# Influence of major PWR radiochemical pollutants on dose rates and dosimetry - Importance of efficiency of retention techniques

Alain Rocher, Martial Berger, Jean-Luc Bretelle, Gilles Bégué, Frédéric Dacquait

► To cite this version:

Alain Rocher, Martial Berger, Jean-Luc Bretelle, Gilles Bégué, Frédéric Dacquait. Influence of major PWR radiochemical pollutants on dose rates and dosimetry - Importance of efficiency of retention techniques. International Conference on Water Chemistry of Nuclear Reactor Systems 8, Sep 2000, Bournemouth, United Kingdom. pp.479-481, 10.1680/wconrs8v2.29583.0019. cea-04463822

HAL Id: cea-04463822

https://cea.hal.science/cea-04463822

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
# Influence of major PWR radiochemical pollutants on dose rates and dosimetry - Importance of efficiency of retention techniques

A. Rocher, Electricité de France, FTC, Groupe Prévention des Risques, France

M. Berger, Ph. Ridoux, Electricité de France, SEPTEN, France

J.L. Bretelle, Electricité de France, Groupe des Laboratoires, France

G. Bégué, Electricité de France, CNPE de Tricastin, France

F. Dacquait, Commissariat à l'Energie Atomique, France

## Introduction

In addition to usual systems contamination mostly due to materials generalised corrosion, some pollutions can lead to « over-contamination ».

The major pollutants responsible for over-contamination in EDF plants are : Co 60 coming from stellites degradation, Ag 110 m coming from control rod piercing, and Sb 124 coming from some pumps of auxiliary systems.

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 against penalising pollutions.

Priority is set on prevention.

## Programme to eradicate hot spots (Co 60) carried out in French PWR units

In most cases, hot spots [1] are due to particles of cobalt activated by a neutrons flux (Co 60) mainly from stellite surfaced equipment (rich in cobalt) [2] in the primary cooling circuit (valves, pumps, internals, etc.).

Cobalt 60 emits, in particular, two types of high energy gamma radiation [3] (γ radiation of 1.17 MeV and γ radiation of 1.33 MeV given off by decay). Its decay period is 5.3 years ; it therefore generates high equivalent dose rates over many years if it is not removed from the cooling circuits.

The contribution of hot spots to shutdown dosimetry may appear to be marginal across all French reactors (2 to 4 %) but is significant (15 to 25 %) for the units affected. This excess dosimetry is a significant factor, particularly for the most exposed workers (the population with a dosimetry of ≥ 20 mSv/year). Approximately ten French units have been affected by this phenomenon over the last 12 years.

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

### Prevention

Prevention consists in avoiding the appearance of hot spots. It concerns all units and is based mainly on the following actions:

* limiting the use of cobalt based components in contact with effluent from the primary circuit ;

* removing stellite particles produced during certain maintenance operations (e.g. lapping) ;

* filtering effluent liable to carry hot spots outside the Reactor Building.

The simplified diagram below illustrates the principles of the preventive filtration methods proposed (Fig. 1).

<figure><img src="image_1.png" /><figcaption>Fig. 1 : Preventive filtration methods</figcaption></figure>

### 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.

### Curative actions
# Influence of major PWR radiochemical pollutants on dose rates and dosimetry - Importance of efficiency of retention techniques

A. Rocher, Electricité de France, FTC, Groupe Prévention des Risques, France

M. Berger, Ph. Ridoux, Electricité de France, SEPTEN, France

J.L. Bretelle, Electricité de France, Groupe des Laboratoires, France

G. Bégué, Electricité de France, CNPE de Tricastin, France

F. Dacquait, Commissariat à l'Energie Atomique, France

## Introduction

In addition to usual systems contamination mostly due to materials generalised corrosion, some pollutions can lead to « over-contamination ».

The major pollutants responsible for over-contamination in EDF plants are : Co 60 coming from stellites degradation, Ag 110 m coming from control rod piercing, and Sb 124 coming from some pumps of auxiliary systems.

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 against penalising pollutions.

Priority is set on prevention.

## Programme to eradicate hot spots (Co 60) carried out in French PWR units

In most cases, hot spots [1] are due to particles of cobalt activated by a neutrons flux (Co 60) mainly from stellite surfaced equipment (rich in cobalt) [2] in the primary cooling circuit (valves, pumps, internals, etc.).

Cobalt 60 emits, in particular, two types of high energy gamma radiation [3] (γ radiation of 1.17 MeV and γ radiation of 1.33 MeV given off by decay). Its decay period is 5.3 years ; it therefore generates high equivalent dose rates over many years if it is not removed from the cooling circuits.

The contribution of hot spots to shutdown dosimetry may appear to be marginal across all French reactors (2 to 4 %) but is significant (15 to 25 %) for the units affected. This excess dosimetry is a significant factor, particularly for the most exposed workers (the population with a dosimetry of ≥ 20 mSv/year). Approximately ten French units have been affected by this phenomenon over the last 12 years.

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

### Prevention

Prevention consists in avoiding the appearance of hot spots. It concerns all units and is based mainly on the following actions:

* limiting the use of cobalt based components in contact with effluent from the primary circuit ;

* removing stellite particles produced during certain maintenance operations (e.g. lapping) ;

* filtering effluent liable to carry hot spots outside the Reactor Building.

The simplified diagram below illustrates the principles of the preventive filtration methods proposed (Fig. 1).

<figure><img src="image_2.png" /><figcaption>Fig. 1 : Preventive filtration methods</figcaption></figure>

### 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.

### Curative actions
The curative actions proposed are gathered into a set of sheets, approved by operators and which are broken down into two parts. Actions to be taken in the reactor pool (whether filled with water or not) and actions to be taken on the cooling circuits (flushing, biological protection, etc.).

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

## Metastable silver 110 contamination in Tricastin NPP

### Context

In some 900 MWe PWR units, the primary and auxiliary systems are subject to metastable silver 110 (Ag 110 m) contamination, due to silver 109 neutronic activation. Silver comes from control rods untightness (Ag, Cd, In parts).

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

- in ionic form in solution;

- in colloïd form;

- in metallic form.

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 and CVCS coolers;

- the purification system downstream the CVCS demineralisers;

- the reactor cavity.

### Preventive actions

Preventive actions to control silver root source are:

- control rods examination;

- and primary Ag 110 m volumic activity monitoring during operation.

**Control rods examination** : Eddy current and ultrasonic non destructive control rod examination is performed every three cycles. Doubtful rods or rods with piercing signals are systematically replaced with either chrome 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 16000 MBq/t during primary coolant oxygenation. Such an activity called for unscheduled examination of the control rods, revealing an early piercing of 10 standard rods, which were rejected and replaced with coated rods.

**Primary Ag 110 m volumic activity monitoring** : 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 a). 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_2.png" /><figcaption>Fig. a : Ag 110 m activity in Tricastin 2 unit</figcaption></figure>

The correlation between Ag 110 m activity released during operation and activity integrated during primary coolant oxygenation allows to forecast the activity level likely to be observed during the end of cycle shutdown.

### Remedial actions

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

- purification during end of cycle shutdown;

- elution of activity confined in dead legs;

- other complementary actions (system cleaning).

**Primary coolant purification** : Ag 110 m purification is performed on a dedicated ion exchanger operating at the maximum possible flowrate. Depending on the estimated Ag 110 m activity level and on the number of end of cycle shutdowns «lived» by the demineraliser, a second demineraliser can be set into service. Such a configuration allows to improve purification and to prevent from a possible unefficiency of the shutdown dedicated demineraliser.

**Crossover legs drainage** : 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.
The curative actions proposed are gathered into a set of sheets, approved by operators and which are broken down into two parts. Actions to be taken in the reactor pool (whether filled with water or not) and actions to be taken on the cooling circuits (flushing, biological protection, etc.).

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

## Metastable silver 110 contamination in Tricastin NPP

### Context

In some 900 MWe PWR units, the primary and auxiliary systems are subject to metastable silver 110 (Ag 110 m) contamination, due to silver 109 neutronic activation. Silver comes from control rods untightness (Ag, Cd, In parts).

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

- in ionic form in solution;

- in colloïd form;

- in metallic form.

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 and CVCS coolers;

- the purification system downstream the CVCS demineralisers;

- the reactor cavity.

### Preventive actions

Preventive actions to control silver root source are:

- control rods examination;

- and primary Ag 110 m volumic activity monitoring during operation.

**Control rods examination** : Eddy current and ultrasonic non destructive control rod examination is performed every three cycles. Doubtful rods or rods with piercing signals are systematically replaced with either chrome 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 16000 MBq/t during primary coolant oxygenation. Such an activity called for unscheduled examination of the control rods, revealing an early piercing of 10 standard rods, which were rejected and replaced with coated rods.

**Primary Ag 110 m volumic activity monitoring** : 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 a). 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. a : Ag 110 m activity in Tricastin 2 unit</figcaption></figure>

The correlation between Ag 110 m activity released during operation and activity integrated during primary coolant oxygenation allows to forecast the activity level likely to be observed during the end of cycle shutdown.

### Remedial actions

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

- purification during end of cycle shutdown;

- elution of activity confined in dead legs;

- other complementary actions (system cleaning).

**Primary coolant purification** : Ag 110 m purification is performed on a dedicated ion exchanger operating at the maximum possible flowrate. Depending on the estimated Ag 110 m activity level and on the number of end of cycle shutdowns «lived» by the demineraliser, a second demineraliser can be set into service. Such a configuration allows to improve purification and to prevent from a possible unefficiency of the shutdown dedicated demineraliser.

**Crossover legs drainage** : 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 Residual Heat Removal System 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.

A pretreatment of the effluents prior to release can also be performed. It consists in precipitating silver together with aluminum or iron hydroxide. It allows to catch Ag 110 m on filters prior to purification on demineralisers and prevents resins contamination. Such a treatment leads to Ag 110 m final volumic activity lower than 0.5 MBq/t.

## Antimony pollution

In some French PWRs, ¹²²Sb and ¹²⁴Sb volume activity peaks are observed. These peaks, sometimes greater than 110 GBq/t, can be higher than ⁵⁸Co peaks. They impact 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 radioactive half-life, ¹²²Sb has a low impact on health physics, but it allows to determine antimony sources.

### Antimony sources

Two sources have been identified in French PWRs :

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

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

Some 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.

### Antimony pollution impact

This 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 1.

<table><caption>Table 1 : 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 rate can reach 10 % in some parts of circuits. And the dosimetry for maintenance during a plant shutdown can increase by about 5 % because of the ¹²⁴Sb contamination.

### Remedies

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 2), that allows to make the ¹²⁴Sb peaks decrease.

<table><caption>Table 2 : Bearing replacement during the 14th cycle</caption><tr><td>Cycles</td><td>13</td><td>14*</td><td>15</td></tr><tr><td>¹²⁴Sb (GBq/t)</td><td>100</td><td>60</td><td>20</td></tr></table>

During oxygenation, antimony is soluble (< 0.45 µm). If an antimony pollution occurs, to improve purification by the ion exchange resins, reactor coolant lithium content has to be inferior to 1 ppm [4]. In order to obtain an acid 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 acid reactor coolant seems to increase antimony deposition on the primary circuit surfaces [4].

## Conclusion

The presence of some pollutions in the systems (Co 60, Ag 110 m, Sb 124) leads to significant increase of shutdown dosimetry (10 to 30 %) in affected units.

Early detection of pollutants, in operation as well as during shutdown, is the best way to set up appropriate solutions. The early replacement of possibly polluting materials, the implementation of a specific chemistry or the choice of specific resins or filters are among the available answers to minimise over-contamination risks in PWR units.

At the present time, an optimisation of the chemical and physical properties of the environment during shutdown is being examined, in order to minimise contamination. Studies on filters and resins types and on the impact of purification flowrate are being performed. Minimising systems contamination by retaining pollutants such as Ag 110 m is of the major importance for EDF.

## References

[1] A. Rocher et al, Programme to eradicate hot spots (Co-60) carried out in French PWR units, JAIF International Conference on Water Chemistry in Nuclear Power Plants, Kashiwasaki 1998

[2] S. Anthoni, Synthèse Points Chauds Origine et Comportement, Note 96041 - CEA/DNR/DEC/SECA/LTC 1996

[3] K-H. Neeb, The Radio of Nuclear Power Plants with Light Water Reactors p 153-154, Walter de Gruyter, Berlin, New York 1997

[4] Hauser, "Retour d'expérience sur la pollution en antimoine des tranches du Bugey, EDF, D5118/NT/96342
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 Residual Heat Removal System 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.

A pretreatment of the effluents prior to release can also be performed. It consists in precipitating silver together with aluminum or iron hydroxide. It allows to catch Ag 110 m on filters prior to purification on demineralisers and prevents resins contamination. Such a treatment leads to Ag 110 m final volumic activity lower than 0.5 MBq/t.

## Antimony pollution

In some French PWRs, ¹²²Sb and ¹²⁴Sb volume activity peaks are observed. These peaks, sometimes greater than 110 GBq/t, can be higher than ⁵⁸Co peaks. They impact 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 radioactive half-life, ¹²²Sb has a low impact on health physics, but it allows to determine antimony sources.

### Antimony sources

Two sources have been identified in French PWRs :

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

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

Some 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.

### Antimony pollution impact

This 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 1.

<table><caption>Table 1 : 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 rate can reach 10 % in some parts of circuits. And the dosimetry for maintenance during a plant shutdown can increase by about 5 % because of the ¹²⁴Sb contamination.

### Remedies

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 2), that allows to make the ¹²⁴Sb peaks decrease.

<table><caption>Table 2 : Bearing replacement during the 14th cycle</caption><tr><td>Cycles</td><td>13</td><td>14*</td><td>15</td></tr><tr><td>¹²⁴Sb (GBq/t)</td><td>100</td><td>60</td><td>20</td></tr></table>

During oxygenation, antimony is soluble (< 0.45 µm). If an antimony pollution occurs, to improve purification by the ion exchange resins, reactor coolant lithium content has to be inferior to 1 ppm [4]. In order to obtain an acid 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 acid reactor coolant seems to increase antimony deposition on the primary circuit surfaces [4].

## Conclusion

The presence of some pollutions in the systems (Co 60, Ag 110 m, Sb 124) leads to significant increase of shutdown dosimetry (10 to 30 %) in affected units.

Early detection of pollutants, in operation as well as during shutdown, is the best way to set up appropriate solutions. The early replacement of possibly polluting materials, the implementation of a specific chemistry or the choice of specific resins or filters are among the available answers to minimise over-contamination risks in PWR units.

At the present time, an optimisation of the chemical and physical properties of the environment during shutdown is being examined, in order to minimise contamination. Studies on filters and resins types and on the impact of purification flowrate are being performed. Minimising systems contamination by retaining pollutants such as Ag 110 m is of the major importance for EDF.

## References

[1] A. Rocher et al, Programme to eradicate hot spots (Co-60) carried out in French PWR units, JAIF International Conference on Water Chemistry in Nuclear Power Plants, Kashiwasaki 1998

[2] S. Anthoni, Synthèse Points Chauds Origine et Comportement, Note 96041 - CEA/DNR/DEC/SECA/LTC 1996

[3] K-H. Neeb, The Radio of Nuclear Power Plants with Light Water Reactors p 153-154, Walter de Gruyter, Berlin, New York 1997

[4] Hauser, "Retour d'expérience sur la pollution en antimoine des tranches du Bugey, EDF, D5118/NT/96342