Failure Knowledge Database / 100 Selected Cases

Burst of Steam Turbine Rotor in Nuclear Power Plant
Sep. 19th, 1969, Somerset, England, UKNITTA, Akito (Central Research Institute of Electric Power Industries)
KOBAYASHI, Hideo (Tokyo Institute of Technology)(Summary)
At the Hin kley Point ' A' nuclear p ower statio n (cl osed in May 20 00) in S omerset locat ed in thesouthwest part of En gland, the No. 5 t urbine generator suffered a catastrophic failure on Se ptember 19th,
1969, 33,360 ho urs after commercial servic e w as st arted in A pril 1 965. The failur e w as caused byspontaneous b rittle fa cture of a shr unk-on low-pressure disc. The No.5 u nit was d isconnected from thegrid for overs peed tests a few minutes before the acc ident. The fa ilure occurred as the speed reache d3,200rpm.

The rot or s haft fract ured c ompletely in fi ve p ositions, and t hree discs of th e low -pressure A

rotor came free from the unit.

The cause is thought to be stress corrosion cracking (SCC) in a disc keyway.

This was t he first ca tastrophic fail ure of a turbine-generator c aused by the brittle frac ture of a ro tating
component in the UK.

Fortunately, there were no casualties although seven operators were in the vicinityof the unit at that time.
1. Component
The Hinkley Point 'A' nuclear power station has two Calder Hall type Magnox nuclear reactors and six
turbine-generators with a n ominal capacity of 87MW. The design conditions of t he steam supplied fromthe reactors were 4.24MPa, 360℃ for a high-pressure turbine, and 1.07MPa, 346℃ for a low-pressure one.
As a measure to counter CO2 corrosion of mild steel in the reactor, the steam temperatures were reduced by
10℃ before the accident in 1969.

The design rotation speed of the turbines was 3000rpm.

2. EventAt th e Hi nkley Po int ' A' nuclear po wer st ation, th e tu rbine-generator of the No.5 un it su ffered acatastrophic failure on September 19, 1 969, 33,360h after the unit started in commercial service in A pril
1965.

The cross-sectional diagram of the turbine-generator is shown in Fig.1. The failure was caused byspontaneous brittle fracture of a s hrunk-on low-pressure disc.

Figure 2 shows the appearance of the unit

just after th e failure .

The N o.5 un it was disconnected from the gri d for ov erspeed tests a few minutes

before the accident.

The failure occurred a t the t ime when t he rot ation s peed re ached 3,200rpm .

rotor shaft fra ctured completely in fi ve p ositions.

As shown i n Fi g.3, three discs of t he l ow-pressure A

rotor came free from the unit, and large fragments of them were found scattered in the machine hall.
was the first catastrophic failureof a turbi ne-generator ca used by the brittle fracture ofcomponent in the United Kingdom.

TheThisa rota tingFortunately, there were no casualties although seven operators werein the vicinity of the unit at that time.
The following paragraphs show the results from the Fault Tree Analysis.
1Failure Knowledge Database / 100 Selected Cases(1) Fault tree diagram for mode, mechanism and process of fracture (Fig. 4)
The l ow-pressure tur bine d isc, w hich w as the si te of th e fracture , w as made of a 3 Cr-0.5Mo s teel byacid o pen hearth pr ocess. Also, t he dis c w as fou nd fro m chem ical analyses to have a high s ulfur a nd
phosphorus content, although the levels were within the specifications.

Thus, the material was apt to havea lower fract ure tou ghness as a resul t of the temper em brittlement cause d d uring hea t trea tment pr ocess.
Furthermore, in order t o shrink the disc onto the shaft, the disc had a semicircular keyway that a cted as astress co ncentration res ervoir.

Under s uch conditions, th e str ess c orrosion crac king (SCC) rel ated t oenvironmental factors such as NaOH i n the steam can occur, although it was very difficult to identify thetrue cause of the SCC .

Finally, when the crack re ached only 1.6mm in dept h, the brittle fracture of t hedisc occurred.
(2) Fault tree diagram for design and manufacturing errors (Fig. 5)
No pr oblems were found in either the for ging a nd heat treatment processes a t t he tim e of the disc
manufacture or in the results of n ondestructive (i.e., magnetic and ultrasonic) and tensile acceptance tests.
Furthermore, review of the desi gn s tressing of t he m ain r otating c omponents a nd in vestigation of t hepossible modes of the failure by additional or a bnormal stressing did not find any problems in th e design.
Therefore, the fo llowing tests we re ca rried out to id entify the r oot ca use of the fa ilure: the detailed
fractographic observation o f the frac tured dis cs, a nd chemical anal yses, tensi le tests, fat igue a nd cra ck
propagation tests, Charpy impact tests, fracture toughness tests and SCC tests by using the test pieces takenfrom the disc s.

From these resul ts, t he brit tle frac ture of th e l ow-pressure t urbine was thought to b ecaused by the initiation and growth of SCC re lated to environmental factors in the low-pressure steam at
the key way ( i.e., stress concentrated p ortion) of the shru nk-on disc, together with the l ow fracture
toughness of the 3Cr-0.5Mo steel made by acid open hearth process.
(3) Event tree diagram for the low-pressure steam turbine rotor burst attributed to the stress concentration
in a shrunk-on disc keyway and the low toughness of a disc material (Fig. 6)
The SCC, which was thought to be related to environmental factors in the low-pressure steam, occurredon the bore surface of a sh runk-on disc of 3Cr-0.5Mo steel made by acid o pen hearth process.

The disc

keyway was a stress c oncentration reservoir, and the toughness of the material of the disc was lowered bytemper embrittlement during its heat treatment process. The compound effect of these two factors resulted
in the brittle fracture of the low-pressure turbine rotor.
3. Course
The N o. 5 unit of the H inkley Poi nt ' A' nu clear p ower stat ion w as disc onnected from the grid f oroverspeed tests a few minutes before the accident.

The speed of the unit was raised gradually. A numberof o perators who were wa tching the t achometer af firmed th at th e failure o ccurred as the ro tation s peed
reached 3,200rpm. At the time, flames came out from the area of the low-pressure turbine together with aloud bang, and within a few seconds an explosion occurred.
At the time of the accident, all six turbine-generators were operational.
were no t damaged a nd o peration w as co ntinued.

The No.1, No.2 and No.3 units

However, the N o.4 a nd N o.6 u nits w ere sli ghtly

2Failure Knowledge Database / 100 Selected Cases
damaged and were temporarily shut down.

Fortunately, there were no casualties although seven operators

were in the vicinity of the un it at th at t ime.

Immediately after the acc ident, t he Ce ntral Electricity

Generating Board (CEGB) established a formal board of inquiry to conduct a technical investigation into
the nature and cause of t he failure.

The cause was thought to be caused by the SCC in the keyway crownof the disc, and the SCC was observed on many other discs.
4. Cause
The failure was caused by the SCC in the keyway crown.
observed on many other di scs.

The SCC in the disc bore and keyway wasThe disc made of 3C r-0.5Mo steel met the material specifications, but itsfracture toughness was reduced by temper embrittlement during the furnace cooling after the heat treatment
process. Thus, a crack only about 1.6mm in depth was large enough to initiate the brittle fracture becauseof the stress concentration in the keyway crown.
5. Immediate Action
This fai lure w as the first of its ki nd to be ex perienced in t he UK.
technical i nvestigating c ommittee was form ed of
manufacturers and from the CEG B.

expert representatives fro m the t urbine-generator

After that, n umerous examinations, experiments and analyses w erestrongly conducted to identify the c ause of the failure.
not reached.

Immediately after the acc ident, aHowever, a clear identification of t he cause was

It was concluded from circumstantial evidences that the failure was caused by the SCC.

6. Countermeasure
A defini te c ountermeasure w as not o btained from the i nvestigations c onducted by t he t echnicalinvestigating committee, so add itional i nvestigations will be re quired for esta blishing pre ventive ac tions.
However, more ge neral c ountermeasures, inc luding th e elimination o f the en vironmental fac tors tha t had
contributed to the SCC as well as the adoption of a disc material with higher fracture toughness and a discwithout a keyway, were introduced.
7. Knowledge(1) Stress corros ion crac king i s a troublesome form of dam age th at occurs t hrough t he t hree way
combination ofmaterial, env ironment andstress.

It is particularly dif ficult to id entify theenvironmental factors contributing to stress corrosion cracking.
(2) Avoid stress concentration reservoirs in the structural design.

If stress concentrations occur, they willcause problems later.
8. BackgroundNo prob lem was found in the for ging a nd h eat tr eatment processe s at the time of manufacturing thediscs as w ell as in t he res ults of nondestructive, te nsile an d impact tests.

Also, after i nvestigating t he

design s tressing of t he main ro tating components a nd th e p ossible modes of f ailure by a dditional or
abnormal st ressing, no p roblems rel ated t o the d esign we re found.
3Therefore, it can be sai d t hat thereFailure Knowledge Database / 100 Selected Cases

were no technological problems in the design and manufacturing as well as the examinations carried out atthat tim e.

On th e o ther h and, it was r ecognized t hat som e of the discs were made of low- toughness

3Cr-0.5Mo st eel processed by the air open h earth m ethod. Also, t he shru nk-on disc h ad a k eyway that
acted as a str ess conc entration reservoir .

However, although t hese facts w ere c ertainly rel ated t o theaccident, they were co mmon in t he UK a nd other countries at the time.

Thus, no one coul d foresee t hefailure from them . In a ny case, it wa s t he in itiation of the SC C t hat triggered the fa ilure. S tresscorrosion cracking is tr oublesome form of d amage that oc curs t hrough t he thr ee w ay c ombination ofmaterial, environment and stress.

As seen in the result of the failure investigation, it is particularly hard inmany cases to ide ntify the environmental factors.

In other w ords, t he SCC is an un predictable form ofdamage that cannot be prevented completely.
9. Information Source(1) D.Kaideron, "Steam T urbine Fai lure at H inkley Poi nt ' A'," Proc. Instn.
pp.341-377(1972).
10. Primary Scenario
01. Ignorance
02. Insufficient Knowledge
03. Poor Experience
04. Production
05. Hardware Production
06. Manufacturing of Machinery and Equipment
07. Low-Pressure Steam Turbine Disc08. 3Cr-0.5Mo Steel by Air Open Hearth Process
09. Temper Embrittlement
10. Usage
11. Operation/Use
12. Operation of Machine
13. Keyway of Disc
14. Bad Event
15. Chemical Phenomenon
16. Corrosion
17. Failure
18. Fracture/Damage19. Stress Corrosion Cracking
20. Brittle Fracture
21. Bust of Rotor (Flying-off)

4Mech. Engrs. , V ol.186,

Failure Knowledge Database / 100 Selected Cases

Fig. 1

Overview of the burst TVA Gallatin No.2 IP-LP turbine rotor.

Fig. 2 Appearance of the No.5 unit after the failure.

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Failure Knowledge Database / 100 Selected Cases

Fig. 3 Flying-off paths of three large disc fragments.

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Failure Knowledge Database / 100 Selected Cases

Burst of rotor
or GateBrittle fracture

Ductile fractureor Gate

or GateLarge temperembrittlement

Little temper
embrittlement

Crack growth

Crack arrest

or GateHigh S/Pcontents

Low S/Pcontentsor Gate

Acid open
hearth process

Basic electricarc process

or GateHigh stressregionKeywayor Gate

Low stressregionTime-dependent
fracture

Time-independent
fractureor GateCreepStress corrosion cracking

Fatigue

Otherand Gate

3.5Cr-Mo steel

Fig. 4

Shrunk-on disk bore

Corrosive environment

Fault tree diagram for mode, mechanism and process of fracture.

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Failure Knowledge Database / 100 Selected Cases

Burst of rotor
or GateLow pressure turbine

High/intermediate pressure turbine
or GateManufacturing

DesignMaterial

Repair/replacement

or Gateor GateAcid open
hearth process(3Cr-0.5Mo)

Inspection

Basic electricarc process(3Cr-0.5Mo)

KeywayNo keyway

or GateIntegrated disk

Shrunk-on disk

Fig. 5 Fault tree diagram for design and manufacturing errors.

Low pressure
turbine3Cr-0.5Mo
steel

Shrunk-on
disk

Corrosive
environment

KeywayStress corrosion
cracking

Stressconcentration

Crack
growthTemperembrittlement

Results

Yes

Burst(Brittle fracture)

Yes
NoYes
NoNo
Yes

Yes

NoYes

Yes

Initial flaw
Initial flaw

No problem

NoNo problem

NoFig. 6

Initial flaw

No problem

NoYesCrackNo problem

Event tree diagra m for the low-pressure s team turbine rotor burst at tributed to stressconcentration in a shrunk-on disc keyway and the low toughness of a disc material.

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