A Study on the Straight Double-Walled Tube Steam Generator ...

A STUDY ON THE STRAIGHT DOUBLE-WALLED TUBE STEAM GENERATOR DESIGN AGAINST SODIUM-WATER REACTION IN DECS

Hironori Kanda Japan Atomic Energy Agency

Oarai, Higashi-ibaraki-gun, Ibaraki, Japan E-mail: [email protected]

Yasuhiro Enuma Japan Atomic Energy Agency

Oarai, Higashi-ibaraki-gun, Ibaraki, Japan

Satoshi Futagami Japan Atomic Energy Agency Oarai, Higashi-ibaraki-gun,

Ibaraki, Japan

Masaya Kawamura Mitsubishi FBR Systems, INC.

Shibuya, Tokyo, Japan

Hiroshi Ushiki Mitsubishi FBR Systems, INC.

Shibuya, Tokyo, Japan

Shinya Ogumo Mitsubishi Heavy Industries, LTD

Kobe, Hyogo, Japan

Takashi Ichihara Mitsubishi Heavy Industries, LTD

Kobe, Hyogo, Japan

Takashi Nakashima Nippon Steel & Sumitomo Metal

Chiyoda, Tokyo, Japan

ABSTRACT The Sodium-cooled Fast Reactor (SFR) is one of the most

promising concepts suggested for Generation-IV nuclear reactor systems. Some SFRs adopt Steam Generator (SG) as their heat exchange system between sodium and water. Sodium-water reaction occurs in the tube failure accident of a SG. The tube failure may propagate to adjacent tubes resulting in a large scale tube failure by this reaction. In an advanced loop-type SFR design promoted by Japan Atomic Energy Agency (JAEA), a straight double-walled tube SG is adopted to prevent this sodium-water reaction [1], [2]. The double-walled tube is expected to prevent water leakage by acting as double wall boundary and mitigate consequences of the sodium-water reaction. It is expected for the outer tubes to practically behave as waste resistant for the adjacent tubes to mitigate sodium water reaction consequences. Mitigation is expected in Design Extension Conditions (DECs) such as the loss of the mitigation function which might lead an initial water leakage to large scale tube failure. In addition to the prevention of the initial leakage, the initial water leakage rate is practically suppressed because of the narrow gap between the inner and the outer tube.

In this paper, tube failure propagation has been calculated to assess property protection performance on outer tubes. The evaluation results showed that the total leakage rate is limited to one double-ended guillotine scale hence the double-walled tube SG has the property protection performance. By additional

calculations assuming the loss of the mitigation function, a sever event in DECs is cleared. These calculations suggest that increase of the reliability of water blowdown system and enhancement of the pressure release system are effective for the boundary integrity between primary and secondary cooling systems.

There is an issue to be addressed to adopt the concept described above, that is, the decrease of temperature difference between exchange tubes especially for structural integrity of the straight double-walled tube SG for its thermal contact resistance between double-walled tubes and its lack of bending part to release thermal stress. The dispersion of thermal contact resistance between tubes causes temperature difference there due to their heat transfer rate difference. To suppress this dispersion, the oxidized scale is reduced on the interface between the inner and the outer tubes by applying heat treatment using hydrogen furnace for the tube element production. Then, thermal contact resistance of the double-walled tube is successfully reduced at laboratory scale.

Thus, these results suggest that the double-walled tube SG may suppress water leakage rate and sodium-water reaction consequences in DECs. Furthermore, temperature difference between exchange tubes due to oxidized scale on the interface between the inner and the outer tubes can be reduced at laboratory scale. Hereafter, the specifications of the double-

Proceedings of the 2016 24th International Conference on Nuclear Engineering ICONE24

June 26-30, 2016, Charlotte, North Carolina

ICONE24-60156

1 Copyright © 2016 by ASME

walled tube SG will be determined including tolerance reinforcement of sodium boundary.

1. INTRODUCTION The major concern for the Sodium-cooled fast reactor

(hereinafter referred to as SFR) is the prevention and mitigation of sodium water reaction (hereinafter referred to as sodium-water reaction) and their consequences. Sodium-water reaction is caused by heat transfer tube failure accident in Steam Generator (hereinafter referred to as SG). A sodium-water reaction generates reaction heat, corrosive reaction product and hydrogen gas. Reaction heat and corrosive reaction product may failure heat transfer tubes around the initially leaked tube to propagate the heat transfer tube failure. Hydrogen gas generated by sodium-water reaction increases pressure of secondary coolant system and that may affect the structural integrity of the boundaries of the primary and the secondary coolant system.

JAEA aims to improve economic efficiency for a single SG by adopting the advanced loop-type SFR for its enlarged thermal capacity, and the scale merit and the two-loop configuration. Following is the postulated benefits achieved by the SGs of the advanced loop-type SFR.

・Promotion of public acceptance, ・Improvement of the availability of SG, ・Property protection performance. There are issues to be concerned to enlarge the SG power,

namely large equipment productivity and property protection performance. Larger SG brings delay in the water leak detection for its increased sodium flow rate and sodium inventory. The in-Sodium detector detects a rate of hydrogen concentration rise in the sodium caused by water leak where slower concentration rise due to inventory increases detection delay in the sodium-water reaction. Thus the failure earns more time to propagate. In order to restrain large scale water leak, it is important that consequences of sodium-water reaction are mitigated and tube failure propagation is suppressed even in the heat transfer tube failure of large-scaled SGs.

Generation IV International Forum (hereinafter referred to as GIF) has promoted construction of the Safety Design Criteria (hereinafter referred to as SDC) of SFRs [3]. The SDC demands consideration to deal with Design Extension Conditions (hereinafter referred to as DECs). That is, consideration for measures against sodium -water reaction is necessary, assuming events severer than those which have been considered in conventional designs. As a measure against the issue described above, straight double-walled tube is planned to be applied to SGs of the advanced loop-type SFR. This paper describes a concept of measures against sodium-water reaction in the double-walled tube SG with the measures considered assuming DECs.

In addition, there is a risk that buckling occurs when the temperature difference between tubes is generated in the straight tube type SGs. Measures for reducing the mutual temperature difference of heat transfer tubes is such as

uniformity of sodium flow in the straight tube type SGs. Concerning the double-walled tube, heat transfer performance (thermal contact resistance between inner and outer tube) also becomes a factor of the temperature difference among tubes. The basic production conditions of the double-walled tube have been grasped and basic data such as heat transfer performance has been obtained in previous studies. The decrease of heat transfer performance has been confirmed in some tubes that contact pressure of the interface between inner and outer tube is low. It has been found that an improvement on the contact state of the interface is necessary. This paper also reports on experimental test results to reduce the temperature difference between heat transfer tubes.

2. CONCEPT OF SODIUM-WATER REACTION

2.1 EVENT TRANSITION OF WATER LEAK Fig.1 shows an event transition of heat transfer tube failure. Water leak (tube failure)

Water leak detection

Reactor trip SG isolation (water/steam) SG blow down (pressure relief

of water/steam system) (pressure relief by rupture

disk)

Reactor shut down

Water leak stop

Water leak termination

Increase of hydrogen concentration in sodium Increase of cover gas pressure

Decay heat removal operation by sound loop

Nitrogen gas replacement

Fig. 1 Event transition of heat transfer tube failure

If a heat transfer tube fails for some reason, sodium and

water react rapidly around the leak hole and hydrogen is generated. When leak rate is relatively large, two types of pressure rises are generated in the secondary coolant system. One is initial spike pressure which affects within a very short period. The other is quasi steady pressure which lasts a relatively long time. There is a risk that these two types of pressure rises may cause failure of the sodium boundaries to components of the secondary coolant system, pipes, heat transfer tubes of intermediate heat exchanger and so on. If the boundary between the first and the secondary coolant system should be broken, SFR plants would become CV bypass state and the basic safety functions related to the integrity of the core and fuel would be affected. Measures for an early detection of water leak and safety shutdown of reactor is necessary for


prevention of CV bypass by SG tube rupture (hereinafter referred to as SGTR).

Thus, the following are operated with measures against sodium-water reaction. Water leak is detected early by censing increase of hydrogen concentration or cover gas pressure rise during sodium-water reaction. Water/steam system is isolated and blown down rapidly after water leak detection, and sodium-water reaction is terminated immediately. In addition, the reactor is shutdown automatically by safety protection system that is operated by trip of the secondary main circulating pump.

2.2 MEASURES AGAINST SODIUM-WATER REACTION As measure to mitigate sodium-water reaction

consequences caused by SGTR, the following systems are installed.

2.2.1 WATER LEAK DETECTION SYSTEM Water leak is detected by the following systems for water

leak signal to be sent. ・Hydrogen detection system in sodium：2 out of 4, detect

the rate of rise of hydrogen concentration rise in sodium “high”

・Cover gas pressure gauge：2 out of 4, detect the cover gas pressure “high”

・Rupture disk：2 out of 4 (relay circuit), detect the rupture disk “open”

2.2.2 WATER/STEAM BLOWDOWN SYSTEMS After detecting water leak into SG by water leak detection

system described above, the following water/steam blowdown systems are installed for early termination of sodium-water reaction.

・Outlet release valve: 2trains and multiplexing in parallel in terms of reliability of blowdown

・Inlet release valve: 2trains and multiplexing in parallel in terms of reliability of blowdown

・Main feed-water isolation valve: 2trains and multiplexing in series in terms of reliability of water feed stop

・Main steam isolation valve: 2trains and multiplexing in series in terms of reliability of isolation steam

・Check valve: equipment to function dependently at the time of loss of function of the isolation valve. (The check valve is installed respectively to the steam outlet side.)

2.2.3 MITIGATION OF PRESSURE INCREASE Pressure of the secondary coolant system is increased by

sodium-water reaction consequences in case of relatively large water leak caused by SGTR. A relief system of sodium-water reaction products is installed in cover gas region of SG in order to mitigate pressure increase described above. Rupture disk is placed in the release system of sodium-water reaction products, where the disk bursts by cover gas pressure increase. Pressure increase of the secondary coolant system and the cover gas

system is suppressed by burst of the rupture disk so that the structural integrity of the boundary of the primary and the secondary coolant system is ensured.

3. CONCEPT OF DOUBLE-WALLED TUBE FOR MEASURES AGAINST SODIUM-WATER REACTION

3.1 CHARACTERISTICS OF DOUBLE-WALLED TUBE Fig.2 describes a summary of a SG applied double-walled

tube.

Fig. 2 Summary of a SG applied double-walled tube A heat balance of the double-walled tube SG at the rated

power state is as follows. ・sodium flow rate： 14700t/h/loop ・water/steam flow rate： 1442t/h ・sodium temperature (outlet/inlet)： 350/520°C ・water/steam temperature (outlet/inlet)： 497/240°C ・water/steam pressure (outlet)： 19.2MPa[abs] The double-walled tube behaves as a double wall boundary

between sodium and water. The double wall boundary is expected to prevent water leak and mitigate consequences of sodium-water reaction. Fig.3 shows a concept of double-walled tube for measures against sodium-water reaction.

For water leak prevention, the double wall boundary is expected to prevent a same position failure of the outer tube and heat transfer tube (inner tube) due to common causes. The double wall boundary is also expected to prevent a dependent failure of outer tube by internal pressure at the time of heat transfer tube (inner tube) failure. Water leak into sodium doesn’t occur by the prevention function described above, even if heat transfer tube failed. Measures for prevention of common cause failure are as follows. Against the same position failure, heat transfer tube (inner tube) and outer tube are brought into close contact by cold working though they are structurally separated. Measures are taken to those events concerned in the design, such as DNB vibration, flow-induced oscillation,

Type Double-walled tube

Fluid

（inside tube/outside tube）water・steam/sodium

Thermal capacity 882.5MWt

Heat transfer area 6750m2

Material Mod.9Cr-1Mo[4]

Number of tubes 3920

Tube pitch 41mm

Outer diameter

（inner tube/outer tube）15.6/19.0mm

Thickness

（inner tube/outer tube）2.3/1.7mm


fretting wear, seismic motion, and uneven temperature distribution of heat transfer tube.

For prevention of a dependent failure, outer tubes are designed thick enough to hold inner pressure of water/steam during a fixed inspection interval in case of heat transfer tube (inner tube) failure.

For mitigation of sodium-water reaction consequences, outer tubes are expected as wastage resistance in DECs such as loss of mitigation function which would cause large scale failure from an initial water leak. Because a gap between heat transfer tube (inner tube) and outer tube is narrow, water leak rate from an initial failed tube is practically suppressed, too.

3.2 ANALYSIS FOR TUBE FAILURE PROPAGATION IN THE DOUBLE-WALLED TUBE SG

The number of propagated failed tubes is evaluated to evaluate the property protection performance of the double-walled tube SG. In this study, functions as wastage resistance and overheating rupture resistance are expected in analysis for tube failure propagation in terms of evaluation of property protection performance. As differently from the safety assessment, the following are expected.

・thickness of outer tubes (considered such as corrosion allowance for 60 years and wear allowance)

・acceleration of blowdown after water leak detection (from 30s to 20s)

Table.1 shows analysis results for tube failure propagation. Initial water leak rate between 0.56 to 5.7 g/s resulted in the tube failure propagation by wastage. It was one tube that failure propagated. In other ranges, no tube failure by wastage or overheating rupture has propagated.

These results suggest that sodium-water reaction consequences can be mitigated by the functions of double-walled tubes. Tube failure propagation to neighbor tubes is also

suppressed significantly. Hence it seems that extension to large scale leak which affects the structural integrity of the boundary

of the primary and the secondary coolant system can be prevented.

Table.1 Analysis results for tube failure propagation

1 0.06 10.3 2022 2042

2 0.11 0.6 1158 1179

3 0.56 37.9 279 299

4 1.1 105 197 218

5 5.7 1100 70 90

6 11.5 55.3 70 90

7 26.7 108 70 90

8 63.3 253 70 90

9 132 500 70 90

10 244 826 70 90

11 617 1102 7.5 28

12 1157 1263 1.5 22

13 3392 3392 0.2 21 0 0

0 0

0 0

0 0

0 0

0 0

0 0

0 1

0 1

0 0

0 0

0 0

0 1

CaseNo.

initialwater leak

rate(g/s)

maximumwater leak

rate(g/s)

detectiontime(s)

termination time

(s)

Propagated failure tube

overheatingrupture

wastage

4. MEASURES AGAINST SODIUM-WATER REACTION IN DEC

A prospect property protection performance of the double-walled tube SG shown in Section 3.2 is based on the evaluation assuming DBA. On the other hand, SDC under discussion in GIF demands measures against severer event than DBA.

Section 4.1 shows analysis results for tube failure propagation assuming loss of mitigation function such as blowdown system for sodium-water reaction consequences. Section 4.2 describes an event that is reasonable as DEC concerning SG tube rupture in the advanced loop-type SFR

Fig. 3 Concept of double-walled tube for measures against sodium-water reaction

Double-walled tubeInner tube

Water/steam（inside tube）

Sodium(outside tube)

【Prevention of water leak】【Mitigation of sodium-water reaction】

・double-walled tube made of Mod.9Cr-1Mo which has a narrow gap

Inner tube

Double-walled tube

Outer tube

・sodium-water reaction is practically eliminated by ISI of inner tube and maintenance of outer tube (operation early stage )

Outer tube

・water leak rate corresponding to small leak～1DEG

・reinforcing surrounding tube against wastage by outer tube・practically suppression of tube failure propagation(outer tube inspection )


promoted by JAEA. Section 4.3 shows mitigation measures for quasi steady pressure and initial spike pressure in case of more severe water leak than DBA or DEC.

4.1 ANALYSIS FOR TUBE FAILURE PROPAGATION IN DEC

Assuming loss of mitigation function for sodium-water reaction consequences as DEC of SG tube rupture, sodium-water reaction analysis (evaluation of tube failure propagation) is performed. The number of failure tubes propagated by wastage and overheating rupture is calculated so that an event transition and consequences in DEC of SG tube rupture are grasped.

4.1.1 ANALYSIS CONDITIONS ・A water leak accident is assumed that the function of

water leak suppression is lost by dependent failure of outer tubes at the time of initial tube failure.

・Initial water leak rate is up to 1 Double Ended Guillotine (DEG) from the minute leak.

・The hydrogen detection system in sodium, cover gas pressure gauge and rupture disk are supposed to be water leak detection systems.

・After blowdown of water/steam, nitrogen is injected into heat transfer tubes in order to prevent sodium infiltration into water/steam system facilities. At this point, the event is deemed to have terminated.

・Tube failure propagation is evaluated assuming loss of two functions from among the water leak detection system and blowdown system.

4.1.2 ANALYSIS RESULTS Table.2 shows case No. and results of tube failure

propagation. Assuming loss of mitigation function of sodium-

water reaction consequences, it is indicated that a loss of the function of the hydrogen detection system in sodium has a relatively large influence to create an overheating rupture. In the cases where initial leak rate is small, tube failure propagation continues until it is possible for the cover gas pressure gauge to detect water leak by loss of detection function of hydrogen detective system in sodium. And overheating rupture has occurred.

When the blowdown function of outlet and inlet release valve is lost, overheating rupture has caused the largest tube failure. 38 tubes failed due to overheating rupture and the maximum water leak rate is about 130kg/s. In this case, it is possible to detect water leak with hydrogen detection system, rapture disk and so on. However, it is impossible to terminate water leak event by water/steam blowdown because the blowdown function of outlet and inlet release valve will be lost. Since water/steam system is isolated by actuation of isolation valves, it is presumed that water/steam system holds high pressure. Accordingly, it seems that overheating rupture has easily propagated heat transfer tube failure.

Combinations of loss of mitigation function for sodium-water reaction are comprehensively sorted out concerning the double-walled tube SG. Subsequently, tube failure propagation is evaluated using tube failure propagation analysis program. From these results, it is estimated that the loss of blowdown function (inlet and outlet release valves) is the most severe supposition which causes the largest scale tube failure.

4.2 SUPPOSED EVENT IN DEC The failures to be assumed as a DBE and DEC in sodium-

water reaction are exhaustively extracted. Events to be assumed in the evaluation are organized in Table. 3. Table. 3 is based on measures against sodium-water reaction and their operation, as well as, the mitigation function against sodium-water reaction

Table.2 Results of tube failure propagation

1 × ○ ○ ○ ○ ○ ○ 3 14

2 ○ × ○ ○ ○ ○ ○ 0 3.4

3 ○ ○ × ○ ○ ○ ○ 0 3.4

4 ○ ○ ○ × ○ ○ ○ 0 3.4

5 ○ ○ ○ ○ × ○ ○ 0 3.5

6 ○ ○ ○ ○ ○ × ○ 0 3.4

7 ○ ○ ○ ○ ○ ○ × 0 3.4

8 × × ○ ○ ○ ○ ○ 3 14

9 × ○ ○ × ○ ○ ○ 3 14

10 × ○ ○ ○ × ○ ○ 5 22

11 × ○ ○ ○ ○ × ○ 3 14

12 × ○ ○ ○ ○ ○ × 3 14

13 ○ × × ○ ○ ○ ○ 0 3.4

14 ○ ○ ○ × × ○ ○ 38 130

15 ○ ○ ○ × ○ × ○ 0 3.4

number ofpropageted failure

tubes

Maximum waterleak rate(kg/s)

Hydrogen Detector(2 out of 4)

Cover gaspressure gauge

(2 out of 4)

Rupture Disk(2 out of 4 : relay

circuit)

Inlet releasevalve

(2trains)

Outlet releasevalve

(2trains)

Feed-waterisolation valve

(2trains)

Steam isolationvalve

(2trains)

caseNo.

Water leak detection system Water/steam blowdown facilities


whose function is expected in the double-walled tube SG. As well as DBA, failure from small damage causing

instantaneous DEG of one heat transfer tube is assumed as an initial heat transfer tube failure in DEC.

Initial spike pressure and quasi steady pressure is also considered. Even though it is sufficiently low probability of occurrence, the assumption of multiple loss of mitigation function is taken into account in order to confirm that there is no cliff edge.

Table.3 Events to be assumed in the evaluation

Assumed event

DBE ・water leak occurrence by inner and outer tube failure・disregard outer tube in calculation of tube failure propagation ・failure of 1train of inlet and outlet release valves・water leak detection system operates normally

DEC ・water leak occurrence by inner and outer tube failure・fail・expect outer tube in calculation of tube failure propagation ・water leak detection system, release valves and isolation valves are supposed to

fail in addition to initial tube failure・suppose that close arrangement causes release valves to fail simultaneously

4.3 PRESSURE INCREASE CAUSED BY SODIUM-WATER REACTION IN DEC

Evaluation result in DEC would be same as DBA in the initial spike pressure because the number of initial heat transfer tube failure is also one. On the other hand, water leak scale for quasi steady pressure evaluation becomes severer than DBA since multiple loss of mitigation function is assumed in DEC. In this case, the number of failed tubes is 38 and maximum water leak rate is approximately 130kg/s. The initial water leak has been assumed to occur at the upper end of the effective heat transfer zone. Quasi steady pressure is calculated using time change of water leak rate described above. Fig.4 shows calculation results. If a loss of inlet and outlet release valve function is assumed (case no.14 shown in Table.2), maximum water leak rate is approximately 130kg/s.

However, pressure increase of the secondary coolant system by quasi steady pressure is estimated about 3.0MPa with a calculation using water leak rate based on behavior of realistic tube failure propagation. The reason why behavior of realistic tube failure propagation has been assumed is that heat transfer tubes are unlikely to be broken at the completely same time by sodium-water reaction. The structural integrity of the boundary of the primary and the secondary coolant system is presumed to be ensured against the above pressure increase. And it is supposed to be possible to prevent CV bypass even in DEC.

4.4 MITIGATION MEASURES FOR PRESSURE INCREASE IN MULTIPLE FAILURE OF HEAT TRANSFER TUBE

In the advanced loop-type SFR promoted by JAEA, DEC related to sodium-water reaction will assume the following:

a number of initial leak tube is one, multiple loss of mitigation functions.

If multiple heat transfer tube failure is assumed as an initial leak, pressure increase (initial spike pressure and quasi steady pressure) caused by sodium-water reaction is more excessive than DBA. The pressure increase may affect the structural integrity of the boundary of the primary and the secondary coolant system. In this chapter, pressure relief measures and their effects is investigated in the case of assuming a simultaneous failure of the large number of heat transfer tubes, though this assumption is realistically unlikely to occur.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

0 50 100 150 200

press

ure

(M

Pa)

wat

er

leak

rat

e (

g/s)

time (s)

Water Leak RateQuasi Steady Pressure

Fig. 4 Calculation results for quasi steady pressure

4.4.1 MEASURES FOR QUASI STEADY PRESSURE RELEASE

Enlargement of a diameter of discharging line in cover gas and installation of discharging line in sodium are suggested as relief measure for quasi steady pressure. Pressure increase was calculated under the following conditions, assuming multiple simultaneous failures of heat transfer tubes as the initial heat transfer tube breakage.

・Current design: a diameter of the relief system in cover gas is 16B.

・Case1: a diameter of the relief system in cover gas is 30B.

・Case2: a diameter of the relief system in cover gas is 30B and the relief system in sodium is additionally installed.

In case 1, pressure reduction effect to current design is about 34-58%. In case 2, pressure reduction effect to current design is about 53-76%. But water leak rate is expected to increase by reducing the pressure generated in sodium (back pressure), if the relief system is reinforced. It is necessary to confirm above influence hereafter. When the relief system in sodium is installed, it is also necessary to pay attention to the following:

・Implementation of the installation position of the relief system in the sodium,

・Understanding the rupture characteristics of rupture disk in sodium at the time of individual usage or parallel usage with the relief system in cover gas,


・ Structure detailing of the relief system in sodium (Productivity assessment, structural integrity evaluation and so on),

・Embodiment Inspection method, replacement interval and implementation of the jig.

4.4.2 MEASURES FOR INITIAL SPIKE PRESSURE RELEASE

At the time of heat transfer tube failure, spike-shaped pressure wave is also generated. Measures have been examined in DEC when multiple simultaneous failure of heat transfer tube is considered though initial spike pressure hardly occurs beyond 1DEG. Because a branch line with a liquid surface is placed, reflected waves (Decompression waves) are generated at expansion of flow path area or liquid surface. Then, they are expected to reduce initial spike pressure. Pressure wave propagation is calculated in the following conditions:

・a conventional condition that there is no liquid surface, ・a condition installed the branch line(tank) which has a

liquid surface A diameter and a length of the branch line are 1.57m and

20m. (Both are temporary conditions.) Calculation results have suggested that pressure increase at the inlet of IHX can be reduced to about 42%. In order to apply the above measure for initial spike pressure, it is important to investigate a reasonable control method of liquid surface.

5. PRODUCTION METHOD OF HEAT TRANSFER TUBE

Buckling due to temperature difference between exchange tubes is a concern for the straight tube SG design. Previous studies have found that dispersion of the thermal contact resistance of the inner and outer tube leads to the mutual temperature differences of heat transfer tubes. They have also suggested that dispersion of the thermal contact resistance of the inner and outer tube depends on the contact state of the inner and outer tubes. Reduction of the oxide scale at the interface which prevents a smooth contact with the inner and outer tubes is estimated to be effective for suppressing the dispersion and decreasing an upper limit of the thermal contact resistance. In the present study, the hydrogen furnace is applied to the heat treatment at elemental tube manufacturing

(normalizing and tempering) as a measure for reduction oxide scale. By comparing with the tubes with the interfaces not reduced in the oxide scale, it is confirmed that reduction of the oxide scale affects the contact thermal resistance of the inner and outer tubes.

5.1 PRODUCTION TEST OF HEAT TRANSFER TUBE Fig.5 shows an overview of a prototypical manufacturing

process of the double-walled tube.

Hot rolling

Cold drawing

Heat treatment

Straightening

Tube combiningBoring

Manufacturing processof elemental tube

Manufacturing processof double-walled tube

Cold drawing

Straightening

Fig. 5 Prototypical manufacturing process of the double-walled

tube Target dimension of the double-walled tube is 19mm in

diameter and 4.2mm (inner tube: 2.4mm, outer tube: 1.8mm) in thickness. Target outer diameter of elemental tubes is 20.2mm (outer tube) and 15.6mm (inner tube). In order to reduce amount of the oxide scale generated, the double-walled tubes have been produced experimentally under the conditions shown in Table.4.

5.2 MEASUREMENT OF CONTACT THERMAL RESISTANCE

To understand the heat transfer performance of the prototypical double-walled tubes, the basic data such as contact pressure and thermal contact resistance are acquired for the following tubes.

・Double-walled tube that simulates the relaxation of

Table.4 Test product condition of double-walled tube case1 case2 case3 case4 Not heat treated

Outer Tube

normalizing hydrogen furnace ← ← ← bright furnace tempering hydrogen furnace bright furnace ← ← ← Polishing

(inner face) Not polished ← ← ← ←

Inner Tube

normalizing hydrogen furnace ← ← bright furnace ← tempering hydrogen furnace bright furnace ← ← ← Polishing

(outer face) Not polished Not polished +buff finished

Not polished polished Not polished


the contact pressure in service ・Double-walled tube without heat treatment

Double-walled tube that simulates the relaxation of the contact pressure in service is heat-treated under 650°C (1h). This condition corresponds to operating conditions in plants (515°C×60 years).

5.2.1 MEASUREMENT METHOD (a) Summary

Saturated steam heated the outside of the double-walled tube and a reference tube (single tube) at the same time. Then, the cooling water flowing inside the tubes is heated. The heat transfer rates are calculated from temperature difference of the cooling water flowing through the double-walled tube and a reference tube. The contact thermal resistance of the double-walled tube is calculated from a comparison of the heat transfer rates between the double-walled tube and a reference tube. (b) Equipment

Measurement device for the thermal contact resistance adopted a multi-tube heat exchanger structure. Fig.6 shows a concept of testing part of test equipment. Measurement points of the test equipment are as follows.

Steam

Steam

SteamDouble-walled tube

Condensed water

Reference tube

Steam inlet

Steam outlet

Heat exchanger

Condensedwater outlet

Coolingwater

380mm

160mm

37mm

1500mm

Fig. 6 concept of testing part of test equipment

Temperature ・ reference tube：cooling water（inlet, outlet）・ double-walled tube：cooling water（inlet, outlet）・ heat exchanger：steam （inlet, center, outlet） Flow rate ・ reference tube：cooling water（inlet）・ double-walled tube：cooling water（inlet）・ heat exchanger：condensation water

(c) Calculation of contact thermal resistance From the measurement results of the cooling water

temperature flowing through the reference tube and the double-walled tube, the heat transfer coefficient [4] of heat transfer tubes is calculated. (as shown in Eq.1)

( )−= = p ⅱ ⅰ f

m m

C T T GQK

AΔT AΔT （Eq.1）

Heat transfer rate of the reference tube is calculated from the following equation.

out1in1

out1

1

out1

in1

out1

in11 α

1

r

rln

λ

r

r

r

α

1

K

1+

×+= （Eq.2）

Assuming the thermal contact resistance of the interface as R, heat transfer rate of the double-walled tube is determined from the following formula.

Rα

1

r

rln

λ

r

r

r

α

1

K

1

out2in2

out2

2

out2

in2

out2

in22

++

×+= （Eq.3）

Heat transfer rate (K1、K2) and heat transfer coefficient (α in1 、 α in2) are calculated from measurement results of temperature and flow rate. Accordingly, thermal contact resistance (R) is calculated from the following equation.

out2out1in1

out1

1

out1

in2

out2

2

out2

in1

out1

in1in2

out2

in212 α

1

α

1

r

rln

λ

r

r

rln

λ

r

r

r

α

1

r

r

α

1

K

1

K

1R −+

−−

−−−=

（Eq.4）

Reference tube and the double-walled tubes are heated in the same steam conditions,

（Eq.5）

Contact thermal resistance is calculated from the following

equation.

−−

−−−=

in1

out1

1

out1

in2

out2

2

out2

in1

out1

in1in2

out2

in212 r

rln

λ

r

r

rln

λ

r

r

r

α

1

r

r

α

1

K

1

K

1R

（Eq.6）

5.2.2 MEASUREMENT (a) Measurement of contact thermal resistance

Using the double-walled tubes (length: 2150mm) shown in Table.4, the contact thermal resistance has been measured under the following conditions.

・flow velocity of cooling water：4.0m/s ・flow velocity of steam outside tube：26m/s

(b) Measurement of contact thermal resistance after contact pressure reduction

The double-walled tubes which are measured for the contact thermal resistances in Sec. (a) are heat-treated (650°C, 1hour) to decrease the contact pressure that is supposed after 60 years operation.

5.2.3 MEASUREMENT RESULTS OF THE CONTACT THERMAL RESISTANCE

Fig.7 shows measurement results of the contact thermal resistance. Fig.7 also shows measurement results for the double-walled tube which is not treated to reduce the oxide scale.

Fig.7 indicates that the contact thermal resistance of the double-walled tubes which have been treated to reduce the

1𝛼𝛼𝑜𝑜𝑜𝑜𝑜𝑜1

−1

𝛼𝛼𝑜𝑜𝑜𝑜𝑜𝑜 2= 0


oxide scale achieves a provisional target value despite the prototypical production conditions.

Fig.8 shows measurement results of the double-walled tubes which have been treated to reduce the oxide scale selectively. It is found that the prototypical production conditions hardly affect the contact thermal resistance. Fig.8 also indicates that heat treatment to simulate the operation condition in the actual plant scarcely changes the contact thermal resistance.

Cont

act t

herm

al re

sista

nce

(m2 K

/W)

3.0E-4

2.5E-4

2.0E-4

1.5E-4

1.0E-4

0.5E-4

0.0E-4

Surface pressure (MPa)0 20 40 60 80

Relaxation

-walled tube

Without

With

-walled tube

Without

With

Fig. 7 Measurement results of the contact thermal resistance

3.0E-4

2.5E-4

2.0E-4

1.5E-4

1.0E-4

0.5E-4

0.0E-4

Surface pressure (MPa)0 20 40 60 80

Cont

act t

herm

al re

sista

nce

(m2 K

/W)

With/withoutRelaxation

1 Without

With

2 Without

With

3 Without

With

4 Without

With

Fig. 8 Measurement results of the double-walled tubes

(In case of reduction the oxide scale) From the above, it is confirmed that the double-walled

tubes which have been produced under all prototypical condition achieve the provisional target value of the contact thermal resistance. The basic data measured for the prototype double-walled tube offered the following findings.

・ Despite the prototypical condition and the heat treatment, the double-walled tubes with the reduced oxide scale achieve the provisional target value of the contact thermal resistance.

・Because the test produced double-walled tubes achieve the provisional target value of the contact thermal resistance despite its conditions, case no. 4 with much less hydrogen furnace use seems to be the good prospective for the industrial manufacturing method.

The followings are subjects for the future investigation. ・ Manufacturing facility, which includes heat treating

furnace for elemental tubes, for long double-walled tubes is needed to be prepared. Quality control method of the hydrogen furnace has to be investigated in order to reduce the oxide scale even from the center of inner face of the long double-walled tubes.

・Industrial manufacturing and quality control methods which guarantee interface properties of inner and outer tubes are needed.

・In case that contact pressure control is necessary for the structural integrity of the double-walled tubes, quality control methods of contact pressure are required.

6. CONCLUSION In order to mitigate occurrences and consequences of

sodium-water reaction, double-walled tube is proposed to be applied to large SGs program for the advanced loop-type SFR. The most severe event is extracted in the SG with the double-walled tube by evaluating failure propagation comprehensively postulating countermeasure facility failure as DEC. The results suggested that assumption of loss of both inlet and outlet release valve blowdown function brings significant effect for the structural integrity of the boundary of the primary and the secondary coolant system. Pressure increase is calculated under the above condition and the structural integrity of the boundary of the primary and the secondary coolant system is estimated to be ensured in DEC. In addition, the mitigation measures for pressure increase caused by sodium-water reaction have been investigated in case of multiple simultaneous failures of heat transfer tubes despite its unlikelihood.

One of the issue concerning about adopting the double-walled tube SG is reduction of the mutual temperature difference between heat transfer tubes. It is necessary for reduction of the mutual temperature difference between heat transfer tubes to decrease the dispersion of the thermal contact resistance of the inner and outer tube. In order to reduce the dispersion of the thermal contact resistance, the production method of the double-walled tubes has been changed and the thermal contact resistance has been measured. It has been confirmed that the thermal contact resistance can be reduced by adopting the hydrogen furnace for heat treatment of elemental tubes.

It is necessary to substantiate the mitigation measures against sodium-water reaction in DEC and the productivity of


large scale SG with double-walled tubes to pursue the realization of the advanced loop-type SFR,.

ACKNOWLEDGMENTS This document is the result of "Technology development of

fast breeder reactors, etc.” entrusted to Japan Atomic Energy Agency (JAEA) by the Ministry of Economy, Trade and Industry (METI)

NOMENCLATOR K： heat transfer rate (W/(m2・K)) Q： heat amount (W) Cp： specific heat of coolant water (J/(kg・K)) Gf： flow rate (kg/s) A： heat transfer area (m2) T： temperature of coolant water (°C) ΔTm： log-mean temperature difference (°C) r： radius (m) λ： heat conductivity of case metal (W/(m・K)) R： thermal contact resistance (m2・K/W) α： heat transfer coefficient (W/(m2・K)) in： inside tube out： outside tube 1： reference tube 2： double-walled tube i： inlet ii： outlet

REFERENCES [1] Kurome, K., et al., 2010, ICAPP, San Diego, 13-17

June. 10247 [2] Futagami, S., et al., 2009, ICAPP, Tokyo, 9169 [3] GIF 2013 Annual Report, NEA [4] ASME SECTION II Materials Part D-Properties 1992

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