The Concept of Passive Cooling Systems for Inherently Safe

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

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

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 ABSTRACT

The 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 cope with a long-term station blackout (SBO). Both these cooling systems remove relatively high decay heat for the initial 10 days after an accident, and then the infinite-time air-cooling system continues to remove attenuated decay 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) and inlet 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 data obtained from the element heat transfer tests. The heat transfer performance increased at least 100% with the enhancement technologies compared with a bare tube. From these test results, we confirmed good feasibility for application of the cooling systems.

*Corresponding author, E-mail: [email protected]

KEYWORDS

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

ARTICLE INFORMATION

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 the long-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 to many 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 results for 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

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

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systems of both the water- and the air-cooling system types begin to work by simply opening the start-up valves and no valve operation is needed to control them. The main features of the cooling systems 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 by

a choking orifice. The water pool can be located at a lower level because water can be pumped up to the RPV

from 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 and heat-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 the water-cooling system operation. The air-cooling system uses the atmosphere as a heat sink, so the attenuated decay heat can be removed for an infinite time. The cooling pool of the water-cooling system, 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 the air-cooling system.

Wetwell

RCIC

ICHeat exchangerfor air cooling system

PCCS

Suppression pool

Reactor buildingAir cooling system

PCV

Heat exchanger

Main steam line

New water coolingsystem

Cooling pool

RPV

Elapsed time [day]

0

10

40

5 10 150

Air-cooling system

Water-coolingsystem

20

30

Deca

y hea

t [M

W]

(470

0MW

t cla

ss re

acto

r)

Activation period of both systems

Alternative feed water system

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 the thermocouple location at each measuring position. Three single tubes were installed in the water pool.


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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 new water-cooling system. The IC operating conditions could be extrapolated by considering the equivalent Re number with an inlet steam velocity of more than 30m/s at 3.0MPa. The superheated steam was used to assure the accuracy of the steam enthalpy because the steam wetness was not measured in the tests.

5.7m

Baffle plate

Exhaust

Condensation tube

Waterpool

Steam injection tube

Optical void sensor

Straingage TCsSteam

Condensate Accelerometer

Condensationtube TC

Detailed location of TCs

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

60

50

40

30

20

10

00.0 0.5 1.0 2.0 3.01.5 2.5 3.5

Pressure [MPa]

Inle

t ste

am v

eloc

ity [m

/s]

New systemPCCS

Equivalent Re number with IC condition

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 the tube. 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 to 122 oC at the outlet.

Figure 5 shows the boundary of the 130 oC region of the outlet condensate temperature for the test conditions that were shown in Fig. 3. The red triangles indicate outlet condensate temperature was over 130 oC. The chain line shows that the boundary ultimate PCV pressure equaled the PCV design pressure. 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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100

120

140

160

180

200

0 2 4 6 8 10 12

Inne

r flu

id te

mpe

ratu

re [

o C]

Distance from the inlet [m]

Condensation complete

Pressure: 1.0MPaInlet velocity: 33.6m/s

Saturated temperature at 1.0MPa

130 oC

0

10

20

30

40

50

60

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

10

60

50

40

30

20

0

Pressure [MPa]

Inle

t ste

am v

eloc

ity[m

/s]

PCCS New system

▲：Over 130℃◆：Under 130℃

Fig. 4. Inner fluid temperature distribution Fig. 5. Boundary of the outlet temperature at 130 oC

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 performance with 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 and heat-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 copper and 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 performance on the micro-fabrication surface, the turbulence-enhancing ribs were attached on the pipes at a uniform interval. We expected that the micro-fabrication surface would provide the air-cooling enhancement 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 were investigated using the pipes to which they were applied.

A A

B B

100mm

Air

Steam Heat-transfer fins

Turbulence-enhancing ribs

Micro-fabrication surface

Micro-fabrication surface for copper

Heat exchanger pipe

Heat-transfer fins

Turbulence-enhancing ribs

Micro-fabrication surface for SUS

1μm1μm

Fig. 6. Air-cooling enhancing technologies Fig. 7. Micro-fabrication surfaces


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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 diameter corresponding 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 and the measured temperature difference between the pipe surface and air bulk. Pressure loss of the test section was measured by the differential pressure gage.

T

T

F

2400mm

ΔP

950mm

1500mm

T

T

T

300mm

300mm

300mm

P

T

Thermocouple

Air compressor

Reducing valve

Flow control valve

Air heater

Current plates

Insulator

Heat exchanger pipe

82.4mm

25.4mm

Cross-sectional view

Insulator

Thermocouple

Sheath heater（1.5kW）

Heat exchanger pipe

Outer tube

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 the pipe 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 to the 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 and resulted 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, we confirmed that heat transfer performance was increased at least 100% with the technologies with consideration 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 the micro-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 these experimental and analysis results, we demonstrated the feasibility for implementing the infinite-time air-cooling system.


95

0

0.2

0.4

0.6

0.8

1

1.2

5000 15000 25000 35000 45000 55000 65000

Pres

sure

-loss

coef

ficie

nt[-]

Reynolds number [-]

0

50

100

150

200

250

300

350

5000 15000 25000 35000 45000 55000 65000

Nus

selt

num

ber[

-]

Reynolds number [-]

Pipe with developed technologies

Bare pipe

Pipe with developed technologies

Bare pipe

140%

70%

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 the water-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 the tests for the air-cooling system, heat transfer was increased at least 100% by implementing the enhancement 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 the infinite-time cooling.

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The Concept of Passive Cooling Systems for Inherently Safe