Better
Plant
Response
By Ronald Jarrett, Tennessee Valley
Authority.
Ronald Jarrett
Ronald Jarrett is an I&C Engineering
Specialist and Digital Project Manager
in the Corporate Design Engineering
Staff for TVA
Nuclear (NPG). He
is responsible for
engineering expertise
regarding nuclear site
Instrumentation and
Control (I&C) Systems.
Jarrett is the
Program Manager
responsible for NPG’s
Instrumentation
Setpoint Program and
for NPG’s Engineering
program for the
design, procurement,
qualification, and
application of Digital
Instrumentation and Systems.
He is Co-Chairman of ISA 67.06 Work
Group for Performance Monitoring,
Chairman of ISA 67.04 Work Group
for Data Quality, Co-Chairman of
PWROG Transmitter Calibration
Extension Project, an NEI Digital
Work Group Representative for NRC
ISG#4 development, and part of NEI
Digital Federal Regulation 10CFR50.59
“Changes, tests and experiments” and
Common Cause Failure Working Groups.
Jarrett received a BSEE in Electrical
Engineering from the University of
Tennessee with centralized study in the
areas of Electronic design and Control
System design.
An interview by Newal Agnihotri, Editor
of Nuclear Plant Journal, at the Utility
Working Conference in Amelia Island,
Florida on August 15, 2016.
1.
What is the scope of a digital upgrade
project at TVA?
For TVA Nuclear, we put our formal
digital program in place in the mid-‘90s
due to a lot of lessons learned during
the early ‘90s in the non safety area.
We look at what tasks add value, what
doesn’t, what things to do and don’t do.
We developed a set of defensive measures
to be used in our designs. For example,
vendors will have built in features that
they say will take care of this failure and
that failure. You’ll say okay, that’s fine,
but when we do our application software,
we design in an extra level of redundancy
with our preventative design measures.
These features such
as, if an input signal
fails out of range, high
or low, the system itself
will take the failed
input signal out of scan
automatically. We don’t
rely on these built-in
features because they
are for gross failures
and
not
bumpless.
That’s kind of an extra
backup feature. We have
had some issues with
one of these features
recently. The use of 3
redundant inputs and the
medium signal select is
a very powerful tool that we utilize in our
application software.
We design these defensive measures
for availability. In the non safety area,
plant availability is our key focus only
second to safety. We want to eliminate
our single point vulnerabilities. When
we did our feedwater upgrade, just in the
feedwater control system alone 37 single
points of vulnerability were eliminated.
That system had been tripping us,
probably, on an average, once per year.
To date during the past 5 to 7 years, we
have not had any trips. Due to feedwater
control system failures the improved
plant performance has prevented several
trips.
We now have a low power control
system that automated the bypass
controls. All the bypass valves were
manual requiring constant monitoring
and time consuming adjustments. Our
operators would get to a point where they
would transfer from the bypass valves
to the main feedwater control valves.
That was a very sensitive control area
due to the valve sizing differences and
the high gain of the colder water at low
power (aka. shrink and swell effect).
When our operators would crack the
main valve open manually, you would
get significantly more flow than the
full open bypass valves. In our control
scheme, we went to three element control
(feedwater flow, steam flow, and SG
level).The flow signals are not of high
quality at low power, so we start control
in single element mode, SG level only,
until at least one feedwater and one steam
flow signal is on-scale and is acting in a
reliable manner. As these input signals
come on scale with good quality, they are
automatically placed in service. SG level
control transitions to three element control
mode. We’re in a load following mode so
as the turbine comes up, feedwater just
follows it on up as we increase power.
The operator brings in the condensate
and main feedwater pumps during power
ascension. Bringing in the pumps are
still manual functions but as far as steam
generator level control, it just follows
providing significant improved control
response during the normal startup
progression and not just on transients.
We’ve had trips and transients due to
other plant events and we go back and
look at how steam generator level control
functioned during these events. We have
a lot less shrink and swell problems.
Our steam generator levels variations
are smaller and recover to set point a lot
quicker on a transient condition. So, it’s a
significant improvement.
2.
Which plants are in this scope?
Watts Bar, Sequoyah and Browns
Ferry. Browns Ferry was our most
digitalized plant before Watts Bar
Nuclear Unit 2. We’ve been doing digital
mod since the mid-‘90s. So, a lot of
the plant has been upgraded in the non
safety area. For the safety area, we’re still
using analog Rosemount trip units and
transmitters. At Watts Bar and Sequoyah,
we put in digital Eagle-21 Reactor
Protection System (RPS) and Engineered
Safety Features and Actuation Systems
(ESFAS) during the late 80s’.
We started up Watts Bar with
Eagle-21. I wouldn’t say it’s a simple
system, but compared to today’s systems,
32
NuclearPlantJournal.com Nuclear Plant Journal, September-October 2016
