloy grids) or exhausted at about cycle 10, the dose rate index dropped down to the CPY index.

The quasi stability of the CPY index for about ten cycles is due to the ⁵⁸Co contamination decrease counter-balanced by the relatively high ⁶⁰Co contamination. Then, thanks to the reduction of the average ⁵⁸Co and ⁶⁰Co contaminations after about 10 cycles, the 900 MWe PWR indices decrease. The contamination reduction is probably due to the release of SG tubing being lower than the release during the first cycles, to the replacement of the alloy 718 spacer grids by Zircaloy grids and to the improvement of chemistry (coordinated bore-lithium pH₃₀₀°C 7.0 and then 7.2). The SGRs contribute to contamination reduction, especially for ⁶⁰Co deposited activities. Their favourable effect on the ⁵⁸Co contamination is not generally immediate.

The lower dose rate indices of 1300 MWe PWRs are probably due to the improvement in SG tubing material (alloy 600TT and then 690TT), to the chemistry and grid replacement.

The 1450 MWe PWRs have benefited from the improvements of prior generations as early on as the first operating cycle: pH₃₀₀°C of 7.2, Zry spacer grids, 690TT SG tubing with a low cobalt content, reduction by about 20% of Stellite valves, removal of the temperature sensor bypass piping.

As seen in para. IV, ⁶⁰Co is the greatest contributor to the dose rates around the primary systems after several cycles, between 65 and 80%, against 15 and 30% for ⁵⁸Co. Even if we try to act on both radionuclides, in order to further reduce the dose rates around the primary systems, it would be better to act on ⁶⁰Co contamination rather than on ⁵⁸Co contamination, the potential profit being greater. Zinc injection could be a good candidate. Two experiments are planned on two CP0 PWRs and should provide answers.

## VI. CONCLUSIONS

To characterize primary system contamination in French PWRs, EDF, like foreign power utilities, uses dose rate indices.

However, behind constant values of dose rate index, there can be great variations in contamination that counter-balance each other, thus masking the phenomena. In particular, that is why the French strategy has been to develop a deposited activity measurement system (EMECC) under any conditions (all types of components with or without volume activities) and use it for more than 30 years in France and abroad.

Thus, this system has contributed to the studies of PWR design and operating parameters in order to reduce the dose rates around PWR primary systems. And, thanks to the improvements of French PWR design and operation, the dose rate indices tend to decrease with PWR generations and operating cycles.

## REFERENCES

1. S. ANTHONI and P. RIDOUX, "Origin of Corrosion Products in PWR Primary Circuits", Eurocorr '96, Nice 24-26 September 1996, Session IX Nuclear Corrosion and Protection, OR1, Nice (1996).

2. P.L. FRATTINI, J. BLOCK, S. CHAUFFRIAT, J. SAWICKI and J. RIDDLE, "Axial Offset Anomaly: Coupling PWR Primary Chemistry with Core Design", Proceedings of the International Conference of Water Chemistry of Nuclear Reactor systems 8, Bournemouth 22-26 October 2000, Vol. 1, p. 24-33, BNES, London (2000).

3. "Waterside Corrosion of Zirconium Alloys in Nuclear Power Plants", IAEA-TECDOC-996, p. 225-235, IAEA, Vienna (1998).

4. P. BESLU, G. FRÉJAVILLE and A. LALET, "A Computer Code PACTOLE to Predict Activation and Transport of Corrosion Products in a PWR", Proceedings of the International Conference of Water Chemistry of Nuclear Reactors Systems 1, Bournemouth 24-27 October 1977, p. 195-201, BNES, London (1978).

5. F. NGUYEN, H. MARTEAU, F. DACQUAIT, N. PEROT, G. RANCHOUX, L. GUINARD and A. LONG, "First Numerical Simulations of Contamination of the PWR Primary Circuit by Activated Corrosion Products with the PACTOLE V3.0 Code", this conference, (2004)

6. R. EIMECKE and S. ANTHONI, "Ensemble de Mesures et d'Etude de la Contamination des Circuits", Proceedings of the 7th International Conference on Radiation Shielding, Bournemouth 12-16 September 1988, Vol. 1, p.103-113, UKAEA & OECD, Winfrith (1988).

7. F. DACQUAIT, C. ANDRIEU, M. BERGER, J.L. BRETELLE and A. ROCHER, "Corrosion Product Transfer in French PWRs during Shutdown", Proc. of the International Conference on Water Chemistry in Nuclear Reactors Systems, Avignon 22-26 April 2002, paper 113, SFEN, Paris (2002).

8. F. DACQUAIT, S. ANTHONI, P. RIDOUX, C. BRUN and N. ENGLER, "Impact of Steam Generator Replacement on PWR Primary Circuit Contamination by Radioactive Corrosion products", Proceedings of the International Conference of Water Chemistry of Nuclear Reactor systems 8,
Bournemouth 22-26 October 2000, Vol. 1, p. 53-58,
BNES, London (2000).

9 S. ANTHONI, P. RIDOUX, O. MENET and C. WEBER, “Effects of pH of Primary Coolant on PWR Contamination”, Proceedings of the International Conference of Water Chemistry of Nuclear Reactor systems 6, Bournemouth 12-15 October 1992, Vol. 2, p. 9-15, BNES, London (1992).

10 J.L. BRETELLE, L. GUINARD, C. ANDRIEU and F. DACQUAIT, “EDF Strategy for the PWR Primary Coolant pH”, this conference, (2004)

11 P. BESLU, S. ANTHONI, A. BRISSAUD, P. RIDOUX, C. CHEVALIER and P. SAURIN, “Trend of Plant Radiation Field of French reactors: Analysis and perspectives”, Proceedings of the International Conference of Water Chemistry of Nuclear Reactor systems 5, Bournemouth 23-27 October 1989, Vol. 2, p. 1-6, BNES, London (1989).

12 P. BESLU, Discussion on Papers 1-5, Proceedings of the International Conference of Water Chemistry of Nuclear Reactor systems 5, Bournemouth 23-27 October 1989, Vol. 2, p. 19-21, BNES, London (1989).

13 J. HAAG, D. NIEDER, P. SCHNEIDER-KUHNLE, D. WOLTER and B. STELLWAG, “Operational Experience with Zinc Addition in a German PWR”, Proceedings of the 1998 JAIF International Conference on Water Chemistry in Nuclear Power Plants, Kashiwasaki 13-16 October 1998, p. 505-509, JAIF, Tokyo (1998).

14 H. OCKEN, K. FRUZZETTI, P. FRATTINI and C.J. WOOD “Recent Developments in PWR Zinc Injection”, Proceedings of the International Conference on Water Chemistry in Nuclear Reactor systems, Avignon 22-26 April 2002, paper 41, SFEN, Paris (2002).

15 B. STELLWAG, M. JUERGENSEN and D. WOLTER “Zinc Injection in German PWR Plants”, Proceedings of the International Conference on Water Chemistry in Nuclear Reactor systems, Avignon 22-26 April 2002, paper 93, SFEN, Paris (2002).

16 C. SUTEAU, M. CHIRON and G. ARNAUD "Improvement of MERCURE-6's General Formalism for Calculating Gamma-Ray Buildup Factors in Multilayer Shields", Nucar Science and Engineering 147, p. 43-55 (2004)

17 J.L. BRETELLE, A. ROCHER, M. BERGER, F. DACQUAIT and R.ROSSET “Study of Various Chemical Species Behaviour for Contamination Risk”, Proceedings of the International Conference on Water Chemistry in Nuclear Reactor systems, Avignon 22-26 April 2002, paper 123, SFEN, Paris (2002).

18 A. ROCHER, P. RIDOUX, S. ANTHONI and C. BRUN, “Programme to Eradicate Hot Spots (Co-60) Carried out in French PWR Units”, Proceedings of the 1998 JAIF International Conference on Water Chemistry in Nuclear Power Plants, Kashiwasaki 13-16 October 1998, p. 161-166, JAIF, Tokyo (1998).
Bournemouth 22-26 October 2000, Vol. 1, p. 53-58,
BNES, London (2000).

9 S. ANTHONI, P. RIDOUX, O. MENET and C. WEBER, “Effects of pH of Primary Coolant on PWR Contamination”, Proceedings of the International Conference of Water Chemistry of Nuclear Reactor systems 6, Bournemouth 12-15 October 1992, Vol. 2, p. 9-15, BNES, London (1992).

10 J.L. BRETELLE, L. GUINARD, C. ANDRIEU and F. DACQUAIT, “EDF Strategy for the PWR Primary Coolant pH”, this conference, (2004)

11 P. BESLU, S. ANTHONI, A. BRISSAUD, P. RIDOUX, C. CHEVALIER and P. SAURIN, “Trend of Plant Radiation Field of French reactors: Analysis and perspectives”, Proceedings of the International Conference of Water Chemistry of Nuclear Reactor systems 5, Bournemouth 23-27 October 1989, Vol. 2, p. 1-6, BNES, London (1989).

12 P. BESLU, Discussion on Papers 1-5, Proceedings of the International Conference of Water Chemistry of Nuclear Reactor systems 5, Bournemouth 23-27 October 1989, Vol. 2, p. 19-21, BNES, London (1989).

13 J. HAAG, D. NIEDER, P. SCHNEIDER-KUHNLE, D. WOLTER and B. STELLWAG, “Operational Experience with Zinc Addition in a German PWR”, Proceedings of the 1998 JAIF International Conference on Water Chemistry in Nuclear Power Plants, Kashiwasaki 13-16 October 1998, p. 505-509, JAIF, Tokyo (1998).

14 H. OCKEN, K. FRUZZETTI, P. FRATTINI and C.J. WOOD “Recent Developments in PWR Zinc Injection”, Proceedings of the International Conference on Water Chemistry in Nuclear Reactor systems, Avignon 22-26 April 2002, paper 41, SFEN, Paris (2002).

15 B. STELLWAG, M. JUERGENSEN and D. WOLTER “Zinc Injection in German PWR Plants”, Proceedings of the International Conference on Water Chemistry in Nuclear Reactor systems, Avignon 22-26 April 2002, paper 93, SFEN, Paris (2002).

16 C. SUTEAU, M. CHIRON and G. ARNAUD "Improvement of MERCURE-6's General Formalism for Calculating Gamma-Ray Buildup Factors in Multilayer Shields", Nucar Science and Engineering 147, p. 43-55 (2004)

17 J.L. BRETELLE, A. ROCHER, M. BERGER, F. DACQUAIT and R.ROSSET “Study of Various Chemical Species Behaviour for Contamination Risk”, Proceedings of the International Conference on Water Chemistry in Nuclear Reactor systems, Avignon 22-26 April 2002, paper 123, SFEN, Paris (2002).

18 A. ROCHER, P. RIDOUX, S. ANTHONI and C. BRUN, “Programme to Eradicate Hot Spots (Co-60) Carried out in French PWR Units”, Proceedings of the 1998 JAIF International Conference on Water Chemistry in Nuclear Power Plants, Kashiwasaki 13-16 October 1998, p. 161-166, JAIF, Tokyo (1998).