Nuclear corrosion modeling : the nature of CRUD

195 Pages·2010·1.842 MB·English

by  Roy Castelli

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INTRODUCTION
Contents
1. WhyDoWeCare? xiii
2. BoundingtheDiscussion xiii
TheReactor xiv
MaterialsofConstruction xiv
pHControlAgents andCoolantAdditives xv
Clarifying theDefinition xv
3. TheAnalyticalDomain xvi
1. WHY DO WE CARE?
It would be pointless to proceed without a brief word about why anyone
in the nuclear industry might care enough about Chalk River unidentified
deposit(CRUD)tohavespentthepastsixdecadestryingtounderstandits
behavior. There are two basic reasons. First, deposits of any kind on heat
transfer surfaces are (in general) a bad thing. Second, CRUD, in almost all
its forms, becomes highly radioactive and is the source of high radiation
fields, which are most troublesome during reactor servicing evolutions.
The first reason (basically fouling) is not unique to nuclear reactors, but
the radioactive nature of CRUD is quite unique and shall be discussed
in detail herein.
Therefore, one can understand that if designers had a tool that would
allow them to predict the behavior of CRUD within the primary reactor
coolant circuit, then it might be possible to design equipment, processes,
and/or procedures to mitigate (or at least minimize) these detrimental
qualities. Thus the quest to understand the nature of CRUD.
2. BOUNDING THE DISCUSSION
Manydifferentreactordesignsusewater(bothlightandheavy)asboththe
moderator for the thermal fission reaction and the primary heat transfer
working fluid. Similarly, these different designs may use a wide variety
xiii
xiv Introduction
of structural materials and coolant chemistries, and they may operate in
vastly different thermal-hydraulic domains with different operating objec-
tives and conditions.
This book is not intended to be the universal text to explain the nature
ofCRUDinalltypesofreactorsandphysicaldomains.Infact,thecurrent
horizon of scientific understanding is such that we are only just beginning
to understand how to model its behavior in limited circumstances and
conditions. Nonetheless, all types of water reactors share many similarities
and thus there is value in understanding the nature of CRUD in a known
setofcircumstancessothatothersmightdiscoverthesecretsofitsbehavior
under different conditions.
The Reactor
The discussion of the nature of CRUD herein is limited to the context of
pressurized light water reactors (PWRs) and specifically to those in which
theprimaryreactorcoolantisalwaysintheliquidphase(except,ofcourse,
in the pressurizer). Thus, the discussion does not focus on boiling water
reactors, heavy water reactors, or groups of PWRs in which a certain
degree of boiling is allowed to occur within the nuclear core.
Heavy water reactors are excluded here because there is a considerably
smaller body of fundamental corrosion kinetic data published for materials
in heavy water relative to that of light water. Two-phase light water reac-
torsare excluded becausethe modeling of CRUDin the presenceof boil-
ing will add a significant complication to the discussion of fundamental
driving forces for hydrothermal crystallization. The discussion of these
fundamental models will, however, suggest how and where one might
expand the model to account for a system in which boiling occurs.
Materials of Construction
The materials of construction (i.e., the wetted surface areas of the primary
reactor coolant circuit) will define the source terms in our mathematical
modeling,whichisbasicallyasetofequationsthatwillbeusedtocompute
a mass balance. These materials define the sources of metal oxides that
make up the composition of CRUD and the sources of cobalt, which is
one of the most significant sources of radiation exposure from CRUD.
In this text, the discussion focuses primarily on nickel- and iron-based
corrosion-resistant alloys and high-cobalt content-bearing materials. Only
those metal oxides that are either loose-adherent or have significant solu-
bility in high-temperature, high-pH, hydrogenated water are important
Introduction xv
to the understanding of the nature of CRUD. Thus, for example, the
oxide that forms on zircaloy alloys is not discussed because it is neither
loose-adherent nor does it have a significant solubility.
pH Control Agents and Coolant Additives
ModernPWRreactorcoolantsarecloselycontrolledtomaintainverylow
oxygen concentrations and very high pH (e.g., 9.0 < pH < 11.5 at room
temperature). This has not always been the case. In the early years when
neutral pH water was used, scientists and engineers learned that in time,
the coolant of operating reactors would turn alkaline as hydrogen, formed
by radiolysis, would combine with dissolved nitrogen in the coolant to
form ammonia (NH ). It was also observed that the more alkaline the
3
coolant became, the less CRUD was formed. Thus, throughout the years,
several different kinds of coolant additives have been used to control the
coolant pH (e.g., LiOH, BaOH, and NH ).
3
The pH of the reactor coolant (and the presence of hydrogen in the
water) is important because it determines the stable solid phases of metal
oxide corrosion products in the reactor coolant system. The method of
maintaining the coolant pH is only of secondary importance for funda-
mental model development because it only affects the calculation of metal
ion solubilities, as discussed herein.
Two other important coolant additives have been employed in some
commercial PWRs—boron (in the form of boric acid) and zinc. Boron
is used as a neutron poison to aid in the control of the neutronics of the
reactor, and zinc is sometimes used to reduce the absorption of cobalt into
corrosion films and thus reduce pipewall radiation levels. Neither of these
isdiscussedindetailexcepttosaythathowandwhyzincworksasacobalt
inhibitor is directly related to the substitution of divalent metal ions into
corrosion films and, thus, it will be discussed briefly.
Clarifying the Definition
Within this text, the definition of CRUD is somewhat broader than the
historic interpretation of the acronym. For more than 50 years, CRUD,
and its hydraulic behavior, was thought to be a particulate phenomenon.
It is true that CRUD often manifests itself in a solid phase, either as metal
oxide films or as micrometer-sized particles of metal oxide. However,
these solid phases are directly in contact with high-temperature reactor
coolant where Mother Nature wants there to be an equilibrium between
the solid and liquid phases.
xvi Introduction
Virtually all transition metals have some solubility in hot water (except
goldand platinum,which donotform oxides);thus, hydrolyzedspeciesof
metal oxides do exist in the aqueous phase. Not modeling CRUD in the
soluble phase ignores one of the most important mechanisms for mass
transport—hydrothermal crystallization/dissolution. This is partly the rea-
son why early attempts to model CRUD as a particulate phenomena have
not been successful.
Thus, in this text, CRUD is defined as metal (in either solid or soluble
phases) that has been transformed from the metallic state to an oxide state
bythediffusionofoxygenintothebasemetalsofconstructioninaprocess
known as corrosion. One important distinction between this definition
and the historic interpretation is that CRUD, taken in this sense, does
not have to be an oxide. This definition is chosen because some of the
hydrolytic equilibria between the metal oxide solid phases and aqueous
phases simply result in divalent metal ions being the soluble species. Con-
sequently, one of the assumptions that will ultimately be used to frame the
modeling equations is that mass balances will be constructed to conserve
grams of metal.
3. THE ANALYTICAL DOMAIN
An important requirement in the construction of a successful fundamentals-
based model is that all flowing hydraulic regions of the primary reactor
coolant circuit (and, more precisely, all sources and sinks of CRUD) must
bemodeled.Theconsequenceofthisrequirementisthatthemathematical
models used to describe the fluxes of mass to and from the solid and aque-
ous phases must be able to accurately perform over a huge range of
thermal-hydraulic and chemical conditions for many decades of operation.
A brief example of these analytical domains is summarized in Table 1.
Regarding the temperature domain, which is purposely restricted to
(cid:1)
300 Fonthelowerendoftheband,thereareoftenCRUDsinksincool-
(cid:1)
ant purification systems where the coolant temperature falls below 300 F;
however, these regions require special analytical handling for several rea-
sons.First,thecorrosionkineticsforstructuralmaterialsbecomeverysmall
(andthusinsignificantassources)atlowertemperaturesand,second,metal
ion solubilities can no longer be predicted based on thermodynamics as
they become limited by kinetic processes at low temperatures.
At first glance, these domains do not appear to present technical chal-
lenges. However, most scientific investigations of mass transfer coefficients
Introduction xvii
Table1 Chemical,Thermal-Hydraulic, andTemporalDomain
Domain Range Units
Temperature 300 < T < 650 (cid:1) F
Coolant velocity 1 < U < 102 ft/sec
Reynolds number 102 < N < þ107 Unitless
Re
Hydraulic sheer 1 < t < þ102 Dynes/cm2
pH 9 < pH < 11.5 At room temperature
H concentration 10 < H < 102 scc/kg
2 2
Time 0 < t < þ30 Years
are conducted within the typical hydraulic domain of heat exchangers
where 104 < N < 105. Thus, in many ways, these modeling efforts will
Re
push the limits of the current horizon of scientific understanding. None-
theless, one must always start at the beginning, where the learning curve
is the steepest and the science is at its infancy. It is from this vantage point
that the potential for success and growth is the greatest.
PREFACE
At the birth of the nuclear age in the early 1950s, scientists and engineers
set out to harness the power of the atom for peaceful use as a clean alter-
natesourceofthermalenergy.IntheUnitedStates,almostallofthisinitial
research was conducted by the government and would eventually be
administrated by the U.S. Navy under NAVSEA08 (Naval Reactors).
At that time, teams of the nation’s finest scientists and engineers were
commissioned to investigate everything related to the task of converting
the energy of nuclear fission to useable steam. This included the search
for the optimum working fluid to be used as a heat transfer medium,
and although many were tried and tested, light pressurized water would
dominatethefieldofstudyandultimatelyseethelion’sshareofuseduring
the next half century.
Although water had been used for centuries as a versatile heat transfer
working fluid, there were many new technical challenges relating to its
use in nuclear reactors, including the high radiation fields within the
nuclear core to which the coolant would be exposed.
All of the initial research and investigation was performed in the
absence of most of the fundamental science that is discussed in this book.
What was understood, however, was that cleanliness was of paramount
importance (to prevent the spread of radiation); thus, water purity was a
critical concern.
This understanding led to the use of the most corrosion-resistant struc-
tural materials of the time (stainless steels)and ultrapure, neutral pHwater.
(Note that within this context, the word “ultrapure” is a relative term.
Chemists ofthe 1950scouldnotimaginethe puritythatis achievablewith
today’s technology.)
At one of the earliest Canadian reactors at Chalk River Laboratories,
the entire primary coolant system was found to be fouled with a highly
radioactive black “gunk” of unknown origin. The discovery spawned
the acronym CRUD, which stands for Chalk River unidentified deposit.
To this day, when the nuclear industry discusses primary reactor coolant
corrosion products and their consequences, the acronym is used, even in
international references.
xi
xii Preface
For more than 60 years, governments, companies, universities, and
agencies have employed thousands of engineers and scientists to under-
standthenature ofCRUD, what causesit, how it buildsupandtransports
in primary reactor circuits, what might be done to mitigate its undesirable
effects, and how to predict its behavior in these aqueous systems.
There is a remarkable body of excellent research in the public domain
inthisfieldofstudy,rivaledonlybythegreaterbodyofworkthatissome-
what less accomplished. Many have tried to develop predictive models of
the physicochemical phenomena involved, but most have fallen short
because the physical domain of chemistry and thermal-hydraulics required
tomodelMotherNaturewithintheprimarycoolantofanuclearreactoris
almost unfathomable. Most notable are attempts to build correlative mod-
els of CRUD behavior; as discussed in this book, the forces and phenom-
ena that affect the behavior of CRUD are far too complex to correlate
based on a few physical parameters.
Nonetheless, some success has been achieved, and although most of the
validation of that work is shrouded in the secrecy of government classified
documents, much of the fundamental science and understanding has been
published (piecewise) in the public domain.
This book attempts to string those pieces together into a cohesive state-
mentofwhatisrequiredtoconstructafirst-principles,fundamentals-based
analytical model to use as a predictive tool.
11
CHAPTER
The Corrosion Source
Contents
1. TheProcess 1
2. TheForm 2
3. WhyaDouble-LayeredFilm? 3
4. IonSitePreference 5
5. Kinetics 7
ModelingtheBehavior 7
ACloserLookatkp 9
ElementalSpeciationofkpandkr 20
6. TheCobalt Source 22
TrampCobaltinConstructionMaterial 23
HighCobalt ContentAlloys 24
7. APlaceToStart 30
1. THE PROCESS
Understanding the nature of Chalk River unidentified deposit (CRUD)
must start at the source—that is, with the discussion of how CRUD is
formed and the physical forms it takes once it is formed. As the definition
implies, CRUD is born as a result of the oxidation (or corrosion) of the
materials of construction in the primary reactor coolant circuit that come
in contact with the reactor coolant.
An affordable structural material that will not corrode in the presence of
high-temperature, high-pH, hydrogenated water has not been discovered.
Eventhebestcorrosion-resistantalloys“rust”;theonlythingthatmakesone
morecorrosionresistantthananotheristherateatwhichthecorrosionoccurs.
Theprocessisthesameforbothiron-andnickel-basedalloysinprototyp-
icalprimaryreactorcoolant.Whenthesealloysareexposedtoprimaryreactor
coolant at high temperature under prototypic coolant chemistries, oxygen
willdiffuseintothebasemetalatthewettedsurfaceandconverttheelements
inthealloyfromthemetallicstatetoanoxidestate.Intheprocess,divalent
metalionsarereleasedintothewaterassolublemetalions.Thetermsmost
#2009ElsevierInc.Allrightsreserved.
doi:10.1016/B978-1-85617-802-0.00001-3
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