ML20062E692

Forwards Results of Studies Carried Out in Effort to Determine Reason Behind Apparent Discrepancy Between BNL & GE Void Reactivity Feedback Models

ML20062E692
Person / Time
Site:	Peach Bottom
Issue date:	10/17/1978
From:	Cook K GENERAL ELECTRIC CO.
To:	Schroeder F Office of Nuclear Reactor Regulation
Shared Package
ML20062E688	List: ML20062E685 ML20062E690 ML20062E692
References
781010, MFN-389-78, NUDOCS 7812110277
Download: ML20062E692 (7)

Text

.____ ________ __

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L GENER AL h ELECTRIC nuCosta susaar PROJsCTS DIVISION GENERAL ELECTRIC CCMPANY,175 CURTNER AVE., SAN JCSE. CAUFORNIA 95125 MM-384-78 MC 682, (408) 925-5003 October 10, 1978 U. S. Nuclear Regulatory Commission

(')

Division of Systems Safety Office of Nuclear Reactor Regulation Washington, D. C. 20555 Attention:

Mr. Frank Schroeder Assistant Director for Reactor Safety l

Gentlemen:

SUBJECT:

POTENTIAL DIFFERENCES BETWEEN GE AND ENL MODELS Since the GE-BNL-NRC meeting at Brookhaven on September 27-29, several studies have been carried out at GE in an effort to determine the reason behind the apparent discrepancy between the BNL and GE void reactivity feedback mocels.

The first study concerns the effects of density changes in the bypass m

channel water, which are neglected by BNL.

Additional edits have been extracted from the 00YN analysis of the Peach Bottom 2 end of cycle 2 i

license basis transient.

These edits indicate that the bypass density changes reduce the void reactivity feedback by less than 3% over the important region of the transient and hence does not explain the apparent BNL-GE void reactivity difference.

The second study deals with the effects of~ exposure on void reactivity response.

During the September 27-29 meeting it was stated that although the 2 group BNL TdIGL cross sections were exposure dependent, the variation of the cross sections with void fraction and temperature were obtained from beginning of life (BOL)' studies.

The BOL cross section changes with void fraction were then used for all exposures.

, In order to test the validity of this assumption, the few group cross sections for a 7x7 3G3-lG4 type cell, which is dominant in the Peach Bottom Reactor, have been examined as a function of exoosure.

The

), greco cross sections have been obtained frcm the GE ;attice pnysics code and represent averages over the folicwing energy ranges:

'7812N

- ~... _ _.

r

[

GEN ER AL $ ELECTRIC U. S. Nuclear Regulatory Commission

. Attention of Mr. F. Schroeder October 10, 1978 Page 2 Group 1 5.53 kev to 10 MeV Group 2

.625 eV to 5.53 kev Group 3 0 to.625 eV The absorption, slowing down, and re-fission cross sections for each group are available as a function of void fraction and exposure.

Figures 1, 2, and 3 show plots of

(

1 dI 15 as a function of exposure.

The quantity S

a=

is the derivative of a cross section with respect to void fraction evaluated at a void fraction of = = 0.4.

Examination of Figures 1 and 2 show hS A om

(

to be fairly constant with exposure. The group 1 yI and group 2 f

slowing down derivatives have the largest exposure dependence varying by 8 and 5 percent over the exposure range of 20,000 mwd /t.

This constant behavior is not observed in the thermal group, however.

Specifically, the void fraction derivative of YI changes by a factor of 2.5 over the 3

first 20,000 mwd /t. This change in sensitvity to void fraction is caused by the buildup of P2ss with exposure.

The Plutonium fission resonance at 0.3 eV causes the average thermal fission cross section to be less sensitive to thermal spectrum changes and therefore to lattice ;

void fraction.

Changes in cross section void sensitivity with exposure can have a significant effect on lattice void coefficient.

Figure 4 shows lattice void coefficients as a function of exposure, assuming expcsure decendent S

a=

a constant 1E, I c=

c.

I GENER AL $ ELECTRIC U. S. Nuclear Regulatory Commission Attention of Mr. F. Schroeder October 10, 1978 Page 3 and a constant da Depending on the approximation used, it is evident that very large errors in void coefficient could be generated by neglecting the exposure effects.

In both approximations, the void coefficient is overestimated._

by from 10 to 60 percent.

The primary contributor to this error is the (3

change in the thermal fission void dependence.

The constant 1 dI 13E produces smaller errors because changes in the group 2 slowing down derivative tend to cancel the group 3 yI effects.

f We currently do not know the specific method BNL uses to compute exposure and void dependent cross sections.

BNL uses 2 group cross sections rather than 3 group cross sections.

Also, it is possible some exposure corrections are made that we are not aware of. The potential errors in void coefficient are large enough, however, that GE would like to know specifically how exposure effects on S

(x da m.-

are handled by BNL so they can be compared to GE methods.

Due to the potentially large impact of this approximation on core void coefficient, we see no useful purpose in carrying out the ramp pressure studies agreed to at BNL until this issue is resolved.

We would appreciate quick NRC response to this comment in order to reschedule the GE work-load accordingly.

If you have any questions regarding this transmittal, please contact me.

Very truly yours, W'

K. W. Cook Senior Licensing Engineer l

Special Projects Licensing Safety and Licensing Operation KWC:sj/2X l

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