ML19318A907
| ML19318A907 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 06/19/1980 |
| From: | Goodwin C PORTLAND GENERAL ELECTRIC CO. |
| To: | Clark R Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8006240365 | |
| Download: ML19318A907 (7) | |
Text
rGE June 19, 1980 Trojan Nuclear Plant Docket 50-344 License NPF-1 Director of Nuclear Reactor Regulation ATTN:
Ifr. Robert A. Clark, Chief Operating Reactors Branch No. 3 Division of Licensing U. S. Nuclear Regulatory Commission Washington, D. C.
20555
Dear Sir:
Attached is our response to your request for information, dated May 15, 1980, related to turbine disc cracking. As discussed in our response, we have developed a schedule for performing an ultrasonic wheel bore inspection of at least one of the low pressure turbines during the 1981 Trojan Nuclear Plant refueling outage.
Sincerely,
,h 8
C. Goodwin, Jr.
Assistant Vice President Thermal Plant Operation and Maintenance CG/DRS/4jed6A2 Attachment 0
('e c:
Mr. Lynn Frank, Director 9
\\
State of Oregon t
\\
Department of Energy g
\\
Mr. R. H. Engelken, Director
\\
S U. S. Nuclear Regulatory g4 [k Commission Region V 8 o 062 4 0 3bt 6
e PORTLAND GENERAL ELECTRT.C COMPANY TROJAN NUCLEAR PLANT Response to Request for Information Related to-Turbine Discs
_ _ _(NRC Letter Dated May 15, 1980)
SITE SPECIFIC GENERAL QUESTIONS - To Be Completed in 30 Days I.
Provide the -following information for each LP turbine:
A.
. Turbine type.
B.
Number of hours of operation for each LP turbine at time of last turbine inspection or if not inspected, postulated to turbine inspection.
C.
Number of turbine trips and overspeeds.
D.
For each dise:
1.
Type of material including material specifications.
2.
Tensile properties data.
3.
Toughness properties data including Fracture Appear-ance Transition Temperature and Charpy upper steel energy and temperature.
4.
Keyway temperatures.
5.
Critical crack size and basis for the calculation.
6.
Calculated bore and keyway stress at operating design overspeed.
7.
Calculated Klc data.
8.
Minimum yield strength specified for each disc.
Response
A.. The Trojan Nuclear Plant has a tandem-compound unit with six flow low pressure stages.
It consists of one double-flow noncondensing high pressure turbine and three double flow low pressure turbines.
The exhaust of_the high pressure turbine is sent through a moisture separator / reheater (MSR). The MSR removes entrained moisture and superheats the steam to within 25'F of its initial cemperature before it enters the
~
. low pressure turbines.
B.
The number of hours of operation for each LP turbine at the time of the last inspection is as follows:
- LP turbine "A" was inspected in 1977 af ter 5514 hr.
of. operation.
- LP turbine "B" was inspected in 1978 af ter 11,575 hr.
of operation.
- LP turbine "C" was inspected in 1977 af ter 5514 hr.
of operation.
C.
There has been a total number of 53 unscheduled turbine trips.. Of this number, 14 occurred at less than 10 per-cent full power,10 occurred between 10 and 50 percent power, and 29 occurred at greater than 50 percent power.
To date, there have been no turbine trips with resultant overspeeding.
The turbine has been overspeed tested six times since 1975. All tests were successful with the trip points being within the 1960 and 2002 rpm setpoints for normal and backup trips, respectively.
D.
Answers to these questions involve data which are pro-priety to the General Electric Company.
Information was provided directly to the NRC during a meeting between General Electric and NRC representatives on April 21, 1980.
It is our understanding that the NRC has inforecd General Electric that this information is an acceptable reply to this question.
II.
Provide details of the results of any completed inservice inspec-tion of LP turbine rotors, including areas examined, since issuance of an operating license. For each indication detected, provide details of the location of the indication, its orientation, size and postulated cause.
Response
All LP rotor inspections performed to date have consisted of magnetic particle, dye penetrant and visual examinations of the following areas: buckers, shrouds, tenons, tie wires, tie wire holes, journals, couplings and erosion snields.
In addition, the "L" and "L-1" bucket pins have been visually and
.sonically tested.
The "A" and "C" LP rotors were inspected in 1977. One tenon in each rotor. in stages #9, #10 and #11 had an indication. The indication was very minor and was determined to have been caused C an the tenon was peened over the shroud.
In accordance with General Electric Company's rotor design group, the tenons were left as is.
G The "B" LP. rotor was inspected in 1978. The only areas found to have any indications were the shrouds on the generator end of stages #8, #9 and #10..These were caused by the passage of foreign material at low velocity. These areas were repaired.
No ultrasonic wheel bore examinations have been performed to date.
III._ Provide the nominal water chemistry conditions for each LP tur-bine and describe any condenser inleakages or other significant changes in water chemistry to this point in its operating life.
Response
Chemistry is maintained within tb= following limits:
h Steam
! bin Stsam Generator Header pH 8.5 - 9.2 6.9 - 9.2 Cation Conductivity 12.0 mho/cm 11.0 mho/cm Sodium 140 ppb
$1.0 ppb Conductivity 14.0 sho/cm 14.0 sho/cm Ammonia 1 25 ppm 1 5 ppm Hydrazine
>20 ppb Chloride 1 05 ppm Flouride 1 05 ppm Oxygen
$ minimum detectable 1.
Suspended Solids 11000 ppb Silica 11.0 ppm 20 ppb No samples can be taken directly of the low-pressure turbine steam. Normal conditions for the low pressure turbines is relc-tively pure steam with some hydrazine contamination of chemistry control.
lunor condenser inleakage occurred in August 1979. The full flow demineralizer system provided sufficient protection to allow continued operation without violating our limits.
The. leak was repaired during a scheduled outage in October.
Another signifi-cant occurrence was a high condenser offgas flow rate (>80 scfs) i which existed throughout plant life until the leak was found and repaired in' January 1980.
IV.
If your plant has not been inspected, describe your proposed schedule and approach to ensure that turbine cracking does not exist in your turbine.
j
Response
'Although each LP turbine-has had one major inservice inspection to date, the ultrasonic wheel bore inspection has not yet been perf ormed. Our present schedule for performing this inspection is :
p.
y.'
- LP "C", 1981
'In light of the fact that no General Electric turbines have been found to tuve turbine cracking, we feel confident that there is no reason 'for us to assume that our turbines would be any different.
It is our belief that preventative maintenance practices and strin-gent chemistry control minimize the chances of crack initiation and propagation and that our schedule for wheel bore inspection will provide the necessary assurances that a turbine cracking problem does not develop.
V.
If your plant has,been inspected and plans to return or has returned to power with sracks or other defects, provided your proposed schedule for the next turbine inspection and the basis for this inspection schedule, including postulated def ect growth rate.
Response
As explained in our response to Item II above, indications to date have been repaired or have been so minor as to obviate repair.
The ultrasonic wheel bore inspection schedule is denoted in our response to Item IV.
VI.
Indicate whether an analysis and evaluation regarding turbine missiles have been performed for your plant and provided to the staff.
If such an analysis and evaluation has been performed and reported, please provide appropriate references to the available documentation.
In the event that such studies have not been made, consideration should be given to scheduling such an action.
Response
An analysis and evaluation regarding turbine missiles has been
. performed for the Trojan Nuclear Plant and provided to the NRC Staff. This is documented in Section 3.5.3 of the FSAR.
Further experience gained since these analyses were performed suggests an acceptably low risk of turbine missile damage and supports the design adequacy of the Plant. This view is based on (a) the low probability of turbine missile generation in light of the excellent service experience of General Electric's large steam turbine generators with shrunk-on wheels, and (b) detailed probabilistic studies of turbinc missiles for the proposed Pebble Springs Nuclear Plant', which has a plant layout and turbine arrangement similar to Trojan. The latter demonstrated an over-all risk of a wheel failute compromising safe plant shutdown on the order of 10-7 per year.
In our judgment, the corresponding probabilities _for Trojan would be comparable because:
, 1
.~, -.
., y
- e:
+
.s t
4 1.3 Trojan utilizes' smaller low ~ pressure turbines (38-in.
vs 43-in. -last stage buckets) which' result in a "sof ter" missile ' spectrum, i.e'.,
lighter wheel fragments of lesser velocity.-
ti
- 2.
Design' speed turbine. failures, which is the item of Limmediate concern _in this request, produce much lower 4
energy nissiles than-a destructive overspeed turbine
~
- failure." The latter was identified as the-dominant riskEcont'ributor in the Pebble Springs analysis.
E t
3.
Recent data from the EPRI turbine missile impact program k
indicatelthat' nuclear plant structural components are much more effective -in preventing spallation and perfor-
- mation than heretofore predicted by ballistic penetration
. formulae.
Because of ' the' low overall' risk of damaging essential systems, we
{
- believe ' current Plant design provides adequate protection against the extremely.unlikely occurrence of turbine failure missiles.
I GENERIC QUESTIONS = To Be Completed in 30 Days
~_ I. Describe what - quality control and inspection procedures are used for the disc - bore and keyway areas.
f.
I
Response
- Af ter. the. rough machined wheel / disc forging has been tempered, material is removed from surface locations to measure mechanical properties. The forging is then subjected to a 100 percent volu-metric ultrasonic inspection.. If the test results meet stringent accepta'nce standards, the forging is released for final machining.
~
~
During final [ machining, attention is continually paid-to the
' finish, contour and dimensions of every surface. For instance,
.the keyway depth, width,11ocation, radii, and surface finish for every wheel is_ checked for conformance to drawings. Quality controlL personnelL assure that tolerances are maintained. Any
~
. deviation 4from accepted tolerances are reported to engineering
. for'. disposition.
Only. coolants'and. lubricants approved by engineering are used in
-the manufacturing and assembly process.' These coolant and lubri-cants.imve undergone extensive laboratory corrosion testing to 4
ensure their. acceptability prior to their approval for use in
.L manufacturing. LPeriodic sampling is done on all such. fluids to l verify:that;theirLchemistry is'within acceptable limits.
If
' required, co' erective! actions are taken to maintain the chemistry within limitsi.
- After[ finisbimachining,' each wheel is thoroughly cleaned and given 4-a-magnetic; particle inspection of all surfaces.
If acceptable,
~ zthe buckets 'are" assembled and the wheel is static balanced. After
, assembly on !the: shaf t, each ~ wheel is xinspected and measurements m,
- e.
i b
%.3 - c--W m.
x are made to assure its proper location. The assembled rotor is then spun to'20-percent overspeed following a high speed balance.
- Finally, af ter a magnetic particle inspection of the buckets, the rotor is cleaned to prepare for shipment.
II.
Provide details of the General Electric repair / replacement procedures for faulty discs.
Response-Stress corrosion cracks have not been observed to date in nuclear wheels manufactured by General Electric and we do not anticipate that removal or replacement of wheels will be required because of this phenomenon. The water erosion which has been observed in the keyways on several non-reheat machines is _ being studied intensively.
We currently believe that the erosion process is self-limiting and should not require the replacement of any wheels.
III. What immediate and long-term actions are being taken by General Electric to minimize future " water cutting" problems with turbine discs? What actions are being recommended to utilities to minimize
" water cutting" of discs?
Response
No immediate actions are required to minimize water erosion because of the apparent self-limiting nature of the phenomenon. However, if future inspections show an unexpected progression of the water erosion, appropriate operating restrictions and/or modifications will be recommended.
IV.
Describe fabrication.and heat treatment sequence for discs, including thermal exposure during shrinking operations.
- Responce, The wheel / disc forgings are heat treated in the rough machined condition. The heat treatment consists of soaking at a temperature above the upper critical temperature with the time and temperature sufficient to ensure complete austenitization throughout the forging, followed by a quench in cold, vigorously circulated water for a sufficient time to ensure complete transformation throughout the section. The forgings are heated uniformly to a tempering sufficient to ensure complete austenitization throughout the forging, followed by a quench in cold, vigorously circulated water for a sufficient time to ee tre complete transformation throughout the section. The forgings
.4 heated uniformly to a tempering temperature below the -lower critical temperature and held for a sufficient time to. sof ten to the desired tensile range.
Af ter tempering, the forgings are still-air cooled to room temperature.
After final machining, the wheels (discs) are uniformly heated in an electric furnace to a temperatore below the embrittling range, but sufficiently high to increr.se the wheel diameter enough to assemble on the shalt with the required shrink fit.
DRS/jed/ 4sa66.12A22