# Chemical degassing on EDF units - Feed back experience and method

Florence Bardet, Jean-Luc Bretelle, Nathalie Perdrigé, Frédéric Dacquait

► To cite this version:

Florence Bardet, Jean-Luc Bretelle, Nathalie Perdrigé, Frédéric Dacquait. Chemical degassing on EDF units - Feed back experience and method. International Conference on Water Chemistry of Nuclear Reactors Systems, Oct 2006, Jeju, South Korea. session 2 paper 9. cea-04464348

HAL Id: cea-04464348

https://cea.hal.science/cea-04464348

Submitted on 18 Feb 2024

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Distributed under a Creative Commons Attribution 4.0 International License
# Chemical degassing on EDF units - Feed back experience and method

Florence Bardet, Jean-Luc Bretelle, Nathalie Perdrigé, Frédéric Dacquait

► To cite this version:

Florence Bardet, Jean-Luc Bretelle, Nathalie Perdrigé, Frédéric Dacquait. Chemical degassing on EDF units - Feed back experience and method. International Conference on Water Chemistry of Nuclear Reactors Systems, Oct 2006, Jeju, South Korea. session 2 paper 9. cea-04464348

HAL Id: cea-04464348

https://cea.hal.science/cea-04464348v1

Submitted on 18 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
# Chemical Degassing on EDF units – Feed Back Experience and Method

F. Bardet¹, J-L. Bretelle², N. Perdrige³, F. Dacquait⁴

¹EDF, CEIDRE, 2 rue Ampère, F- 93206 Saint-Denis, France, florence.bardet@edf.fr

²EDF, CAPE, 1 Place Pleyel, F- 93206 Saint-Denis, France, jean-luc.bretelle@edf.fr

³EDF – CNPE de Civaux - BP 64– F-86320 Civaux, France, nathalie.perdrige@edf.fr

⁴CEA, Centre de Cadarache, DEN, 13108 Saint Paul lez Durance, France, frederic.dacquait@cea.fr

## Abstract

The chemical degassing feasibility of all French units RCS during outages is usually performed on the four 1500 MWe French units since July 2004 and is planned for 2007 on the 34 units of 900 MWe and the 20 units of 1300 MWe units. It helps to an optimized shutdown planning management, especially in case of unplanned shutdowns, and helps to limit the thermal stress corrosion of RHR at temperature higher than 120°C by avoiding temperature step extension after bubble collapsing to remove hydrogen increase.

Chemical degassing is performed at temperature equal or lower than 80°C, by hydrogen peroxide injection if $3 < H_{2\text{dissolved}} < 35 \text{ cm}^3/\text{kg}$. $H_2O_2$ calculation can be stoichiometric or increased by 20 % depending on the SG tubes material and the full power operation duration before the shutdown. Purification flow is adapted to each design type units characteristics to easily manage a hypothetical early oxygenation. Early oxygenation management consists in forecasting the dilution of the VCT (Volume Control Tank) gaseous phase by nitrogen to avoid dangerous $H_2/O_2$ gaseous mixture and to obtain oxygenation criteria on VCT. Three kinds of dilution can be used: nitrogen burping with high levels amplitude, continuous nitrogen flushing or discontinuous flushing with a maximal constant VCT level but pressure variation.

## I. INTRODUCTION

The chemical degassing feasibility of all design French units RCS during outages has been proved by tests performed from 2002 to 2004 on EDF units. It has been usually performed on the four 1500 MWe French units since July 2004 and is planned for 2007 on the 34 units of 900 MWe and the 20 units of 1300 MWe units.

The use of chemical degassing improves the management of refueling outages and of unplanned shutdowns on operation conditions and radioprotection point of view. It is also used:

* to limit the thermal stress corrosion of RHR by shorter operation time of RHR at temperatures higher than 120°C,

* to better manage $H_2$ increases after bubble collapsing,

* to perform a new way of bubble collapsing.

This paper describes the main characteristics of the French chemical degassing method. The method considers:

* the main risks associated with the chemical degassing and the usual outages like material, surface contamination, $H_2/O_2$ gaseous mixture ...,

* in-core corrosion products quantity with which the hydrogen peroxide may react,

* the SG tubes material.

It is adapted to the main characteristics of each type of French units. To improve the chemical degassing

duration and to limit the contamination risk, $H_2O_2$ volume calculation takes into account the reaction between $H_2O_2$ and corrosion products and the purification flow rate is adapted. New criteria have been established for the gaseous phases management.

## II. CONTEXT

During cold shutdown for refuelling outages, hydrogen is removed from RCS and biphasic capacities before oxygenation to avoid any dangerous gaseous $H_2/O_2$ mixture. RCS dissolved hydrogen is usually degassed by physical means: nitrogen burplings on VCT (Volume Control Tank), degassing of pressurizer gaseous phase by the sampling line between pressurizer gaseous phase and BRS head tank, BRS degasser. In gaseous phases, gaseous hydrogen must be less than 2% and in RCS, oxygenation criteria for dissolved hydrogen is $H_{2\text{dissolved}} < 3 \text{ cm}^3/\text{kg}$ at $T \le 80^\circ\text{C}$ if oxygenation is performed by oxygen peroxide injection and achieved by VCT venting. On French units, hydrogenated gas ($H_{2\text{gaseous}} > 2\%$) are set to hydrogenated gaseous wastes tanks and aerated gas are evacuated by the chimney.

In some operation cases, high hydrogen concentrations can be observed during shutdowns.

* Bubble collapsing at 130°C needs to maintain high hydrogen concentrations ($H_2 > 10 \text{ cm}^3/\text{kg}$) in order to limit the RHR operating duration at temperatures higher than 120°C without increasing RCS contamination risk
# Chemical Degassing on EDF units – Feed Back Experience and Method

F. Bardet¹, J-L. Bretelle², N. Perdrige³, F. Dacquait⁴

¹EDF, CEIDRE, 2 rue Ampère, F- 93206 Saint-Denis, France, florence.bardet@edf.fr

²EDF, CAPE, 1 Place Pleyel, F- 93206 Saint-Denis, France, jean-luc.bretelle@edf.fr

³EDF – CNPE de Civaux - BP 64– F-86320 Civaux, France, nathalie.perdrige@edf.fr

⁴CEA, Centre de Cadarache, DEN, 13108 Saint Paul lez Durance, France, frederic.dacquait@cea.fr

## Abstract

The chemical degassing feasibility of all French units RCS during outages is usually performed on the four 1500 MWe French units since July 2004 and is planned for 2007 on the 34 units of 900 MWe and the 20 units of 1300 MWe units. It helps to an optimized shutdown planning management, especially in case of unplanned shutdowns, and helps to limit the thermal stress corrosion of RHR at temperature higher than 120°C by avoiding temperature step extension after bubble collapsing to remove hydrogen increase.

Chemical degassing is performed at temperature equal or lower than 80°C, by hydrogen peroxide injection if $3 < H_{2\text{dissolved}} < 35 \text{ cm}^3/\text{kg}$. $H_2O_2$ calculation can be stoichiometric or increased by 20 % depending on the SG tubes material and the full power operation duration before the shutdown. Purification flow is adapted to each design type units characteristics to easily manage a hypothetical early oxygenation. Early oxygenation management consists in forecasting the dilution of the VCT (Volume Control Tank) gaseous phase by nitrogen to avoid dangerous $H_2/O_2$ gaseous mixture and to obtain oxygenation criteria on VCT. Three kinds of dilution can be used: nitrogen burping with high levels amplitude, continuous nitrogen flushing or discontinuous flushing with a maximal constant VCT level but pressure variation.

## I. INTRODUCTION

The chemical degassing feasibility of all design French units RCS during outages has been proved by tests performed from 2002 to 2004 on EDF units. It has been usually performed on the four 1500 MWe French units since July 2004 and is planned for 2007 on the 34 units of 900 MWe and the 20 units of 1300 MWe units.

The use of chemical degassing improves the management of refueling outages and of unplanned shutdowns on operation conditions and radioprotection point of view. It is also used:

* to limit the thermal stress corrosion of RHR by shorter operation time of RHR at temperatures higher than 120°C,

* to better manage $H_2$ increases after bubble collapsing,

* to perform a new way of bubble collapsing.

This paper describes the main characteristics of the French chemical degassing method. The method considers:

* the main risks associated with the chemical degassing and the usual outages like material, surface contamination, $H_2/O_2$ gaseous mixture ...

* in-core corrosion products quantity with which the hydrogen peroxide may react,

* the SG tubes material.

It is adapted to the main characteristics of each type of French units. To improve the chemical degassing

duration and to limit the contamination risk, $H_2O_2$ volume calculation takes into account the reaction between $H_2O_2$ and corrosion products and the purification flow rate is adapted. New criteria have been established for the gaseous phases management.

## II. CONTEXT

During cold shutdown for refuelling outages, hydrogen is removed from RCS and biphasic capacities before oxygenation to avoid any dangerous gaseous $H_2/O_2$ mixture. RCS dissolved hydrogen is usually degassed by physical means: nitrogen burplings on VCT (Volume Control Tank), degassing of pressurizer gaseous phase by the sampling line between pressurizer gaseous phase and BRS head tank, BRS degasser. In gaseous phases, gaseous hydrogen must be less than 2% and in RCS, oxygenation criteria for dissolved hydrogen is $H_{2\text{dissolved}} < 3 \text{ cm}^3/\text{kg}$ at $T \le 80^\circ\text{C}$ if oxygenation is performed by oxygen peroxide injection and achieved by VCT venting. On French units, hydrogenated gas ($H_{2\text{gaseous}} > 2\%$) are set to hydrogenated gaseous wastes tanks and aerated gas are evacuated by the chimney.

In some operation cases, high hydrogen concentrations can be observed during shutdowns.

* Bubble collapsing at 130°C needs to maintain high hydrogen concentrations ($H_2 > 10 \text{ cm}^3/\text{kg}$) in order to limit the RHR operating duration at temperatures higher than 120°C without increasing RCS contamination risk
* For some kinds of unplanned shutdowns, without RCS opening, hydrogen is maintained in RCS at high levels (> 10 cm³/kg). If RCS opening is finally needed, large quantities of dissolved hydrogen must be degassed. Slow hydrogen degassing could be detrimental to the contamination risk management and to the work planning.

* Some hydrogen increases can occur during refuelling shutdown:

- At RHR connexion, if gaseous hydrogen has come from pressurizer gaseous phase by common sampling lines because of valves which are no more hydrogen tight,

- after the bubble collapsing, if the pressurizer has not been completely degassed.

In those cases, physical degassing of high hydrogen concentrations is performed at temperatures higher than 120°C. It takes a lot of time, increases the thermal stress corrosion of RHR and delays the operation planning.

After keeping high hydrogen concentrations as long as possible, chemical degassing by hydrogen peroxide injection at temperature equal or lower than 80°C is performed in some foreign countries [1]. The chemical degassing feasibility on all design French units RCS has been proved by four tests (one per each design).

Considering the positive feed back experience of the four tests and the solutions accorded by the chemical degassing in the fields of operation management improvement and of RHR thermal stress limitation, it was decided in 2003 to propose the chemical degassing to all design French units, with a priority to the 1500 MWe units because of their pressurizer degassing problems. The industrial chemical degassing method takes into account the feed back experience of the four tests and the results of the risks analysis about staff security, materials, fission products behaviour, corrosion products management, contamination and gaseous H₂/O₂ mixtures.

### III. RISKS ANALYSIS

#### III.A. Materials

Hydrogen at concentrations in a range from 3 to 35 or 50 cm³/kg at temperature equal or lower than 120°C and for short steps (few hours) has no influence on SCC of stainless steel materials or Alloy 600.

Chemical degassing increases injected H₂O₂ volumes during shutdowns but the injection temperature is lower than 120°C, which limits the impact of higher H₂O₂ volume on corrosion.

#### III.B. Fission products behaviour

Gaseous fission products like xenon are removed by physical degassing (hydrogen or nitrogen burpings on VCT, BRS degasser and use of the degassing line between pressurizer gaseous phase and the surge head tank). Iodine mainly reacts as xenon does but can be

removed by lithiated mixed bed resins in reducing conditions [2]. Fission products can increase into RCS depending on RCS pressure decreases. It means that fission gas physical degassing must be performed during the first shutdown steps. The gaseous wastes are set to the hydrogenated gaseous wastes tanks.

Because of environment, planning and security points of view, fission products must be mainly removed before oxygenation and H₂O₂ injection.

Chemical degassing uses the same reactant, H₂O₂, as the oxygenation. The temperature of both operations are closed and there is no pressure variation between those operations. Chemical degassing can not remove fission products and because of the gas treatment, fission gas must be removed before chemical degassing. Fission products activities must respect oxygenation criteria (¹³³Xe< 4 GBq/t and ¹³¹I < 2 GBq/t) before chemical degassing. The figure 1 presents the behaviour of iodine and xenon during the chemical degassing and the oxygenation performed on Civaux 2 NPP in august 2004.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Behaviour of fission products during chemical degassing and oxygenation on Civaux 2 NPP RF04 – August 2004</figcaption></figure>

#### III.C. Corrosion products

The four tests and the first chemical degassings on 1500 MWe-units have shown that more than 70% of the injected H₂O₂ for chemical degassing react with dissolved hydrogen. The other part reacts with corrosion products, illustrated by an increase of volumic activities, as shown by the figure 2.

Volumic activities increase represents a small part of the corrosion products available to be dissolved. The chemical degassing has a low influence on the activity peak during oxygenation : Saint-Laurent B 2 NPP RF18 oxygenation peak after chemical degassing : 140 GBq/t, to be compared to a calculated 160 GBq/t without chemical degassing peak (calculation made by the CEA RELTRANS code).
* For some kinds of unplanned shutdowns, without RCS opening, hydrogen is maintained in RCS at high levels (> 10 cm³/kg). If RCS opening is finally needed, large quantities of dissolved hydrogen must be degassed. Slow hydrogen degassing could be detrimental to the contamination risk management and to the work planning.

* Some hydrogen increases can occur during refuelling shutdown:

- At RHR connexion, if gaseous hydrogen has come from pressurizer gaseous phase by common sampling lines because of valves which are no more hydrogen tight,

- after the bubble collapsing, if the pressurizer has not been completely degassed.

In those cases, physical degassing of high hydrogen concentrations is performed at temperatures higher than 120°C. It takes a lot of time, increases the thermal stress corrosion of RHR and delays the operation planning.

After keeping high hydrogen concentrations as long as possible, chemical degassing by hydrogen peroxide injection at temperature equal or lower than 80°C is performed in some foreign countries [1]. The chemical degassing feasibility on all design French units RCS has been proved by four tests (one per each design).

Considering the positive feed back experience of the four tests and the solutions accorded by the chemical degassing in the fields of operation management improvement and of RHR thermal stress limitation, it was decided in 2003 to propose the chemical degassing to all design French units, with a priority to the 1500 MWe units because of their pressurizer degassing problems. The industrial chemical degassing method takes into account the feed back experience of the four tests and the results of the risks analysis about staff security, materials, fission products behaviour, corrosion products management, contamination and gaseous H₂/O₂ mixtures.

### III. RISKS ANALYSIS

#### III.A. Materials

Hydrogen at concentrations in a range from 3 to 35 or 50 cm³/kg at temperature equal or lower than 120°C and for short steps (few hours) has no influence on SCC of stainless steel materials or Alloy 600.

Chemical degassing increases injected H₂O₂ volumes during shutdowns but the injection temperature is lower than 120°C, which limits the impact of higher H₂O₂ volume on corrosion.

#### III.B. Fission products behaviour

Gaseous fission products like xenon are removed by physical degassing (hydrogen or nitrogen burpings on VCT, BRS degasser and use of the degassing line between pressurizer gaseous phase and the surge head tank). Iodine mainly reacts as xenon does but can be

removed by lithiated mixed bed resins in reducing conditions [2]. Fission products can increase into RCS depending on RCS pressure decreases. It means that fission gas physical degassing must be performed during the first shutdown steps. The gaseous wastes are set to the hydrogenated gaseous wastes tanks.

Because of environment, planning and security points of view, fission products must be mainly removed before oxygenation and H₂O₂ injection.

Chemical degassing uses the same reactant, H₂O₂, as the oxygenation. The temperature of both operations are closed and there is no pressure variation between those operations. Chemical degassing can not remove fission products and because of the gas treatment, fission gas must be removed before chemical degassing. Fission products activities must respect oxygenation criteria (¹³³Xe< 4 GBq/t and ¹³¹I < 2 GBq/t) before chemical degassing. The figure 1 presents the behaviour of iodine and xenon during the chemical degassing and the oxygenation performed on Civaux 2 NPP in august 2004.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Behaviour of fission products during chemical degassing and oxygenation on Civaux 2 NPP RF04 – August 2004</figcaption></figure>

#### III.C. Corrosion products

The four tests and the first chemical degassings on 1500 MWe-units have shown that more than 70% of the injected H₂O₂ for chemical degassing react with dissolved hydrogen. The other part reacts with corrosion products, illustrated by an increase of volumic activities, as shown by the figure 2.

Volumic activities increase represents a small part of the corrosion products available to be dissolved. The chemical degassing has a low influence on the activity peak during oxygenation : Saint-Laurent B 2 NPP RF18 oxygenation peak after chemical degassing : 140 GBq/t, to be compared to a calculated 160 GBq/t without chemical degassing peak (calculation made by the CEA RELTRANS code).
<figure><img src="image_3.png" /><figcaption>Fig. 2. Evolution of volumic activities during chemical degassing and oxygenation on Bugey 5 NPP</figcaption></figure>

On units whose shutdowns have no several hours step at temperatures between 170°C and 80°C, the part of corrosion products reacting with H₂O₂ during chemical degassing is about 20 to 30 %, whatever the injections number. For units with 690 SG tubes and whose ⁵⁸Co oxygenation peaks are lower than 50 GBq/t after a full cycle, actual feed back experience indicates that this part is about 10-15%. For units where shutdowns present very long steps at temperature lower than 170°C and with hydrogen concentration higher than 10 cm³/kg, like Bugey 5 NPP in 2003, the corrosion part can increase to 30-40%. This increase could be explained by structure modifications of some corrosion products kinds, usually insoluble during normal shutdowns, to some soluble species in presence of oxidizing reactants. [1]

Chemical degassing must take into account the corrosion products behaviour which depends on SG tubes material and power duration.

The forms and the quantities of corrosion products produced during a cycle and having an influence on shutdown management, mainly depend on SG tubes material and on full power duration. Thus, fore one kind of material with a given history, the longer is the full power duration, the greater is the activated corrosion products quantity. Activated corrosion products are usually considered as a simple part of RCS total corrosion products.

When an oxygenation is performed during unplanned shutdown after more than six full power months, ⁵⁸Co oxygenation peak value mainly represents more than 50% of the peak expected after a full production cycle. Corrosion products quantities which can react with dissolved oxygen are very important. For oxygenation performed during the first six months of a cycle, ⁵⁸Co oxygenation peak values have a very large range and the feed back experience is not enough to can determine such a rule. [3]

For SG tubes made of 600 nickel based alloy, ⁵⁸Co oxygenation peaks stay in the same range, as the figure 3 shows it for some 1300 MWe-units.

Smaller values can be explained by unplanned oxygenation performed during the first cycle months and which has reduced the RF oxygenation peak value.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Evolution of ⁵⁸Co refuelling oxygenation peaks on some 1300 MWe-NPPs with Alloy 600 SG tubes</figcaption></figure>

On units whose SG tubes are made of 690 nickel based alloy (SGR or original SGs), ⁵⁸Co oxygenation peaks are mainly about 200 to 300 GBq/t during the first two or three outages, then they decrease and reach very low values like less than 5 GBq/t, as it is described by the figure 4. Smaller oxygenation peak means lower release of 690 SG tubes. Corrosion products quantities reacting during oxygenations are low on 690 SG tubes units whose ⁵⁸Co oxygenation peak are smaller than 50 GBq/t after a full power cycle (cycle length equal or higher than nine months). [4]

<figure><img src="image_5.png" /><figcaption>Fig. 4. Evolution of ⁵⁸Co refuelling oxygenation peaks on some NPPs with Alloy 690 SG tubes</figcaption></figure>

Even if ⁵⁸Co oxygenation peak can be very low, corrosion products available for reaction with oxygen or H₂O₂ are still present and have influence on the oxygen value. As an example, 30 L of H₂O₂ at 30% injected in a 1500 MWe-unit RCS at 80°C give about 10 mg/kg of dissolved oxygen. On those units, where ⁵⁸Co oxygenation peak are very low (3 GBq/t at Civaux 2 NPP RF 05), the maximal oxygen value after H₂O₂ injection is about 7 mg/kg (figure 5), proving that corrosion products have reacted with H₂O₂. The oxygen saturation is reached by VCT venting.
<figure><img src="image_3.png" /><figcaption>Fig. 2. Evolution of volumic activities during chemical degassing and oxygenation on Bugey 5 NPP</figcaption></figure>

On units whose shutdowns have no several hours step at temperatures between 170°C and 80°C, the part of corrosion products reacting with H₂O₂ during chemical degassing is about 20 to 30 %, whatever the injections number. For units with 690 SG tubes and whose ⁵⁸Co oxygenation peaks are lower than 50 GBq/t after a full cycle, actual feed back experience indicates that this part is about 10-15%. For units where shutdowns present very long steps at temperature lower than 170°C and with hydrogen concentration higher than 10 cm³/kg, like Bugey 5 NPP in 2003, the corrosion part can increase to 30-40%. This increase could be explained by structure modifications of some corrosion products kinds, usually insoluble during normal shutdowns, to some soluble species in presence of oxidizing reactants. [1]

Chemical degassing must take into account the corrosion products behaviour which depends on SG tubes material and power duration.

The forms and the quantities of corrosion products produced during a cycle and having an influence on shutdown management, mainly depend on SG tubes material and on full power duration. Thus, fore one kind of material with a given history, the longer is the full power duration, the greater is the activated corrosion products quantity. Activated corrosion products are usually considered as a simple part of RCS total corrosion products.

When an oxygenation is performed during unplanned shutdown after more than six full power months, ⁵⁸Co oxygenation peak value mainly represents more than 50% of the peak expected after a full production cycle. Corrosion products quantities which can react with dissolved oxygen are very important. For oxygenation performed during the first six months of a cycle, ⁵⁸Co oxygenation peak values have a very large range and the feed back experience is not enough to can determine such a rule. [3]

For SG tubes made of 600 nickel based alloy, ⁵⁸Co oxygenation peaks stay in the same range, as the figure 3 shows it for some 1300 MWe-units.

Smaller values can be explained by unplanned oxygenation performed during the first cycle months and which has reduced the RF oxygenation peak value.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Evolution of ⁵⁸Co refuelling oxygenation peaks on some 1300 MWe-NPPs with Alloy 600 SG tubes</figcaption></figure>

On units whose SG tubes are made of 690 nickel based alloy (SGR or original SGs), ⁵⁸Co oxygenation peaks are mainly about 200 to 300 GBq/t during the first two or three outages, then they decrease and reach very low values like less than 5 GBq/t, as it is described by the figure 4. Smaller oxygenation peak means lower release of 690 SG tubes. Corrosion products quantities reacting during oxygenations are low on 690 SG tubes units whose ⁵⁸Co oxygenation peak are smaller than 50 GBq/t after a full power cycle (cycle length equal or higher than nine months). [4]

<figure><img src="image_5.png" /><figcaption>Fig. 4. Evolution of ⁵⁸Co refuelling oxygenation peaks on some NPPs with Alloy 690 SG tubes</figcaption></figure>

Even if ⁵⁸Co oxygenation peak can be very low, corrosion products available for reaction with oxygen or H₂O₂ are still present and have influence on the oxygen value. As an example, 30 L of H₂O₂ at 30% injected in a 1500 MWe-unit RCS at 80°C give about 10 mg/kg of dissolved oxygen. On those units, where ⁵⁸Co oxygenation peak are very low (3 GBq/t at Civaux 2 NPP RF 05), the maximal oxygen value after H₂O₂ injection is about 7 mg/kg (figure 5), proving that corrosion products have reacted with H₂O₂. The oxygen saturation is reached by VCT venting.
Hydrogen and oxygen evolution - Civaux 2 RF05

<figure><img src="image_6.png" /><figcaption>Fig. 5. Evolution of dissolved oxygen during oxygenation on Civaux 2 NPP RF 05</figcaption></figure>

As a conclusion, the quantities of solubilized species during chemical degassing are :

* Smaller than those of the oxygenation, because available oxygen quantity for corrosion products is lower during chemical degassing,

* A small part of the global species quantity, which can react with oxidizing agents. The main part is solubilized during oxygenation.

Chemical degassing has a low influence on oxygenation peak values.

### III.D. Contamination risk

Saint-Laurent B2 NPP test feed back experience has shown that a RCS contamination had occurred during the five hours period from $H_{2dissolved} < 3 \text{ cm}^3/\text{kg}$ criterion reaching to the oxygenation. This time period has been used to obtain $H_{2gaseous} < 2\%$ on VCT gaseous phase and to prepare air venting on VCT. The three other tests and the 1500 MWe-units feed back experience show that one to three hours after the chemical degassing can be needed to perform oxygenation ($H_2O_2$ injection). This period can be very critical for the contamination risk management. Thus, performing a nitrogen burping on VCT before reaching 80°C and having well trained operators can help to quickly obtain $H_{2gaseous} < 2\%$ on VCT after chemical degassing.

To manage contamination risks during chemical degassing, it is important to keep in mind that, the more important are the out of flux zones contamination risks during temperature steps, the longer is the step, the lower is the temperature, the higher is the volumic activity" [3]. This behaviour is particularly important when the primary coolant contains very low level of dissolved hydrogen (less than $5 \text{ cm}^3/\text{kg}$) and is not oxygenated. Contamination can occur during sudden decreases of purification flow rate and during sudden temperature decreases. It means that, according to the general shutdowns procedures,

* Temperature step at temperature lower than 180-170°C without lithium and with small amount of

hydrogen, are reduced or avoided, to limit corrosion products solubilization,

* Purification flow rate is as high as possible but takes into account the management of the other risks (activity peak at the chimney, $H_2/O_2$ gaseous mixture ...) which can ask to reduce the flow rate for a short while.

Chemical degassing is performed at temperature equal or lower than 80°C, which are a good compromise between planning respect and contamination risk. The temperature is low and this helps to the corrosion products dissolution. The higher is the number of $H_2O_2$ injections, the longer is the temperature step. It is important to limit the chemical degassing duration by limiting the $H_2O_2$ injection number.

As a consequence, to limit the injections number, the $H_2O_2$ volume calculation must take into account the reaction between $H_2O_2$ and the corrosion products.

* For units with more than six months of full power if they have 600 SG tubes or 690 SG tubes with $^{58}$Co oxygenation peak higher than 50 GBq/t, an excess of 20 % of $H_2O_2$ compared to the stoechiometric volume is injected.

* For units with less than six months of full power, the injected $H_2O_2$ volume is stoechiometric.

* For units with 690 SG tubes and with $^{58}$Co peak lower than 50 GBq/t, the injected $H_2O_2$ volume is stoechiometric.

In a second step, $H_2O_2$ volume calculation can be reviewed, after feed back experience (four shutdowns with full power duration between some hours to four months).

Purification flow rate must be as high as possible and must take into account the management of others risks.

### III.E. $H_2/O_2$ gaseous mixture

#### III.E.1 Normal situation

On biphasic capacities, a dangerous $H_2/O_2$ gaseous mixture is a risk usually managed during shutdowns on French units because of air venting on main capacities. This risk is associated too to the chemical degassing. This risk is managed for the chemical degassing in the same way as for oxygenation : biphasic capacities, VCT excepted, must respect the usual oxygenation criteria (commonly $H_{2gaseous} < 2\%$) before chemical degassing at temperature equal or lower than 80°C. VCT gaseous phase has its own criterion : $H_{2gaseous} < 15\%$, which is a compromise between continuous hydrogen introduction in VCT due to the sprinkling flow rate, the later oxygenation criterion reaching ($H_{2gaseous} < 2\%$), the gaseous wastes volume and the VCT gaseous phase nitrogen dilution duration. For oxygenation, criterion on VCT remains $H_{2gaseous} < 2\%$. Nitrogen burpings with a great levels range help to quickly obtain this criterion. Nitrogen burping can be performed before reaching 80°C.
Hydrogen and oxygen evolution - Civaux 2 RF05

<figure><img src="image_6.png" /><figcaption>Fig. 5. Evolution of dissolved oxygen during oxygenation on Civaux 2 NPP RF 05</figcaption></figure>

As a conclusion, the quantities of solubilized species during chemical degassing are :

* Smaller than those of the oxygenation, because available oxygen quantity for corrosion products is lower during chemical degassing,

* A small part of the global species quantity, which can react with oxidizing agents. The main part is solubilized during oxygenation.

Chemical degassing has a low influence on oxygenation peak values.

### III.D. Contamination risk

Saint-Laurent B2 NPP test feed back experience has shown that a RCS contamination had occurred during the five hours period from $H_{2\text{dissolved}} < 3 \text{ cm}^3\text{/kg}$ criterion reaching to the oxygenation. This time period has been used to obtain $H_{2\text{gaseous}} < 2\%$ on VCT gaseous phase and to prepare air venting on VCT. The three other tests and the 1500 MWe-units feed back experience show that one to three hours after the chemical degassing can be needed to perform oxygenation ($H_2O_2$ injection). This period can be very critical for the contamination risk management. Thus, performing an nitrogen burping on VCT before reaching 80°C and having well trained operators can help to quickly obtain $H_{2\text{gaseous}} < 2\%$ on VCT after chemical degassing.

To manage contamination risks during chemical degassing, it is important to keep in mind that, the more important are the out of flux zones contamination risks during temperature steps, the longer is the step, the lower is the temperature, the higher is the volumic activity" [3]. This behaviour is particularly important when the primary coolant contains very low level of dissolved hydrogen (less than 5 $\text{cm}^3\text{/kg}$) and is not oxygenated. Contamination can occur during sudden decreases of purification flow rate and during sudden temperature decreases. It means that, according to the general shutdowns procedures,

* Temperature step at temperature lower than 180-170°C without lithium and with small amount of

hydrogen, are reduced or avoided, to limit corrosion products solubilization,

* Purification flow rate is as high as possible but takes into account the management of the other risks (activity peak at the chimney, $H_2/O_2$ gaseous mixture ...) which can ask to reduce the flow rate for a short while.

Chemical degassing is performed at temperature equal or lower than 80°C, which are a good compromise between planning respect and contamination risk. The temperature is low and this helps to the corrosion products dissolution. The higher is the number of $H_2O_2$ injections, the longer is the temperature step. It is important to limit the chemical degassing duration by limiting the $H_2O_2$ injection number.

As a consequence, to limit the injections number, the $H_2O_2$ volume calculation must take into account the reaction between $H_2O_2$ and the corrosion products.

* For units with more than six months of full power if they have 600 SG tubes or 690 SG tubes with $^{58}$Co oxygenation peak higher than 50 GBq/t, an excess of 20 % of $H_2O_2$ compared to the stoechiometric volume is injected.

* For units with less than six months of full power, the injected $H_2O_2$ volume is stoechiometric.

* For units with 690 SG tubes and with $^{58}$Co peak lower than 50 GBq/t, the injected $H_2O_2$ volume is stoechiometric.

In a second step, $H_2O_2$ volume calculation can be reviewed, after feed back experience (four shutdowns with full power duration between some hours to four months).

Purification flow rate must be as high as possible and must take into account the management of others risks.

### III.E. $H_2/O_2$ gaseous mixture

#### III.E.1 Normal situation

On biphasic capacities, a dangerous $H_2/O_2$ gaseous mixture is a risk usually managed during shutdowns on French units because of air venting on main capacities. This risk is associated too to the chemical degassing. This risk is managed for the chemical degassing in the same way as for oxygenation : biphasic capacities, VCT excepted, must respect the usual oxygenation criteria (commonly $H_{2\text{gaseous}} < 2\%$) before chemical degassing at temperature equal or lower than 80°C. VCT gaseous phase has its own criterion : $H_{2\text{gaseous}} < 15\%$, which is a compromise between continuous hydrogen introduction in VCT due to the sprinkling flow rate, the later oxygenation criterion reaching ($H_{2\text{gaseous}} < 2\%$), the gaseous wastes volume and the VCT gaseous phase nitrogen dilution duration. For oxygenation, criterion on VCT remains $H_{2\text{gaseous}} < 2\%$. Nitrogen burpings with a great levels range help to quickly obtain this criterion. Nitrogen burping can be performed before reaching 80°C.
The 20 % H₂O₂ excess is injected on units with very large amount of corrosion products and compensates the H₂O₂/corrosion products reaction. There is no risk of oxygen increase.

Nevertheless, on line oxygen measurement is performed and recorded. The devices (hydrogen-meter and oxygen-meter) allow on-line measurements and help to an early detection of an “early oxygenation”.

### III.E.2 Early oxygenation

If H₂O₂ injected volume is over evaluated, hydrogen disappears and dissolved oxygen could appear in RCS if no reaction between H₂O₂ and corrosion products took place, which is a very hypothetic case. This dissolved oxygen would degas into the VCT which contains more than 2% of gaseous hydrogen. A H₂/O₂ gaseous mixture would form in VCT gaseous phase. Such a situation is called “early oxygenation”.

To prevent an “early oxygenation” :

* H₂gaseous < 15 % on VCT

* H₂gaseous < oxygenation criteria on other biphasic capacities (mainly, H₂gaseous < 2%)

* Availability of the oxygen monitor on the inlet to gaseous wastes treatment system,

* Availability of redundancy mean for on-line monitors.

Early oxygenation is declared when hydrogen has disappeared and oxygen appears and is stable, although there is more than 2 % of gaseous hydrogen in the VCT. The management of an “early oxygenation” consists to forecast to dilute the VCT gaseous phase by nitrogen to avoid dangerous gaseous mixture and to obtain as quickly as possible H₂gaseousVCT< 2 %. VCT gaseous wastes are set to hydrogenated gaseous wastes tanks. To manage an early oxygenation in good conditions (gas volumes, duration ...), gaseous oxygen concentration must remain smaller than 3.0 %. Three methods can be used to dilute the VCT gaseous phase :

* Nitrogen burping (levels variation)

* Continuous nitrogen flushing

* Discontinuous nitrogen flushing (constant level but pressure variation by nitrogen inlet and gas discharge to gaseous wastes treatment system).

One of the three solutions is operated as soon as the early oxygenation is detected and confirmed by the chemist. The first dilution must have been performed before gaseous oxygen concentration becomes higher than 3.0%. It means that after a burping, the maximal VCT level, just before decreasing the level to introduce nitrogen, must be reached before O₂gaseous = 3.0 %. The minimal value of the time period called Δt1 in the figure 6 must be higher than the monitor information added to the chemistry time period (detection and confirmation) and to the level increase.

$$t = \frac{P_G V_G}{R T_G} \times \frac{M(O_2)}{c_{O_2 RCPmesure}} \times \frac{1}{Q_{RCV}} \times 100 \times O_{2gaseousVCT}$$

c(O₂RCPmesure) : RCS dissolved oxygen concentration (mg/kg)

M (O₂) : dioxygen molar mass (g.mol⁻¹)

QRCV : CVCS flow rate (t/h)

t : time period to obtain a specific gaseous oxygen concentration (hours)

P_G : VCT gaseous phase pressure in Pa (× 1,013 10⁵bar)

V_G : VCT gaseous phase volume (m³)

R : thermodynamic constant = 8,314

T : VCT temperature (K)

<figure><img src="image_7.png" /><figcaption>Fig.6. Evolution of VCT (Volume Control Tank) levels during early oxygenation management by N₂ burping</figcaption></figure>

The design case is taken for a maximal volume equal to the reactant injection tank volume which integers an error of five litres, an excess of 20% and an error on the hydrogen measurements. The table 1 gives the main characteristics of an early oxygenation management.

<table><caption>Table 1. Main characteristics of an early oxygenation management according to the design-type units.</caption><thead><tr><th></th><th>CP0</th><th>CPY</th><th>1300 MWe</th><th>1500 MWe</th></tr></thead><tbody><tr><td>M<sub>RCS</sub></td><td>304 t</td><td>299 t</td><td>433 t</td><td>440 t</td></tr><tr><td>V<sub>VCT (total)</sub></td><td>8.5 m³</td><td>8.9 m³</td><td>19 m³</td><td>19 m³</td></tr><tr><td>Q<sub>RCV</sub></td><td>14 t/h</td><td>14 t/h</td><td>36 t/h</td><td>36 t/h</td></tr><tr><td>V<sub>max</sub> H₂O₂</td><td>30 L</td><td>20 L</td><td>40 L</td><td>50 L</td></tr><tr><td>O₂max (mg/kg)</td><td>8.1</td><td>6.7</td><td>8.4</td><td>10.3</td></tr><tr><td>Minimal time period to perform the first treatment</td><td>60+20+30 = 110 min</td><td>60+20+30 = 110 min</td><td>20+20+50 = 90 min</td><td>20+20+30 = 70 min</td></tr></tbody></table>

To avoid oxygen concentration higher than 3.0 % in a time period shorter than the minimal time period, purification flow rate must be reduced to 14 t/h on 3 loops reactors. On four loops reactors, VCT volumes are greater than those of the three loops reactors and the reaction is more rapidly detected by monitors.
The 20 % H₂O₂ excess is injected on units with very large amount of corrosion products and compensates the H₂O₂/corrosion products reaction. There is no risk of oxygen increase.

Nevertheless, on line oxygen measurement is performed and recorded. The devices (hydrogen-meter and oxygen-meter) allow on-line measurements and help to an early detection of an “early oxygenation”.

### III.E.2 Early oxygenation

If H₂O₂ injected volume is over evaluated, hydrogen disappears and dissolved oxygen could appear in RCS if no reaction between H₂O₂ and corrosion products took place, which is a very hypothetic case. This dissolved oxygen would degas into the VCT which contains more than 2% of gaseous hydrogen. A H₂/O₂ gaseous mixture would form in VCT gaseous phase. Such a situation is called “early oxygenation”.

To prevent an “early oxygenation” :

* H₂gaseous < 15 % on VCT

* H₂gaseous < oxygenation criteria on other biphasic capacities (mainly, H₂gaseous < 2%)

* Availability of the oxygen monitor on the inlet to gaseous wastes treatment system,

* Availability of redundancy mean for on-line monitors.

Early oxygenation is declared when hydrogen has disappeared and oxygen appears and is stable, although there is more than 2 % of gaseous hydrogen in the VCT. The management of an “early oxygenation” consists to forecast to dilute the VCT gaseous phase by nitrogen to avoid dangerous gaseous mixture and to obtain as quickly as possible H₂gaseousVCT< 2 %. VCT gaseous wastes are set to hydrogenated gaseous wastes tanks. To manage an early oxygenation in good conditions (gas volumes, duration ...), gaseous oxygen concentration must remain smaller than 3.0 %. Three methods can be used to dilute the VCT gaseous phase :

* Nitrogen burping (levels variation)

* Continuous nitrogen flushing

* Discontinuous nitrogen flushing (constant level but pressure variation by nitrogen inlet and gas discharge to gaseous wastes treatment system).

One of the three solutions is operated as soon as the early oxygenation is detected and confirmed by the chemist. The first dilution must have been performed before gaseous oxygen concentration becomes higher than 3.0%. It means that after a burping, the maximal VCT level, just before decreasing the level to introduce nitrogen, must be reached before O₂gaseous = 3.0 %. The minimal value of the time period called Δt1 in the figure 6 must be higher than the monitor information added to the chemistry time period (detection and confirmation) and to the level increase.

$$t = \frac{P_G V_G}{R T_G} \times \frac{M(O_2)}{c_{O_2 RCPmesure}} \times \frac{1}{Q_{RCV} \times 100} \times O_{2gaseousVCT}$$

c(O₂RCPmesure) : RCS dissolved oxygen concentration (mg/kg)

M (O₂) : dioxygen molar mass (g.mol⁻¹)

QRCV : CVCS flow rate (t/h)

t : time period to obtain a specific gaseous oxygen concentration (hours)

P_G : VCT gaseous phase pressure in Pa (× 1,013 10⁵bar)

V_G : VCT gaseous phase volume (m³)

R : thermodynamic constant = 8,314

T : VCT temperature (K)

<figure><img src="image_7.png" /><figcaption>Fig.6. Evolution of VCT (Volume Control Tank) levels during early oxygenation management by N₂ burping</figcaption></figure>

The design case is taken for a maximal volume equal to the reactant injection tank volume which integers an error of five litres, an excess of 20% and an error on the hydrogen measurements. The table 1 gives the main characteristics of an early oxygenation management.

<table><caption>Table 1. Main characteristics of an early oxygenation management according to the design-type units.</caption><thead><tr><th></th><th>CP0</th><th>CPY</th><th>1300 MWe</th><th>1500 MWe</th></tr></thead><tbody><tr><td>M<sub>RCS</sub></td><td>304 t</td><td>299 t</td><td>433 t</td><td>440 t</td></tr><tr><td>V<sub>VCT (total)</sub></td><td>8.5 m³</td><td>8.9 m³</td><td>19 m³</td><td>19 m³</td></tr><tr><td>Q<sub>RCV</sub></td><td>14 t/h</td><td>14 t/h</td><td>36 t/h</td><td>36 t/h</td></tr><tr><td>V<sub>max</sub> H₂O₂</td><td>30 L</td><td>20 L</td><td>40 L</td><td>50 L</td></tr><tr><td>O₂max (mg/kg)</td><td>8.1</td><td>6.7</td><td>8.4</td><td>10.3</td></tr><tr><td>Minimal time period to perform the first treatment</td><td>60+20+30 = 110 min</td><td>60+20+30 = 110 min</td><td>20+20+50 = 90 min</td><td>20+20+30 = 70 min</td></tr></tbody></table>

To avoid oxygen concentration higher than 3.0 % in a time period shorter than the minimal time period, purification flow rate must be reduced to 14 t/h on 3 loops reactors. On four loops reactors, VCT volumes are greater than those of the three loops reactors and the reaction is more rapidly detected by monitors.
The calculation have used conservative hypothesis by not decreasing at first the VCT level, to introduce nitrogen and to dilute the VCT gaseous phase. But, for an easier early oxygenation management, it is recommended to decrease first the VCT level as low as possible and then to perform maximal nitrogen burping levels variations (for an 1500 MWe unit : levels variation from 20 to 80%).

If continuous or discontinuous nitrogen flushing are the management way, the VCT level must be increased to 80% to have a maximal efficiency (small gaseous space to dilute). The normal nitrogen flow rate is higher than the needed dilution flow rate. The time period to perform the level increase is the same for the three methods. The main difference between the three methods is the gaseous wastes volume.

### III.F. Environment

During the four feasibility tests, hydrogen was remained in RCS. Hydrogenated gaseous wastes volumes were smaller than the usual volumes after a physical degassing (reduction from 25 to 40%). This conclusions still remains for units which must perform unplanned oxygenation and have no time to physically degas the RCS. In case of hydrogen increase after bubble collapsing, chemical degassing avoids greater gaseous wastes volume production.

Chemical degassing uses hydrogen peroxide which is the reactant used for the oxygenation. There is no new waste type. The injected H₂O₂ volume is too small to have an influence on the liquid wastes volume. By reducing burpings during the first times of the shutdown, it can reduce the liquid wastes to be treated in the BRS head tank.

As a conclusion, chemical degassing does not increase the volumes of gaseous and liquid wastes and could limit them when hydrogen is remained in RCS.

## IV. METHOD

Because of the specific French gaseous wastes management, chemical degassing is strictly separated from oxygenation. It is performed at temperatures equal or less than 80°C by hydrogen peroxide injection. It can be used on units with fission products if their volumic activities respect the oxygenation criteria before the chemical degassing.

Hydrogen management till the end of the bubble collapsing depends on the nature of the shutdown (planned or unplanned) and on the available volume in gaseous wastes tanks.

After bubble collapsing, RCS dissolved hydrogen is measured and according to the concentration value, a check up is done before temperature decreasing to 80°C.

* 3 < H₂dissolved < 35 cm³/kg in RCS,

* H₂gaseous < 15% on VCT,

* H₂gaseous < oxygenation criteria on other biphasic capacities (mainly, H₂gaseous < 2%)

* Availability of the oxygen monitor on the inlet to gaseous wastes treatment system and of RCS oxygen and hydrogen monitors

* Availability of redundancy mean for on-line monitors,

* Availability of hydrogenated gaseous wastes system tanks,

* H₂O₂ volume enough for chemical degassing and oxygenation.

During temperature decrease to 80°C, biphasic capacity which has collected the gaseous phase of the pressurizer is burped or flushed by nitrogen to reach the oxygenation criteria before starting the chemical degassing. At about 100°C, a nitrogen burping can be performed on VCT to reduce hydrogen gaseous concentration in VCT gaseous phase.

At 80°C, if:

* Radiochemical oxygenation criteria are respected (¹³³Xe < 4 GBq/t and ¹³¹I < 2 GBq/t),

* All biphasic capacities respect the oxygenation criteria on gaseous hydrogen (mainly H₂gaseous < 2%),

* Purification and sprinkling flow rate = 36 t/h on 4-loops reactors and = 14 t/h on 3 loops reactors,

* H₂O₂ volume calculation

* For units with more than six months of full power if they have 600 SG tubes or 690 SG tubes with ⁵⁸Co oxygenation peak higher than 50 GBq/t, an excess of 20% of H₂O₂ compared to the stoechiometric volume is injected,

* For units with less than six months of full power, the injected H₂O₂ volume is stoechiometric,

* For units with 690 SG tubes and with ⁵⁸Co peak lower than 50 GBq/t, the injected H₂O₂ volume is stoechiometric.

$$V_{\text{H2O2 stoechiom.}} = \frac{\Delta C_{\text{H2OREN}} \times M_t}{V_{\text{OREN}} \times n_{\text{H2O2}}} = \frac{\Delta C_{\text{H2OREN}}(\text{cm}^3/\text{kg}) \times M_t}{V_{\text{OREN}}(\text{L/mol}) \times 9,794} \quad (\text{L})$$

$$\text{If } \Theta_{\text{REN}} = 25^{\circ}\text{C}, V_{\text{OREN}}(\text{L/mol}) = 24,8 \text{ l/mol} \\ \text{Mt} = \text{RCS mass in tons ;}$$

$$9,794 : \text{H}_2\text{O}_2 \text{ moles number per injected liter}$$

$$V_{\text{H2O2 major}} =$$

$$1,2x \frac{\Delta C_{\text{H2OREN}} \times M_t}{V_{\text{OREN}} \times n_{\text{H2O2}}} = 1,2x \frac{\Delta C_{\text{H2OREN}}(\text{cm}^3/\text{kg}) \times M_t}{V_{\text{OREN}}(\text{L/mol}) \times 9,794} \quad (\text{L})$$

$$\text{If } \Theta_{\text{REN}} = 25^{\circ}\text{C}, V_{\text{OREN}}(\text{L/mol}) = 24,8 \text{ l/mol} \\ \text{Mt} = \text{RCS mass in tons ;}$$

$$9,794 : \text{H}_2\text{O}_2 \text{ moles number per injected liter}$$

If dissolved hydrogen remains higher than 3 cm³/kg, a new injection is performed with similar calculation rules. If the hydrogen peroxide volume introduced into the reactants injection tank is wrong, the tank is drained, rinsed and filled with the right H₂O₂ volume.
The calculation have used conservative hypothesis by not decreasing at first the VCT level, to introduce nitrogen and to dilute the VCT gaseous phase. But, for an easier early oxygenation management, it is recommended to decrease first the VCT level as low as possible and then to perform maximal nitrogen burping levels variations (for an 1500 MWe unit : levels variation from 20 to 80%).

If continuous or discontinuous nitrogen flushing are the management way, the VCT level must be increased to 80% to have a maximal efficiency (small gaseous space to dilute). The normal nitrogen flow rate is higher than the needed dilution flow rate. The time period to perform the level increase is the same for the three methods. The main difference between the three methods is the gaseous wastes volume.

### III.F. Environment

During the four feasibility tests, hydrogen was remained in RCS. Hydrogenated gaseous wastes volumes were smaller than the usual volumes after a physical degassing (reduction from 25 to 40%). This conclusions still remains for units which must perform unplanned oxygenation and have no time to physically degas the RCS. In case of hydrogen increase after bubble collapsing, chemical degassing avoids greater gaseous wastes volume production.

Chemical degassing uses hydrogen peroxide which is the reactant used for the oxygenation. There is no new waste type. The injected H₂O₂ volume is too small to have an influence on the liquid wastes volume. By reducing burpings during the first times of the shutdown, it can reduce the liquid wastes to be treated in the BRS head tank.

As a conclusion, chemical degassing does not increase the volumes of gaseous and liquid wastes and could limit them when hydrogen is remained in RCS.

## IV. METHOD

Because of the specific French gaseous wastes management, chemical degassing is strictly separated from oxygenation. It is performed at temperatures equal or less than 80°C by hydrogen peroxide injection. It can be used on units with fission products if their volumic activities respect the oxygenation criteria before the chemical degassing.

Hydrogen management till the end of the bubble collapsing depends on the nature of the shutdown (planned or unplanned) and on the available volume in gaseous wastes tanks.

After bubble collapsing, RCS dissolved hydrogen is measured and according to the concentration value, a check up is done before temperature decreasing to 80°C.

* 3 < H₂dissolved < 35 cm³/kg in RCS,

* H₂gaseous < 15% on VCT,

* H₂gaseous < oxygenation criteria on other biphasic capacities (mainly, H₂gaseous < 2%)

* Availability of the oxygen monitor on the inlet to gaseous wastes treatment system and of RCS oxygen and hydrogen monitors

* Availability of redundancy mean for on-line monitors,

* Availability of hydrogenated gaseous wastes system tanks,

* H₂O₂ volume enough for chemical degassing and oxygenation.

During temperature decrease to 80°C, biphasic capacity which has collected the gaseous phase of the pressurizer is burped or flushed by nitrogen to reach the oxygenation criteria before starting the chemical degassing. At about 100°C, a nitrogen burping can be performed on VCT to reduce hydrogen gaseous concentration in VCT gaseous phase.

At 80°C, if:

* Radiochemical oxygenation criteria are respected (¹³³Xe < 4 GBq/t and ¹³¹I < 2 GBq/t),

* All biphasic capacities respect the oxygenation criteria on gaseous hydrogen (mainly H₂gaseous < 2%),

* Purification and sprinkling flow rate = 36 t/h on 4-loops reactors and = 14 t/h on 3 loops reactors,

* H₂O₂ volume calculation

* For units with more than six months of full power if they have 600 SG tubes or 690 SG tubes with ⁵⁸Co oxygenation peak higher than 50 GBq/t, an excess of 20% of H₂O₂ compared to the stoechiometric volume is injected,

* For units with less than six months of full power, the injected H₂O₂ volume is stoechiometric,

* For units with 690 SG tubes and with ⁵⁸Co peak lower than 50 GBq/t, the injected H₂O₂ volume is stoechiometric.

$$V_{\text{H2O2 stoechiom.}} = \frac{\Delta C_{\text{H2OREN}} \times M_t}{V_{\text{OREN}} \times n_{\text{H2O2}}} = \frac{\Delta C_{\text{H2OREN}}(\text{cm}^3/\text{kg}) \times M_t}{V_{\text{OREN}}(\text{L/mol}) \times 9,794} \quad (\text{L})$$

$$\text{If } \Theta_{\text{REN}} = 25^{\circ}\text{C}, V_{\text{OREN}}(\text{L/mol}) = 24,8 \text{ l/mol} \\ \text{Mt} = \text{RCS mass in tons ;}$$

$$9,794 : \text{H}_2\text{O}_2 \text{ moles number per injected liter}$$

$$V_{\text{H2O2 major}} =$$

$$1,2x \frac{\Delta C_{\text{H2OREN}} \times M_t}{V_{\text{OREN}} \times n_{\text{H2O2}}} = 1,2x \frac{\Delta C_{\text{H2OREN}}(\text{cm}^3/\text{kg}) \times M_t}{V_{\text{OREN}}(\text{L/mol}) \times 9,794} \quad (\text{L})$$

$$\text{If } \Theta_{\text{REN}} = 25^{\circ}\text{C}, V_{\text{OREN}}(\text{L/mol}) = 24,8 \text{ l/mol} \\ \text{Mt} = \text{RCS mass in tons ;}$$

$$9,794 : \text{H}_2\text{O}_2 \text{ moles number per injected liter}$$

If dissolved hydrogen remains higher than 3 cm³/kg, a new injection is performed with similar calculation rules. If the hydrogen peroxide volume introduced into the reactants injection tank is wrong, the tank is drained, rinsed and filled with the right H₂O₂ volume.
When $0 < H_{2\text{dissolved}} < 3 \text{ cm}^3\text{/kg}$, purification flow rate is increased to 27 t/h on three loops reactors and nitrogen burping or flushing is performed on VCT. For a maximal efficiency, burpings present a maximal levels variation (20 to 80%).

<figure><img src="image_8.png" /><figcaption>Figure 7. $H_2O_2$ volume to remove dissolved hydrogen on a 1500 MWe French unit</figcaption></figure>

## V. CONCLUSIONS AND FURTHER USES

Chemical degassing has been successfully performed on the French 1500 MWe units since July 2004 and the feasibility has been proved on the three other design type units. It helps to an optimized shutdown planning management, especially in case of unplanned shutdowns, when physical degassing cannot be easily and efficiently performed. Chemical degassing helps to limit the thermal stress corrosion of RHR at temperature higher than 120°C by avoiding temperature step extension after bubble collapsing to mitigate hydrogen concentration increase.

Chemical degassing and oxygenation still remain separated operations because of the gaseous wastes.

Chemical degassing is simply performed at temperature equal or lower than 80°C, by hydrogen peroxide injection if dissolved hydrogen is in the range 3 - 35 $\text{cm}^3\text{/kg}$. $H_2O_2$ calculation can be:

* Stoechiometric if the power duration is less than six months,

* Stoechiometric if SG tubes are made in 690 nickel based Alloy and the unit has $^{58}$Co oxygenation peak smaller than 50 GBq/t after a full cycle,

* Increased by 20 % if the power duration is higher than six months and if the SG tubes are made of 600 nickel based Alloy or of 690 nickel based Alloy with $^{58}$Co oxygenation peak greater than 50 GBq/t after a full cycle

Purification flow rate which is the VCT sprinkling flow rate is adapted to each design type units characteristics to easily manage a hypothetic early oxygenation. Three kinds of dilution can be used: nitrogen burping with high levels amplitude, continuous nitrogen flushing with a maximal VCT level and a flow rate monitors on gaseous wastes or discontinuous flushing with a maximal constant VCT level but pressure variation.

Chemical degassing, by its very rapid hydrogen removing, gives more choices for chemical conditioning for unplanned shutdowns. To limit the thermal stress corrosion of RHR, bubble collapsing is performed again at 130°C but with 10 to 35 $\text{cm}^3\text{/kg}$ of hydrogen to limit contamination risk (expected value: 15 $\text{cm}^3\text{/kg}$).

## VI. REFERENCES

[1] Pressurized Water Reactor Primary - Water Chemistry Guidelines – Volume 2 Revision 5 – EPRI, Palo Alto, CA. 12884.

[2] J-L. BRETELLE and alii, Study of various Chemical Species Behaviour for Contamination Risk – Water Chemistry in nuclear Reactor Systems – SFEN Avignon, France, April 2002.

[3] F. DACQUAIT and alii, Corrosion Products transfer in French PWRs Shutdown Systems – SFEN Avignon, France, April 2002.

[4] F. CARRETTE and alii, Impact of the Surface State of Steam Generator Tubes on the Release of Corrosion Products in Pressurized Water Reactors, Water Chemistry in nuclear Reactor Systems –Jeju Island, Korea, October 2006.
When $0 < H_{2\text{dissolved}} < 3 \text{ cm}^3\text{/kg}$, purification flow rate is increased to 27 t/h on three loops reactors and nitrogen burping or flushing is performed on VCT. For a maximal efficiency, burpings present a maximal levels variation (20 to 80%).

<figure><img src="image_8.png" /><figcaption>Figure 7. $H_2O_2$ volume to remove dissolved hydrogen on a 1500 MWe French unit</figcaption></figure>

## V. CONCLUSIONS AND FURTHER USES

Chemical degassing has been successfully performed on the French 1500 MWe units since July 2004 and the feasibility has been proved on the three other design type units. It helps to an optimized shutdown planning management, especially in case of unplanned shutdowns, when physical degassing cannot be easily and efficiently performed. Chemical degassing helps to limit the thermal stress corrosion of RHR at temperature higher than 120°C by avoiding temperature step extension after bubble collapsing to mitigate hydrogen concentration increase.

Chemical degassing and oxygenation still remain separated operations because of the gaseous wastes.

Chemical degassing is simply performed at temperature equal or lower than 80°C, by hydrogen peroxide injection if dissolved hydrogen is in the range 3 - 35 cm³/kg. $H_2O_2$ calculation can be:

* Stoechiometric if the power duration is less than six months,

* Stoechiometric if SG tubes are made in 690 nickel based Alloy and the unit has ⁵⁸Co oxygenation peak smaller than 50 GBq/t after a full cycle,

* Increased by 20 % if the power duration is higher than six months and if the SG tubes are made of 600 nickel based Alloy or of 690 nickel based Alloy with ⁵⁸Co oxygenation peak greater than 50 GBq/t after a full cycle

Purification flow rate which is the VCT sprinkling flow rate is adapted to each design type units characteristics to easily manage a hypothetic early oxygenation. Three kinds of dilution can be used: nitrogen burping with high levels amplitude, continuous nitrogen flushing with an maximal VCT level and a flow rate monitors on gaseous wastes or discontinuous flushing with a maximal constant VCT level but pressure variation.

Chemical degassing, by its very rapid hydrogen removing, gives more choices for chemical conditioning for unplanned shutdowns. To limit the thermal stress corrosion of RHR, bubble collapsing is performed again at 130°C but with 10 to 35 cm³/kg of hydrogen to limit contamination risk (expected value: 15 cm³/kg).

## VI. REFERENCES

[1] Pressurized Water Reactor Primary - Water Chemistry Guidelines – Volume 2 Revision 5 – EPRI, Palo Alto, CA. 12884.

[2] J-L. BRETELLE and alii, Study of various Chemical Species Behaviour for Contamination Risk – Water Chemistry in nuclear Reactor Systems – SFEN Avignon, France, April 2002.

[3] F. DACQUAIT and alii, Corrosion Products transfer in French PWRs Shutdown Systems – SFEN Avignon, France, April 2002.

[4] F. CARRETTE and alii, Impact of the Surface State of Steam Generator Tubes on the Release of Corrosion Products in Pressurized Water Reactors, Water Chemistry in nuclear Reactor Systems –Jeju Island, Korea, October 2006.