# Auxiliary system contamination in French PWRs

Frédéric Dacquait, Luc Guinard, Stephane Taunier, Jean-Luc Bretelle, Alain Rocher

► To cite this version:

Frédéric Dacquait, Luc Guinard, Stephane Taunier, Jean-Luc Bretelle, Alain Rocher. Auxiliary system contamination in French PWRs. NPC 08 - International Conference on Water Chemistry of Nuclear Reactors Systems, Sep 2008, Berlin, Germany. paper L11-4. cea-04463630

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https://cea.hal.science/cea-04463630

Submitted on 17 Feb 2024

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Distributed under a Creative Commons Attribution 4.0 International License
Auxiliary system contamination in French PWRs

Frédéric Dacquait, Luc Guinard, Stephane Taunier, Jean-Luc Bretelle, Alain
Rocher

► To cite this version:

Frédéric Dacquait, Luc Guinard, Stephane Taunier, Jean-Luc Bretelle, Alain Rocher. Auxiliary system contamination in French PWRs. NPC 08 - International Conference on Water Chemistry of Nuclear Reactors Systems, Sep 2008, Berlin, Germany. paper L11-4. cea-04463630

HAL Id: cea-04463630

https://cea.hal.science/cea-04463630v1

Submitted on 17 Feb 2024

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

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

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

Distributed under a Creative Commons Attribution 4.0 International License
# Auxiliary System Contamination in French PWRs

F. Dacquait¹, L. Guinard², S. Taunier³, J.L. Bretelle⁴, A. Rocher⁴

¹CEA/DEN, ²EDF/SEPTEN, ³EDF/CEIDRE, ⁴EDF/UNIE

CEA, DEN, F-13108 Saint-Paul-Lez-Durance, France.

E-mail: frederic.dacquait@cea.fr

## Abstract

In order to study the impact of design and operating parameters on French PWR contamination, the CEA, by request of EDF, has been performing in-situ gamma spectrometry measurements using the so-called EMECC device for 37 years. These measurements have allowed researchers to characterize contamination levels accurately (surface activities by radionuclide), mainly in primary systems, but also in some reactor auxiliary systems such as the Chemical and Volume Control System (CVCS), the Residual Heat Removal System (RHRS), the Nuclear Sampling System (NSS) or the Fuel Pool Cooling and Cleanup System (FPCS).

Based on EMECC campaigns performed in different standardized plant series, this paper shows that the contamination of nuclear auxiliary systems presents differences in comparison to the contamination of the primary system:

- The ⁵⁸Co and ⁶⁰Co deposited activities on the CVCS, RHRS, NSS and FPCS are generally lower. On the other hand, the ¹¹⁰mAg contamination is higher with a considerable recontamination resulting during cold shutdowns.

- The dose rates around the auxiliary systems are generally higher.

- The ⁵⁸Co contribution and especially the ¹¹⁰mAg contribution are both higher. For auxiliary systems that do not operate for several months, the ⁶⁰Co contribution emerges as the major one.

- The nuclear auxiliary systems are the only circuits that allow us to measure the ⁵¹Cr surface activities.

These characteristics are due to different design and operating conditions, particularly the following : the presence of heat exchangers and lower fluid temperature, different flow rates, thinner thickness of pipes and of exchanger shells, the presence of filtering systems and operations during a short period.

A final observation is that, the peculiar behavior of ¹¹⁰mAg inside the NSS generally results in a poor estimation of the ¹¹⁰mAg volume activity in the primary coolant.

## 1 INTRODUCTION

In order to study the impact of design and operating parameters on the French PWR contamination by Activated Corrosion Products (ACP), the CEA, by request of EDF, has been performing in-situ gamma spectrometry measurements using the so-called EMECC device [1] for the past 37 years. These measurements have allowed researchers to characterize the contamination levels accurately (surface activities by radionuclides), mainly in primary systems [2] but also in some nuclear auxiliary systems. Knowledge of the contamination levels and their types in these systems is important for many reasons:

- The maintenance work carried out in the vicinity of these systems can influence the dosimetry greatly.

- Some radionuclides contaminate these systems preferentially. Thus, they can highlight a peculiar contamination, which could not be pinpointed by the EMECC measurements of the primary system.

- The surface activities of radionuclides emitting low energy gamma rays can only be measured in these systems.

- It contributes to studies on corrosion product transfer in PWR systems.

This paper will present:

- the surface activities in some auxiliary systems and their comparison to those in the primary system,

- the reasons for the contamination differences between auxiliary systems and the primary system,

- and the explanation of the peculiar behavior of ¹¹⁰mAg.

## 2 EMECC CAMPAIGNS OF NUCLEAR AUXILIARY SYSTEMS

In this paper, nuclear auxiliary systems are defined as systems, other than the primary system, in which primary coolant circulates. These systems are thus contaminated and in particular by ACP. The surface activities of the components in these circuits (piping, heat exchangers, valves...) are measured by gamma spectrometry measurements using the EMECC device.
# Auxiliary System Contamination in French PWRs

F. Dacquait¹, L. Guinard², S. Taunier³, J.L. Bretelle⁴, A. Rocher⁴

¹CEA/DEN, ²EDF/SEPTEN, ³EDF/CEIDRE, ⁴EDF/UNIE

CEA, DEN, F-13108 Saint-Paul-Lez-Durance, France.

E-mail: frederic.dacquait@cea.fr

## Abstract

In order to study the impact of design and operating parameters on French PWR contamination, the CEA, by request of EDF, has been performing in-situ gamma spectrometry measurements using the so-called EMECC device for 37 years. These measurements have allowed researchers to characterize contamination levels accurately (surface activities by radionuclide), mainly in primary systems, but also in some reactor auxiliary systems such as the Chemical and Volume Control System (CVCS), the Residual Heat Removal System (RHRS), the Nuclear Sampling System (NSS) or the Fuel Pool Cooling and Cleanup System (FPCS).

Based on EMECC campaigns performed in different standardized plant series, this paper shows that the contamination of nuclear auxiliary systems presents differences in comparison to the contamination of the primary system:

- The ⁵⁸Co and ⁶⁰Co deposited activities on the CVCS, RHRS, NSS and FPCS are generally lower. On the other hand, the ¹¹⁰mAg contamination is higher with a considerable recontamination resulting during cold shutdowns.

- The dose rates around the auxiliary systems are generally higher.

- The ⁵⁸Co contribution and especially the ¹¹⁰mAg contribution are both higher. For auxiliary systems that do not operate for several months, the ⁶⁰Co contribution emerges as the major one.

- The nuclear auxiliary systems are the only circuits that allow us to measure the ⁵¹Cr surface activities.

These characteristics are due to different design and operating conditions, particularly the following : the presence of heat exchangers and lower fluid temperature, different flow rates, thinner thickness of pipes and of exchanger shells, the presence of filtering systems and operations during a short period.

A final observation is that, the peculiar behavior of ¹¹⁰mAg inside the NSS generally results in a poor estimation of the ¹¹⁰mAg volume activity in the primary coolant.

## 1 INTRODUCTION

In order to study the impact of design and operating parameters on the French PWR contamination by Activated Corrosion Products (ACP), the CEA, by request of EDF, has been performing in-situ gamma spectrometry measurements using the so-called EMECC device [1] for the past 37 years. These measurements have allowed researchers to characterize the contamination levels accurately (surface activities by radionuclides), mainly in primary systems [2] but also in some nuclear auxiliary systems. Knowledge of the contamination levels and their types in these systems is important for many reasons:

- The maintenance work carried out in the vicinity of these systems can influence the dosimetry greatly.

- Some radionuclides contaminate these systems preferentially. Thus, they can highlight a peculiar contamination, which could not be pinpointed by the EMECC measurements of the primary system.

- The surface activities of radionuclides emitting low energy gamma rays can only be measured in these systems.

- It contributes to studies on corrosion product transfer in PWR systems.

This paper will present:

- the surface activities in some auxiliary systems and their comparison to those in the primary system,

- the reasons for the contamination differences between auxiliary systems and the primary system,

- and the explanation of the peculiar behavior of ¹¹⁰mAg.

## 2 EMECC CAMPAIGNS OF NUCLEAR AUXILIARY SYSTEMS

In this paper, nuclear auxiliary systems are defined as systems, other than the primary system, in which primary coolant circulates. These systems are thus contaminated and in particular by ACP. The surface activities of the components in these circuits (piping, heat exchangers, valves...) are measured by gamma spectrometry measurements using the EMECC device.
The background noise and the volume activities are taken into account for each EMECC measurement in order to determine the surface activities. Usually, the primary system contamination is the main interest of EMECC campaigns but some of them focus on the auxiliary system contamination. The main auxiliary systems analyzed are:

- CVCS: Chemical and Volume Control System

- RHRS: Residual Heat Removal System

- NSS: Nuclear Sampling System

- FPCS: Fuel Pool Cooling and Cleanup System

The measurement points are determined according to the probability of contamination changes: the presence of heat exchangers and lower fluid temperature, the presence of filtering systems, operation during a short period...

Two EDF PWR cases are presented below: a 900 MWe PWR (Reactor A) and a 1300 MWe PWR (Reactor B).

## 2.1 Reactor A

This 900 MWe PWR is equipped with steam generator tubing in alloy 600TT. A boron-lithium coordinated chemistry at pH 7.0 was applied until cycle 3 and since then the aimed pH has been 7.2 at 300°C.

Several EMECC campaigns of the nuclear auxiliary system contamination were carried out in this PWR at cycle 11:

a. During the power operation, 4 months before the refueling outage,

b. Before the primary system oxygenation at the beginning of the refueling shutdown,

c. After the primary system oxygenation,

d. At the end of the shutdown.

### 2.1.1 Surface activities and dose rates

The list of the EMECC measurement points and their dose rates measured in contact with components of the CVCS, RHRS and FCPS is presented in Table 1. The values of dose rates in italics were measured with a shielded probe to mitigate the ambient dose rates. The EMECC measurement points of the Reactor Coolant System (RCS) are also indicated in this table.

Figure 1, Figure 2, Figure 3 and Figure 4 present the ⁵⁸Co, ⁶⁰Co, ⁵¹Cr and ¹¹⁰mAg surface activities in the RCS, CVCS and RHRS respectively; and Figure 5 in the FPCS. The ⁵⁸Co/⁶⁰Co surface activity ratio in the circuits is described in Figure 6.

<table><caption>Table 1: EMECC measurement points and dose rates</caption><thead><tr><th rowspan="2">EMECC Points</th><th colspan="4">Dose rates (mSv/h)</th></tr><tr><th>a</th><th>b</th><th>c</th><th>d</th></tr></thead><tbody><tr><td colspan="5">CVCS</td></tr><tr><td>Letdown line / Upstream Regenerative heat exchanger</td><td></td><td>2.7</td><td>3.0</td><td></td></tr><tr><td>Regenerative heat exchanger</td><td></td><td>2.1</td><td>2.5</td><td></td></tr><tr><td>Upstream Non-regenerative heat exchanger</td><td>1.4</td><td>2.4</td><td>2.4</td><td></td></tr><tr><td>Non-regenerative heat exchanger</td><td>2.7</td><td>11</td><td>20</td><td></td></tr><tr><td>Upstream Filter</td><td>1.8</td><td>6.0</td><td></td><td></td></tr><tr><td>Downstream Filter</td><td>0.65</td><td>1.2</td><td></td><td></td></tr><tr><td>Downstream Charging pump</td><td>1.2</td><td></td><td></td><td></td></tr><tr><td colspan="5">RHRS</td></tr><tr><td>Upstream Main valves</td><td></td><td>1.5</td><td>1.0</td><td></td></tr><tr><td>Upstream Pumps</td><td></td><td>0.21</td><td></td><td></td></tr><tr><td>Heat exchanger Bypass</td><td></td><td></td><td></td><td></td></tr><tr><td>Heat exchanger</td><td></td><td></td><td>0.58</td><td></td></tr><tr><td>Downstream Heat exchanger</td><td></td><td></td><td>0.64</td><td></td></tr><tr><td colspan="5">FPCS</td></tr><tr><td>Cooling/Downstream Spent fuel pool</td><td>0.03</td><td></td><td></td><td>0.09</td></tr><tr><td>Cooling/Upstream heat exchanger</td><td>0.19</td><td></td><td></td><td>0.26</td></tr><tr><td>Cooling/Downstream Heat exchanger</td><td>0.02</td><td></td><td></td><td>0.08</td></tr><tr><td>Cleanup/Upstream Filter</td><td>0.03</td><td></td><td></td><td>0.10</td></tr><tr><td>Cleanup/Downstream Filter</td><td>0.06</td><td></td><td></td><td>0.10</td></tr><tr><td colspan="5">RCS</td></tr><tr><td>Hot leg</td><td></td><td>0.84</td><td>0.78</td><td></td></tr><tr><td>SG tubing</td><td></td><td>0.32</td><td>0.32</td><td></td></tr><tr><td>Crossover leg</td><td></td><td>0.23</td><td></td><td></td></tr><tr><td>SG bypass</td><td></td><td></td><td></td><td></td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>Figure 1: Reactor A - Cycle 11 - ⁵⁸Co surface activities in RCS, CVCS and RHRS</figcaption></figure>
The background noise and the volume activities are taken into account for each EMECC measurement in order to determine the surface activities. Usually, the primary system contamination is the main interest of EMECC campaigns but some of them focus on the auxiliary system contamination. The main auxiliary systems analyzed are:

- CVCS: Chemical and Volume Control System

- RHRS: Residual Heat Removal System

- NSS: Nuclear Sampling System

- FPCS: Fuel Pool Cooling and Cleanup System

The measurement points are determined according to the probability of contamination changes: the presence of heat exchangers and lower fluid temperature, the presence of filtering systems, operation during a short period...

Two EDF PWR cases are presented below: a 900 MWe PWR (Reactor A) and a 1300 MWe PWR (Reactor B).

## 2.1 Reactor A

This 900 MWe PWR is equipped with steam generator tubing in alloy 600TT. A boron-lithium coordinated chemistry at pH 7.0 was applied until cycle 3 and since then the aimed pH has been 7.2 at 300°C.

Several EMECC campaigns of the nuclear auxiliary system contamination were carried out in this PWR at cycle 11:

a. During the power operation, 4 months before the refueling outage,

b. Before the primary system oxygenation at the beginning of the refueling shutdown,

c. After the primary system oxygenation,

d. At the end of the shutdown.

### 2.1.1 Surface activities and dose rates

The list of the EMECC measurement points and their dose rates measured in contact with components of the CVCS, RHRS and FCPS is presented in Table 1. The values of dose rates in italics were measured with a shielded probe to mitigate the ambient dose rates. The EMECC measurement points of the Reactor Coolant System (RCS) are also indicated in this table.

Figure 1, Figure 2, Figure 3 and Figure 4 present the ⁵⁸Co, ⁶⁰Co, ⁵¹Cr and ¹¹⁰mAg surface activities in the RCS, CVCS and RHRS respectively; and Figure 5 in the FPCS. The ⁵⁸Co/⁶⁰Co surface activity ratio in the circuits is described in Figure 6.

<table><caption>Table 1: EMECC measurement points and dose rates</caption><thead><tr><th rowspan="2">EMECC Points</th><th colspan="4">Dose rates (mSv/h)</th></tr><tr><th>a</th><th>b</th><th>c</th><th>d</th></tr></thead><tbody><tr><td colspan="5">CVCS</td></tr><tr><td>Letdown line / Upstream Regenerative heat exchanger</td><td></td><td>2.7</td><td>3.0</td><td></td></tr><tr><td>Regenerative heat exchanger</td><td></td><td>2.1</td><td>2.5</td><td></td></tr><tr><td>Upstream Non-regenerative heat exchanger</td><td>1.4</td><td>2.4</td><td>2.4</td><td></td></tr><tr><td>Non-regenerative heat exchanger</td><td>2.7</td><td>11</td><td>20</td><td></td></tr><tr><td>Upstream Filter</td><td>1.8</td><td>6.0</td><td></td><td></td></tr><tr><td>Downstream Filter</td><td>0.65</td><td>1.2</td><td></td><td></td></tr><tr><td>Downstream Charging pump</td><td>1.2</td><td></td><td></td><td></td></tr><tr><td colspan="5">RHRS</td></tr><tr><td>Upstream Main valves</td><td></td><td>1.5</td><td>1.0</td><td></td></tr><tr><td>Upstream Pumps</td><td></td><td>0.21</td><td></td><td></td></tr><tr><td>Heat exchanger Bypass</td><td></td><td></td><td></td><td></td></tr><tr><td>Heat exchanger</td><td></td><td></td><td>0.58</td><td></td></tr><tr><td>Downstream Heat exchanger</td><td></td><td></td><td>0.64</td><td></td></tr><tr><td colspan="5">FPCS</td></tr><tr><td>Cooling/Downstream Spent fuel pool</td><td>0.03</td><td></td><td></td><td>0.09</td></tr><tr><td>Cooling/Upstream heat exchanger</td><td>0.19</td><td></td><td></td><td>0.26</td></tr><tr><td>Cooling/Downstream Heat exchanger</td><td>0.02</td><td></td><td></td><td>0.08</td></tr><tr><td>Cleanup/Upstream Filter</td><td>0.03</td><td></td><td></td><td>0.10</td></tr><tr><td>Cleanup/Downstream Filter</td><td>0.06</td><td></td><td></td><td>0.10</td></tr><tr><td colspan="5">RCS</td></tr><tr><td>Hot leg</td><td></td><td>0.84</td><td>0.78</td><td></td></tr><tr><td>SG tubing</td><td></td><td>0.32</td><td>0.32</td><td></td></tr><tr><td>Crossover leg</td><td></td><td>0.23</td><td></td><td></td></tr><tr><td>SG bypass</td><td></td><td></td><td></td><td></td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>Figure 1: Reactor A - Cycle 11 - ⁵⁸Co surface activities in RCS, CVCS and RHRS</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2: Reactor A - Cycle 11 - <sup>60</sup>Co surface activities in RCS, CVCS and RHRS</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 5: Reactor A – Cycle 11 – Surface activities in FPCS</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 3: Reactor A - Cycle 11 - <sup>51</sup>Cr surface activities in RCS, CVCS and RHRS</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 6: Reactor A – Cycle 11 – <sup>58</sup>Co/<sup>60</sup>Co surface activity ratio in the systems</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 4: Reactor A – Cycle 11 – <sup>110m</sup>Ag surface activities in RCS, CVCS and RHRS</figcaption></figure>

### 2.1.2 Comments

Compared to the primary system, the dose rates of the CVCS are higher, those of the RHRS are similar and those of the FPCS are generally lower except for the pipe upstream from the heat exchanger. The surface activities of the CVCS and the RHRS are similar or lower than those of the RCS except for the <sup>110m</sup>Ag contamination, which is higher in the CVCS. Thus, when compared to the RCS, the higher or similar dose rates in the CVCS and the RHRS (see Table 1) are due to the lessened thickness of piping or to the higher <sup>110m</sup>Ag contamination level.

In the CVCS, downstream from the regenerative heat exchanger, the <sup>58</sup>Co deposited activities are less than those upstream (factor about 2) and they are much lower downstream from the particle filter (factor about 200) (see Figure 1). Downstream from the charging pump and therefore the ion exchanger resins, the ACP surface activities are higher than upstream. This could be attributed to the saturated ion exchanger or to the fact that the ion exchanger bypass valve was not closed properly.

The <sup>60</sup>Co surface activities tend to decrease along the CVCS (factor 10) particularly downstream from the particle filter (factor about 40) (see Figure 2).

The <sup>58</sup>Co/<sup>60</sup>Co ratios increase along the CVCS, particularly in the regenerative heat exchanger and the
<figure><img src="image_3.png" /><figcaption>Figure 2: Reactor A - Cycle 11 - <sup>60</sup>Co surface activities in RCS, CVCS and RHRS</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 5: Reactor A – Cycle 11 – Surface activities in FPCS</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 3: Reactor A - Cycle 11 - <sup>51</sup>Cr surface activities in RCS, CVCS and RHRS</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 6: Reactor A – Cycle 11 – <sup>58</sup>Co/<sup>60</sup>Co surface activity ratio in the systems</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 4: Reactor A – Cycle 11 – <sup>110m</sup>Ag surface activities in RCS, CVCS and RHRS</figcaption></figure>

## 2.1.2 Comments

Compared to the primary system, the dose rates of the CVCS are higher, those of the RHRS are similar and those of the FPCS are generally lower except for the pipe upstream from the heat exchanger. The surface activities of the CVCS and the RHRS are similar or lower than those of the RCS except for the <sup>110m</sup>Ag contamination, which is higher in the CVCS. Thus, when compared to the RCS, the higher or similar dose rates in the CVCS and the RHRS (see Table 1) are due to the lessened thickness of piping or to the higher <sup>110m</sup>Ag contamination level.

In the CVCS, downstream from the regenerative heat exchanger, the <sup>58</sup>Co deposited activities are less than those upstream (factor about 2) and they are much lower downstream from the particle filter (factor about 200) (see Figure 1). Downstream from the charging pump and therefore the ion exchanger resins, the ACP surface activities are higher than upstream. This could be attributed to the saturated ion exchanger or to the fact that the ion exchanger bypass valve was not closed properly.

The <sup>60</sup>Co surface activities tend to decrease along the CVCS (factor 10) particularly downstream from the particle filter (factor about 40) (see Figure 2).

The <sup>58</sup>Co/<sup>60</sup>Co ratios increase along the CVCS, particularly in the regenerative heat exchanger and the
pipe downstream (see Figure 6). They are higher than the ratios of the primary system.

The $^{51}$Cr surface activities decrease by a factor of 3 in the non-regenerative heat exchanger and are not detectable downstream from the particle filter (see Figure 3).

On the other hand, during the shutdown before the primary system oxygenation, the $^{110m}$Ag deposited activities are practically constant along the CVCS up to the particle filter (see Figure 4). Downstream from the filter, they are lower by a factor of about 10.

The EMECC measurements in the nuclear auxiliary building (from the pipe upstream from the non regenerative heat exchanger) show that the surface activities have increased during power operation except for the $^{110m}$Ag contamination which tends to decrease.

The primary system oxygenation by injection of H₂O₂ has not led to a change in the contamination level of $^{58}$Co (peak of 100 GBq/t) and $^{60}$Co (peak of 0.62 GBq/t) in the CVCS nor in the RCS, thanks to the oxygenation temperature of 80°C [3]. On the other hand, the $^{110m}$Ag surface activities have decreased upstream from the non-regenerative heat exchanger and have increased by a factor of 5 in this exchanger (with a measured maximum value of $^{110m}$Ag volume activity after oxygenation of 0.63 GBq/t).

In the **RHRS**, before its startup, the ACP surface activities on the pipe upstream from the main four valves are similar to those on the primary system since the primary coolant circulates in this part of the RHRS during power operation. After its startup, the $^{58}$Co and $^{51}$Cr contaminations tend to decrease (probably, erosion of a deposit enriched in $^{58}$Co and $^{51}$Cr) and the $^{60}$Co surface activity is constant (see Figure 1, Figure 2 and Figure 3). It should be noted that in the second half of cycle 11, the $^{58}$Co and $^{51}$Cr volume activities under particle form have increased. These particles have formed a deposit not very adhesive that could be eroded with flow rate changes. Downstream from the main four valves, before their opening, the very low or undetectable contamination level of short radioactive half-life radionuclides ($^{58}$Co: 70.8 days, $^{51}$Cr: 27.7 days) shows that this contamination is old and formed during the previous unit outage (the primary coolant did not circulate in the RHRS during the reactor power operation). On the other hand, after its startup, the RHRS parts downstream from the main valves are re-contaminated. However, the contamination levels in the RHRS remain low compared to the primary system, but with a similar $^{58}$Co/$^{60}$Co ratio (see Figure 6). The $^{58}$Co, $^{60}$Co and $^{51}$Cr surface activities are lower and lower along the RHRS particularly in the heat exchanger. However, it is the opposite case for the $^{110m}$Ag: after the H₂O₂ injection, the deposited activity is higher in the heat exchanger (see Figure 4).

In the **FPCS** (see Figure 5), during reactor power operation, the main radionuclide is $^{60}$Co due to its long radioactive half-life (5.27 years) so the contamination is old. The contamination levels are practically constant in the different parts of the system except in the cooling line

upstream from the heat exchanger, which is also downstream from the main FPCS tank and in which radioactive water circulates. During the refueling shutdown, due to the filling of the refueling cavity and the shifting of fuel in the spent fuel pit, the ACP contamination increases in the FPCS, particularly the $^{110m}$Ag contamination, which has become predominant.

At the end of the refueling shutdown, the $^{58}$Co/$^{60}$Co ratio in the FPCS is similar to the ratio in the RCS.

What must be pointed out is that generally the $^{51}$Cr surface activities can not be measured in the **RCS** due to its low gamma energy of 320 keV and the great thicknesses of the primary coolant pipes and steam generator shell. Thanks to a “nuclear auxiliary system”, the steam generator bypass, the EMECC measurement of this thin pipe provides the $^{51}$Cr surface activity of the primary system (see Figure 3).

## 2.2 Reactor B

This 1300 MWe PWR is equipped with steam generator tubing in alloy 600TT. A boron-lithium coordinated chemistry at pH 7.0 was applied until cycle 5 and since then the aimed pH has been 7.2 at 300°C.

An EMECC campaign of the nuclear auxiliary system contamination was performed in this PWR at cycle 10 at the beginning of the refueling shutdown before and after the primary system oxygenation.

### 2.2.1 Surface activities and dose rates

The list of the EMECC measurement points and their dose rates measured in contact with components of the CVCS, RHRS and NSS is presented in Table 2. The values of dose rates in italics were measured with a shielded probe to mitigate the ambient dose rates. The EMECC measurement points of the Reactor Coolant System (RCS) are also indicated in this table.

Figure 7 and Figure 8 present the $^{58}$Co and $^{60}$Co surface activities in the RCS, CVCS, RHRS and NSS respectively. The radionuclide contributions to the dose rates around the systems are presented in Figure 9 before the H₂O₂ injection and in Figure 10 after the primary system oxygenation.
pipe downstream (see Figure 6). They are higher than the ratios of the primary system.

The $^{51}$Cr surface activities decrease by a factor of 3 in the non-regenerative heat exchanger and are not detectable downstream from the particle filter (see Figure 3).

On the other hand, during the shutdown before the primary system oxygenation, the $^{110m}$Ag deposited activities are practically constant along the CVCS up to the particle filter (see Figure 4). Downstream from the filter, they are lower by a factor of about 10.

The EMECC measurements in the nuclear auxiliary building (from the pipe upstream from the non regenerative heat exchanger) show that the surface activities have increased during power operation except for the $^{110m}$Ag contamination which tends to decrease.

The primary system oxygenation by injection of H₂O₂ has not led to a change in the contamination level of $^{58}$Co (peak of 100 GBq/t) and $^{60}$Co (peak of 0.62 GBq/t) in the CVCS nor in the RCS, thanks to the oxygenation temperature of 80°C [3]. On the other hand, the $^{110m}$Ag surface activities have decreased upstream from the non-regenerative heat exchanger and have increased by a factor of 5 in this exchanger (with a measured maximum value of $^{110m}$Ag volume activity after oxygenation of 0.63 GBq/t).

In the **RHRS**, before its startup, the ACP surface activities on the pipe upstream from the main four valves are similar to those on the primary system since the primary coolant circulates in this part of the RHRS during power operation. After its startup, the $^{58}$Co and $^{51}$Cr contaminations tend to decrease (probably, erosion of a deposit enriched in $^{58}$Co and $^{51}$Cr) and the $^{60}$Co surface activity is constant (see Figure 1, Figure 2 and Figure 3). It should be noted that in the second half of cycle 11, the $^{58}$Co and $^{51}$Cr volume activities under particle form have increased. These particles have formed a deposit not very adhesive that could be eroded with flow rate changes. Downstream from the main four valves, before their opening, the very low or undetectable contamination level of short radioactive half-life radionuclides ($^{58}$Co: 70.8 days, $^{51}$Cr: 27.7 days) shows that this contamination is old and formed during the previous unit outage (the primary coolant did not circulate in the RHRS during the reactor power operation). On the other hand, after its startup, the RHRS parts downstream from the main valves are re-contaminated. However, the contamination levels in the RHRS remain low compared to the primary system, but with a similar $^{58}$Co/$^{60}$Co ratio (see Figure 6). The $^{58}$Co, $^{60}$Co and $^{51}$Cr surface activities are lower and lower along the RHRS particularly in the heat exchanger. However, it is the opposite case for the $^{110m}$Ag: after the H₂O₂ injection, the deposited activity is higher in the heat exchanger (see Figure 4).

In the **FPCS** (see Figure 5), during reactor power operation, the main radionuclide is $^{60}$Co due to its long radioactive half-life (5.27 years) so the contamination is old. The contamination levels are practically constant in the different parts of the system except in the cooling line

upstream from the heat exchanger, which is also downstream from the main FPCS tank and in which radioactive water circulates. During the refueling shutdown, due to the filling of the refueling cavity and the shifting of fuel in the spent fuel pit, the ACP contamination increases in the FPCS, particularly the $^{110m}$Ag contamination, which has become predominant.

At the end of the refueling shutdown, the $^{58}$Co/$^{60}$Co ratio in the FPCS is similar to the ratio in the RCS.

What must be pointed out is that generally the $^{51}$Cr surface activities can not be measured in the **RCS** due to its low gamma energy of 320 keV and the great thicknesses of the primary coolant pipes and steam generator shell. Thanks to a “nuclear auxiliary system”, the steam generator bypass, the EMECC measurement of this thin pipe provides the $^{51}$Cr surface activity of the primary system (see Figure 3).

## 2.2 Reactor B

This 1300 MWe PWR is equipped with steam generator tubing in alloy 600TT. A boron-lithium coordinated chemistry at pH 7.0 was applied until cycle 5 and since then the aimed pH has been 7.2 at 300°C.

An EMECC campaign of the nuclear auxiliary system contamination was performed in this PWR at cycle 10 at the beginning of the refueling shutdown before and after the primary system oxygenation.

### 2.2.1 Surface activities and dose rates

The list of the EMECC measurement points and their dose rates measured in contact with components of the CVCS, RHRS and NSS is presented in Table 2. The values of dose rates in italics were measured with a shielded probe to mitigate the ambient dose rates. The EMECC measurement points of the Reactor Coolant System (RCS) are also indicated in this table.

Figure 7 and Figure 8 present the $^{58}$Co and $^{60}$Co surface activities in the RCS, CVCS, RHRS and NSS respectively. The radionuclide contributions to the dose rates around the systems are presented in Figure 9 before the H₂O₂ injection and in Figure 10 after the primary system oxygenation.
<table><caption>Table 2: EMECC measurement points and dose rates</caption><thead><tr><th rowspan="2">EMECC Points</th><th colspan="2">Dose rates (mSv/h)</th></tr><tr><th>Before O<sub>2</sub></th><th>After O<sub>2</sub></th></tr></thead><tbody><tr><td colspan="3"><strong>CVCS</strong></td></tr><tr><td>Letdown line / Upstream Regenerative heat exchanger</td><td>0.45</td><td></td></tr><tr><td>Upstream Non-regenerative heat exchanger</td><td>0.76</td><td>1.0</td></tr><tr><td>Non-regenerative heat exchanger</td><td>0.95</td><td>1.0</td></tr><tr><td>Upstream Filter</td><td>0.32</td><td>0.51</td></tr><tr><td>Upstream Charging pump</td><td>0.070</td><td>0.046</td></tr><tr><td>Charging line / Downstream Regenerative heat exchanger</td><td>0.090</td><td>0.11</td></tr><tr><td colspan="3"><strong>RHRS</strong></td></tr><tr><td>Upstream Pumps</td><td>0.20</td><td>0.35</td></tr><tr><td>Heat exchanger</td><td>0.090</td><td>0.14</td></tr><tr><td>Downstream Heat exchanger</td><td>0.12</td><td>0.52</td></tr><tr><td colspan="3"><strong>NSS</strong></td></tr><tr><td>RCS Hot leg / Upstream First heat exchanger</td><td>0.15</td><td>0.095</td></tr><tr><td>RCS Hot leg / Second heat exchanger</td><td>0.70</td><td>1.1</td></tr><tr><td>RCS Hot leg / Downstream Second heat exchanger</td><td>0.15</td><td>0.20</td></tr><tr><td>CVCS Upstream filtration / Upstream Heat exchanger</td><td>0.29</td><td>0.35</td></tr><tr><td>CVCS Upstream filtration / Heat exchanger</td><td>0.46</td><td>0.9</td></tr><tr><td>CVCS Upstream filtration / Downstream Heat exchanger</td><td>0.14</td><td>0.25</td></tr><tr><td colspan="3"><strong>RCS</strong></td></tr><tr><td>Hot leg</td><td>0.39</td><td>0.22</td></tr><tr><td>SG tubing</td><td>0.10</td><td>0.090</td></tr><tr><td>Cold leg</td><td>0.22</td><td>0.22</td></tr><tr><td>SG bypass</td><td></td><td>0.69</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>Figure 7: Reactor B - Cycle 10 - <sup>58</sup>Co surface activities in RCS, CVCS, RHRS and NSS</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Figure 8: Reactor B - Cycle 10 - <sup>60</sup>Co surface activities in RCS, CVCS, RHRS and NSS</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 9: Reactor B - Cycle 10 - Before oxygenation - Radionuclide contribution to the dose rates</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 10: Reactor B - Cycle 10 - After oxygenation - Radionuclide contribution to the dose rates</figcaption></figure>

### 2.2.2 Comments

Compared to the primary system, the dose rates of the CVCS and NSS are generally higher while those of the RHRS are similar. The <sup>58</sup>Co surface activities of the CVCS pipes up to the particle filter are similar to those of the RCS. Those of the RHRS and the NSS are lower, as is also the case of the <sup>60</sup>Co deposited activities. Thus, compared to the RCS, the higher dose rates in the auxiliary systems are due to either the lessened thickness of the piping or to the higher <sup>110m</sup>Ag contamination level (see Figure 9 and Figure 10).
<table><caption>Table 2: EMECC measurement points and dose rates</caption><thead><tr><th rowspan="2">EMECC Points</th><th colspan="2">Dose rates (mSv/h)</th></tr><tr><th>Before O<sub>2</sub></th><th>After O<sub>2</sub></th></tr></thead><tbody><tr><td colspan="3"><strong>CVCS</strong></td></tr><tr><td>Letdown line / Upstream Regenerative heat exchanger</td><td>0.45</td><td></td></tr><tr><td>Upstream Non-regenerative heat exchanger</td><td>0.76</td><td>1.0</td></tr><tr><td>Non-regenerative heat exchanger</td><td>0.95</td><td>1.0</td></tr><tr><td>Upstream Filter</td><td>0.32</td><td>0.51</td></tr><tr><td>Upstream Charging pump</td><td>0.070</td><td>0.046</td></tr><tr><td>Charging line / Downstream Regenerative heat exchanger</td><td>0.090</td><td>0.11</td></tr><tr><td colspan="3"><strong>RHRS</strong></td></tr><tr><td>Upstream Pumps</td><td>0.20</td><td>0.35</td></tr><tr><td>Heat exchanger</td><td>0.090</td><td>0.14</td></tr><tr><td>Downstream Heat exchanger</td><td>0.12</td><td>0.52</td></tr><tr><td colspan="3"><strong>NSS</strong></td></tr><tr><td>RCS Hot leg / Upstream First heat exchanger</td><td>0.15</td><td>0.095</td></tr><tr><td>RCS Hot leg / Second heat exchanger</td><td>0.70</td><td>1.1</td></tr><tr><td>RCS Hot leg / Downstream Second heat exchanger</td><td>0.15</td><td>0.20</td></tr><tr><td>CVCS Upstream filtration / Upstream Heat exchanger</td><td>0.29</td><td>0.35</td></tr><tr><td>CVCS Upstream filtration / Heat exchanger</td><td>0.46</td><td>0.9</td></tr><tr><td>CVCS Upstream filtration / Downstream Heat exchanger</td><td>0.14</td><td>0.25</td></tr><tr><td colspan="3"><strong>RCS</strong></td></tr><tr><td>Hot leg</td><td>0.39</td><td>0.22</td></tr><tr><td>SG tubing</td><td>0.10</td><td>0.090</td></tr><tr><td>Cold leg</td><td>0.22</td><td>0.22</td></tr><tr><td>SG bypass</td><td></td><td>0.69</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>Figure 7: Reactor B - Cycle 10 - <sup>58</sup>Co surface activities in RCS, CVCS, RHRS and NSS</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Figure 8: Reactor B - Cycle 10 - <sup>60</sup>Co surface activities in RCS, CVCS, RHRS and NSS</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 9: Reactor B - Cycle 10 - Before oxygenation - Radionuclide contribution to the dose rates</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 10: Reactor B - Cycle 10 - After oxygenation - Radionuclide contribution to the dose rates</figcaption></figure>

### 2.2.2 Comments

Compared to the primary system, the dose rates of the CVCS and NSS are generally higher while those of the RHRS are similar. The <sup>58</sup>Co surface activities of the CVCS pipes up to the particle filter are similar to those of the RCS. Those of the RHRS and the NSS are lower, as is also the case of the <sup>60</sup>Co deposited activities. Thus, compared to the RCS, the higher dose rates in the auxiliary systems are due to either the lessened thickness of the piping or to the higher <sup>110m</sup>Ag contamination level (see Figure 9 and Figure 10).
During the shutdown procedure ($^{58}$Co peak of 48 GBq/t), the $^{58}$Co contamination has increased by a factor 2 to 8 in the NSS and it is practically constant in the other systems except for the pipe upstream from the pumps of the RHRS. The $^{60}$Co surface activities are constant in all the circuits and the $^{110m}$Ag contamination has increased in some cold parts of the RHRS, CVCS and NSS (with a measured maximum value of $^{110m}$Ag volume activity after oxygenation of 39 MBq/t).

Figure 9 and Figure 10 show that the main radionuclides contributing to the dose rates around the auxiliary systems are $^{58}$Co, $^{60}$Co and $^{110m}$Ag.

In the RHRS, before its startup, the main radionuclides are $^{60}$Co and $^{110m}$Ag because of their long half-life, the RHRS does not operate during reactor power operation. After oxygenation, the $^{110m}$Ag contribution increases in the cold parts; it reaches about 70% in the heat exchanger.

In the CVCS, upstream from the filter, the two main contributors are $^{58}$Co and $^{110m}$Ag, with the $^{110m}$Ag contribution increasing after the oxygenation. The contribution upstream from the regenerative heat exchanger is similar to the one of the primary system (close conditions): $^{58}$Co and $^{60}$Co are the two main contributors. Downstream from the filter, the two main contributors are $^{60}$Co and $^{110m}$Ag.

In the NSS, $^{58}$Co, $^{60}$Co and $^{110m}$Ag are the three main contributors to the dose rates. The $^{58}$Co contribution increases after oxygenation.

### 3 DISCUSSION

The EMECC campaigns carried out in different standardized plant series allow us to define the principal characteristics of the contamination of the main auxiliary systems.

The $^{58}$Co and $^{60}$Co deposited activities on the CVCS, RHRS, NSS and FPCS are generally lower than those on the primary system. They can be similar, especially in the upstream parts and they do not decrease inevitably along the circuits. Of course, they decrease downstream from the CVCS purification system (particle and mixed bed filters) unless there is an opened bypass valve or a problem such as a saturated filter.

The $^{58}$Co and $^{60}$Co contaminations of the CVCS and NSS build up essentially during reactor power operation, but the $^{58}$Co contamination can also increase during the oxygenation. The RHRS and FPCS contaminations occur during the shutdown, when these systems operate or are particularly requisite. Before their operation, they are contaminated by ACP with long radioactive half-life, mainly $^{60}$Co. During their operation, they are contaminated by ACP with short half-life. Thus, the RHRS is contaminated after its startup during cold shutdown and the FPCS during the refueling shutdown due to the filling of the refueling cavity and the shifting of fuel in the spent fuel pit. The comparison of the $^{58}$Co/$^{60}$Co ratios of deposited activities on the primary system and on auxiliary systems shows that the types of contamination are different: different because of their

thermal hydraulic conditions, their components or their operation.

Sometimes during shutdown, the $^{58}$Co and $^{51}$Cr contaminations or those of the $^{60}$Co decrease in the auxiliary systems. This is due to the movement of particles that were deposited and that have been swept away by the flow rate changes.

The nuclear auxiliary systems are the only circuits that allow us to measure the $^{51}$Cr deposited activities. The thicknesses of primary pipes or steam generator shells are generally too great (more than about 7 cm) for the 320 keV gamma emitted by $^{51}$Cr. In order to know the $^{51}$Cr surface activity in the primary system, the SG bypass pipe, when it exists (in 1450 MWe PWRs, there is not any SG bypass), or the CVCS letdown pipe upstream from the regenerative heat exchanger (thickness of about 1 cm) are measured with an EMECC device. As the contamination levels by the other radionuclides of the primary pipes and of these parts of auxiliary systems are similar, the $^{51}$Cr contamination level can be extrapolated to the primary system.

One main characteristic of the nuclear auxiliary systems, which is not shared with the primary system, is its potentially high recontamination by $^{110m}$Ag during the primary system oxygenation.

Silver is soluble in oxidizing conditions and its solubility increases with temperature. Thus, during oxygenation, it dissolves in the primary coolant from the deposits on the primary surfaces (e.g. the $^{110m}$Ag deposited activities on the primary pipes and SG tubes decrease during oxygenation, see Figure 4 and Figure 12), then it precipitates on the cold parts of the auxiliary systems, in particular on the heat exchangers of the RHRS and the NSS and on the non-regenerative heat exchanger of the CVCS and the pipes downstream. The $^{110m}$Ag surface activity can become the highest one, e.g. in the FPCS at the end of the refueling shutdown of reactor A (see Figure 5).

The fact that silver precipitates on the cold parts, particularly on the NSS, can lead to an underestimation of the $^{110m}$Ag volume activity for some hours after the oxygenation and then an overestimation. Figure 11 illustrates this behavior. This figure presents the variations of the $^{110m}$Ag and $^{58}$Co deposited activities on the second heat exchanger of the RCS sampling line, the $^{110m}$Ag and $^{58}$Co volume activities sampled with this line and the primary system temperature of an EDF unit (Reactor C).
During the shutdown procedure ($^{58}$Co peak of 48 GBq/t), the $^{58}$Co contamination has increased by a factor 2 to 8 in the NSS and it is practically constant in the other systems except for the pipe upstream from the pumps of the RHRS. The $^{60}$Co surface activities are constant in all the circuits and the $^{110m}$Ag contamination has increased in some cold parts of the RHRS, CVCS and NSS (with a measured maximum value of $^{110m}$Ag volume activity after oxygenation of 39 MBq/t).

Figure 9 and Figure 10 show that the main radionuclides contributing to the dose rates around the auxiliary systems are $^{58}$Co, $^{60}$Co and $^{110m}$Ag.

In the RHRS, before its startup, the main radionuclides are $^{60}$Co and $^{110m}$Ag because of their long half-life, the RHRS does not operate during reactor power operation. After oxygenation, the $^{110m}$Ag contribution increases in the cold parts; it reaches about 70% in the heat exchanger.

In the CVCS, upstream from the filter, the two main contributors are $^{58}$Co and $^{110m}$Ag, with the $^{110m}$Ag contribution increasing after the oxygenation. The contribution upstream from the regenerative heat exchanger is similar to the one of the primary system (close conditions): $^{58}$Co and $^{60}$Co are the two main contributors. Downstream from the filter, the two main contributors are $^{60}$Co and $^{110m}$Ag.

In the NSS, $^{58}$Co, $^{60}$Co and $^{110m}$Ag are the three main contributors to the dose rates. The $^{58}$Co contribution increases after oxygenation.

### 3 DISCUSSION

The EMECC campaigns carried out in different standardized plant series allow us to define the principal characteristics of the contamination of the main auxiliary systems.

The $^{58}$Co and $^{60}$Co deposited activities on the CVCS, RHRS, NSS and FPCS are generally lower than those on the primary system. They can be similar, especially in the upstream parts and they do not decrease inevitably along the circuits. Of course, they decrease downstream from the CVCS purification system (particle and mixed bed filters) unless there is an opened bypass valve or a problem such as a saturated filter.

The $^{58}$Co and $^{60}$Co contaminations of the CVCS and NSS build up essentially during reactor power operation, but the $^{58}$Co contamination can also increase during the oxygenation. The RHRS and FPCS contaminations occur during the shutdown, when these systems operate or are particularly requisite. Before their operation, they are contaminated by ACP with long radioactive half-life, mainly $^{60}$Co. During their operation, they are contaminated by ACP with short half-life. Thus, the RHRS is contaminated after its startup during cold shutdown and the FPCS during the refueling shutdown due to the filling of the refueling cavity and the shifting of fuel in the spent fuel pit. The comparison of the $^{58}$Co/$^{60}$Co ratios of deposited activities on the primary system and on auxiliary systems shows that the types of contamination are different: different because of their

thermal hydraulic conditions, their components or their operation.

Sometimes during shutdown, the $^{58}$Co and $^{51}$Cr contaminations or those of the $^{60}$Co decrease in the auxiliary systems. This is due to the movement of particles that were deposited and that have been swept away by the flow rate changes.

The nuclear auxiliary systems are the only circuits that allow us to measure the $^{51}$Cr deposited activities. The thicknesses of primary pipes or steam generator shells are generally too great (more than about 7 cm) for the 320 keV gamma emitted by $^{51}$Cr. In order to know the $^{51}$Cr surface activity in the primary system, the SG bypass pipe, when it exists (in 1450 MWe PWRs, there is not any SG bypass), or the CVCS letdown pipe upstream from the regenerative heat exchanger (thickness of about 1 cm) are measured with an EMECC device. As the contamination levels by the other radionuclides of the primary pipes and of these parts of auxiliary systems are similar, the $^{51}$Cr contamination level can be extrapolated to the primary system.

One main characteristic of the nuclear auxiliary systems, which is not shared with the primary system, is its potentially high recontamination by $^{110m}$Ag during the primary system oxygenation.

Silver is soluble in oxidizing conditions and its solubility increases with temperature. Thus, during oxygenation, it dissolves in the primary coolant from the deposits on the primary surfaces (e.g. the $^{110m}$Ag deposited activities on the primary pipes and SG tubes decrease during oxygenation, see Figure 4 and Figure 12), then it precipitates on the cold parts of the auxiliary systems, in particular on the heat exchangers of the RHRS and the NSS and on the non-regenerative heat exchanger of the CVCS and the pipes downstream. The $^{110m}$Ag surface activity can become the highest one, e.g. in the FPCS at the end of the refueling shutdown of reactor A (see Figure 5).

The fact that silver precipitates on the cold parts, particularly on the NSS, can lead to an underestimation of the $^{110m}$Ag volume activity for some hours after the oxygenation and then an overestimation. Figure 11 illustrates this behavior. This figure presents the variations of the $^{110m}$Ag and $^{58}$Co deposited activities on the second heat exchanger of the RCS sampling line, the $^{110m}$Ag and $^{58}$Co volume activities sampled with this line and the primary system temperature of an EDF unit (Reactor C).
<figure><img src="image_12.png" /><figcaption>Figure 11: Reactor C - Deposited activities on the second heat exchanger of the RCS sampling line</figcaption></figure>

Generally, the ¹¹⁰mAg volume activity variation can be described in 4 steps:

1. Just after the H₂O₂ injection, the ¹¹⁰mAg surface activity increases in the heat exchanger<fnref n="1" />, whereas the ¹¹⁰mAg volume activity sampled downstream from this exchanger has not yet increased. This means that the ¹¹⁰mAg precipitates on the sampling line before its sampling at the end of this line.

2. Then, when the deposits become more and more saturated by silver, the sampling begins to show an increase in the ¹¹⁰mAg volume activity.

3. Gradually, the ¹¹⁰mAg deposited on the sampling line dissolves which leads to a plateau of ¹¹⁰mAg volume activity. This dissolution compensates the decrease in the volume activity of the primary coolant.

4. Finally, the decrease in the ¹¹⁰mAg volume activity is observed, the dissolution does not compensate any more.

In step 3, one might think that the CVCS purification system efficiency is poor, the ¹¹⁰mAg volume activity remaining constant. This seems to be confirmed by the ¹¹⁰mAg volume activity sampled downstream from the mixed bed demineralizers which tends to increase. In fact, this increase is due to the silver dissolution deposited on the pipe downstream from the ion exchangers, as it can be observed in Figure 12. This figure presents the ¹¹⁰mAg surface activities before and after the primary coolant oxygenation in the CVCS of reactor C.

The silver contamination of this pipe occurs during power operation since silver can be under a colloidal form and poorly trapped by the purification system [4].

<figure><img src="image_13.png" /><figcaption>Figure 12: Reactor C - ¹¹⁰mAg surface activities in CVCS</figcaption></figure>

During step 1 and perhaps step 2 as well, the sampled ¹¹⁰mAg volume activity is underestimated. On the other hand, during steps 3 and 4, it is overestimated.

One can note that this issue concerns ¹¹⁰mAg and not the other ACP (see Figure 11). The ⁵⁸Co volume activity has increased just after the H₂O₂ injection and the ⁵⁸Co deposited activity on the heat exchanger remains constant.

Because of the peculiar behavior of silver, ¹¹⁰mAg is often the major contributor to the dose rates around the auxiliary systems, sometimes at a level greater than 90%. Even in the 1300 MWe PWRs, which is less affected by this kind of pollution, its contribution is not negligible and can be the major one (see Figure 10) but the dose rates are relatively low.

The dose rates around the auxiliary systems are generally higher than those around the primary system. There are two main reasons for this: the lesser thickness of pipes and exchanger shells and the high ¹¹⁰mAg contamination level.

Last it would be worthwhile to point out that the ⁵⁸Co contribution to the dose rates in comparison to the ⁶⁰Co contribution is higher around the auxiliary systems than it is around the primary system. It is still due to the lesser thickness of the pipes and exchangers (see Figure 9). However, for the auxiliary systems which do not operate for several months, ⁶⁰Co becomes the major contributor owing to its long radioactive half-life.

## 4 CONCLUSION

Based on EMECC campaigns carried out in different standardized plant series, this paper shows that the contamination of the nuclear auxiliary systems presents differences in comparison to the contamination of the primary system:

- The ⁵⁸Co and ⁶⁰Co deposited activities on the CVCS, RHRS, NSS and FPCS are generally lower. On the other hand, the ¹¹⁰mAg contamination is higher with a great recontamination during cold shutdowns.

- The dose rates around the auxiliary systems are generally higher.

- The ⁵⁸Co contribution and especially the ¹¹⁰mAg contribution are higher. For the auxiliary systems

<footnote n="1">¹ thereby proving that the ¹¹⁰mAg volume activity is increasing.</footnote>
<figure><img src="image_12.png" /><figcaption>Figure 11: Reactor C - Deposited activities on the second heat exchanger of the RCS sampling line</figcaption></figure>

Generally, the ¹¹⁰mAg volume activity variation can be described in 4 steps:

1. Just after the H₂O₂ injection, the ¹¹⁰mAg surface activity increases in the heat exchanger<fnref n="1" />, whereas the ¹¹⁰mAg volume activity sampled downstream from this exchanger has not yet increased. This means that the ¹¹⁰mAg precipitates on the sampling line before its sampling at the end of this line.

2. Then, when the deposits become more and more saturated by silver, the sampling begins to show an increase in the ¹¹⁰mAg volume activity.

3. Gradually, the ¹¹⁰mAg deposited on the sampling line dissolves which leads to a plateau of ¹¹⁰mAg volume activity. This dissolution compensates the decrease in the volume activity of the primary coolant.

4. Finally, the decrease in the ¹¹⁰mAg volume activity is observed, the dissolution does not compensate any more.

In step 3, one might think that the CVCS purification system efficiency is poor, the ¹¹⁰mAg volume activity remaining constant. This seems to be confirmed by the ¹¹⁰mAg volume activity sampled downstream from the mixed bed demineralizers which tends to increase. In fact, this increase is due to the silver dissolution deposited on the pipe downstream from the ion exchangers, as it can be observed in Figure 12. This figure presents the ¹¹⁰mAg surface activities before and after the primary coolant oxygenation in the CVCS of reactor C.

The silver contamination of this pipe occurs during power operation since silver can be under a colloidal form and poorly trapped by the purification system [4].

<figure><img src="image_13.png" /><figcaption>Figure 12: Reactor C - ¹¹⁰mAg surface activities in CVCS</figcaption></figure>

During step 1 and perhaps step 2 as well, the sampled ¹¹⁰mAg volume activity is underestimated. On the other hand, during steps 3 and 4, it is overestimated.

One can note that this issue concerns ¹¹⁰mAg and not the other ACP (see Figure 11). The ⁵⁸Co volume activity has increased just after the H₂O₂ injection and the ⁵⁸Co deposited activity on the heat exchanger remains constant.

Because of the peculiar behavior of silver, ¹¹⁰mAg is often the major contributor to the dose rates around the auxiliary systems, sometimes at a level greater than 90%. Even in the 1300 MWe PWRs, which is less affected by this kind of pollution, its contribution is not negligible and can be the major one (see Figure 10) but the dose rates are relatively low.

The dose rates around the auxiliary systems are generally higher than those around the primary system. There are two main reasons for this: the lesser thickness of pipes and exchanger shells and the high ¹¹⁰mAg contamination level.

Last it would be worthwhile to point out that the ⁵⁸Co contribution to the dose rates in comparison to the ⁶⁰Co contribution is higher around the auxiliary systems than it is around the primary system. It is still due to the lesser thickness of the pipes and exchangers (see Figure 9). However, for the auxiliary systems which do not operate for several months, ⁶⁰Co becomes the major contributor owing to its long radioactive half-life.

## 4 CONCLUSION

Based on EMECC campaigns carried out in different standardized plant series, this paper shows that the contamination of the nuclear auxiliary systems presents differences in comparison to the contamination of the primary system:

- The ⁵⁸Co and ⁶⁰Co deposited activities on the CVCS, RHRS, NSS and FPCS are generally lower. On the other hand, the ¹¹⁰mAg contamination is higher with a great recontamination during cold shutdowns.

- The dose rates around the auxiliary systems are generally higher.

- The ⁵⁸Co contribution and especially the ¹¹⁰mAg contribution are higher. For the auxiliary systems

<footnote n="1">¹ thereby proving that the ¹¹⁰mAg volume activity is increasing.</footnote>
that do not operate for several months, the ⁶⁰Co
contribution becomes the major one.

- The nuclear auxiliary systems are the only circuits that allow us to measure the ⁵¹Cr surface activities.

These differences are due to different design and operating conditions, in particular the presence of heat exchangers and lower fluid temperature, different flow rates, lesser thickness of pipes and exchanger shells, the presence of filtering systems, operations carried out during a short period.

Last the peculiar behavior of ¹¹⁰mAg inside the NSS
generally results in a poor estimation of the ¹¹⁰mAg
volume activity in the primary coolant.

5 REFERENCES

[1] 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, 12-16 September 1988, Bournemouth (GB), Vol. 1, p.103-113, UKAEA & OECD, Winfrith (1988)

[2] F. DACQUAIT, L. GUINARD, C. ANDRIEU, J.L. BRETELLE, F. BARDET, A. ROCHER and 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).

[3] 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, 22-26 April 2002, Avignon (France), paper 113, SFEN, Paris (2002).

[4] J.L. BRETELLE, A. ROCHER, M. BERGER, F. DACQUAIT and R.ROSSET "Study of Various Chemical Species Behaviour for Contamination Risk", Proceedings of the International Conference on Water Chemistry in Nuclear Reactor systems, 22-26 April 2002, Avingon (France), paper 123, SFEN, Paris (2002).
that do not operate for several months, the ⁶⁰Co
contribution becomes the major one.

- The nuclear auxiliary systems are the only circuits that allow us to measure the ⁵¹Cr surface activities.

These differences are due to different design and operating conditions, in particular the presence of heat exchangers and lower fluid temperature, different flow rates, lesser thickness of pipes and exchanger shells, the presence of filtering systems, operations carried out during a short period.

Last the peculiar behavior of ¹¹⁰mAg inside the NSS
generally results in a poor estimation of the ¹¹⁰mAg
volume activity in the primary coolant.

5 REFERENCES

[1] 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, 12-16 September 1988, Bournemouth (GB), Vol. 1, p.103-113, UKAEA & OECD, Winfrith (1988)

[2] F. DACQUAIT, L. GUINARD, C. ANDRIEU, J.L. BRETELLE, F. BARDET, A. ROCHER and 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).

[3] 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, 22-26 April 2002, Avignon (France), paper 113, SFEN, Paris (2002).

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