ML20235C563

Informs That NRC Currently Reviewing Fuel Channel Hydrodynamic,Reactor Core Reactivity & Total Reactor Sys Stability Margins for Certain GE Bwrs.Addl Info Needed Re BWR Stability Margins Identified in Encl

ML20235C563
Person / Time
Site:	Millstone, Peach Bottom, Browns Ferry, 05000000
Issue date:	02/20/1976
From:	Butler W Office of Nuclear Reactor Regulation
To:	Stuart I GENERAL ELECTRIC CO.
Shared Package
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References
FOIA-87-40 NUDOCS 8707090463
Download: ML20235C563 (5)

Text

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Mr. Ivan F. Stuart, Manager GUCT:k i . .

Safety and Licensing

W.Q Ug[gl Nuclear Energy Division General Electric Ccmpany 175 Curtner Avenue San Jose, California 95114

Dear Mr. Stuart:

The NRC staff is currently reviewing fuel channel hydrodynamic, reactor core reactivity, and total reactor system stability margins i for certain General Electric (GE) boiling water reactors. This review is being conducted because certain utilities have proposed to modify the previously licensed designs for the flow referenced rod block monitors and fuel channel bypass cooling arrangements for their boiling water reactors (e.g., Edwin I. Hatch, Unit 1; Peach Bottom, Units 2 and 3; Browns Ferry, Units 1, 2, and 3; and Millstone, Unit 1).

A discussion of specific areas of interest to the NRC staff in its review of boiling water reactor stability margins is prov'ided in the enclosure. We believe.that the review of this subject would be expedited if GE could file an amendment to its existing topical report on stability margins, APED-5652 dated April 1969.

Specific emphasis on startup and low power operating stability margins, including phase margin and frequency response data along with Nyquist and Bode plots, should be added to the following sections:

Section V - Total Plant Stability - Product Line Application Section VI - Reactor Core Performance - Product Line Application section VII - Channel Hydrodynamic Performance - Product Line Application 1 - ,.

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( l Mr. Ivan F. Stuart ,

f f.Il 2 0 1976 The information we have identified above and in the enclosure is necessary for the NRC staff to complete its review of individual plant modifications which affect stability margins. The procedures to be followed by the NRC staff in conducting this review are contained in the NRC Standard Review Plan,. Sections 4.3 and 7.~2.

Future licensing actions by the NRC will.be affected by the j availability of this information. For these reasons we request that you consider fil'ing a revision to APED-5652 and inform us of your decision within two weeks of receipt of thf e letter.

Sincerely,

($ ll Walter *R. Buticr, Chief

.< (O Light Water' Reactors Branch No. 4 Division of Project Management

Enclosure:

BWR Stability Margins cc: Mr. L. Gifford General Electric Company 4720 Montgomery Lane, Suite 1107 Bethesda; Maryland 20014 .

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ENCLOSURE BWR STABILITY MARGINS Discussion Stability analysis of boiling water systems is generally concerned with three topics:

1. Channel hydrodynamic ctability:

2. Reactor core (reactivity) stabilit/* and

3. Total reactor system stability.

Channel hydrodynamic instability is charactet! zed by flow oscillations which may impede heat transfer to the moderatoc and drive the reactor into power oscillations. Reactor core instability is concerned with the reactivity feedback of the entire reactor core which could drive.

the reactor into power oscillations. Channel hydrodynamic and reactor core stability are inherent properties of the reactor design.

Stability margins, which are measures of the degree of stability, vary as a function of operating power level, cycle status, and design parameters. Typical parameters of interest are axial power distribution, fuel pin time constants, flow and void sweep times as well as feedback reactivity coefficients.

Total reactor system stability is affected by the dynamics of the control system in conjunction with the inherent dynamics of the reactor plant. It is recognized that a control system, properly designed, can stabilize a marginally stable or unstable process. This design technique can'be used to obtain high performance from a' system.

However, application of these design techniques to a reactor could

. present a safety problem in the event of a failure of the control system.

The stability of BWR systems has recevied intensive study and operational evaluation *, The majority of these evaluations have been conducted at design operating power, where stability margins are generally the largest over the operating spectrum of the plant.

The use of a second order model response characteristic to evaluate channel hydrodynamic and reactor core stability appears to be adequate for design operating conditions. The use of the same criteria for low power operation and post startup conditions appears to be less conservative.

Two operational states where stability margins are of concern are:

1. Natural circulation flow at a power corresponding to the rod block power limit condition; and

2. End of cycle power distributions at low power operation.

Stability margins for these operational states are considerably less than for design power conditions. This raises a concern about the adequacy of the models and criteria used to evaluate stability at low power. Presenting. stability criteria in terms of decay ratios

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I and damping' coefficients is not conclusive, especially when the dacay

' ratio is large and the damping coefficient is small. This concern is compounded when model. verification is presented in terms of the same criteria.

A more. effective evaluation of stability would consider gain and phase margins. 'This has the benefit of being model independent and presents results in terms of' classical control theory. Also, the. comparison of experimental data with simulated data cansbe made in the frequency domain, allowing for an evaluation of the model. Finally, sensitivity-analyses with a verified model can be used to establish stability-margins as a function of design parameters and operational states.

It appears that the hydrodynamic and the reactor core stability are

, evaluated using the STABLE and FABLE codes. These codes are frequency domain ~ codes. The system stability, which includes the controls, is , evaluated from a time. domain code, described in NED0-10802**.

The compatibility and conservativeness'of the codes relative to each other-is an additional concern. Finally, the validity of the parameters used in the models to establish-stability margins.is also of concern to the staff.

Conclusions and Recommendations Several recent reload submittals have requested authorization for l additional control rod maneuverability during post startup operations.

The reason given is to achieve rod patterns favorable to transient xenon poison conditions. However, in general, this has.resulted in <

relaxation of the rod-block natural circulation intersection, resulting l in reduced second order stability margins.- These conditions, in addition to the end of cycle operational states at low power, warrant i further evaluations of models used and stability criteria applied. i Accordingly, you should provide an updated version of topical report APED-5652 to address'these concerns generically. In addition to the above expressed concerns,.the report should also address:

1. What is the relationship between second order analysis and measured  !

frequency responses? Is phase margin improved or reduced? Address ,

the conservativeness of second order modeling at low power, low j flow conditions. l

2. What is the variation of stability margins (gain and phase) as a ]

function of design parameters? (e.g., fuel time constant, void j transport time, low power operation, etc.),

3. What is the variation of stability margin as a function of control rod pattern?

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4. Provide the listings of the . (STABLE, FABLE) models, and typical data used to conduct analysis that evaluate stability margins, Where it exists, provide substantiation of:the models.and parameters as a function of experimental data.

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• " Stability and Dynamic Performance of the GE Boiling Water Reactor" APED-5652, Apr 69, L. A. Carmichael, G. L. Catena.

• • NEDO-10802 " Analytical Methods of Plant Transie'nt Evaluations for I the General Electric Boiling Water Reactor" R. B. Linford, February 1973.

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