Nuclear Plant Journal, July-August 2015 NuclearPlantJournal.com
27
Fatigue Limits
EPRI is collaborating with Korea
Hydro & Nuclear Power Co. (KHNP) and
Mitsubishi Heavy Industries (MHI) to
collect data on environmentally assisted
fatigue (EAF) under conditions that
closely simulate plant operation. This
data will enable more realistic predictions
of EAF degradation and inform the
calculation of fatigue usage, which is
used to guide decisions related to new
plant design and long-term operations.
Light water reactors were designed
for cyclic fatigue using acceptance
criteria based on simplified laboratory
testing in an air environment. Follow-on
research later revealed, however, that the
water environment in nuclear reactors
accelerates fatigue degradation. The
U.S. Nuclear Regulatory Commission
generated fatigue test data in water
environments using similar simplified
methods to quantify this acceleration,
and subsequently published fatigue
acceptance requirements for new plants
and for long-term operation of existing
plants. The potential conservatism
introduced through the use of data
from simplified test methods – when
combined with the accelerated fatigue
in water environments – can result in
fatigue degradation predictions that
do not correlate with field experience.
This discrepancy, in turn, can impact
inspection and analysis requirements.
A suspected reason for the difference
is that the NRC laboratory tests did
not simulate plant operation. In 2012,
four EPRI programs – PWR Materials
Reliability Program, BWR Vessel and
Internals Project, Advanced Nuclear
Technology, and Primary Systems
Corrosion Research – joined forces to
develop additional test data, publishing
a report documenting needed research
and testing (Environmentally Assisted
Fatigue Gap Analysis and Roadmap for
Future Research (1026724)). Although
initiated in response to U.S. concerns,
this research will be applicable in other
countries that have similar requirements.
The KHNP testing focuses on
evaluating the effect of steady-state
plant operation between fatigue-causing
transients (such as startup/shutdown)
on EAF degradation. This phenomenon
was not accounted for in the test data
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underlying the NRC guidance. KHNP
also is exploring the effect of surface
roughness on EAF. After almost two years
of testing, initial results are emerging,
and testing will be completed by the end
of 2016.
MHI also is addressing the effect of
steady operation, focusing on longer hold
times and experiments to confirm test
results by other researchers. Additionally,
MHI is examining the effect of plant
transients that cause metal temperatures
to change either in- or out-of-phase with
material stresses and strains – more
effectively simulating plant operation.
The MHI testing began in late 2014 and
is scheduled for completion in 2016.
Contact: Al Ahluwalia, EPRI,
telephone: (973) 396-2777, email:
.
Source: Electric Power Research
Institute's (EPRI) Nuclear Executive
Update, May 2015.
