E-Journal of Advanced Maintenance Vol.7-1 (2015) 90-95
Japan Society of Maintenology

The Concept of Passive Cooling Systems for Inherently Safe BWRs
Naoyuki ISHIDA1,*, Akinori TAMURA2, Toshinori KAWAMURA1, Kazuaki KITOU1,and Mamoru KAMOSHIDA1
1

Hitachi, Ltd.,Hitachi Research Laboratory,7-1-1 Omika-cho, Hitachi-shi, Ibaraki-ken, 319-1292,
Japan

2

Hitachi Europe, Ltd., European Research Centre, 25 Chapel Street, London

ABSTRACTThe Fukushima Daiichi Nuclear Power Plant accident and its consequences have led to extensive rethinking
about the safety technologies used in boiling water reactors (BWRs). As one of the options of the safety
technologies, we have been developing passive cooling systems consisting of a water-cooling system and an
infinite-time air-cooling system. These systems achieve core cooling without electricity and are intended to copewith a long-term station blackout (SBO). Both these cooling systems remove relatively high decay heat for theinitial 10 days after an accident, and then the infinite-time air-cooling system continues to remove attenuateddecay heat after this period.
To obtain heat transfer data for the design of the water-cooling system, we conducted heat transfer tests using
a full-scale U-shaped single tube. The data were obtained at a system pressure of 0.2 to 3.0MPa (absolute) andinlet steam velocity of 5 to 56m/s. To enhance heat transfer of the air-cooling system, we successfully
implemented some air-cooling enhancing technologies. The performance was evaluated by heat transfer dataobtained from the element heat transfer tests. The heat transfer performance increased at least 100% with theenhancement technologies compared with a bare tube. From these test results, we confirmed good feasibility for
application of the cooling systems.
KEYWORDS

ARTICLE INFORMATION

BWR, Inherently safe, Passive cooling system, Water-cooling, Air-cooling

Article history:
Received 31 October 2014
Accepted 27 March 2015

1.

Introduction

A severe accident occurred at the Fukushima Daiichi Nuclear Power Station (1F) because of thelong-term station blackout (SBO) caused by damage from the great earthquake and tsunami on March
11, 2011 [1-3]. In the reactor design work, the assumed duration of the SBO had been a maximum of
8 hours, but in fact electric power supply was not recovered for 9 days because serious damage tomany facilities and infrastructures prevented recovery work.
Building from the experiences in the 1F nuclear accident, we have been developing inherently
safe BWRs which could cool the core and avert overpressurization of the primary containment vessel(PCV) for a SBO continuing 10 or more days.
In this paper, we describe our new concept of passive cooling systems for the inherently safe
BWRs which can remove the decay heat for an infinite time. We also present heat transfer test resultsfor both the water- and the air-cooling systems to confirm the feasibility.

2.

Concept of passive cooling systems and heat transfer tests

2.1. Concept of passive cooling systems
Figure 1 shows a schematic view of the passive cooling systems. The new passive cooling*Corresponding author, E-mail: naoyuki.ishida.sx@hitachi.com

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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N. Ishida, et al./The Concept of Passive Cooling Systems for Inherently Safe BWRs

systems of both the water- and the air-cooling system types begin to work by simply opening thestart-up valves and no valve operation is needed to control them. The main features of the coolingsystems are listed as follows.
The water-cooling system: The new water-cooling system can remove up to 40MW of decay heat.
 The steam flow rate from the reactor pressure vessel (RPV) can be passively controlled bya choking orifice.
 The water pool can be located at a lower level because water can be pumped up to the RPVfrom the suppression pool (S/P) using the reactor core isolation cooling system (RCIC)
with a turbine-driven pump or supplied through an alternative water injection line.
 The condensate is cooled below 130 oC to reduce the PCV pressure by supplying cooled
condensate to the S/P.
 If an isolation condenser (IC) or a passive containment cooling system (PCCS) is needed,
it may be included in the new water-cooling system.
The air-cooling system: The air-cooling system can remove up to 10MW of decay heat.
 The heat transfer performance is increased at least 100% with some air-cooling enhancing
technologies such as a micro-fabrication surface, turbulence-enhancing structures andheat-transfer fins.
The new water-cooling system removes the relatively high decay heat during the first 10 days
after an incident. The decay heat can be removed by the air-cooling system after 10 days without thewater-cooling system operation. The air-cooling system uses the atmosphere as a heat sink, so theattenuated decay heat can be removed for an infinite time. The cooling pool of the water-coolingsystem, operating simultaneously with the air-cooling system, has enough capacity for 10 days
cooling, while its capacity (2800t of water) is sufficient for 3 days cooling without operating theair-cooling system.
Reactor building

Air cooling system

PCCSIC
New water coolingsystem
PCVRPV

Main steam line
Alternative feedwater system

RCICWetwell

Heat exchanger
Cooling pool

Suppression pool

Decay heat [MW](4700MWt class reactor)

Heat exchanger
for air cooling system

40

Water-coolingsystem

30
20
100

Air-cooling system
0105
Elapsed time [day]15

Activation period of both systems

Fig. 1. Schematic view of the passive cooling systems

2.2. Heat transfer tests for the water-cooling system
2.2.1. Outline of the tests
Heat transfer tests were conducted to obtain heat transfer data using full-scale U-shaped tubes.
Figure 2 shows the test facility overall, the 6 measuring positions for heat transfer and details of thethermocouple location at each measuring position. Three single tubes were installed in the water pool.

91

N. Ishida, et al./The Concept of Passive Cooling Systems for Inherently Safe BWRs

The tube diameters were: 22.2mm I.D. with 2.5mm thickness; 28.4mm I.D. with 2.8mm thickness;and 35.5mm I.D. with 3.6mm thickness. The heat transfer coefficient of both boiling outside the tubes
and condensation inside the tubes were estimated from measured temperature on the tube surfaces.
Figure 3 shows the test conditions with the pressure on the horizontal axis and the inlet steam velocity
on the vertical axis. The test conditions included operating conditions of the PCCS and the newwater-cooling system. The IC operating conditions could be extrapolated by considering theequivalent Re number with an inlet steam velocity of more than 30m/s at 3.0MPa. The superheatedsteam was used to assure the accuracy of the steam enthalpy because the steam wetness was notmeasured in the tests.
ExhaustWaterpool
Steam

Opticalvoid sensor

5.7m
Condensation tube
StrainBaffle plate
gage

Condensate

Condensation
tube

TCTCs

Accelerometer
Detailed location of TCs
Steam injection tube

Fig. 2. Test facility of the water-cooling system and TC locations

Inlet steam velocity [m/s]

6050
40

Equivalent Re number
with IC condition

302010PCCS
00.0 0.5

New system
1.0 1.5 2.0 2.5
Pressure [MPa]

3.0

3.5

Fig. 3. Test conditions of the water-cooling system

2.2.2. Results and discussion
Figure 4 shows an example of the inner fluid temperature distribution from the inlet to the outlet.
The temperature was the saturated temperature at the pressure during condensation occurred in thetube. After condensation was completed, the temperature of condensate that had lost its latent heat
dropped below the saturated temperature. For this condition, the condensate temperature decreased to122 oC at the outlet.
Figure 5 shows the boundary of the 130 oC region of the outlet condensate temperature for thetest conditions that were shown in Fig. 3. The red triangles indicate outlet condensate temperature wasover 130 oC. The chain line shows that the boundary ultimate PCV pressure equaled the PCV designpressure. The new system and PCCS operating conditions were within the area where outlet
condensate temperature was under 130 oC. We confirmed that the condensation tube with 11m length,
which is an existing IC tube design value, could cool the condensate enough to avoid PCV
overpressurization and our water-cooling concept was practical.

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2006060

Saturated temperature at 1.0MPa

Inlet steam velocity [m/s]

Inner fluid temperature [oC]

N. Ishida, et al./The Concept of Passive Cooling Systems for Inherently Safe BWRs

180160120100

Condensation
complete130 oC

140

Pressure: 1.0MPa
Inlet velocity: 33.6m/s

0246810◆：Under 130℃

40403030
2020
101000120.0
0.0

Distance from the inlet [m]▲：Over 130℃

5050

PCCS
0.5
0.5

New system
1.0
1.0

1.5
2.0
2.5
1.5
2.0
2.5
Pressure [MPa]

3.0
3.0

3.5
3.5

Fig. 5. Boundary of the outlet temperature at 130 oC

Fig. 4. Inner fluid temperature distribution

2.3. Heat transfer tests for the air-cooling system
2.3.1. Heat transfer enhancement technologies for the air-cooling system
Many studies [4-9] regarding air-cooling enhancement technologies have been investigated.
Examples of the technologies include fins and micro-fins, porous media, vortex generators, vibrating
surfaces, rough surfaces, and swirl flow devices. In general, there is a trade-off relationship between
the air-cooling enhancement and the pressure-loss increment. To enhance the air-cooling performancewith a small pressure-loss increment, we developed an air-cooling enhancement technology using a
micro-fabrication surface and combined it with the conventional turbulence-enhancing ribs andheat-transfer fins. The air-cooling pipe with those technologies is shown in Fig. 6. The heat-transfer
fins were fixed to the pipe along the flow direction. Two micro-fabrication surfaces for use on copperand stainless steel (SUS) are shown in Fig. 7. These surfaces were formed easily on the heating
surface, including the fins, by chemical treatments. In order to increase the heat transfer performanceon the micro-fabrication surface, the turbulence-enhancing ribs were attached on the pipes at auniform interval. We expected that the micro-fabrication surface would provide the air-coolingenhancement without a pressure loss increment because its structure height was sufficiently smaller
than the thickness of the laminar sub-layer in the turbulent boundary layer. The synergetic effects
using the micro-fabrication surface, the turbulence-enhancing ribs and the heat transfer fins wereinvestigated using the pipes to which they were applied.

SteamA

Heat exchanger
pipe

Heat-transfer finsA

Heat-transfer fins

100mm

Turbulenceenhancing ribs

Turbulenceenhancing ribs
BAirB

Micro-fabrication
surfaceFig. 6. Air-cooling enhancing technologies

93

1μm

1μm

Micro-fabrication
surface for copper

Micro-fabrication
surface for SUS

Fig. 7. Micro-fabrication surfaces

N. Ishida, et al./The Concept of Passive Cooling Systems for Inherently Safe BWRs

2.3.2. Outline of the tests
Figure 8 shows the test apparatus of the air-cooling system. The air was supplied to the test
section with the same velocity as assumed by natural circulation in a real plant. The sheath heater
inside the heat exchanger pipe was used as the heat source. The pipe with 25.4mm O.D. and 1500mm
length was located at the center of the test section. The outer tube had an inner diametercorresponding to the thermal equivalent diameter of the air-cooling system. The forced convection
heat transfer coefficient was estimated from the heat flux calculated by the sheath heater power andthe measured temperature difference between the pipe surface and air bulk. Pressure loss of the test
section was measured by the differential pressure gage.
Thermocouple
T

300mm

82.4mm

T300mmAircompressorT

2400mmTHeat exchanger
pipeThermocouple
25.4mmFlow control
valve

P

Insulator

1500mm

300mm

Reducingvalve

T950mmΔP

Sheath heater（1.5kW）

Heat exchanger
pipe
Currentplates

FAir heater

Outer tube

Cross-sectional view

TInsulator

Fig. 8. Test apparatus of the air-cooling system

2.3.3. Results and discussion
Figure 9 compares the Nu number and pressure-loss coefficient between the bare pipe and thepipe with all of the enhancement technologies of the micro-fabrication surface, turbulence-enhancing
ribs and heat-transfer fins. The Re number was defined using the average air velocity around the pipe.
Compared to the bare pipe, we saw that the Nu number increased by 140% with a synergic effect of
the enhancement technologies (left graph) and the pressure-loss coefficient also increased 70% due tothe influence of the ribs and fins (right graph). The micro-fabrication contributed to increase Nu
number by up to 10%. The increase of the pressure loss coefficient decreased the air flow rate andresulted in a reduction of heat transfer.
Therefore, we carried out the 1D analysis using these test results to evaluate the real heat transfer
rate when the enhancement technologies were applied in a real heat exchanger. From the analysis, weconfirmed that heat transfer performance was increased at least 100% with the technologies withconsideration of the decrease of air circulation flow rate by the pressure-loss increment. Fouling factor
should be considered in the detailed system design, and we are studying a fouling factor of themicro-fabrication surfaces. However, since the system was designed with double safety ratio in
consideration of uncertainties and aircraft crash, the system may have enough heat removal capacity
even if fouling occurs.
The enhancement technologies can be adopted at low cost in the infinite-time air-cooling system
for a real nuclear power plant and they will provide an additional margin of safety. From theseexperimental and analysis results, we demonstrated the feasibility for implementing the infinite-time
air-cooling system.

94

350

Bare pipe

300

Pipe RiPh developedPechnologies

1.2
1250

140%

200
150
100
50

Pressure-loss coefficient [-]Nusselt number [-]N. Ishida, et al./The Concept of Passive Cooling Systems for Inherently Safe BWRs

05000 15000 25000 35000 45000 55000 65000

Bare pipe
Pipe RiPh developed
Pechnologies

0.80.6

70%
0.4
0.20
5000 15000 25000 35000 45000 55000 65000

Reynolds number [-]Reynolds number [-]Fig. 9. Comparison of Nu number and pressure-loss coefficient

3.

Conclusion

The concept of passive cooling systems for inherently safe BWRs was presented in this paper.
We conducted heat transfer tests for both the water- and the air-cooling systems. In the tests for thewater-cooling system, heat transfer data were obtained over a wide range of thermal hydraulics
conditions including the conditions for the PCCS, the IC and the new water-cooling system. In thetests for the air-cooling system, heat transfer was increased at least 100% by implementing theenhancement technologies in heat exchanger pipes. From these test results, we confirmed the good
feasibility for application of the passive cooling systems as inherently safe technologies for theinfinite-time cooling.

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