ML20141J679
| ML20141J679 | |
| Person / Time | |
|---|---|
| Issue date: | 06/13/1989 |
| From: | Stello V NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| To: | |
| References | |
| REF-GTECI-015, REF-GTECI-NI, TASK-015, TASK-15, TASK-OR, TASK-PII, TASK-SE SECY-89-180, NUDOCS 8906190110 | |
| Download: ML20141J679 (28) | |
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POLICY ISSUE June 13, 1989
" 18 (InfOrmatiOn)
For:
The Comissioners From:
Victor Stello, Jr.
Executive Director for Operations
Subject:
GENERIC SAFETY ISSUE 15, " RADIATION EFFECTS ON REACTOP, VESSEL SUPPORTS"
Purpose:
To inform the Ccmmissioners of the staff plans for resolution of GSI-15. This information paper responds to the Comission's request for additional information about this Generic Safety Issue (SRM dated May 2,1989 - M890425).
Sumary:
Generic Safety IssLe 15 (GSI-15) addresses the potential problem of radiation embrittlement of reactor pressure vessel (RPV) support structures. Originally identified as a candidate Unresolved Safety Issue in NUREG-0705, June 1981, it was prioritized in November 1983 and assigned a LOW Ridge National Laboratory (ORNL) yses developed by the Oakin April 1988 priority. Based on data and anal concluded that the potential for RPV support embrittlement from neutron radiation damage could be greater than predictions basad on pre-1988 data. A reevaluation of the issue was
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requested in September 1988; in December 1988 it was concluded that the issue should be given a HIGH priority ranking. Although the more recent ORNL radiation data suggested that a potential problem may exist for RPV supports, preliminary analyses by the staff led to the conclusion that this problem does not pose an immediate concern to public safety. At the same time, there are several reasons to resolve the issue, as detailed below.
Contact:
R. E. Johnson, RES 492-3909
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t TASK ACTION PLAN Revision 0 May,_, 1989 Generic Sa,*ety Issue 15 Radiation Effects on Reactor Vessel Supports Issue Type: High/ Generic Safety Issca Lead Organization: Engineering Issues Branch Division of Safety Issue Resolution Office of Nuclear Regulatory Research Task Manager: Richard E. Johnson Engineering Issues Branch Division of Safety Issue Resolution Of fice of Nuclear Regulatory Research lead Supervisor: Frank C, Cherny, Section Leader, Section B Engineering Issues Branch Division of Safety Issue Resolution Office of Nuclear Regulatory Research Contractor: ORNL INEL LLNL ANL MEA NIST Applicability: All LWP Nuclear Power Plants Projected Completion Date: DATE Approved , Task.!anager Richard E Johnson , Section Leader Frank C. Cherny , Chief, EIB Robert L. Baer ,, Deputy Director, DSIR Warren Minners , Director, OSIR R. Wayne Houston I s? gj I. Description of Problem A. Statement of Issue Generic Safety Issue 15 (GSI-15), " Radiation Effects on Reactor Vessel Supports," will address a concern related to a recent Department of Energy (DOE)/0ak Ridge National Laboratory (ORNL) discovery that the embrittlement of reactor pressure vessel (RPV) supports exposed to low-temperature, low-flux rad'ation may be more rapid than previously expected. A preliminary neutron radiation review by the NRC indicated that the safety concern in this issue might not require an immediate fix and, therefore, can be treated in the context of long term aging effects and/or license extension of plants. Further investigation of this concern, therefore, is needed under GSI-15 to assess the short-term and long-term radiation ef fects on RPV supports exposed to low-temperature, low-flux radiation. The problem is complicated by uncertainties in the chemical composition and hence in the mechanical properties of the reactor support structure both prior to, and resulting from, irradiation. B
Background
GSI-15 addresses the potential problem of radiation embri' lement of RPV supports.
It was originally identified as a candidate Unresolved Safety Issue (US1) in NUREG-0705 (Ref.1) where it was recommended for further study before a judgment was made on its designation as a USI.
In a prioritization of the issue in November 1983, it was 1
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concluded that the total occupational dose associated with resolving the issue outweighed the potential decrease in public risk.
As a result, the issue was assigned a LOW priority until additional data on the problem became available.
In June of 1987, the Advisory Committee on Reactor Safeguards (ACRS) asked the NRC to consider the possibility of more rapid than expected embrittlement of RPV supports when exposed to low-temperature, low-flux irradiation.
The ACRS was reacting to information derived from a Department of Energy investigation into the embrittlement and fitness for continued service of the High Flux Isotope Reactor (HFIR)-
located at ORNL.
That investigation *, described in ORNL/TM-10444 (Ref. 2), revealed significantly more embrittlement of the HFIR pressure vessel than would be expected based on traditional irradiation damage data.
At the end of the HFIR sttidy, the researchers were left with the conclusion that the more rapid than expected embrittlement was due to low-temperature (~120*F), low-flux 10'3 _n/cm2-sec, E > 1MeV) irradiation.
This has been called a (108
" fluence-rate effect" in that low tlux irradiation produces more damage than high flux for a given fluence.
Since RPV supports operating temperatures are typically in the range of 100"F to 200 F and the cavity flux in the vicinity of the core mid plane is similar to that for the HFIR vessel wall, the ACRS was questioning the potential for more rapid than expected embrittlement of these supports.
An initial scoping study perfortred by the ORNL k.'\\ l..
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u6 researchers that had studied HFIR indicated that there wcs a significant potential for these support structures to become embrittled.
Based on this new information, a reassessment of the priority of this issue was performed by the staff in December 1988, and the issue was subsequently reclassified and given a HIGH priority ranking.
A more detailed, but still preliminary, study was undertaken by the ORNL researchers as a task under the Heavy Section Steel Technology (HSST) program.
This study focused on evaluating the structural integrity of specific plant support structures.
As part of the study, a survey of the various types of support structures was completed.
Five support types were identified--skirts, shield tanks, long columns, e
short columns, and suspens'on studs.
The skirt type supports were not considered further because of the extremely low fluence they receive.
Both shield tanks and long columns were not considered in the ORNL study, because industry analyses had indicated that there were no problems.
Suspension studs are used on only one BWR; apparently they operate at a relatively high temperature and were not considered further. This lef t only the short column type supports.
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The short column support designs were reviewed and two plants (Trojan and Turkey Point Unit 3) were selected based on a review of the Final Safety Analysis Report (FSAR) drawings.
These support structures both had members that w *e in tension, that were exposed to neutron irradiation fluxes similar to those in the HFIR, and that operated at low temperatures.
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\\nI 81 I The results of the analysis of these two plants were reported in NUREG/CR-5320 (Ref. 3).
A number of assumptions were required to complete these analyses.
These included assumptions regarding the material and its mechanical properties, the size flaws that could have resulted from the fabrication process, the loadings, the neutron flux and energy spectrum at the tension members in the supports, the attenuation of that flux due to the concrete biological shield wall, and the effects of low-temperature, low-flux irradiation.
The analyses of the Trojan and Turkey Point RPV supports led to the conclusion that within the plants' design lives, small flaws could result in structural failure under certain accident loadings.
- However, owing to the number of assumptions needed in performing the analyses, the uncertainty in the conclusion was significant.
In this regard, the fracture analyses essentially were inconclusive.
The conclusion in Ref. 3 that neutron radiation could significantly reduce the fracture resistance of the Trojan RPV supports depended to a large degree on the assumption that the HFIR data were influenced by a fluence-rates effect.
HFIR has a much larger ratio of thermal neutrons to fast neutrons than typical materials testing reactors (MTRs) used to obtain radiation damage data.
Therefore, another possibility was that therma? neutrons were active in damaging the steel. The NRC staf f performed an initial eveluation of this possibility based on approximations of the HFIR flux spectrum and integration to lower neutron energies than were used in Ref. 3 to
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DR1FT calculate the expected danage to the structural supports.
These calculations indicated that the high thermal flux was a plausible explanation for the data obtained at HFIR.
Further work will be undertaken to resolve the discrepancy.
On January 11, 1989, the Office of Nuclear Reactor Regulation (NRR) issued a request to the Office of Nuclear Regulatory Research (RES),
to perform:
(1) a failure consequence evaluation of RPV suppcrt failure; (2) a probabilistic fracture mechanics risk analysis of the limiting RPV supports; and (3) structural tests, if necessary, to show the capacity of cracked RPV supports.
The Division of Engineering, RES, responded on January 31 1989 stating that the first of the 1
three items was to begin immediately, the second would begin after the-staff had made more progress and the third would be deferred pending the results of the first two.
The RES staff identitied additional work that will be needed to resolve many of the uncertaintias in the ORNL analyses.
The results of the investigation reported in Reference 3 and more recent preliminary analyses, including materials and structural evaluations performed by the staff and its contractors, were presented at the joint ACRS Materials and Metallurgy / Structural Engineering Subcommittee Meeting on March 23, 1989 and at the full ACRS Meeting on April 6, 1989.
As stated at the ACRS meetings, further research and analysis is needed to conclusively evaluate the integrity of the supports.
Preliminary structural evaluations of bb 5
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EA:I the Trojan PRV supports, including the effects of high stresses from accident loadings and significant low temperature irradiation, led to the conclusion that there is no immediate safety concern.
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Purpose
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l The purpose of this task action plan (TAP) is to assess the short-term and long-term radiation effects on reactor pressure vessel supports when exposed to low-temperature, low-flux radiation.
However, GSI-15 will be closed out with recommendations for future research efforts if:
4 (1) It is clearly determined that the safety concern is only one of long-term aging, and (2) Extensive research must be performed to determine what licensee actions are needed prior to license ext 3nsion.
The TAP is based on the current knowledge of the issue, including
- results of work performed previously and the preliminary results of on going efforts at the time this plan was developed.
The plan wil.
be periodically reviewed, and revised if appropriate, as results of various tasks and subtasks become available.
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Plan For Problem Resolution A.
General Approach The general approach for the resolution of GSI-15 is shown schematically in Figure 1.
Task 1 represents the main effort:
develop general RPV support review criteria; prepare and issue the GSI-15 resolution package.
There are a number of supporting tasks, some of which were initiated prior to the preparation of this TAP.
ORNL performed an initial survey of the support structures of domestic reactors and identified Trojan as th'e plant that appears to have the most immediate potential pr blem.
Further assessment of the Trojan reactor support structure indicates that the problem is 'ne of long-term aging rather than an immediate safety concern.
However, the analysis should be considered preliminary because of large uncertainties in the original mechanical properties of '
A-36 steel from which the structure was fabricated, the unknown effect of flame-cut holes in the structure, and the actual radiation exposure of the structure and its effect on the material properties.
Task 1, in part, will include a multi-disciplineo team review of the ORNL survey to verify that the vulnerabilities of various types cf reactor support structures have t,een correctly characterized, and that the crucial locations in those structures have been identified.
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Tasks 2 through 4 involve further evaluation of the Trojan plant to reduce the uncertainties in the initial ORNL study.
This effort includes (1) further neutron radiation review. (2) material properties review, (3) Trojan RPV support analyses (consequence evaluation of embrittled RPV supports and probabilistic fracture mechanics analysis), and (4) Trojan cavity dosimetry measurement / flux attenuation study in the biological shield wall.
The knowledge obtained by further study of the Trojan plant is expected to greatly assist in providing a basis for resolution of GSI-15.
Task 5 consists of post-irradiation testing of RPV support steels (testing of Shippingport neutron shi" eld tank, testing of A-36 steel irradiated in MTR, and testing of Belgium BR-3 shield tank steel).
Task 6 deals with the development of low-temperature, low-flux damage correlations.
Task 7 involves NRR activities to interface with the licensees regarding GSI-15 activities.
The detailed description of these tasks are presented below.
B.
Specific Tasks for Issue Resolution 1.
Task 1 - Develop General RPV Support Review Criteria and Resolution Package Estimated Level of Effort:
staff-months (contractor)
Estimated Completion Date:
FY 94 Contractor: ORNL, INEL (DSIR)
DBMT 8
MMI This efft c is the main task of the TAP which will develop the resolution for GSI-15.
An initial survey of domestic reactors reoarding RPV supports was alreado conducted by ORNL and reported in Ref. 3.
All the domestic reactor types will be independently reviewed by another contractor to verify that the most vulnerable plant (s) has(have) been identified.
In addition, the most critical location (s) of the RPV support structures in the most vulnerable plant (s) will be verified.
These reviews should be performed with an integrated approach.
The review team should be multidisciplined.
As a minimum, the review team members collectively should have backgrounds in reactor physics / nuclear engineering, material science /e'ngineering, and structural mechanics / stress analysis.
The specific potential problems will then be identified.
The results of the Trojan study, the low-temperature, low-flux damage correlations, and post-irradiation tests of RPV support steels will provide the basis of the technical findings.
A value/ impact assessment of the potential solution (s) to the issue will be developed based on both risk and cost analyses.
All of the preceeding will be employed to formulate the GSI-15 resolution.
Depending on whether the result of the Belgium BR-3 test will be used or not, the issuance of the final resolution for GSI-15 is expected to be FY.93 or FY 94.
2.
Task 2 - Evaluation of Trojan Plant RPV Support Irradiation Effects 9
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Estimated Level of Effort:
staff-months (contractor)
Estimated Completion Date:
FY 91 Contractor:
to be determined, & INEL (DSIR)
Tnis task involves an ',iitial neutron radiation review by DSIR of the ORNL Trojan results and follow-on, a more detailed neutron irradiation review by a yet-to-be-determined contractor.
In the meantime, the Trojan RPV support material p operties will be revieweo by DSIR/INEL (Task 2) and the specific plan for Trojan will be developed.
Using the results of Trojan RPV support analysis (Task 3), the Trojan cavity dosimetry measurements and
- neutron flux attenuation calcul'ations within the biological shield wall-(Task 4), the low-temperature, low-flux damage correlations (Task 6), and post-irradiation testing of RPV support steels (Task 5), a more accurate assessment of neutron radiation cmbrittlement and fracture vulnerability of the RPV supports at Trojan will be made.
3.
Task '.i - Trojan RPV Support Analyses Subtask 3.1 Consequence Evaluation of Embrittled RPV Supports Estimated Level of Effort:
staff-months (contrac. tor)
Estimated Completion Date:
FY 91 Contractor:
LLN' (DE)
FIN:
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I This program will evaluate tne reserve load-bearing capacity of the Trojan RPV supports, working from the assumption that one or more of the horizontal support beams is cracked completely and that the SSE and "small break" LOCA occur independently.
The initial phase of the study will consider the consequences of the bounding case of complete failure of all of the Trojan RPV supports.
This work, taken to completion, will address qualitatively the consequences of RPV support failure for support structure designs dif ferent from those at Trojan--shield tanks for example.
The rationale is that while Trojan's design may be limiting from a fracture standpoint, the consequences of support failure at Trojan may not be the limiting case.
The work was initiated in FY 1989 and the " bounding case" analysis of Trojan is expected to be completed by the end of FY 1989.
The more detailed analysis, if warranted, will begin in FY 1990 and is scheduled to be completed in FY 1991.
Subtask 3.2 Probat,ilistic Fracture Mechanics Analysis of the Trojan Support Structures Estimated Level of Effort:
staff-months (contractor)
Estimated Completion Date:
FY 91 Contractor:
To be determined (DE)
This study will evaluate the probability of failure of the supports at the Trojan plant as a pilot study.
However, the i
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work has beer deferred until some of the other hardware-related studies (Tasks 4, 5 and 6) can be completed to help reduce some of the uncertainty in the results.
Special emphasis will be plac.1 on C ' eloping reasonable distributions for initial flaw size ana vperating temperatures for the support structures.
The deterministic analysis reported in NUREG/CR-5320 will serve as a referenca study for this effort.
However, it is anticipated that additional work will be necessary to develop more appropriate fracture mechanics models and to improve the models describing loading rate effects on fracture toughness.
This work currently is schedulid to begin in FY 19o" and to be completed in FY 1991.
The final schedule will depend on the completions of the hardware-related research tasks.
4.
Task 4 - Trojan Cavity Dosimetry Measurement and Flux Attenuation in Biological Shield Wall Subtask 4.1 Tr'ojan Cavity Dosimetry Measurement Estimated Level of Effort:
staff-months (contractor)
Estimated Completion Date:
Late FY 90 or early FY 91 Contractor:
National Institute for Standards and Technology (DE)
FIN:
B6224 Using the techniques developed under other NRC funded research, this program will seek to determine the actual neutron flux and 12 WL L
I energy spectrum in the vicinity of the RPV supports in the Trojan plant.
This work will reduce one significant source of uncertainty in the earlier ORNL analyses since the flux and energy spectrum used in those analyses were based on expert opinion and inference from resu'.ts obtained in other plants.
Reducing this source of uncertainty will allow a better assessment of the " critical" crack size for the accident loads considered.
Further, it will help reduce the uncertainty in the probabilistic fracture mechanics analysis to be performed for the Trojan plant.
A meeting was held with the utility on April 12, 1989 to determine if it is feasible to make these measurements at the Trojan facility.
The utility appeared to be very favorably disposed to make the measurements and agreed to supply NRC with design information and power distribution measurer.ents that might be needed to design a test rig.
It appears possible to place rea.
lux monitor sets at several locations of the cavity region to obtain data permitting a more precise characterization of the fluences and fluence rates at the vessel supports.
The test rig could be installed at the next outage in April 1990 which would allow enough time to design and construct the test rig.
The measurements would be completed and results reported in late FY 1990 or early FY 1991.
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o Subtask 4.2 Flux Attenutation in Trcjan Biological Shield ' Wall Estimated Level of Effort:
staff-morths (contractor)
Estimated Completion Date:
FY 90 Contractor:
FIN:
80415 This study will determine the attenuation of fast neutron flux as a frction of position along the horizontal support beam in the Trojan RPV supports.
The study would use state-of-the-art transport computations, and would consider not only the att enuation of the concrete but'also transport of neutrons along the support beam.
This work would resolve one uncertainty in the earlier ORNL analyses of the Trojan plant, allowing emphasis on the controlling variables and a more accurate assessment of the " critical" crack size for the accident loads consider These results also would reduce the uncertainty in the probabilistic fracture mechanics analysis to be performed for the Trojan plant.
The work will be performed and reported in FY 1990.
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Task 5 - Post-Irradiation Testing of RPV Support Steels Subtask 5.1 Testing of Shippingport Neutron Shield Tank Estimated level of Effort:
staff-months (contractor)
Estimated Completion Date:
Later FY 89 or early FY 90 Contractor:
ANL (DE)
FIN:
A2256 As part of a larger effort, samples from the inner and outer cylinders of the shield tank from the decommissioned Shippingport reactor have been obtained.
These samples are being used to determine the degree of embrittlement for that shield tank using Charpy specimens, microhardness measurements, and metallographic examination.
Also, an attempt is being made to experimentally determine the total fluence received by the inner cylinder.
These measurements, coupled with the revised transport calculations performed by Westinghouse, will provide a reasonably clear picture of the neutron flux and energy spectrum for the shield tank over the life of the plant. This information, coupled with the embrittlement data obtained from the Charpy spe:imens, will provide another data point for evaluating the effects of low-temperature, low-fl ux embrittlement.
The work was started in FY 1988 and should be completed in late FY 1989 or early FY 1990.
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Subtask 5.2 MTR Irradiation of Structural Steels Estimated Leval of Effort:
staff-months (contractor)
Estimated Completion Date:
FY 90 Contractor:
Materials Engineering Associates (DE)
FIN:
85848 This study will determine the shif t in nil-ductility temperature after high flux neutron irradiation, providing a comparison between the HFIR pressure vessel materials and materials commonly used in RPV supports.
Irradiation will be performed in a Materials Test Reactnr (MTR) that has bee'n used in earlier NRC Benchmark Experiments to provide comparability with other MTR irradiations of similar materials, and in particular with the MTR irradiation of the HFIR material.
It is expected that soe'i:.an-from several heats of A-36 and A-2128 steel will be irradiated at approximately 120 F to a fluence of approximately 1019 n/cm2, E > 1 HeV.
The shift in nil-ductility temperature will be determined from the shif t in the Charpy energy transition at the 15 f t-lb energy level.
The earlier ORNL analysis of the Trojan plant RPV supports assumed that the A-36 matetrial used in those supports would respond to neutron irradiation in the same manner as the A-212B material used in the HFIR.
Since there are no irradiation damage data on A-36, it is impassiolt. to validate this assump-tion.
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The work to be performed will only provide an assessment of how A-36 responds to low-temperature, high-flux irradiation.
It must then be assumed that the A-36 will respond to low-temperature, low-flux irradiation similarly as the A-2128 from the HFIR.
Although this work will help resolve uncertainty in the ORNL analysis of the Trojan RPV supports, it will leave open the question of how these materials -
i respond to low-flux irradiation.
It also will leave open the question of whether or not other RPV suppor' r els, such as A588 used in the Turkey Point suppoits, will behave similarly to A-36.
4 The results will be used in the probabilistic fracture mechanics analysis of the Trojan plant and will contribute to the overall understanding of the susceptibility to embrittlement of RPV support structures.
The work is expected to be performed and reported in FY 1990.
Subtask 5.3 Embrittlement Effects in the BR-3 Shield Tank Estimated Level of Effort:
staf f-months (contractor)
Estimated Completion Date:
Mid FY 92 Contractor:
To be determined (DE)
The Belgians are decommissioning the BR-3 reactor and currently are making plans for using componerts taken from the facility to assess plant aging.
Specimens taken from the shield tank mnI 17
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<w cou1d be used in the same way as those taken from the Shippingport ShieIn tank to rvaluate low temperature, low-flux e
embrittlement.
This suggestion has been 1,ade to the appropriate Belgian cuthorities and further discussions are p l a nt.e d.
Since the Belgian plans are not flhalized, a firm plan for NRC's participation cannot be developed.
This activity is being pursued actively, and it is hoped that the effort could begin in early FY 1990 and could be completed by mid-FY 1992.
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Task 6 - Low-Temperature Low-Ffux Damage Correlation Development Estimated Level of Effort:
staff-months (con w etor)
Estimated Completion Date:
End of FY 91 Contractor:
To be determined (DE)
One of the greatest sources of uncertainty in the ORNL analyses of RPV supports is the correlation between neutron flux, fluence, and shift in nil-ductility ttmperature.
The correlation used in the ORNL analyses was based on a significant extrapolation of the results obtaintd from the HFIR.
The approach taken in developing this correlation may be unduly conservative for RPV support structures.
This possibility is suppurted by tho pre-limir':ry resulta
,ni the Shippingport reactor shield tank inves-tigation which indicate Liiuch less embrittlement than observed in the HFIR surve
'ance specimens. While 11e preliminary 0
Shippingport results are not conclusive, they do clearly show that there is a tisic lack of understanding of low-temperature, 10w-flux embrittlement.
The results of the HFIR study, the Shippingport study, the BR-3 study, and other DOE and NRC funded work evaluating low-temperature, low-flux embrittlement will be used to develop a mathematical model to describe this embrittlemen's, This model would then be used in lieu of the ORNL extrapolation in the pr obabilistic fracture mechanics analysis of the Trojan plant as well as other specific plant analyses perforrhed in resolving G51-15.
It is expected that this work would begin late in FY 1990 and would be completed and repurted by the end of FY 1993, 7.
Task 7 - NRR Interface With Licensees Estimated Level of Effort:
staff-months (contractor)
Estimated Completion Date:
Contractor:
To be determined (NRR)
This contract will provide technical assistance to NRR relative to the NRR interface with licensees regarding GSI-15 activities, DWT 19
REFERENCES 1.
NUREG-0705, " Identification of New Unresolved Safety Issue: Relating to Nuclear Power Plant Stations," U.S. Nuclear Regulatory Commission, June 1981.
2.
ORNL/1M-10444, " Evaluation of HFIR Pressure Vessel Integrity Considering Radiation Embrittlement," Oak Ridge National Laboratory,1988.
3.
NUREG/CR-5320, " Impact of Radiation Erubrittlement on Integrity of Pressure Vessel Supports for Two PWR Plants," Oak Ridge National Laboratory, 1989.
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Dot wnen t Names TASK ACTION PLAN /G1 15 REV 0 Requestor's ID:
DEVAN Author's Name:
RBaer Document Comments:
Radiation Elfects on Reactor Vessel Supports D
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