Fixed
In-core
Assembly
By Martin Parece, AREVA North
America.
Martin Parece
Martin Parece is Vice President,
Products & Technology for the Reactors
& Services Business
Group of AREVA in
North America. He
is responsible for
development, technical
oversight and
configuration control
of pressurized water
and high temperature
gas reactor designs
planned for
deployment in North
America.
In addition, Mr.
Parece directs
product development,
fabrication and
testing at AREVA’s
U.S. Technical Center. Mr. Parece
has B.S. and M.S. degrees in Nuclear
Engineering and is a member of the
American Nuclear Society. During
the last 33 years with AREVA and
predecessor companies, he has gained
extensive experience in plant operations,
safety and performance analyses,
computer code development, accident
mitigation, thermal-hydraulics, plant
auxiliary systems, Class 1 component
design, reactor design, and licensing.
Prior to his appointment as Vice
President, he served as Chief Engineer
for AREVA.
Responses to questions by Newal
Agnihotri, Editor of Nuclear Plant
Journal.
1.
What are the current applications for
ICDAs?
An In-core Detector Assembly
(ICDA), also known as a “fixed in-core
assembly,” is an assembly that allows
for the permanent or fixed positioning of
temperature and radiation sensors inside
a reactor. This provides for continuous
monitoring of localized reactor power in
all regions of the core.
Each ICDA has self-powered neutron
detectors distributed along the length of
the assembly, which allows operators to
measure power from the top to the bottom
of the core. Depending on the design,
between 40 and 60 ICDAs are distributed
around the core to allow
operators to get axial
power shapes in many
locations in the core,
which yields a 3-D
power map. The ICDAs
also have a thermal-
couple that provides
fluid temperature at the
core exit during normal
operation and for post-
accident monitoring.
2.
Do the displays for
these detectors need
power to operate?
ICDAs
need
power to operate. The
self-powered neutron
detectors operate by an (n, ß) reaction,
whereby the emitter absorbs a neutron
(n) and decays (beta) by emitting an
electron. The electrons are collected by an
electrode within the detector, providing a
current that is proportional to the power.
Hence the name self-powered neutron
detector. However, this current is very
small and must be amplified, conditioned
and sent to the plant process computer,
all of which require external power. In
the event of loss of offsite power, the
thermal-couple signals are powered by
emergency power supplies and batteries.
3.
Describe tests to ensure operability
of these detectors in a beyond-design-
basis accidents?
During production, we test a sample
of the thermal-couples up to 1750 degrees
F and we calibrate each thermal-couple
up to 750 degrees F. We also perform a
pressure test up to 3200 psi, which is 150
percent of operating pressure.
4.
Describe the displays designed to
support the ICDAs in a beyond-design-
basis accident?
Only the thermal-couple of the ICDA
is designed to operate during an accident.
In this case, the normal control room
displays would be used to monitor core
exit fluid temperatures. These displays are
powered by emergency sources in the case
of loss of offsite power. Core exit thermal-
couple readings from the ICDAs, coupled
with fluid temperature measurements
from the reactor coolant system piping
and various loop flow indications, give
operators an understanding of fluid flows
and core cooling during a postulated or
beyond design basis accident.
ICDA thermal-couple temperatures
that significantly exceed saturation
temperature of the reactor coolant
indicate inadequate core cooling. In that
event, plant operators would implement
inadequate core cooling procedures.
Temperature readings approaching the
failure point of the thermal-couples
(>1,750 degrees F) indicate fuel damage
is imminent, causing the plant operators to
implement severe accident management
guidelines.
5.
Do ICDAs have self-diagnostic
feature?
Loss of signal due to a bad connection
or failures in the signal conditioning
electronics is detected immediately. More
subtle failures of the individual detectors
are identified by the operator or software.
This is done by comparing detectors in
the same ICDA or by comparing detector
signals from a suspect ICDA to those
signals from an ICDA in the matching
location in a different quadrant of the core
(i.e., match signals from “symmetrical
pairs” of ICDAs).
Monitoring detector signals and
declaring suspect detectors “out of
service” is necessary because the detector
readings are used to monitor power peaks
within the core and to ensure that power
variations from quadrant to quadrant are
within limits during power operation.
6.
Which plants worldwide are
currently using ICDAs?
22
NuclearPlantJournal.com Nuclear Plant Journal, January-February 2016
