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# Low primary system contamination levels in some French PWRs

Frédéric Dacquait, Luc Guinard, Florence Bardet, Jean-Luc Bretelle, Alain Rocher

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Frédéric Dacquait, Luc Guinard, Florence Bardet, Jean-Luc Bretelle, Alain Rocher. Low primary system contamination levels in some French PWRs. Nuclear Plant chemistry conference, Oct 2006, Jeju, South Korea. session 2 paper 17. cea-04463615

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# Low primary system contamination levels in some French PWRs

F. Dacquait¹, L. Guinard², F. Bardet³, J.L. Bretelle⁴, A. Rocher⁴

¹CEA (Commissariat à l'Energie Atomique), Centre de Cadarache, DEN, 13108 Saint Paul Lez Durance,
France, frederic.dacquait@cea.fr

²EDF (Electricité De France), SEPTEN, 12-14 rue Dutrievoz, 69628 Villeurbanne, France

³EDF, CEIDRE, 2 rue Ampère, 93206 Saint Denis, France

⁴EDF, CAPE, 1 place Pleyel, 93282 Saint Denis, France

**Abstract ---** PWRs equipped with Steam Generator tubing in a nickel-based alloy (alloy 600 or 690) are generally characterized by higher ⁵⁸Co peaks during cold shutdowns (order of magnitude of 100 GBq/m³) and higher ⁵⁸Co deposited activities (several GBq/m²) compared to PWRs equipped with SG tubing in an iron-based alloy (alloy 800 or stainless steel). However, the trends are different for the Steam Generator Replacement (SGR) with alloy 690 carried out in the last decade, in particular for the SAINT-LAURENT-B1 PWR: after an increase in the ⁵⁸Co contamination just after the SGR, the contamination on the primary system decreases and reaches a very low level several cycles after the SGR. The ⁵⁸Co peak during cold shutdowns follows the same variation, it drops by a factor of 10 to 100. Similar levels are also measured in the last French operating PWR generation (N4). The key factor of these low ⁵⁸Co deposited and released activities is the surface finish of alloy 690TT SG tubing by means of an improved manufacturing process. The nickel release is low enough to limit the presence of metallic nickel or nickel oxide, which tend to form deposits on fuel elements. In consequence, nickel deposits on fuel elements and then ⁵⁸Co volume and surface activities are reduced.

## I. INTRODUCTION

PWRs equipped with steam generator tubing in a nickel-based alloy (alloy 600 or 690) are generally characterized by higher ⁵⁸Co peaks during cold shutdowns and higher ⁵⁸Co deposited activities compared to PWRs equipped with SG tubing in an iron-based alloy (alloy 800 or stainless steel).

However, in some French PWRs with alloy 690TT SG tubing, the ⁵⁸Co deposited and released activities are very low and close to those of PWRs with alloy 800.

This paper presents the variation of the deposited and volume activities of a French PWR, SAINT-LAURENT-B1, which has low contamination levels. After the presentation of typical volume and surface activities in French PWRs and of the SAINT-LAURENT-B1 case, the analysis and the reasons for these low contamination levels in some PWRs are given.

## II. USUAL VOLUME AND SURFACE ACTIVITIES IN FRENCH PWRs

### II.A. Volume activities during cold shutdowns

The physicochemical conditions vary significantly during cold shutdown in French PWRs: the power and temperature decrease and the chemical conditions are modified (decrease of pH and change from reducing to oxidizing conditions). These changes involve corrosion

product releases in the primary water (dissolving and crud bursts), which can be seen in general by a considerable corrosion product increase, from 2 to 3 orders of magnitude, in the activity concentrations (about a hundred GBq/m³ of ⁵⁸Co and about one GBq/m³ of ⁶⁰Co...) (see Fig. 1) and in the chemical species concentrations (in particular, several ppm of nickel) (see Fig. 2)¹.

The large quantity of corrosion products released during shutdown mainly comes from the dissolution of the in-core deposits due to primary system oxygenation, in particular the dissolution of metallic nickel (Ni⁰) or nickel oxide (NiO), leading to the dissolution of the ⁵⁸Co derived from the nickel sites in the crud. Indeed, the high ⁵⁸Co/⁶⁰Co ratio (> about 100) and the ⁵⁸Co specific activity, ⁵⁸Co/Ni, (> about 25 GBq/g) show that an in-core nickel deposit dissolves (see Fig. 1 and Fig. 2). During power operation, the nickel tends to precipitate on fuel elements because of the nickel solubility in relation to the Ni⁰ or NiO, which decreases as temperature increases (retrograde solubility). The order of magnitude of the ⁵⁸Co total released activity during a typical shutdown is several 10 TBq or even about 100 TBq.

Oxygenation thus creates high activity peaks. For example, the statistical data of the ⁵⁸Co peak values of the French PWRs in 2000 are given in Table 1.
# Low primary system contamination levels in some French PWRs

F. Dacquait¹, L. Guinard², F. Bardet³, J.L. Bretelle⁴, A. Rocher⁴

¹CEA (Commissariat à l'Energie Atomique), Centre de Cadarache, DEN, 13108 Saint Paul Lez Durance,
France, frederic.dacquait@cea.fr

²EDF (Electricité De France), SEPTEN, 12-14 rue Dutrievoz, 69628 Villeurbanne, France

³EDF, CEIDRE, 2 rue Ampère, 93206 Saint Denis, France

⁴EDF, CAPE, 1 place Pleyel, 93282 Saint Denis, France

**Abstract ---** PWRs equipped with Steam Generator tubing in a nickel-based alloy (alloy 600 or 690) are generally characterized by higher ⁵⁸Co peaks during cold shutdowns (order of magnitude of 100 GBq/m³) and higher ⁵⁸Co deposited activities (several GBq/m²) compared to PWRs equipped with SG tubing in an iron-based alloy (alloy 800 or stainless steel). However, the trends are different for the Steam Generator Replacement (SGR) with alloy 690 carried out in the last decade, in particular for the SAINT-LAURENT-B1 PWR: after an increase in the ⁵⁸Co contamination just after the SGR, the contamination on the primary system decreases and reaches a very low level several cycles after the SGR. The ⁵⁸Co peak during cold shutdowns follows the same variation, it drops by a factor of 10 to 100. Similar levels are also measured in the last French operating PWR generation (N4). The key factor of these low ⁵⁸Co deposited and released activities is the surface finish of alloy 690TT SG tubing by means of an improved manufacturing process. The nickel release is low enough to limit the presence of metallic nickel or nickel oxide, which tend to form deposits on fuel elements. In consequence, nickel deposits on fuel elements and then ⁵⁸Co volume and surface activities are reduced.

## I. INTRODUCTION

PWRs equipped with steam generator tubing in a nickel-based alloy (alloy 600 or 690) are generally characterized by higher ⁵⁸Co peaks during cold shutdowns and higher ⁵⁸Co deposited activities compared to PWRs equipped with SG tubing in an iron-based alloy (alloy 800 or stainless steel).

However, in some French PWRs with alloy 690TT SG tubing, the ⁵⁸Co deposited and released activities are very low and close to those of PWRs with alloy 800.

This paper presents the variation of the deposited and volume activities of a French PWR, SAINT-LAURENT-B1, which has low contamination levels. After the presentation of typical volume and surface activities in French PWRs and of the SAINT-LAURENT-B1 case, the analysis and the reasons for these low contamination levels in some PWRs are given.

## II. USUAL VOLUME AND SURFACE ACTIVITIES IN FRENCH PWRs

### II.A. Volume activities during cold shutdowns

The physicochemical conditions vary significantly during cold shutdown in French PWRs: the power and temperature decrease and the chemical conditions are modified (decrease of pH and change from reducing to oxidizing conditions). These changes involve corrosion

product releases in the primary water (dissolving and crud bursts), which can be seen in general by a considerable corrosion product increase, from 2 to 3 orders of magnitude, in the activity concentrations (about a hundred GBq/m³ of ⁵⁸Co and about one GBq/m³ of ⁶⁰Co...) (see Fig. 1) and in the chemical species concentrations (in particular, several ppm of nickel) (see Fig. 2)¹.

The large quantity of corrosion products released during shutdown mainly comes from the dissolution of the in-core deposits due to primary system oxygenation, in particular the dissolution of metallic nickel (Ni⁰) or nickel oxide (NiO), leading to the dissolution of the ⁵⁸Co derived from the nickel sites in the crud. Indeed, the high ⁵⁸Co/⁶⁰Co ratio (> about 100) and the ⁵⁸Co specific activity, ⁵⁸Co/Ni, (> about 25 GBq/g) show that an in-core nickel deposit dissolves (see Fig. 1 and Fig. 2). During power operation, the nickel tends to precipitate on fuel elements because of the nickel solubility in relation to the Ni⁰ or NiO, which decreases as temperature increases (retrograde solubility). The order of magnitude of the ⁵⁸Co total released activity during a typical shutdown is several 10 TBq or even about 100 TBq.

Oxygenation thus creates high activity peaks. For example, the statistical data of the ⁵⁸Co peak values of the French PWRs in 2000 are given in Table 1.
<figure><img src="image_1.png" /><figcaption>Fig. 1: Typical <sup>58</sup>Co and <sup>60</sup>Co volume activities and ratio during a shutdown</figcaption></figure>

<figure><img src="image_2.png" /><figcaption>Fig. 2: Typical <sup>58</sup>Co and Ni concentrations and ratio during a shutdown</figcaption></figure>

<table><caption>Table 1: Mean peaks during refuelling shutdowns in 2000</caption><thead><tr><th>PWR</th><th><sup>58</sup>Co peak + 1 σ<br>(GBq/m<sup>3</sup>)</th><th><sup>60</sup>Co peak<br>(GBq/m<sup>3</sup>)</th></tr></thead><tbody><tr><td>900 MWe</td><td>103 ± 47</td><td>0.7</td></tr><tr><td>1300 MWe</td><td>165 ± 64</td><td>1.0</td></tr></tbody></table>

## II.B. Volume activities during cycles

From the third cycle of French PWRs, the range of the typical mean volume activities in the primary coolant during cycles is shown in Table 2.

<table><caption>Table 2: Range of typical mean volume activities during cycles in French PWRs<sup>2</sup></caption><thead><tr><th></th><th>MBq/m<sup>3</sup></th><th></th><th>Ratio</th></tr></thead><tbody><tr><td><sup>58</sup>Co</td><td>8-25</td><td></td><td></td></tr><tr><td><sup>60</sup>Co</td><td>1-3</td><td><sup>58</sup>Co/<sup>60</sup>Co</td><td>5-12</td></tr><tr><td><sup>54</sup>Mn</td><td>1-2.5</td><td></td><td></td></tr><tr><td><sup>51</sup>Cr</td><td>6-15</td><td><sup>58</sup>Co/<sup>51</sup>Cr</td><td>1.25-3</td></tr></tbody></table>

In some cases, particularly in case of AOA, the <sup>58</sup>Co and <sup>51</sup>Cr volume activities can be greater than 100 MBq/m<sup>3</sup> and are mainly transported by particles.

## II.C. Surface activities

The deposited activities in many PWR primary systems have been measured by the EMECC device<sup>3</sup> for more than 30 years. The values are presented in Fig. 6 to Fig. 11 (see DACQUAIT *et al.*<sup>4</sup> for more information and explanations about the contamination levels and variations). The mean values of the primary system contamination of French 900 MWe PWRs after the 10<sup>th</sup> cycle are provided in Table 3.

<table><caption>Table 3: Average deposited activities in the primary system of French 900 MWe PWRs after the 10<sup>th</sup> cycle</caption><thead><tr><th>GBq/m<sup>2</sup></th><th><sup>58</sup>Co ± 1 σ</th><th><sup>60</sup>Co ± 1 σ</th><th><sup>58</sup>Co/<sup>60</sup>Co</th></tr></thead><tbody><tr><td>Hot legs</td><td>3.3 ± 1.6</td><td>2.3 ± 1.6<sup>a</sup></td><td>1.4</td></tr><tr><td>Crossover/cold legs</td><td>4.9 ± 2.4</td><td>2.0 ± 1.0<sup>a</sup></td><td>2.5</td></tr><tr><td>SG tubing</td><td>4.3 ± 2.1</td><td>1.4 ± 0.5</td><td>3.3</td></tr></tbody></table>

## III. LOW FRENCH PWR CONTAMINATION – SLB1 CASE

### III.A. SLB1 - Volume activities during cold shutdowns

Over the last few years, the <sup>58</sup>Co peak during cold shutdowns has decreased significantly in some French PWRs: it is below 10 GBq/m<sup>3</sup> or even 1 GBq/m<sup>3</sup>.

In the SAINT-LAURENT-B1 PWR (SLB1)<sup>b</sup>, the <sup>58</sup>Co peak is 834 MBq/m<sup>3</sup> during the refueling shutdown at the end of cycle 20 (see Fig. 3). This low peak is not due to an incomplete oxygenation; the O<sub>2</sub> content in the primary circuit reaches several ppm just after the H<sub>2</sub>O<sub>2</sub> injection. Nor is it due to an unscheduled shutdown with oxygenation just before the end of cycle 20<sup>1</sup>.

The <sup>58</sup>Co total released activity during this shutdown is about 700 GBq as opposed to the usual several 10 TBq.

The <sup>58</sup>Co/<sup>60</sup>Co ratio remains practically constant, at a value below around 5, during the entire cold shutdown, whereas usually it exceeds 100 after the H<sub>2</sub>O<sub>2</sub> injection.

<footnote><sup>a</sup> The <sup>60</sup>Co contamination values of the CP0 primary pipes are not taken into account</footnote>

<footnote><sup>b</sup> SLB1 is a 900 MWe PWR, which has undergone a Steam Generator Replacement (SGR) during the refueling shutdown at the end of cycle 11 (old SG tubing in 600MA / new SG tubing in 690TT).</footnote>
<figure><img src="image_3.png" /><figcaption>Fig. 1: Typical <sup>58</sup>Co and <sup>60</sup>Co volume activities and ratio during a shutdown</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2: Typical <sup>58</sup>Co and Ni concentrations and ratio during a shutdown</figcaption></figure>

<table><caption>Table 1: Mean peaks during refuelling shutdowns in 2000</caption><thead><tr><th>PWR</th><th><sup>58</sup>Co peak + 1 σ<br>(GBq/m<sup>3</sup>)</th><th><sup>60</sup>Co peak<br>(GBq/m<sup>3</sup>)</th></tr></thead><tbody><tr><td>900 MWe</td><td>103 ± 47</td><td>0.7</td></tr><tr><td>1300 MWe</td><td>165 ± 64</td><td>1.0</td></tr></tbody></table>

### II.B. Volume activities during cycles

From the third cycle of French PWRs, the range of the typical mean volume activities in the primary coolant during cycles is shown in Table 2.

<table><caption>Table 2: Range of typical mean volume activities during cycles in French PWRs<sup>2</sup></caption><thead><tr><th></th><th>MBq/m<sup>3</sup></th><th></th><th>Ratio</th></tr></thead><tbody><tr><td><sup>58</sup>Co</td><td>8-25</td><td></td><td></td></tr><tr><td><sup>60</sup>Co</td><td>1-3</td><td><sup>58</sup>Co/<sup>60</sup>Co</td><td>5-12</td></tr><tr><td><sup>54</sup>Mn</td><td>1-2.5</td><td></td><td></td></tr><tr><td><sup>51</sup>Cr</td><td>6-15</td><td><sup>58</sup>Co/<sup>51</sup>Cr</td><td>1.25-3</td></tr></tbody></table>

In some cases, particularly in case of AOA, the <sup>58</sup>Co and <sup>51</sup>Cr volume activities can be greater than 100 MBq/m<sup>3</sup> and are mainly transported by particles.

### II.C. Surface activities

The deposited activities in many PWR primary systems have been measured by the EMECC device<sup>3</sup> for more than 30 years. The values are presented in Fig. 6 to Fig. 11 (see DACQUAIT *et al.*<sup>4</sup> for more information and explanations about the contamination levels and variations). The mean values of the primary system contamination of French 900 MWe PWRs after the 10<sup>th</sup> cycle are provided in Table 3.

<table><caption>Table 3: Average deposited activities in the primary system of French 900 MWe PWRs after the 10<sup>th</sup> cycle</caption><thead><tr><th>GBq/m<sup>2</sup></th><th><sup>58</sup>Co ± 1 σ</th><th><sup>60</sup>Co ± 1 σ</th><th><sup>58</sup>Co/<sup>60</sup>Co</th></tr></thead><tbody><tr><td>Hot legs</td><td>3.3 ± 1.6</td><td>2.3 ± 1.6<sup>a</sup></td><td>1.4</td></tr><tr><td>Crossover/cold legs</td><td>4.9 ± 2.4</td><td>2.0 ± 1.0<sup>a</sup></td><td>2.5</td></tr><tr><td>SG tubing</td><td>4.3 ± 2.1</td><td>1.4 ± 0.5</td><td>3.3</td></tr></tbody></table>

## III. LOW FRENCH PWR CONTAMINATION – SLB1 CASE

### III.A. SLB1 - Volume activities during cold shutdowns

Over the last few years, the <sup>58</sup>Co peak during cold shutdowns has decreased significantly in some French PWRs: it is below 10 GBq/m<sup>3</sup> or even 1 GBq/m<sup>3</sup>.

In the SAINT-LAURENT-B1 PWR (SLB1)<sup>b</sup>, the <sup>58</sup>Co peak is 834 MBq/m<sup>3</sup> during the refueling shutdown at the end of cycle 20 (see Fig. 3). This low peak is not due to an incomplete oxygenation; the O<sub>2</sub> content in the primary circuit reaches several ppm just after the H<sub>2</sub>O<sub>2</sub> injection. Nor is it due to an unscheduled shutdown with oxygenation just before the end of cycle 20<sup>1</sup>.

The <sup>58</sup>Co total released activity during this shutdown is about 700 GBq as opposed to the usual several 10 TBq.

The <sup>58</sup>Co/<sup>60</sup>Co ratio remains practically constant, at a value below around 5, during the entire cold shutdown, whereas usually it exceeds 100 after the H<sub>2</sub>O<sub>2</sub> injection.

<footnote><sup>a</sup> The <sup>60</sup>Co contamination values of the CP0 primary pipes are not taken into account</footnote>

<footnote><sup>b</sup> SLB1 is a 900 MWe PWR, which has undergone a Steam Generator Replacement (SGR) during the refueling shutdown at the end of cycle 11 (old SG tubing in 600MA / new SG tubing in 690TT).</footnote>
<figure><img src="image_3.png" /><figcaption>Fig. 3: SLB120 - ⁵⁸Co and ⁶⁰Co volume activities and ratio during the refueling shutdown</figcaption></figure>

The maximum nickel concentration is 0.292 ppm during the primary system oxygenation (see Fig. 4) whereas usually it is several ppm. And the ⁵⁸Co specific activity is practically constant during the shutdown, between 2 and 3 GBq/g, whereas it is about 10 times greater after oxygenation during a typical shutdown.

<figure><img src="image_4.png" /><figcaption>Fig. 4: SLB120 - ⁵⁸Co and Ni concentrations and ratio during the refueling shutdown</figcaption></figure>

### III.B. SLB1 – Volume activities during cycle 20

Respectively, Fig. 5 and Table 4 show the volume activities during cycle 20 of SLB1 and their average.

It is to be noted that the ⁵⁸Co volume activities are low during cycle 20. Indeed, compared to Table 2, the average ⁶⁰Co, ⁵⁴Mn and ⁵¹Cr values are included in the typical volume activity ranges of French PWRs, but the average ⁵⁸Co volume activity and the ratios ⁵⁸Co/⁶⁰Co and ⁵⁸Co/⁵¹Cr are lower by a factor of 3 to 10.

<figure><img src="image_5.png" /><figcaption>Fig. 5: SLB1 – Volume activities during cycle 20</figcaption></figure>

<table><caption>Table 4: Average volume activities in SLB120</caption><thead><tr><th></th><th>MBq/m³</th><th></th><th>Ratio</th></tr></thead><tbody><tr><td>⁵⁸Co</td><td>3.2</td><td></td><td></td></tr><tr><td>⁶⁰Co</td><td>2.1</td><td>⁵⁸Co/⁶⁰Co</td><td>1.5</td></tr><tr><td>⁵⁴Mn</td><td>1.5</td><td></td><td></td></tr><tr><td>⁵¹Cr</td><td>9.5</td><td>⁵⁸Co/⁵¹Cr</td><td>0.3</td></tr></tbody></table>

### III.C. SLB1 – Surface activities

The ⁵⁸Co and ⁶⁰Co deposited activities in the primary system (hot legs, cold legs and steam generator tube bundles) of SAINT-LAURENT-B1 and their comparison to the contamination level ranges of the different French PWR types are shown in Fig. 6 to Fig. 11. EMECC campaigns were carried out in SLB1 before the SGR of cycle 11 (cycles 9, 10, 11) and after the SGR (cycles 13 and 20).

The values of the SLB1 primary system surface activities at cycle 20 are given in Table 5.

<table><caption>Table 5: Deposited activities in the SLB1 primary system at cycle 20</caption><thead><tr><th>GBq/m²</th><th>⁵⁸Co ± 2σ</th><th>⁶⁰Co ± 2σ</th><th>⁵⁸Co/⁶⁰Co</th></tr></thead><tbody><tr><td>Hot legs</td><td>1.4 ± 32%</td><td>1.5 ± 23%</td><td>0.93</td></tr><tr><td>Crossover legs</td><td>2.0 ± 32%</td><td>1.2 ± 23%</td><td>1.7</td></tr><tr><td>SG tubing<br>(average hot and cold sides)</td><td>0.45 ± 33%</td><td>0.26 ± 23%</td><td>1.7</td></tr></tbody></table>

The SGR impact on the SLB1 primary system contamination is consistent with the analysis presented in 2000⁵.

The ⁵⁸Co deposited activities increase in the primary pipes after the SGR, then decrease between cycle 13 and 20 by a factor of about 6 in the crossover legs and 4 in the hot legs. They reach some of the lowest levels of the French nuclear fleet, whereas they were within the average or even among the highest before the SGR (a
<figure><img src="image_5.png" /><figcaption>Fig. 3: SLB120 - <sup>58</sup>Co and <sup>60</sup>Co volume activities and ratio during the refueling shutdown</figcaption></figure>

The maximum nickel concentration is 0.292 ppm during the primary system oxygenation (see Fig. 4) whereas usually it is several ppm. And the <sup>58</sup>Co specific activity is practically constant during the shutdown, between 2 and 3 GBq/g, whereas it is about 10 times greater after oxygenation during a typical shutdown.

<figure><img src="image_6.png" /><figcaption>Fig. 4: SLB120 - <sup>58</sup>Co and Ni concentrations and ratio during the refueling shutdown</figcaption></figure>

### III.B. SLB1 – Volume activities during cycle 20

Respectively, Fig. 5 and Table 4 show the volume activities during cycle 20 of SLB1 and their average.

It is to be noted that the <sup>58</sup>Co volume activities are low during cycle 20. Indeed, compared to Table 2, the average <sup>60</sup>Co, <sup>54</sup>Mn and <sup>51</sup>Cr values are included in the typical volume activity ranges of French PWRs, but the average <sup>58</sup>Co volume activity and the ratios <sup>58</sup>Co/<sup>60</sup>Co and <sup>58</sup>Co/<sup>51</sup>Cr are lower by a factor of 3 to 10.

<figure><img src="image_7.png" /><figcaption>Fig. 5: SLB1 – Volume activities during cycle 20</figcaption></figure>

<table><caption>Table 4: Average volume activities in SLB120</caption><thead><tr><th></th><th>MBq/m³</th><th></th><th>Ratio</th></tr></thead><tbody><tr><td><sup>58</sup>Co</td><td>3.2</td><td></td><td></td></tr><tr><td><sup>60</sup>Co</td><td>2.1</td><td><sup>58</sup>Co/<sup>60</sup>Co</td><td>1.5</td></tr><tr><td><sup>54</sup>Mn</td><td>1.5</td><td></td><td></td></tr><tr><td><sup>51</sup>Cr</td><td>9.5</td><td><sup>58</sup>Co/<sup>51</sup>Cr</td><td>0.3</td></tr></tbody></table>

### III.C. SLB1 – Surface activities

The <sup>58</sup>Co and <sup>60</sup>Co deposited activities in the primary system (hot legs, cold legs and steam generator tube bundles) of SAINT-LAURENT-B1 and their comparison to the contamination level ranges of the different French PWR types are shown in Fig. 6 to Fig. 11. EMECC campaigns were carried out in SLB1 before the SGR of cycle 11 (cycles 9, 10, 11) and after the SGR (cycles 13 and 20).

The values of the SLB1 primary system surface activities at cycle 20 are given in Table 5.

<table><caption>Table 5: Deposited activities in the SLB1 primary system at cycle 20</caption><thead><tr><th>GBq/m²</th><th><sup>58</sup>Co ± 2σ</th><th><sup>60</sup>Co ± 2σ</th><th><sup>58</sup>Co/<sup>60</sup>Co</th></tr></thead><tbody><tr><td>Hot legs</td><td>1.4 ± 32%</td><td>1.5 ± 23%</td><td>0.93</td></tr><tr><td>Crossover legs</td><td>2.0 ± 32%</td><td>1.2 ± 23%</td><td>1.7</td></tr><tr><td>SG tubing<br>(average hot and<br>cold sides)</td><td>0.45 ± 33%</td><td>0.26 ± 23%</td><td>1.7</td></tr></tbody></table>

The SGR impact on the SLB1 primary system contamination is consistent with the analysis presented in 2000⁵.

The <sup>58</sup>Co deposited activities increase in the primary pipes after the SGR, then decrease between cycle 13 and 20 by a factor of about 6 in the crossover legs and 4 in the hot legs. They reach some of the lowest levels of the French nuclear fleet, whereas they were within the average or even among the highest before the SGR (a
decrease by a factor of about 2 between cycles 11 and 20). At cycle 20, the ⁵⁸Co deposited activity is about twice lower than the French average (see Table 3 and Table 5).

The SG tubing ⁵⁸Co contamination is already very low 2 cycles after the SGR. It then decreases and reaches practically the lowest level of the French nuclear PWRs at any cycle. Between cycle 11 and 20, the SG ⁵⁸Co contamination decreases by a factor of 4 to 20. At cycle 20, the ⁵⁸Co deposited activity is about 9 times lower than the French average (see Table 3 and Table 5).

The ⁶⁰Co deposited activities on the primary pipes are constant. They are within the average of the French PWRs.

The ⁶⁰Co contamination increases slightly in the new SG tube bundles and is very low 9 cycles after the SGR. It is about 2 to 6 times lower at cycle 20 (SGR+9) compared to cycle 9. At cycle 20, the ⁶⁰Co deposited activity is about 5 times lower than the French average (see Table 3 and Table 5).

The effect of the 600MA SG tubing manufacturer is to be noted⁴ (see Fig. 8 and Fig. 11). The SG tubes manufactured by A are about twice as contaminated as the SG tubes manufactured by B.

We can also observe the impact of the primary coolant temperature of the SG tubing. The hot side of the SG tube bundles (H) is about 2 times less contaminated than the cold side (C) (see Fig. 8 and Fig. 11). This is due to the lower cobalt equilibrium concentration in relation to the SLB1 SG oxides at 280°C than at 320°C.

<figure><img src="image_6.png" /><figcaption>Fig. 6: ⁵⁸Co deposited activities on the hot legs of French PWRs</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 7: ⁵⁸Co deposited activities on the cold and crossover legs of French PWRs</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 8: ⁵⁸Co deposited activities on the steam generator tubing of French PWRs</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 9: ⁶⁰Co deposited activities on the hot legs of French PWRs</figcaption></figure>
decrease by a factor of about 2 between cycles 11 and 20). At cycle 20, the ⁵⁸Co deposited activity is about twice lower than the French average (see Table 3 and Table 5).

The SG tubing ⁵⁸Co contamination is already very low 2 cycles after the SGR. It then decreases and reaches practically the lowest level of the French nuclear PWRs at any cycle. Between cycle 11 and 20, the SG ⁵⁸Co contamination decreases by a factor of 4 to 20. At cycle 20, the ⁵⁸Co deposited activity is about 9 times lower than the French average (see Table 3 and Table 5).

The ⁶⁰Co deposited activities on the primary pipes are constant. They are within the average of the French PWRs.

The ⁶⁰Co contamination increases slightly in the new SG tube bundles and is very low 9 cycles after the SGR. It is about 2 to 6 times lower at cycle 20 (SGR+9) compared to cycle 9. At cycle 20, the ⁶⁰Co deposited activity is about 5 times lower than the French average (see Table 3 and Table 5).

The effect of the 600MA SG tubing manufacturer is to be noted⁴ (see Fig. 8 and Fig. 11). The SG tubes manufactured by A are about twice as contaminated as the SG tubes manufactured by B.

We can also observe the impact of the primary coolant temperature of the SG tubing. The hot side of the SG tube bundles (H) is about 2 times less contaminated than the cold side (C) (see Fig. 8 and Fig. 11). This is due to the lower cobalt equilibrium concentration in relation to the SLB1 SG oxides at 280°C than at 320°C.

<figure><img src="image_8.png" /><figcaption>Fig. 6: ⁵⁸Co deposited activities on the hot legs of French PWRs</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7: ⁵⁸Co deposited activities on the cold and crossover legs of French PWRs</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 8: ⁵⁸Co deposited activities on the steam generator tubing of French PWRs</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 9: ⁶⁰Co deposited activities on the hot legs of French PWRs</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 10 : <sup>60</sup>Co deposited activities on the cold and crossover legs of French PWRs</figcaption></figure>

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

## IV. DISCUSSION

In paragraph III.A, we have seen that, as opposed to usual cold shutdowns, the primary coolant oxygenation during the refueling shutdown at the end of cycle 20 of SAINT-LAURENT B1 does not lead to a massive corrosion product release: the <sup>58</sup>Co peak, the <sup>58</sup>Co total released activity, the <sup>58</sup>Co/<sup>60</sup>Co ratio, the nickel concentration and the <sup>58</sup>Co specific activity are very low.

Whereas the released corrosion products during usual oxygenation come mainly from the in-core Ni⁰ or NiO deposits, this body of proof shows that there is practically no Ni⁰ or NiO deposit on the fuel elements. Almost all the nickel is in a nickel ferrite form, which is relatively stable in an oxidizing medium, as opposed to Ni⁰ or NiO⁶. Thus, compared to the in-core deposits, the out-of-core deposit contribution to the release is significant: one can remark that the <sup>58</sup>Co/<sup>60</sup>Co ratio of released activities is close to the

<sup>58</sup>Co/<sup>60</sup>Co ratio of out-of-flux surfaces (see Table 5), but its contribution is low compared to the total out-of-core activities, at the most a few percentage points.

The very low <sup>58</sup>Co peak and primary system contamination of SLB1 are not due to the boron-lithium coordinated chemistry at pH<sub>300°C</sub> 7.2 applied from cycle 14 (see the water chemistry during cycle 20 in Fig. 12 and Fig. 13). All French PWRs operate with this water chemical conditioning and their surface and volume contamination levels are not as low.

<figure><img src="image_12.png" /><figcaption>Fig. 12 : SLB120 – Chemical conditioning</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 13 : SLB120 – pH<sub>300°C</sub></figcaption></figure>

Compared to other PWRs, however, the 690TT SG tube manufacturing process of SLB1 has been improved, as those of the most recent 1450 MWe PWRs (in particular, no more sandblasting and complementary heat treatments). The very low <sup>58</sup>Co peak and primary system contamination of SLB1 are mainly due to the SG tubes in alloy 690TT. In connection with that, the <sup>58</sup>Co primary system contaminations of the two units of CIVAUX, the most recent 1450 MWe PWRs, are similar to the one of SAINT-LAURENT-B1.

Concerning the <sup>60</sup>Co contamination, the SG tube manufacturing process improvement only impacts the SG tubing contamination (see Fig. 11), the SG tubing trapping corrosion products very few. But the <sup>60</sup>Co piping
<figure><img src="image_12.png" /><figcaption>Fig. 10 : <sup>60</sup>Co deposited activities on the cold and crossover legs of French PWRs</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 11 : <sup>60</sup>Co deposited activities on the steam generator tubing of French PWRs</figcaption></figure>

IV. DISCUSSION

In paragraph III.A, we have seen that, as opposed to
usual cold shutdowns, the primary coolant oxygenation
during the refueling shutdown at the end of cycle 20 of
SAINT-LAURENT B1 does not lead to a massive
corrosion product release: the <sup>58</sup>Co peak, the <sup>58</sup>Co total
released activity, the <sup>58</sup>Co/<sup>60</sup>Co ratio, the nickel
concentration and the <sup>58</sup>Co specific activity are very low.

Whereas the released corrosion products during usual
oxygenation come mainly from the in-core Ni⁰ or NiO
deposits, this body of proof shows that there is practically
no Ni⁰ or NiO deposit on the fuel elements. Almost all the
nickel is in a nickel ferrite form, which is relatively stable
in an oxidizing medium, as opposed to Ni⁰ or NiO⁶. Thus,
compared to the in-core deposits, the out-of-core deposit
contribution to the release is significant: one can remark
that the <sup>58</sup>Co/<sup>60</sup>Co ratio of released activities is close to the

<sup>58</sup>Co/<sup>60</sup>Co ratio of out-of-flux surfaces (see Table 5), but
its contribution is low compared to the total out-of-core
activities, at the most a few percentage points.

The very low <sup>58</sup>Co peak and primary system contamination of SLB1 are not due to the boron-lithium coordinated chemistry at pH<sub>300°C</sub> 7.2 applied from cycle 14 (see the water chemistry during cycle 20 in Fig. 12 and Fig. 13). All French PWRs operate with this water chemical conditioning and their surface and volume contamination levels are not as low.

<figure><img src="image_14.png" /><figcaption>Fig. 12 : SLB120 – Chemical conditioning</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 13 : SLB120 – pH<sub>300°C</sub></figcaption></figure>

Compared to other PWRs, however, the 690TT SG tube manufacturing process of SLB1 has been improved, as those of the most recent 1450 MWe PWRs (in particular, no more sandblasting and complementary heat treatments). The very low <sup>58</sup>Co peak and primary system contamination of SLB1 are mainly due to the SG tubes in alloy 690TT. In connection with that, the <sup>58</sup>Co primary system contaminations of the two units of CIVAUX, the most recent 1450 MWe PWRs, are similar to the one of SAINT-LAURENT-B1.

Concerning the <sup>60</sup>Co contamination, the SG tube
manufacturing process improvement only impacts the SG
tubing contamination (see Fig. 11), the SG tubing
trapping corrosion products very few. But the <sup>60</sup>Co piping
contamination remains constant after the SGR (see Fig. 9 and Fig. 10). The reasons why the SLB1 SGR has not had as significant an effect on the ⁶⁰Co contamination as on the ⁵⁸Co contamination, are:

* The memory effect due to the longer ⁶⁰Co radioactive half-life (5.3 years for ⁶⁰Co versus 71 days for ⁵⁸Co),

* The relatively low cobalt content of the former 600MA SG tubes of SLB1, 240 ppm on average. It was shown that below a cobalt content of about 200-300 ppm, other cobalt sources become preponderant⁴,

* The peculiar behaviour of nickel (nuclear parent of ⁵⁸Co) compared with the behaviour of cobalt (nuclear parent of ⁶⁰Co) as explained hereafter.

As seen in paragraph III.A, the ⁵⁸Co peak during refueling shutdown at the end of cycle 20 is extremely low, less than 1 GBq/m³. After the increase usually observed one or two cycles after SGR⁵, the ⁵⁸Co peaks in SLB1 start sharply decreasing and reach values below 15 GBq/m³ at cycle 18 (see Fig. 14)

<figure><img src="image_14.png" /><figcaption>Fig. 14: SLB1 – ⁵⁸Co peak during shutdowns</figcaption></figure>

The ⁵⁸Co peak of cycle 20 is typical of KONVOI PWRs (alloy 800 SG tubing). The ⁵⁸Co peak is due to the dissolution of Ni⁰ or NiO deposits on fuel elements (retrograde solubility) during oxygenation, which involves the dissolution of ⁵⁸Co derived from the nickel sites in the crud (⁵⁸Ni (n,p) ⁵⁸Co). In PWRs such as KONVOI PWRs or SAINT-LAURENT-B1 at cycle 20, there is practically no Ni⁰ or NiO present. The SLB1 SG tubing release is low enough for that practically all the nickel forms nickel ferrites such as CoₓNiᵧFe₃₋ₓ₋ᵧO₄ (with x + y ≤ 1 and x << y) for the outer oxide layer, which are more stable in oxidizing medium and of which the nickel solubility is not retrograde (no preferential deposits on fuel elements). The cobalt, which forms ⁶⁰Co by neutron activation, is also incorporated into nickel ferrites.

In SLB1 at cycle 20, as ⁵⁸Co and ⁶⁰Co are part of the same oxides, their behaviour is similar. Moreover, it is shown by the ⁵⁸Co/⁶⁰Co ratio, which varies little during the cold shutdown procedure (see paragraph III.A).

Because of the lower in-core deposits of nickel and therefore of ⁵⁸Co in SLB1, the ⁵⁸Co activities deposited in the primary circuit and released during the cold shutdown are lower and therefore the decrease in ⁵⁸Co contamination is more significant than the one in ⁶⁰Co, without even taking into account the radioactive half-life.

This lower ⁵⁸Co source term can also be observed by the ⁵⁸Co volume activity variation during cycles. We have seen that this activity is low during cycle 20 (see paragraph III.B) and it falls for several cycles as it is shown in Fig. 15. The ⁶⁰Co volume activityc is practically constant during the cycles after the SGR, but the ⁵⁸Co volume activity starts decreasing around cycle 15.

<figure><img src="image_15.png" /><figcaption>Fig. 15: SLB1 – ⁵⁸Co and ⁶⁰Co volume activities during cycles</figcaption></figure>

It is to be noted that ⁵⁸Co peaks during oxygenation under 10 GBq/m³ have also been observed in other French PWRs equipped with 690TT SG tubes like those of SLB1 or more recent ones. Such peaks have only been observed after several operating cycles with these SGs. During the first cycle, the SG tubing release is higher during the formation of the inner protective oxide layer. Then, as the nickel release and the nickel quantity deposited inside the circuits are decreasing, the ⁵⁸Co surface and volume activities decrease as well.

The circuit cleanliness after an SGR or a commissioning must have an impact on the deposited and released activities during the first following cycles, but not after several cycles. According to Cubbit *et al*⁷, the hot functional test passivation procedure used at SIZEWELL-B is the key factor in obtaining low deposited and released activities. But, in SAINT-LAURENT-B1 or in other French PWRs like CIVAUX-1 or CIVAUX-2, the deposited and released activities are similar to those of SIZEWELL-B and there was not any hot functional test passivation procedure during the SGR shutdown or the commissioning of these French PWRs.

<footnote>c As well as the ⁵¹Cr and ⁵⁴Mn volume activities</footnote>
contamination remains constant after the SGR (see Fig. 9 and Fig. 10). The reasons why the SLB1 SGR has not had as significant an effect on the ⁶⁰Co contamination as on the ⁵⁸Co contamination, are:

* The memory effect due to the longer ⁶⁰Co radioactive half-life (5.3 years for ⁶⁰Co versus 71 days for ⁵⁸Co),

* The relatively low cobalt content of the former 600MA SG tubes of SLB1, 240 ppm on average. It was shown that below a cobalt content of about 200-300 ppm, other cobalt sources become preponderant⁴,

* The peculiar behaviour of nickel (nuclear parent of ⁵⁸Co) compared with the behaviour of cobalt (nuclear parent of ⁶⁰Co) as explained hereafter.

As seen in paragraph III.A, the ⁵⁸Co peak during refueling shutdown at the end of cycle 20 is extremely low, less than 1 GBq/m³. After the increase usually observed one or two cycles after SGR⁵, the ⁵⁸Co peaks in SLB1 start sharply decreasing and reach values below 15 GBq/m³ at cycle 18 (see Fig. 14)

<figure><img src="image_16.png" /><figcaption>Fig. 14: SLB1 – ⁵⁸Co peak during shutdowns</figcaption></figure>

The ⁵⁸Co peak of cycle 20 is typical of KONVOI PWRs (alloy 800 SG tubing). The ⁵⁸Co peak is due to the dissolution of Ni⁰ or NiO deposits on fuel elements (retrograde solubility) during oxygenation, which involves the dissolution of ⁵⁸Co derived from the nickel sites in the crud (⁵⁸Ni (n,p) ⁵⁸Co). In PWRs such as KONVOI PWRs or SAINT-LAURENT-B1 at cycle 20, there is practically no Ni⁰ or NiO present. The SLB1 SG tubing release is low enough for that practically all the nickel forms nickel ferrites such as CoₓNiᵧFe₃₋ₓ₋ᵧO₄ (with x + y ≤ 1 and x << y) for the outer oxide layer, which are more stable in oxidizing medium and of which the nickel solubility is not retrograde (no preferential deposits on fuel elements). The cobalt, which forms ⁶⁰Co by neutron activation, is also incorporated into nickel ferrites.

In SLB1 at cycle 20, as ⁵⁸Co and ⁶⁰Co are part of the same oxides, their behaviour is similar. Moreover, it is shown by the ⁵⁸Co/⁶⁰Co ratio, which varies little during the cold shutdown procedure (see paragraph III.A).

Because of the lower in-core deposits of nickel and therefore of ⁵⁸Co in SLB1, the ⁵⁸Co activities deposited in the primary circuit and released during the cold shutdown are lower and therefore the decrease in ⁵⁸Co contamination is more significant than the one in ⁶⁰Co, without even taking into account the radioactive half-life.

This lower ⁵⁸Co source term can also be observed by the ⁵⁸Co volume activity variation during cycles. We have seen that this activity is low during cycle 20 (see paragraph III.B) and it falls for several cycles as it is shown in Fig. 15. The ⁶⁰Co volume activityc is practically constant during the cycles after the SGR, but the ⁵⁸Co volume activity starts decreasing around cycle 15.

<figure><img src="image_17.png" /><figcaption>Fig. 15: SLB1 – ⁵⁸Co and ⁶⁰Co volume activities during cycles</figcaption></figure>

It is to be noted that ⁵⁸Co peaks during oxygenation under 10 GBq/m³ have also been observed in other French PWRs equipped with 690TT SG tubes like those of SLB1 or more recent ones. Such peaks have only been observed after several operating cycles with these SGs. During the first cycle, the SG tubing release is higher during the formation of the inner protective oxide layer. Then, as the nickel release and the nickel quantity deposited inside the circuits are decreasing, the ⁵⁸Co surface and volume activities decrease as well.

The circuit cleanliness after an SGR or a commissioning must have an impact on the deposited and released activities during the first following cycles, but not after several cycles. According to Cubbit *et al*⁷, the hot functional test passivation procedure used at SIZEWELL-B is the key factor in obtaining low deposited and released activities. But, in SAINT-LAURENT-B1 or in other French PWRs like CIVAUX-1 or CIVAUX-2, the deposited and released activities are similar to those of SIZEWELL-B and there was not any hot functional test passivation procedure during the SGR shutdown or the commissioning of these French PWRs.

<footnote>c As well as the ⁵¹Cr and ⁵⁴Mn volume activities</footnote>
The surface finish<sup>d</sup> of the SAINT-LAURENT-B1 SG tubing by means of an improved manufactured process is the key factor, and probably as for SIZEWELL-B<sup>e</sup>. On the other hand, a hot functional test passivation procedure followed by a complete water purification stage (circuit cleanliness) should lower the contamination level more quickly, because the corrosion product deposits and release must be lower during the first cycles after an SGR or a commissioning. Moreover, EDF is studying the advisability of using a hot functional test passivation procedure followed by a complete water purification for the EPR (European Pressurized water Reactor) commissioning at the FLAMANVILLE nuclear power station.

## V. CONCLUSIONS

French PWRs equipped with SG tubing in a nickel-based alloy (alloy 600 or 690) are generally characterized by high <sup>58</sup>Co peaks during cold shutdowns (order of magnitude of 100 GBq/m<sup>3</sup>) and high <sup>58</sup>Co deposited activities (several GBq/m<sup>2</sup>). However, the trends are different for the SGR with alloy 690 carried out in the last decade, in particular the SAINT-LAURENT-B1 PWR: after an increase in the <sup>58</sup>Co contamination just after the SGR, the contamination on the primary system decreases and reaches a very low level several cycles after the SGR. The <sup>58</sup>Co peak during cold shutdowns follows the same variation; it drops by a factor of about 10 to 100. Similar levels are also measured in the last French operating PWR generation (N4).

The key factor of this low <sup>58</sup>Co contamination level is the surface finish of alloy 690TT SG tubing by means of an improved manufacturing process. The nickel release is low enough to limit the presence of metallic nickel or nickel oxide, which tend to form deposits on fuel elements. In consequence, nickel deposits on fuel elements and then <sup>58</sup>Co volume and surface activities are reduced.

## VI. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the chemists of the Nuclear Power Plant of SAINT-LAURENT, in particular J.M. Lavigne.

## REFERENCES

1. F. DACQUAIT, C. ANDRIEU, M. BERGER, J.L. BRETELLE and A. ROCHER, "Corrosion Product

Transfer in French PWRs during Shutdown", Proceedings of the International Conference on Water Chemistry in Nuclear Reactors Systems, Avignon 22-26 April 2002, paper 113, SFEN, Paris (2002)

2. D. BORDES, "Etude du comportement des produits de corrosion dans l'eau du circuit primaire des réacteurs à eau pressurisée", PhD thesis, Université Claude Bernard-Lyon 1 (1995)

3. 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)

4. F. DACQUAIT, L. GUINARD, C. ANDRIEU, J.L. BRETELLE, F. BARDET, A. ROCHER, C. BRUN, "Status of primary system contamination in French PWRs", Proceedings of the International Conference on Water Chemistry in Nuclear Reactors Systems, 11-14/10/04, San Francisco (USA), Session 2, paper 4, EPRI, Palo Alto (2005)

5. 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).

6. D. YOU, E. BLANCHARD, D. FÉRON, C. MASSON, "Corrosion product transfer in PWR primary circuits during cold shutdowns", Proceedings of the International Conference on Water Chemistry in Nuclear Reactors Systems, Avignon 22-26 April 2002, paper 118, SFEN, Paris (2002)

7. M. CUBITT, K. GARBETT, G. A. LANCASTER, G. A. RENN and E. W. THORNTON, "Shutdown Corrosion Product, Radiation Field and Personnel Dose Development at the SIZEWELL B PWR", Proceedings of the International Conference on Water Chemistry in Nuclear Reactors Systems, 11-14/10/04, San Francisco (USA), Session 2, paper 5, EPRI, Palo Alto (2005)

<footnote><sup>d</sup> The surface finish is mainly defined by crystallographic, chemical, physicochemical and microgeometric characteristics</footnote>

<footnote><sup>e</sup> The tubing manufacturers of SIZEWELL-B and SAINT-LAURENT-B1 are different.</footnote>
The surface finish<sup>d</sup> of the SAINT-LAURENT-B1 SG tubing by means of an improved manufactured process is the key factor, and probably as for SIZEWELL-B<sup>e</sup>. On the other hand, a hot functional test passivation procedure followed by a complete water purification stage (circuit cleanliness) should lower the contamination level more quickly, because the corrosion product deposits and release must be lower during the first cycles after an SGR or a commissioning. Moreover, EDF is studying the advisability of using a hot functional test passivation procedure followed by a complete water purification for the EPR (European Pressurized water Reactor) commissioning at the FLAMANVILLE nuclear power station.

## V. CONCLUSIONS

French PWRs equipped with SG tubing in a nickel-based alloy (alloy 600 or 690) are generally characterized by high <sup>58</sup>Co peaks during cold shutdowns (order of magnitude of 100 GBq/m<sup>3</sup>) and high <sup>58</sup>Co deposited activities (several GBq/m<sup>2</sup>). However, the trends are different for the SGR with alloy 690 carried out in the last decade, in particular the SAINT-LAURENT-B1 PWR: after an increase in the <sup>58</sup>Co contamination just after the SGR, the contamination on the primary system decreases and reaches a very low level several cycles after the SGR. The <sup>58</sup>Co peak during cold shutdowns follows the same variation; it drops by a factor of about 10 to 100. Similar levels are also measured in the last French operating PWR generation (N4).

The key factor of this low <sup>58</sup>Co contamination level is the surface finish of alloy 690TT SG tubing by means of an improved manufacturing process. The nickel release is low enough to limit the presence of metallic nickel or nickel oxide, which tend to form deposits on fuel elements. In consequence, nickel deposits on fuel elements and then <sup>58</sup>Co volume and surface activities are reduced.

## VI. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the chemists of the Nuclear Power Plant of SAINT-LAURENT, in particular J.M. Lavigne.

## REFERENCES

1. F. DACQUAIT, C. ANDRIEU, M. BERGER, J.L. BRETELLE and A. ROCHER, "Corrosion Product

Transfer in French PWRs during Shutdown", Proceedings of the International Conference on Water Chemistry in Nuclear Reactors Systems, Avignon 22-26 April 2002, paper 113, SFEN, Paris (2002)

2. D. BORDES, "Etude du comportement des produits de corrosion dans l'eau du circuit primaire des réacteurs à eau pressurisée", PhD thesis, Université Claude Bernard-Lyon 1 (1995)

3. 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)

4. F. DACQUAIT, L. GUINARD, C. ANDRIEU, J.L. BRETELLE, F. BARDET, A. ROCHER, C. BRUN, "Status of primary system contamination in French PWRs", Proceedings of the International Conference on Water Chemistry in Nuclear Reactors Systems, 11-14/10/04, San Francisco (USA), Session 2, paper 4, EPRI, Palo Alto (2005)

5. 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).

6. D. YOU, E. BLANCHARD, D. FÉRON, C. MASSON, "Corrosion product transfer in PWR primary circuits during cold shutdowns", Proceedings of the International Conference on Water Chemistry in Nuclear Reactors Systems, Avignon 22-26 April 2002, paper 118, SFEN, Paris (2002)

7. M. CUBITT, K. GARBETT, G. A. LANCASTER, G. A. RENN and E. W. THORNTON, "Shutdown Corrosion Product, Radiation Field and Personnel Dose Development at the SIZEWELL B PWR", Proceedings of the International Conference on Water Chemistry in Nuclear Reactors Systems, 11-14/10/04, San Francisco (USA), Session 2, paper 5, EPRI, Palo Alto (2005)

<footnote><sup>d</sup> The surface finish is mainly defined by crystallographic, chemical, physicochemical and microgeometric characteristics</footnote>

<footnote><sup>e</sup> The tubing manufacturers of SIZEWELL-B and SAINT-LAURENT-B1 are different.</footnote>