Failure Knowledge Database / 100 Selected Cases

Leakage of Sodium Coolant from Secondary Cooling Loopin Prototype Fast Breeder Reactor MONJU
Dec. 8th, 1995, Tsuruga, Fukui pref.
KOBAYASHI, Hideo (Tokyo Institute of Technology)(Summary)
On December 8th, 1995, a leakage of sodium coolant occurred at the terminal of the thermometer sensor
on the secondary sodium cooling loop in the prototype fast breeder reactor MONJU, of Power Reactor andNuclear Fuel Development Corporation (PNC) in Tsuruga city. The leakage resulted from the break of thethermometer well c aused by fati gue d ue t o t he fl uid vi bration. T he re actor w as stop ped m anually i n
response to the alarm of sodium leakage, and the sodium in the loop pipe was fed back to the sodium tank.
The f atigue fr acture was attributed t o a defective de sign st andard of th e thermometer we ll th at ha dspecified that the vi bration by cyclic vortex should be evaluated only in the direction perpendicular to theflow.
1. Component
Terminal of t he t hermometer sensor o n t he o utlet piping of the s econdary main coo ling l oop in t heintermediate heat exchanger. The sc hematic diagram of the e quipment, the appearance of t he site, and thebroken thermometer w ell t ogether w ith the rou te of the le akage a re show n i n Figures 1, 2 an d 3,
respectively.
2. Event
Results of fault-tree analysis are as follows;〇 Figure 4 - Fault-tree diagram based on morphology, mechanism, and process of fracture.
Leakage of s odium occurr ed as a res ult of a break i n the t hermometer well. T he bre ak poi nt
corresponded to a part of the thermometer well where its diameter had been changed sharply. Investigationand observation of the fracture surface confirmed the cause of the break to be a high frequency fatigue due
to fl uid vibration in the d irection para llel t o the fl ow. Th e d irection of the fat igue cra ck pr opagation
coincided to that of the sodium flow, which had caused the well to vibrate in a direction parallel to the flow.
The flow-induced vibration was clarified afterwards as being due to a symmetrical vortex.
〇 Figure 5 - Fault-tree diagram based on defective design standard and fabrication of equipments.
Because of a mistake in the desi gn standard, the designer had ev aluated only the vibration in th e
direction perpendicular to the flow that is caused by an alternative vortex. In addition, the shape of the well
had been designed t o h ave a part w here the direction cha nged sh arply, a nd a st ress con centration w as
induced at the region of diameter change that caused the fatigue crack.
Results of event-tree analysis are as follows;1

Failure Knowledge Database / 100 Selected Cases〇 Figure 6 - Event-tree diagram on the break of the thermometer-well caused by a flow-induced vibration
due to a symmetrical vortex.
There are two kinds of flow-induced vibrations that are induced by Karman's vortex row, a vibration in
the di rection perpendicular t o th e flo w caused b y an alternative vortex, and a vibra tion in t he dire ction
parallel to the flow caus ed by a symm etrical vortex. The relation between the amplitude and flow rate i nvibration of t he t hermometer w ell is sh own i n Fi gure 7. Base d on t he defective design s tandard, t he
designer had evaluated only the former perpendicular vibration in the design of the thermometer well, notthe later parallel vibration. However, the vibration in the direction parallel to the flow that was caused by
the symmetrical vortex resulted in a high frequency fatigue fracture in the well due to the resonance and thestress concentration. The resulting break in the well caused the leakage of sodium.
3. Course
On December 8, 1995, the nuclear reactor Monju was operated in raising power for the plant trip test, asa p art of the 4 0% output test. At 18:47, an al arm w as trig gered th at i ndicated a h igher than normal
temperature of the sodium at the outlet of the intermediate heat exchanger in the secondary loop C. At thesame time, the fire alarm was a lso sounded. After one minute, an alarm indicating a leak of sodium in thesecondary m ain l oop C w as also tr iggered. In res ponse to the alarm, operators opened t he d oor of t hesecondary main loop C pipe work room, where they observed a smoke, in the expansion of t he fire alarm.
They decided th at t he sm oke and alarms w ere c aused by a le ak o f sod ium, so they tripped the re actor
manually at 21:20, and drained the sodium that was in the pipe to the stock tank. An investigation in thepipe work room C confirmed the leak of sodium from the terminal of the thermometer in the outlet pipingof the secondary main cooling system for the intermediate heat exchanger.
4. Cause(1) Defective design standard
The recommended practice for preventing vibration due to Karman's vortex row was introduced into thedesign st andards for deca des ago. In 1974, t

he AS ME(American Society of

Mechanical E ngineers)

Standard, which is widely used on all over the world, provided the following regulation about the vibrationin the direction perpendicular to the flow caused by an alternative vortex; ASME Perform ance Test Code,Supplement on Instruments and Apparatus Part 3, Temperature Measurements, 1974.
(Frequency of vortex row) ＜ 0. 8×(Frequency of proper vibration for cylinder)
In addition, ASME added a supplementary regulation about the vibration parallel to the flow caused by
the symmetrical vortex; ASME Boiler and Pressure Vessel Code, Section Ⅲ, Division 1, Appendix N-1300,
Flow-induced vibration of tubes and tube-banks, 1995.
(Velocity of flow)／(Diameter of cylinder)＜(Frequency of proper vibration for cylinder)
In the Japanese version of the standards, the additional regulation about the vibration parallel to the flow
failed to be noticed.

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Failure Knowledge Database / 100 Selected Cases(2) Thermometer well with a sharply changing step-wise diameterStress concentration at the point where t he diameter of the well changed shar ply in a st ep-wise caused
the fatigue crack fracture. In order to reduce the stress concentration, it is necessary to take a large radius ofcurvature at the changing point of the well's diameter. Inspection for the design with the step-wise shaped
thermometer well was insufficient.
5. Immediate Action
The following actions were conducted to counter the causes mentioned above.
(1) It w as confir med that t he technical information sy stem would b e kept up- to-dated, a nd t he n ewest
design standards were consolidated.
(2) The m ethod for exam ination of t he design w as i mproved i n order t o stre ngthen th e d esign
management, and the outline of the design process was rechecked.
6. Countermeasure
The design s tandards s hould b e p eriodically ree xamined and u pdated w ith t he new est technicalinformation. In 1997, the PNC established a design guideline for pre venting the flow-induced vibration of
the thermometer (PNC TN94 10-97-042). Based on t his gui deline, the J apan S ociety of M echanicalEngineers established the guideline for evaluating the flow-induced vibration of a cylindrical structure on
piping (JSME S 012-1998) in 1998.
7. Knowledge〇 Lack of collaboration among specialist fields;
The technical specialists designing the structure of a plant such as the Monju belong to three fields ofexpertise, that is, materials and strength, thermal fluid flow, and vibration. Each specialist restricts his or
her job and area of responsibility ito that specialist's own field of expertise. In particular, the specialist fails
to make the e ffort to develop a mutual u nderstanding and exchange of inf ormation w ith spe cialists fromother fields. The mismatching between the specialists fields is an important cause of accidents such as theone described here. A typical example of the importance of collaboration between different specialist fieldis the evaluation of fatigue fractures due to flow-induced vibration versus those due to thermal stress.
8. Discussion
A vortex is defined as the motion of a fluid to rotate. There are various kinds of vortices. When a spoon
stirs water in a c up, v ortices are formed beh ind t he sp oon. In t he case of air planes an d m otor vehicles,
vortices of air are formed behind their bodies in the same way. Smoke from a chimney or cigarette sm okefrom a human mouth forms a ring-shaped vortex. The rotation of the screw of a ship forms a spiral vortex.
The source of the fluid vibration is the row of vortices
If a physical solid, for example a cylindrical rod exists in a flowing fluid, vortices are formed behind thesolid. The same phenomenon occurs when the solid moves in a static fluid. A row of the vortices is m ost

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Failure Knowledge Database / 100 Selected Cases

easily formed if the solid has a shape that is symmetric to the flowing direction，and that does not have any
sharp point at the rear, such as that of a cylindrical rod (a cylindrical pole).
Behind a cy lindrical rod, if th e first vortex is form ed on the ri ght h and sid e, the second one w ill beformed on the left, an d the next one on the right, and so on, alternating between left and right (Figure 8).
The direction of the vortices in the row on the right hand side is the opposite of that on the left. The vorticesof the two rows occur at even intervals, but rather than being aligned, the vortices on the left and right arearranged in a zigzag fashion. The reason for why this phenomenon occurs was first elucidated theoreticallyby Th. von Karman, so that this phenomenon is called Karman's vortex row.
If vel ocity of the fl uid is V (m /sec) and t he diameter of t he cylindrical ro d is D (m ), the number ofvortices f (sec-1=Hz) generated per second can be roughly estimated by the following equation;
f = 0.2 V/D
The vortices generated on the left and right are unsymmetrical, so the distribution in the velocity of flow
is different on the left and on the right. The different distributions of velocity cause different distributionsof the hydraulic pressure (Bernoulli's theorem), so that a composite force is generated by hydraulic pressurein the d irection p erpendicular to t he fl ow. Alter native vortices on t he lef t a nd rig ht cause t he a lternative
composed forces by hydraulic pressure, so that a vibrating force having the same frequency f (H z) acts on
the solid. The solid vibrates in the direction perpendicular to the flow (left and right), that vibration called
the fluid vibration. Because there is also a difference in the distribution in flow velocity in front of the solidand behind the solid, the solid also vibrates in the direction parallel to the flow. The trembling of reeds in ariver is caused not always by the wind but also by the fluid vibration, especially in the c ase of the shakingof th e heads of th e re eds. Fluid v ibration c auses dam age in t he f orm of noise a nd fa tigue. A vi brating
bamboo rod makes a s ound. High tension wires make sound as a result of vibration induced by the wind.
The bo ughs an d t wigs o f t rees wail violently in a t yphoon. A t wig with a diameter D= 5 mm v ibrates at
f=400Hz in response to a s trong wind of V=10m/sec, causing a w ave of air (an acoustic wave, sound) tovibrates t he h uman e ardrum making a gentle s ound. A ty phoon wind of V = 30m/sec m akes a shri eking
sound of f= 1200Hz. The frequency given by a row of v ortices coincides with the proper frequency of thesolid to produce the resonance and to generate a strong sound. This is the same principle that causes thestrings of a kite to moan. A solid experiencing sympathetic vibration suffers from fatigue and failure easily.
Damage fro m fluid vibration occ urs in h igh te nsion w ires, chim ney stacks, th e su pports of a br idge,
hydraulic t urbines, pumps, screw of a shi p, an d the periscope ofa s ubmarine. Th e fu ndamental

countermeasure is to avoid the sympathetic vibration that gives rise to resonance, by raising the frequencyof the proper vibration of the solid over the freq uency of the row of vortices. A large diameter raises t heformer, and lowers the latter, so increasing the diameter of the solid is a simple solution. Another solution is
to break the row of vortices by changing the shape of the solid, such as winding a rope around a cylindricalpole, attaching a fin at the rear of the solid, to streamlining the cross sectional shape, and so on.
On December 8, 1995, a leakage of sodium coolant occurred in the secondary main cooling loop of the
prototype fast breeder reac tor MONJU. The l eakage was caus ed by a brea k in the thermometer well that
had been inserted into the pipe. The break was caused by fluid vibration (Figure 3).

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Failure Knowledge Database / 100 Selected Cases

The flow of sodi um makes tw o row s of vorti ces b ehind t he w ell, c ausing t he w ell t o vi brate i n th e
direction perpendicular to the flow. This vibration is c alled fluid vibration. The frequency of t he vortices
row is very h igh. For e xample if flow 's vel ocity is 5 m/sec and the diam eter of well is 10m m, then th e
frequency is 0.20×5／0.01=100Hz. The total number ofcycles is, therefore, 8.64×106=107 per day. If

the amplitude of the vibration, i.e. the amplitude of the stress on the well is small, fatigue failure can occur
easily. Of course, if t he frequency of t he vortices row coincides with the proper frequency of the well, theamplitude of t he stress o n t he well wi ll i ncrease by resonance causing rap id breakdown in a n extremely
short period of time.
There are two difficulties in evaluating fluid vibration fatigue. One is the existence of the two directionsin the vibration, that is, the well vibrates not only in the direction perpendicular to the flow but also in the
direction parallel to the flo w. The condition b elow c an sto p reso nance from occurring w ith vibrations
perpendicular to the flow.
(Frequency of vortex row) ＜ 0.8(Frequency of proper vibration for the well)
However, e ven w ith a freq uency of vor tex row t hat is eno ugh t o avoid res onance w ith vi brations
perpendicular t o the fl ow, reson ance can occur w ith th e v ibration i n t he dire ction parallel t o th e flow. In
order to avoid this resonance, the additional condition below is necessary.
(Velocity of flow)／(Diameter of the well)＜(Frequency of proper vibration for the well)Furthermore, the vibration under a low frequency of the vortex row continues to vibrate the well in thesame direction to the flow, so that it causes the fatigue failure sooner or later. The fracture surface shown in
Figure 9 shows that the vibration in the direction parallel to the flow caused the well to break by the fatigue
failure.
Another pro blem is the material of th e well. The rel ation b etween the stress a mplitude of t he wellmaterials a nd the num ber of cycles t o fa ilure for the w ell (S-N curves) is s hown i n Fi gure 1 0. The w ellmaterial w as an austenitic stainless steel, w hich h as a lower strength against high frequency fatigue thancarbon st eel o r low al loy st eels, and sh ows a monotonic decreases in stre ngth w ithout any fat igue lim it.
Therefore, fa tigue fa ilure w ill alw ays occur n o matter how small t he stress amplitude is. T he pr oblem isthat we do not currently have data for S-N curves where the number of cycle to failure is 10 7〜1011, whichis the range that corresponds to objects subjected to fluid vibration fatigue.
Could w e n ot have predicted th e flu id v ibration fati gue on t he w ell?

If th e di ameter of the w ell i s

increased, the frequency of the row of vortices row wi ll decrease so that the proper frequency of the wellshould increase resulting in avoiding the resonance, and the s tress amplitude of the well decreases so thatthe number of cycles to failure should increase. Furthermore, if the radius of the curvature at the part of thewell that has a sharply stepwise changed diameter is increased, the factor of the stress concentration will
decrease and the number of frequency to failure will increase. The shape and dimensions of the well for the
JOYO, an experimental type fast breeder reactor, which had not caused any problems, had been changed inmany bad w ays in the design for MO NJU. It can' t be hel ped th at i nsufficient m easures aga inst the fl uid
vibration fatigue were called to account.
Some acci dents ca used by the fluid vibration fa tigue have occ urred previously i n the nuc lear power
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Failure Knowledge Database / 100 Selected Cases

field. In 1983, a sim ilar ac cident i n a t hermometer well for a re- circulation pump w as e xperienced in a
nuclear re actor overseas. I n 1991, the f racture of t he hea t e xchanger t ubes in the st eam generator of
Mihama No.2 react or of Ka nsai Electric Co. was also attributed to t he fretting fatigue caused by the fl uid
vibration. The Kashiwazaki-Kariwa nuclear reactor No.6 and 7 generators, which are improved type boilingwater reactors, constructed by Tokyo Electric Co. have re-circulation pumps inside the pressure vessel ofthe nucl ear r eactor (i nternal pump type). Bec ause of concer ns regarding fl uid vibration fati gue from
unpredicted flows of fluids, experiments were conducted for a long period in order to assess the reliability.
The knowledge a nd experience ga ined f rom those res ults, however, w ere no t refl ected i n t he design of
MONJU.
There are not a few a ccidents by the fluid vibration fatigue in other field except the nuclear reactor. In
Europe, there are m any ac cidents ca used by f luid vibration fa tigue in sta cks. There are fewer acc idents
involving sta cks in Japan, because the s trict regul ations for the earthquake-proof designs res ult in st acksthat are built with strong structures. However, there are no regulations regarding earthquake-proof design ofthe thermometer wells. But overlooking and miscalculations for events to be supposed as a matter of course
are attributed to the fault of the engineers.
9. Information Source(1) Materials in Research committee to utilize fau lt knowledge (N o.4), Leak of so dium fro m secon daryloop in prototype fast b reeder reactor MONJU, Power Reactor and Nuclear Fuel Cycle Development
Corporation, December 12, 2000.
(2) Hideo Kobayashi: Fluid Vibration, High Pressure Gas, 33-9(1996), 755.
(3) Hideo Kobayashi: Fluid Vibration Fatigue, High Pressure Gas, 33-11(1996), 950-951.
10. Primary Scenario
01. Misjudgment
02. Narrow Outlook
03. Lacked Standard
04. Planning and Design
05. Poor Planning
06. Bad Design
07. Thermometer Well
08. Bad Event
09. Thermo-Fluid Event
10. Fluid Event
11. Fluid Vibration
12. Karman's Vortex
13. Vibration Parallel to Flow
14. Failure

6Failure Knowledge Database / 100 Selected Cases

15. Fracture/Damage
16. Fatigue
17. Crack
18. Penetrate through Wall Thickness
19. Leak of Sodium

7Failure Knowledge Database / 100 Selected Cases

Secondary main
cooling system(sodium)

Shelter

Reactorcontainment

Primary main cooling loop
Secondary main cooling loop
Water・steam loop

Air cooler

Intermediate heat exchanger
Control rod
drive system

Leak point

TurbinePrimary main
circulation pump
Super heater

Generator

Condenser
Water outlet

Secondarymain circulation
pump
Control rod

Circulating Cooling water
water(sea water)
pump

Evaporator

Feed waterpump

Water・steam loop
(Water・steam)Fig. 1

Schematic diagram of reactor.

Thermometer(Sodium leakage point)
Perforation
part ofshelterOutlet piping of secondary loop
for intermediate heat exchanger

Flow direction of Sodium

Hole in duct
Hole in steel walkway
Steel walkway

Sodium compoundon steel floor

Fig. 2 Appearance of leakage point of sodium.

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Failure Knowledge Database / 100 Selected Cases

Route of sodium leakage

Insulator

Thermocouple

Secondary loop piping

Bent thermometer
Flow of sodium

Broken wellFig. 3

Broken thermometer well and leakage route of sodium.

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Failure Knowledge Database / 100 Selected Cases

Leakage accident(Terminal of thermometer sensor)
Or GateothersBreak of thermometer sensor

Or GateTime independent fractureTime dependent fracture

Or gateDuctile fracture

Or GateBrittle fracture

FatigueStress corrosion cracking

High frequency fatigue

Creep

Low frequency fatigue

Or gateOthersFlow- induced vibration

Or GateAlternative vortex

Fig.4

Symmetrical vortex

Fault-tree diagram based on morphology, mechanism, and process of fracture.

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OthersFailure Knowledge Database / 100 Selected Cases

Leakage accident(Terminal of thermometer sensor)
Or GateThermometer well

OthersOr GateDesignMaterial

Production

Inspection

Repair・Renewal

and / or Gate

Structure

Shape

Step-wise shape

Fig. 5Strength

Stiffness

OthersLacked design
and piping

Fault-tree diagram based on wrong design and fabrication of equipments.

11Failure Knowledge Database / 100 Selected Cases

Fluid

Alternative vortex
Flow-induced vibration

Thermometer well
Lacked design standard

Resonance

ResultStep-wise shape
Yes

Break, Leakage

Yes
No

Yes

No

Yes

No

Yes

Soundness

Soundness

No

Soundness

No

Fig. 6Crack

Soundness

Event-tree diagram on the break of thermometer-well caused by a flow-induced vibration due to asymmetrical vortex

12Failure Knowledge Database / 100 Selected Cases

Broken thermometer↓・Parallel direction vibration in operation due tosymmetrical vortex.
・Perpendicular direction vibration due to alternative
vortex considered at design

Amplitude of thermometer well

Symmetrical vortex

Thermometer well

Alternative vortex

Imaginative illustration of flow-induced
vibration of thermometer well by vortex

Velocity of flow

Fig. 7

Relation between amplitude and flow rate in vibration of thermometer.

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Failure Knowledge Database / 100 Selected Cases

V
D

Fig. 8

Karman’s vortex row

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Failure Knowledge Database / 100 Selected Cases

Main crack
Subsidiary crack

Na upstream

Na downstream

1 mm
Subsidiary crack
Fig. 9Fracture surface of well and direction of crack .

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Failure Knowledge Database / 100 Selected Cases

Stress amplitude

Austenitic stainless steel

Fatigue limit
Carbon steel, Low alloy, steel

10

4

10

5

10

6

10

7

10

8

Number of cycle to failure

Fig. 10

S-N curve of material (log-log plot)

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10

9

10

10