ML19257C213
| ML19257C213 | |
| Person / Time | |
|---|---|
| Issue date: | 01/08/1980 |
| From: | Harold Denton Office of Nuclear Reactor Regulation |
| To: | Thompson A THOMPSON, A.S. |
| Shared Package | |
| ML19257C214 | List: |
| References | |
| NUDOCS 8001250265 | |
| Download: ML19257C213 (8) | |
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NUCLEAR REGULATORY COMMISSION
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JAN O g 79gg Dr. A. Stanley Thompson P. O. Box 118 Sperryville, VA 22740
Dear Dr. Thompson:
Thank you for your letter of October 29, 1979 to Commissioner Peter A. Bradford which transmitted your proposal entitled, " Instability of Power in Nuclear Reactors."
We believe that you will be pleased to learn that reactor stability is a subject of NRC staff review in all reactor license applications and that we have identi-fied this as a generic concern which is receiving a great deal of attention via ongoing research effort sponsored by the NRC. However, there is a consensus among the industry, NRC, and academic experts that the only significant instabi-lity problems with Light Water Reactors are associated with the flow and power instabilities possible during conditions of two phase flow (i.e., boiling in the reactor core). The reactor protection system (including overpower trips) and the negative reactivity response to fuel temperature as well as moderator density which is inherent in LWR designs would not permit severe nuclear transients of the nature described in your paper for any realistic conditions of operation. However, we can not dismiss local instabilities or conditions of power and temperature oscillation which would have potential for loss of fuel integrity. This is an unacceptable operating characteristic for licensed.
nuci nr reactors and is addressed in the Code of Federal Regulations, 10 CFR 50, Appendix A, by the General Design Criteria 11 and 12 which follow:
GDC 11 Reactor inherent protection "The reactor core and associated coolant systems shall be designed so that in the power operating range the net effect of the prompt inherent nuclear feedback characteristics tends to compensate for a rapid increase in reactivity."
GDC 12 Suopression of reactor power oscillations "Thereactohcoreandassociatedcoolant, control,andprotectionsystemsshall be designed to assure that power osci?.'tions which can result in conditions exceeding specified acceptable fuel design limits are not possible or can be reliably and readil, detected and suppressed.
With respect tv your recommendation for further investigation of reactor stability:
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s' JAN 0 81980 Dr. A. Stanley Thompson (1) Collect design and operating data -
Wa feel that the NRC staff is well informed of.the history for instability incidents via our normal liaison with reactor vendors and licensees.
Licensees are required to repo; t incidents of this nature.
-0ther than Ft. St. Vrain, which you have noted, the only known instance of an instability with commercial reactors occurred during stability testing of the Garigliano Nuclear Reactor, which is a General Electric BWR lccated in Italy (described in GEAP-5534 " Control Rod Oscillation Tests: Garigliano Nuclear Reactor," August 1967 and GEAP-5473 " Development & Program on the Garigliano Nuclear Reactor" Quarterly Report No. 18, April 1967). This test was able to produce sustained local power oscillations of + 10% from the 67% power level. The oscillations were attributed to atypical flow conditions in one fuel assembly due to incorporation of a flow meter.
Test data on other BWRs such as the Peach Bottom transient tests (described in EPRI WP-564 '! Peach Bottom 2 Transient and Stability Tests," 1978 and EPRI Report (to be published) " Peach Bottom 2 Cycle 3 Stability Tests") show that the oscillation characteristics over the reactor operating range are inherently damped and are. reasonably well predicted using existing design analysis methods. However, a program is continuing to assess the capability of BWR analytical models to predict stability test data..In order to provide additional margin to stability limits natural circulation operation of high power density BWRs is prohibited until the stability evaluation program is complete.
(2) Develop methods for theoretical analysis of reactor stability - describes the work scope for ongoing work being performed by our contractors. As you may note _,_this is_ consistent with your recommendation.
(3) Develop a computer code for reactor stability analysis -
This recommendation is also consistent with the program described in Enclosure 1.
The staff comments on the experience examples cited in your report are addressed in Enclosure 2.
The staff feels that the analysis provided in your report correctly presents the neutron kinetics equations and the classical mechanics solution to oscillatory motion. The analysis results have been. calculated for a reactor system with assumed instable characteristics. However, the analysis does not show that the assumed instability is characteristic of any licensed reactors.
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JAN 0 81980 Dr. A. Stanley Thompson
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l' In sumary, the staff feels that our current knanledge of reactor operating history and our existing program for reactor stability evaluation are respcosive to the needs identified in,your proposal. We trust that this response will alleviate your concerns and thank you again for your letter l
and offer of assistance.
l Sincerely.
Original Signed By:
i Harold R. Denton, Director Office of Nuclear Reactor Regulation bec:
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JAN 0 g 1989 Dr. A. Stanley Thompson l l'
In saunary the stafT feels that our current knowledge of reactor operat'sg history and our existing program for reactor stability evaluation are responsive to the needs identified in your proposal. We trust that this response will alleviate your concerns and thank you again for your letter
-and offer of assistance.
Sincasely.
Harold R. Denton. Director i
Office of Hoclear Reactor Regulation l
CENTRAL FILES
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Technical Assistance Program for Thermal-Hydraulic Stability Analysis Statement of Work Thermal-Hydraulic stability is an important area relative to light water reactor safety. The importance is a result of the fact that instabilities can adversely affect: cladding integrity (due to premature ONB); hydraulic loads; LOCA reflood hydraulics; and can also adversely affect other reactor safety calculations such as core reactivity and power distribution calculations.
There is a need to determine those areas in which thermal-hydraulic instabilities can adversely affect ceactor safety. This requires an understanding of the fundamental phenomena which cause instabilities and also reqtires the ability to predict conditions which would lead to instabilities.
Objectives The objective of this program is to provide the staff with licensing support in the area of thermal-hydraulic stability. This support will consist of a review of tests, experiments and analytical methods used in this area and will also include the development of fundamental techniques and criteria which the staff can use to identify areas of potential thermal-hydraulic instabilities. This will allow the staff to assure that these concerns are adequately considered in the reactor design.
Specific Tasks The contractor shall furnish the ne',:essary qualified personnel and services to perform the following tasks:
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, 1.
Perform a comprehensive review of the presently available data from.
fundamental experiments in two-phase flow and from reactor stability tests.
2.
Perform a general review of the analytical methods, related to thermal-hydraulic stability, used in reactor design and safety analyses. This review should include the following information: mathematical approach (frequency domain code, etc.); simplifying assumptions; code applications (reactor design, ECCS design, accident analysis, etc.); types of insta-bilities considered (density wave instabilities, etc.); code limitations (closed channel vs open channel), flow regime limitations, parameter range limitations, nodal limitations. Provide recommendations for further tests, experiments, analytical studies, and code development needed to perform a detailed licensing audit of reactor stability.
3.
Develop and evaluate simple, fundamental techniques to identify areas of potential thermal-hydraulic instabilities. These techniques should not be computer codes developed for the purpose of establishing detailed information about the onset instabilities. These techniques should identify the flow regimes, geometries and parameter ranges for which instabilities are expected.
4.
Provide on-call assistance in licensing reviews.
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Comments on Dr. Thompson's Review of Experience with Power Oscillations Spert Experiment The staff does not agree with Dr. Thompson's interpretation of the SPERT results.
In the final test (test-run 54), a power excursion with a 3.2 msec period and a total energy of 30.7 Mw-sec resulted in some melting of all 270 U-Al alloy plates with the average melt being 35%. The nuclear characteristics of the shutdown were essentially identical to earlier SPERT transients. We believe that the blast pressure pulse, which occurred about 15 msec after the nuclear transient was terminated, was produced by a self-propagating steam explosion that resulted from the dispersal of the molten plates into the water throughout the core.
SL-1 Reactor The staff does not agree with Dr. Thompson's evaluation of the SL-1 accident.
The simple explanation is that a rod withdrawal caused a supercritical power excursion which resulted in violent vaporization of the water in the reactor
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core. We believe that a more complex scenario is not suggested by the evidence.
Rover (Nerva) Reactor Although we have not performed a review of the Rover experience, Dr. Thompson's analysis of the Rover instability seems plausible. The Ft. St. Vrain oscillation problem has been attributed to a similar mechanism.
General Atomic Reactor The Ft. St. Vrain oscillation problem has not been the primary reason for limi-tations on the licensed power level. Fluctuations in the outlet helium tem-perature from various regions have been attributed to a redistribution of the gapsbetweengraphiteblockmodulesundercertainconditionsofcoreflow7 d
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_2-pressure drop. Core thermal conditions are believed to be a contributory factor in the initiation of the oscillations. However, no large power oscillations have occurred; apparent power fluctuations indicated by individual ex-core detectors were due to streaming of neutrons through changing gaps in the side reflector modules at the core periphery.
The Ft. St. Vrain reactor is not permitted to operate in the fluctuating mode except when conducting approved experiments to map threshold operating conditions for fluctuations and to evaluate the cause and effect of the instabilities.
Limits are imposed on the amplitude of oscillations to protect the fuel integrity.
The staff has currently approved the installation of region restraint devices which tie together the various regions of the core by restraining movement at the top. Testing will be performed to determine if this design modification will eliminate the instabilities previously observed.
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