# EMECC Campaign at TRILLO: Results and Comparison with French Feedback

Gilles Ranchoux, Frédéric Dacquait, Eduardo Velasco-Marcelo, José Manuel Garcés de Marcilla Bayo

## ► To cite this version:

Gilles Ranchoux, Frédéric Dacquait, Eduardo Velasco-Marcelo, José Manuel Garcés de Marcilla Bayo. EMECC Campaign at TRILLO: Results and Comparison with French Feedback. NPC 2010 - Nuclear Plant Chemistry 2010 - International Conference on Water Chemistry of Nuclear Reactors Systems, Jul 2010, Quebec CIty, Canada. cea-04473996

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EMECC Campaign at TRILLO: Results and
Comparison with French Feedback

Gilles Ranchoux, Frédéric Dacquait, Eduardo Velasco-Marcelo, José Manuel

Garcés de Marcilla Bayo

► To cite this version:

Gilles Ranchoux, Frédéric Dacquait, Eduardo Velasco-Marcelo, José Manuel Garcés de Marcilla Bayo. EMECC Campaign at TRILLO: Results and Comparison with French Feedback. NPC 2010 - Nuclear Plant Chemistry 2010 - International Conference on Water Chemistry of Nuclear Reactors Systems, Jul 2010, Quebec CIty, Canada. cea-04473996

HAL Id: cea-04473996

https://cea.hal.science/cea-04473996v1

Submitted on 22 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
# EMECC CAMPAIGN AT TRILLO :
RESULTS AND COMPARISON WITH FRENCH FEEDBACK

G. Ranchoux

EDF/SEPTEN, 12-14 avenue DUTRIEVOZ, 69628 Villeurbanne, France

F. Dacquait

CEA/Cadarache, LMTR, 13108 Saint-Paul-Lez-Durance

E. Velasco-Marcelo, J.M. Garces de Marcilla Bayo

CNAT/Trillo, Spain

## ABSTRACT

The reduction of collective dose is an important objective for all nuclear plant operators. One of the support for studying the impact of design and operation parameters is the measurement campaigns of contamination in PWRs. EDF performs consequently gamma spectrometry measurements with CEA and his EMECC system, with more than 300 campaigns in EDF plants and also some campaigns in collaboration with other operators.

Commissioned in 1988, the only unit of the nuclear power plant of TRILLO, located in Spain, is a 3-loop PWR of 1066 MWe designed by KWU-Siemens. TRILLO-1, equipped with alloy 800 tubed steam generators, is characterized by a low dosimetry. In order to compare the contamination level of TRILLO-1 to that of French PWRs, an EMECC campaign was performed within the framework of a contract between EDF and the CEA, and with the agreement of CNAT/TRILLO and the support of the CSN (Consejo de Seguridad Nuclear).

This campaign took place during the refueling shutdown at the end of cycle 21 from 9 to 16 February 2009. A previous EMECC campaign was performed in 1992 during the refueling shutdown of cycle 4.

At the first time, this paper will present the main conclusions of this EMECC campaign :

* deposited activities measured on legs walls and steam generator tubes walls,

* dose rates measurements.

The evolution of the contamination since cycle 4 will be also analyzed.

Furthermore, a comparison with the French unit St-LAURENT B1 will be achieved in order to better understand the origin of the differences (source term or outage management) and to improve good operational practices in the future.

## 1. INTRODUCTION

Commissioned in 1988, the only unit of the nuclear power plant of TRILLO, located in Spain, is a 3-loop PWR of 1066 MWe designed by KWU-Siemens. TRILLO-1 equipped with alloy 800 tubed steam generators, is characterized by a low dosimetry (382 Man.mSv in 2008).

In order to compare the contamination level of TRILLO-1 to that of the French unit St-LAURENT B1, an EMECC campaign [1.] was carried out within the framework of a contract
# EMECC CAMPAIGN AT TRILLO :
## RESULTS AND COMPARISON WITH FRENCH FEEDBACK

G. Ranchoux

EDF/SEPTEN, 12-14 avenue DUTRIEVOZ, 69628 Villeurbanne, France

F. Dacquait

CEA/Cadarache, LMTR, 13108 Saint-Paul-Lez-Durance

E. Velasco-Marcelo, J.M. Garces de Marcilla Bayo

CNAT/Trillo, Spain

## ABSTRACT

The reduction of collective dose is an important objective for all nuclear plant operators. One of the support for studying the impact of design and operation parameters is the measurement campaigns of contamination in PWRs. EDF performs consequently gamma spectrometry measurements with CEA and his EMECC system, with more than 300 campaigns in EDF plants and also some campaigns in collaboration with other operators.

Commissioned in 1988, the only unit of the nuclear power plant of TRILLO, located in Spain, is a 3-loop PWR of 1066 MWe designed by KWU-Siemens. TRILLO-1, equipped with alloy 800 tubed steam generators, is characterized by a low dosimetry. In order to compare the contamination level of TRILLO-1 to that of French PWRs, an EMECC campaign was performed within the framework of a contract between EDF and the CEA, and with the agreement of CNAT/TRILLO and the support of the CSN (Consejo de Seguridad Nuclear).

This campaign took place during the refueling shutdown at the end of cycle 21 from 9 to 16 February 2009. A previous EMECC campaign was performed in 1992 during the refueling shutdown of cycle 4.

At the first time, this paper will present the main conclusions of this EMECC campaign :

* deposited activities measured on legs walls and steam generator tubes walls,

* dose rates measurements.

The evolution of the contamination since cycle 4 will be also analyzed.

Furthermore, a comparison with the French unit St-LAURENT B1 will be achieved in order to better understand the origin of the differences (source term or outage management) and to improve good operational practices in the future.

## 1. INTRODUCTION

Commissioned in 1988, the only unit of the nuclear power plant of TRILLO, located in Spain, is a 3-loop PWR of 1066 MWe designed by KWU-Siemens. TRILLO-1 equipped with alloy 800 tubed steam generators, is characterized by a low dosimetry (382 Man.mSv in 2008).

In order to compare the contamination level of TRILLO-1 to that of the French unit St-LAURENT B1, an EMECC campaign [1.] was carried out within the framework of a contract
between EDF and CEA and with the agreement of CNAT/TRILLO and the support of the CSN (Consejo de Securidad Nuclear i.e. the Spanish Nuclear Safety Authority).

This campaign took place during the refueling shutdown at the end of cycle 21 from February 9th to 16th 2009. A previous EMECC campaign was conducted in 1992 during the refueling shutdown of cycle 4.

# 1. OPERATIONNAL PARAMETERS AT TRILLO

## 1.1 Reactor coolant chemistry

In TRILLO-1 during cycle 21 (281 EFPD), the reactor coolant was based on a boron-lithium coordinated chemistry with a pH₃₀₀°C target of 7.5. Almost immediately at the beginning of cycle 21, pH₃₀₀°C rose above 6.9 whereas the maximum Li content has remained quasi-stable at about 2.2 ppm for 80% of the cycle duration. This procedure is slightly different than the French one for which the pH₃₀₀°C target is 7.2 (optimum chosen by EDF between contamination risk and potential PWSCC long term risk with high lithium concentration).

During cycle 21, H₂ concentration in primary coolant ranged between 22 and 28 cm³/kg (TPS), with an average of 24 cm³/kg.

## 1.2 Cold shutdown procedure

Various shutdown parameters are shown in Figure 1 (temperature, letdown flow-rate, boron, hydrogen and oxygen concentrations and ⁵⁸Co volume activity).

Considering the very low activity level of ⁵⁸Co (1.33 GBq/t), the last RCP pump shutdown occurred only 2.5 hours after RHRS connection.

<figure><img src="image_2.png" /><figcaption>Figure 1. TRILLO 1 Cycle 1 - Cold shutdown procedure</figcaption></figure>
between EDF and CEA and with the agreement of CNAT/TRILLO and the support of the CSN (Consejo de Securidad Nuclear i.e. the Spanish Nuclear Safety Authority).

This campaign took place during the refueling shutdown at the end of cycle 21 from February 9th to 16th 2009. A previous EMECC campaign was conducted in 1992 during the refueling shutdown of cycle 4.

# 1. OPERATIONNAL PARAMETERS AT TRILLO

## 1.1 Reactor coolant chemistry

In TRILLO-1 during cycle 21 (281 EFPD), the reactor coolant was based on a boron-lithium coordinated chemistry with a pH₃₀₀°C target of 7.5. Almost immediately at the beginning of cycle 21, pH₃₀₀°C rose above 6.9 whereas the maximum Li content has remained quasi-stable at about 2.2 ppm for 80% of the cycle duration. This procedure is slightly different than the French one for which the pH₃₀₀°C target is 7.2 (optimum chosen by EDF between contamination risk and potential PWSCC long term risk with high lithium concentration).

During cycle 21, H₂ concentration in primary coolant ranged between 22 and 28 cm³/kg (TPS), with an average of 24 cm³/kg.

## 1.2 Cold shutdown procedure

Various shutdown parameters are shown in Figure 1 (temperature, letdown flow-rate, boron, hydrogen and oxygen concentrations and ⁵⁸Co volume activity).

Considering the very low activity level of ⁵⁸Co (1.33 GBq/t), the last RCP pump shutdown occurred only 2.5 hours after RHRS connection.

<figure><img src="image_2.png" /><figcaption>Figure 1. TRILLO 1 Cycle 1 - Cold shutdown procedure</figcaption></figure>
# 2. EMECC CAMPAIGN RESULTS AT TRILLO-1 AND COMPARISON WITH SAINT-LAURENT B1

## 2.1 Volume activities

### *During cycle 21*

A comparison of volume activities during TRILLO 1 cycle 21 and St LAURENT B1 is shown in table 1.

<table><caption>Table 1. Comparison CNT121/SLB120 – Average volume activities during cycle</caption><thead><tr><td>MBq/t</td><td>CNT 121</td><td>SLB 120</td></tr></thead><tbody><tr><td><sup>58</sup>Co</td><td>1.8</td><td>3.1</td></tr><tr><td><sup>60</sup>Co</td><td>1.2</td><td>1.8</td></tr><tr><td><sup>54</sup>Mn</td><td>0.9</td><td>1.5</td></tr><tr><td><sup>51</sup>Cr</td><td>6.4</td><td>8.9</td></tr></tbody></table>

One can observe that the volume activities in CNT121 are slightly lower than those measured in SLB1.

### *During shutdown*

RCS oxygenation is the most important stage of the French cold shutdown procedure because it usually causes a very large increase in the <sup>58</sup>Co volume activity (order of magnitude of 100 GBq/t), which can lead to an increase in the activities deposited on the out of flux surfaces [8]. To control this stage, H<sub>2</sub>O<sub>2</sub> is injected when the primary coolant temperature is 80°C. Nevertheless, due to an improved manufacturing process of the alloy 690 TT SG tubes (Ni : 60%; Fe : 10%; Cr : 30%) of St-LAURENT B1 [4], the <sup>58</sup>Co peak is very low at cycle 20 : 0.8 GBq/t.

In TRILLO 1, thanks to the alloy 800 SG tubes (Ni : 35%; Fe : 45%; Cr : 20%), the primary system oxygenation leads to a very low <sup>58</sup>Co peak (order of magnitude of 1 GBq/t) typical of Konvoy PWRs, therefore there is less need to control it. At cycle 21, the primary system oxygenation occurred during the start of RHRS, perhaps during the draining of RCS (increase of <sup>110m</sup>Ag volume activities) and of course during the primary system ventilation to atmosphere.

The <sup>58</sup>Co peaks in PWRs equipped with alloy 690 SG tubing, like St-LAURENT B1, can be as low as those of the Konvoy PWRs thanks to a very low SG tubing release. Therefore, more than the material (alloy 690 or alloy 800), the key factor leading to very low <sup>58</sup>Co peaks is the SG tubing surface state obtained by means of an improved tubing manufactured process.

Moreover, in TRILLO 1, the low <sup>58</sup>Co peak could be explained by the fact that practically all the nickel released by the alloy 800 SG tubes generates nickel ferrites. Thus, there is almost no Ni<sup>0</sup> or NiO deposited on the fuel elements and potentially releasable during the oxygenation peak. Nickel ferrites are more stable in oxidizing medium and their solubility increases with temperature in optimum conditions (no preferential deposits on fuel elements). In St-LAURENT B1, the lack of Ni<sup>0</sup> or NiO is due to a very low release of alloy 690 SG tubing [7].
## 2. EMECC CAMPAIGN RESULTS AT TRILLO-1 AND COMPARISON WITH SAINT-LAURENT B1

### 2.1 Volume activities

#### *During cycle 21*

A comparison of volume activities during TRILLO 1 cycle 21 and St LAURENT B1 is shown in table 1.

<table><caption>Table 1. Comparison CNT121/SLB120 – Average volume activities during cycle</caption><thead><tr><td>MBq/t</td><td>CNT 121</td><td>SLB 120</td></tr></thead><tbody><tr><td><sup>58</sup>Co</td><td>1.8</td><td>3.1</td></tr><tr><td><sup>60</sup>Co</td><td>1.2</td><td>1.8</td></tr><tr><td><sup>54</sup>Mn</td><td>0.9</td><td>1.5</td></tr><tr><td><sup>51</sup>Cr</td><td>6.4</td><td>8.9</td></tr></tbody></table>

One can observe that the volume activities in CNT121 are slightly lower than those measured in SLB1.

#### *During shutdown*

RCS oxygenation is the most important stage of the French cold shutdown procedure because it usually causes a very large increase in the <sup>58</sup>Co volume activity (order of magnitude of 100 GBq/t), which can lead to an increase in the activities deposited on the out of flux surfaces [8]. To control this stage, H<sub>2</sub>O<sub>2</sub> is injected when the primary coolant temperature is 80°C. Nevertheless, due to an improved manufacturing process of the alloy 690 TT SG tubes (Ni : 60%; Fe : 10%; Cr : 30%) of St-LAURENT B1 [4], the <sup>58</sup>Co peak is very low at cycle 20 : 0.8 GBq/t.

In TRILLO 1, thanks to the alloy 800 SG tubes (Ni : 35%; Fe : 45%; Cr : 20%), the primary system oxygenation leads to a very low <sup>58</sup>Co peak (order of magnitude of 1 GBq/t) typical of Konvoy PWRs, therefore there is less need to control it. At cycle 21, the primary system oxygenation occurred during the start of RHRS, perhaps during the draining of RCS (increase of <sup>110m</sup>Ag volume activities) and of course during the primary system ventilation to atmosphere.

The <sup>58</sup>Co peaks in PWRs equipped with alloy 690 SG tubing, like St-LAURENT B1, can be as low as those of the Konvoy PWRs thanks to a very low SG tubing release. Therefore, more than the material (alloy 690 or alloy 800), the key factor leading to very low <sup>58</sup>Co peaks is the SG tubing surface state obtained by means of an improved tubing manufactured process.

Moreover, in TRILLO 1, the low <sup>58</sup>Co peak could be explained by the fact that practically all the nickel released by the alloy 800 SG tubes generates nickel ferrites. Thus, there is almost no Ni<sup>0</sup> or NiO deposited on the fuel elements and potentially releasable during the oxygenation peak. Nickel ferrites are more stable in oxidizing medium and their solubility increases with temperature in optimum conditions (no preferential deposits on fuel elements). In St-LAURENT B1, the lack of Ni<sup>0</sup> or NiO is due to a very low release of alloy 690 SG tubing [7].
## 2.2 Surface activities

Average surface activities measured at TRILLO 1 cycle 21 are presented in table 2 for legs, SG tubes and CVCS non regenerative heat exchanger (NRHE).

<table><caption>Table 2. Average surface activities for some radionuclides in TRILLO 1</caption><thead><tr><th>GBq/m²</th><th><sup>58</sup>Co</th><th><sup>60</sup>Co</th><th><sup>54</sup>Mn</th><th><sup>124</sup>Sb</th><th><sup>110m</sup>Ag</th><th><sup>58</sup>Co/<sup>60</sup>Co ratio</th></tr></thead><tbody><tr><td>Legs</td><td>0.76</td><td>2.69</td><td>0.16</td><td>0.08</td><td></td><td>0.28</td></tr><tr><td>SG tubes</td><td>0.2</td><td>0.49</td><td></td><td>0.07</td><td></td><td>0.4</td></tr><tr><td>CVCS letdown line (NRHE)</td><td>0.004</td><td>0.006</td><td></td><td>0.004</td><td>0.022</td><td>0.66</td></tr></tbody></table>

Average surface activities measured at St LAURENT B1 cycle 20 are presented in table 3 for legs, SG tubes and CVCS letdown line.

<table><caption>Table 3. Average surface activities for some radionuclides in St LAURENT B1</caption><thead><tr><th>GBq/m²</th><th><sup>58</sup>Co</th><th><sup>60</sup>Co</th><th><sup>54</sup>Mn</th><th><sup>124</sup>Sb</th><th><sup>110m</sup>Ag</th><th><sup>58</sup>Co/<sup>60</sup>Co ratio</th></tr></thead><tbody><tr><td>Legs</td><td>1.52</td><td>1.34</td><td>0.21</td><td>0.21</td><td>0.15</td><td>1.1</td></tr><tr><td>SG tubes</td><td>0.53</td><td>0.29</td><td>0.04</td><td>0.07</td><td></td><td>1.8</td></tr><tr><td>CVCS letdown line (NRHE)</td><td>0.04</td><td>0.04</td><td>0.006</td><td>0.047</td><td>0.32</td><td>1.0</td></tr></tbody></table>

At TRILLO 1, one can observe that the main contributors to activity are <sup>58</sup>Co (≈ 20%) and <sup>60</sup>Co (≈ 65%) in RCS. The same radionuclides also mainly contribute to surface activity at St-LAURENT B1 but the <sup>58</sup>Co impact is significantly higher (40%-60% for <sup>58</sup>Co and ≈ 35% for <sup>60</sup>Co) as shown by the different ratios <sup>58</sup>Co/<sup>60</sup>Co.

One can also notice a specific contribution of <sup>110m</sup>Ag (≈ 50-60%) in CVCS non regenerative heat exchanger. A same tendency is observed at St-LAURENT B1. This phenomenon, well-known in French PWRs [2.], can be explained by thermochemical considerations. Silver is soluble in oxidizing conditions and its solubility increases with temperature. Thus, during oxygenation, it dissolves in the primary coolant from RCS deposits and then precipitates on the cold parts of auxiliary systems, in particular heat exchangers.

Deposited activities in <sup>58</sup>Co and <sup>60</sup>Co are presented in figure 2 for hot legs, in figure 3 for crossover legs, in figure 4 for SG tubes and in figure 5 for CVCS non-regenerative heat exchanger.
## 2.2 Surface activities

Average surface activities measured at TRILLO 1 cycle 21 are presented in table 2 for legs, SG tubes and CVCS non regenerative heat exchanger (NRHE).

<table><caption>Table 2. Average surface activities for some radionuclides in TRILLO 1</caption><thead><tr><th>GBq/m²</th><th><sup>58</sup>Co</th><th><sup>60</sup>Co</th><th><sup>54</sup>Mn</th><th><sup>124</sup>Sb</th><th><sup>110m</sup>Ag</th><th><sup>58</sup>Co/<sup>60</sup>Co ratio</th></tr></thead><tbody><tr><td>Legs</td><td>0.76</td><td>2.69</td><td>0.16</td><td>0.08</td><td></td><td>0.28</td></tr><tr><td>SG tubes</td><td>0.2</td><td>0.49</td><td></td><td>0.07</td><td></td><td>0.4</td></tr><tr><td>CVCS letdown line (NRHE)</td><td>0.004</td><td>0.006</td><td></td><td>0.004</td><td>0.022</td><td>0.66</td></tr></tbody></table>

Average surface activities measured at St LAURENT B1 cycle 20 are presented in table 3 for legs, SG tubes and CVCS letdown line.

<table><caption>Table 3. Average surface activities for some radionuclides in St LAURENT B1</caption><thead><tr><th>GBq/m²</th><th><sup>58</sup>Co</th><th><sup>60</sup>Co</th><th><sup>54</sup>Mn</th><th><sup>124</sup>Sb</th><th><sup>110m</sup>Ag</th><th><sup>58</sup>Co/<sup>60</sup>Co ratio</th></tr></thead><tbody><tr><td>Legs</td><td>1.52</td><td>1.34</td><td>0.21</td><td>0.21</td><td>0.15</td><td>1.1</td></tr><tr><td>SG tubes</td><td>0.53</td><td>0.29</td><td>0.04</td><td>0.07</td><td></td><td>1.8</td></tr><tr><td>CVCS letdown line (NRHE)</td><td>0.04</td><td>0.04</td><td>0.006</td><td>0.047</td><td>0.32</td><td>1.0</td></tr></tbody></table>

At TRILLO 1, one can observe that the main contributors to activity are <sup>58</sup>Co (≈ 20%) and <sup>60</sup>Co (≈ 65%) in RCS. The same radionuclides also mainly contribute to surface activity at St-LAURENT B1 but the <sup>58</sup>Co impact is significantly higher (40%-60% for <sup>58</sup>Co and ≈ 35% for <sup>60</sup>Co) as shown by the different ratios <sup>58</sup>Co/<sup>60</sup>Co.

One can also notice a specific contribution of <sup>110m</sup>Ag (≈ 50-60%) in CVCS non regenerative heat exchanger. A same tendency is observed at St-LAURENT B1. This phenomenon, well-known in French PWRs [2.], can be explained by thermochemical considerations. Silver is soluble in oxidizing conditions and its solubility increases with temperature. Thus, during oxygenation, it dissolves in the primary coolant from RCS deposits and then precipitates on the cold parts of auxiliary systems, in particular heat exchangers.

Deposited activities in <sup>58</sup>Co and <sup>60</sup>Co are presented in figure 2 for hot legs, in figure 3 for crossover legs, in figure 4 for SG tubes and in figure 5 for CVCS non-regenerative heat exchanger.
<figure><img src="image_3.png" /><figcaption>Figure 2. Comparison CNT121/SLB120 - <sup>58</sup>Co and <sup>60</sup>Co surface activity on Hot Legs</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 3. Comparison CNT121/SLB120 - <sup>58</sup>Co and <sup>60</sup>Co surface activity on Crossover Legs</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4. Comparison CNT121/SLB120 - <sup>58</sup>Co and <sup>60</sup>Co surface activity on SG tubing</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2. Comparison CNT121/SLB120 - <sup>58</sup>Co and <sup>60</sup>Co surface activity on Hot Legs</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 3. Comparison CNT121/SLB120 - <sup>58</sup>Co and <sup>60</sup>Co surface activity on Crossover Legs</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4. Comparison CNT121/SLB120 - <sup>58</sup>Co and <sup>60</sup>Co surface activity on SG tubing</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5. CVCS letdown pipe surface activity – Comparison Trillo 1 / Saint Laurent B1</figcaption></figure>

There is no difference in contamination level between SG tubes hot and cold sides at TRILLO 1 whereas the SG tube cold side at St LAURENT is about 2 times higher than the hot side. This phenomenon has rarely been pointed out on the French fleet and has been mainly observed after a steam generator replacement [3]. This could be explained by a probable strong dependence of the new SG tube oxides solubility to temperature due to differences into stoichiometric ratios between old and new SG tubes.

On one hand, $^{58}$Co surface activity at TRILLO 1 has decreased by a factor 3.8 on average inside primary system since cycle 4. On the other hand, $^{60}$Co surface activity has increased by a factor 2.6 on average. These tendencies are consistent with those generally observed in French PWR [4].

## 2.3 Dose rates

Dose rate measurements were performed around the RCS in order to estimate CEA indexes (figure 8) on primary piping and on SG tubes bundle. The results are reported in table 4.

<figure><img src="image_7.png" /><figcaption>Figure 8. Primary piping and SG tube bundle : CEA dose index</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5. CVCS letdown pipe surface activity – Comparison Trillo 1 / Saint Laurent B1</figcaption></figure>

There is no difference in contamination level between SG tubes hot and cold sides at TRILLO 1 whereas the SG tube cold side at St LAURENT is about 2 times higher than the hot side. This phenomenon has rarely been pointed out on the French fleet and has been mainly observed after a steam generator replacement [3]. This could be explained by a probable strong dependence of the new SG tube oxides solubility to temperature due to differences into stoichiometric ratios between old and new SG tubes.

On one hand, $^{58}$Co surface activity at TRILLO 1 has decreased by a factor 3.8 on average inside primary system since cycle 4. On the other hand, $^{60}$Co surface activity has increased by a factor 2.6 on average. These tendencies are consistent with those generally observed in French PWR [4].

## 2.3 Dose rates

Dose rate measurements were performed around the RCS in order to estimate CEA indexes (figure 8) on primary piping and on SG tubes bundle. The results are reported in table 4.

<figure><img src="image_7.png" /><figcaption>Figure 8. Primary piping and SG tube bundle : CEA dose index</figcaption></figure>
<table><caption>Table 4. Comparison CNT121/SLB120 – CEA dose rate index</caption><thead><tr><th>mSv/h</th><th>CNT 121</th><th>SLB120</th><th>CNT121 / SLB120 Ratio</th></tr></thead><tbody><tr><td>Primary piping</td><td>0.49</td><td>0.27</td><td>1.8</td></tr><tr><td>SG tube bundle</td><td>0.02</td><td>0.09</td><td>0.2</td></tr><tr><td>CVCS NRHE</td><td>0.23</td><td>2.3</td><td>0.1</td></tr></tbody></table>

**Primary piping :**

The average dose rate measured around primary system is higher in TRILLO 1 Cycle 21 than at St LAURENT B1 Cycle 20. The higher dose rate index of TRILLO primary piping is due to higher $^{60}$Co surface activities (factor 2) and to a lower wall thickness (about 6 cm at TRILLO compared to about 7 cm at St-LAURENT). However, one can notice that the dose rate index at TRILLO would have been similar to St-LAURENT if the component thickness had been identical in spite of a higher level of $^{60}$Co contamination because the ambient dose rate was lower at TRILLO than at St-LAURENT (contaminated by-pass pipes influence).

**SG Tubes :**

The lower dose rate index of the TRILLO SG tube bundle is due to a higher SG shell thickness (10 cm in TRILLO compared to 8.5 cm in St-LAURENT). The dose rate index of TRILLO would have been similar to the St-LAURENT one if the component thickness was identical.

**CVCS Regenerative Heat Exchanger :**

The dose rate close to the non regenerative heat exchanger is 10 times lower in TRILLO 1 than in St-LAURENT B1 : 0.23 mSv/h compared to 2.3 mSv/h in SLB120. This is essentially due to the contamination caused by $^{110m}$Ag which is the major contributor (about 65% to 80%) whereas $^{60}$Co contribute for only 15% to 20%.

**Contributors to dose rate :**

As shown in figures 9 and 10, $^{60}$Co is proved to be the major contributor to the dose rates around the primary system at CNT121 (between 80% and 90%), and SLB120 (between 65% and 75%).

Contribution from $^{58}$Co was about 5% in CNT121 and ranged between 10% and 25% in SLB120.

Around SGs, $^{124}$Sb the contribution is not higher than 10%.

<figure><img src="image_8.png" /><figcaption>Figure 9. TRILLO 1 Cycle 21 – Radionuclide contribution to dose rates</figcaption></figure>
<table><caption>Table 4. Comparison CNT121/SLB120 – CEA dose rate index</caption><thead><tr><th>mSv/h</th><th>CNT 121</th><th>SLB120</th><th>CNT121 / SLB120 Ratio</th></tr></thead><tbody><tr><td>Primary piping</td><td>0.49</td><td>0.27</td><td>1.8</td></tr><tr><td>SG tube bundle</td><td>0.02</td><td>0.09</td><td>0.2</td></tr><tr><td>CVCS NRHE</td><td>0.23</td><td>2.3</td><td>0.1</td></tr></tbody></table>

**Primary piping :**

The average dose rate measured around primary system is higher in TRILLO 1 Cycle 21 than at St LAURENT B1 Cycle 20. The higher dose rate index of TRILLO primary piping is due to higher $^{60}$Co surface activities (factor 2) and to a lower wall thickness (about 6 cm at TRILLO compared to about 7 cm at St-LAURENT). However, one can notice that the dose rate index at TRILLO would have been similar to St-LAURENT if the component thickness had been identical in spite of a higher level of $^{60}$Co contamination because the ambient dose rate was lower at TRILLO than at St-LAURENT (contaminated by-pass pipes influence).

**SG Tubes :**

The lower dose rate index of the TRILLO SG tube bundle is due to a higher SG shell thickness (10 cm in TRILLO compared to 8.5 cm in St-LAURENT). The dose rate index of TRILLO would have been similar to the St-LAURENT one if the component thickness was identical.

**CVCS Regenerative Heat Exchanger :**

The dose rate close to the non regenerative heat exchanger is 10 times lower in TRILLO 1 than in St-LAURENT B1 : 0.23 mSv/h compared to 2.3 mSv/h in SLB120. This is essentially due to the contamination caused by $^{110m}$Ag which is the major contributor (about 65% to 80%) whereas $^{60}$Co contribute for only 15% to 20%.

**Contributors to dose rate :**

As shown in figures 9 and 10, $^{60}$Co is proved to be the major contributor to the dose rates around the primary system at CNT121 (between 80% and 90%), and SLB120 (between 65% and 75%).

Contribution from $^{58}$Co was about 5% in CNT121 and ranged between 10% and 25% in SLB120.

Around SGs, $^{124}$Sb the contribution is not higher than 10%.

<figure><img src="image_8.png" /><figcaption>Figure 9. TRILLO 1 Cycle 21 – Radionuclide contribution to dose rates</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 10. St-LAURENT B1 Cycle 20 – Radionuclide contribution to dose rates</figcaption></figure>

## 2.4 Collective dosimetry

The comparison between TRILLO 1 and St-LAURENT B1 indicates that TRILLO 1 had a very lower collective dosimetry than St-LAURENT B1 until 1999. This is due on one hand to higher contamination levels in $^{58}$Co and $^{60}$Co at St-LAURENT B1 mainly on SG tubes (see figure 2 to 5) and on the other hand probably to outage management. St-LAURENT B1 staff have performed a significant improvement program for the last decade, due to efficient ALARA plans. As a result of these actions, figure 9 shows quite similar collective dosimetry for both units in the last 3 years.

<figure><img src="image_10.png" /><figcaption>Figure 9: Collective dose in TRILLO-1 and St-LAURENT B1 from 1989 to 2008</figcaption></figure>

## 3. DISCUSSION

The $^{60}$Co surface activity inside the primary system of TRILLO 1 at cycle 21 was higher than that of St-LAURENT B1 at cycle 20 and that of Konvoy PWRs. The possible reasons for this higher $^{60}$Co contamination level in TRILLO 1 are examined in this section hereafter.
<figure><img src="image_9.png" /><figcaption>Figure 10. St-LAURENT B1 Cycle 20 – Radionuclide contribution to dose rates</figcaption></figure>

## 2.4 Collective dosimetry

The comparison between TRILLO 1 and St-LAURENT B1 indicates that TRILLO 1 had a very lower collective dosimetry than St-LAURENT B1 until 1999. This is due on one hand to higher contamination levels in $^{58}$Co and $^{60}$Co at St-LAURENT B1 mainly on SG tubes (see figure 2 to 5) and on the other hand probably to outage management. St-LAURENT B1 staff have performed a significant improvement program for the last decade, due to efficient ALARA plans. As a result of these actions, figure 9 shows quite similar collective dosimetry for both units in the last 3 years.

<figure><img src="image_10.png" /><figcaption>Figure 9: Collective dose in TRILLO-1 and St-LAURENT B1 from 1989 to 2008</figcaption></figure>

## 3. DISCUSSION

The $^{60}$Co surface activity inside the primary system of TRILLO 1 at cycle 21 was higher than that of St-LAURENT B1 at cycle 20 and that of Konvoy PWRs. The possible reasons for this higher $^{60}$Co contamination level in TRILLO 1 are examined in this section hereafter.
First of all, a chemical conditioning such as the one at cycle 21 can not be the cause of this specific high ⁶⁰Co contamination level. In effect, it has been shown that primary chemistry may have a potential positive (and not negative) impact on contamination [5.] and there is no reason for the impact to only affect ⁶⁰Co.

Stellite hard-facing alloys could be responsible of this elevated ⁶⁰Co source term in case of abnormal wear, for instance during the grinding of valve seats. An abnormal wear of a component such as pump bearings (made of stainless steel with increased Co content) could also be a possible origin of contamination. However, an abnormal wear is generally characterized by the presence of hot spots inside the circuits, which is not the case at TRILLO 1.

Furthermore, Inconel 718 fuel grids could be the main cause of that observed ⁶⁰Co contamination. In fact, since the beginning, fuels assemblies at TRILLO 1 have always contained Inconel 718 grids. During the four first cycles, all fuel spacer grids was made of Inconel 718. Afterwards and until cycle 17, only bottom and top grids were made of Inconel 718, the others grids being made in Zircaloy-4. More recently, the refueling assemblies have been made with a bottom fuel grid made of Inconel 718 and the others made of HPA-4. Since the Co specification of alloy 718 grids was high (1000 ppm), it is possible that the grids (activated by intense neutronic flux) have directly released Co into the primary coolant leading to a ⁶⁰Co contamination build-up.

It should also be pointed out that ⁶⁰Co and especially ⁵⁸Co surface activities were already relatively high at the end of cycle 4. Thus, this contamination, that probably occurred during the first cycles, could involve a nickel-based material as Inconel 718 grids (about 52% of Ni).

Moreover, the nickel concentration has been high in operation since cycle 16 at least (between 1 and 10 ppb) and do not decrease as cycles go along. This level is not common, particularly in a PWR equipped with alloy 800 SG tubes (the usual values are measured between 0.1 and 1 ppb [9.]) and seems to show that Ni source term is “regenerated” after each outage, what could be consistent with refueling assemblies equipped with Inconel 718 grids.

Contamination levels at TRILLO-1 look, to a lesser extent, like those of BIBLIS-B, a pre-KONVOY PWR i.e. low ⁵⁸Co contamination levels (surface and released) and relatively high ⁶⁰Co surface activity for TRILLO-1 or very high for BIBLIS-B. The very high ⁶⁰Co contamination level in BIBLIS-B is due to the use of nickel-plated fuel spacers with an increased cobalt content [6.]. Inconel 718 fuel grids could also be the cause in TRILLO-1.

A priori, the SG tubing Co content is not responsible for this high ⁶⁰Co source term : the specified mean value was 150 ppm in TRILLO, which is similar to St-LAURENT-B1(measured content of 122 ppm) [4.].

Finally, The fact that the Ni concentration has remained at a high level for several cycles could be also due to the CVCS design : there is no particle filter upstream from the ion exchange resins. It seems that neither the mixed bed resins nor the mechanical filter downstream from them could trap particles efficiently.

# 4. CONCLUSION

The EMECC campaign performed during the beginning of the refuelling outage planned at the end of cycle 21 in TRILLO-1 revealed the following points in particular :
First of all, a chemical conditioning such as the one at cycle 21 can not be the cause of this specific high ⁶⁰Co contamination level. In effect, it has been shown that primary chemistry may have a potential positive (and not negative) impact on contamination [5.] and there is no reason for the impact to only affect ⁶⁰Co.

Stellite hard-facing alloys could be responsible of this elevated ⁶⁰Co source term in case of abnormal wear, for instance during the grinding of valve seats. An abnormal wear of a component such as pump bearings (made of stainless steel with increased Co content) could also be a possible origin of contamination. However, an abnormal wear is generally characterized by the presence of hot spots inside the circuits, which is not the case at TRILLO 1.

Furthermore, Inconel 718 fuel grids could be the main cause of that observed ⁶⁰Co contamination. In fact, since the beginning, fuels assemblies at TRILLO 1 have always contained Inconel 718 grids. During the four first cycles, all fuel spacer grids was made of Inconel 718. Afterwards and until cycle 17, only bottom and top grids were made of Inconel 718, the others grids being made in Zircaloy-4. More recently, the refueling assemblies have been made with a bottom fuel grid made of Inconel 718 and the others made of HPA-4. Since the Co specification of alloy 718 grids was high (1000 ppm), it is possible that the grids (activated by intense neutronic flux) have directly released Co into the primary coolant leading to a ⁶⁰Co contamination build-up.

It should also be pointed out that ⁶⁰Co and especially ⁵⁸Co surface activities were already relatively high at the end of cycle 4. Thus, this contamination, that probably occurred during the first cycles, could involve a nickel-based material as Inconel 718 grids (about 52% of Ni).

Moreover, the nickel concentration has been high in operation since cycle 16 at least (between 1 and 10 ppb) and do not decrease as cycles go along. This level is not common, particularly in a PWR equipped with alloy 800 SG tubes (the usual values are measured between 0.1 and 1 ppb [9.]) and seems to show that Ni source term is “regenerated” after each outage, what could be consistent with refueling assemblies equipped with Inconel 718 grids.

Contamination levels at TRILLO-1 look, to a lesser extent, like those of BIBLIS-B, a pre-KONVOY PWR i.e. low ⁵⁸Co contamination levels (surface and released) and relatively high ⁶⁰Co surface activity for TRILLO-1 or very high for BIBLIS-B. The very high ⁶⁰Co contamination level in BIBLIS-B is due to the use of nickel-plated fuel spacers with an increased cobalt content [6.]. Inconel 718 fuel grids could also be the cause in TRILLO-1.

A priori, the SG tubing Co content is not responsible for this high ⁶⁰Co source term : the specified mean value was 150 ppm in TRILLO, which is similar to St-LAURENT-B1(measured content of 122 ppm) [4.].

Finally, The fact that the Ni concentration has remained at a high level for several cycles could be also due to the CVCS design : there is no particle filter upstream from the ion exchange resins. It seems that neither the mixed bed resins nor the mechanical filter downstream from them could trap particles efficiently.

## 4. CONCLUSION

The EMECC campaign performed during the beginning of the refuelling outage planned at the end of cycle 21 in TRILLO-1 revealed the following points in particular :
* The ⁵⁸Co surface activities on the primary coolant pipes and on the steam generator tubing in TRILLO-1 at cycle 21 were 2 times lower than those in St-LAURENT-B1 at cycle 20 and similar to those in the Konvoy PWRs. On the other hand, the Ni concentration has been abnormally high for several cycles in TRILLO-1.

* The ⁵⁸Co peak during the outage 21 in TRILLO-1 was very low and similar to the one in SAINT-LAURENT-B1 at cycle 20 and to those in the Konvoy PWRs.

* The reason for the low ⁵⁸Co surface activities and ⁵⁸Co peak in TRILLO-1 is the SG tubing alloy 800 (low Ni content), whereas the low ⁵⁸Co peak in St-LAURENT-B1 at cycle 20 is due to the alloy 690 SG tubing surface finish.

* The ⁶⁰Co surface activities of the primary system were higher by a factor 2 in TRILLO-1 at cycle 21 compared to SAINT-LAURENT-B1 at cycle 20. The use of Inconel 718 fuel grids with a high Co impurity content must have played a great part in this high ⁶⁰Co contamination level.

* The main contributor to the dose rates around the primary circuit in TRILLO-1 at cycle 21 was ⁶⁰Co, between 80% and 90%. As the ⁶⁰Co radioactive half-life is 5.3 years, the dose rates will remain relatively high for many cycles to come.

**5. REFERENCES**

[1.] Eimecke, R. and al., "Ensemble de Mesure et d'Etude de la Contamination des Circuits (EMECC)", 7th International Conference on Radiation Shielding, Bournemouth (England), 12-15/09/88.

[2.] Dacquait, F. and al., "Auxiliary System Contamination in French PWRs", Proceedings of the International Conference on Water Chemistry of Nuclear Reactors Systems, 15-18/09/08, Berlin (Germany), L11-4, 2008.

[3.] Dacquait, F. and al., "The Impact of Steam Generator Replacement on PWR Primary System Contamination", International Conference on Water Chemistry of Nuclear Reactors Systems, 4-7/10/10, Quebec (Canada), This Conference.

[4.] Dacquait, F. and al., "Status of primary system contamination in French PWRs", Proceedings of the International Conference on Water Chemistry of Nuclear Reactors Systems, 11-14/10/04, San Francisco (USA), Session 2 Paper 4.

[5.] Bretelle, J.L. and al., "EDF strategy for the primary coolant pH", Proceedings of the 1998 JAIF International Conference on Water Chemistry in Nuclear Power Plants, Kashiwasaki, 11-14/10/04, San Francisco (USA).

[6.] Haag, J. and al., "Operational Experience with Zinc Addition in German PWR", Proceedings of the 1998 JAIF International Conference on Water Chemistry in Nuclear Power Plants, Kashiwasaki, 13-16/98/10, Tokio (Japan), p. 505-509

[7.] Dacquait, F. and al., "Low primary system contamination levels in some PWRs", Proceedings of the International Conference on Water Chemistry of Nuclear Reactors Systems, 23-26/10/06, Jeju Island (Korea), Session 2 Paper 17.

[8.] Dacquait, F. and al., "Corrosion product transfer in French PWRs during shutdown", Proceedings of the International Conference on Water Chemistry of Nuclear Reactors Systems, 22-26/04/02, Avignon (France), Paper 113.
* The ⁵⁸Co surface activities on the primary coolant pipes and on the steam generator tubing in TRILLO-1 at cycle 21 were 2 times lower than those in St-LAURENT-B1 at cycle 20 and similar to those in the Konvoy PWRs. On the other hand, the Ni concentration has been abnormally high for several cycles in TRILLO-1.

* The ⁵⁸Co peak during the outage 21 in TRILLO-1 was very low and similar to the one in SAINT-LAURENT-B1 at cycle 20 and to those in the Konvoy PWRs.

* The reason for the low ⁵⁸Co surface activities and ⁵⁸Co peak in TRILLO-1 is the SG tubing alloy 800 (low Ni content), whereas the low ⁵⁸Co peak in St-LAURENT-B1 at cycle 20 is due to the alloy 690 SG tubing surface finish.

* The ⁶⁰Co surface activities of the primary system were higher by a factor 2 in TRILLO-1 at cycle 21 compared to SAINT-LAURENT-B1 at cycle 20. The use of Inconel 718 fuel grids with a high Co impurity content must have played a great part in this high ⁶⁰Co contamination level.

* The main contributor to the dose rates around the primary circuit in TRILLO-1 at cycle 21 was ⁶⁰Co, between 80% and 90%. As the ⁶⁰Co radioactive half-life is 5.3 years, the dose rates will remain relatively high for many cycles to come.

## 5. REFERENCES

[1.] Eimecke, R. and al., "Ensemble de Mesure et d'Etude de la Contamination des Circuits (EMECC)", 7th International Conference on Radiation Shielding, Bournemouth (England), 12-15/09/88.

[2.] Dacquait, F. and al., "Auxiliary System Contamination in French PWRs", Proceedings of the International Conference on Water Chemistry of Nuclear Reactors Systems, 15-18/09/08, Berlin (Germany), L11-4, 2008.

[3.] Dacquait, F. and al., "The Impact of Steam Generator Replacement on PWR Primary System Contamination", International Conference on Water Chemistry of Nuclear Reactors Systems, 4-7/10/10, Quebec (Canada), This Conference.

[4.] Dacquait, F. and al., "Status of primary system contamination in French PWRs", Proceedings of the International Conference on Water Chemistry of Nuclear Reactors Systems, 11-14/10/04, San Francisco (USA), Session 2 Paper 4.

[5.] Bretelle, J.L. and al., "EDF strategy for the primary coolant pH", Proceedings of the 1998 JAIF International Conference on Water Chemistry in Nuclear Power Plants, Kashiwasaki, 11-14/10/04, San Francisco (USA).

[6.] Haag, J. and al., "Operational Experience with Zinc Addition in German PWR", Proceedings of the 1998 JAIF International Conference on Water Chemistry in Nuclear Power Plants, Kashiwasaki, 13-16/98/10, Tokio (Japan), p. 505-509

[7.] Dacquait, F. and al., "Low primary system contamination levels in some PWRs", Proceedings of the International Conference on Water Chemistry of Nuclear Reactors Systems, 23-26/10/06, Jeju Island (Korea), Session 2 Paper 17.

[8.] Dacquait, F. and al., "Corrosion product transfer in French PWRs during shutdown", Proceedings of the International Conference on Water Chemistry of Nuclear Reactors Systems, 22-26/04/02, Avignon (France), Paper 113.
[9.] Stellwag, B. and al., "Short-term and long-term effect of Zinc injection on RCS chemistry and dose rates at Siemens PWR plants", Proceedings of the International Conference on Water Chemistry of Nuclear Reactors Systems, 11-14/10/04, San Francisco (USA), Session 2 Paper 6.
[9.] Stellwag, B. and al., "Short-term and long-term effect of Zinc injection on RCS chemistry and dose rates at Siemens PWR plants", Proceedings of the International Conference on Water Chemistry of Nuclear Reactors Systems, 11-14/10/04, San Francisco (USA), Session 2 Paper 6.