Page 166 - Handbook of Materials Failure Analysis

162 CHAPTER 7 Investigation of failure behavior of tubular components

In nuclear reactors, thin-walled tubes are used as fuel-clad tubes as well as pressure
tubes and calandria tubes, respectively. Zirconium alloys are widely used as the
fuel-cladding tubes and other core internals in light-water reactors such as boiling water
reactors, pressurized water reactors as well as pressurized heavy-water reactors.
Understanding of failure behavior of these fuel-clad tubes is important from
safety point of view. Failure of fuel-clad can release enormous amount of radioac-
tivity into the coolant and this in turn will contaminate other components in the reac-
tor. In case leakage of coolant at flanges or other joints, the activity may be released
to reactor containment. The decontamination of rector containment and other sys-
tems in case of large-scale failure of fuel-clad tubes can add a huge economic penalty
on the utility. Hence, assessment of clad-integrity is an important concern to oper-
ators of nuclear power plants as well as designers and safety analysts.
These zirconium-based alloys have favorable properties such as high mechanical
strength, good ductility, very-low neutron absorption cross-section, and good corro-
sion resistance [1–3]. Fuel-clad tubes are of small thickness in order to have less
resistance in the path of heat flow from the fuel to the coolant.
Under reactor operating conditions, the deformation behavior of fuel cladding is
strongly influenced by the mechanical properties of the material. The mechanical
properties also significantly change with years of reactor operation because of irra-
diation, high temperature, high stress, and corrosive environment, etc. Nonstandard
tests (e.g., ring-tension test) on very small specimens need to be performed as it is
difficult to extract standard specimens from these tubes. A ring tensile test (RTT) is
one of the tests that use a ring specimen to obtain the hoop tensile properties of the
tube-shaped structures.
The ring specimen has a curvilinear shape, and bending moment and deformation
appear during the ring test. To reduce the bending moment, Arsene and Bai [4] pro-
posed a central piece-inserted ring tensile specimen. Josefsson and Grigoriev [5]
have used the ring-tension specimen to determine the mechanical properties of irra-
diated fuel pins. Limited bending stresses appear during pellet-clad mechanical inter-
action in the fuel-clad. The bending phenomenon rather affects the ring-tension test
due to the specimen geometry, diameter difference between the ring and the mandrel,
space (i.e., gap) between the semi-cylindrical mandrels, etc. [6,7]. The gauge geom-
etry is not very well defined when plain rings are used in the tests. For this reason,
many laboratories have developed RTTs with machined gauge section for the ring
[4,6]. This specimen along with its modified versions and several other designs have
been used in literature [8–19] to evaluate the ductility, creep and strain rate sensitiv-
ity, delayed hydrogen cracking velocity, and fracture resistance behavior of various
types of Zircaloy fuel claddings.
It is known that the standard ASTM test techniques cannot be used for these thin
and small-diameter tubular specimens. The results from these tests can also not be
directly used to determine the mechanical properties and hence, a finite-element (FE)
analysis is essential for this purpose. In this work, ring-tension tests were carried out
on unirradiated Zircaloy-4 cladding tube specimens. Specimens for these tests were
machined from the fuel pins in the form of rings. A split-mandrel was used to load
these ring specimens in tension. Both the specimen and the mandrel were modeled to