4' Results Contd

1
Fracture Toughness of Hydrided Zircaloy-4 Sheet
4. Results (Contd)
1. Introduction
3. Experimental Procedures
Production of a Single-Edge Surface Crack To
perform fracture toughness testing, a single
crack must initially be present across the width
of a bend specimen. A unique technique was
developed to achieve this by localizing hydrogen
absorption in order to generate a linear hydride
blister (Fig.5). Radial hydrides are present
directly under the blister and circumferential
hydrides are present in the rest of the substrate
Elastic-Plastic Fracture Mechanics (EPFM)
Zirconium the Nuclear Industry Nuclear energy
provides 20 of the USAs electric power, and
this is likely to increase in the future. A fuel
assembly in a light-water nuclear reactor (Fig.1)
consists of a series of fuel rods containing
fissile material in the form of uranium dioxide
(UO2) pellets. The UO2 pellets are contained in
cladding tubes made of a zirconium alloy which
serves as a barrier between the pellets and the
coolant (water). Zirconium and its alloys are
selected for this application mainly because of
their very low neutron absorption cross-section,
which optimizes the operating efficiency of the
reactor. Zircaloy-4 has been the most widely used
alloy for fuel cladding in pressurized water
reactors (PWR).
KQ fracture toughness values are calculated using
the J-integral approach
Bn specimen width b remaining ligament Apl
plastic energy spent by the applied load K
stress intensity factor at crack tip E Youngs
modulus ? Poissons ratio ? ratio of the
plastic J-integral to the total plastic work done
per unit uncracked area ? geometry factor
Figure 5 cross-section of a cracked test specimen
All specimens are pre-cracked at room temperature
while monitoring the acoustic emission. The
majority of the signals scale with the load and
are attributed to the load frame. (Fig.6) Higher
energy signals occurring between 50 and 100 of
the yield load are attributed to blister
cracking. Hence prior to testing, all specimens
are loaded up to the onset of yielding at room
temperature to ensure the presence of an initial
crack. Crack length is measured via micrographs
prior to testing.
K-R Resistance Curves The load-displacement data
from a four-point bend fracture toughness test
can be correlated to the crack length to produce
crack growth resistance (K-R) curves. Because
the crack length is known at each instant, the
fracture toughness can be related to the
microstructure of the substrate into which the
crack grows, as shown in Fig.10 for a room
temperature test. By knowing the fracture
toughness, it is possible to predict failure for
a given potential crack length, which generally
corresponds to a certain service life for a fuel
rod in the reactor (usually in years), after
which it must be replaced. Safety guidelines can
thus be defined based on the fracture toughness
failure criterion.
In-service Microstructural Evolution Immersed in
water in the reactor core at 300C, in
conditions of strong heat flux, aggressive
chemistry and radiation field, Zr alloy cladding
undergoes uniform corrosion in its outer
diameter Unlike radiation damage, which
appears to saturate rather early in reactor
exposure (months), hydrogen pickup continues as
long as corrosion is taking place. This results
in the precipitation of hydride platelets in the
Zircaloy-4 matrix, as well as the formation of
reduced ductility zones such as hydride rims or
blisters (Fig.2).
Figure 6 Load and corresponding acoustic
emission versus time for a test to failure
Crack Length Measurement The electric potential
drop (EPD) technique was used to measure crack
length. This technique uses the relationship
between the normalized EPD across the crack and
the normalized crack length (Johnsons equation)
Figure 10 K-R resistance curve for a room
temperature fracture toughness test, i.e.
fracture toughness KQ as a function of crack
length increment da.
5. Conclusions
Figure 7 Current and potential lead placement
locations for the EPD technique

• A 4-point bend test configuration was designed
  for fracture toughness testing of hydrided CWSR
  Zircaloy-4 sheet under through-thickness crack
  growth conditions at room temperature and up to
  400C. The specimens contain a narrow strip of
  brittle hydride blister across their width, which
  form a pre-crack upon initial loading, allowing
  for the use of elastic-plastic fracture mechanics
  to determine crack growth initiation conditions.
• Acoustic emission was used to monitor
  pre-cracking, and the electric potential drop
  (EPD) method was used to measure crack length at
  each instant during a fracture toughness test.
• At 25C, mode I crack growth through the
  thickness of the specimen occurs. The crack
  growth behavior and fracture toughness are
  sensitive to the microstructure of the hydride
  particles beneath the solid hydride blister. For
  crack growth within radial or mixed hydrides, Kq
  ? 25 MPavm. In contrast, Kq is much higher, Kq ?
  75 MPavm, when the crack grows through a zone of
  circumferential hydrides. Crack growth then
  occurs by linking the large hydride-initiated
  primary voids.
• At 300C, stable mixed mode III crack growth
  through the thickness of the specimen occurs with
  extensive plastic deformation. Initial data
  indicate Kq ? 83 MPavm, independent on the
  microstructure. Despite the similarity of this
  Kq-value to that for in-plane crack growth at
  room temperature, the fracture surfaces formed at
  300ºC suggest little direct involvement of
  hydrides in the crack growth process, which is
  quite unlike that at room temperature. In fact,
  the hydride precipitates do not crack.

• Fracture Toughness Testing
• A specially designed 4-point bend fixture (Fig.8)
  allows for fracture toughness testing up to
  400C.
• Uniform bending is achieved between the center
  loading-pins
• Stress fields resulting from the loading pins do
  not interact with the crack tip
• Stable crack growth achieved in displacement
  control

Reactivity Initiated Accident (RIA) During a RIA,
the ejection or drop of a control rod would cause
the fission rate to suddenly increase, resulting
in a significant amount of energy being deposited
in the fuel over a very short period of
time. This results in thermal expansion of the
fuel, which then comes in contact with the
cladding. Further loading can cause crack
initiation in a highly hydrided spot such as a
hydride rim or hydride blister, and subsequent
crack growth through the substrate material under
the rim or blister (Fig.3). If failure were to
occur, fuel channels may be blocked, resulting in
a loss of coolable geometry, fuel damage, or even
a partial meltdown.
4. Results
Fracture Behavior

• At 300C
• Mixed mode III crack growth at 45 angle due to
  shear deformation
• No hydride cracking
• Ductile failure of the Zircaloy-4 substrate
  (large ductile dimples)

• At 25C
• Mode I crack growth along hydrides
• Fractured hydrides alongside the crack
• Hydrides crack and form large primary voids
• Ductile failure in between hydrides (ductile
  dimples)

2. Motivation for this Study
At operating temperatures, cladding failure
during a RIA appears to be limited by crack
initiation and growth. It is assumed the strain
to failure in high-burnup cladding is defined by
the relationship ?total ?crack initiation
?crack propagation Previous
studies This study The strain to
initiate or grow a crack is directly related to
the fracture toughness of the material, thus it
is critical to determine the fracture toughness
(KQ) of hydrided Zircaloy-4 for through thickness
crack propagation. No such data is currently
available.
6. Future Work

• Additional testing at 300C and 375C while using
  the electric potential drop technique for crack
  length measurement. Work in progress.
• Finite element modeling of the 4-point bend test
  must be performed to validate the EPFM analysis
  used. In particular, J-integral calculations must
  be implemented to determine the geometric factors
  ? and ? and compare them to the literature. Work
  in progress.
• The applicability of this fracture toughness
  failure criterion to existing life prediction
  codes used for power plant licensing must be
  investigated. Guidelines to implement the
  criterion must be defined.

Acknowledgements References
Reference P. Raynaud, Fracture Toughness of
Hydrided Zircaloy-4 Sheet, M.Sc. Thesis in
Materials Science and Engineering, The
Pennsylvania State University, 2005
The financial support for this research is
provided by the N.R.C.
Figure 4 Finite element modeling of a hydride
blister submitted to a four point bend fracture
toughness test (Von Mises stress, yielding is
represented in red). The failure of the cladding
due to crack growth is controlled by its fracture
toughness.
Figure 9 Fracture behavior at 25C and 300C.