LCC7 SPECIAL TOPIC REPORT PWR/VVER Primary Side Coolant Chemistry Volume I – Technical Basis and Recent Discussions
PWR/VVER PRIMARY SIDE COOLANT CHEMISTRY – VOLUME I
PWR/VVER Primary Side Coolant Chemistry Volume I Technical Basis and Recent Discussions Authors
Rolf Riess Neunkirchen, Germany
Suat Odar Erlangen, Germany
Jan Kysela Prague, Czech Republic
Francis Nordmann Beauchamp, France Reviewed by
Francis Nordmann Beauchamp, France
© December
2011
Advanced Nuclear Technology International Analysvägen 5, SE-435 33 Mölnlycke Sweden
[email protected] www.antinternational.com
PWR/VVER PRIMARY SIDE COOLANT CHEMISTRY – VOLUME I
Disclaimer The information presented in this report has been compiled and analysed by Advanced Nuclear Technology International Europe AB (ANT International®) and its subcontractors. ANT International has exercised due diligence in this work, but does not warrant the accuracy or completeness of the information. ANT International does not assume any responsibility for any consequences as a result of the use of the information for any party, except a warranty for reasonable technical skill, which is limited to the amount paid for this assignment by each LCC program member.
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PWR/VVER PRIMARY SIDE COOLANT CHEMISTRY – VOLUME I
Contents Preface
V
1
Introduction (Rolf Riess)
1-1
2
Background Information (Rolf Riess)
2-1
2.1 2.1.1 2.1.2 2.1.3 2.2
2-3 2-3 2-3 2-4 2-6
3
4
5
Areas of concern – Actual status Material degradation Fuel performance Radiation field control Areas of concern in VVER units
Design and materials used in RCS
3-1
3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.2 3.2.1 3.2.2 3.2.3 3.3 3.4
3-1 3-3 3-4 3-5 3-7 3-10 3-10 3-10 3-12 3-18 3-18 3-20
PWR plant design (Rolf Riess) Reactor Pressure Vessel Main coolant pump and fuel assemblies Steam Generators CVCS Evaluation of the PWR design PWR RCS materials (Rolf Riess) Reactor Pressure Vessel and Low Alloy Steels Material impact on activity build-up Evaluation of PWR RCS materials VVER plant design (Jan Kysela) VVER plant material (Jan Kysela)
Formation of protective layers and formation of crud (Rolf Riess)
4-1
4.1 4.2 4.3 4.3.1 4.3.2 4.3.3
4-1 4-4 4-25 4-28 4-42 4-43
Material integrity Metal release Activity build-up Activity build-up by activated CPs Hot Functional Tests Evaluation of Section 4.3
PWR reactor coolant chemistry (Suat Odar)
5-1
5.1 Background Information 5.2 Recent discussions to improve coolant chemistry 5.2.1 Control of material degradation 5.2.1.1 Dissolved hydrogen 5.2.1.1.1 PWSCC Crack Growth Rate (CGR) 5.2.1.1.2 PWSCC crack initiation 5.2.1.1.3 Discussions and recommendations 5.2.1.1.3.1 Discussions 5.2.1.1.3.2 Recommendations 5.2.1.2 Lithium and coolant pH value 5.2.1.2.1 PWSCC crack initiation 5.2.1.2.2 PWSCC CGR 5.2.1.2.3 IASCC in stainless steels 5.2.1.2.4 Discussions and recommendations 5.2.1.2.4.1 Discussions 5.2.1.2.4.2 Recommendations
5-1 5-2 5-3 5-4 5-5 5-10 5-14 5-14 5-19 5-20 5-20 5-22 5-24 5-25 5-25 5-26
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5.2.1.3 Zinc 5.2.1.3.1 PWSCC crack initiation 5.2.1.3.2 PWSCC CGR 5.2.1.3.3 Discussions and recommendations 5.2.1.3.3.1 Discussions 5.2.1.3.3.2 Recommendations 5.2.2 Radiation field control 5.2.2.1 Dissolved hydrogen 5.2.2.1.1 Field experience 5.2.2.1.1.1 Tsuruga-2 investigation: 5.2.2.1.1.2 Information from Ikata PWR plant 5.2.2.1.1.3 Beznau investigation 5.2.2.1.2 Discussions and recommendations 5.2.2.1.2.1 Discussions 5.2.2.1.2.2 Recommendations 5.2.2.2 Lithium an coolant pH value 5.2.2.2.1 Field experience with different lithium/pHT control strategies 5.2.2.2.2 Discussions and recommendations 5.2.2.2.2.1 Discussions 5.2.2.2.2.2 Recommendations 5.2.2.3 Zinc addition 5.2.2.3.1 Discussions based on recent field observations 5.2.2.3.2 Recommendations 5.2.3 Control of fuel performance 5.2.3.1 Improvements in fuel cladding materials 5.2.3.2 Fuel cladding degradation 5.2.3.2.1 Cladding corrosion 5.2.3.2.2 Cladding HPU and embrittlement: 5.2.3.3 Influence of coolant chemistry on fuel cladding degradation 5.2.3.3.1 Dissolved hydrogen 5.2.3.3.2 Lithium and coolant pH Value 5.2.3.3.2.1 Lithium and boric acid influence on corrosion of zirconium alloys 5.2.3.3.2.2 Recent field data with elevated lithium chemistry 5.2.3.3.2.3 Recommendations 5.2.3.3.3 Zinc 5.2.3.3.3.1 Field experience: Influence of Zn on fuel clad corrosion 5.2.3.3.3.2 Recommendations 5.2.3.4 Axial offset anomalies/Crud induced power shift 5.2.3.4.1 General information 5.2.3.4.2 Influence of water chemistry parameters 5.2.3.4.2.1 Dissolved hydrogen 5.2.3.4.2.2 Lithium and coolant pH value 5.2.3.4.2.3 Zinc 5.2.3.4.3 Recommendations 5.2.4 Conclusive summary 5.2.5 Recommendations 5.2.5.1 PWR plants with nickel base alloys sensitive to PWSCC in RCS 5.2.5.2 PWR plants with iron base alloys or nickel base alloys non-sensitive to PWSCC in RCS
5-26 5-26 5-29 5-30 5-30 5-32 5-33 5-35 5-36 5-36 5-38 5-39 5-39 5-39 5-40 5-40 5-40 5-46 5-46 5-52 5-52 5-53 5-57 5-57 5-57 5-58 5-58 5-62 5-65 5-65 5-70 5-70 5-74 5-79 5-79 5-81 5-90 5-90 5-90 5-93 5-94 5-95 5-95 5-97 5-97 5-99 5-100
Influence of impurities in the reactor coolant (Rolf Riess)
6-1
6.1 6.1.1 6.1.2 6.1.3 6.2 6.3
6-3 6-3 6-4 6-6 6-7 6-8
Material degradation Modes of corrosion SCC and IASCC mechanism Influence of impurities other than chloride on SCC Influence on fuel cladding integrity Radiation field
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7
8
9
VVER reactor coolant chemistry (Jan Kysela)
7-1
7.1 7.2 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.4 7.4.1 7.4.2 7.4.3 7.4.4 7.4.5 7.4.6
7-1 7-3 7-5 7-5 7-8 7-10 7-13 7-15 7-15 7-16 7-18 7-19 7-20 7-22
Basics of VVER coolant chemistry Control of radiolysis Recent discussions to improve VVER coolant chemistry Fuel cladding corrosion Radiation build-up Antimony and silver behaviour Transportation and impact of crud in RCS Conclusions and recommendations regarding recent discussions KOH as pH control agent Ammonia as a contributor to the pH Influence of ammonia on potassium behaviour in ion exchange resins Hydrogen to replace ammonia as H2 source Zinc addition and possible consequences Sampling technology and location at VVER
Summary, conclusions and recommendations (Rolf Riess, Suat Odar, and Francis Nordmann)
8-1
8.1 8.2
8-1 8-2
Design and materials Chemistry and corrosion
9-1
References
Nomenclature Unit conversion
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PWR/VVER PRIMARY SIDE COOLANT CHEMISTRY – VOLUME I
Preface This is Volume I of Pressurized Water Reactor (PWR) and Voda Voda Energo Reactor (VVER) Reactor Coolant Chemistry which will be followed in LCC8 in 2013, by Volume II. In order to ensure the integrity of the Reactor Coolant System (RCS) materials, the fuel cladding behaviour and performances, maintaining the trend towards a reduced radiation field and minimizing the waste releases, require the continued optimization of reactor coolant chemistry. The optimization of coolant chemistry by Utilities is important in the light of the trend towards extended fuel cycles, higher duty core, increasing stringent dose rate control, decreasing the refuelling outage duration, and reducing operating cost. This document is intended to provide a detailed description of the PWR/VVER Primary Side Coolant Chemistry. Furthermore, it should provide a strong support to the utilities for establishing a responsive plant specific chemistry program. It may also help the Manufacturers and Regulators at having a detailed approach of primary water chemistry and corresponding issues. In the first two Sections, there are introductory remarks and background information which shall describe the existing problems in the operating plants. In Section 3 the design and the materials used in the RCS of PWRs and VVERs are described in detail. This will include an overview about the different material concepts which are used for the construction of the plant. In Section 4 the formation of protective layers and the formation of crud are described in detail. This illustration shall demonstrate the importance of the source term for Corrosion Products (CPs) and it shall indicate the possible optimizations. It is followed by a description (Section 5) of the basis of coolant chemistry including the possible measures to control radiolysis and a description of the properties of boric acid, zinc, and potential pH control agents. Connected to Sections 3 and 4, there is an outline of recent discussions in Section 5 to improve coolant chemistry. This includes the latest discussions in the nuclear industry regarding material degradation as it is influenced by the dissolved hydrogen, lithium and zinc concentrations. Section 5 also includes a discussion about the latest development of radiation field control and fuel performance. In order to specifically address these recent discussions of the nuclear industry there will be a separate Section with conclusions and recommendations regarding the recent discussions. In Section 6, the influence of impurities in the reactor coolant on material degradation, radiation field build-up and fuel performance are discussed in detail. This discussion is followed by a description of the VVER reactor coolant chemistry (Section 7), whereby previous descriptions of PWR plants shall be considered. Finally, Volume I is ending with a summary, conclusions and recommendations (Section 8). This current Special Topical Report is built on previous ANT International Reports, for example in LCC1 there are various contributions [Lundgren et al, 2005] to the reactor coolant chemistry with the following content:
Coolant quality and control issues,
Materials selection for the primary circuit,
Primary circuit corrosion,
Dose rate build-up and control,
Fuel/Water chemistry interaction.
This document is followed by ANT International LCC2 Report [Riess et al, 2006] dealing with the following relevant topics:
Coolant Quality and Control Issues
Dose-rate build-up and Control
Effect of Water Chemistry
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In the LCC3 Annual Report there is only one contribution to Reactor Coolant Chemistry by [Riess et al, 2007]: “Effect of Water Chemistry on Fuel Cladding”. This STR is followed by the LCC4 Annual Report, 2008 [Riess et al, 2008] which focused on the plant experience in the US by Wood in Section 2.1 and in Europe by Riess in Section 2.3. It also contains a Review of: “Water Chemistry Sampling and Monitoring, PWR, VVER, and BWR (Primary side)” by Odar in Section 3. This LCC4 document includes also an update on Fuel/Water Chemistry Interaction by Riess in Section 6. The LCC5 Annual Report of 2009 [Riess et al, 2009] contains short descriptions of the Primary Side Coolant Chemistry by Riess in Section 2 and the VVER Primary Side Coolant Chemistry, which is a contribution by Kysela in Section 3. In 2010 there was no LCC Annual Report because the subjects of interest are covered in Special Topical Reports, one of these Reports being entitled: “Effect of Zinc in BWRs and PWRs/VVER on Activity Build-up Intergranular Stress Corrosion Cracking (IGSCC) and Fuel Performance” [Odar et al, 2010]. In the current STR, all the previous data shall be recognized and discussed under a specific aspect, which is a communication between ANT International and some PWR utilities that acquires detailed answers to urgent questions arising in the operation of PWR plants. The expectation is that the detailed answers will also cover the concerns of operating PWR plants worldwide. The results of this evaluation will be summarized and recommendations can be given. This Volume I will be followed in 2013, with LCC8, by Volume II.
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PWR/VVER PRIMARY SIDE COOLANT CHEMISTRY – VOLUME I
1
Introduction (Rolf Riess)
The objective of a water chemistry control strategy has been to optimize the chemistry between the often conflicting requirements of controlling materials degradation, avoiding fuel performance issues and minimizing radiation field [Nordmann et al, 2010]. The requirements and the available tools for corrective actions are illustrated in Figure 1-1.
Figure 1-1:
ANT International’s view for chemistry optimizations.
According to [Nordmann et al, 2010], the objectives still remain today. However, the emphasis on each of the three constraints has changed over time. As will be described, the early emphasis in PWRs was to minimize build-up in radiation field in out-of-core areas. This period was followed by concerns over fuel crud problems. However, after improving the cladding materials this concern decreased. Due to subsequent increases in fuel cycle duration, the move in the past 10-15 years to greater fuel duty, increased the Sub-cooled Nucleate Boiling (SNB) again and continued to be challenging issues for PWR/VVER chemistry. Recently materials degradation concerns have received higher priority, following problems with primary side Stress Corrosion Cracking (SCC). Since there is a continued effort in the nuclear industry to optimize Reactor Coolant Chemistry, this STR will describe the actual situation and will provide recommendations for future plant optimizations. Such optimizations must be plant specific.
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Background Information (Rolf Riess)
In the US there are currently 70 PWR units in operation, build by the following NSSS vendors: Westinghouse (49 units), Combustion Engineering (CE) (14 units), and Babcock and Wilcox (B&W) (7 units). There are differences in the design of these units whereby the most notable is the use of Once-Through Steam Generators (SGs) in B&W plants. The feature of CE-units is the size of the SGs, which offer twice of the heat transfer surface compared with W-units. However, these design differences of the RCS have no influence on the chemistry issues to be discussed. In Western Europe 90 PWR units are in operation, build by Westinghouse and its former licensees Siemens and Framatome (now AREVA). VVER plants can still be found in Russia, Ukraine, Hungary, Slovakia, Czech Republic, Bulgaria, Armenia, Finland and China and the total number of units is 63. The 440 MWe plants are contributing to 36 units and the 1000 MWe stations to 27 units. These designs of PWRs differ appreciably in their materials of construction and generally all adopt somewhat different philosophies in the specification of the primary circuit chemistry for nominal power operation and in the methods of start-up and shutdown. In the case of the VVER-440 units the basic layout and operating temperature is also different. This STR reviews the materials inventories and important design features of the different types of reactor, describes the basic processes of corrosion, corrosion release and activation occurring in the primary circuits of these reactors and, finally, reviews the primary circuit chemistry specifications and start-up/shutdown chemistry requirements of the different reactors. Reactor Coolant Chemistry has grown over the years by solving the problems emerging during the operation of the plants, e.g. Activity Build-up. This was the first severe issue that had to be resolved by the early PWRs.
Figure 2-1:
Transport and activation of CPs in PWR primary systems, after [Wood, 2008].
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Figure 2-1 illustrates the activity build-up mechanism which can be described as follows: From out-of-core surfaces, CPs are released especially from SG tubing surfaces. This “source-term” problem will be discussed in detail in Section 4. These CPs are transported to the reactor core where they are deposited and activated. After a re-dissolution they are transported back to the out-of-core surfaces where they are integrated and lead to increased radiation fields. In order to mitigate this phenomenon, hydrogen over-pressure was introduced to the RCS, to compensate for the radiolysis and the SCC risk in presence of oxygen, while the pH of the coolant was adjusted by the use of LiOH. The details of these countermeasures will be discussed in Section 5 of this STR. Beside the activity build-up, the plant operation had shown that there are two additional issues which created concerns. These are: Primary Water Stress Corrosion Cracking (PWSCC) and Fuel Integrity. The concerns existed not only in the early operation of PWRs and VVERs, they still exist today. Figure 2-2 may illustrate the PWR primary chemistry changes at US plants as shown by [Wood, 2008].
Figure 2-2:
PWR primary chemistry changes at US plants [Wood, 2008].
The major reason that the issues of concern are still existing today is that PWR Primary Chemistry has become complicated by demands of longer fuel cycle, increased sub-cooled boiling, and higher burn-up. An additional parameter to be considered in the optimization processes is the existence of CPs which can be a problem due to its volume/mass of material and the complex chemical composition of the participating elements. From the fuel perspective, the resulting area of concern is Axial Offset Anomaly (AOA) or Crud Induced Power Shift (CIPS). This phenomenon occurs preferentially in US plants using Ni-based alloys as SG tubing material. From a plant operational view,, material integrity and radiation exposure may have a higher priority than AOA or CIPS. In a separate Section, this CRUD problem will be discussed in more detail.
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2.1
Areas of concern – Actual status
2.1.1
Material degradation
In LCC4 AR, Wood and Riess covered the material degradation area. They stated that the RCS design is of negligible influence on primary side chemistry, whereas the selection of the material has a major impact. An example is the difference in the material selection in the US, and in Europe (i.e. Germany). The preferred materials in US primary systems is the use of 304 or 316 type stainless steels in combination with Ni-based Alloys for SG tubing (Alloy 600 then 690). The German concept prefers the 347 type stainless steel in combination with Alloy 800NG, which is immune against PWSCC as the latest US material, Alloy 690. Another important issue is the source of Co-60 coming from activation of Co-59. There are two most important sources of Cobalt, i.e. the use of Stellites as hard-facing materials containing a large proportion of cobalt but present in small locations and Cobalt-59 as an impurity in the austenitic stainless steel and in the SG tubing material, present in small proportion but in large components. The major countermeasure against Co-60 formation is the removal (stellites) and minimization (impurity) of Cobalt from the construction materials.
2.1.2
Fuel performance
A general plant operating experience emerged when fuel duty was increased. It is the observation that the margins to tolerate crud have to be reduced. Nevertheless, it is also obvious that crud is not a prime reason for fuel integrity problems, as can be seen from Figure 2-3.
Figure 2-3:
US PWR fuel failures by mechanism [Wood, 2008].
However, there is an additional phenomenon called AOA, which created concerns during the past 10 years. Axial Offset is a measure of the relative power produced in the upper and lower part of the core and is normally expressed in %. A positive percentage indicates that more power is produced in the upper part of the core. This problem occurs, when the boron concentration in crud in the upper part is increased because the fuel assemblies undergo nucleate boiling. This mechanism is causing a reduction in the neutron flux. An example is shown in Figure 2-4.
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Design and materials used in RCS
3.1
PWR plant design (Rolf Riess)
This subject was already discussed to some extent in the LCC1 Annual Report see Strasser in Section 3 in [Lundgren et al, 2005]. In accordance with this document, the objective of this Section is to identify the materials used for the major components of the primary coolant circuit. In particular the reasons for this selection of the materials are discussed under the aspect of minimizing the Co-60 source term. This subject is discussed here in order to separate from the discussion about the Ni-source-term which is the main subject of Section 4. The RCS constructing materials were selected based on the Plant operating experience at the time of construction starting with the information available in the early 1960s or 1970s. They were considered to perform for the life of the reactor, but for a variety of reasons their integrity was impaired and they had to be replaced or repaired. The major reason for such actions was corrosion behaviour of the selected materials, i.e. general corrosion, erosion corrosion, SCC, and Irradiation Assisted Stress Corrosion Cracking (IASCC). In addition, these types of corrosion were accelerated by specific types of impurity, changes in water chemistry practices, enhanced stress levels or sensitization. One consequence out of such material behaviour is to replace the component which is a costly and time consuming process. Because an identification of all materials used in the RCS of the various NSSS vendors is not possible, this Section is focused on the major components. A typical primary system arrangement is illustrated in Figure 3-1which shows the reactor vessel and his internals containing the fuel core, the piping of the coolant research system, the pressurizer, steam-generators, pumps and valves.
Figure 3-1:
Primary coolant system for a large PWR [Cohen, 1985].
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The primary system operates at a pressure of 2.250 PSIA (15.5 MPa) and the pressure boundary consists of the Reactor Pressure Vessel (RPV), the recirculation piping, pressurizer, SG tubing, and pumps as shown in Figure 3-1. There are heat transfer surfaces within the system that consist of the fuel cladding transferring heat to the primary coolant (approximately 25% of the RCS surface) and the SG tubing transferring heat to the secondary system (about 65% of the total RCS surface). The gamma heating of all the components within the reactor vessel is relatively minor but is also removed by the coolant.
Figure 3-2:
PWR primary system – Pressure boundary designed by Westinghouse [Garbett, 2006].
The pipework of each loop is normally sub-divided into a hot leg (RPV to SG), a crossover or intermediate leg (SG to Reactor Coolant Pump (RCP)) and a cold leg (RCP to RPV). A pressurizer is connected to the hot leg of one of the loops via a surge line and a spray line is taken from the cold leg of the same loop; an auxiliary spray line is taken from the cold leg of a second loop. Typical operating conditions are Thot 325/329°C, Tcold 292/293°C, 15.5 MPa (155 bar).
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3.1.1
Reactor Pressure Vessel
The key component in the RCS is the Reactor Pressure Vessel (RPV). In Westinghouse and Framatome Reactors the Reactor Pressure Vessel, the pressurizer and the SG channel heads are made from low alloy carbon steels (type A-508 or equivalent and the SG channel heads from carbon steel (type A-216) or low alloy carbon steel (type A-508) all weld clad internally with stainless steel, see Table 3-1 and Table 3-2. The clad usually has an inner layer of type 309L SS and an outer layer of type 308L SS. Weld deposited type 308L SS is equivalent to type 304L. The main coolant pipework, reactor internals, main coolant pumps bowls and pump internals and the Chemical and Volume Control System (CVCS) and RHR system pipework are made from various grades of types 304 or 316 non-stabilized stainless steels. In Siemens designed reactors the Reactor Pressure Vessel, pressurizer, the main coolant pumps, the loop pipework and the SG channel heads are made from Low Alloy Steels (LAS) (types 22NiMoCr37, 20 MnMoNi55 18NiMoCr37 or equivalents), all weld clad internally with the Niobium stabilized stainless steel 1.4550 (X10 CrNiNb 18 9, equivalent to 347SS).
Figure 3-3:
PWR pressure vessel internals, see Strasser in Section 3 in [Lundgren et al, 2005], courtesy by Westinghouse.
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Formation of protective layers and formation of crud (Rolf Riess)
4.1
Material integrity
For a safe and economic operation of a NPP it is mandatory to minimize:
The risk and occurrence of SCC,
the exposure of the staff and other people to ionizing radiation,
the risk of fuel deposits and the consequences thereof e.g. AOA.
Key factors allowing to achieve this goal are associated with the protective layers and the CPs in the primary coolant of both PWRs and VVERs. However, these CPs play an ambivalent role in the plant operation: On one hand, the CPs are forming tenacious oxide films on the structural materials and thereby assure the integrity of the systems. On the other hand, CPs are released to the coolant, where they are transported to the core, activated, re-dissolved and re-deposited on out-of-core surfaces. Thus, they are contributing to an enhanced risk for the occurrence of the three main issues mentioned above. In order to illustrate the importance of the protective layers and crud for the operation of power plants, the model of [Clauzel et al, 2010] has been selected.
Figure 4-1:
Cross-section of the oxide layer on nickel-base alloys [Clauzel et al, 2010].
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According to Figure 4-1 the surface film is formed due to an oxidation reaction at the interface between the base metal and the coolant. This surface film can be separated into the “Inner oxide layer” and the “Outer oxide layer” forming together the so-called duplex oxide layer. The inner layer is compact and protective and can be divided in two sub-layers. The first sub-layer consists mainly of Cr2O3 and is often called “barrier oxide layer” and has a thickness of a few nanometres. The second sub-layer consists of mixed spinel oxides and may be described by the following formula (Ni i-x Fe x)(Cr 2-y Fe y)O4.
Figure 4-2:
Illustration of metal/metal oxide interface on nickel base Alloy 600 at different dissolved hydrogen concentrations (5, 15 and 25 cc/kg2) [Molander et al, 2009b].
An example for providing more insight into the oxidation of Ni-base alloys in PWR water is given by [Combrade et al, 2005]. They are summarizing information on the oxide layers and the associated damage to the base metal. This subject has been of specific interest to AREVA, supporting for several years studies on the oxidation of Alloys 600 and 690, due to the following motivation:
The radioactivity of the primary circuit is determined by the cations released by the corrosion of the SG tubes and
the oxidation process is at the origin of the initiation of IGSCC in Alloy 600, 82 and 183.
2 cubic centimetre (per kg): used for H2 concentration in RCS = ml/kg (under normal pressure and temperature)
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In this sense several PhD theses where summarized by [Combrade et al, 2005] with the following main findings:
The oxide is made up of three layers:
An internal layer of a few nm of Cr2O3 which is the main protective layer.
An intermediate layer of a spinel type, Cr rich oxide containing both Ni and Fe whose protective function is not clear.
An external layer made of Ni ferrite that forms only in metal cation saturated environments. The composition and thickness is strongly dependent on the hydrogen concentration in water.
On bare metal surfaces, the oxidation exhibits several growth stages typical for Ni base Alloys:
The Cr2O3 internal layer is formed very rapidly and its thickness increases with the Cr content of the Alloy.
The oxide growth stops for a period of time probably due to a temporary lack of Cr at the metal/oxide interface.
The oxide layer starts growing again with an oxidation rate controlled by ion transport through the inner oxide layer.
The base metal suffers at least two types of damage that probably involve accelerated mass transport due to vacancy injection from the metal/oxide interface:
Intergranular penetration of oxygen.
Selective oxidation of Cr on the whole surface.
The formations of oxide layers were determined on Alloys 600, 690 and 800 and on pure Cr in the same primary coolant at 325°C and at a partial pressure of hydrogen of 30 kPa that corresponds to the maximum susceptibility to PWSCC. With these coupons further findings were made as for example:
The formation of the Cr2O3 layer is formed in very early stages of oxidation (< 1 min).
The thickness and composition of the inner oxide layer depends on the nature of the base metal and the corrosion potential.
On Alloy 690 the thickness is almost independent of corrosion potential and is thinner than on Alloy 600.
The thickness of the inner oxide layer depends on the surface condition, e.g. electro-polished surfaces have thinner oxide layers compared to mechanical polished surfaces. Summarizing the findings related to the Cr-rich inner layer, it has to be stated, that it is not clear, whether the protective character of the oxide is only due to the thin Cr2O3 sub-layer or if the Cr-rich spinel layer also has a protective character and it is also not clear whether these films are porous as claimed by some authors.
An external Cr-free layer does not contribute to the protection of the base metal and contains only Fe and Ni. Their nature and composition depends on the surface condition of the base metal and the saturation of the aqueous environment in Fe and Ni cations.
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PWR reactor coolant chemistry (Suat Odar)
5.1
Background Information
Demineralized water is used as reactor coolant in the PWR plants to moderate the fast neutrons to thermal neutrons, which are needed for the nuclear fission reaction to produce energy in the core. Another function of the reactor coolant is to transport the heat of nuclear fission energy produced in the core to SGs. In addition boric acid is added to the reactor coolant as chemical shim to control the core reactivity. Based on field experience gained during the early PWR operation in the 1960s at several research and power plants, an operation with reactor coolant without further chemical treatment may result in several serious problems with respect to safe plant operation, radiation field control and compatibility of the structural materials. This is due to the fact that coolant is exposed to radiation field in the core, where it decomposes by radiolysis. As a result of this, radiolysis products are generated, some of which are extremely strong oxidants and can jeopardize the compatibility of the structural materials by corrosion. Oxidizing conditions can cause enhanced fuel cladding corrosion or PWSCC in Alloy 600MA that is used as SG tubing material or in numerous reactor penetrations in many PWR plants designed and constructed by US vendors or their licenses worldwide. In addition oxidizing conditions enhance CP (so called “crud”) deposition on fuel rods, which may cause crud induced fuel clad corrosion and increase the radiation field exposure. In addition to these problems that are created solely by coolant radiolysis, the insufficient alkalinity of the reactor coolant, which would happen if only boric acid would be added to coolant as chemical shim, similar problems again with respect to fuel performance and radiation field control cannot be prevented. This is because, boric acid as a weak acid, undergoes polycondensation processes with increasing temperatures. Thereby the pH-value is increasing, but this pH increase is not sufficient to minimize the metal release and corrosion rates of the structural materials at operating temperatures. As confirmed by old-field experience in the 1960s and early 1970s, the result is the heavy deposition on fuel rods with possible crud induced fuel clad corrosion and radiation field increase. Therefore, application and control of coolant chemistry is indispensable by adding additional chemicals to avoid the problems explained above. The objective of the coolant chemistry in PWR plants for safe plant operation is in detail to:
Avoid the radiolytic decomposition of the reactor coolant (suppression of the water radiolysis),
Avoid or minimize the crud deposition on fuel rods.
Maintain the fuel integrity without jeopardizing its compatibility.
Protect the integrity of RCS by maintaining the corrosion compatibility of the structural materials.
Control the radiation field at low levels.
To fulfil these objectives, alkaline water chemistry under reducing conditions is selected for the reactor coolant. For this purpose, hydrogen gas is added to the reactor coolant to suppress the coolant radiolysis (the net decomposition of the water), which also provides sufficient reducing conditions. Adequate alkalinity is achieved by adding depleted Lithium-hydroxide (6Li depleted LiOH; i.e. 7LiOH). Since the last fifteen years, many PWR plants are now also injecting in addition zinc to the RCS with the aim to reduce the radiation field exposure and/or to improve the PWSCC resistance of the Alloy 600MA/TT if that is used as SG tubing material and for reactor system penetrations.
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This Section is presenting in detail the recent discussions that are going on in the PWR industry to modify the coolant chemistry with the aim to improve the material compatibility of the nickel base alloys used in RCS and to cover the needs raised due to introduction of high duty cores for extended fuel cycles. The discussions consider the effect of chemistry parameter modifications on:
Fuel performance,
Material compatibility, and
Radiation field control.
Finally, all these recent discussions are evaluated and plant design specific recommendations for coolant chemistry are given. Even though similar coolant conditions exist in VVER plants, different chemicals are used to control the VVER coolant chemistry. This is explained in Section 7.
5.2
Recent discussions to improve coolant chemistry
The main objectives of the coolant chemistry is to:
Maintain the integrity of the primary system pressure boundary,
Assure the fuel-cladding integrity with achievement of design fuel performance, and
Minimize out-of-core radiation fields.
In order to fulfil these objectives, an alkaline chemistry under reducing conditions was introduced in the late 1960s and 1970s, as described in Section 5.1. Application of this chemistry was the right step in right direction to enable safe and reliable PWR plant operation by fulfilling the objectives. However, at that time all PWR plants had low duty core designs. Because of economical reasons, the PWR industry was forced since then to push their core designs all the time to higher boiling core duties. This requires significantly higher coolant boron concentration at the Beginning Of Cycle (BOC), so that the required minimum coolant pH300 of 6.9 to avoid excessive crud deposition on the fuel assemblies could not be achieved at BOC by practiced lithium concentration that was limited to 2.2 mg/kg (ppm) due to Alloy 600MA/TT material compatibility and fuel cladding corrosion issues (see Figure 5-1). Hence, the chemistry established in the 1970s was no more optimum to fulfil the objectives and had to be improved with respect to pHT control to avoid the excessive crud deposition on fuel rods and to minimize the radiation fields. In the late 1970s and 1980s PWSCC in nickel base Alloy 600MA SG tubes started to be experienced at the field, which was followed by PWSCC indications and leaks in the Alloy 600 Reactor Pressure Vessel penetrations. This was a serious problem for the PWR plants using nickel base Alloy 600MA and their weld metals Alloy 182 and Alloy 82 in RCS. Therefore comprehensive investigation work was started especially in US PWR industry to establish strategies with the goal of mitigating materials degradation. Besides the selection of adequate material concept for SG tubes, for RCS penetrations and for fuel cladding, coolant chemistry plays a quite important role to avoid or mitigate these problems described briefly above. Especially in the last 10 years, a lot of effort was done to improve the coolant chemistry by optimizing the control of its key parameters: pHT, lithium and dissolved hydrogen concentrations, and zinc addition. In the following Sections the recent discussions of the PWR industry in the last 10-15 years in this area are summarized and the laboratory results with field experience gained are discussed with respect to material compatibility (Section 5.2.1), radiation field control (Section 5.2.2) and fuel performance (Section 5.2.3). Finally conclusions and recommendations are given in Section 5.2.4 for PWR plants considering the RCS design and material concept; i.e. use of nickel or iron base alloys.
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Figure 5-1:
5.2.1
Schematic description for influence of coolant lithium concentration on fuel cladding corrosion and on PWSCC of Alloy 600MA, after [Riess & Millet, 1994].
Control of material degradation
Within the almost 50 years of PWR operation there were worldwide severe material degradation problems in RCS only with nickel base Alloy 600 that was used in many PWRs designed and/or licensed by US vendors (Westinghouse, CE and B&W). Beside this, to some small extent fuel cladding corrosion was also experienced, which will be discussed with respect to the influence of coolant chemistry (Section 5.2.3). The PWR plants that avoided using nickel base Alloy 600 with its weld materials, like Siemens-KWU3 designed PWRs and some CANDU4 plants, are free of such material degradation problems in their RCS. In the following Sections the influence of coolant chemistry parameters on the PWSCC in nickel base alloys (Alloy 600, weld metal Alloys 182, 82 and Alloy X750) is discussed based on the recent results and field experience of last 10 to 15 years.
3 4
Kraftwerk Union (NPP Section of Siemens) Canada Deuterium Uranium type of Reactor
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6
Influence of impurities in the reactor coolant (Rolf Riess)
The areas of concern for water chemistry in the PWR Primary Coolant are described in Section 2 of this STR. They are related to material integrity, fuel performance, and radiation field build-up. Regarding the presence of impurities it can be stated that the impurities are enhancing the concerns in at least one area if not in all three. In order to give an overview on the influence of impurities which may show up in the reactor coolant, the Table 6-1 provides a matrix of issues to be discussed in more detail. Table 6-1:
Overview on the influence of impurities/additives in reactor coolant.
Impurities and additives Chlorides, Fluorides, Sulphates, Oxygen
Areas of concern Remark Material degradation
Fuel integrity
Radiation build-up
PWSCC General corrosion
CPs like Fe, Ni, Cr, Co
Biggest concern regarding impurities Zr corrosion AOA Pressure drop
Special elements like: Sb and Ag
High radiation fields after activation
See Section 4.3,
After activation
Highly activable problem during RFO
Water and Impurities, see Table 4-12
Table 4-9
Tritium and C-14 may become a waste problem
Al, Ca, Mg, SiO2
Cladding corrosion
Organics
Risk: Functional groups
Additives B, Li, H2, Zn
PWSCC enhancement by H2 different for initiation and propagation
Risk of zeolithe formation No problem when pure carbon hydrates
Zr corrosion
Necessity to use Li-7 to avoid Tritium from Li-6 ANT International, 2011
Detailed discussions of the influence of impurities in reactor coolant are made by following the items shown in the overview. However, before starting this evaluation it should be mentioned that the major emphasis is given to the coolant and not to the material side. Among the known impurities the halides and sulphates are the most dangerous substances especially in the presence of oxygen. In all Guidelines they are specified, whereas other impurities are rarely specified but they are measured in many cases. In the present STR not only the impurities shall be discussed but also the chemical additives may be mentioned. The Guidelines describing the quality of the water that is in contact with the materials are e.g. containing several sets of specifications and recommendations. Examples for Guidelines are:
The NSSS vendor’s recommendations and perhaps warranty conditions for the plant materials,
the fuel vendor’s specifications for maintaining the fuel warranties,
the EPRI Guidelines – a good review was made by [Fruzzetti et al, 2004],
the VGB Guidelines,
the EdF set of specifications,
the utility’s internal guidelines that may or may not be identical to some of the above,
the details of the water chemistry are discussed in Section 5.
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At this point only the general operating conditions for the Primary Coolant Chemistry are given as they are described in such Specifications. A reducing atmosphere is maintained in the coolant by the addition of 25-50 cc/kg of hydrogen (H). Oxygen limits have been set at < 2-5 μg/kg (ppb). The pHT is controlled by the balance of the B in the form of boric acid and Li in the form of lithium hydroxide in the range of 6.9 to 7.4. The quantity of soluble B shim in the coolant is adjusted to control the reactivity and has been as high as 1800-1900 mg/kg (ppm) at the beginning of the cycle and decreased approaching zero at the end of the cycle. The Li content varies to control the desired pH program throughout the cycle and the current maximum practice of about 4 ppm is limited by the potential effect of zirconium alloy cladding corrosion with increased Li levels (see Section 5). Additions of Zn are made to control initiation of SCC and to slow down CGR as well. In addition activity transport is reduced to low levels by Zn incorporation in the oxide films of the structural materials and piping, a mechanism that reduced cobalt (Co) and Ni transport from the stainless steels. Levels of 4-5 μg/kg (ppb) Zn are used for this purpose. Higher levels of 10-20 ppb have been used for reducing SCC of SG tubing, but inconsistent results have reduced this practice (see Section 5). Impurities are present for reasons other than a purposeful addition, such as:
Cl, F, SO4, as well as oxidizing agents in ionic forms as the result of leakage from other systems, such as the demineralizers, pump seals, instrument lines, etc.
Fe, Ni, Cr, Co and potentially other metallic materials in solid or solution form, the product of uniform corrosion of the primary circuit materials (see also Table 4-9 for the complete set of activation possibilities).
Al, Ca, Mg, Si from contamination during maintenance work, impurities in the boric acid or Li additions or other sources such as Boroflex spent fuel racks, make up water.
Special radionuclides like Sb-122. Sb-124 and Ag 110m. They are important in the radiation field control.
Organic compounds.
Many of these impurities were not known and taken into account at the time the plant materials were selected and perhaps even if known, the same materials may have been chosen. As a result of experience with these impurities new recommended limits have evolved, some of which are noted below. The impurities with the most significant effect on plant materials corrosion, specifically SCC, are the halides (Cl, F) and the anion SO4. The usual expected value of halides is < 10 or 20 μg/kg (ppb) with an historical limit < 150 (or 100) μg/kg. The reason for such a difference between the target and the limit value is that under normal power operation, there should not be any pollution and the target value is much lower than the historical limit of 150 μg/kg based on the risk of SCC. A limit for SO4 has also been set at similar values as chloride (< 10 ppb or higher). Its appearance seems to occur primarily during shutdown. Limits for the zeolithes (Al, Ca, Mg) have been suggested as 5 ppb to prevent their deposit on the fuel and their potential densification of the crud that would increase the thermal resistance of the crud. It seems that the risk of zeolithe may occur in simultaneous presence of silica and Ca, Mg or Al. Limits on other impurities have not been set; however the transport of Ni and Co for example is being influenced by Zn injection to limit activity transport. The remaining elements will have an effect on fuel crud formation and cladding performance rather than have an effect on plant materials. In addition to the discussion of the influence of impurities in the RCS Coolant, the influence of impurities in the metals shall be mentioned. This discussion shall include the impact of the additives in the primary coolant on RCS materials.
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6.1
Material degradation
This Section has to be seen as an amendment to Section 5.2.1.
6.1.1
Modes of corrosion
General corrosion is not important for system integrity due to the thickness of components required for the pressure resistance. It is an oxide formation on the surface of materials until a steady state condition is reached between oxide film thickness and the general corrosion rate. Some of the CPs will be released and will form deposits on fuel elements. They will be activated, redistributed and deposited on out-of-core surfaces. General corrosion is important for radiation field control and fuel integrity (See Section 4.3). Therefore this issue is described in more detail in the relevant Sections. Chemistry plays an important role for this type of corrosion, especially during the HFT. Intergranular and/or Transgranular SCC of Alloy 600 and austenitic steels (e.g. A 304, 316L) can be caused by oxygen and chlorides as can be seen from Figure 6-1 [Gordon, 1980]. However, under the RCS-conditions of a PWR, which is an oxygen-free and reducing environment, the risk for this type of corrosion is pretty low. Nevertheless, occurrences of oxygen ingress should be avoided as far as possible. In addition, operational experience shows that impurity excursions in PWR primary systems are very unlikely since the pressure in RCS is in most cases higher than the pressure in connected systems, preventing contaminants from entering RCS, even in case of tightness failure.
Oxygen (ppm)
As can be taken from Figure 6-1 the operating range of PWRs is far away from impurity concentrations which are a risk for system integrity.
Figure 6-1:
Chloride and oxygen induced SCC of stainless steel in high temperature water [Gordon, 1980].
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7
VVER reactor coolant chemistry (Jan Kysela)
7.1
Basics of VVER coolant chemistry
Primary coolant circuit of VVER reactors is characterized by the presence of boron in water, introduced in the form of boric acid. Boron concentration varies widely depending on reactor power and fuel burn up. To compensate for the acidic properties of H3B03, strong KOH alkali is introduced. In addition boron irradiated with neutrons produces Li-7 through the following reaction: 10
5
B + 10n 73Li + 42He which may also be written as 10B (n, ) 7Li
To suppress water radiolysis and oxygen formation, an excess of hydrogen is maintained by ammonia introduction and following radiochemical decay of it. Water quality standards for VVER and PWR reactors are presented in Table 4-1 [Kritsky, 1999], Western standards for the primary coolant circuit of PWR-type reactors provide for water chemistry conditions controlled by additives of H3B03,given by reactor physics, LiOH, H2, and N2H4, H202, if necessary. The standards are constantly being improved depending on the plant power; materials selected; and accumulation of data on the mechanisms of corrosion, mass transfer, and radioactivity. Table 7-1:
Parameters Country
Summary of water chemistry standards for the primary coolant circuit PWR Concentrations in mg/kg (ppm), conductivity in µS/cm. EPRI
Westinghouse
VGB
KWU
J-PWR
EdF
VVER 440/1000
VVER 440
New project Russia
USA
USA
FRG
FRG
Japan
France
SU
Finland
0.2-2.2*
0.7-2.2*
0.2-2.2*
0.2-2*
0.2-2.2*
0.6-2.2
*
0.45-2.2**
KOH
—
—
—
2-26.5#
2-22#
NH3
—
—
—
>5
>5
>5
H2
2.2-4.5
2.2-4.4
1 -4
2-4
2.2-3.15
2.2-4.4
2.7-4.5
2.2-4.5
2.7-5.5
Oxygen
