Heat exchanger cleaning at French nuclear plants
27 May 2013Inspections on moisture separator reheaters in the French CP0 reactor series has shown evidence of significant clogging due to deposits of magnetite inside tube bundle tubes whose presence could damage equipment and harm productivity. We have developed a procedure that consists of a mechanical pre-cleaning of all tubes of the MSR in order to unblock them, followed by chemical cleaning where magnetite is dissolved and flushed out of the tube bundle. By José T. Ruiz, Patrice Guerra and Cristina Carreres
French nuclear power plant MSRs, particularly the oldest ones (Fessenheim and Bugey), are subject to the build-up of magnetite deposits, resulting from steam eroding the tube system. As deposits build up on the inner surfaces of the tubes, they impede flow, and increase the pressure drop between inlet and outlet of the MSR. In some cases, the pressure drop amount has been found to be near the maximum design criterion. This effect could result in equipment damage and loss of plant productivity. Consequently, the MSR must be cleaned periodically.
The main object of the project is to provide a method to remove magnetite and oxide deposits from the inner tubes of the MSR in order to recover a passage diameter and an exchange surface equivalent to the original and to prevent loss of plant productivity. The method basically comprises a first step of mechanical pre-cleaning by running high-pressure water, then introducing a rotary air-operated tube cleaner through the tubes of the MSR to unblock the tubes without damaging them. The last step of mechanical pre-cleaning is a final rinse with high-pressure water to eliminate all the residues resulting from mechanical cleaning and to facilitate the next step of chemical cleaning.
The second step refers to a chemical cleaning at low temperature (around 65°C) by using different combination of acids, preferably organic ones (citric acid, ascorbic acid, formic acid), and at low concentration, in order to dissolve the deposits from the inner side of the tubes. The chemical cleaning is carried out by recirculating the solution through the tube systems. The mechanical pre-cleaning must be prior to the chemical cleaning to facilitate the contact and the circulation of the chemical solution over the surfaces to be cleaned.
The reaction of dissolution of magnetite by citric acid (C6H8O7 = AH3) is based on the following equation [1]:
Fe3O4 + 4AH2 - ? 2Fe(III)-A+Fe(II)-A-+4H2O+A3-
AH2 - and A3- are chemical species resulting from citric acid (AH3), but the following acid-base reactions:
AH3 ? AH2 - + H+ pKa1 = 3.15
AH2 - ? AH2- + H+ pKa2 = 4.77
AH2- ? A3- + H+ pKa3 = 6.40
The chemical cleaning operation also includes the steps of passivation by introducing a passivating agent to create a thin oxide layer on the metallic surfaces to prevent posterior corrosion. A final rinsing step at alkaline pH is then carried out to eliminate all the passivation residues and to leave the MSR in good operational condition.
Procedure description
Before starting mechanical pre-cleaning, the MSR is sealed off temporarily with pipe end and pneumatic plugs, in order to isolate the system (Figure 1).
The first step of the process consists of checking for leaks by filling the system with water to a certain pressure. The leak test is passed when there is no leak after 20 minutes under pressure.
The second step is mechanical pre-cleaning. Each tube is cleaned with a high-pressure water jet (100-200 bar) from an electrical hydrodynamic unit (GHDE). This operation also enables creation of a map of clogged tubes.
The next step of the mechanical pre-cleaning process consists of using a rotary air-operated tube cleaner made of a flexible shaft, and a cleaning tool mounted to the tip of the flexible shaft. The unit feeds water and air through the casing to the cleaning tool, flushing out deposits, while the flexible shaft snakes through each tube. A final rinse with high-pressure water (400-600 bar) is carried out using the electrical hydrodynamic unit.
In the third step of the process, after a leak check (at 30°C), chemical cleaning with a recirculated aqueous cleaning solution ('acid phase') is introduced into the system.
The cleaning solution used to dissolve the magnetite layers contains citric acid, formic acid, a corrosion inhibitor and other chemicals. In order to improve the cleaning efficiency, the cleaning solution is heated to 65°C. This is done by an electrical heat exchanger connected in parallel with the chemical cleaning system.
During the chemical cleaning process, analytical controls are carried out periodically (at least each hour) to follow the following parameters:
- Temperature
- Acidity
- pH
- Ferric iron concentration
- Total iron concentration
- Inhibitor efficiency
The end of the acid phase is determined by the criterion of total iron concentration <14 g/L and by the stability of four measurements of total iron concentration. If the total iron concentration becomes = 14 g/L or saturation occurs (practical experience shows iron saturation occurs around 14 g/L), a second acid phase should be carried out. A rinsing phase is then carried out to eliminate all the acid residues and to prepare the system for the next step.
The final step of chemical cleaning is passivation, which is also carried out with recirculation. The aim is to prevent corrosion of the tubes' metallic surfaces by creating a thin oxide layer which protects the surfaces.
The passivating solution contains hydrogen peroxide and ammonia. The system is first filled with water, and is heated to 55°C. Ammonia is added until a pH ranging between 9.5 and 10 is reached. Hydrogen peroxide is then introduced.
During the passivation process, analytical controls are carried out periodically (at least each hour) to monitor the following parameters:
- pH
- Temperature
- Ferric Iron concentration
- Total Iron concentration
- REDOX potential
Three hours later, the solution is drained out of the MSR and the system is rinsed with alkaline solution (pH around 9.5) until it reaches a conductivity variation below
30 µS/cm. In order to leave the MSR in an operating condition and to prevent corrosion by humidity, a drying process is carried out by means of a dehumidifier unit until reaching a humidity level lower than 40%.
Implementing this method on site has removed between 100 and 110 kg of magnetite without damaging the tube material (corrosion less than 10 microns)[2]
Results
This procedure has been applied successfully in 14 MSR type heat exchangers in Fessenheim and Bugey nuclear power plants in France between 2009 and 2011. The results obtained during the various MSR cleaning campaigns allowed the validation of the elimination of magnetite deposits. The total quantity of eliminated magnetite is about 100 kg per MSR, as shown in Table 1. The process generated 70 m3 of waste water per MSR, which was managed by an authorized waste management company. On sample coupons placed in the interior of the top of the MSR tube bundle, the loss of thickness caused by the cleaning process averaged about 10 µm. The load after cleaning (a measurement of delta P after restarting the unit) was comparable to new equipment.
| Year | Plant | Reactor | Number of MSRs | Duration |
|---|---|---|---|---|
| 2009 | Fessenheim | unit 2 | 2 | 3 weeks |
| 2009 | Bugey | units 2&3 | 6 | 5 weeks |
| 2011 | Bugey | unit 4 | 3 | 3.5 weeks |
| 2011 | Bugey | unit 5 | 3 | 3.5 weeks |
| Acid phase 1 (7 hr) | Acid phase 2 (5 hr) | Passivation (2 hr) | Total | |
|---|---|---|---|---|
| Dissolved Fe in ppm | 14600 | 3200 | 2650 | |
| Dissolved Fe in kg | 87.6 | 19.2 | 15.9 | |
| Fe dissolved by corrosion | 21.85 | 15.6 | 6.24 | |
| Fe as deposit | 65.75 | 3.6 | 9.66 | |
| Removed deposit (kg of magnetite) | 90.88 | 4.97 | 13.35 | 109.2 |
Conclusions
EDF completed a follow-up of the fouling of the MSR by measuring the pressure delta (?P). We noticed that the plugging of the bundle begins to be significant from a threshold of ?P of 2.5 bar; the maximum ?P to avoid generation of plastic distortion, and thus MSR damage, is 3.4 bar. It can take four to six years to go from ?P of 2.7 bar to 3.4 bar. According to the maintenance strategy envisaged by EDF, when a MSR crosses the threshold of ?P > 2.5 bar, a cleaning operation is scheduled for the most convenient shutdown less than six years away. The possibility of cleaning all the unit's MSRs in the same outage will be considered.
The cleaning process designed and developed by LAINSA and SOLARCA for mechanical and chemical cleaning the MSR tube bundle has been demonstrated to totally eliminate tube-side deposits and deposits in MSR tube bend regions. This process has been retained in EDF's MSR maintenance strategy.
References
[1] 'Química para centrales termoeléctricas,' Asinel, Diciembre 1981, p 819
[2] Report 08/2495 : Metallographic analysis of inner tubes of MSR after mechanical and chemical cleaning- ARGOS
[3] ETV MSR FES2: Synthèse des résultats des expertises télévisuelles mises en oeuvre sur le 2 GSS 001ZZ après nettoyage mécanique et chimique sur FESSENHEIM tranche 2
José T. Ruiz, Logistica y Acondicionamientos Industriales SAU (LAINSA), C/Belgrado, 6, 28232 Las Rozas, Spain. Patrice Guerra, LAINSA France, 59 Avenue André Roussin, ZAC Saumaty Séon 13016 Marseille, France. Cristina Carreres, SOLARCA, C/ De la Quimica no 3. Poligono Industrial Xalamec, Parcela C1 La Selva del Camp. 43470 Tarragona, Spain.
