Nuclear Technology
ISSN: 0029-5450 (Print) 1943-7471 (Online) Journal homepage: http://www.tandfonline.com/loi/unct20
Nanofluids for Enhanced Economics and Safety of
Nuclear Reactors: An Evaluation of the Potential
Features, Issues, and Research Gaps
Jacopo Buongiorno, Lin-Wen Hu, Sung Joong Kim, Ryan Hannink, Bao Truong
& Eric Forrest
To cite this article: Jacopo Buongiorno, Lin-Wen Hu, Sung Joong Kim, Ryan Hannink, Bao Truong
& Eric Forrest (2008) Nanofluids for Enhanced Economics and Safety of Nuclear Reactors: An
Evaluation of the Potential Features, Issues, and Research Gaps, Nuclear Technology, 162:1,
80-91
To link to this article: http://dx.doi.org/10.13182/NT08-A3934
Published online: 10 Apr 2017.
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Date: 18 June 2017, At: 01:07
NANOFLUIDS FOR ENHANCED
ECONOMICS AND SAFETY OF
NUCLEAR REACTORS: AN EVALUATION
OF THE POTENTIAL FEATURES,
ISSUES, AND RESEARCH GAPS
THERMAL HYDRAULICS
KEYWORDS: nanofluids, reactor
coolant, critical heat flux
JACOPO BUONGIORNO,* LIN-WEN HU,* SUNG JOONG KIM,
RYAN HANNINK, BAO TRUONG, and ERIC FORREST
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received March 23, 2007
Accepted for Publication May 12, 2007
Nanofluids are engineered colloidal suspensions of
nanoparticles in water and exhibit a very significant enhancement (up to 200%) of the boiling critical heat flux
(CHF) at modest nanoparticle concentrations (ⱕ0.1%
by volume). Since CHF is the upper limit of nucleate
boiling, such enhancement offers the potential for major
performance improvement in many practical applications that use nucleate boiling as their prevalent heat
transfer mode. The Massachusetts Institute of Technology is exploring the nuclear applications of nanofluids,
specifically the following three:
1. main reactor coolant for pressurized water reactors (PWRs)
2. coolant for the emergency core cooling system
(ECCS) of both PWRs and boiling water reactors
3. coolant for in-vessel retention of the molten core
during severe accidents in high-power-density light
water reactors.
The main features and potential issues of these applications are discussed. The first application could enable
significant power uprates in current and future PWRs,
I. INTRODUCTION
Addition of solid nanoparticles to common fluids
such as water is an effective way to increase the critical
heat flux ~CHF!. The resulting colloidal suspensions are
known in the literature as nanofluids.1 Materials used for
*E-mail: jacopo@mit.edu and lwhu@mit.edu
80
thus enhancing their economic performance. Specifically, the use of nanofluids with at least 32% higher CHF
could enable a 20% power density uprate in current plants
without changing the fuel assembly design and without
reducing the margin to CHF. The nanoparticles would
not alter the neutronic performance of the system significantly. A REL AP5 analysis of the large-break loss-ofcoolant accident in PWRs has shown that the use of a
nanofluid in the ECCS accumulators and safety injection
can increase the peak-cladding-temperature margins (in
the nominal-power core) or maintain them in uprated
cores if the nanofluid has a higher post-CHF heat transfer rate. The third application can increase the margin to
vessel breach by 40% during severe accidents in highpower density systems such as Westinghouse AP1000 and
the Korean APR1400. In summary, the use of nanofluids
in nuclear systems seems promising; however, several
significant gaps are evident, including, most notably,
demonstration of the nanofluid thermal-hydraulic performance at prototypical reactor conditions and the
compatibility of the nanofluid chemistry with the reactor
materials. These gaps must be closed before any of the
aforementioned applications can be implemented in a
nuclear power plant.
nanoparticles include chemically stable metals ~e.g., gold,
silver, copper!, metal oxides ~e.g., alumina, zirconia, silica, titania!, and carbon in various forms ~e.g., diamond,
graphite, carbon nanotubes, fullerene!. Nanoparticles are
relatively close in size to the molecules of the base fluid
and thus, if properly prepared, can realize very stable
suspensions with little erosion and gravitational deposition over long periods of time. As of today, more than ten
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studies of CHF in nanofluids have been reported in the
literature.2–12 The findings can be summarized as follows:
1. Significant CHF enhancement ~up to 200%! occurs with various nanoparticle materials, including silicon, aluminum, and titanium oxides.
2. The CHF enhancement occurs at relatively low
nanoparticle concentrations, typically, ⱕ0.1 vol%.
3. During nucleate boiling, some nanoparticles precipitate on the surface and form a layer whose morphology depends on the nanoparticle materials. The CHF
enhancement mechanism is poorly understood but appears to be related to the presence of this layer.
4. A study of nanofluid post-CHF ~film-boiling! heat
transfer also reports greatly accelerated quenching of hot
copper spheres in nanofluids.13
At the Massachusetts Institute of Technology ~MIT!,
we are conducting basic research on nanofluid heat transfer,14,15 including pool-boiling heat transfer and CHF
~Refs. 16 and 17!, as well as flow boiling CHF ~Ref. 18!,
and are also assessing the feasibility of water-based nanofluids for light water reactors ~LWRs!. The potential applications include
1. use of a nanofluid as the primary coolant in pressurized water reactors ~PWRs! to increase the
power density in the core
2. use of a nanofluid in the accumulators and safety
injection of the emergency core cooling systems
~ECCS! to increase margins during design-basis
events
3. use of a nanofluid for reactor cavity flooding to
increase safety margins during severe accidents.
This paper provides an overview of the features, issues, and research gaps of these applications. The PWR
coolant application is presented in Sec. II, the ECCS
application, in Sec. III and the severe-accident application, in Sec. IV.
II. PWR COOLANT APPLICATION
Increasing the power density of operating and0or
future PWRs is an effective approach to improving their
economic attractiveness. Because the capital cost of a
typical PWR accounts for ;65% of the levelized busbar
cost of electricity, extracting more energy from an existing reactor or reducing the physical size of future PWRs
may reduce the total cost of nuclear power considerably.
Power uprates at LWRs have been implemented for decades. The approved, pending, and expected power uprate applications in the United States currently amount to
;5000, 1000, and 1400 MW~electric!, respectively, as
reported by the U.S. Nuclear Regulatory Commission
~NRC! website. The objective of our research is to demNUCLEAR TECHNOLOGY
VOL. 162
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onstrate that a water-based nanofluid coolant could be
used to increase the power density in a PWR plant with
only modest changes to plant design, coolant chemistry,
and operation.a
The main feasibility issues are as follows:
1. chemical and physical stability of the nanoparticle suspension in typical PWR chemistry
2. nanoparticle effects on reactivity and coolant
activation
3. thermal-hydraulic performance of the nanofluids.
At relatively high nanoparticle loadings ~.1 vol%!, waterbased nanofluids tend to have much higher viscosity than
pure water. This would not be acceptable in nuclear systems. However, the CHF gains appear to be associated
with low nanoparticle loadings. At such low loadings,
the viscosity, thermal conductivity, surface tension, and
specific heat of a water-based nanofluid are similar to
those of pure water. This is an ideal situation in which
only the CHF is affected by the nanoparticles, while other
properties basically are unchanged. The nanoparticle materials initially considered are alumina, zirconia, silica,
carbon, gold, platinum, and iridium. Of these materials,
alumina, silica, and carbon were chosen based on the
literature review, gold, platinum, and iridium for their
expected stability, and zirconia based on engineering judgment. Nanoparticles made of all these materials are commercially available.
II.A. Chemical and Physical Stability
Particle sedimentation can occur via a variety of mechanisms, including gravity, inertia, thermophoretic and
electric effects, boiling, etc.19 Agglomeration increases
the size of the particles, thus greatly increasing the potential for gravity and inertial deposition. Dilute suspensions of nanoparticles ~such as those being studied in our
project! tend to be more stable, because the probability of
particle-to-particle collision is low. However, even in
dilute suspensions, thermal agitation and flow mixing are
generally not sufficient to prevent agglomeration. In nanofluids with oxide nanoparticles, agglomeration can be
largely reduced by adjusting the pH to create like electric
charges on the nanoparticle surface so that the nanoparticles reject each other on contact. With nonoxide
nanoparticles, one typically has to resort to surfactants
to keep the particles from agglomerating. However, pH
is not an independent variable in PWRs; it plays an important role in managing corrosion and thus has to be
a Our
initial focus is on the PWR. The coolant application for
the boiling water reactor ~BWR! does not seem very promising, because nanoparticle carryover to the turbine and condenser could raise erosion and fouling concerns. However,
note that small quantities of zinc and noble metals in dissolved form are introduced in current BWRs for corrosion
control.
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NANOFLUIDS FOR ENHANCED ECONOMICS AND SAFETY OF REACTORS
controlled within a certain range, typically, 6.9 to 7.4 at
room temperature. Therefore, it is necessary to find a
nanofluid that is stable within the same allowable pH
range, as changing the PWR water chemistry to accommodate the nanofluid is not a viable option. Furthermore,
surfactants may undergo serious radiolysis when exposed to core radiation, thus not performing their intended duty. The University of Florida, an MIT partner in
the nanofluid project, has obtained some initial encouraging results for carbon ~diamond! nanoparticles without
surfactants in prototypical PWR water chemistry,20 but
much more work in this area is clearly needed, including
the investigation of radiation effects on nanofluid stability, as well as the impact of nanoparticle deposition on
corrosion of the fuel cladding.
II.B. Nuclear Performance
Two potential issues have been analyzed in this area:
neutron activation of the nanoparticles and reactivity
effects.
The equilibrium coolant activity A due to neutron
activation of the nanoparticles was calculated by means
of the well-known equation
A⫽
FsNT ~e ltC ⫺ 1!
e l~tc⫹toc ! ⫺ 1
,
~1!
where
f ⫽ neutron flux
s ⫽ capture cross section
Representative values of the activity for various nanoparticles are reported in Table I. All activities are calculated for 0.001 vol% nanoparticle loading, 5 ⫻ 10 13
n0cm 2 {s thermal neutron flux, an in-core transit time of
0.6 s, and an out-of-core residence time of ;16 s. Discussions with a health physicist at the Seabrook nuclear
power station have established that a reasonable target
for coolant activity during refueling is ,0.1 mCi0cm 3.
Therefore, nanofluids with carbon nanoparticles should
meet this criterion easily, even at higher loadings. Silica
and alumina nanoparticles should also meet the criterion
because their activity decays rapidly after shutdown. Zirconia nanoparticles are within the limit only at very low
loadings, while gold, platinum, and iridium are not acceptable because their activities would remain high during an outage. Thus, the noble metals are discarded from
further consideration.
For a traditional 17 ⫻ 17 PWR fuel assembly with
5% enriched UO 2 fuel, the effect of the nanoparticle
loading on the beginning-of-life ~BOL! reactivity is shown
in Fig. 1. The calculations were performed with the popular LWR neutronic code CASMO ~Ref. 21!. It is apparent that the effect on reactivity is minimal. For example,
we estimate that the boron equivalent concentration for 1
vol% Al2O 3 nanoparticles is ;3 ppm, i.e., three orders of
magnitude lower than the actual boron concentration in
PWRs at BOL. The void and Doppler reactivity coefficients were also calculated with CASMO and found practically unchanged from the pure water situation.
II.C. Thermal-Hydraulic Performance
NT ⫽ target nucleus number density
The target is to uprate by 20% the core power density
for existing PWR plants. In principle, even more ambitious targets could be set for new plants, as discussed in
Ref. 22. For uprates in existing PWRs, the postulated
constraints are as follows:
l ⫽ radioactive decay constant
tc ⫽ in-core transit time
toc ⫽ out-of-core residence time.
TABLE I
Activation of Nanoparticles*
Nanoparticle
Material
Dominant Radionuclide,
Decay
Half-Life
Carbon
Silica ~SiO 2 !
Alumina ~Al2O 3 !
Zirconia ~ZrO 2 !
Gold
Platinum
Iridium
C, b ⫺ ~no g!
b ⫺ ~g: 1.3 MeV!
28
Al, b ⫺ ~g: 1.8 MeV!
95 Zr, b ⫺ ~g: 0.7 MeV!
198
Au, b ⫺ ~g: 0.4 MeV!
197 Pt, b ⫺ ~g: 0.2 MeV!
194 Ir, b ⫺ ~g: 0.3 MeV!
5700 yr
2.6 h
2 min
64 days
2.7 days
18 h
19 h
14
31 Si,
Activity
~per unit coolant volume!
0.001 mCi0cm 3
0.04 mCi0cm 3
10 mCi0cm 3
0.2 mCi0cm 3
3000 mCi0cm 3
6 mCi0cm 3
2500 mCi0cm 3
a
*At 0.001 vol% loading and for 5 ⫻ 10 13 n0cm 2 {s thermal flux.
a Due to its long half-life, the activity of 14 C does not reach equilibrium during irradiation, so Eq. ~1! cannot be used. The reported
activity is based on 60 effective full-power years of operation.
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3. Case 3. A combination of cases 1 and 2, e.g.,
increase the flow and temperature rise by ;10% each.
Fig. 1. The BOL reactivity in a PWR fuel assembly with nanofluid coolants. ~The curves for alumina and zirconia
nanofluids overlap.!
1. Maintain the traditional 17 ⫻ 17 fuel assembly
design to avoid certification of the new fuel design and
retooling of the fuel manufacturing facilities.
2. Minimize the changes to the nuclear island components, i.e., no replacement of the reactor pressure vessel, steam generators, and main piping. Replacement of
the main coolant pumps is considered tolerable.
3. Maintain the same core outlet temperature to mitigate materials concerns.
The reference plant is a standard four-loop Westinghouse PWR. There are three approaches to accommodating the 20% power density uprate:
1. Case 1. Increase the core flow by 20%.
2. Case 2. Increase the core average temperature rise
by 20%.
A more modest 5% power upgrade, accommodated with
a 5% core flow increase, is also evaluated and referred to
as Case 4.
The main operating parameters for the reference plant
and cases 1, 2, 3, and 4 are shown in Table II. The values
were calculated with RELAP5 ~Ref. 23! and verified
with hand calculations. Replacement of the main coolant
pumps is probably needed in cases 1 and 3. Major upgrade of the turbine-generator module is probably needed
in cases 1, 2, and 3. The need to replace the pumps and
upgrade the turbine-generator module in case 4 will vary
from plant to plant, depending on their current margin.
Since the steam generators and the core outlet temperature are left unchanged in all cases, the temperature ~and
pressure! on the secondary side of the steam generators
must be lowered somewhat in order to transfer the higher
thermal power. The lower secondary temperature results
in a small decrease in plant thermal efficiency, which we
estimate to be ;1%, based on the Rankine cycle efficiencypressure curves in Ref. 24. In uprating the power density,
we seek, where possible, to maintain or improve the margin to the key thermal-hydraulic limits, i.e., fuel melting
during transient overpower and CHF during transient overpower, the latter typically being dominant in PWRs.
To evaluate the margin to fuel melting and CHF, we
have performed a subchannel analysis of the core using
the VIPRE code, developed by the Electric Power Research Institute ~EPRI! for LWR core thermal-hydraulic
analysis.25 The code solves the set of finite difference
conservation equations for mass, momentum, and energy
of the coolant in interconnected subchannels, assuming
incompressible homogeneous flow, and calculates the
coolant velocity, pressure, and enthalpy fields and fuel
rod temperatures. A suite of CHF correlations is provided
by VIPRE for the evaluation of the minimum CHF ratio
~MCHFR!, calculated for the given operating conditions.
For our calculations, we used the W-3 correlation with
TABLE II
Operating Parameters for the Reference and Uprated Plants
Parameter
Reference
Case 1
Case 2
Case 3
Case 4
Power @MW~thermal!#
Total core flow rate ~kg0s!
Effective core flow rate ~kg0s!
Bypass flow rate ~kg0s!
Tin ~8C!
Tout ~8C!
Tout ⫺ Tin ~8C!
Primary system pressure ~MPa!
Secondary system pressure ~MPa!
3 411
18 712
17 799
913
293.2
324.7
31.5
15.50
6.07
4 093 ~⫹20%!
22 291 ~⫹20%!
21 205
1 084
293.1
324.9
31.8
15.50
5.70
4 093 ~⫹20%!
18 979
18 050
929
286.8
324.7
37.9 ~⫹20%!
15.50
5.21
4 093 ~⫹20%!
20 689 ~⫹10%!
19 680
1 009
290.2
324.7
34.5 ~⫹10%!
15.50
5.47
3 582 ~⫹5%!
19 628 ~⫹5%!
18 670
958
293.2
324.7
31.5
15.50
5.98
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Fig. 2. VIPRE subchannel model representing one-eighth of a 17 ⫻ 17 PWR fuel assembly. ~MCHFR typically occurs in
subchannel 2 due to the cold-wall effect.!
correction factors accounting for the cold wall ~i.e., unheated surfaces, such as the control-rod guide tubes! effect, the nonuniform axial flux, and the grid spacers. The
model simulates the whole core by means of lumped
channels for the peripheral fuel assemblies and a detailed
subchannel description of the hot fuel assembly. Figure 2
shows the VIPRE subchannel model representing oneeighth of the hot fuel assembly with a standard 17 ⫻ 17
PWR fuel assembly geometry. This model was benchmarked successfully with previous VIPRE simulations
performed at MIT ~Ref. 26!.
The results of the VIPRE analysis are reported in
Table III. It is no surprise that the margin to fuel melting
decreases considerably in the uprated cases with respect
to the reference case because the number of pins per
fuel assembly is the same and the linear power is 20%
higher. Since the centerline temperature mostly depends
on the fuel thermal conductivity, this margin reduction
could not be helped by the use of nanofluids. However,
the calculated values of the maximum fuel temperature
appear to be well below the 28008C melting point of
UO 2 at BOL, i.e., when the maximum linear power
occurs.
With respect to the reference case ~MCHFR ⫽1.76!,
the maximum MCHFR decrease is for case 2 ~MCHFR ⫽
1.33!. Thus, use of nanofluids with at least ~1.76 ⫺ 1.60!0
1.60 ' 32% higher CHF than pure water would enable
the postulated power uprate without a reduction of the
margin to CHF. Given the results of our low-pressure
pool and flow-boiling CHF experiments 17,18 and the
TABLE III
Subchannel Analysis Results for the Reference and Uprated Cases*
Parameter
Reference
Case 1
Case 2
Case 3
Case 4
Maximum fuel temperature ~8C!
MCHFR
Maximum exit quality
Maximum exit void fraction
Core pressure drop ~kPa!
2377
1.76
0.06
0.26
125
2704
1.60
0.06
0.26
167
2704
1.33
0.11
0.39
126
2704
1.49
0.08
0.32
145
2468
1.72
0.06
0.26
135
*Analysis assumes radial peaking factor Fh ⫽ 1.65, hot spot factor FQ ⫽ 2.5 ~chopped cosine!, ⫹12% overpower, ⫺5% flow,
⫹2.28C inlet temperature, ⫺200-kPa pressure swing.
84
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data published so far, this target would seem reasonable.
However, verification of CHF enhancement at prototypical PWR pressure, flow, and geometric conditions is
clearly needed.
The core pressure drop increases by 34, 16, and 8%
in cases 1, 3, and 4, respectively, and does not increase in
case 2 with respect to the reference case. The corresponding core pumping power increase in cases 1, 3, and 4 is
60, 28, and 13%, respectively, which would probably
make it necessary to replace the main coolant pumps.
III. ECCS APPLICATION
Following a large-break loss of coolant accident
~LB-LOCA! in current LWRs, the primary system inventory is discharged to the containment and the core is
uncovered. To prevent excessive fuel overheating in the
ensuing transient, the ECCS is actuated so that vessel
reflood occurs and core cooling is restored. Because the
fuel can be initially very hot ~.7008C!, its rewetting
occurs slowly through the development of a quench front,
which advances upward in the core. The speed of the
quench front and thus the peak temperature reached during the reflood transient depend on a combination of
factors, including film-boiling heat transfer, wettability
of the fuel surface by the coolant, and localized axial
conduction within the cladding near the quench front.
The use of nanofluids could afford a significant increase
of the quench speed for two reasons:
1. Boiling-induced deposition of nanoparticles on
the surface greatly enhances surface wettability. This has
been experimentally demonstrated in our laboratory.16
2. Deposited nanoparticles of high-conductivity material ~e.g., alumina! improve localized axial conduction
in the cladding near the quench front.
The enhanced wettability, especially, is expected to
increase the Leidenfrost temperature for the cladding and
promote return to nucleate boiling. Quench experiments
of small copper balls, whose surface was fouled with
nanoparticles, have demonstrated that return to nucleate
boiling can be greatly accelerated in nanofluids.13 If this
behavior can be reproduced for cylindrical rods, the use
of nanofluids in the ECCS offers the potential for enhanced safety margins or even power uprates for those
LWRs whose power is limited by the fuel response during the LB-LOCA. For this application, the issue of
chemical-physical stability of the nanofluids is much less
important than for the PWR coolant application because
the ECCS coolant spends most of its lifetime at room
temperature and is not exposed to continuous irradiation.
Also, the ECCS is actuated in response to seriously abnormal events during which coolant chemistry control is
not a priority.
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The main figure of merit for the ECCS performance
is the peak cladding temperature ~PCT!. During a LOCA,
the PCT must be kept below 12008C ~22008F or 1473 K!
to prevent rapid oxidation of the cladding. To evaluate
the PCT during a LB-LOCA, we have used a previously
developed and benchmarked RELAP5 input deck for a
PWR. The model of the plant, representing the average
and hot fuel assemblies in the core, the reactor vessel, the
pressurizer, the steam generators and the pumps, is shown
in Fig. 3. Note that the three intact loops are simulated as
a single lumped loop. The accumulators and the longterm safety injection ~SI! system are also modeled. The
six-group point-kinetics model with typical PWR reactivity coefficients is included. The balance of plant is
represented by time-dependent junctions connected to
the secondary side of the steam generators. The model
was first used to generate the steady-state operating conditions reported in Table II. The following assumptions
were made for the LB-LOCA analysis:
1. At t ⫽ 5 s, a 100% break occurs in the cold leg of
one loop.
2. At t ⫽ ;5.1 s, the reactor scram signal is sent,
based on low pressure in the pressurizer and low mass
flow rate in the core
3. The SI signal is sent automatically when the pressure falls below 12.9 MPa.
4. At t ⫽ 8 to 9 s, the main coolant pumps trip.
5. The SI flow ~155 kg0s! starts 27 s after the SI
signal.
The behavior of the PCT during the LB-LOCA for
the reference case and cases 1, 2, 3, and 4 is shown in
Fig. 4, and a summary is provided in Table IV. The blowdown and reflood peaks are clearly visible in Fig. 4. The
maximum value of the PCT is reached during reflood in
all cases. As expected, the uprated cases have generally
higher PCTs than the reference case during both blowdown and reflood. The core is successfully quenched in
all cases, but the time-to-quench is longer for the 20%
uprate cases ~cases 1, 2, and 3! than for the 5% uprate
case ~case 4!, as expected.
To quantify the effect of a nanofluid-based ECCS,
RELAP5 was modified for us by the Idaho National Laboratory, to include user-specified multipliers enabling manipulation of the boiling curve. That is, the CHF and the
post-CHF heat transfer coefficient can be arbitrarily increased throughout the accident to simulate the effect of
a nanofluid. The results of the analysis with the manipulated boiling curve are also reported in Table IV ~bottom four rows! and Fig. 5. It is apparent that a large
increase of CHF helps significantly during blowdown
but not as much during reflood. On the other hand, an
increase in post-CHF heat transfer is key to reducing the
PCT during reflood but does not have a dramatic effect
85
Buongiorno et al.
NANOFLUIDS FOR ENHANCED ECONOMICS AND SAFETY OF REACTORS
Fig. 3. RELAP5 model for the cold-leg LB-LOCA analysis of the PWR.
Fig. 4. The PCT history during a LB-LOCA.
on blowdown. It should be emphasized that for PWR
plants that are not LOCA-limited but CHF-limited, an
improvement of the CHF will be sufficient to enable the
power uprate. A systematic experimental study of the
post-CHF heat-transfer characteristics of nanofluids at
prototypical reactor conditions is clearly needed before
the merit of nanofluids for the ECCS application can be
fully judged.
86
IV. SEVERE-ACCIDENT APPLICATION
Advanced LWR systems, such as Westinghouse’s
AP1000 and the Korean APR1400, employ an in-vessel
retention ~IVR! strategy to mitigate the consequences
of hypothetical severe accidents in which the core melts
and relocates to the bottom of the reactor vessel. The
IVR strategy consists of flooding the reactor cavity and
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TABLE IV
LB-LOCA Analysis Results
Blowdown Peak
Reflood Peak
PCT
~8C!
Time
~s!
PCT
~8C!
Time
~s!
Reference
Case 1
Case 2
Case 3
Case 4
674
723
853
766
687
7.1
7.7
7.4
7.0
7.0
726
951
924
966
813
85.4
185.0
130.7
122.8
105.0
Case 1 ~⫹100% CHF!
Case 1 ~⫹100% post-CHF!
Case 4 ~⫹100% CHF!
Case 4 ~⫹100% post-CHF!
680
712
635
697
8.2
7.2
7.1
7.1
936
825
749
614
123.0
116.5
92.3
100.45
Case
Fig. 5. The PCT history with enhanced CHF and post-CHF during a LB-LOCA ~20% uprate!.
removing the residual heat from the molten core through
the reactor vessel lower head. Heat removal is limited by
the occurrence of CHF on the reactor vessel outer surface, so the use of a nanofluid instead of water can help
to better mitigate the consequences of the severe accident.
The AP1000 was selected as the reference plant for
the analysis. Following core melting, the AP1000 operator floods the reactor cavity with water from the incontainment refueling water storage tank ~IRWST! if the
IRWST has not already been drained. Therefore, we envision a system in which small tanks containing a concentrated nanofluid would inject it into the reactor cavity.
The resulting diluted nanofluid would then flow along
the vessel and remove the heat. The system comprises
2 ⫻100% capacity tanks located above the reactor cavity
near the IRWST ~Fig. 6!. Each tank contains ;0.5 m 3 of
concentrated nanofluid. A risk-informed analysis of this
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injection system design has shown that the nanofluid
IVR failure probability, given an initiating event leading
to core relocation, has a reasonably low value of 0.01
~Ref. 27!. Note that in this application, there is great
flexibility in selecting the type of nanofluid with little
concern for its compatibility with the reactor coolant
chemistry, because the nanofluid tanks are completely
separated from the primary coolant and the ECCS. Therefore, we have selected an alumina nanoparticle nanofluid
at 20 wt% concentration, which exhibits excellent colloidal stability. When diluted in the ;2500-m 3 vessel
cavity water, this nanofluid gives a target concentration
of ;0.001 vol%. To ensure reliable and effective nanofluid injection into the vessel cavity, the tanks are kept at
slight overpressure with inert gas accumulators, and the
injection lines are equipped with radial spargers. Moreover, effective mixing occurs in the cavity due to the
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Fig. 6. Schematic of the nanofluid injection system for severe-accident management.
coolant flow induced by boiling on the vessel surface.
Finally, the time available for mixing is of the order of
tens of minutes. Computational Fluid Dynamics ~CFD!
simulations using AP1000 as a reference case suggest
that nanofluid mixing in the reactor cavity can be realized within a few minutes. The nanofluid tank discharge
signal is automatically tied to the operator’s action of
manually flooding the vessel cavity, but it can also be
actuated independently if the cavity has already been
flooded. During normal operating conditions, the nanofluid in the tanks is stagnant and at room temperature, so
sampling for quality control is straightforward. To guarantee long-term stability of the nanoparticle suspension
in the tanks, the pH is adjusted to ;4 by adding nitric
acid to the nanofluid. Given the acidity of the concentrated nanofluid and the presence of nitrates, the material
of choice for the injection tanks is titanium grade 2
~Ref. 28!. Once the nanofluid is diluted in the vessel
cavity, the pH shifts toward neutral and nanoparticle stability may become an issue; however, under these circumstances, good stability is needed for only ,1 day,
that being the typical timescale of severe accidents. To
evaluate the nanoparticle stability when pH changes, we
have conducted dilution experiments and measure nanoparticle size changes as a function of time. Large increases in nanoparticle size would indicate agglomeration.
The results are shown in Table V. It is apparent that the
size changes are minimal, i.e., within the experimental
uncertainty ~620 nm! of the dynamic-light-scattering particle sizer chosen for these measurements. Other factors
that need to be considered before making a final selection
of a nanofluid are ~a! the response of the nanofluid to the
dose of radiation it would be exposed to during IVR and
~b! the response of the nanofluid to the coolant chemistry
it would encounter in the reactor cavity. Preliminary re88
TABLE V
Average Nanoparticle Diameter in Dilution Experiments
with Alumina Nanofluids*
Time After
Dilution
~h!
Concentration
20 wt%
0.01 vol%
0.001 vol%
0
1
6
24
37.8 nm
n0a
n0a
n0a
n0a a
42.6 nm
32.6 nm
46.7 nm
n0a
52.3 nm
33.4 nm
46.7 nm
*The experimental uncertainty is 620 nm.
a
Not applicable.
sults of experiments conducted at MIT suggest that Nyacol alumina nanofluid is stable when exposed to doses of
radiation it would encounter in IVR ~Ref. 29! but might
be unstable when combined with trisodium phosphate
~which would be present in a flooded reactor cavity!.
More work is needed in this area.
In AP1000 IVR, the coolant boils in a gap between
the vessel outer surface and the vessel thermal insulation. The resulting two-phase mixture creates a density
difference between the gap and the flooded vessel cavity,
which drives a natural circulation flow. A one-dimensional
model for the two-phase flow and heat transfer on the
vessel outer surface has been developed and used to quantify the residual heat that can be removed by using a
nanofluid. This model defines the gap between the reactor vessel outer surface and the thermal insulation as the
rising portion coolant flow path ~riser!. The falling portion of the flow path ~downcomer! is outside this gap in
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TABLE VI
Key Input for the IVR Heat Transfer Model
Parameter
Value
Vessel inner diameter
Vessel height
Lower head height
Vessel0insulation gap
Inlet temperature
Inlet pressure
4.02 m
6.14 m
2.31 m
0.076 m
1008C
0.157 MPa a
a Corresponding to a depth of ;15 m from the water level in the
reactor cavity.
the reactor cavity. Table VI shows several key parameters
used in the model. The Levy correlation 30 is used to
determine the flow quality in the heated section, and the
EPRI correlation 31 is used to determine the void fraction.
The momentum equation, including the gravity, acceleration, and wall friction terms, is solved to provide the
flow rate for a given heat-removal rate. Finally, the SULTAN CHF correlation,32 which accounts for the effect of
the channel inclination, is used to determine the CHF
ratio. This model reproduces the power-flow curve of the
University of California at Santa Barbara experiments
within 2% ~Ref. 33!. A multiplier is used to account for
the CHF enhancement effect in a nanofluid. Experimental results at MIT found that a 0.001 vol% alumina nanofluid enhances the pool-boiling and flow-boiling CHFs
by 50 and 30%, respectively, compared to the pure water
case.17,18 So, heuristically, the value of the multiplier was
assumed to be an intermediate 1.4.
The results are summarized in Fig. 7. The curve that
shows the flow rate as a function of power has a peak,
which is due to the conflicting effects of increased density head and increased two-phase pressure loss in the
gap. Figure 7 shows the minimum CHF ratio for pure
water and nanofluid. It is apparent that if the margin to
CHF must be retained, the maximum allowable power
that can be removed from the vessel outer surface with a
nanofluid is ;26.8 MW, or some 42% higher than water.
In determining the maximum operating power, the stability of the system was also considered as large flow
oscillations could cause premature CHF. Three types of
instability were considered: ~a! flow excursion ~static!,
~b! density-wave oscillation ~dynamic!, and ~c! pressuredrop oscillations ~dynamic!. The criteria developed by
Rohatgi and Duffey 34 were used to calculate the onset of
flow excursions. Operation at the higher heat-removal
rate attained with nanofluids was found to be stable ~with
a good margin! against this type of instability ~see the
32-MW line in Fig. 7!. Using Saha et al.’s 35 criteria, it
was determined that the system would be stable against
density-wave instabilities at heat-removal rates up to
47 MW. Dynamic instabilities of the pressure-drop oscillation type may occur if the curve that shows the pressure drop as a function of mass flow rate displays a
negative slope.36 That slope was inspected and found
positive at all power levels up to the 32-MW static instability limit and beyond. Therefore, flow excursion
Fig. 7. Operating range of the IVR system. @Departure from nucleate boiling ratio ~DNBR! is the same as CHF ratio.#
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NANOFLUIDS FOR ENHANCED ECONOMICS AND SAFETY OF REACTORS
seems to be the most limiting type of two-phase flow
instability in this application.
V. CONCLUSIONS AND FUTURE WORK
Three nuclear reactor applications have been explored for nanofluids. The findings are as follows:
1. PWR coolant application: The use of nanofluids
with at least 32% higher CHF could enable a 20% power
density uprate in existing plants without changing the
fuel assembly design and without reducing the margin to
CHF. The nanoparticles would not alter the neutronic
performance of the system significantly. Of the seven
nanoparticle materials considered, silica, alumina, and
carbon would result in acceptable coolant radioactivity
during shutdown, while gold, platinum, and iridium would
not. Zirconia is borderline.
2. ECCS application: A RELAP5 analysis of the
LB-LOCA in PWRs has shown that the use of a nanofluid in the ECCS accumulators and safety injection can
increase PCT margins ~in the nominal-power core! or
maintain them in uprated cores if the nanofluid has a
higher post-CHF heat transfer rate.
3. Severe accident application: Use of nanofluids
could enable a more than 40% heat removal rate increase
on the outer surface of the vessel during severe accidents
with core relocation to the bottom of the vessel. This
would result in higher margins for IVR in state-of-theart systems such as AP1000 or APR1400, or enable IVR
for higher power density reactors. Among the three reactor applications considered, this one seems to present
the fewest constraints and thus is likely to be implemented first.
Although this exploratory study has shown that nanofluids are indeed promising, major uncertainties remain:
1. The heat transfer gains observed in nanofluid poolboiling and flow-boiling experiments at low pressure must
be demonstrated at prototypical conditions. For example,
CHF tests in bundle geometry at high pressure, temperature, and mass flux will be needed for the PWR coolant
application. Also, CHF tests for nanofluids flowing on
the outer surface of a hemispherical heater will be needed
for the severe-accident application.
2. The chemical-physical stability of the nanoparticle suspension in PWR chemistry will have to be tested
over long periods of time, including the effects of radiation.
ACKNOWLEDGMENTS
The following sponsors are gratefully acknowledged:
AREVA, the Idaho National Laboratory, the Nuclear Regulatory Commission, the U.S. Department of Energy’s Innovation
in Nuclear Infrastructure and Education Program, and the Korea
Science and Engineering Foundation.
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