Fundamentals Handbook - Chemistry Volume 2

DOE-HDBK-1015/2-93

JANUARY 1993

DOE FUNDAMENTALS HANDBOOK CHEMISTRY Volume 2 of 2

U.S. Department of Energy Washington, D.C. 20585

FSC-6910

Distribution Statement A. Approved for public release; distribution is unlimited.

This document has been reproduced directly from the best available copy. Available to DOE and DOE contractors from the Office of Scientific and Technical Information. P.O. Box 62, Oak Ridge, TN 37831. Available to the public from the National Technical Information Services, U.S. Department of Commerce, 5285 Port Royal., Springfield, VA 22161. Order No. DE93012222

DOE-HDBK-1015/2-93 CHEMISTRY

ABSTRACT

The Chemistry Handbook was developed to assist nuclear facility operating contractors in providing operators, maintenance personnel, and the technical staff with the necessary fundamentals training to ensure a basic understanding of chemistry. The handbook includes information on the atomic structure of matter; chemical bonding; chemical equations; chemical interactions involved with corrosion processes; water chemistry control, including the principles of water treatment; the hazards of chemicals and gases, and basic gaseous diffusion processes. This information will provide personnel with a foundation for understanding the chemical properties of materials and the way these properties can impose limitations on the operation of equipment and systems.

Key Words: Training Material, Atomic Structure of Matter, The Periodic Table of the

Elements, Chemical Bonding, Corrosion, Water Chemistry Control, Water Treatment Principles, Chemical Hazards, Gaseous Diffusion Processes

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DOE-HDBK-1015/2-93 CHEMISTRY

FOREWORD

The Department of Energy (DOE) Fundamentals Handbooks consist of ten academic subjects, which include Mathematics; Classical Physics; Thermodynamics, Heat Transfer, and Fluid Flow; Instrumentation and Control; Electrical Science; Material Science; Mechanical Science; Chemistry; Engineering Symbology, Prints, and Drawings; and Nuclear Physics and Reactor Theory. The handbooks are provided as an aid to DOE nuclear facility contractors. These handbooks were first published as Reactor Operator Fundamentals Manuals in 1985 for use by DOE category A reactors. The subject areas, subject matter content, and level of detail of the Reactor Operator Fundamentals Manuals were determined from several sources. DOE Category A reactor training managers determined which materials should be included, and served as a primary reference in the initial development phase. Training guidelines from the commercial nuclear power industry, results of job and task analyses, and independent input from contractors and operations-oriented personnel were all considered and included to some degree in developing the text material and learning objectives. The DOE Fundamentals Handbooks represent the needs of various DOE nuclear facilities' fundamental training requirements. To increase their applicability to nonreactor nuclear facilities, the Reactor Operator Fundamentals Manual learning objectives were distributed to the Nuclear Facility Training Coordination Program Steering Committee for review and comment. To update their reactor-specific content, DOE Category A reactor training managers also reviewed and commented on the content. On the basis of feedback from these sources, information that applied to two or more DOE nuclear facilities was considered generic and was included. The final draft of each of the handbooks was then reviewed by these two groups. This approach has resulted in revised modular handbooks that contain sufficient detail such that each facility may adjust the content to fit their specific needs. Each handbook contains an abstract, a foreword, an overview, learning objectives, and text material, and is divided into modules so that content and order may be modified by individual DOE contractors to suit their specific training needs. Each handbook is supported by a separate examination bank with an answer key. The DOE Fundamentals Handbooks have been prepared for the Assistant Secretary for Nuclear Energy, Office of Nuclear Safety Policy and Standards, by the DOE Training Coordination Program. This program is managed by EG&G Idaho, Inc.

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DOE-HDBK-1015/2-93 CHEMISTRY

OVERVIEW

The Department of Energy Fundamentals Handbook entitled Chemistry was prepared as an information resource for personnel who are responsible for the operation of the Department's nuclear facilities. An understanding of chemistry will enable contractor personnel to understand the intent of the chemical concerns within their facility. A basic understanding of chemistry is necessary for DOE nuclear facility operators, maintenance personnel, and the technical staff to safely operate and maintain the facility and facility support systems. The information in the handbook is presented to provide a foundation for applying engineering concepts to the job. This knowledge will help personnel understand the impact that their actions may have on the safe and reliable operation of facility components and systems. The Chemistry handbook consists of five modules that are contained in two volumes. The following is a brief description of the information presented in each module of the handbook. Volume 1 of 2 Module 1 - Fundamentals of Chemistry Introduces concepts on the atomic structure of matter. Discusses the periodic table and the significance of the information in a periodic table. Explains chemical bonding, the laws of chemistry, and chemical equations. Appendix A - Basic Separation Theory Introduces basic separation theory for the gaseous diffusion process. Discusses converter construction and basic operating principals. Module 2 - Corrosion Supplies basic information on the chemical interaction taking place during the corrosion process between the environment and the corroding metal.

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OVERVIEW (Cont.)

Volume 2 of 2 Module 3 - Reactor Water Chemistry Describes the chemical measures taken to retard the corrosion often found in water systems. The consequences of radioactivity on facility cooling water systems are also addressed. Module 4 - Principles of Water Treatment Details the principles of ion exchange in the context of water purity. Discusses typical water treatment methods and the basis for these methods. Module 5 - Hazards of Chemicals and Gases Explains why certain chemicals are considered hazardous to facility personnel. Includes general safety rules on handling and storage. The information contained in this handbook is by no means all encompassing. An attempt to present the entire subject of chemistry would be impractical. However, the Chemistry Handbook does present enough information to provide the reader with a fundamental knowledge level sufficient to understand the advanced theoretical concepts presented in other subject areas, and to better understand basic system and equipment operation.

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TABLE OF CONTENTS

TABLE OF CONTENTS

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . EFFECTS OF RADIATION ON WATER CHEMISTRY (SYNTHESIS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv v

1

Interaction of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 CHEMISTRY PARAMETERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Specific Parameters . . . . . . . . . pH . . . . . . . . . . . . . . . . . . . . . Dissolved Oxygen . . . . . . . . . . Hydrogen . . . . . . . . . . . . . . . . Total Gas . . . . . . . . . . . . . . . . Conductivity . . . . . . . . . . . . . . Chlorides . . . . . . . . . . . . . . . . Fluorine . . . . . . . . . . . . . . . . . Radioactivity . . . . . . . . . . . . . . Tritium . . . . . . . . . . . . . . . . . . Abnormal Chemistry Conditions Injection of Air . . . . . . . . . . . . Fuel Element Failure . . . . . . . . Resin Overheating . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 14 15 16 18 19 21 22 23 23 25 25 28 28 30

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LIST OF FIGURES

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LIST OF FIGURES

Figure 1 Change in pH, Gas Concentration, and Nitrogen Compounds With Excess Oxygen Added . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 2 Corrosion Rate vs. pH for Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 3 Pressurizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 4 pH and Conductivity as a Function of NH3 Concentration . . . . . . . . . . . . . . . 19 Figure 5 Theoretical Conductivity as a Function of pH . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 6 Facility Start-up with Air in Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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LIST OF TABLES

LIST OF TABLES

Table 1 Summary of Reactor Coolant Chemistry Control . . . . . . . . . . . . . . . . . . . . . . 13 Table 2 Hydrogen Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

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REFERENCES

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REFERENCES

Donald H. Andrews and Richard J. Kokes, Fundamental Chemistry, John Wiley & Sons, Inc., 1963 Compressed Gas Association, Inc., Handbook of Compressed Gases, 2nd Edition, Reinhold Publishing Corporation, 1981. R. A. Day, Jr. and R. C. Johnson, General Chemistry, Prentice Hall, Inc., 1974. Dickerson, Gray, Darensbourg and Darensbourg, Chemical Principles, 4th Edition, The Benjamin Cummings Publishing Company, 1984. Academic Program for Nuclear Plant Personnel, Volume II, Chemistry, Columbia, MD, General Physics Corporation, Library of Congress Card A 326517, 1972. General Physics Corporation, Fundamentals of Chemistry, General Physics Corporation, 1982. Glasstone and Sesonske, Nuclear Reactor Engineering, 3rd Edition, Van Nostrand Reinhold Company, 1981. McElroy, Accident Prevention Manual for Industrial Operations Engineering and Technology, Volume 2, 8th Edition, National Safety Council, 1980. Sienko and Plane, Chemical Principles and Properties, 2nd Edition, McGraw and Hill, 1974. Underwood, Chemistry for Colleges and Schools, 2nd Edition, Edward Arnold, Ltd., 1967. Norman V. Steere and Associates, CRC Handbook of Laboratory Safety, 2nd Edition, CRC Press, Inc., 1971.

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OBJECTIVES

TERMINAL OBJECTIVE

1.0 Without references, DESCRIBE the effects of radiation on reactor water and methods of treatment for the products.

ENABLING OBJECTIVES

1.1 1.2 DESCRIBE the process of radiolytic decomposition and recombination of water. DESCRIBE the process of radiolytic decomposition and recombination of nitric acid and ammonia. STATE the advantage of maintaining excess hydrogen in reactor water. STATE the three sources of radioactivity in reactor water and each one's decay product. STATE the following for reactor water chemistry. a. b. c. 1.6 Nine parameters controlled Reason for controlling each parameter Method of controlling each parameter

1.3 1.4 1.5

STATE the possible effects of abnormal chemistry on core conditions.

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EFFECTS OF RADIATION ON WATER CHEMISTRY (SYNTHESIS)

EFFECTS OF RADIATION ON WATER CHEMISTRY (SYNTHESIS)

Radiation synthesis is a process that takes place in the reactor coolant system. This phenomenon is limited to the reactor coolant system because of the high flux (radiation) levels that exist in the core region and further complicate chemistry control of the reactor plant. EO 1.1 DESCRIBE the process of radiolytic decomposition and recombination of water. DESCRIBE the process of radiolytic decomposition and recombination of nitric acid and ammonia. STATE the advantage of maintaining excess hydrogen in reactor water. STATE the three sources of radioactivity in reactor water and each one's decay product.

EO 1.2

EO 1.3

EO 1.4

Interaction of Radiation

As reactor coolant water passes through the core region of an operating reactor, it is exposed to intense radiation. The major components of the radiation field are neutrons, protons, gamma rays, and high energy electrons (beta particles). These types of radiation interact with the coolant water primarily by an ionization process and have a marked effect on the water itself and on the chemical reactions between substances dissolved in the water. This section discusses these effects, and in particular the effects that involve gases dissolved in reactor coolant. The interaction of radiation with matter produces ion pairs. Usually, the negative member of the ion pair is a free electron and the positive member is a polyatomic cation, the exact nature of which depends on the particular substance being irradiated. For example, the interaction of radiation with water is illustrated by the following reaction. H2O radiation e H2O (3-1)

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EFFECTS OF RADIATION ON WATER CHEMISTRY (SYNTHESIS)

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Both of these species are very reactive chemically, and there are several reaction pathways available to each. Some of these mechanisms are very complex and are usually of little practical value to the reactor operator, who is more concerned with the overall, observable effects. In the case of water, the overall effect of irradiation is shown in the following reaction. 2H2O radiation 2H2 O2 (3-2)

Because this result is not at all apparent from Reaction (3-1), the following section describes the intermediate processes in some detail. This discussion is presented only to illustrate the types of reaction mechanisms that occur in irradiated solutions. Subsequent discussions primarily involve only the overall effects of these processes. Reaction (3-1) shows that irradiation of pure water produces an electron and a H2O+ ion. As stated, both species are highly reactive. The H2O+ ion rapidly reacts with a water molecule as follows. H2O H2O H3O OH (3-3)

The species OH is an uncharged hydroxyl group. Neutral groups such as this, in which all chemical bonding capacity is not satisfied, are common intermediate species in chemical reactions and are called radicals or sometimes free radicals. The electron produced by Reaction (3-1) first forms a species called the hydrated electron, denoted by eaq-. The hydrated electron may be thought of as resulting from the interaction of a negative electron with the positive end of a polar water molecule. This is analogous to the formation of hydronium ions by interaction of a positive proton (H+) with the negative end of a water molecule. Because the water molecules associated with hydrated electrons do not participate in subsequent chemical reactions, they are not shown in chemical equations, and the hydrated electron (eaq-) is used instead. Hydrated electrons may interact with H3O+ ions in solution or with water molecules. Both reactions produce another reactive species, atomic hydrogen. eaq or

eaq H2O H OH

H3O

H

H2O

(3-4)

(3-5)

Reaction (3-4) usually predominates.

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EFFECTS OF RADIATION ON WATER CHEMISTRY (SYNTHESIS)

Because Reactions (3-4) and (3-5) are slow compared to that in Reaction (3-3), there are three reactive species present at any one time: hydroxyl radicals (OH), hydrated electrons (eaq-), and hydrogen atoms (H). These species may undergo any of several possible reactions such as the following. OH OH H2O2 (hydrogen peroxide) (3-6)

OH

H

H2O

(3-7)

H

H H2O

H2

(3-8) (3-9) (3-10)

H H2

eaq

H2 H2O

OH H

OH

Hydrogen peroxide, formed by Reaction (3-6), may also react with the original reactive species, but at high temperatures H2O2 is unstable, and the predominant reaction is decomposition. 2H2O2 O2 2H2O (3-11)

To illustrate the overall result of these reactions, let us assume that each of the reactive species produced by successive steps in the irradiation of water reacts in only one way. That is, whenever several reactions of a particular substance are possible, assume that one predominates to such an extent that the others are negligible. The following set of reactions is one possibility. In some cases, entire reactions are multiplied by a factor to allow cancellation of terms when the reactions are summed. 4 ( H2O 4 ( H2O eaq radiation H2O H3O

OH

e H3O H

H2O ) OH ) H2O

H2O2 )

(3-1) (3-3) (3-4) (3-6)

2 ( OH

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EFFECTS OF RADIATION ON WATER CHEMISTRY (SYNTHESIS)

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2 (H 2H2O2

H O2 2H2

H2 ) 2H2O O2 6H2O

(3-8) (3-11)

Net reaction:

8 H2O

radiation

or

2 H2O radiation 2H2 O2

(3-12)

The net result of these reactions is simply the decomposition of water. If H2 and O2 are allowed to escape from solution as gases, the reaction continues as written. If, however, the water is contained in a closed system under pressure (as in a reactor coolant system), H2 and O2 are confined, and an equilibrium state is reached because radiation also causes the reverse of Reaction (3-2) to take place. Primarily neutron and gamma radiation induce both the decomposition of water and the recombination of H2 and O to form water. Thus, it is 2 appropriate to write Reaction (3-2) as a radiation-induced equilibrium reaction.

radiation 2H2O radiation 2H2 O2

(3-12)

To arrive at the overall effect of radiation on water, the above process involved the assumption that only one reaction pathway is available to each reactive species. This was done primarily for convenience because inclusion of every possible reaction in the summation process becomes rather cumbersome. Even if all the reactions are taken into account, the net result is the same as Reaction (3-12), which is reasonable because inspection of Reactions (3-3) through (3-11) shows that the only stable products are H2, O2, and H2O (H3O+ and OH- combine to form water, and H2O2 decomposes at high temperature). Perhaps not as obvious, more water is consumed than is produced in these reactions, and the net result is the initial decomposition of water that proceeds until equilibrium concentrations of H2 and O2 are established. Before discussing the effects of radiation on other processes, chemical equilibrium in the presence of ionizing radiation should be mentioned. Equilibrium processes in aqueous solutions are discussed briefly in Module 1, which states that temperature influences the equilibrium. Ionizing radiation also influences the equilibrium of these solutions.

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EFFECTS OF RADIATION ON WATER CHEMISTRY (SYNTHESIS)

Radiation has an effect on the equilibrium in the case of water. In the absence of radiation, water does not spontaneously decompose at 500 F and the equilibrium lies far to the right.

2H2 O2 2H2O

When irradiated, however, water does decompose, as shown above. Also, H2 and O2 do not normally react at 500 F because a large activation energy is required to make the reaction occur. Radiation, in effect, supplies this activation energy, and the reaction takes place readily. Thus, radiation increases the rates of both forward and reverse reactions, although not by the same factor. In general, the effect of radiation on the equilibrium for a given reaction cannot be predicted quantitatively. The situation is further complicated by the observation that the effect on the equilibrium may vary with the intensity of the radiation. In nuclear facilities, the effect may vary with the power level of the facility. In most cases, this complication is not a severe problem because the direction of the effect is the same; only the degree or magnitude of the effect varies with the intensity of the radiation. As noted several times previously, reactor coolant is maintained at a basic pH (in facilities other than those with aluminum components or those that use chemical shim reactivity control) to reduce corrosion processes. It is also important to exclude dissolved oxygen from reactor coolant for the same reason. As shown in the preceding section, however, a natural consequence of exposing pure water to ionizing radiation is production of both hydrogen and oxygen. The addition of a base to control pH has essentially no effect on this feature. To prevent the formation of oxygen in reactor coolant, hydrogen is added. Hydrogen suppresses the formation of oxygen primarily by its effect on the reactions that OH radicals, produced by Reaction (3-3), undergo. In the presence of excess hydrogen, hydroxyl radicals react predominantly by Reaction (3-10) rather than as in Reactions (3-6) through (3-8).

H2 OH H2O H

(3-10)

Hydrogen atoms from this equation subsequently react to form H2 and H2O by Reactions (3-7), (3-8), and (3-9). None of these reactions leads to O2, or H2O2, which decomposes to form O2 and H2O at high temperatures. Thus, the addition of H2 to reactor coolant largely eliminates production of free oxygen.

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EFFECTS OF RADIATION ON WATER CHEMISTRY (SYNTHESIS)

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Another way of viewing the effect of hydrogen on reactor coolant is through its effect on the equilibrium of the reaction. 2 H2O radiation 2H2 O2 (3-12)

By LeChatelier's principle, the addition of excess hydrogen forces the equilibrium to the left, which requires that O2 be consumed. If the dissolved hydrogen concentration is sufficiently large, only a very small amount of oxygen will be present at equilibrium. Normally, therefore, reactor coolant contains excess dissolved hydrogen, and there is no significant net decomposition of water by radiation. Reactor coolant makeup water usually contains a small amount of air, which is composed primarily of nitrogen and oxygen in a volume ratio of 4:1 (80 percent nitrogen, 20 percent oxygen). These gases undergo radiation-induced reactions. The reactions are the same as those that occur in certain accident situations and are included in the following discussion. In addition to the small amount of air normally dissolved in makeup water, there is a small possibility that air may be accidentally injected directly into the reactor coolant system. Whenever air enters the reactor coolant system, and the reactor is operating, the most immediate reaction involves oxygen from the air and hydrogen, which is normally present in the coolant.

radiation radiation

2H2

O2

2H2O

(3-13)

That is, the addition of O2 disturbs the above equilibrium and causes the equilibrium to shift to the right, consuming both H2 and O2 . The concentration of hydrogen normally maintained in reactor coolant is such that small amounts of oxygen will be rapidly consumed before any excess oxygen can cause severe corrosion problems to occur. Reaction (3-13) also consumes oxygen added to the reactor coolant as a natural consequence of air dissolved in makeup water. Other than initial fill of the reactor coolant system, the situations that require the largest amounts of makeup water are feed and bleed operations to correct an abnormal chemistry parameter or cooldown after some period of reactor operation. In this case, gamma radiation from the decay of fission products in the reactor core continues to induce the H2 - O2 reaction for some period after shutdown. During initial fill and long shutdown periods, chemicals other than hydrogen (e.g. hydrazine) may be added to reactor coolant to remove any dissolved oxygen.

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EFFECTS OF RADIATION ON WATER CHEMISTRY (SYNTHESIS)

After essentially all of the oxygen has been consumed by reaction with hydrogen, the nitrogen contained in air will remain. For small air additions, some hydrogen will also remain; thus, the reactor coolant will contain both dissolved hydrogen and dissolved nitrogen. These two gases do not react in an unirradiated solution at low temperature and pressure. When exposed to radiation, however, the gases do react by the following reaction. radiation (3-14)

3H2

N2

2NH3 (ammonia)

Again, this is an equilibrium reaction, and radiation induces the reaction in both directions. Ammonia (NH3) produced by this reaction combines with water to form ammonium hydroxide (NH4OH).

NH3 H2O NH4 OH

(3-15)

Under the operating conditions of reactor coolant, Reaction (3-14) is far from complete. In most cases, less than about 10 percent of the nitrogen will be converted to ammonia. If no additional base were added to reactor coolant, Reaction (3-14) would be sufficient to cause the coolant to be mildly basic, pH 9. In the presence of added base, however, the reaction has only a very slight and negligible effect on pH. If the base NH3 were used to control reactor coolant pH, the reverse of Reaction (3-14) would be more important. The reverse step of this reaction requires that some of the ammonia added to the coolant decompose into N2 and H2. Because operating conditions favor this step of the equilibrium, rather than formation of NH3, it would be expected that most of the ammonia added would decompose. However, the rate of the ammonia decomposition reaction is slow, and the pH of reactor coolant can be maintained in the required range. It should also be noted that the decomposition of NH3 would produce hydrogen gas in significant concentrations in reactor coolant (sufficient to satisfy normal H2 requirements). In the event that a large quantity of air is injected into the reactor coolant system, the inventory of dissolved hydrogen would be rapidly depleted by Reaction (3-13). If the amount of air injected is sufficiently large, there could be oxygen remaining in the coolant after depletion of the hydrogen. In this case, another reaction is available to the oxygen and nitrogen in the air.

radiation

2 N2

5O2

2 H2O

4HNO3

(3-16)

Nitric acid (HNO3) produced by this reaction will neutralize any base contained in the coolant, and if sufficient acid is produced, the coolant will acquire an acidic pH.

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EFFECTS OF RADIATION ON WATER CHEMISTRY (SYNTHESIS)

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Reactor Water Chemistry

Normally, the amount of hydrogen maintained in the reactor coolant, in conjunction with other precautions employed, greatly reduces the probability that the amount of oxygen entering the coolant will be sufficient to lead to Reaction (3-16). If a large amount of air were accidentally added to the reactor coolant, one solution would be to add more hydrogen. The added hydrogen would react with remaining oxygen, disrupting the equilibrium of Reaction (3-16) causing the reverse step of that reaction to occur. When all the oxygen has been removed, H2 and N2 could react by Reaction (3-14) and help reestablish a basic pH. The relationship between these reactions and pH following the initial oxygen addition, and a subsequent hydrogen addition, is illustrated in Figure 1.

Figure 1 Change in pH, Gas Concentration, and Nitrogen Compounds With Excess Oxygen Added

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EFFECTS OF RADIATION ON WATER CHEMISTRY (SYNTHESIS)

In the preceding discussion, the reactions possible after the addition of air to reactor coolant containing hydrogen were described. These are Reactions (3-13), (3-14), and (3-16). The relative rates of these reactions are of considerable importance. Briefly, Reaction (3-13) is much more rapid than either of the others, and Reaction (3-16) is faster than Reaction (3-14). Thus, the sequence of reactions is as described in the preceding sections. H2 and O2 react to form water. If hydrogen remains, it undergoes an incomplete reaction with N2 to form small amounts of NH3. If O2 remains after all the H2 has been consumed, the O2 - N2 reaction produces nitric acid. The flux of neutrons and protons in a nuclear reactor core region leads to several important nuclear reactions with the constituent atoms of water. Most of these reactions involve oxygen isotopes and fast neutrons or protons. In many cases, the absorption of a fast neutron by a nucleus is immediately followed by ejection of a proton. These reactions are called neutron-proton or n-p reactions and are commonly written (using the 16O reaction to illustrate) in the following manner.

16 8

O (n, p) 16N (t1/2 = 7.13 seconds) 7

(3-17)

In this notation, the original isotope that undergoes the reaction is written first, the product isotope is last, and the two are separated by, in order, the particle absorbed and the particle emitted. The isotope 16N decays to 16O with a 7.13-second half-life by emitting a beta particle 7 8 ( ) and a high-energy gamma ray (6 Mev predominantly).

16 7

N

16 8

O+

+

Oxygen-17 undergoes a similar reaction.

17 8 17 7

O (n, p) 17N (t1/2 = 4.1 seconds) 7

(3-18)

The isotope

N decays by emission of a beta particle, a neutron, and a gamma ray.

17 7

N

16 8

O+

+ 1n + 0

Reactions (3-17) and (3-18) have no significant chemical effect on reactor coolant because of the relatively small number of atoms that undergo these reactions. They are of considerable importance, however, because the radioactive species 16N and 17N are carried outside the core 7 7 region by the flow of reactor coolant. The neutrons and high-energy gamma rays emitted by these isotopes easily penetrate the piping and components that contain the coolant and are important considerations in the design of shielding for nuclear facilities. Because the half-lives of these isotopes are very short, they decay to low levels very rapidly after shutdown and are, therefore, of little concern during such periods.

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Two other nuclear reactions with oxygen isotopes are shown below.

18 8

O (p, n) 18F (t1/2 = 112 minutes) 9 O (p, ) 13N (t1/2 = 10 minutes) 7

16 8

An ejected alpha particle is indicated by . The protons that cause these reactions result from inelastic collisions of fast neutrons with hydrogen atoms in water molecules. The radioactivity levels of these isotopes are much lower than the levels of 16N and 17 N during reactor facility 7 7 operation. However, during the period from a few minutes to about five hours after reactor shutdown or after removing a coolant sample from the system, 13N and 18 F are the principal 7 9 sources of radioactivity in the reactor coolant of most reactor facilities.

13 7

N F

13 6 18 8

0 C + +1

18 9

0 O + +1

The only significant nuclear reaction that occurs with hydrogen involves deuterium (2H), which 1 comprises about 0.015 percent of natural hydrogen.

2 1

H (n, ) 3H (t1/2 = 12.3 years) 1

Tritium (3H) decays by emission of a very weak particle (0.02 Mev) and no gamma rays. 1 Thus, tritium is not a radiological hazard unless it enters the body in significant amounts. Tritium can enter the body through inhalation or ingestion. It is also possible to absorb forms of tritium through the skin.

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Summary

The important information in this chapter is summarized below.

Effects of Radiation on Water Chemistry (Synthesis) Summary

The intense radiation inside the core of an operating nuclear reactor produces several chemical effects on the reactor coolant itself and on substances dissolved in the coolant. Radiation causes pure water to decompose into H2 and O . The 2 decomposition is suppressed by adding excess hydrogen. 2H2O 2H2 + O2

Excess hydrogen is added to suppress the decomposition of reactor water. It also reacts with any oxygen that enters the reactor coolant system, usually as a component of air in makeup water, provided the amount of oxygen is not excessive. If the amount of oxygen is more than enough to deplete the hydrogen, the excess oxygen reacts with nitrogen (also a component of air) and forms nitric acid. In the case of addition of very large amounts of air, the amount of nitric acid produced may be more than enough to neutralize the normally basic coolant and cause it to become acidic. Radiation induces the combination of N2 and H2 to form ammonia, although the extent of this reaction is small and usually has a negligible effect on the pH of reactor coolant. All of the reactions in this chapter are reversible and reach an equilibrium state under normal operating conditions. Changes in the concentrations of any of the reactants disturb the equilibrium and causes the reaction to shift in the direction which restores the equilibrium. N2 + 3H2 2NH3

Radiation also produces several nuclear reactions in reactor coolant. The products 16 17 7N and 7N, of two of these reactions, contribute radioactivity to the reactor coolant, adding significantly to the shielding requirements for nuclear reactors. Others, 13N 7 and 18F, are also major contributors to the radioactivity in reactor coolant. 9

16 7 13 7

N N

16 8 13 6

O+

0 C + +1

+

17 7

N

16 8

O+ F

18 8

1 + 0n + 0 O + +1

18 9

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CHEMISTRY PARAMETERS

The reasons for control of selected chemistry parameters, and some of the more common methods of controlling these parameters will be discussed. No attempt will be made to identify specific values of any of the parameters discussed because of the number of different reactor facilities involved, but an overview concerning the bases and common methods used will be included. For operating values and specifications, users should refer to local facility publications. In addition, some information on tritium is provided. EO 1.5 STATE the following for reactor water chemistry. a. b. c. EO 1.6 Nine parameters controlled Reason for controlling each parameter Method of controlling each parameter

STATE the possible effects of abnormal chemistry on core conditions.

Specific Parameters

Specific chemical parameters vary from facility to facility but generally include the following: pH, dissolved oxygen, hydrogen, total gas content, conductivity, chlorides, fluorine, boron, and radioactivity. For the parameters indicated, control is generally achieved by one or more of three basic processes. (1) Ion exchange in the primary system demineralizer(s) or by supplemental chemical additions Oxygen scavenging by hydrogen or hydrazine addition Degassification

(2) (3)

Table 1 lists the more common chemistry parameters measured and/or controlled, the reasons each is measured and/or controlled, and control methods utilized.

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TABLE 1 Summary of Reactor Coolant Chemistry Control

Parameter Reason To inhibit corrosion To protect corrosion film To preclude caustic stress corrosion To inhibit corrosion To scavenge oxygen To suppress radiolytic decomposition of water To scavenge nitrogen To preclude hydrogen embrittlement Total Gas Content To protect pumps To indicate air in leakage To minimize scale formation Conductivity To indicate increased corrosion To preclude chloride stress corrosion To preclude corrosion of Zr cladding To control reactivity To indicate increased corrosion To indicate a crud burst Radioactivity To indicate a core fuel defect To monitor effectiveness of demineralizer Feed and bleed Degassification Degassification Deaeration of makeup water Ion exchange Feed and Bleed Ion exchange Chlorides Feed and bleed Ion exchange Fluorine Boron Feed and Bleed Boric acid addition Ion exchange Hydrogen addition Method of Control Ion exchange Ammonium hydroxide addition Nitric acid addition Hydrogen addition Hydrazine addition

pH Dissolved Oxygen

Hydrogen

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pH

The reason for controlling pH in the reactor coolant system is to minimize and control corrosion. As discussed in Module 1, the presence of excess H+ ions in solution results in an acidic condition. In reactor facilities (except those containing aluminum components), acidic conditions are detrimental to the materials of construction in a number of ways. An acidic condition in the primary coolant results in processes that are potentially harmful to the system as follows. First, a low pH promotes rapid corrosion by deteriorating or "stripping off" the protective corrosion film, and second, corrosion products such as ferrous oxide (Fe3O4), which is predominant in the corrosion film, are highly soluble in an acidic solution. Figure 2 shows how the corrosion rate increases as the pH decreases. Thus for facilities not using aluminum components, a neutral or highly basic pH is less corrosive.

Figure 2 Corrosion Rate vs. pH for Iron

In nuclear facilities that do not use chemical shim to control reactivity, pH is normally maintained at a relatively high value, such as a pH of about 10. In these facilities the upper limit for pH is set based on caustic stress corrosion considerations because caustic stress corrosion becomes more probable as higher pH values are approached. In facilities that use chemical shim reactivity control (chemical shim involves the addition of boron in the form of boric acid) the pH is maintained at a much lower value. A low pH is necessary because of the large amounts of boric acid added to the reactor coolant. Accordingly, pH in these facilities is maintained as high as possible consistent with the reactivity requirements of the nuclear facility, with pH range from 5 to 7 being common.

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In facilities using aluminum components, pH is maintained on the acidic side of the scale because of the corrosion characteristics of aluminum discussed in Module 2. In these facilities pH may be controlled by the addition of a dilute nitric acid (HNO3) solution to the reactor coolant system in conjunction with an ion exchange system of some type. Regardless of the pH range maintained, most facilities use an ion exchange process (described in Module 4) to help control pH. For the high pH facilities, the most common means of control is the use of a lithium or an ammonium form cation and a hydroxyl form anion. When lithium is used, it must be 7Li because other lithium isotopes produce tritium, which represents a significant biological hazard to personnel. In facilities that employ high pH chemistry control and do not use chemical shim reactivity control, it is sometimes necessary to add a strong base solution such as ammonium or lithium hydroxide. When chemical additions are used for pH control, facility design and operating procedures are utilized to preclude overconcentration at any point in the system, which may lead to caustic stress corrosion conditions. Many reactions that take place in the reactor coolant system can affect pH; therefore chemistry control must be considered carefully to preclude upsetting the pH balance provided by the ion exchanger.

Dissolved Oxygen

Control of the dissolved oxygen content in the reactor facility system is of paramount importance because of its contribution to increased corrosion. The base reactions of concern regarding high concentrations of dissolved oxygen are the following. 3 Fe 2O2 Fe3O4 (3-19)

4Fe

3 O2

2Fe2O3

(3-20)

They are dependent on both the concentration of oxygen and temperature. Reaction (3-19) is predominant at high temperatures ( 400 F) in the presence of lower oxygen concentrations. This corrosion film, ferrous oxide, is also known as magnetite and is a black, generally tightly-adherent film that provides a protective function to surfaces within the facility. Reaction (3-20) occurs at temperatures below about 400 F in the presence of higher oxygen concentrations. Ferric oxide (Fe2O 3) is more commonly known as rust and is generally a reddish color. This corrosion product adheres loosely to surfaces and is therefore easily removed and transported throughout the system for subsequent deposition and possible irradiation. In either of the reactions, the corrosion rate is accelerated by increased concentrations of dissolved O2 and can be aggravated further by the presence of other substances that may be present in the system.

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In addition to the direct contribution to corrosion, oxygen reacts with nitrogen to lower the pH of the reactor water, which also results in an increased rate of corrosion. Oxygen and nitrogen react to form nitric acid by the following reaction. radiation

2N2

5 O2

2H2O

4 HNO3 (nitric acid)

In all the reactions presented, it can be seen that oxygen concentrations promote corrosion. It follows then that if corrosion is to be minimized, oxygen concentrations must be maintained as low as possible. In most nuclear facility reactor coolant systems, the limit for dissolved oxygen concentrations is expressed in ppb (parts per billion). Concentration may be monitored on a continuous basis by using an in-line analyzing system or periodically by withdrawing a sample volume and analyzing that sample. Monitoring oxygen levels is done not only to ensure that no oxygen is available for corrosion, but also to indicate the introduction of air into the system.

Hydrogen

Because the presence of dissolved oxygen contributes to most mechanisms of corrosion, the concentration of oxygen is controlled and reduced by the addition of scavenging agents in most facilities. Hydrogen gas (H2) and hydrazine (N2H4) are the scavenging agents normally used to eliminate dissolved oxygen from the reactor coolant system. These substances scavenge oxygen by the following reactions. 2H2 N2H4 O2 O2 radiation radiation 2H2O N2 (3-13) (3-21)

2H2O

Because hydrazine decomposes rapidly at temperatures above about 200 F (forming NH3, H2, and N2), hydrogen gas is used as the scavenging agent during hot operation and hydrazine is used when the reactor coolant system is cooled below 200oF. (Heat) 2NH3 N2 2 N2H4 (decomposition of hydrazine) H2

The decomposition reactions of hydrazine pose additional problems in chemistry control. Even if sufficient hydrazine were added to overcome the loss due to decomposition, instability of coolant pH would probably occur by the following reactions. 2N2 3H2 5 O2 N2 2H2O 2 H2O 4 HNO3 (acid) 2NH4OH (base) (3-16) (3-22)

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The use of hydrogen gas at temperatures above 200 F precludes the generation of the compounds formed by Reactions (3-16) and (3-22). In addition, hydrogen is compatible with the high flux levels present in the reactor core. Accordingly, advantage may be taken of the reversibility of the radiolytic decomposition of water. The following reaction illustrates the scavenging process utilizing hydrogen. 2H2 O2 radiation radiation 2H2O (3-13)

As indicated, the reaction is an equilibrium process and will therefore depend on the relative concentrations of the reactants and the products. By maintaining an excess of hydrogen (H2), the reaction is forced to shift to the right and theoretically eliminates any dissolved oxygen that may be present. As long as an inventory of H2 is present in the coolant, dissolved oxygen will be eliminated or forced to recombine immediately after radiolytic decomposition, thereby being unavailable for corrosion reactions. A boiling water reactor (BWR) facility is susceptible to corrosion, resulting from dissolved oxygen, in the same reactions as are present in a pressurized water reactor (PWR). However, because of the design of these facilities the use of chemical additives is prohibited because continuous concentration would occur in the reactor vessel due to boiling. Boiling would result in a plating out process, and the irradiation of these concentrated additives or impurities would create an extreme environment of radiation levels as well as adverse corrosion locations. By the very nature of operation of a BWR facility, the buildup of high concentrations of dissolved oxygen is prevented. Because boiling is occurring in the reactor vessel and the steam generated is used in various processes and subsequently condensed, removal of dissolved gases is a continual process. As stated, boiling is an effective means of removing gases from a solution. If we were to compare the oxygen content of the steam and the water in a BWR, we would find typical concentrations of 100 ppb to 300 ppb in the water and 10,000 ppb to 30,000 ppb in the steam. This concentration process is continuous during operation, and the dissolved oxygen remains in the gaseous state and is subsequently removed in the condensing units along with other noncondensible gases. As with PWR facilities, BWR facilities minimize the introduction of dissolved oxygen by pretreating makeup water by some method. The large oxygen concentrations measured in the steam system result primarily from the radiolysis of water according to Reaction (3-12), and as operation is continued, the equilibrium concentration of 100 ppb to 300 ppb is established. This concentration of oxygen is consistent with the objective of minimizing corrosion.

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Total Gas

Total gas concentration in the reactor coolant system is another parameter of concern. Total gas is the sum of all gases contained in the coolant system and is made up primarily of hydrogen (H2), nitrogen (N2 ), argon (Ar), and oxygen (O2). The small amounts of fission gases (Kr and Xe) normally present in the system may also contribute to the total gas concentration; however, under normal conditions these are essentially undetectable. Total gas is of concern because high concentrations can result in the formation of gas pockets in areas that are high points of the system where low or stagnant flow conditions exist. Of particular concern in PWR facilities are the erosion and corrosion that may occur on the impellers of the primary coolant pumps. As the concentration of gas is increased, the probability of the gas coming out of solution in significant amounts in areas of low pressure is also increased. This low pressure condition exists at the inlet to the primary coolant pump impeller (where centrifugal pumps are utilized). As these gas bubbles are forced back into solution on the high pressure side of the impeller, erosion can occur as a result of the gas bubble impinging on the impeller. In extreme concentrations of total gas, loss of pump priming and cavitation can occur with resultant mechanical pump damage.

Figure 3 Pressurizer

Reduction of total gas concentrations in PWRs is normally accomplished by the venting of a steam space. In those facilities utilizing a pressurizer, the steam space in the top of the pressurizer is designed to accomplish this venting operation either continuously or intermittently. This process of reducing the total gas concentration is generally referred to as degassification. A typical PWR pressurizer with degassification piping is shown in Figure 3. Degassification is not normally required in a BWR because of its design. As discussed previously, the boiling action in the reactor vessel strips dissolved gases from the water, and they are continuously removed in the condensing phase of the energy cycle.

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Conductivity

Conductivity of reactor facility water is measured to provide an indication of dissolved ionic substances in the coolant. Conductivity measurements provide quantitative rather than qualitative information because it is possible to determine the total conductivity of the ions present, but not the specific types of ions present. Because many ions such as iron (Fe+), chromium (Cr+), copper (Cu+) and aluminum (Al+) are susceptible to forming oxides and plating out as scale on heat transfer surfaces, reactor coo

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