Failure Knowledge Database / 100 Selected Cases

Steam Eruption from Secondary System Piping
in Nuclear Power Plants
August 9th, 2004, Mihama-cho, Fukui-pref.
KOBAYASHI, Hideo (Tokyo Institute of Technology)(Summary)
On 9th August 2004, piping ruptured and a steam eruption occurred in the turbine building (3 stories) of
the n uclear p ower pla nt u nder o peration. A n ins pection team on the 2 nd fl oor suf fered th e erupt ion,resulting in 4 deaths, 2 severe injuries and 5 injuries. One of the 2 severe injuries died later. As an accidentof t he nuclear pow er pl ant under operation, it w as the w orst e ver happen. As t he rup tured piping w as
secondary system piping, there was no radioactive contamination.
The cause of the eruption was localized metal loss of piping due to erosion and/or corrosion. In order tomeasure t he c oolant fl ow ra te in t he p iping, a n orifice flow m eter was ins talled. Th is caused a l ocalized
metal l oss in cross-sectional are a of the piping year -to-year, and t he stress le vel became hi gher, at last,
plastic c ollapse w as oc curred an d th e pi ping rup tured. As a resu lt, large quantities of h igh temperaturewater became steam, and blew off.
This part of the piping was widely recognized as a suspected part for erosion and/or corrosion, and it
should be ins pected per iodically. H owever, th e p ipe t hickness w as no t c hecked for 27 y ears du e t ooversights.
1. Component
Pressurized Water Reactor (PWR), a turbine for a n electric power ge neration, a c ondensate pipe whichconnects a condenser and a steam generator
2. Event
On 9th August 2004, a piping ruptured and a steam eruption occurred in the turbine building (3 stories)of the "mihama nuclear power plant No.3" under operation, which belongs to The Kansai Electric Powerco., inc, Japan. An inspection team on t he 2nd fl oor suffered the eruption, resulting in 4 deat hs, 2 severe
injuries and 5 inj uries. One of the 2 se vere injuries died later. As an acci dent of t he nuclear power plan tunder operation, it was the worst ever happen. As the ruptured piping was secondary system piping, there
was no radioactive contamination.
The rupture occurred at the carbon steel pipe which was connecting the condenser and steam generator.
The s pecifications are as follows; outer diameter: 5 60mm, thicknes s: 10mm , coolant pr essure: 10 MPa,
temperature: 142 degrees Celsius. Eruption makes maximally 570mm fissure, which resulted in blow off of
800 tons coolant from the pipes. The thickness of the ruptured pipe was reduced to 2〜3mm. The minimum
thickness w as ab out 0 .4mm. In order t o sat isfy safety stan dards, the cross-section sh ould have had athickness of at least 4.7mm.
1

Failure Knowledge Database / 100 Selected Cases

The cause of the eruption was localized metal loss of piping due to erosion and/or corrosion. In order tomeasure t he c oolant fl ow ra te in t he p iping, a n orifice flow m eter was ins talled. Th is caused a l ocalized
metal l oss in cross-sectional are a of the piping year -to-year, and t he stress le vel became hi gher, at last,
plastic collapse was occurred and piping ruptured. As a result, large quantities of high temperature water
became steam, and blow off.
This p art of t he p iping is widely rec ognized as a sus pected part fo r erosion and/or corrosi on, and it
should be inspected periodically. The bylaw of The K ansai Electric Power Company orders t o inspect the
thickness of a fourth of all secondary piping every ten years. Inspection of all secondary piping, therefore,
complete by 40 years. The burst section of pipe has never been inspected for 27 years since the plant started
operation. O n th e other hand, Th e K ansai E lectric Po wer Com pany ins pected other ei ght nuclear p ower
plants w hich have i dentical sy stem of seco ndary p iping from 199 0 t o 2 003. A s a resul t, they foun dreduction of thickness of pipe in "Ohama nuclear plant No.1" and "Takahama nuclear plant No.2" whichhave same orifice a t same position to " Mihama nuclear plant N o.3". Those pipes were replaced w ith thepipe m ade of an Aust enite stai nless s teel. In t he case of the M ihama nucle ar power pl ant, becaus e ofoversights by The Ka nsai Electric Powe r Com pany a nd t he ins pection companies (Mi tsubishi Hea vy
Industries and Nihon Arm), the thickness of the pipe was not inspected.
Investigation headquarters of Tsuruga station, Fukui prefectural police, opened a criminal investigationof fatal professional negligence.
3. Course
In the latter half of 1975, thickness reduction of pipes due to erosion and/or corrosion was occurred insome pressurized water-reactor (PWR) a nd wall-thickness investigations were conducted.

Subsequent tothis, Sally nuclear plant accident was occurred in the U.S. in December 1986, then power plant companies
investigated a management method for the thickness reduction of pipe by evaluating the inspection resultsof a ll PWR plants. As a c onsequence of th is st udy, in May 1990, a m anagement g uideline f or PWR' ssecondary piping was decided. Although the guidelines were decided in 1990, it has not revised more thanten years even though they got many data regards thickness reduction.
From 1985 t o 19 89, T he Kansai Electric Pow er Company had o utsourced t he pi pe w all -t hickness
measurement and d ata c ollection to M itsubishi Hea vy I ndustries. Th is da ta was us ed for formulating th e
PWR's maintenance standard in 1990. T he section which caused the accident was erroneously omitted on
the inspection list of Mihama nuclear power plant No.3 made by Mitsubishi Heavy Industries. A total ofthree sect ions, tw o s ections at condensation orifice fl ow meter at downstream and one p art at st eam
converter heating steam orifice flow meter at downstream, out of 39 sections in Mihama nuclear plant No.3
were erroneously omitted on the inspection list.

It is unknown why those three parts were not listed. The

Kansai Electric Power Company did not check the inspection list, either.
In 19 96, The Kan sai El ectric Po wer Company ch anged the contra ct of an ins pection service from
Mitsubishi He avy Ind ustries to Nih on Ar m.

However, eve n a t th e t ime, the i nspection l ist had still notbeen revised.

2

Failure Knowledge Database / 100 Selected Cases

Nihon A rm carried out maintenance of the inspection l ist from 2001 t o 2002. In A pril 2003, amaintenance worker found the omitted section and revised the mistake of the inspection list. Nihon Arm put
the om itted s ection i nto t heir 2 0th p eriodic ins pection report a nd proposed 2 1st periodic inspection plan
which contains the omitted section to the Kansai Electric Power Company (November 2003). The Kansai
Electric Power Company did not recognize that the ruptured pipe section erroneously had not been in theinspection list before.
In summary, although the ruptured section was known to be important to monitor wall-thickness, it waserroneously omitted on the inspection list. As a result, wall-thickness had not been inspected for 27 years.
Ironically the omission was found in the previous year of the accident and the wall thickness inspection wasscheduled in the year which just caused the accident.
4. Cause
The central system of PWR and location of the damaged section is shown in Fig 1. The damaged sectionof pi ping w as situa ted ne ar the downstream of orifice flow meter in " A" sy stem condenser pi ping. Twocondenser piping system labeled "A" and "B" were in stalled, which connect low pressure water absorption
heater No.4 near ceiling to deaerator in t he 2nd floor turbine room. Orifice is a m echanism which re ducescross sectional area of pipe to measure the fluid flow.
The detailed appearance of the damage is shown in Fig. 2. The material used for the piping was Carbon
Steel (SB42), having an outer diameter of 538.8mm, a nominal thickness of 10mm, a temperature of around
140 de grees Celsius, a pressure of aro und 0.93 MPa , and a flow r ate of 17 00 m3/h. The reduct ion of
thickness w as sign ificant a t upp er part . The m aximum size of fissure was 515mm in axis direct ion and
930mm in radial direction. The thinnest wall-thickness was 0.4mm.
Later investigation re ported th at the t hickness of p ipe w as pe netrated, and re duction of th ickness o fflange w hich supports t he orifice w as al so observed at t he downstream of a vent. T he si tuation of t hedownstream of the vent is shown in Fig. 4. The vent is installed at upper part of orifice to release air. In thiscase, the diameter was 4mm.
The scale pattern, a feature of erosion and/or corrosion, was ob served at downstream of orifice except
for the lower part of a pipe. On the other hands, a nominal thickness was remained at the lower part of apipe and the section had the thick surface coating (0.4mm). The scale pattern was not observed there.
The results of investigation can be summarized as follows.
○ The damaged pi ping w as made by easi ly erode d/corroded c arbon steel. Th e d amaged p art is at
downstream of the orifice at which turbulent flow was often occurred.
○ Data rela ted to w ater qu ality (pH , percen tage of diss olved oxygen, e tc.) in fe eder and c ondenser
was kept within the specified value.
○

At the area of eventual rupture, the temperature was around 140 degrees Celsius, which is ideal for
erosion and/or corrosion.

○

Large t hickness reduct ion was observe d insid e the pipe. The sc ale pat tern was also observe dentirely.
3

Failure Knowledge Database / 100 Selected Cases○ Large thickness reduc tion and sc ale p attern w ere al so observed at downstream of orific e of "B "system.
It can be supposed that the pipe rupture was caused by the plastic collapse at thinner part by pressure inservice as a result of reduction of pipe thickness reduction due to erosion and/or corrosion. Here, reductionof pipe thickness due to erosion and/or corrosion at downstream of vent occurred before the reduction ofpipe thickness at d ownstream of orifice. Fortunately, the leakage was not happened at downstream of thevent because the flange was installed to support the orifice.

This was typical case of th e localized metal

loss and the rate of reduction is very high. The part is not in the inspection list because the inspection ofthis part is impossible. The cause of erosion and/or corrosion at downstream of vent must be clarified. The
localized metal loss at downstream of vent indicates the other parts of erosion and/or corrosion which were
not re alized. Furthermore, it is su pposed that the tur bulent fl ow tha t caus ed er osion a nd/or c orrosion a t
downstream of the vent combined with the flow that caused erosion and/or corrosion at downstream of theorifice, hence accelerating the erosion and/or corrosion.
From a techn ological point of vi ew, the caus e of t he prob lem w as reduc tion in t hickness d ue t oerosion/corrosion. However, the direct cause of the ac cident was negligence by The Kansai Electric Power
Company, Mitsubishi Heavy Industries, and Nihon Arm. There was a fault in inspection list of s econdary
piping and it could not be revised until the accident happened. The poor quality assurance and maintenanceof The Kansai Electric Power Company resulted in the accident.
Main cause of the accidents is as follows.
○

The damaged section was erroneously omitted on the inspection list.

○

This has not been revised for a long time.

○

Even after the omitted section on the inspection list was realized, inadequate liaison with relevant
persons led to the lack of an appropriate inspection plan.

5. Immediate Action
The Nucle ar and Industria l Safety Agency establis hed an accident analysis com mittee of s econdary
piping of Mihama power plant No.3 after the accidents. In a series of meetings between 11th August 2004
and 27th September 2004 (1st to 6th meetings), committee submitted an interim report.
The report shows about the current management method for w all-thickness of piping in PWR's nuclear
power pl ants, BWR' s nucle ar pow er pl ants, an d th ermal p ower pla nts. They pro posed fo llowing revise dmanagement plan.
〇Reduction of pipe thickness in PWR
The maintenance method o f PWR was examined. Validity of the guideline of PW R maintenance wa s
investigated. The problems of the guideline were indicated.
〇Reduction of pipe thickness in BWR
Thickness reduction in BWR piping due to erosion and/or corrosion was also recognized. Oxygen was
injected to feed and condensed water to improve the water quality. Hematite coating (Fe203) was appliedon the surface of the pipes in order to help prevent erosion and/or corrosion.
4

Failure Knowledge Database / 100 Selected Cases

The accident of secondary piping of Sally reactor plant gave a m otivation to electric power c ompanies
to m easure t he re duction of pi pe thickness. T he e lectric power companies u niquely established th emaintenance rule based on the data.
If the maintenance rules of BWR's by each electric power company are compared with those of PWR's,
the inspection range is wider in BWR's and the inspection frequency is higher in PWR's.
The ra te of re duction of p ipe t hickness i s less i n a BWR's than i n a PWR' s. It is considered that thedifference is cause d by the difference of the water qua lity. The secondary pi ping of PWR e mploys adeaeration alkali treatment. Oxide layer of magnetite (Fe3O4) is also made on the surface of carbon steel.
〇Reduction of pipe thickness in thermal power plant
By the report submitted by electric power companies (8th August 2004), out of the 1467 units in 802
power plants, 704 units enforced pipe thickness measurement and 763 did not.
The rep ort su bmitted by el ectric industries (21 Sep tember) shows that, by t he p ublic wal l-thickness
inspection pl an for 20-year-olds t hermal power pla nts, about 249,000 s ections should be
although, 213,000 se ctions has not y et b een ins pected.

inspected,

They wil l s equentially i nspect t hese re maining

sections.
6. Countermeasure
Many data about the reduction of pipe thickness has been accumulated by the inspection which based on
guidelines for PWR plants. It is su pposed from the d ata that the guideline is almost appropriate. However,in order to make assurance doubly sure, nuclear power plant companies and concerned parties should makenew gu ideline. The guideline sh ould be made by a neu tral 3rd p arty and t he process of making t heguideline must be re leased to the public. The accumulated data in Japan and other countries are of grea t
value to refer. The following factors should also be considered.
○ Thickness reduction rate should be supposed from past measurement○ The range to be inspect should be supposed from past measurement○ The cl assification of i nspected sect ions int o c omplete i nspection and sam pling insp ection, and thenumber of sampling points.
○ The frequency of inspection in accordance with the evaluation results of the remaining lifetime.
○ The required minimum thickness and the integrity evaluation method based on the new knowledge
such as the minimum thickness, the maximum rate of t hickness reduction, and the change ratio of
thickness reduction rate regarding localized metal loss phenomenon.
○ The measurement method (addition of the detailed measurement method to the guideline).
Since it is hoped t hat every power com pany cond uct a n i nspection accordance w ith t he unified
management method, BWR companies and concerned parties should consider the management method incooperating with PWR companies.
Furthermore, even though a t the present time there are no common technical guidelines covering pipe
thickness in t hermal power plants, from now on al l measurement data taken by e very company should becolleted and used to set technical guidelines for appropriate management of pipe thickness.
5Failure Knowledge Database / 100 Selected Cases

Under present m easurement te chniques, 8 or 4 p oints a re sel ected t o m easure the thickness of every
cross section. If the thickness fails to meet the specified criteria, more detailed measurements are performed
and the m inimum thickness was measured.

If the minimum thickness is l ess than technical standards, itwill be replaced. This maintenance method is supposed that the reduction of the pipe thickness is progress
uniformly in circumference.
Although this maintenance method is effective if the measured point is thinnest point, in practice it has
been found that the rates of localized metal loss vary greatly from point to point in a cross section.
It is n ecessary to e xtract the s ection w hich localized m etal loss tends to o ccur an d develop themeasurement m ethod resp onse to the extract resu lt. Integrity eva luation method for the piping w ith
localized metal l oss sho uld be a lso considered.

These po ints m ust be c onfirmed whe n t he n eutral 3r dparty is organized.
When considering the dire ct ca use of t he sec ondary pip ing a ccident by The K ansai Ele ctric Pow er
Company, Mi tsubishi H eavy Industri es a nd N ihon A rm, it is cl ear tha t t here w ere fail ures i n q uality
assurance and maintenances by The Kansai Electric Power Company.
By th e re vision of ins pection re gulation sy stem in O ctober 2003, concrete re quirements t o q uality
assurance and maintenance were regulated by a law. It was al so decided to conduct a peri odical operator
inspection. The com missioners are req uired t o de velop q uality as surance an d maintenance system b ycomplying withthe l aw. Nucle ar and Industri al Safety Agency is com

missioned to inspect t hecommissioners' duti es by maintenance i nspection an d peri odic safe ty management i nquisition. Fro m theview p oint of qua lity assur ance and maintenance for the wal l-thickness red uction of pi ping, foll owing
countermeasures are required.
○ Preparation of Inspection lists and unified maintenance system○ Proper subcontract management system○ Regulations on pipe thickness maintenance○ Information sharing in order to prevent accidents before happen

7. Knowledge○ What is erosion and/or corrosion?
The w all-thickness of pi ping w ith flow ing fl uid is re duced a nd this leads to a leakage of flui d or an
eventual ru pture of piping.

The w all-thickness re duction m echanisms are corros ion a nd er osion. One

representative example of erosion is "cavitation erosion".
Erosion and corrosion occur at the s ame time, and it is difficult to determine which one is d ominant.
Therefore, th e term , " erosion a nd/or cor rosion," is used. In t he a tomic ener gy field, the ter ms "Flow
Accelerated Corrosion (F AC)" and "Flow Induced

Corrosion (FIC)" are a lso use d. This means themechanism that an oxidized lay er (Fe3O4 ) on car bon st eel is dissolved by water fl ow. Th is is si milar tocorrosion. Er osion a nd/or corrosion do not re duce the pi pe t hickness un iformly in c ircumference, b ut
reduces a ccumulatively i n specific point in a cross se ction.
corrosion.
6

This i s the c haracteristics of er osion a nd/orFailure Knowledge Database / 100 Selected Cases○ Localized metal loss is a defectThickness red uction is c lassified into "g eneral m etal loss" and "l ocalized m etal l oss". On th e otherhands, defect is classified into "crack-like flaws" a nd "localized metal loss". Then, localized metal loss is
defect, a nd t he l ocalized m etal l oss should be inspected as well as fati gue crac k a nd s tress corrosion
cracking to keep structural integrity.
○ Difficulty of prediction of localized metal loss rate
The rate of localized metal loss is much higher than the rate of general metal loss. Localized metal loss
only occurs at part icular secti on (flow turbulence) under p articular operation co ndition (temperature,chemical species, flow velocity). The rate of localized metal loss cannot be predicted by general corrosion
data because it is very se nsitive to t he c hanges in pro cess c ondition and m aterial, which is j ust same tostress corrosion cracking.
○ The situation of thickness reduction control
For the light water reactor, thickness reduction of carbon steel piping in secondary system of PWR a ndfeed-condenser sy stem of BWR sho uld b e monitored and controlled. It is c ontrolled by cha nging w ater
quality ( temperature, dissolved o xygen, pH) an d m aterials ( low a lloy ste el, a ustenitic st ainless stee l).
Localized metal loss is caused if carbon steel is used while stress corrosion cracking is caused if austenitic
stainless steel is used. Completely safe cannot be realized with materials.
○ Current situation of inspection.
Wall-thickness at representative point of a pipe is measured by the ultrasonic inspection.

There is nodiscrimination between general metal loss and localized metal loss in the inspection. Measured thickness iscompare with required thickness of permissible level.
○ Problem of localized metal loss
Localized metal loss sh ould be list ed in the standards of integrity evaluation in the fitness for s ervice.
Permissible le vel ( decided by plas tic c ollapse) s hould be re gulated de pendent on t he s hape a nd s ize of
localized metal l oss. Th e major secti on which sust ains lo calized metal loss e asily sho uld b e clar ified.
Improvement in accuracy of localized metal loss rate estimation is required for the accurate remaining life
predictions.

The structural heath monitoring can be applied tentatively.

○ Si milar accidents
The accident of the H-II F6 rocket on 29th Novem ber 2003 was caused by localized er osion. Due to a
turbulence of combustion gas, a roc ket booster nozzle made by carbon fiber reinforced plastic was eroded
away leading to a hole eventually opening in the nozzle wall t hrough which the combustion gas escaped.
The rate of local erosion is very difficult to predict.
8. Sequel(a) The case of Tohoku-Electric Power Co., Inc
On 29th September 2004, a report was s ubmitted by Tohoku-Electric Power Com pany to the Nuclear
and Industrial Safety Agency concerning the thickness reduction of pipe at orifice downstream of the ventof high pressure feed water heater in the Onagawa nuclear plant No.1 and No.2.
7

Failure Knowledge Database / 100 Selected Cases

At the Onagawa nuclear plant No.1, the pipe repeatedly replaced due to thickness reduction. The pipe in
the Onagawa nuclear plant No.2 was also recognized the thickness reduction and replaced.
The d etails o f the i nspections a nd rec ord of repl acements are present ed below (1st and 2nd h ighpressure feed water heater systems have two heaters respectively, those are labeled A and B)○Onagawa nuclear power plant No.1
・ The 5th inspection in 1989 led to the replacement of pipes at A and B in 1st high pressure feed water
heater system. (Carbon steel → Carbon steel)・ The 6th Ins pection in 1990 led to the replacement of pi pes at A and B in 1st and 2nd high pressure
feed water heater system. (Carbon steel → Low alloy steel)・ The 9th inspection in 1993 led to the replacement of pipes at A in 1st and 2nd high pressure feed water
heater system. (Low alloy steel → Low alloy steel)・ The 11th inspection in 1997 led to the replacement of pipes at A and B in 1st and 2nd high pressure
feed water heater system. (Low alloy steel → S tainless steel)○Onagawa nuclear power plant No.2
・ The 2nd inspection in 1997 led to t he replacement of pipes at A and B in 1st and 2nd high pressure
feed water heater system. (Low alloy steel → S tainless steel)
Onagawa nu clear power plant No .1 co uld no t st op the redu ction in pi pe th ickness alt hough it was
replaced with carbon steel and low alloy steel. Austenite stainless steel was used finally. The piping i n the
Onagawa n uclear power plant N o.2 c ould n ot be sto pped t he re duction of pi pe t hickness, e ither. O n t heother ha nds, Onagawa nuclear p ower p lant N o.3 has been us ing a ustenitic sta inless steel since it s tarted
operation. The reduction of the pipe thickness has not yet been observed in No.3. In "Kashiwazaki Kariwa
nuclear power plant No.1" (Tokyo Electric Power Company), the reduction of pipe thickness has not yet
been observed, although the carbon steel pipe were replaced with low alloy steel pipe.
Although above reduction of pipe thickness was lo calized metal loss, the cause was not erosion and/or
corrosion but erosion due to jet blast of condensed water. The mechanisms leading to the erosion are shown
below;
1. There was a vent in the feed water heater to draw in steam and condensable gas. The vent also drewin condensed water.
2. The condensed water arrived at orifice with steam flow in vent.
3. The condensed water w hich arrived at orifice is eru pted as dro plets to downstream of orifice. Its
speed was close to the sound speed.
4. As the droplets were erupted along the orifice direction (at 45 degree angle), the pipe thickness waseroded, causing a belt shaped section of erosion.
The taper of the orifice edges in Onagawa nuclear power plant No.1 and No.2 are 45 degree whereas theorifice edge in Onagawa nuclear power plant No.3 is straight. The 45-degree of taper is one of the reasonsof localized metal loss.
The pipes in Onagawa nuclear power plant No.1 and No.2 have already replaced to Austenite stainless
8

Failure Knowledge Database / 100 Selected Cases

steel pipes. Tohoku-Electric Power Company is going to do the remaining life assessment by every periodic
inspection. The shape of orifices is also going to be considered.
The m ain ca use of t he Kansai E lectric P ower C ompany acc ident w as FAC (corro sion is d ominant),
whereas t hat in the T ohoku-Electric Power Com pany accident was erosion. T he f act t hat the a bove two
diametrically opposite ac cidents were occurred m eans that many i n-between er osion and/or corrosion,
although it has not yet recognized, have been occurring. It is, therefore, important to consider fundamental
counter-measures against the problems.
(b) The case of thermal power plant
After the accident at t he Kansai E lectric Power Company, anot her a ccident occurred on 15th August
2004. " Shinchi th ermal pow er pla nt N o.2" in Shi nchi, Fukushima Prefect ure st opped o peration after t heleak of steam due to the burst of piping in turbine building. The burst location was at th e downstream ofvalve of carbon steel pipe (300mm diameter and 10mm thick). The pipe thickness of the burst location was
reduced to approximately 1.4mm. The localized metal loss was due to a typical erosion and/or corrosion.
This rupt ure had oc curred whil e t he N uclear a nd Industrial Safe ty Agency wa s in vestigating t heinspection c onditions f or p iping of thermal pow er plant in Ja pan. The piping th at ru ptured h ad s tarted
operation in 1995 and the plant has never experienced an inspection.
Erosion and/or corrosi on problems are no t l imited t o nuclear power plants b ut als o to t hermal pow erplants, petrochemical plants, oil refining facilities, general chemical factories, etc.
9. On the Side
The investigation of t he damage examples of eros ion and/or corrosion except for nuclear power plants
were conducted
Under several laws an d regulations (High Pressure Gas Safety Law , Petroleum Use Law, Fire D efense
low, In dustrial Safety a nd Health low, e tc.) a ccidents such as l eakage, rupture , and fire dis aster m ust bereported t o t he go vernment. Corrosi on is most often the c ause of t hese a ccidents. 53 accident du e t ocorrosion and 19 pot ential accidents due to were re ported from 1971 t o 1997. Out of these 72 cases, 22
were erosion and/or corros ion. It is considere d that erosion an d/or corrosi on is direc tly related to th e
accidents.
Figure 4 s hows the resul ts of all 2 2 ero sion a nd/or c orrosion case s. The or dinate in dicates t he p ipe
thickness, and the abscissa indicates the duration of service in year. If it is assumed that the localized metal
reduction penetrates the entire thickness and the accident occurs, the gradient in Fig. 4 indicates the rate ofthickness reduction. The rate of thickness reduction (mm/year) that can cause accidents is in the following
range.
・ General corrosion: 0.15-0.3 mm/year
・ Accelera ted corrosion: 0.3-0.5 mm/year
・ General erosion and/or corrosion: 0.5-1.0 mm/year
・ Accelerated erosion and/or corrosion: 4 mm/year
The accelerated corrosion represents stress corrosion cracking or the corrosion that affected by seawater9

Failure Knowledge Database / 100 Selected Casesor groundwater. The accelerated erosion and/or corrosion represent the erosion and/or corrosion under thecondition that th e stru cture or t he m aterial is i nappropriate. At a ny rate, the rat e of th ickness redu ction
caused by erosion and/or corrosion is much higher than that caused by corrosion only. In the general design,
although the applicable corrosion allowance is ta ken into account, although the reduct ion rate of few mm
per ten year is not take into account.
The causes of erosion and/or corrosion are classified as follows:・ Position (T-junction, curve, joint elbow, downstream of valve, downstream of an orifices etc.)
・

Fluid events (water injection, collision, and hot water flushing)・ Design (inappropriate structure, unsuitable materials)
Erosion and/or corrosion are rarely caused by just one of the above factors. It is most often caused by a
combination of several factors. Some of these combination causes are described below:
・ Inappropriate structure composite, e.g T-junction and water injection
・ Fluid-position composite, e.g. hot water flushing through an orifice・ Unsuitable structure and unsuitable materials
Figure 5 s hows a com bination e xample of w ater injection a nd an e lbow part. Th e p osition of water
injection was struct urally i nappropriate. The turbulent flow of the i njected wat er stream accel erated t heerosion and/ or corrosion at the el bow pa rt. In addit ion to th e en vironmental af fect, th e rate of thickness
reduction was at 4 m m/year. By a nalogy wit h this case, the accident occurred a t Kansai El ectric Power
Company was supposed that the vent performed as water injection and the injection accelerated the erosion
and/or corrosion at the orifice downstream.
Figure 6 shows an example of hot flushing combined with an orifice. Under the condition that the hotwater flus hing (sep aration betw een li quid an d gas phases) occ urs, erosi on and/or corrosion wereaccelerated at downstream of the orif ice. Compared this example and the Kansa i Electric Power Company
accident, the opening appearance caused by the rupt ure and the thickness reduction at downstream orifice
was analogous to each other.
In gen eral, the l ocalized metal loss d ue t o eros ion and /or c orrosion occurs preferentia lly at the
specialized equipment and part. The question arises why the Kansai Electric Power Company accident was
occurred prior to other plants which involve similar equipments and parts.

Through the analysis of cas estudies fo cused on histories of o perating condition, it w as f ound t hat ch anges in operating c ondition(intentional changing) cause the changes in the flow velocity and the environment, and it accelerate the rateof thi ckness r eduction ( unintentional c hanging or acc umulative ch anging). A lthough t he l ocalized m etal
loss due t o er osion a nd/or corrosion itself is probabilistic event, c hanges i n operating condition is als omajor factor of the problems.
10. Information Source(1) Nuclear and Industrial Safety, "Kansai E lectric Power Company number 3 turbine sec ondary cooling
accident interim report", 27th September 2004
(2) Hideo K obayashi, "S tandardized Proce dures ne cessary for accura te T echnical Control", Nuclear
10

Failure Knowledge Database / 100 Selected Cases

energy Culture 35-10, 11-13, October 2004
(3) Tohoku E lectric Pow er Company, " On the O nnagawa pl ant high p ressure fee d water h eaters w all
thickness reduction," 29th September 2004
(4) Hideo K obayashi e t al , " Research into s tressed c omponent damage due t o c orrosion a nd corrosion
speed," High Pressure Gas 35-3, 203/214 (1998)(5) Hideo K obayashi et al , " Research i nto s tressed component erosi on/corrosion," High Pressur e G as,
36-8, 720/728 (1999)
11. Primary Scenario
01. Organizational Problems
02. Poor Management
03. Insufficient Analysis or Research
04. Insufficient Prior Research
05. No Inspection Register
06. Usage
07. Maintenance/Repair
08. Inspection/examination
09. Lack of inspection
10. Usage
11. Operation/Use
12. Piping
13. Bad Event
14. Thermo-Fluid Event
15. Fluid Phenomenon
16. Fluid turbulence
17. Orifice
18. Failure
19. Abrasion
20. Erosion and/or corrosion
21. Failure
22. Fracture/Damage
23. Burst
24. Steam Eruption
25. Bodily Harm
26. Death
27. Accidental Death
28. Loss to Organization
29. Social Loss

11

Failure Knowledge Database / 100 Selected Cases

30. Loss the Confidence

12Failure Knowledge Database / 100 Selected Cases

Reactor Container
Control Rod

Pressurizer

DamageNuclear FuelFig. 1 Main damaged area of PWR.

Support
Orifice

Support
Orifice

Weld Line

Flow direction
Flow direction

Fig. 2 Piping Damage.

13

Failure Knowledge Database / 100 Selected Cases

FrangeFrangeHole
Cavityshape
Maximum cavity depth(bore through the wall)

Orifice

GasketCornerweld

Nominalthickness(10t)

CavityFlow

Flow direction

Vent hole

CavityEnlargemant

Enlargemant

A line's downstream frange

Flange outer diameter
FrangeFrangeMaximum cavity depth

Cavityshape

Orifice
GasketCornerweld

Nominalthickness
(10t)

Flow
Vent h ole
Cavity direction
Enlargemant

B line's downstream frange

Fig. 3 Downstream of Vent.

14

CavityEnlargemantFailure Knowledge Database / 100 Selected Cases

Nominal thickness (mm)● Damage example○ Near miss example

Years in use (year)

to aircooledheat exchanger

Fig. 4 Erosion/Corrosion Reduction Speed.

from combined cycle
heat exchanger

Damaaged area

Water injection
point

Fig. 5

Damage from cold water Injection.

15

Failure Knowledge Database / 100 Selected Cases

pressure measurement
nozzleFig. 6

Damage from hot water flushing.

16