ON-LINE NDE FOR ADVANCED REACTOR DESIGNS
N. Nakagawa1, F. Inane1, R. B. Thompson1, W. R. Junker2, F. H. Ruddy2, J. M. Beatty2,
andN.G.Arlia 2
^mes Laboratory and Center for NDE, Iowa State University, Ames, IA 50011
Science & Technology Department, Westinghouse Electric, Pittsburgh, PA 15235
2
ABSTRACT. This expository paper introduces the concept of on-line sensor methodologies for
monitoring the integrity of components in next generation power systems, and explains general
benefits of the approach, while describing early conceptual developments of suitable NDE
methodologies. The paper first explains the philosophy behind this approach (i.e. the design-forinspectability concept). Specifically, we describe where and how decades of accumulated knowledge
and experience in nuclear power system maintenance are utilized in Generation IV power system
designs, as the designs are being actively developed, in order to advance their safety and economy.
Second, we explain that Generation IV reactor design features call for the replacement of the current
outage-based maintenance by on-line inspection and monitoring. Third, the model-based approach
toward design and performance optimization of on-line sensor systems, using electromagnetic,
ultrasonic, and radiation detectors, will be explained. Fourth, general types of NDE inspections that
are considered amenable to on-line health monitoring will be listed. Fifth, we will describe specific
modeling developments to be used for radiography, EMAT UT, and EC detector design studies.
INTRODUCTION
The purpose of this paper is to outline a recently initiated project, funded by the
DOE as a part of the Nuclear Energy Research Initiative (NERI). The project involves the
design and development of an on-line health-monitoring system, incorporating NDE
sensors, applicable to next-generation nuclear power system design. Decades of
maintenance and NDE experience have been accumulated from existing commercial
reactors. The project aims to advance nuclear power system safety and economy by the
proactive use of this experience to impact future reactor designs while they are being
developed. Here, the design-for-inspectability concept is put into practice; namely, by
identifying the potential failure modes for each of the critical reactor components, we can
affect the design to ensure that any component failure can be reliably detected and
addressed before it reaches the critical stage.
Among various Generation IV reactor designs, we pay particular attention to the
International Reactor Innovative and Secure (IRIS). IRIS is a scale-up version of IRIS-50
[1] that uses the same basic design principles, and is currently under development for
commercial use by an international collaboration. In what follows, we will review briefly
the IRIS design targets and features, and show that on-line monitoring systems are
indispensable in meeting the design goals, particularly for safe and continuous long-term
reactor operations. We will subsequently present a current list of on-line monitoring needs
and candidate NDE methodologies, and describe model-based studies in the areas of
electromagnetic, EMAT UT, and radiography methods. Finally, we draw conclusions
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
1728
about our on-line NDE concept for health monitoring of next generation nuclear power
systems.
REACTOR DESIGN AND ON-LINE MONITORING
IRIS Design Features
Generation IV reactors have several general design goals in common; a) passive
safety features, b) increased availability and economy, c) long-term, uninterrupted
operation, d) environmental friendliness, and e) proliferation resistance. To meet these
design requirements, most Generation IV reactors use compact, integrated designs, and are
made compatible to operating with extended refueling cycles.
IRIS, for example, is a descendant of the pressurized water reactor (PWR), but
integrates the reactor core, steam generators (SG), and pumps in a single reactor pressure
vessel (RPV) [1]. By design, the IRIS reactor is compact and cost-effective, and requires
less maintenance because it has no large primary-water loop piping outside RPV. In
addition, there is less likelihood that the SG tubes will develop stress-corrosion cracking
(SSC) since they operate in compression.
However, the long refueling cycle (every 4 years for IRIS-based design) poses a
maintenance challenge because there will be fewer opportunities for periodic outage-based
maintenance, as has been practiced for existing commercial reactors.
Call for Oil-Line Maintenance Strategy
In our view, the key design features of future reactors such as IRIS, namely the
long-term, uninterrupted, and safe operations, call for a new maintenance strategy, not
relying solely on outage-based maintenance, but also actively performing on-line
inspection and monitoring. The advantages of on-line health monitoring are multi-fold: 1)
It is done while the reactor is in operation, not requiring shutdown for
inspection/monitoring activities. 2) It is done remotely, thus greatly reducing exposure
levels. 3) It allows us to perform continuous or on-demand system integrity verification,
meaning that deviation from normal operation can be detected in real time, thus
maximizing potential options for issue resolution.
We have, therefore, initiated this project with the objective to develop conceptual
on-line sensor systems that will replace/augment outage-based maintenance. Technically,
we review the IRIS design to identify critical inspection needs, and then conceive on-line
monitoring systems that can address monitoring needs. This is followed by model-based
performance analyses. Among usual electromagnetic, ultrasonic, and radiation sensors and
detectors, we select those which are potentially compatible with on-line use in the reactor
environment.
MONITORING NEEDS AND ON-LINE NDE
In order to be specific to a Generation IV reactor system of significant maturity, we
examined a version of the IRIS design and identified several potential needs for on-line
integrity monitoring. Although the monitoring needs identified are from this specific
design, we believe that they are common among most of the advanced water-cooled
reactor designs. Typical examples of monitoring needs and potential on-line approaches
are listed in Table 1. Experience shows that the maintenance of steam generator (SG)
tubing is critical for long-term safe operation of nuclear power systems. Similar
requirements are expected to exist in the new designs as well. For instance, magnetite
1729
TABLE 1. Identified monitoring needs and applicable on-line NDE methods:
pressure vessel, EC=eddy current, EMAT=electromagnetic acoustic transducer.
Component
RPV=reactor
Monitoring Needs
NDE Method
Magnetite deposits
(inside tubes)
EC,EMATUT
Tube/tube sheet integrity
EMATUT
Tube integrity
(tubes themselves)
EC, EMAT UT
RPV
Cracking
EMAT UT
Reactor core
Fuel activity anomaly
y-ray radiography
Steam Generator
deposit build-up is anticipated where the secondary coolant evaporates. For the IRIS
design, the problem will occur on the inner-diameter (ID) tube surface. Existing outagebased maintenance experience indicates that eddy current (EC) inspection and guidedwave ultrasonic testing (UT) will be effective tools for deposit detection. The attachment
of the tubes to the headers another area anticipated to require monitoring, for which UT
has been demonstrated as effective. The EC and UT methods will also be applicable to
monitoring of tube integrity itself. Experience has shown that the UT method can address
issues of degradation in reactor pressure vessels (RPV), as well as attachments to RPV.
The y-ray radiography will be most effective to monitor potential fuel activity anomaly and
other reactor-core integrity.
EC Monitoring Sensors
Eddy current sensors are known to work well for detecting magnetite deposits in
SG tubing, as well as for SCC detection. On-line extension of this NDE modality is
expected to work equally well. Eddy current coils are suitable for on-line usage because
they are sufficiently robust to survive the reactor environment. Conventional EC
inspection, done during reactor shutdown with EC probes inserted into the tubes to scan
the interior wall, may be applicable while the reactor is in operation. However, for on-line
monitoring, it is more practical to build fixed-site coil arrays into the structure itself at
critical locations. Figure 1 illustrates a conceptual design example. The inspection is
designed to monitor potential deposit buildup in regions where the secondary water
evaporates, i.e., in the middle of tubes in the axial direction. An encircling solenoid coil
may be placed to surround several tubes, selected by sampling.
To estimate EC coil impedance responses, we considered a simple 2D model (Fig.
IB), with a single-layer coil encircling an infinitely long tube coaxially, while the
magnetite deposit is modeled as a uniform layer on the ID surface. The calculated
impedance results are shown in Figs. 2 and 3, where we used a conductivity of
250 £Tlcm~l and a relative permeability of 130 at 327°C for the magnetite (Fe3O4) crystal
[2]. The coil was assumed to have 50 turns in the length of 10 mm. The tube cross section
is similar in dimensions to the existing SG tubes in the version of IRIS design examined.
1730
(B)
FIGURE 1. Conceptual encircling-coil design of an on-line EC inspection for magnetite deposit detection:
(A) A specific design uses an encircling solenoid coil surrounding several tubes chosen by sampling. (B)
A simple schematic model for analytical signal estimation, involving (1) a SG tube, (2) a deposit layer on
the ID wall, and (3) a coaxial solenoid coil.
The results in Fig. 2 show that the impedance shift peaks at approximately 20 kHz.
The existence of the peak is expected because the induction vanishes in low frequencies,
while, in high frequencies, the interrogating fields fail to reach the deposit layer due to the
skin effect. This peak frequency value is favorable because it is sufficiently low that
background lift-off signals are suppressed. The signal discrimination is further simplified
by the fact that the phase difference approaches 90° near the peak. Indeed, Figure 3 shows
the detailed 27kHz result that exhibits 90° phase difference, while still having significant
signal magnitudes. The fact that the magnetite permeability is twice as high at 327°C as
the room-temperature value [2] is also helpful.
UT Monitoring Sensors
We consider the use of electromagnetic acoustic transducers (EMATs; see Ref. [3]
20-i
Lift-off signal
1
1.0E+3
1.0E+4
1.0E+5
1.0E+6
Frequency [Hz]
FIGURE 2. Calculated deposit signal magnitude and phase vs. EC frequency. The plotted phase signals
are actually the phase difference between deposit and lift-off (diameter variation) signals. The deposit
layer thickness was assumed 1% of the tube wall thickness.
1731
Deposit signal
0,0
-0.5
0,0
1
0.5
'
1"
1.0
I
I
1,5
2.0
Resistance [Ohm]
FIGURE 3. Impedance plane plot of the EC deposit signals, calculated at the fixed frequency of 21 kHz,
while the deposit layer thickness was varied from 0% to 1% of the tube wall thickness. Also shown is the
phase separation by 90° from the lift-off (diameter-variation) phase.
and the references cited therein) for UT monitoring sensors in order to meet the
requirements posed by the reactor environment. Like EC sensors, EMATs are made of
coils that are generally robust and easily ruggedized to survive the environment. A variety
of applications are conceivable. For SG tubing inspections, EMAT-generated UT guided
waves can be used for magnetite deposit detection, for monitoring the integrity of tube
mounts, and perhaps for determination of liquid-gas phase transition location. The reactor
pressure vessel is another system component for EMAT UT applications. The principal
monitoring targets are various welded attachments where sensors may be mounted for online monitoring.
The guided-wave modes for a straight tube have been known for decades [4]. For
SG tubing applications, the torsional modes are the most practical to use. The challenge is
to generate such modes by EMATs of all-coil designs (i.e., the quasi-static magnetic bias is
produced by passing a pulsed current through an appropriate coil), in order to survive
reactor environment. In Fig. 4, we present one such design concept: the asymmetrical
Helmholtz coil pair is placed to contain the tube in cross section, so that it creates the bias
DC field B pointing approximately perpendicular to the tube wall. A conventional RFdriven meandering coil placed parallel to the tube length will induce eddy current in the
axial direction. The external DC field B will act on the EC density j in the tube wall
with the force density / according to the Lorentz force relation
(1)
FIGURE 4. Schematics of an EMAT system designed to generate torsional modes in tubing. This
conceptual design uses a combination of an asymmetrical Helmholtz coil pair (1) with a meandering
coil (2).
1732
f = j*B.
(1)
Thus, the force density / approximately points in the circular direction, launching the
torsional-mode waves predominantly.
It should be noted that there are several potential complications in actual SG
designs because the tubes will be wound in spiral and immersed into the primary coolant
while their interior will be filled partially with the secondary coolant. The tube curvature,
for instance, creates mode mixing between torsional and general flexural modes, where the
latter have tendency to leak their energy into the surrounding water. The tube bend also
has a potential to modify the ovality of the tube cross section, which will cause mode
mixing as well. In addition, it has been pointed out that there may be caustic issues
because of the curved tube geometry. Generally, these potential effects are expected to be
small and may not modify the unloaded straight tube results significantly; the modification
is, at most, on the order of the ratio between the tube diameter and the spiral radius. This
ratio is small for the practical designs. However, near the tube mounts, the curvature may
become especially tight, and the effects may become significant. It may, therefore,
become necessary at the later stage of the project to estimate the amount of these effects.
Radiation-based Integrity Monitor
By nature, the reactor core generates abundant penetrating radiation. It is our view
that there are potential on-line applications of radiation detectors, such as monitoring
reactor-fuel activities and their anomalies. It may even be possible to monitor the in-vessel
structural integrity, utilizing the penetrating radiation. For these types of applications,
robust, reactor-compatible detectors that can measure radiation intensities at in-core, excore, and perhaps ex-vessel locations are needed.
Recent radiation detector developments have produced y-ray and neutron detectors
that are compatible with in-situ use inside RPV. Specifically, we consider the use of
proprietary on-chip detectors based on silicon carbide (SiC) technology [Fig. 5]. They
have been developed by Westinghouse for a, |3, y, X-ray, and neutron (thermal,
epithermal, and fast) measurements. They are not only operable at elevated temperatures
(up to 700°C), but also much more resistant to radiation damage than conventional
semiconductor detectors. Given such in-vessel capabilities, the use of radiation detectors
becomes practical in several on-line monitoring applications, such as reactor core integrity,
FIGURE 5. Photograph of a patented radiation detector fabricated on SiC semiconductor chip.
1733
fuel activity anomalies, and fuel rod integrity.
To estimate in-vessel radiation intensities as functions of locations over the reactor
life, radiation propagation models are needed. There exist both Monte Carlo and
deterministic codes, available in the public domain, to perform some of the needed
numerical calculations. However, for our considerations on in-core and near-core detector
placement, it is important to account for hard y-rays, i.e. the spectrum itself as well as their
effect on soft y-ray spectra. The hard y-rays affect the soft y-ray spectra through photon
interactions with charged particles (electrons and positrons). The photon spectrum
calculation, therefore, must be coupled with the charged-particle sector to estimate photon
spectra accurately at various locations of interest. Development of a deterministic
transport-equation model including the charged-particle sector is a part of this project, and
the progress is reported in a companion paper to describe the above and related issues
concerning hard y-rays [5].
CONCLUSIONS
In this paper we address maintenance issues associated with next-generation
nuclear power systems that aim to achieve specific design goals, including safety and
economy, by the use of compact, integrated designs and extended refueling cycles. Our
first conclusion is that on-line integrity monitoring systems are indispensable elements of
next-generation reactors for safe and long-term operations without interruption, and they
can be conceived as built-in devices based on known NDE technologies. Second? through
a case study of a next-generation design (IRIS), we found several specific on-line
inspection needs (cf. Table 1). We believe that these are generic among various lightwater Generation IV reactor designs in order to ensure long-term integrity and safety while
allowing reduction of excessive system redundancy. We also have made preliminary
conceptual designs and model-based studies of on-line NDE monitoring devices for each
of the perceived monitoring needs, based on eddy current, EMAT ultrasonic, and
radiography techniques.
ACKNOWLEDGMENT
This work is supported by the US DOE Nuclear Energy Research Initiative (NERI)
Program, Project Number 2001-076.
REFERENCES
1. "Study outlines reactor designs that may be ready for deployment by decade's end,"
Nuclear News 44,25-33 (2001).
2. J. L. Snoek, New developments in ferromagnetic materials, Elsevier, Amsterdam,
1947.
3. R. B. Thompson, Physical Acoustics 19, 157 (1990).
4. D. C. Gazis, J. Acoust. Soc. Am. 31, 568 (1959).
5. F. Inane, "Issues with the High Energy Radiography Simulations," in this Volume.
1734