ML19312C834
| ML19312C834 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 04/19/1977 |
| From: | Seyfrit K NRC OFFICE OF INSPECTION & ENFORCEMENT (IE) |
| To: | Long F NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| Shared Package | |
| ML19312C835 | List: |
| References | |
| NUDOCS 8001100750 | |
| Download: ML19312C834 (4) | |
Text
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Cc-plVA Q/a.,
APR 101977 i
Docket 40. 50-287 i
MDt0RANDU;4 FOR:
F. J. Long, Chief, Reactor Operations and Huclear Support Branch, RII FR0ft:
K. V. Seyfrit, Chief, Reactor Technical Assistance Branch, IF
SUBJECT:
OCONEE DOPPLER COEFFICIENT (AITS F21371Hl) 4 The issue of pemitting operation with a core negative doppler coef-ficient than that used in the steata line break analysis was reviewed as you recomended.
In addition, the differences in the Westinghouse and D&W power coefficients were evaluated.
A review of the steam line break analyses in a variety of SARs, shows that a core negative doppler coefficient does not significantly increase the hazcrd.
In the core serious cases assumed, a more negative doppler coefficient is,in fact, beneficial.
The analyses involves a variety of assumptions and consequences, but the more serious cases all have certain for.tures in coraaon:
x 1.
Following the break the reactor scraus from one of a variety of causes, leaving the most reactive rod stuck out of the core.
The reactivity available to be inserted by the reaaining rods is equil to the power defect plus the required hot shutdown margin (1% to 1.77%
arJK).
t 2.
The affected steam generator boils dry, rapidly lowering the I
primary coolant r.eaperature.
The reactivity increase during this i
phase is governed by the end of life temperature coefficient.
I j
3.
The increased reactivity due to cooldown overccmes the shutdown i
cargin returning the reactor to power.
l is reduced due to the negative power coefficient.As power increases, reactivity
- 4. - The power reaches its peak when the reactivity increase due to'
',the' reduced coolant temperature ' equals the shutdown reactivity plus
' the' reduction in' reactivity ~due to the power increase.
" reactivity reductions due to boron injection and water voids are also(In scce
~1ncluded.).
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F. J. Long 2
The important fact in this analysis is that the doppler effect plays a st.all part in the reactivity increase due to cooldown, but has a i
r.och larger role in the reactivity reduction caused by the return to power. The main contributors to the overall tenperature coefficient, include the nederatcr as well as the doppler temperature coefficients.
1 At end of life the roderator coefficients are an order of magnitude larger than the doppler coefficiants.
This is
.w.in in Table I in i
which coefficients are listed for the plants.eviewed. A 10% change i
in the end of life doppler tcuperatura coefficient at Oconce, results
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in only a 0.4% change in the overall temperature coefficient.
TABLE I - EMD OF LIFE TEiiPEPATURE COEFFICIENTS l
PLANT lloderator Doppler Source in Respective SAR j
AK/K *Fx 104 AK/K/*Fx104 Oconce
-3
.12 SLB Analysis P 14-19 (Assunedinanalysis)
B Sar 205
-3
.2 Table 15.1.14 - 1 l
(AssumedinSLBanalysis)
+
Surry
-3 at 540*F
.1 to
.17 Fig. 3.3-8 and Fig. 3.3-9 to -1.6 at 400*F I
Beaver
-3 at 550*F
.2 Figs. 3.3-31 and 3.3-27 Valley to 1.2 at 350*F f
The doppler effect is much more important in the power coefficient, due t
to the fact that the fuel tenperature increase is Greater than the noderator temperature increase as the power is raised. This is shown by the values in Table II which give the doppler and coderator contributions, as well as the total reactivity decrease in going from 0 to 100% power.
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TABLE II - REDUCTI0ti Ifl REACTIVITY DUE TO POWER IriCREASE (AK/K from 1
0 - 100%) AT BEGIliHI!iG AND EHD OF LIFE l
SAR-Doppler defect 1 lioderator defect Total d'efect" BOL i EOL :
DOL E0L.
800.
EOL:
~
Oconee 1.3 1.4-0.2 1.0 1.5 2.4,
i i. -...
BSAR 205
_l.2 1.4 0.2 1.2
.l.4 2.6 a
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APR 131977 f
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F. d. Low) 5
-1 A 10% increase in the doppler effect at Oconce would cause a 0 I
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Although this would cause change in the total 100% power defect.
an increase in the a ount of reactivity needed to shutdown from J'
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100% power initially, this initial reactivity requirement does not effect the accident analysis, since it is already compensated T
for by the requirement to maintain the specified shutdown margin.
l A more negative doppler effect, h:yaever, would reduce the tcagnitude This concern of any return to power following the steam line break.
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was discussed with cognizant personnel in GRR uho agree with this conclusion.
I The differences between Westinghouse and BW power coefficient As illustrated by-trends with burn up is also shown in Table II.
the last two colu:ms all of these PWR's have larger power defects This is due to the moderator contribution to the I
at end of life.
power coefficient, which always becomes nore negative with bor i
depletion.
of the dopplcr contribution varies, becoming more negative in B&W j
The doppler effect plants and less negative in Westinghouse plants.is caused by an the doppler broadening of the resonance absorption peaks in U 238 and j
Pu 240, as the fuel temperature increases.
The two main causes for change in the doppler coefficicnt with burn up f
I are:
i 1.
A reduction in the average fuel temperature, which causes the coefficient to be less negative.
I The buildup of Pu 240, which ccuses the coefficient to be 2.
uoyr.g negative.
The_ fuel temperature depends or several contributing variables, such j-as, pellet conductivity and ex r.sion; initial fill gas pressure and fission gas release rate; and lad defonastion and the fraction of the The difference in results can be explained pellet contacting the clad.
by the fact that the doppler trend is due to two roughly equal competing I
processes which vary such that the sathe one is not always dominant.
The significant facts are,that the doppler power coefficients remain j
strongly negative in either case and that the doppler changes are not l
large when compared to the overall end of life power coefficients, '
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Based on the above discussion, we conclude that the opcration of the Oconee reactors with a 6. ore negative doppler coefficient than predicted, does not involve an unreviewed safety question.
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Or15Dal diPO-K.V. Seyfrit K. V. Seyfrit, Chief Reactor Technical Assistance Branch 1
Office of Inspection and Enforce.m.cnt cc:
H. J. Richings, DSS P. T. Duraett, RII I
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