# Contamination simulation of DOEL-4 PWR using The OSCAR V1.2 code and parametrical studies during cycles or cold shutdowns

Benoit Habert, Frédéric Dacquait, Raphaël Lecocq, Kim Schildermans

► To cite this version:

Benoit Habert, Frédéric Dacquait, Raphaël Lecocq, Kim Schildermans. Contamination simulation of DOEL-4 PWR using The OSCAR V1.2 code and parametrical studies during cycles or cold shutdowns. NPC 2014- Nuclear Plant Chemistry - International Conference on Water Chemistry of Nuclear Reactors Systems, Oct 2014, Sapporo, Japan. cea-04473972

HAL Id: cea-04473972

https://cea.hal.science/cea-04473972

Submitted on 22 Feb 2024

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# Contamination simulation of DOEL-4 PWR using The OSCAR V1.2 code and parametrical studies during cycles or cold shutdowns

Benoit Habert, Frédéric Dacquait, Raphaël Lecocq, Kim Schildermans

► To cite this version:

Benoit Habert, Frédéric Dacquait, Raphaël Lecocq, Kim Schildermans. Contamination simulation of DOEL-4 PWR using The OSCAR V1.2 code and parametrical studies during cycles or cold shutdowns. NPC 2014- Nuclear Plant Chemistry - International Conference on Water Chemistry of Nuclear Reactors Systems, Oct 2014, Sapporo, Japan. cea-04473972

HAL Id: cea-04473972

https://cea.hal.science/cea-04473972v1

Submitted on 22 Feb 2024

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

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

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

Distributed under a Creative Commons Attribution 4.0 International License
# Contamination Simulation of DOEL-4 PWR Using The OSCAR V1.2 Code and Parametrical Studies During Cycles or Cold Shutdowns

Benoît HABERT¹, Frédéric DACQUAIT¹, Raphaël LECOCQ², Kim SCHILDERMANS²

1. CEA, DEN, France

2. GDF SUEZ, Laborelec, Belgium

## ABSTRACT

The purpose of the OSCAR code, developed by the CEA in cooperation with EDF and AREVA NP, is to predict the contamination of the PWR primary system. The OSCAR code allows researchers to analyze corrosion products and fission products behavior and to calculate the activity in the fluid and the surface activity in the primary system. Nowadays, the OSCAR code is considered to be not only a tool for numerical simulations and predictions but also one that might combine and organize all new knowledge useful to progress on contamination caused by Activated Corrosion Products (ACPs).

The OSCAR V1.2 code is used for ACPs simulation studies of DOEL-4 reactor. The 22 first cycles have been simulated and compared with measurements [1] of surface activities on the loops. Since, this simulation is the reference of DOEL-4 parametrical studies done with OSCAR V1.2.

The effect on contamination by ACPs of a 4-month operation at 50% power during a cycle will be introduced. The results will be focused on volume and surface activities during the cycle and the following shutdown. Then several studies on mid-cycle shutdowns of a given length of time or with startup criteria on ⁵⁸Co volume activity will be presented.

Last, results of parametrical studies performed on early oxygenation during cold shutdown will be given.

**Keywords:** contamination transfer, activated corrosion products, OSCAR V1.2, DOEL-4
# Contamination Simulation of DOEL-4 PWR Using The OSCAR V1.2 Code and Parametrical Studies During Cycles or Cold Shutdowns

Benoît HABERT¹, Frédéric DACQUAIT¹, Raphaël LECOCQ², Kim SCHILDERMANS²

1: CEA, DEN, France

2: GDF SUEZ, Laborelec, Belgium

## ABSTRACT

The purpose of the OSCAR code, developed by the CEA in cooperation with EDF and AREVA NP, is to predict the contamination of the PWR primary system. The OSCAR code allows researchers to analyze corrosion products and fission products behavior and to calculate the activity in the fluid and the surface activity in the primary system. Nowadays, the OSCAR code is considered to be not only a tool for numerical simulations and predictions but also one that might combine and organize all new knowledge useful to progress on contamination caused by Activated Corrosion Products (ACPs).

The OSCAR V1.2 code is used for ACPs simulation studies of DOEL-4 reactor. The 22 first cycles have been simulated and compared with measurements [1] of surface activities on the loops. Since, this simulation is the reference of DOEL-4 parametrical studies done with OSCAR V1.2.

The effect on contamination by ACPs of a 4-month operation at 50% power during a cycle will be introduced. The results will be focused on volume and surface activities during the cycle and the following shutdown. Then several studies on mid-cycle shutdowns of a given length of time or with startup criteria on ⁵⁸Co volume activity will be presented.

Last, results of parametrical studies performed on early oxygenation during cold shutdown will be given.

**Keywords:** contamination transfer, activated corrosion products, OSCAR V1.2, DOEL-4
# 1. Introduction

The purpose of the OSCAR code, developed by the CEA in cooperation with EDF and AREVA NP, is to predict the contamination of the PWR primary system. The OSCAR code allows researchers to analyze corrosion products and fission products behavior and to calculate the activity in the fluid and the surface activity in the primary system. Nowadays, the OSCAR code is considered to be not only a tool for numerical simulations and predictions but also one that might combine and organize all new knowledge useful to progress on contamination caused by Activated Corrosion Products (ACPs).

This article presents the results of the simulations of ACPs transfers using the OSCAR code V1.2 [2]. The goals of these simulations are to demonstrate the ability of the OSCAR code to reproduce the ACPs contamination transfer of a given reactor (DOEL-4) by using precise operation data. Parametrical studies simulations are of particular interest because of the huge difficulty to carry out them during the operation. We will present here a selection of parametrical studies simulations, first the effect on contamination by ACPs of a 4-month operation at 50% power during a cycle will be introduced. The results will be focused on volume and surface activities during the cycle and the following shutdown. Then several studies on mid-cycle shutdowns of a given length of time or with startup criteria on $^{58}$Co volume activity will be presented. Last, results of parametrical studies performed on early oxygenation during cold shutdown will be given.

# 2. Contamination simulation of the 22 first cycles of the DOEL-4 reactor

DOEL-4 is a 3-loops PWR equipped with alloy 690 tubed steam generators since the steam generator replacement at cycle 11.

The structure of the DOEL-4 primary circuit has been modeled by control volumes using the design data of the reactor (wetted surfaces, hydraulic diameters, material compositions, nominal temperatures ...). The operation data (water chemistry, reactor power, refueling, steam generator replacement...) of the 22 first cycles have been averaged to obtain a one day step simulation.

As a matter of comparison, we have used surface activities measurements coming from EMECC campaigns [3] carried out during some shutdowns and volume activities measured during the operation.

A good agreement between simulation and measurements of the deposited activities of $^{58}$Co and $^{60}$Co can be observed in Figure 1.

Nevertheless, the $^{58}$Co contamination during the first cycle seems to be overestimated. It comes from an excessive release of corrosion products during the first cycle. This modelled release could have been reduced, but we decided not to do so because of the lack of measurement for the first cycle and the weak influence of the first cycle release on the contamination simulation at the cycle that can be compared with measurements.

Moreover, note that the orders of magnitude being sought, we did not try to optimize the results by an advanced calibration.
# 1. Introduction

The purpose of the OSCAR code, developed by the CEA in cooperation with EDF and AREVA NP, is to predict the contamination of the PWR primary system. The OSCAR code allows researchers to analyze corrosion products and fission products behavior and to calculate the activity in the fluid and the surface activity in the primary system. Nowadays, the OSCAR code is considered to be not only a tool for numerical simulations and predictions but also one that might combine and organize all new knowledge useful to progress on contamination caused by Activated Corrosion Products (ACPs).

This article presents the results of the simulations of ACPs transfers using the OSCAR code V1.2 [2]. The goals of these simulations are to demonstrate the ability of the OSCAR code to reproduce the ACPs contamination transfer of a given reactor (DOEL-4) by using precise operation data. Parametrical studies simulations are of particular interest because of the huge difficulty to carry out them during the operation. We will present here a selection of parametrical studies simulations, first the effect on contamination by ACPs of a 4-month operation at 50% power during a cycle will be introduced. The results will be focused on volume and surface activities during the cycle and the following shutdown. Then several studies on mid-cycle shutdowns of a given length of time or with startup criteria on $^{58}$Co volume activity will be presented. Last, results of parametrical studies performed on early oxygenation during cold shutdown will be given.

# 2. Contamination simulation of the 22 first cycles of the DOEL-4 reactor

DOEL-4 is a 3-loops PWR equipped with alloy 690 tubed steam generators since the steam generator replacement at cycle 11.

The structure of the DOEL-4 primary circuit has been modeled by control volumes using the design data of the reactor (wetted surfaces, hydraulic diameters, material compositions, nominal temperatures ...). The operation data (water chemistry, reactor power, refueling, steam generator replacement...) of the 22 first cycles have been averaged to obtain a one day step simulation.

As a matter of comparison, we have used surface activities measurements coming from EMECC campaigns [3] carried out during some shutdowns and volume activities measured during the operation.

A good agreement between simulation and measurements of the deposited activities of $^{58}$Co and $^{60}$Co can be observed in Figure 1.

Nevertheless, the $^{58}$Co contamination during the first cycle seems to be overestimated. It comes from an excessive release of corrosion products during the first cycle. This modelled release could have been reduced, but we decided not to do so because of the lack of measurement for the first cycle and the weak influence of the first cycle release on the contamination simulation at the cycle that can be compared with measurements.

Moreover, note that the orders of magnitude being sought, we did not try to optimize the results by an advanced calibration.
<figure><img src="image_2.png" /><figcaption>Figure 1. Simulation of the ⁵⁸Co and ⁶⁰Co surface activities on the hot legs, crossover legs and the hot and cold sides of the SGs compared with EMECC measurements</figcaption></figure>

This simulation has proved the ability of the OSCAR code to simulate the contamination transfer of a reactor during a large number of cycles given the modelling with control volumes and the use of close to reality operation data.

### 3. Contamination simulation during a typical cold shutdown of the DOEL-4 reactor

The modelling of the cold shutdown with the OSCAR code [2] allowed reproducing the ⁵⁸Co volume activity peak observed in the primary circuit. This peak is explained by a fast dissolution of nickel simple phases deposited on the fuel cladding leading to the dissolution of ⁵⁸Co formed by activation of ⁵⁸Ni.

The dissolution of the ⁵⁸Co is modelled through a driving mechanism linked to the nickel phase it is trapped into. So the amount of ⁵⁸Co released during the cold shutdown is rigorously determined by the amount of nickel simple phases deposited on the fuel cladding before the shutdown.

The operation data of a typical cold shutdown are drawing in Figure 2. Note that the concentrations of oxygen and hydrogen are fixed to the given values.
<figure><img src="image_2.png" /><figcaption>Figure 1. Simulation of the ⁵⁸Co and ⁶⁰Co surface activities on the hot legs, crossover legs and the hot and cold sides of the SGs compared with EMECC measurements</figcaption></figure>

This simulation has proved the ability of the OSCAR code to simulate the contamination transfer of a reactor during a large number of cycles given the modelling with control volumes and the use of close to reality operation data.

### 3. Contamination simulation during a typical cold shutdown of the DOEL-4 reactor

The modelling of the cold shutdown with the OSCAR code [2] allowed reproducing the ⁵⁸Co volume activity peak observed in the primary circuit. This peak is explained by a fast dissolution of nickel simple phases deposited on the fuel cladding leading to the dissolution of ⁵⁸Co formed by activation of ⁵⁸Ni.

The dissolution of the ⁵⁸Co is modelled through a driving mechanism linked to the nickel phase it is trapped into. So the amount of ⁵⁸Co released during the cold shutdown is rigorously determined by the amount of nickel simple phases deposited on the fuel cladding before the shutdown.

The operation data of a typical cold shutdown are drawing in Figure 2. Note that the concentrations of oxygen and hydrogen are fixed to the given values.
<figure><img src="image_3.png" /><figcaption>Figure 2. Operation data during a typical cold shutdown.</figcaption></figure>

The variation of the ⁵⁸Co volume activity is presented in Figure 3. The peak value, 170 GBq/t, is overestimated (several tens of GBq/t measured) and is explained by a too large simulated mass of metal nickel deposited on the fuel cladding at the beginning of the shutdown (2 kg). The composition of the Ni on the cladding is based on equilibrium calculation and doesn't take into account the kinetics of the phase changes; it might be that phase changes, which give metal Ni, are simulated though they actually never occur because of their low kinetic. Otherwise, no increase of APCs surface activities is observed on the primary loops, this agrees with the feedback.

<figure><img src="image_4.png" /><figcaption>Figure 3. Simulated ⁵⁸Co volume activity in the primary circuit during the cold shutdown.</figcaption></figure>

Despite the overestimation of the ⁵⁸Co volume activity peak, this modelling was used as a reference for parametrical studies that will be further presented.
<figure><img src="image_3.png" /><figcaption>Figure 2. Operation data during a typical cold shutdown.</figcaption></figure>

The variation of the ⁵⁸Co volume activity is presented in Figure 3. The peak value, 170 GBq/t, is overestimated (several tens of GBq/t measured) and is explained by a too large simulated mass of metal nickel deposited on the fuel cladding at the beginning of the shutdown (2 kg). The composition of the Ni on the cladding is based on equilibrium calculation and doesn't take into account the kinetics of the phase changes; it might be that phase changes, which give metal Ni, are simulated though they actually never occur because of their low kinetic. Otherwise, no increase of APCs surface activities is observed on the primary loops, this agrees with the feedback.

<figure><img src="image_4.png" /><figcaption>Figure 3. Simulated ⁵⁸Co volume activity in the primary circuit during the cold shutdown.</figcaption></figure>

Despite the overestimation of the ⁵⁸Co volume activity peak, this modelling was used as a reference for parametrical studies that will be further presented.
4. Impact of a 50%-power operation

We have studied the impact of a 4-month operation at 50% power during a cycle. The operation data of the cycle have been replaced by typical ones (constant power, constant hydrogen concentration and smooth decrease of boron and lithium concentrations), this modelling is the reference for this study.

The 50% power operation is simulated by a neutron activation divided by 2, a decrease of the temperature of the “hot” regions (the average temperature evolves from 302 °C to 293.7 °C; for the hot legs: from 321 °C to 304 °C; while for the cold legs it is stable at 283 °C). The concentrations of boron and lithium are increased during the drop of power; their concentrations are given in Figure 4.

The 50% power operation period starts around 2 months after the beginning and ends 9 months before the end of the cycle.

<figure><img src="image_5.png" /><figcaption>Figure 4. Boron and lithium concentrations for the reference (green) and for the cycle with a 50% power operation (blue)</figcaption></figure>

This study combines the effects of two phenomena:

* the drop of power decreases the temperature (and also the equilibrium concentration) and the source term of the APCs contamination (neutron activation divided by 2),

* the change of the chemical conditions influence the pH_T which decreases slightly from 7.1 to 7.06 at the beginning of the 50% power period).

All the simulated volume and surface activities decrease during the 50% operation period; then they increase when power returns to its nominal value. At the end of the cycle, their values are close to the reference ones (see Figures 5 & 6).
4. Impact of a 50%-power operation

We have studied the impact of a 4-month operation at 50% power during a cycle. The
operation data of the cycle have been replaced by typical ones (constant power, constant
hydrogen concentration and smooth decrease of boron and lithium concentrations), this
modelling is the reference for this study.

The 50% power operation is simulated by a neutron activation divided by 2, a decrease of the temperature of the “hot” regions (the average temperature evolves from 302 °C to 293.7 °C; for the hot legs: from 321 °C to 304 °C; while for the cold legs it is stable at 283 °C). The concentrations of boron and lithium are increased during the drop of power; their concentrations are given in Figure 4.

The 50% power operation period starts around 2 months after the beginning and ends 9 months before the end of the cycle.

<figure><img src="image_5.png" /><figcaption>Figure 4. Boron and lithium concentrations for the reference (green) and for the cycle with a 50% power operation (blue)</figcaption></figure>

This study combines the effects of two phenomena:

* the drop of power decreases the temperature (and also the equilibrium concentration) and the source term of the APCs contamination (neutron activation divided by 2),

* the change of the chemical conditions influence the pH_T which decreases slightly from 7.1 to 7.06 at the beginning of the 50% power period).

All the simulated volume and surface activities decrease during the 50% operation period; then they increase when power returns to its nominal value. At the end of the cycle, their values are close to the reference ones (see Figures 5 & 6).
<figure><img src="image_6.png" /><figcaption>Figure 5. Volume activities of <sup>58</sup>Co and <sup>60</sup>Co for the reference (green) and for the cycle with a 50% power operation (blue)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 6. Surface activities of <sup>58</sup>Co and <sup>60</sup>Co on the hot legs for the reference (green) and for the cycle with a 50% power operation (blue)</figcaption></figure>

The cold shutdown at the end of this cycle has been simulated. The <sup>58</sup>Co and <sup>60</sup>Co volume activities peak are slightly lower (resp. -4% and -10%) than the reference despite an equivalent amount of nickel on the fuel cladding at the end of the cycle but because of a slightly lower global amount of <sup>58</sup>Co and <sup>60</sup>Co located on the cladding (produced by the lower neutron activation).

According to the OSCAR simulation, a 50% power operation during 4 months has a slight influence during this period and few months after. Then 6 months after, the contamination levels are equivalent to those obtained with a nominal power operation. As consequence, if the extended period at 50% occurred less than 6 months prior to refueling outage, an influence on the shutdown release would be noticed.
<figure><img src="image_6.png" /><figcaption>Figure 5. Volume activities of <sup>58</sup>Co and <sup>60</sup>Co for the reference (green) and for the cycle with a 50% power operation (blue)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 6. Surface activities of <sup>58</sup>Co and <sup>60</sup>Co on the hot legs for the reference (green) and for the cycle with a 50% power operation (blue)</figcaption></figure>

The cold shutdown at the end of this cycle has been simulated. The <sup>58</sup>Co and <sup>60</sup>Co volume activities peak are slightly lower (resp. -4% and -10%) than the reference despite an equivalent amount of nickel on the fuel cladding at the end of the cycle but because of a slightly lower global amount of <sup>58</sup>Co and <sup>60</sup>Co located on the cladding (produced by the lower neutron activation).

According to the OSCAR simulation, a 50% power operation during 4 months has a slight influence during this period and few months after. Then 6 months after, the contamination levels are equivalent to those obtained with a nominal power operation. As consequence, if the extended period at 50% occurred less than 6 months prior to refueling outage, an influence on the shutdown release would be noticed.
# 5. Mid-cycle shutdown

## 5.1 14-day shutdown

The goal of this study is to determine the influence of mid-cycle shutdowns on the contamination levels according to the moment of the shutdown during the cycle. The shutdown was simulated at three moments of the cycle: 9, 6 and 3 months before the end of cycle.

The shutdown modelling uses the same operation data than the typical cold shutdown. The restart was roughly modelled: the operation data linearly evolving from the shutdown values to the nominal ones in one day (see Figure 7).

<figure><img src="image_8.png" /><figcaption>Figure 7. Operation data during the 14-day shutdown.</figcaption></figure>

The simulation of the ⁵⁸Co and ⁶⁰Co volume and surface (for the hot legs) activities are presented in Figures 8 & 9 for the cycle with the mid-cycle shutdown and the following cycle. For the other regions of the loops (steam generators, crossover legs...) the variations of the surface activities are similar to the hot legs ones.

A strong influence on the ⁵⁸Co contamination and a weak one for ⁶⁰Co are observed. This comes from the strong impact of the mid-cycle shutdown on the amount of nickel on the fuel cladding which strongly decreased.

No increase in surface activities on the loops is observed during the mid-cycle shutdown. The cobalt peaks of the mid-cycle shutdowns are the highest when the shutdown is the closer to the end of the cycle. The end of cycle cold shutdown cobalt peaks decrease when the mid-cycle is close to the end of cycle. This is illustrated in Table 1 with the ratio of the ⁵⁸Co peaks for the mid-cycle shutdowns and the end of cycle cold shutdown divided by the value of the ⁵⁸Co reference peak at the end of cycle cold shutdown (without mid-cycle shutdown).

<table><caption>Table 1. Ratio of the ⁵⁸Co peaks compared with the reference cold shutdown peak.</caption><thead><tr><th></th><th>9 months</th><th>6 months</th><th>3 months</th></tr></thead><tbody><tr><td>Mid-cycle shutdown</td><td>29%</td><td>60%</td><td>85%</td></tr><tr><td>Cold shutdown</td><td>68%</td><td>47%</td><td>20%</td></tr></tbody></table>

The influence of the mid-cycle shutdown disappears at the beginning of the following cycle for the volume activities and during this cycle for the surface activities (see Figures 7 & 8).
# 5. Mid-cycle shutdown

## 5.1 14-day shutdown

The goal of this study is to determine the influence of mid-cycle shutdowns on the contamination levels according to the moment of the shutdown during the cycle. The shutdown was simulated at three moments of the cycle: 9, 6 and 3 months before the end of cycle.

The shutdown modelling uses the same operation data than the typical cold shutdown. The restart was roughly modelled: the operation data linearly evolving from the shutdown values to the nominal ones in one day (see Figure 7).

<figure><img src="image_8.png" /><figcaption>Figure 7. Operation data during the 14-day shutdown.</figcaption></figure>

The simulation of the ⁵⁸Co and ⁶⁰Co volume and surface (for the hot legs) activities are presented in Figures 8 & 9 for the cycle with the mid-cycle shutdown and the following cycle. For the other regions of the loops (steam generators, crossover legs...) the variations of the surface activities are similar to the hot legs ones.

A strong influence on the ⁵⁸Co contamination and a weak one for ⁶⁰Co are observed. This comes from the strong impact of the mid-cycle shutdown on the amount of nickel on the fuel cladding which strongly decreased.

No increase in surface activities on the loops is observed during the mid-cycle shutdown. The cobalt peaks of the mid-cycle shutdowns are the highest when the shutdown is the closer to the end of the cycle. The end of cycle cold shutdown cobalt peaks decrease when the mid-cycle is close to the end of cycle. This is illustrated in Table 1 with the ratio of the ⁵⁸Co peaks for the mid-cycle shutdowns and the end of cycle cold shutdown divided by the value of the ⁵⁸Co reference peak at the end of cycle cold shutdown (without mid-cycle shutdown).

<table><caption>Table 1. Ratio of the ⁵⁸Co peaks compared with the reference cold shutdown peak.</caption><thead><tr><th></th><th>9 months</th><th>6 months</th><th>3 months</th></tr></thead><tbody><tr><th>Mid-cycle shutdown</th><td>29%</td><td>60%</td><td>85%</td></tr><tr><th>Cold shutdown</th><td>68%</td><td>47%</td><td>20%</td></tr></tbody></table>

The influence of the mid-cycle shutdown disappears at the beginning of the following cycle for the volume activities and during this cycle for the surface activities (see Figures 7 & 8).
This behavior is similar to the one highlighted for unexpected shutdown with oxygenation during a cycle [4] that showed an even more low ⁵⁸Co peak since the shutdown is close to the end of cycle.

<figure><img src="image_9.png" /><figcaption>Figure 8. ⁵⁸Co and ⁶⁰Co volume activities during 2 cycles, without shutdown (green) and with 14-day shutdown 9 months (blue), 6 months (pink) and 3 months (turquoise) before the end of the cycle.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 9. ⁵⁸Co and ⁶⁰Co surface activities on the hot legs during 2 cycles, without shutdown (green) and with 14-day shutdown 9 months (blue), 6 months (pink) and 3 months (turquoise) before the end of the cycle.</figcaption></figure>

## 5.2 Early restarts for mid-cycle shutdown

The influence of an early restart of the mid-cycle shutdown has been studied with two restart criteria: a ⁵⁸Co volume activity in the primary circuit of respectively 7 GBq/t and 50 GBq/t.
This behavior is similar to the one highlighted for unexpected shutdown with oxygenation during a cycle [4] that showed an even more low ⁵⁸Co peak since the shutdown is close to the end of cycle.

<figure><img src="image_9.png" /><figcaption>Figure 8. ⁵⁸Co and ⁶⁰Co volume activities during 2 cycles, without shutdown (green) and with 14-day shutdown 9 months (blue), 6 months (pink) and 3 months (turquoise) before the end of the cycle.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 9. ⁵⁸Co and ⁶⁰Co surface activities on the hot legs during 2 cycles, without shutdown (green) and with 14-day shutdown 9 months (blue), 6 months (pink) and 3 months (turquoise) before the end of the cycle.</figcaption></figure>

## 5.2 Early restarts for mid-cycle shutdown

The influence of an early restart of the mid-cycle shutdown has been studied with two restart criteria: a ⁵⁸Co volume activity in the primary circuit of respectively 7 GBq/t and 50 GBq/t.
The reference case is the mid-cycle shutdown 6 months before the end of cycle for which the ⁵⁸Co volume activity at the restart is 0.5 MBq/t.

For both cases, an increase of the ⁵⁸Co surface activities on the legs of the loops is observed (see Table2); while the contamination on the steam generator tubing surface is stable. There is also an increase of the ⁶⁰Co surface activities, but it is almost negligible, the difference with the ⁵⁸Co is explained with the far lower amount of ⁶⁰Co released in the primary water during the shutdown.

<table><caption>Table 2. Increase in the ⁵⁸Co surface activities during the early restart mid-cycle shutdowns.</caption><thead><tr><th rowspan="2"></th><th colspan="2">50 GBq/t</th><th colspan="2">7 GBq/t</th></tr><tr><th>Hot legs</th><th>Crossover legs</th><th>Hot legs</th><th>Crossover legs</th></tr></thead><tbody><tr><th>⁵⁸Co</th><td>+154%</td><td>+106%</td><td>+29%</td><td>+20%</td></tr></tbody></table>

## 6. Early oxygenation during the cold shutdown

The influence of an early oxygenation during the cold shutdown has been modelled. The OSCAR code doesn't calculate the oxygen consumption, so the modelling of this consumption was made by a return to a reducing condition between the parasitical oxygenation and the main one.

The modelled oxygenation starts at a temperature of 180 °C, the primary circuit stays in an oxidant condition during one hour and then returns in the reducing condition of the reference case. For this study, the only changes compared to the reference are the concentrations in oxygen and hydrogen during the one-hour oxygenation.

A double peak of volume activities is observed for the ⁵⁸Co (see Figure 10); the first peak value is 54% of the peak without oxygenation and the second is 57%.

The simulation of the deposited activities presents a huge increase for the ⁵⁸Co on the cold side of the SGs (+210%) at the end of the early oxygenation, whereas it stays stable on the hot side (+1%) and increases slightly on the legs (+5%). This increase is explained by the huge decrease of cobalt equilibrium concentrations at the return in a reducing condition. The cold side of the SGs is the loops region with the lowest cobalt equilibrium concentration, which explains the local massive precipitation; moreover, no release is observed from the SGs, thus the amount of ⁵⁸Co deposited activities on the SGs stays stable during the main oxygenation. Those results confirm that early oxygenations have to be avoided.

<figure><img src="image_11.png" /><figcaption>Figure 10. Simulated ⁵⁸Co volume activity in the primary circuit during the cold shutdown for the early oxygenation case (in blue) and for the reference case (blue).</figcaption></figure>
The reference case is the mid-cycle shutdown 6 months before the end of cycle for which the ⁵⁸Co volume activity at the restart is 0.5 MBq/t.

For both cases, an increase of the ⁵⁸Co surface activities on the legs of the loops is observed (see Table2); while the contamination on the steam generator tubing surface is stable. There is also an increase of the ⁶⁰Co surface activities, but it is almost negligible, the difference with the ⁵⁸Co is explained with the far lower amount of ⁶⁰Co released in the primary water during the shutdown.

<table><caption>Table 2. Increase in the ⁵⁸Co surface activities during the early restart mid-cycle shutdowns.</caption><thead><tr><th rowspan="2"></th><th colspan="2">50 GBq/t</th><th colspan="2">7 GBq/t</th></tr><tr><th>Hot legs</th><th>Crossover legs</th><th>Hot legs</th><th>Crossover legs</th></tr></thead><tbody><tr><th>⁵⁸Co</th><td>+154%</td><td>+106%</td><td>+29%</td><td>+20%</td></tr></tbody></table>

## 6. Early oxygenation during the cold shutdown

The influence of an early oxygenation during the cold shutdown has been modelled. The OSCAR code doesn't calculate the oxygen consumption, so the modelling of this consumption was made by a return to a reducing condition between the parasitical oxygenation and the main one.

The modelled oxygenation starts at a temperature of 180 °C, the primary circuit stays in an oxidant condition during one hour and then returns in the reducing condition of the reference case. For this study, the only changes compared to the reference are the concentrations in oxygen and hydrogen during the one-hour oxygenation.

A double peak of volume activities is observed for the ⁵⁸Co (see Figure 10); the first peak value is 54% of the peak without oxygenation and the second is 57%.

The simulation of the deposited activities presents a huge increase for the ⁵⁸Co on the cold side of the SGs (+210%) at the end of the early oxygenation, whereas it stays stable on the hot side (+1%) and increases slightly on the legs (+5%). This increase is explained by the huge decrease of cobalt equilibrium concentrations at the return in a reducing condition. The cold side of the SGs is the loops region with the lowest cobalt equilibrium concentration, which explains the local massive precipitation; moreover, no release is observed from the SGs, thus the amount of ⁵⁸Co deposited activities on the SGs stays stable during the main oxygenation. Those results confirm that early oxygenations have to be avoided.

<figure><img src="image_11.png" /><figcaption>Figure 10. Simulated ⁵⁸Co volume activity in the primary circuit during the cold shutdown for the early oxygenation case (in blue) and for the reference case (blue).</figcaption></figure>
## 7. Conclusion

This present article demonstrates the ability of the OSCAR code V1.2 to reproduce the ACPs contamination transfer of the DOEL-4 reactor during 22 cycles. Then parametrical studies that are of particular interest for operators were presented.

The simulation of a 50% power operation during 4 months reveals a slight influence on the contamination which disappears 6 months after the return to the nominal power. Thus a 50% power operation during the 6 last months of a cycle will have effects on the shutdown.

Mid-cycle shutdown simulations show a strong impact during the rest of the cycle, it decreases the ⁵⁸Co contamination (volume and surface activities and peak of the end-of-cycle shutdown); the effects slowly vanish during the next cycle. Those beneficial effects can be wiped out when the restart for the mid-cycle shutdowns happens too early; it leads to an increase of the ⁵⁸Co surface activities on the loops.

Finally, the simulation of an early oxygenation during the cold shutdown produces double cobalt peaks and induces a local precipitation on the cold side of the SGs at the end of the early oxygenation. It confirms the risk of ACPs precipitation when early oxygenations occur.

## REFERENCES

[1] Schnongs P., Lecocq R., Minne A., Girasa E., Wyckmans R., Primary Chemistry as an Action Lever to Reduce the Source Term. Review of the Experience at Doel 3 and 4, Nuclear Plant Chemistry Conference, Quebec, 2010.

[2] Dacquait F., Francescatto J., Broutin F., Génin J.B., Bénier G., You D., Ranchoux G., Bonnefon J., Bachet M., Riot G., Simulations of corrosion product transfer with the OSCAR V1.2 code, Nuclear Plant Chemistry Conference, Paris, 2012.

[3] Eimecke R., Anthoni S., Ensemble de Mesure et d'Etude de la Contamination des Circuits (EMECC), 7th International Conference on Radiation Shielding, Bournemouth (England), 12th-15th September 1988.

[4] Dacquait F., Andrieu C., Berger M., Bretelle J.L., Rocher A., « Corrosion product transfer in French PWRs during shutdown", *Proceedings of the International conference on water chemistry in nuclear reactors systems*, 22-26/04/02, Avignon (France), CD-ROM, paper
## 7. Conclusion

This present article demonstrates the ability of the OSCAR code V1.2 to reproduce the ACPs contamination transfer of the DOEL-4 reactor during 22 cycles. Then parametrical studies that are of particular interest for operators were presented.

The simulation of a 50% power operation during 4 months reveals a slight influence on the contamination which disappears 6 months after the return to the nominal power. Thus a 50% power operation during the 6 last months of a cycle will have effects on the shutdown.

Mid-cycle shutdown simulations show a strong impact during the rest of the cycle, it decreases the ⁵⁸Co contamination (volume and surface activities and peak of the end-of-cycle shutdown); the effects slowly vanish during the next cycle. Those beneficial effects can be wiped out when the restart for the mid-cycle shutdowns happens too early; it leads to an increase of the ⁵⁸Co surface activities on the loops.

Finally, the simulation of an early oxygenation during the cold shutdown produces double cobalt peaks and induces a local precipitation on the cold side of the SGs at the end of the early oxygenation. It confirms the risk of ACPs precipitation when early oxygenations occur.

## REFERENCES

[1] Schnongs P., Lecocq R., Minne A., Girasa E., Wyckmans R., Primary Chemistry as an Action Lever to Reduce the Source Term. Review of the Experience at Doel 3 and 4, Nuclear Plant Chemistry Conference, Quebec, 2010.

[2] Dacquait F., Francescatto J., Broutin F., Génin J.B., Bénier G., You D., Ranchoux G., Bonnefon J., Bachet M., Riot G., Simulations of corrosion product transfer with the OSCAR V1.2 code, Nuclear Plant Chemistry Conference, Paris, 2012.

[3] Eimecke R., Anthoni S., Ensemble de Mesure et d'Etude de la Contamination des Circuits (EMECC), 7th International Conference on Radiation Shielding, Bournemouth (England), 12th-15th September 1988.

[4] Dacquait F., Andrieu C., Berger M., Bretelle J.L., Rocher A., « Corrosion product transfer in French PWRs during shutdown", *Proceedings of the International conference on water chemistry in nuclear reactors systems*, 22-26/04/02, Avignon (France), CD-ROM, paper