# Study of various chemical species behaviour for contamination risk

Jean-Luc Bretelle, Alain Rocher, Martial Berger, Frédéric Dacquait, Robert Rosset

► To cite this version:

Jean-Luc Bretelle, Alain Rocher, Martial Berger, Frédéric Dacquait, Robert Rosset. Study of various chemical species behaviour for contamination risk. International conference on water chemistry in nuclear reactors systems, Apr 2002, Avignon, France. cea-04463847

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https://cea.hal.science/cea-04463847

Submitted on 17 Feb 2024

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Distributed under a Creative Commons Attribution 4.0 International License
# Study of various chemical species behaviour for contamination risk

Jean-Luc Bretelle, Alain Rocher, Martial Berger, Frédéric Dacquait, Robert Rosset

► To cite this version:

Jean-Luc Bretelle, Alain Rocher, Martial Berger, Frédéric Dacquait, Robert Rosset. Study of various chemical species behaviour for contamination risk. International conference on water chemistry in nuclear reactors systems, Apr 2002, Avignon, France. cea-04463847

HAL Id: cea-04463847

https://cea.hal.science/cea-04463847v1

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
# Study of various chemical species behaviour for contamination risk

J.L. Bretelle, *Electricité de France/GDL*

A. Rocher, *Electricité de France/GPR*

M. Berger, *Electricité de France/SEPTEN*

F. Dacquait, *Commissariat a l'Energie Atomique - France*

R. Rosset, *ESPCI - France*

## 1- Introduction

The goal of this paper is to precise the behaviour of different radiochemical species in the primary coolant of PWR plants. Managing these pollutants must lead :

- To limit workers contamination risk to avoid incidental dissemination in the containment atmosphere when the primary circuit is opened ;

- To limit Reactor Coolant Sysytem (RCS) walls "over-contamination" to decrease the dose rates during the maintenance operations.

In French Plants, Co 60, iodine, silver and antimony represent the major radiochemical pollutants which require a good understanding of the different phenomena to ensure the lowest contamination risks.

The stakes deal with the control and the optimization of collective and individual doses including waste treatment with low costs. These stakes represent primordial elements of nuclear acceptability.

## 2- Normal contamination of circuits

Contamination is one of the dose causes only. Nevertheless smearable contamination can impact radiological cleanliness of facilities.

In French PWR plants :

- More than 90% of integrated doses are due to contaminated walls in contact with the primary coolant because of activated corrosion products.

- 80% of total dosimetry are integrated during outages.

In the absence of specific pollution, the mean contribution of out of core deposits to dose rates is as follows (results expressed as percentage equivalent dose rates in Sv/h) :

- Co 60 = 50% (the older the plant, the greater the Co 60 quantity).

- Co 58 = 30% (with a higher level in 1300 MW units)

- Ag 110m + Sb 124 + Cr 51 + ... = 10%

Nota : Considering that Co 60 is the major contaminant in French facilities, sensitive and accurate Co 60 measurement and control devices are needed at the entrance and way out of controlled areas as well as individual electronic dosimeters.

Usual contaminants come mostly from :

- Either materials/coolant interactions leading to soluble, particulate and colloidal products transport and corrosion products activation (Co 58, Cr 51, Mn 54 ....) [1] ;

- Or in core pollutions leading to fission products with a slight impact on contamination and dose rates [table 1].
# Study of various chemical species behaviour for contamination risk

J.L. Bretelle, *Electricité de France/GDL*

A. Rocher, *Electricité de France/GPR*

M. Berger, *Electricité de France/SEPTEN*

F. Dacquait, *Commissariat a l'Energie Atomique - France*

R. Rosset, *ESPCI - France*

## 1- Introduction

The goal of this paper is to precise the behaviour of different radiochemical species in the primary coolant of PWR plants. Managing these pollutants must lead :

- To limit workers contamination risk to avoid incidental dissemination in the containment atmosphere when the primary circuit is opened ;

- To limit Reactor Coolant Sysytem (RCS) walls "over-contamination" to decrease the dose rates during the maintenance operations.

In French Plants, Co 60, iodine, silver and antimony represent the major radiochemical pollutants which require a good understanding of the different phenomena to ensure the lowest contamination risks.

The stakes deal with the control and the optimization of collective and individual doses including waste treatment with low costs. These stakes represent primordial elements of nuclear acceptability.

## 2- Normal contamination of circuits

Contamination is one of the dose causes only. Nevertheless smearable contamination can impact radiological cleanliness of facilities.

In French PWR plants :

- More than 90% of integrated doses are due to contaminated walls in contact with the primary coolant because of activated corrosion products.

- 80% of total dosimetry are integrated during outages.

In the absence of specific pollution, the mean contribution of out of core deposits to dose rates is as follows (results expressed as percentage equivalent dose rates in Sv/h) :

- Co 60 = 50% (the older the plant, the greater the Co 60 quantity).

- Co 58 = 30% (with a higher level in 1300 MW units)

- Ag 110m + Sb 124 + Cr 51 + ... = 10%

Nota : Considering that Co 60 is the major contaminant in French facilities, sensitive and accurate Co 60 measurement and control devices are needed at the entrance and way out of controlled areas as well as individual electronic dosimeters.

Usual contaminants come mostly from :

- Either materials/coolant interactions leading to soluble, particulate and colloidal products transport and corrosion products activation (Co 58, Cr 51, Mn 54 ....) [1] ;

- Or in core pollutions leading to fission products with a slight impact on contamination and dose rates [table 1].
<table><caption>Table 1 - Gamma and beta energies and emission percentage (> 50%) of major nuclides</caption><thead><tr><th>Nuclides /<br>Half-life</th><th>Gamma energy<br>KeV (% emission)</th><th>Beta energy<br>keV (% emission)</th></tr></thead><tbody><tr><td>Co58 / 71 d</td><td>811 (100%)</td><td></td></tr><tr><td rowspan="2">Co60 / 5,3 y</td><td>1173 (100%)</td><td rowspan="2">318 (100%)</td></tr><tr><td>1333 (100%)</td></tr><tr><td rowspan="2">Ag110m / 253 d</td><td>658 (100%)</td><td rowspan="2">86 (55%)</td></tr><tr><td>885 (80%)</td></tr><tr><td>Sb124 / 60 d</td><td>603 (98%)</td><td>610 (51%)</td></tr><tr><td>Sb122 / 2,7 d</td><td>564 (66%)</td><td>1400 (63%)</td></tr><tr><td>Mn54 / 280d</td><td>840 (100%)</td><td></td></tr><tr><td>I131 / 8 d</td><td>364 (82%)</td><td>610 (87%)</td></tr><tr><td>Xe133 / 5 d</td><td>81 (100%)</td><td>350 (99%)</td></tr><tr><td rowspan="2">Cs134 / 2 y</td><td>605 (98%)</td><td rowspan="2">658 (70%)</td></tr><tr><td>796 (86%)</td></tr></tbody></table>

The dose rates can increase in case of more important corrosion products transport or of incidental pollution. In the latter case the ratio of species contributing to dose rates is modified [2].

## 3- Incidental contamination

The major pollutants responsible for over-contamination in EDF plants are :

- Co 60 coming from stellite degradation ;

- Ag 110 m coming from control rod perforation and probably from materials like seals ;

- Sb 124 coming from pumps of auxiliary system ;

- Iodine due to fuel cladding degradation.

Given the importance of over-contamination with respect to dosimetry, representing 10 to 30 % of outages dosimetry in the concerned units, a specific program is being set up in France, in the context of the ALARA project. The goal is to optimise diagnosis, surveillance, prevention and remedies to fight pollutions even though priority is set on prevention.

### 3.1 - Co 60

In most cases, hot spots are due to particles of cobalt activated by a neutron flux (Co 60) mainly from stellite surface equipment (rich in cobalt) [2] in the RCS (valves, pumps, internals, etc.) [3].

The contribution of hot spots to shutdown dosimetry may appear to be marginal in French reactors (2 to 4 %) but becomes heavier (15 to 25 %) for the units affected. This excess dosimetry has to be taken into account, particularly for the most exposed workers. Approximately ten French units have been affected by this phenomenon over the last 15 years.

A set of actions, aimed at eradicating hot spots in PWRs, was set up in 1997. It consists of three complementary phases : prevention, surveillance and curative actions.

#### 3.1.1 - Prevention

Prevention consists in avoiding the occurrence of hot spots. It concerns all the units by performing the following actions:

- limiting the use of cobalt based components in contact with the primary coolant ;

- removing stellite particles produced during some maintenance operations (e.g. lapping) ;

- filtering effluents able to carry hot spots outside the Reactor Building.

The simplified diagram below illustrates the principles of the preventive filtration methods which are proposed (Fig. 1).
<table><caption>Table 1 - Gamma and beta energies and emission percentage (> 50%) of major nuclides</caption><thead><tr><th>Nuclides /<br>Half-life</th><th>Gamma energy<br>KeV (% emission)</th><th>Beta energy<br>keV (% emission)</th></tr></thead><tbody><tr><td>Co58 / 71 d</td><td>811 (100%)</td><td></td></tr><tr><td rowspan="2">Co60 / 5,3 y</td><td>1173 (100%)</td><td rowspan="2">318 (100%)</td></tr><tr><td>1333 (100%)</td></tr><tr><td rowspan="2">Ag110m / 253 d</td><td>658 (100%)</td><td rowspan="2">86 (55%)</td></tr><tr><td>885 (80%)</td></tr><tr><td>Sb124 / 60 d</td><td>603 (98%)</td><td>610 (51%)</td></tr><tr><td>Sb122 / 2,7 d</td><td>564 (66%)</td><td>1400 (63%)</td></tr><tr><td>Mn54 / 280d</td><td>840 (100%)</td><td></td></tr><tr><td>I131 / 8 d</td><td>364 (82%)</td><td>610 (87%)</td></tr><tr><td>Xe133 / 5 d</td><td>81 (100%)</td><td>350 (99%)</td></tr><tr><td rowspan="2">Cs134 / 2 y</td><td>605 (98%)</td><td rowspan="2">658 (70%)</td></tr><tr><td>796 (86%)</td></tr></tbody></table>

The dose rates can increase in case of more important corrosion products transport or of incidental pollution. In the latter case the ratio of species contributing to dose rates is modified [2].

## 3- Incidental contamination

The major pollutants responsible for over-contamination in EDF plants are :

- Co 60 coming from stellite degradation ;

- Ag 110 m coming from control rod perforation and probably from materials like seals ;

- Sb 124 coming from pumps of auxiliary system ;

- Iodine due to fuel cladding degradation.

Given the importance of over-contamination with respect to dosimetry, representing 10 to 30 % of outages dosimetry in the concerned units, a specific program is being set up in France, in the context of the ALARA project. The goal is to optimise diagnosis, surveillance, prevention and remedies to fight pollutions even though priority is set on prevention.

### 3.1 - Co 60

In most cases, hot spots are due to particles of cobalt activated by a neutron flux (Co 60) mainly from stellite surface equipment (rich in cobalt) [2] in the RCS (valves, pumps, internals, etc.) [3].

The contribution of hot spots to shutdown dosimetry may appear to be marginal in French reactors (2 to 4 %) but becomes heavier (15 to 25 %) for the units affected. This excess dosimetry has to be taken into account, particularly for the most exposed workers. Approximately ten French units have been affected by this phenomenon over the last 15 years.

A set of actions, aimed at eradicating hot spots in PWRs, was set up in 1997. It consists of three complementary phases : prevention, surveillance and curative actions.

#### 3.1.1 - Prevention

Prevention consists in avoiding the occurrence of hot spots. It concerns all the units by performing the following actions:

- limiting the use of cobalt based components in contact with the primary coolant ;

- removing stellite particles produced during some maintenance operations (e.g. lapping) ;

- filtering effluents able to carry hot spots outside the Reactor Building.

The simplified diagram below illustrates the principles of the preventive filtration methods which are proposed (Fig. 1).
<figure><img src="image_2.png" /><figcaption>Fig. 1: Preventive filtration methods</figcaption></figure>

The drains of pools are important routes for hot spots, before they spread through the systems. The installation of fine filters, an initial containment barrier, is proposed for the drain orifices of each pool:

- Pools located in the reactor building;

- Pools located in the fuel building;

Appropriate filters are required:

- Truncated sieves;

- 50 µm mesh.

The drain lines of the primary cooling circuit represent the second main vector for the movement of hot spots. As a preventive measure, the installation of sleeves on the main migration channels enables sensitive systems to be filtered. This filtration prevents the spread of hot spots and ensures that they are eliminated.

The filtration system is installed in the reactor building or the fuel building, depending on the shutdown phases, thus enabling all the pools to be treated with the same device.

### 3.1.2 - Surveillance

Surveillance is designed to inform the site, as early as possible, of the presence of hot spots (mapping) in order to take the appropriate measures to prevent their propagation and/or to eradicate them.

During unit operation, most hot spots will become fixed to the fuel. Others may fall, by gravity, to the bottom of the pool or be trapped in the special devices or the low points of the primary coolant system.

The most common locations are as follows:

- Thermal sleeves of the pressuriser

- Steam generator packing glands

- Valves of the primary cooling system.

- etc.

After connection of the Residual Heat Removal System (RHRS), some hot spots may migrate into this circuit and be deposited or fixed. The most common locations are the pumps, heat exchangers and valves of the circuit.

An underwater pool cleaner should pass through the pool out after discharging. In this case, particularly high equivalent dose rates, equal to or greater than 1 Sv per hour, measured in contact with the filters, represent the last indicator of the possible presence of hot spots, before the draining of the pools.

Since no warning signs have been identified yet, to indicate the occurrence of hot spots, it was decided to concentrate on preventive filtering, trapping hot spots as close as possible to their source to eliminate them.

## 3.2 - Metastable silver 110 contamination

In some PWR units, the primary and auxiliary systems are subject to metastable silver 110 (Ag 110 m) contamination, due to silver 109 neutronic activation:

$$ ^{109}\text{Ag} + ^{1}\text{n} \rightarrow ^{110}\text{mAg} + \gamma $$
<figure><img src="image_2.png" /><figcaption>Fig. 1: Preventive filtration methods</figcaption></figure>

The drains of pools are important routes for hot spots, before they spread through the systems. The installation of fine filters, an initial containment barrier, is proposed for the drain orifices of each pool:

- Pools located in the reactor building;

- Pools located in the fuel building;

Appropriate filters are required:

- Truncated sieves;

- 50 µm mesh.

The drain lines of the primary cooling circuit represent the second main vector for the movement of hot spots. As a preventive measure, the installation of sleeves on the main migration channels enables sensitive systems to be filtered. This filtration prevents the spread of hot spots and ensures that they are eliminated.

The filtration system is installed in the reactor building or the fuel building, depending on the shutdown phases, thus enabling all the pools to be treated with the same device.

### 3.1.2 - Surveillance

Surveillance is designed to inform the site, as early as possible, of the presence of hot spots (mapping) in order to take the appropriate measures to prevent their propagation and/or to eradicate them.

During unit operation, most hot spots will become fixed to the fuel. Others may fall, by gravity, to the bottom of the pool or be trapped in the special devices or the low points of the primary coolant system.

The most common locations are as follows:

- Thermal sleeves of the pressuriser

- Steam generator packing glands

- Valves of the primary cooling system.

- etc.

After connection of the Residual Heat Removal System (RHRS), some hot spots may migrate into this circuit and be deposited or fixed. The most common locations are the pumps, heat exchangers and valves of the circuit.

An underwater pool cleaner should pass through the pool out after discharging. In this case, particularly high equivalent dose rates, equal to or greater than 1 Sv per hour, measured in contact with the filters, represent the last indicator of the possible presence of hot spots, before the draining of the pools.

Since no warning signs have been identified yet, to indicate the occurrence of hot spots, it was decided to concentrate on preventive filtering, trapping hot spots as close as possible to their source to eliminate them.

## 3.2 - Metastable silver 110 contamination

In some PWR units, the primary and auxiliary systems are subject to metastable silver 110 (Ag 110 m) contamination, due to silver 109 neutronic activation:

$$ ^{109}\text{Ag} + ^{1}\text{n} \rightarrow ^{110}\text{mAg} + \gamma $$
The most likely root causes for silver leading to the observed Ag 110m pollutions may be as follows :

- the neutron-absorbing alloy Ag-In-Cd contained in control rods ;

- some silver coated seals.

For French plants, the major source for Ag110m is natural silver. Moreover, for the French plants polluted with Ag 110m, the quantity of metallic silver released in the primary system could be about 1-10 grams.

### 3.2.1- Impact on dose rates

Ag 110 m systems contamination can be very penalising since it impacts :

- dosimetry integrated during maintenance operations ;

- shutdown schedule ;

- control rods management (examination and rejection).

It can represent 5 to 15 % of a shutdown total dosimetry.

Systems most sensitive to silver contamination are :

- the Residual Heat Removal System (RHRS) and Chemical and Volume Control System (CVCS) coolers ;

- the purification system downstream the CVCS demineralisers ;

- the reactor cavity.

Ag 110m can contribute to more than 90 % of the dose rates around some parts of auxiliary systems.

### 3.2.2 - Surveillance

The volumic activity of Ag 110m in the primary coolant stays at a low level (1-10MBq/t) due to the low silver solubility in reducing medium. Activity increases higher than 10 MBq/t indicate a poor silver removal in the CVCS or an important silver source.

During the shutdown, in case of silver pollution, the observed levels of activity increase dramatically (several decade) specially after the oxygenation. This can be explained by :

- Changing from reducing to oxidising medium ;

- Simultaneously by temperature and pH changes.

At the opposite from other radioelements, for which dissolution is maximum at the "oxygenation peak", silver dissolution shows a trend to go on after the effective oxygenation (dissolved oxygen close to 1 mg/kg).

Generally, the silver peak activity appears 1 to 12 hours later after the Co 58 peak. The table 2 gives some data of silver peak for the French plants in the year 2000 during shutdown.

<table><caption>Table 2 : Ag 110m, 900 and 1300 MWe French Plants in 2000.</caption><thead><tr><th>Shutdown Year 2000</th><th>Ag110m average</th><th>Ag110m maximum</th><th>Ag110m minimum</th></tr></thead><tbody><tr><td>900 MW</td><td>1.47 GBq/t</td><td>6.17 GBq/t</td><td>0.07 GBq/t</td></tr><tr><td>1300 MW</td><td>0.16 GBq/t</td><td>0.48 GBq/t</td><td>0.04 GBq/t</td></tr></tbody></table>

There is no correlation between the Ag 110 activity peak and the other corrosion products. A first level of assessment can lead to consider that RCS is polluted when Ag 110m peak during shutdown is higher than 0,5 GBq/t.

### 3.2.3 - Silver behaviour

The observation of different and unpredictable behaviour of silver, mostly measured by gamma-spectroscopy and particularly during different cold shutdowns, even in a same plant, can be surprising when the operation schedule seems stable. Measurements demonstrated that it is possible to decrease a lot the steam generator channel head silver contamination and to increase the CVCS exchanger.

Depending on chemical environment and physical properties (pH, redox potential ...) silver can be :

- In ionic form in solution, Ag cation (I) specially ;

- In metallic form, Ag (0) ;

- In colloidal form, 200-600 Å.

Studies, conducted by EDF, demonstrated the essential impact of redox potential, pH, and temperature on silver thermodynamic stability. In a large range of pH (corresponding to the nominal operating conditions or to the shutdown ones at 300°C, 80°C and 30°C) and in different locations of RCS, CVCS and RHR, Ag⁰ and Ag⁺ can be found simultaneously and are very sensitive to the redox variation and the concentration. Actually, it seems that the physical and chemical conditions of the primary coolant set silver mostly on the limits of different existence areas considering the corresponding pH-potential diagrams.
The most likely root causes for silver leading to the observed Ag 110m pollutions may be as follows :

- the neutron-absorbing alloy Ag-In-Cd contained in control rods ;

- some silver coated seals.

For French plants, the major source for Ag110m is natural silver. Moreover, for the French plants polluted with Ag 110m, the quantity of metallic silver released in the primary system could be about 1-10 grams.

### 3.2.1- Impact on dose rates

Ag 110 m systems contamination can be very penalising since it impacts :

- dosimetry integrated during maintenance operations ;

- shutdown schedule ;

- control rods management (examination and rejection).

It can represent 5 to 15 % of a shutdown total dosimetry.

Systems most sensitive to silver contamination are :

- the Residual Heat Removal System (RHRS) and Chemical and Volume Control System (CVCS) coolers ;

- the purification system downstream the CVCS demineralisers ;

- the reactor cavity.

Ag 110m can contribute to more than 90 % of the dose rates around some parts of auxiliary systems.

### 3.2.2 - Surveillance

The volumic activity of Ag 110m in the primary coolant stays at a low level (1-10MBq/t) due to the low silver solubility in reducing medium. Activity increases higher than 10 MBq/t indicate a poor silver removal in the CVCS or an important silver source.

During the shutdown, in case of silver pollution, the observed levels of activity increase dramatically (several decade) specially after the oxygenation. This can be explained by :

- Changing from reducing to oxidising medium ;

- Simultaneously by temperature and pH changes.

At the opposite from other radioelements, for which dissolution is maximum at the "oxygenation peak", silver dissolution shows a trend to go on after the effective oxygenation (dissolved oxygen close to 1 mg/kg).

Generally, the silver peak activity appears 1 to 12 hours later after the Co 58 peak. The table 2 gives some data of silver peak for the French plants in the year 2000 during shutdown.

<table><caption>Table 2 : Ag 110m, 900 and 1300 MWe French Plants in 2000.</caption><thead><tr><th>Shutdown Year 2000</th><th>Ag110m average</th><th>Ag110m maximum</th><th>Ag110m minimum</th></tr></thead><tbody><tr><td>900 MW</td><td>1.47 GBq/t</td><td>6.17 GBq/t</td><td>0.07 GBq/t</td></tr><tr><td>1300 MW</td><td>0.16 GBq/t</td><td>0.48 GBq/t</td><td>0.04 GBq/t</td></tr></tbody></table>

There is no correlation between the Ag 110 activity peak and the other corrosion products. A first level of assessment can lead to consider that RCS is polluted when Ag 110m peak during shutdown is higher than 0,5 GBq/t.

### 3.2.3 - Silver behaviour

The observation of different and unpredictable behaviour of silver, mostly measured by gamma-spectroscopy and particularly during different cold shutdowns, even in a same plant, can be surprising when the operation schedule seems stable. Measurements demonstrated that it is possible to decrease a lot the steam generator channel head silver contamination and to increase the CVCS exchanger.

Depending on chemical environment and physical properties (pH, redox potential ...) silver can be :

- In ionic form in solution, Ag cation (I) specially ;

- In metallic form, Ag (0) ;

- In colloidal form, 200-600 Å.

Studies, conducted by EDF, demonstrated the essential impact of redox potential, pH, and temperature on silver thermodynamic stability. In a large range of pH (corresponding to the nominal operating conditions or to the shutdown ones at 300°C, 80°C and 30°C) and in different locations of RCS, CVCS and RHR, Ag⁰ and Ag⁺ can be found simultaneously and are very sensitive to the redox variation and the concentration. Actually, it seems that the physical and chemical conditions of the primary coolant set silver mostly on the limits of different existence areas considering the corresponding pH-potential diagrams.
On the opposite, pH is determinant on colloids behaviour. Repulsion between them is the lowest at iso-electric point, meaning at pH for which the zeta potential is zero.

Thus, optimization of its removal is difficult, the preferential deposition of Ag 110m taking place in "cold" points of auxiliary exchangers where thermal gradient is important. This also elucidates the apparent unoperating conditions of the purification. The goal will become to manage the silver so as to transport it in the form able to be removed by the purification system.

### 3.2.4 - Preventive actions.

Preventive actions to control silver root source are :

- Control rods examination;

- Primary Ag 110 m volumic activity monitoring during operation.

#### Control rods examination

Eddy current and ultrasonic non destructive control rod examination is performed. Doubtful rods or rods with perforation signals are systematically replaced with either chromium or nitride coated rods.

However, such examinations do not allow for detection of fast evolution kinetics flaws. It was the case in Tricastin 2, at the end of cycle 17 shutdown, as Ag 110 m volumic activity reached 16 GBq/t during primary coolant oxygenation. Such an activity called for unscheduled examination of the control rods, revealing an early perforation of 10 standard rods, which were rejected and replaced with coated rods.

#### Primary Ag 110 m volumic activity monitoring during operation

A tight monitoring of Ag 110 m volumic activity in operation can allow to anticipate an anomaly before the shutdown transient. It consists in comparing the on-going cycle volumic activity with that of the previous cycles.

Ag 110 m activity level likely to be observed as the primary coolant is oxygenated during the end of cycle shutdown can be forecasted by correlating Ag 110 m mean activity during operation to Ag 110 m maximum activity at end of cycle shutdown oxygenation (fig 2). Hence, Ag 110 m maximum volumic activity, as the primary coolant is oxygenated, matches the total activity released during the outage.

<figure><img src="image_3.png" /><figcaption>Fig. 2 : Ag 110 m activity in Tricastin 2 unit</figcaption></figure>

### 3.2.5 - Remedial actions

Remedial actions depend on Ag 110 m activity released during cycle operation or during shutdown. They deal with :

- purification during end of cycle shutdown;

- elution of activity confined in dead legs;

- other complementary actions (cleaning system).

#### Purification during end of cycle shutdown

Ag 110 m purification is performed on a dedicated ion exchanger operating at the maximum possible flowrate.

During shutdown, when the filters are replaced, purification must not be suspended. Filters consistent with colloids removal are needed (lower mesh with polarised membrane). Thus, silver removal is improved and downstream resins pollution avoided (poisoning by colloids). The filter replacement must lead to reduce by half the CVCS flowrate purification to minimize resins pollutions.

The background and the resin features show that macroporous mixed-bed will be the most adapted for soluble silver and silver colloids. The pressure drop must be specially observed.
On the opposite, pH is determinant on colloids behaviour. Repulsion between them is the lowest at iso-electric point, meaning at pH for which the zeta potential is zero.

Thus, optimization of its removal is difficult, the preferential deposition of Ag 110m taking place in "cold" points of auxiliary exchangers where thermal gradient is important. This also elucidates the apparent unoperating conditions of the purification. The goal will become to manage the silver so as to transport it in the form able to be removed by the purification system.

### 3.2.4 - Preventive actions.

Preventive actions to control silver root source are :

- Control rods examination;

- Primary Ag 110 m volumic activity monitoring during operation.

#### Control rods examination

Eddy current and ultrasonic non destructive control rod examination is performed. Doubtful rods or rods with perforation signals are systematically replaced with either chromium or nitride coated rods.

However, such examinations do not allow for detection of fast evolution kinetics flaws. It was the case in Tricastin 2, at the end of cycle 17 shutdown, as Ag 110 m volumic activity reached 16 GBq/t during primary coolant oxygenation. Such an activity called for unscheduled examination of the control rods, revealing an early perforation of 10 standard rods, which were rejected and remplaced with coated rods.

#### Primary Ag 110 m volumic activity monitoring during operation

A tight monitoring of Ag 110 m volumic activity in operation can allow to anticipate an anomaly before the shutdown transient. It consists in comparing the on-going cycle volumic activity with that of the previous cycles.

Ag 110 m activity level likely to be observed as the primary coolant is oxygenated during the end of cycle shutdown can be forecasted by correlating Ag 110 m mean activity during operation to Ag 110 m maximum activity at end of cycle shutdown oxygenation (fig 2). Hence, Ag 110 m maximum volumic activity, as the primary coolant is oxygenated, matches the total activity released during the outage.

<figure><img src="image_3.png" /><figcaption>Fig. 2 : Ag 110 m activity in Tricastin 2 unit</figcaption></figure>

### 3.2.5 - Remedial actions

Remedial actions depend on Ag 110 m activity released during cycle operation or during shutdown. They deal with :

- purification during end of cycle shutdown;

- elution of activity confined in dead legs;

- other complementary actions (cleaning system).

#### Purification during end of cycle shutdown

Ag 110 m purification is performed on a dedicated ion exchanger operating at the maximum possible flowrate.

During shutdown, when the filters are replaced, purification must not be suspended. Filters consistent with colloids removal are needed (lower mesh with polarised membrane). Thus, silver removal is improved and downstream resins pollution avoided (poisoning by colloids). The filter replacement must lead to reduce by half the CVCS flowrate purification to minimize resins pollutions.

The background and the resin features show that macroporous mixed-bed will be the most adapted for soluble silver and silver colloids. The pressure drop must be specially observed.
Until now, the feedback demonstrated that withdrawing lithium of the primary coolant with a non lithiated resin during the shutdown, giving the lower pH, improves the silver removal efficiency. Without cladding failure, the lithium can be eliminated starting from the rods drop (see &5)

### Crossover legs drainage

At Tricastin, forecasting Ag 110 m activity level allows to get ready, before shutdown, for a specific operation to decrease Ag 110 m volumic activity confined in the main primary system crossover legs, prior to reactor cavity filling and refueling. Usually, crossover legs drainage is performed after core unloading. In the presence of Ag 110 m, scheduling a crossover legs rinsing prior to system drainage allows to elute the activity confined in them, and hence to lower the risks for contamination of the water during cavity filling.

Ag 110 m final volumic activity of the water in the reactor cavity can be reduced by a factor of 3, leading to limited dose integrated during fuel handling operations.

### Complementary actions

Other remedial actions can be undertaken depending on the types of operations to be performed during the end of cycle outage (cleaning of some systems parts, such as RHRS and CVCS coolers).

Coolers contamination occurs mostly during outages. Contamination of CVCS pumps and valves dowstream the purification system is mostly due to the lithiated CVCS demineraliser inefficiency in purifying Ag 110 m during operation.

## 3.3 - Antimony contamination

In some French PWRs, ¹²²Sb and ¹²⁴Sb volumic activity peaks have been observed. These peaks were higher than ⁵⁸Co peaks (table 3). They impacted dosimetry and waste management.

¹²²Sb and ¹²⁴Sb are two multigamma emitters with radioactive half-life of 2.7 days and 60.2 days respectively.

Thanks to its short radioactive half-life, ¹²²Sb has a low impact on health physics, but it allows to determine antimony sources.

<table><caption>Table 3 : Mean and maximum volumic activities encountered at the oxygenation peak</caption><thead><tr><th>PWR UNITS (MWe)</th><th></th><th>SB 122 (GBq/t)</th><th>Sb 124 (GBq/t)</th></tr></thead><tbody><tr><td rowspan="2">900 first series</td><td>Mean</td><td>21</td><td>14</td></tr><tr><td>Maximum</td><td>200</td><td>110</td></tr><tr><td rowspan="2">900</td><td>Mean</td><td>25</td><td>18</td></tr><tr><td>Maximum</td><td>160</td><td>140</td></tr><tr><td rowspan="2">1300</td><td>Mean</td><td>14</td><td>11</td></tr><tr><td>Maximum</td><td>70</td><td>55</td></tr></tbody></table>

### 3.3.1 – Antimony sources

Two major sources have been identified in French PWRs :

1. Beryllium-Antimony source rod failure, but there are source rods only for the first reactor cycles ;

2. bearing wear of pumps in the Boron Recycle System. These bearings contain graphite impregnated with about 10 % of antimony.

A few activated antimony grams can explain such peaks of ¹²²Sb and ¹²⁴Sb. So, about 7 grams can lead to a ¹²⁴Sb peak of 100 GBq/t.

### 3.3.2 – Antimony behaviour

EDF studies, elaborating pH-Potential curves at normal operating conditions (300°C) and at forced oxygenation at 80°C confirmed the behaviour observed on plants.

It seems that, in nominal conditions at 300°C, metallic antimony is likely the more stable in aqueous reducing medium (fig 3).
Until now, the feedback demonstrated that withdrawing lithium of the primary coolant with a non lithiated resin during the shutdown, giving the lower pH, improves the silver removal efficiency. Without cladding failure, the lithium can be eliminated starting from the rods drop (see &5)

### Crossover legs drainage

At Tricastin, forecasting Ag 110 m activity level allows to get ready, before shutdown, for a specific operation to decrease Ag 110 m volumic activity confined in the main primary system crossover legs, prior to reactor cavity filling and refueling. Usually, crossover legs drainage is performed after core unloading. In the presence of Ag 110 m, scheduling a crossover legs rinsing prior to system drainage allows to elute the activity confined in them, and hence to lower the risks for contamination of the water during cavity filling.

Ag 110 m final volumic activity of the water in the reactor cavity can be reduced by a factor of 3, leading to limited dose integrated during fuel handling operations.

### Complementary actions

Other remedial actions can be undertaken depending on the types of operations to be performed during the end of cycle outage (cleaning of some systems parts, such as RHRS and CVCS coolers).

Coolers contamination occurs mostly during outages. Contamination of CVCS pumps and valves dowstream the purification system is mostly due to the lithiated CVCS demineraliser inefficiency in purifying Ag 110 m during operation.

## 3.3 - Antimony contamination

In some French PWRs, ¹²²Sb and ¹²⁴Sb volumic activity peaks have been observed. These peaks were higher than ⁵⁸Co peaks (table 3). They impacted dosimetry and waste management.

¹²²Sb and ¹²⁴Sb are two multigamma emitters with radioactive half-life of 2.7 days and 60.2 days respectively.

Thanks to its short radioactive half-life, ¹²²Sb has a low impact on health physics, but it allows to determine antimony sources.

<table><caption>Table 3 : Mean and maximum volumic activities encountered at the oxygenation peak</caption><thead><tr><th>PWR UNITS (MWe)</th><th></th><th>SB 122 (GBq/t)</th><th>Sb 124 (GBq/t)</th></tr></thead><tbody><tr><td rowspan="2">900 first series</td><td>Mean</td><td>21</td><td>14</td></tr><tr><td>Maximum</td><td>200</td><td>110</td></tr><tr><td rowspan="2">900</td><td>Mean</td><td>25</td><td>18</td></tr><tr><td>Maximum</td><td>160</td><td>140</td></tr><tr><td rowspan="2">1300</td><td>Mean</td><td>14</td><td>11</td></tr><tr><td>Maximum</td><td>70</td><td>55</td></tr></tbody></table>

### 3.3.1 – Antimony sources

Two major sources have been identified in French PWRs :

1. Beryllium-Antimony source rod failure, but there are source rods only for the first reactor cycles ;

2. bearing wear of pumps in the Boron Recycle System. These bearings contain graphite impregnated with about 10 % of antimony.

A few activated antimony grams can explain such peaks of ¹²²Sb and ¹²⁴Sb. So, about 7 grams can lead to a ¹²⁴Sb peak of 100 GBq/t.

### 3.3.2 – Antimony behaviour

EDF studies, elaborating pH-Potential curves at normal operating conditions (300°C) and at forced oxygenation at 80°C confirmed the behaviour observed on plants.

It seems that, in nominal conditions at 300°C, metallic antimony is likely the more stable in aqueous reducing medium (fig 3).
<figure><img src="image_4.png" /><figcaption>Fig 3 : Simplified Pourbaix diagram for antimony at 300°C. The dotted line limits the RCS operating conditions</figcaption></figure>

For shutdown conditions at forced oxygenation, it seems that the more stable species is at the limit of the SBO₃⁻ area in aqueous solution. Eliminating antimony as well as possible becomes easier in this zone (fig 4).

<figure><img src="image_5.png" /><figcaption>Fig 4 : Simplified Pourbaix diagram for antimony at 80°C. The dotted line limits the RCS shutdown conditions at forced oxygenation.</figcaption></figure>

### 3.3.3 - Impact on dose rates

The important antimony released activity has an impact on dosimetry and waste management.

¹²⁴Sb deposited activities on the out-of-core surfaces increase a lot as it is shown in table 4.

<table><caption>Table 4 : RCS recontamination after a ¹²⁴Sb peak of 70 GBq/t</caption><thead><tr><th>¹²⁴Sb (GBq/m²)</th><th>Before H₂O₂</th><th>After H₂O₂</th></tr></thead><tbody><tr><td>Hot leg</td><td>0.29</td><td>1.02</td></tr><tr><td>SG tubing</td><td>0.32</td><td>0.95</td></tr></tbody></table>

Thus, the ¹²⁴Sb contribution to the dose rates can reach 10% in some parts of circuits. And the dosimetry for maintenance during a plant shutdown can increase to about 5% because of the ¹²⁴Sb contamination.

### 3.3.4 - Remedial actions

To avoid antimony pollution, the incriminated pump bearings have been replaced by pump bearings without antimony. As it is indicated in the table below (table 5), this allows to make the ¹²⁴Sb peaks decrease.
<figure><img src="image_4.png" /><figcaption>Fig 3 : Simplified Pourbaix diagram for antimony at 300°C. The dotted line limits the RCS operating conditions</figcaption></figure>

For shutdown conditions at forced oxygenation, it seems that the more stable species is at the limit of the SBO₃⁻ area in aqueous solution. Eliminating antimony as well as possible becomes easier in this zone (fig 4).

<figure><img src="image_5.png" /><figcaption>Fig 4 : Simplified Pourbaix diagram for antimony at 80°C. The dotted line limits the RCS shutdown conditions at forced oxygenation.</figcaption></figure>

### 3.3.3 - Impact on dose rates

The important antimony released activity has an impact on dosimetry and waste management.

¹²⁴Sb deposited activities on the out-of-core surfaces increase a lot as it is shown in table 4.

<table><caption>Table 4 : RCS recontamination after a ¹²⁴Sb peak of 70 GBq/t</caption><thead><tr><th>¹²⁴Sb (GBq/m²)</th><th>Before H₂O₂</th><th>After H₂O₂</th></tr></thead><tbody><tr><td>Hot leg</td><td>0.29</td><td>1.02</td></tr><tr><td>SG tubing</td><td>0.32</td><td>0.95</td></tr></tbody></table>

Thus, the ¹²⁴Sb contribution to the dose rates can reach 10% in some parts of circuits. And the dosimetry for maintenance during a plant shutdown can increase to about 5% because of the ¹²⁴Sb contamination.

### 3.3.4 - Remedial actions

To avoid antimony pollution, the incriminated pump bearings have been replaced by pump bearings without antimony. As it is indicated in the table below (table 5), this allows to make the ¹²⁴Sb peaks decrease.
<table><caption>Table 5: Bearing replacement during the 14th cycle</caption><tr><td>Cycles</td><td>13</td><td>14*</td><td>15</td></tr><tr><td><sup>124</sup>Sb (GBq/t)</td><td>100</td><td>60</td><td>20</td></tr></table>

During oxygenation, antimony seems soluble (< 0.45 µm). When an antimony pollution occurs, to improve purification by the ion exchange resins, reactor coolant lithium content has to be lower than 1 ppm [4]. In order to obtain an acidic medium, purification can be carried out thanks to non-saturated lithium cation bed demineralizers of the Boron Recycle System. Nevertheless, it should be noted that an acidic reactor coolant seems to increase antimony deposition on the primary circuit surfaces [4].

As for silver behaviour, to manage an antimony pollution the operators face some difficulties linked to the different physical and chemical conditions of the RCS and the auxiliary systems. Except for removing the root cause, the objective consists in the optimization of the form of the antimony to enable the purification line to remove the pollution.

## 3.4 – Iodine contamination

Iodine released in the primary coolant, when a fuel cladding failure has occurred during load reduction, leads to volumic activity increases of I 131 (several GBq/t :5 GBq/t in Gravelines 1 cycle 12). After the forced oxygenation, the iodine concentration shows decreases which could be explained by the efficiency of the purification system. Actually, on 15 units examined, a decrease of about 70% of I 131 measured by gamma spectroscopy can be observed with no real correlation with the resin capacities. Then absorption becomes an assumption to examine. If studies have been conducted in radioactive accident scenario, little concerned the normal operation. That is why EDF tried to define iodine behaviour [4] [5].

### 3.4.1 - Iodine behaviour

Considering the physical and chemical parameters of the primary coolant at nominal power and during transients, iodine can be found principally in iodide, iodate or molecular form following the equilibrium :

$$3 \text{I}_2 + 3 \text{H}_2\text{O} \underset{\text{amphotericization}}{\overset{\text{dismutation}}{\rightleftharpoons}} 5 \text{I}^- + \text{IO}_3^- + 6 \text{H}^+$$

However, the pH then ranging from 7 to 7.2 at 300°C and the redox conditions of the hydrogenated (thus reducing) RCS, strongly favor iodide formation. At low concentration, which is the RCS situation, these ions are soluble and are removed by the anionic resin of the lithiated mixed bed even at the CVCS temperature. Dismutation reaction is almost total but kinetics is low and increases with temperature. Therefore, the risk of absorption is very weak during the operation.

On the opposite, amphotericization reaction rate is high. It occurs at 80°C when RCS is oxygenated. This was verified on a lot of units. This was studied in French laboratories where the iodine absorption was examined on alloy 600, Zircaloy and stainless steel with the chemical primary coolant conditions.

A large absorption was demonstrated. It depends on temperature and pH. pH interferes with dismutation equilibrium : the lower the pH, the higher the molecular iodine ratio free for absorption. The figure 5 shows the variation of the iodine absorption versus time and pH on alloy 600. Similar tests were conducted versus temperature and the experimentation showed that the lower the temperature, the higher the absorption. Whatever the materials studied, the trend stays exactly the same.

Meanwhile laboratories results demonstrated that in given conditions, materials showed different behaviours concerning absorption leading to the following classification :

*“Stainless steel < Zircaloy < Alloy 600”*

Tests on desorption demonstrate that desorbed iodine increases with temperature and dynamic venting. The desorption is maximum on alloy 600 compared to zircaloy and stainless steel.

Considering that the steam generator tubes represent the major surface, the zone where iodine molecular absorption is greater forms also the area where iodine can potentially desorb the most.
<table><caption>Table 5: Bearing replacement during the 14th cycle</caption><tr><td>Cycles</td><td>13</td><td>14*</td><td>15</td></tr><tr><td><sup>124</sup>Sb (GBq/t)</td><td>100</td><td>60</td><td>20</td></tr></table>

During oxygenation, antimony seems soluble (< 0.45 µm). When an antimony pollution occurs, to improve purification by the ion exchange resins, reactor coolant lithium content has to be lower than 1 ppm [4]. In order to obtain an acidic medium, purification can be carried out thanks to non-saturated lithium cation bed demineralizers of the Boron Recycle System. Nevertheless, it should be noted that an acidic reactor coolant seems to increase antimony deposition on the primary circuit surfaces [4].

As for silver behaviour, to manage an antimony pollution the operators face some difficulties linked to the different physical and chemical conditions of the RCS and the auxiliary systems. Except for removing the root cause, the objective consists in the optimization of the form of the antimony to enable the purification line to remove the pollution.

## 3.4 – Iodine contamination

Iodine released in the primary coolant, when a fuel cladding failure has occurred during load reduction, leads to volumic activity increases of I 131 (several GBq/t :5 GBq/t in Gravelines 1 cycle 12). After the forced oxygenation, the iodine concentration shows decreases which could be explained by the efficiency of the purification system. Actually, on 15 units examined, a decrease of about 70% of I 131 measured by gamma spectroscopy can be observed with no real correlation with the resin capacities. Then absorption becomes an assumption to examine. If studies have been conducted in radioactive accident scenario, little concerned the normal operation. That is why EDF tried to define iodine behaviour [4] [5].

### 3.4.1 - Iodine behaviour

Considering the physical and chemical parameters of the primary coolant at nominal power and during transients, iodine can be found principally in iodide, iodate or molecular form following the equilibrium :

$$3 \text{I}_2 + 3 \text{H}_2\text{O} \underset{\text{amphotericization}}{\overset{\text{dismutation}}{\rightleftharpoons}} 5 \text{I}^- + \text{IO}_3^- + 6 \text{H}^+$$

However, the pH then ranging from 7 to 7.2 at 300°C and the redox conditions of the hydrogenated (thus reducing) RCS, strongly favor iodide formation. At low concentration, which is the RCS situation, these ions are soluble and are removed by the anionic resin of the lithiated mixed bed even at the CVCS temperature. Dismutation reaction is almost total but kinetics is low and increases with temperature. Therefore, the risk of absorption is very weak during the operation.

On the opposite, amphotericization reaction rate is high. It occurs at 80°C when RCS is oxygenated. This was verified on a lot of units. This was studied in French laboratories where the iodine absorption was examined on alloy 600, Zircaloy and stainless steel with the chemical primary coolant conditions.

A large absorption was demonstrated. It depends on temperature and pH. pH interferes with dismutation equilibrium : the lower the pH, the higher the molecular iodine ratio free for absorption. The figure 5 shows the variation of the iodine absorption versus time and pH on alloy 600. Similar tests were conducted versus temperature and the experimentation showed that the lower the temperature, the higher the absorption. Whatever the materials studied, the trend stays exactly the same.

Meanwhile laboratories results demonstrated that in given conditions, materials showed different behaviours concerning absorption leading to the following classification :

*“Stainless steel < Zircaloy < Alloy 600”*

Tests on desorption demonstrate that desorbed iodine increases with temperature and dynamic venting. The desorption is maximum on alloy 600 compared to zircaloy and stainless steel.

Considering that the steam generator tubes represent the major surface, the zone where iodine molecular absorption is greater forms also the area where iodine can potentially desorb the most.
<figure><img src="image_6.png" /><figcaption>Fig. 5 : Iodine absorbed in µg/cm² on passivated alloy 600 versus pH and time (Iodine initial concentration 50 mg/kg)</figcaption></figure>

### 3.4.2 - Impact on dose rates

Contamination measurements, performed by CEA have shown that the percentage of deposited activity due to iodine is weak and the dose rates are not a key problem for the workers. Taking into account the potential desorption of iodine when the RCS is opened, the major issue is to ensure personnel protection against contamination.

### 3.4.3 - Remedial actions

Laboratory tests conducted with fresh mixed bed resins, saturated with lithium and equilibrated with boron and close to CVCS conditions, showed that iodine removal performance was not impacted by lithium concentration in reducing medium. Taking into account this result and the behaviour of iodine, recommendations can be made to improve the operating conditions to limit its effects. The goal is to ensure that the maximum iodine can be eliminated before the forced oxygenation of the primary coolant.

From the beginning of temperature decrease, it is better to purify with the maximum flowrate, avoiding introducing oxygenated water in the primary coolant (during the coolant contraction), keeping the maximum concentration of lithium. Thus, a maximum quantity of soluble iodine can be transported upstream the resins to be removed and the potential quantity of iodine able to absorb on the materials walls is reduced. Then, the operating strategy becomes :

- Optimize chemistry before the primary coolant oxygenation ;

- Keep the coolant hydrogenated the longer ;

- Use a mixed bed resin to favour the purification (the lithium removal leads to a pH decrease increasing the molecular iodine to be potentially absorbed, therefore not removable by ionic exchanger).

Some opposite requirements can be noticed when iodine and silver are simultaneously observed in the primary coolant. In this configuration two ways of managing both pollutions can be proposed.

The first is to focus on iodine removal before primary coolant oxygenation then to manage silver from the beginning of forced oxygenation.

The second is to keep the coolant hydrogenated from the temperature decrease and to remove lithium favourising an acidic medium. In this configuration the major part of iodine can be removed considering the local alkaline conditions of anionic resins able to shift the iodine equilibrium towards dismutation. This second one is not the more efficient to eliminate iodine in the RCS and auxiliaries.

However EDF considers that as soon as primary water equivalent I 131 activity exceeds 50 MBq/t before RCS opening, the personnel protection against contamination have to be settle.

The preventive strategy consists in implementing special equipments implying dynamic venting for risky works. Pressure reduction devices and iodine traps are connected to the containment building venting system.
<figure><img src="image_6.png" /><figcaption>Fig. 5 : Iodine absorbed in µg/cm² on passivated alloy 600 versus pH and time (Iodine initial concentration 50 mg/kg)</figcaption></figure>

### 3.4.2 - Impact on dose rates

Contamination measurements, performed by CEA have shown that the percentage of deposited activity due to iodine is weak and the dose rates are not a key problem for the workers. Taking into account the potential desorption of iodine when the RCS is opened, the major issue is to ensure personnel protection against contamination.

### 3.4.3 - Remedial actions

Laboratory tests conducted with fresh mixed bed resins, saturated with lithium and equilibrated with boron and close to CVCS conditions, showed that iodine removal performance was not impacted by lithium concentration in reducing medium. Taking into account this result and the behaviour of iodine, recommendations can be made to improve the operating conditions to limit its effects. The goal is to ensure that the maximum iodine can be eliminated before the forced oxygenation of the primary coolant.

From the beginning of temperature decrease, it is better to purify with the maximum flowrate, avoiding introducing oxygenated water in the primary coolant (during the coolant contraction), keeping the maximum concentration of lithium. Thus, a maximum quantity of soluble iodine can be transported upstream the resins to be removed and the potential quantity of iodine able to absorb on the materials walls is reduced. Then, the operating strategy becomes :

- Optimize chemistry before the primary coolant oxygenation ;

- Keep the coolant hydrogenated the longer ;

- Use a mixed bed resin to favour the purification (the lithium removal leads to a pH decrease increasing the molecular iodine to be potentially absorbed, therefore not removable by ionic exchanger).

Some opposite requirements can be noticed when iodine and silver are simultaneously observed in the primary coolant. In this configuration two ways of managing both pollutions can be proposed.

The first is to focus on iodine removal before primary coolant oxygenation then to manage silver from the beginning of forced oxygenation.

The second is to keep the coolant hydrogenated from the temperature decrease and to remove lithium favourising an acidic medium. In this configuration the major part of iodine can be removed considering the local alkaline conditions of anionic resins able to shift the iodine equilibrium towards dismutation. This second one is not the more efficient to eliminate iodine in the RCS and auxiliaries.

However EDF considers that as soon as primary water equivalent I 131 activity exceeds 50 MBq/t before RCS opening, the personnel protection against contamination have to be settle.

The preventive strategy consists in implementing special equipments implying dynamic venting for risky works. Pressure reduction devices and iodine traps are connected to the containment building venting system.
## 4 - Conclusion

EDF plants faces radioactive pollutions (i.e. Co 60, Ag 110m, antimony and iodine) leading to significant increases of shutdown dosimetry in affected units (10 – 30%) or increasing the contamination risk for personnel. Early detection in operation as well as during shutdown requires a good knowledge of their behaviour to set up appropriate solutions.

Replacing materials as soon as it will be possible should limit the source term. Limiting pollution in circuits need to implement chemistry and shutdown procedure to minimize over-contamination. To manage iodine behaviour allows to eliminate it and to perform devices to protect personnel before opening the RCS.

It is obvious that implementing specific chemistry and optimizing purification features (filters, resins and flowrate) warrant limiting the effects of this type of pollutions.

That's why studies are yet in progress to propose operators a chemical policy and adapted procedures for each type of radioactive pollution able to reduce the impact on dosimetry.

## References

[1] F. Dacquait et alii. Corrosion Products transport in French PWR'S during cold Shutdown. International conference "Chemistry in water reactors" SFEN, Avignon, 22-26 April 2002.

[2] F. Nordmann et alii. Overview of PWR chemistry options. International conference "Chemistry in water reactors" SFEN, Avignon, 22-26 April 2002.

[3] A. Rocher et alii. Program to eradicate hot spots (Co 60) carried out in French PWR Units. JAIF International Conference on Water Chemistry, 1998.

[4] G. Gaudard et alii. Improvements of Primary Coolant Shutdown Chemistry and Reactor coolant cleanup system. International conference "Chemistry in water reactors" SFEN, Avignon, 22-26 April 2002.

[5] B. Leuthrot et alii; "Iodine Behaviour in the primary Circuit during cold shutdown in PWR". International conference "Chemistry in water reactors", Bournemouth, 13-17 October 1996.
## 4 - Conclusion

EDF plants faces radioactive pollutions (i.e. Co 60, Ag 110m, antimony and iodine) leading to significant increases of shutdown dosimetry in affected units (10 – 30%) or increasing the contamination risk for personnel. Early detection in operation as well as during shutdown requires a good knowledge of their behaviour to set up appropriate solutions.

Replacing materials as soon as it will be possible should limit the source term. Limiting pollution in circuits need to implement chemistry and shutdown procedure to minimize over-contamination. To manage iodine behaviour allows to eliminate it and to perform devices to protect personnel before opening the RCS.

It is obvious that implementing specific chemistry and optimizing purification features (filters, resins and flowrate) warrant limiting the effects of this type of pollutions.

That's why studies are yet in progress to propose operators a chemical policy and adapted procedures for each type of radioactive pollution able to reduce the impact on dosimetry.

## References

[1] F. Dacquait et alii. Corrosion Products transport in French PWR'S during cold Shutdown. International conference "Chemistry in water reactors" SFEN, Avignon, 22-26 April 2002.

[2] F. Nordmann et alii. Overview of PWR chemistry options. International conference "Chemistry in water reactors" SFEN, Avignon, 22-26 April 2002.

[3] A. Rocher et alii. Program to eradicate hot spots (Co 60) carried out in French PWR Units. JAIF International Conference on Water Chemistry, 1998.

[4] G. Gaudard et alii. Improvements of Primary Coolant Shutdown Chemistry and Reactor coolant cleanup system. International conference "Chemistry in water reactors" SFEN, Avignon, 22-26 April 2002.

[5] B. Leuthrot et alii; "Iodine Behaviour in the primary Circuit during cold shutdown in PWR". International conference "Chemistry in water reactors", Bournemouth, 13-17 October 1996.