More on Baffle Bolts
Posted by
NRC on
June 1, 2016
John Lubinski
Director, Division of Engineering
Office of Nuclear Reactor Regulation
We described
degradation in baffle-former bolts at Indian Point Unit 2
in an April 27 blog post. The degraded bolts were discovered by
Entergy, the plant operator, while inspecting the bolts during a
refueling outage. Since then, PSEG, the operator of the Salem nuclear
power plant in New Jersey, discovered some degraded bolts while
inspecting the baffles of Unit 1.
To recap, the bolts hold in place a series of vertical metal plates.
Known as baffle plates, they help direct water up through the nuclear
fuel assemblies, where it is heated and subsequently used for power
production. The baffle plates are attached to eight levels of horizontal
plates called baffle-former plates, which are in turn connected to the
reactor core barrel.
Because of these findings, the NRC has initiated its process for
dealing with emerging issues to evaluate the extent of the problem and
determine how best to address it. Here’s what we know so far:
We are confident this issue lacks an immediate safety concern that
would lead us to shut down U.S. nuclear power plants or prevent the
startup of plants in refueling outages.
We have been aware of the phenomenon. Degraded baffle-former bolts
were detected in French reactors in the late 1980s and 1990s, and the
NRC published an Information Notice in 1998 alerting the U.S. industry.
The higher number of degraded bolts seen recently however, was
unexpected.
The 1998 Information Notice prompted several plants to inspect their
bolts, and some made adjustments to their baffles or replaced bolts as a
result. NRC-approved guidance from the Electric Power Research
Institute calls for visual and ultrasonic inspections during a certain
period in a reactor’s lifespan. The current inspections at Indian Point
and Salem resulted from this operational experience.
It’s important to note that if bolt parts come loose during normal
operations and damage fuel, the condition will be detected by routine
monitoring of radioactivity in the reactor coolant water. During
refueling outages, plant operators look for debris on the bottom of the
reactor vessel as another indication of potential issues. Even during an
accident, the danger of core damage would be minimal. For these
reasons, the NRC does not believe it necessary to shut down any
additional plants and order immediate inspections.
Plants with degraded bolts are required to perform analyses and/or
replace the damaged bolts before restarting. Missing bolt parts (such as
a bolt head) must be accounted for or recovered, or the licensee must
perform a “loose-parts evaluation” prior to restarting. The NRC staff
will independently assess the root-cause and safety significance of the
bolt degradation at each reactor and take appropriate regulatory action.
Indian Point 2 has replaced all of the failed bolts, plus an
additional 51 “good” bolts to add additional safety margin. They intend
to inspect all the baffle-former bolts again during the next refueling
outage.
So how many plants might have this problem? Only
Westinghouse-designed pressurized-water reactors with four reactor
coolant loops have reported significant bolt degradation. Although there
are 29 such plants in the United States, the issue appears to be
further limited by two factors: The degraded bolts have all been of a
certain type of stainless steel, and they’ve all been in reactors with
baffles in a “downflow” configuration, meaning the water entering the
reactor is pushed downward between the baffle and the core barrel, which
creates more pressure across the plates and stress in the bolts.
There are only seven four-loop Westinghouse reactors at four sites
with both the susceptible bolt material and a downflow configuration:
Indian Point Units 2 and 3, Salem Units 1 and 2, D.C. Cook Units 1 and
2, and Diablo Canyon Unit 1. (Diablo’s Unit 2 has been reconfigured to
an upflow baffle.) Our resident inspectors at D.C. Cook and Diablo
Canyon have asked those sites to consider the recent findings and
implications for their plants, including their plans for future bolt
inspections. D. C. Cook Unit 2 observed 42 degraded bolts in 2010, and
as a result replaced 52 bolts. Follow-up visual inspections in 2012
revealed no problems.
Bolt degradation starts to appear sometime after 25 “effective
full-power years” of operation (based on actual operation, not calendar
years since licensing). So the NRC-approved EPRI guidance advises PWR
operators to inspect their baffle-former bolts sometime between 25 and
35 effective full-power years. The NRC requires the inspections as part
of aging management plans for reactors with renewed licenses.
As a result of the findings at Indian Point 2, Entergy decided it
will conduct detailed ultra-sonic testing of baffle-former bolts in
Indian Point 3 during its next outage in spring 2017, instead of March
2019 as previously scheduled. Since Indian Point 3 has operated for a
shorter time and has put less thermal stress on its bolts that would
cause fatigue, waiting a year is acceptable.
PSEG had been conducting visual inspections every other outage, and
because of their discovery of degraded bolts during the current
inspection of Salem 1, they decided to conduct the ultrasonic test now
rather than in 2023 as previously planned. PG&E indicated to the NRC
that it will inspect the baffles on Diablo Canyon 1 during its next
scheduled outage in spring 2017.
While the industry is reacting to the recent findings at Indian Point
2 and Salem 1, the NRC will continue to assess baffle-former bolt
degradation for any potential implications to the rest of the U.S.
commercial reactor fleet.