Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces
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960 Charged Particle and Photon Interactions with Matter
34.1 introduCtion
In recent years, nuclear power has been reevaluated as a sustainable and environmentally benign
energy source to avoid global warming because of low emission of CO
2
. In developed countries,
there is renewed interest in further construction of nuclear power plants, and many developing
countries are planning to introduce nuclear energy. Nuclear energy is always accompanied by
radiation and radioactive materials, and there exist many radiation effects in nuclear reactors
and spent fuel reprocessing. These radiation effects are normally detrimental to the performance
and functionality of the facility and much care should be taken. Therefore, an understanding of
radiation-induced phenomenon is essential to avoid the reduction of the integrity of the facilities.
This chapter is composed of three subjects in nuclear technology: water chemistry in nuclear
power plants, reprocessing of spent nuclear fuel, and high-level radioactive waste. It is well known
that coolant water in nuclear reactors receives high radiation doses under a strong mixed eld of
gamma rays and fast neutrons at high temperature and pressure. Radiation controls the chemical
condition of the coolant and precise prediction of radiolysis effects is essential to avoid detrimen-
tal radiation-induced corrosion processes such as stress corrosion cracking (SCC).
34.2.6 Development of an ECP Sensor for BWR Applications........................................... 981
34.2.6.1 Reference
Electrode................................................................................... 981
34.2.6.2
Working
Electrode..................................................................................... 981
34.2.7
Radiation Effects on Corrosion in Nuclear Reactor Systems................................... 981
34.2.7.1 Radiolysis Effects on the Water Environment........................................... 981
34.2.8 Radiation Effect on Materials................................................................................... 985
34.2.9 Conclusion ................................................................................................................ 986
34.3 Radiation-Induced
Processes in the Spent Nuclear Fuel Reprocessing ...............................986
34.3.1
Radiolytically Produced Reactive Species in the Biphasic System..........................987
34.3.1.1 Produced Species in the Aqueous Phase ...................................................987
34.3.1.2 The Produced Species in the Organic Phase .............................................990
34.3.1.3 The
Mixed Phase .......................................................................................990
34.3.2
PUREX
Process Radiation Chemistry ..................................................................... 991
34.3.2.1
TBP Radiolysis...........................................................................................991
34.3.2.2 PUREX Diluent Degradation ....................................................................993
34.3.3 Radiation Chemistry in the Future Fuel Cycle.........................................................994
34.3.3.1 Amide
and Diamide Radiolysis.................................................................994
34.3.3.2
DIAMEX
and TODGA Process Radiolysis ..............................................995
34.3.3.3
TRUEX Process Radiolysis .......................................................................996
34.3.4 Radiation Chemistry and Actinide Oxidation States ...............................................997
34.3.4.1 Radiolytic Production of Nitrous Acid ......................................................997
34.3.4.2 Actinide
Reactions with Free Radicals......................................................999
34.4
Geological
Disposal of High-Level Radioactive Waste .....................................................1000
34.4.1
Introduction ............................................................................................................1000
34.4.2 Effect of Gamma Radiation on Engineered Barrier Systems..................................1005
34.4.2.1 Effect on Clay Minerals........................................................................... 1005
34.4.2.2 Effect
on Corrosion of Waste Package ....................................................1006
34.4.3
Effect
on Dissolution of Waste Forms....................................................................1007
34.4.3.1
Dissolution of Spent Nuclear Fuel ...........................................................1007
34.4.3.2 Dissolution of HLW Glass ........................................................................1010
34.4.4 Redox Front Migration Induced by Groundwater Radiolysis..................................1011
Acknowledgments........................................................................................................................ 1012
References.................................................................................................................................... 1012
Radiation Chemistry inNuclear Engineering 961
The efciency of nuclear energy production from uranium may be increased by the reprocessing
of spent nuclear fuel. Reprocessing allows the recovery of ssionable Pu and U for use in new reac-
tor fuel. In addition, further separation of the minor actinides, and possibly some ssion products
such as Sr and Cs is desired for future reprocessing. Future reprocessing will also be designed to
ensure
the nonproliferation of nuclear material.
After
spent fuel reprocessing, high-level radioactive waste is separated and must be stored safely.
This is an important issue in the eld of nuclear technology. Again, radiation effects should be
considered. The geological repository is assumed to be a most promising and acceptable method for
disposition and current research relevant to the radiation-induced effects in the geological reposi-
tory
are also reviewed here.
34.2 appliCation oF radiation Chemistry to power plants
34.2.1 introduction
It has been widely recognized that intergranular stress corrosion cracking (IGSCC) is a key issue for
boiling water reactors (BWRs) and occurs depending on the three factors of material susceptibility,
residual stress, and environmental condition. Most of the SCC countermeasures have been taken
from the viewpoint of material selection and stress improvement. On the other hand, SCC mitigation
by changing chemistry has also been adopted by eliminating impurities and decreasing oxidizing
species concentrations to achieve low corrosion potential. In order to evaluate the corrosion environ-
ment from the viewpoint of both modeling and experimental approaches, the application of radiation
chemistry is of vital importance in understanding water radiolysis in the nuclear core. In this chapter,
the radiation chemistry of water in BWRs and its implications for corrosion are described.
34.2.2 water cheMiStry control in light water reactorS
Most commercial nuclear reactors are light water reactors (LWRs) and are categorized into two
types: BWRs and pressurized water reactors (PWRs). Proper coolant water chemistry control is
required
to minimize corrosion and to reduce radiation exposure to personnel.
34.2.2.1
bwr water
Chemistry
34.2.2.1.1
Objective of BWR Water Chemistry Control
The objectives of BWR coolant water chemistry control are to maintain material integrity of both fuel
materials and structural materials, to reduce personnel radiation exposure, and to reduce radioactive waste
generation. To achieve these goals, the principle of the BWR water chemistry control is to use pure neutral
water to suppress corrosion of the materials. For example, the reactor water quality is dened as follows:
Reactor
water conductivity (at 25°C):
1μS
cm
−1
or less
Reactor
water chloride ion:
0.1ppm
or less
Reactor water pH:
5.6–8.6
34.2.2.1.2
Outline of BWR Primary Coolant Loop
In the BWR primary coolant loop, the steam generated at the nuclear core is transported to the turbine
system and is condensed in the main condenser. The condensate is used as feedwater to return to the
reactor core. Figure 34.1 shows the schematic of the BWR primary coolant loop (AESJ, 2000a). The total
system is divided into three systems: the condensate and the feedwater system, the reactor coolant water
system, and the main steam system. Chemistry specications are determined for the condensate and the
feedwater system and the reactor water system. The feedwater should bemaintained as pure neutral water
in order to avoid intrusion of any chemical species into the core. The reactor water is puried with the
reactor water cleanup system, which has a capacity of treating 1%–7% of the feedwater ow. Examples
of the water quality specications for each system are shown in Tables 34.1 through 34.3 (AESJ, 2000a).
962 Charged Particle and Photon Interactions with Matter
Suppression
pool spray
Primary containment vessel
Main steam isolation valve
Reactor
pressure
vessel
Suppression
pool
PLR
Moisture separator
LP
turbine
HP
turbine
Turbine bypass
valve
Low pressure
condensate pump
Steam jet
air ejector
Sea
water
To OG system
Grand
condenser
Feedwater
heater
High pressure
condensate
pump
Condensate
filter
Condensate
demineralizer
Condensate
storage tank
Reactor water cleanup system
Reactor core
isolation cooling
system
High pressure
core spray system
Low
pressure core
spray system
Residual heat
removal system
Standby liquid
control system
Drywell
spray
Reactor
feedwater
pump
Main
condenser
Generator
Figure 34.1 Schematic view of BWR primary cooling system. (From AESJ, Handbook of Water Chemistry of Nuclear Reactor System, K. Ishigure (ed.), Atomic
Energy
Society of Japan, Tokyo, Japan, 2000a. With permission.)
Radiation Chemistry inNuclear Engineering 963
34.2.2.2 pwr water Chemistry
34.2.2.2.1
Objectives of PWR Primary Side Water Chemistry Control
The objectives of PWR primary side water chemistry control are to control the reactivity of the core, to
maintain the material integrity of both fuels and structural materials, and to reduce personnel radiation
exposure. Boron as boric acid is added to the reactor coolant water as a neutron absorber to control
reactivity; lithium hydroxide is added as an alkaline agent to control the reactor water pH; and hydro-
gen gas is added to maintain a reducing environment to suppress SCC of the structural materials.
table 34.1
an
e
xample
of
bwr r
eactor
w
ater
Quality
Controlparameters
items unit Control value standard value
Conductivity
(25°C)
μS cm
−1
<1 <10
pH
(25°C) — 5.6–8.6
4–10
Cl
−
ion ppb <100 <500
Silica ppm <5 —
Source: AESJ, Handbook of Water Chemistry of Nuclear Reactor System, K. Ishigure
(ed.), Atomic
Energy Society of Japan, 2000c.
With
permission.
table 34.2
an
e
xample
of bwr Feedwater Quality
Control
p
arameters
items unit Control value
Dissolved
O
2
ppb 20–200
Metal
impurities (as Fe, Cu, Ni, Cr) ppb <15
Total cupper (as Cu) ppb <2
Source:
AESJ, Handbook of Water Chemistry of Nuclear
Reactor System, K. Ishigure (ed.), Atomic Energy
Society
of Japan, 2000c.
With
permission.
table 34.3
an
e
xample
of bwr Condensate d
emineralizer
o
utlet
w
ater
Quality Control p
arameters
items unit Control value
(25°C)
μS
cm
−1
<0.1
pH
(25°C) —
6.5–7.5
Cl
−
ion ppb <100
Silica ppb <10
Metal
impurity (as Fe, Cu, Ni, Cr) ppb <15
Total cupper (as Cu) ppb <2
Source:
AESJ, Handbook of Water Chemistry of Nuclear Reactor
System, K. Ishigure (ed.), Atomic Energy Society of Japan,
2000c. With
permission.
964 Charged Particle and Photon Interactions with Matter
34.2.2.2.2 Outline of PWR Primary Coolant Loop
The PWR primary coolant loop is under non-boiling conditions and the generated heat is trans-
ferred to the secondary side through the steam generators. Figure 34.2 shows the schematic of the
PWR primary coolant loop (AESJ, 2000b). The primary loop has the chemical volume control
system (CVCS), which controls the volume change of the coolant by temperature change, regulates
the concentrations of boric acid and lithium hydroxide, and controls the dissolved hydrogen concen-
tration to suppress corrosion. An example of the water quality specication for the primary loop is
shown
in Table 34.4 (AESJ, 2000b).
34.2.3 evaluation of corroSion environMent in bwrS
With the increase in operating years of commercial BWRs, it has been widely recognized that the
role of preventive maintenance for plant materials is of great importance. In BWRs stainless steels
and nickel-base alloys are used as structural materials. For plants with longer operating history, con-
siderable attention should be paid to the maintenance of those components, especially the pressure
boundary
components and the reactor core internals.
Steam generator
Containment building
Chemical and volume control system
Reactor coolant
pump
Pressurizer
Reactor
vessel
Pressurizer relief tank
Demineralizer
Filter
Volume
control
tank
Pure water
tank
Boric
acid tank
Containment sprays
Refueling
water storage
tank
Containment
spray pump
Low pressure
injection system
(RHR)
High
pressure
injection
system
Containment sump
Accumulator
RHR heat exchanger
Charging pump
Chemical
addition
tank
Figure 34.2 Schematic view of PWR primary cooling system. (From AESJ, Handbook of Water Chemistry
of Nuclear Reactor System, K. Ishigure (ed.), Atomic Energy Society of Japan, Tokyo, Japan, 2000a. With
permission.)
Radiation Chemistry inNuclear Engineering 965
IGSCC of austenitic stainless steels and nickel-base alloys has been a major issue in BWRs. This
IGSCC occurs when three key factors exist simultaneously: material sensitization, residual stress,
and an oxidizing environment. Countermeasures from the viewpoints of material and residual stress
can be taken for the out-of-core components or the materials of good accessibility. The control of
water chemistry, however, can reduce the possibility IGSCC occurrence in materials, which still
have susceptibility. Figure 34.3 shows the basic concept of IGSCC prevention by removing one or
more
factors of IGSCC occurrence (AESJ, 2000c).
An
approach to simulate the BWR core environment by water radiolysis modeling has attracted
wide attention from the standpoint of IGSCC mitigation of the structural materials. As a result of
the recent progress in modeling, water radiolysis in the BWR primary system is now well described
by radiolysis calculation codes (AESJ, 2000c; Takagi etal., 1989, 1998, 2000; Ichikawa etal., 1992,
1994,
1996; Urata etal., 1998; Takagi and Ichikawa, 1999, 2001).
Corrosion
potential has been regarded as an essential parameter to predict IGSCC behavior
of the structural materials and much effort has been made to establish a prediction of corrosion
table 34.4
an
e
xample
of pwr p
rimary
w
ater
Quality
Control
p
arameters
items unit Control standard
Conductivity (25°C)
μS
cm
−1
a
—
pH
(25°C) —
a
—
Boron ppm
b
—
Cl
−
ion ppm
≤0.05 ≤0.15
F
−
ion ppm
≤0.05 ≤0.15
Dissolved
O
2
ppm
≤0.005 ≤0.1
Dissolved
H
2
cc-STP kg
−1
·H
2
O 25–35
≥15,
≤50
Li
+
ion ppm 0.2–2.2 —
Source: AESJ, Handbook of Water Chemistry of Nuclear Reactor
System, K. Ishigure (ed.), Atomic Energy Society of
Japan, Tokyo,
Japan, 2000b.
With
permission.
a
Dependent on the combination of B and Li concentrations.
b
Dependent on the operation.
Remove critical species
Degassed start-up operation
H
2
injection
Environment
Stress
SCC
Material
Stress-relief
Annealing
Shot-peening
Grid-blasting
Selection
and control
of material
Figure 34.3 Three SCC factors and preventive protection of SCC (stress corrosion cracking). (From
AESJ, Handbook of Water Chemistry of Nuclear Reactor System, K. Ishigure (ed.), Atomic Energy Society of
Japan,
Tokyo, Japan, 2000c. With permission.)
966 Charged Particle and Photon Interactions with Matter
potential distribution in addition to radiolysis modeling. A mixed potential theory has therefore
been
applied to electrochemical corrosion potential (ECP) modeling.
An
application of the radiolysis model and the ECP model to the prediction of the chemical spe-
cies and ECP distributions in the primary system under normal water chemistry and hydrogen water
chemistry
conditions is briey described in this chapter.
34.2.3.1
preventive
w
ater
Chemistry
The
importance of water chemistry control has been widely recognized from the viewpoint of
mitigation of IGSCC especially for the structural materials in the BWR primary system. Hydrogen
water chemistry (HWC) is one of the IGSCC countermeasures and has been applied to many BWR
plants
over the world.
According
to the best current knowledge of material corrosion, corrosion potential control is crit-
ical in order to reduce the IGSCC susceptibility of stainless steels and nickel-base alloys. However
corrosion potential is a function of material, water chemistry, and ow condition of the system.
In this sense, corrosion potential is a key parameter, which connects the water chemistry and the
material
behavior as an IGSCC growth rate.
The
local corrosion environment is different depending on its geometry, dose rate, and ow
rate. As preventive maintenance is based on countermeasures to each of these, an understanding of
the corrosion environment, especially under high ow condition or in a creviced region is of much
interest. One of the possible approaches is to perform a three dimensional water ow and radiolysis
calculation, which will be combined with an ECP model prediction. This development will utilize a
more
detailed and specic prediction of the corrosion potential of each component.
34.2.3.2
basic
a
pproaches
to Control igs
CC
The behavior of material corrosion consists of two reactions. One is an anodic reaction of the metal
dissolution or hydrogen oxidation which will donate electrons. The other is a cathodic reaction
which
is oxygen or hydrogen peroxide reduction which will receive electrons.
Generally a corrosion reaction is described electrochemically as follows:
Anodic reactions:
M M ne= +
+ −n
(34.1)
H H e
2
= +
+ −
2 2
(34.2)
Cathodic reactions:
O H e H O
2
+ + =
+ −
4 4 2
2
(34.3)
H O H e H O
2 22
2 2 2+ + =
+ −
(34.4)
Corrosion potential (E
corr
) is dened as the potential at which the anodic current and the cathodic
current
are balanced.
i i E E
a c corr
at ,= =
(34.5)
where
i
a
is the total anodic current
i
c
is the total cathodic current
E
corr
is the corrosion potential
Radiation Chemistry inNuclear Engineering 967
In order to reduce corrosion potential, there are basically two approaches. One is the cathodic reac-
tion control such as hydrogen water chemistry, and the other is the anodic reaction control such as
noble
metal chemistry.
34.2.3.2.1
Cathodic Reaction Control
When hydrogen water chemistry is applied to a BWR plant, the concentration of the oxidizing
species such as oxygen and hydrogen peroxide will be reduced depending on the injection rate of
hydrogen. The principle of this approach is to reduce oxidizing species concentrations in the system.
On an Evans diagram, it is shown as (1) a decrease in equilibrium potential, and thus (2) a decrease
in
limiting current.
As
shown in Figure 34.4, the corrosion potential is expressed as the crossing point of the anodic
curve and the cathodic curve, which becomes lower compared with the normal water chemistry
condition. In this gure, the anodic curve that is metal dissolution is not changed and the corrosion
current
after hydrogen injection is reduced (Takagi and Ichikawa, 2001).
34.2.3.2.2
Anodic Reaction Control
When the material surface is treated by noble metal, the hydrogen oxidation reaction is much
enhanced by a catalytic effect of the noble metal. Therefore, the total anodic current is largely
increased. Figure 34.5 shows a schematic that illustrates the corrosion potential decrease on an
Evans
diagram (Takagi and Ichikawa, 2001).
The
gure shows the corrosion potential decrease by enhancement of the anodic reaction of hydro-
gen. In this case, the exchange current density becomes large and controls the corrosion potential.
The corrosion current is increased when the potential reaches the corrosion potential. However,
the corrosion potential in this system will be largely reduced. The reduction by anodic reaction
control
is sometimes larger than that by cathodic reaction control.
34.2.4 Modeling approach to bwr corroSion environMent evaluation
34.2.4.1 general modeling method
In order to describe the IGSCC phenomena in BWRs, both crack initiation and crack propaga-
tionshould be discussed. From a practical viewpoint, the quantitative analysis of crack propagation
SS
oxidation
Potential
H
2
oxidation
Corrosion
potential
decrease
O
2
reduction
Current
Figure 34.4 Comprehension of corrosion potential decrease by decrease in O
2
concentration on
Evans diagram.
968 Charged Particle and Photon Interactions with Matter
is recognized as the most important parameter when dealing with plant aging issues. Figure 34.6
shows the general modeling method used to describe the crack growth rate of a specic reactor
component (Takagi and Ichikawa, 2001). This requires both radiolysis modeling and the ECP (elec-
trochemical
corrosion potential) modeling.
34.2.4.2
radiolysis
m
odeling
Radiolysis modeling is done by solving a set of more than 30 differential equations representing chemi-
cal reactions of water radiolysis products. This model calculates the concentrations of oxygen, hydrogen,
hydrogen peroxide, and other radicals along with the water ow in the primary loop of a BWR plant.
Water
radiolysis by neutron and gamma rays produces the primary radiolysis species.
H O neutrons -rays e H H OH H H O
aq2 2 2 2
— ( , ) , , , , ,γ
− + • •
(34.6)
Radiolysis modeling
Water chemistry distribution
ECP modeling
Corrosion potential distribution
Crack growth rate
modeling
Crack growth rate prediction of
a specific component
Figure 34.6 The basic concept of preventive water chemistry in the BWR primary system.
SS
oxidation
Potential
Corrosion
potential
decrease
O
2
reduction
Current
H
2
oxidation
Figure 34.5 Comprehension of corrosion potential decrease by increase in H
2
anodic current on
Evans diagram.
Radiation Chemistry inNuclear Engineering 969
Secondary reactions follow to produce molecular species like hydrogen and hydrogen peroxide.
Those
molecular species will react again with radicals.
• •
H H H+ →
2
(34.7)
• •
H OH H O+ →
2
(34.8)
• •
OH OH H O
2
+ →
2
(34.9)
• •
H H O OH H O
2
+ → +
2 2
(34.10
)
• •
OH H H H O
2
+ → +
2
(34.11)
• •
OH H O HO H O
2 2
+ → +
2 2
(34.12)
Generation
of oxygen will occur in the later stage with HO
2
as a precursor.
HO H H O
2 2
• •
+ →
2
(34.13)
HO OH O H O
2 2
• •
+ → +
2
(34.14)
HO HO O H O
2 2 2 2
• •
+ → +
2
(34.15)
The effect of hydrogen injection is described by H atom reactions that recombine oxidizing species
back
into water molecules as follows:
• •
H O HO
2 2
+ → (34.16)
• •
H HO H O
2 2
+ →
2
(34.13)
• •
H H O OH H O
2
+ → +
2 2
(34.10)
• •
H OH H O+ →
2
(34.8)
Finally, water radiolysis is shown as equilibrium reaction among water, hydrogen peroxide, and
oxygen.
The direction of the reactions determines whether the water chemistry condition is oxidiz-
ing or reducing.
H O OH H O HO O
neutron H OH H OH H OH
2 2 2 2 2
← → ←→ ←→ ←→
γ
•
•
(34.17)
The secondary reactions among the radiolysis products proceed according to initial yields (G-values)
and
the rate constants of the radiolytic product reactions.
The secondary reactions are described as
dC
dt
G P
N
k C C
i i
mn m b
m n
= +
∑
,
(34.18)