L-83-367, Comments on Plant Recovery Program Re Reactor Vessel Internals & Thermal Shield (Gamma Heating Effect of Removal of Thermal Shield).Gamma Heating Slight Benefit to Vessel Integrity
| ML17214A256 | |
| Person / Time | |
|---|---|
| Site: | Saint Lucie  |
| Issue date: | 06/23/1983 |
| From: | Robert E. Uhrig FLORIDA POWER & LIGHT CO. |
| To: | Clark R Office of Nuclear Reactor Regulation |
| References | |
| L-83-367, NUDOCS 8306280356 | |
| Download: ML17214A256 (4) | |
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REGULA Y
INFORMATION DISTRIBUTI YSTEM (RIDS)
AGGESSIQN NBR:8306280356 DOC ~ DATE: 83/06/23 NOTARIZED:
NO FACIL:50-335 St, Lucie Planti Unit 1i Florida Power L Light Co ~
AUTH ~ NAME AUTHOR AFFILIATION UHRIGiR,ED Florida Power 8 Light Co, RBCIP ~ NAME RECIPIENT AFFILIATION CLARKiR,A, Operating Reactors Branch 3
SUBJECT:
Comments on plant recovery program re reactor vessel internals 8
thermal shield (gamma heating effec,t of removal of thermal shield)
~ Gamma heating slight benefit to vessel integritv.
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Steam Generator Sleeving Review for PWR's NOTES:
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iyhl IZ d0 FLORIDAPOWER 5 LIGHT COMPANY June 23, l983 L-83-367 Office of Nuclear Reactor Regulation Attention:
Mr. Robert A. Clark, Chief Operating Reactors Branch j13 Division of Licensing U. S. Nuclear Regulatory Commission Washington, D.C,. 20555 De'ar Mr. Clark:
Re:
St. Lucie Unit I
Docket No. 50-335.
Reactor Vessel Internals and Thermal Shield; Plant Recovery Program (Gamma Heatin Effect of Removal of Thermal Shield)
Ref: (I)
Florida Power & Light letter of April 27, I 983 (L-83-263)
(2)
Florida Power & Light letter of May 3, I 983 (L-83-280)
Reference (I) docum'ente'd Florida Power and Light's safety assessment evaluation of reactor vessel embrittlement effects as a
result of thermal shield
- removal, concluding that St. Lucie I may safely operate without restriction from Pressurized
. Thermal Shock (PTS) considerations.
Reference (2) documented Florida Power and Light's review of pressure-temperature (P-T) operating limits as a result of thermal shield removal, concluding that the IO year P-T limit is valid for the upcoming Cycle 6, thereby negating the need to amend the St. Lucie I technical specifications prior to Cycle 6.
Questions relating to the effect on PTS of increased Gamma heating of fhe pressure
. vessel wall, due to removal of the thermal shield, were raised at meetings with you on April 25, l983, and June 3,
l983.
Florida Power and Light has conducted an evaluation of this area and concluded that the increased Gamma heating due to removal of the thermal shield actually results in a slight improvement with respect to PTS.
This confirms our previous expectations that a license amendment (i:e.,
technical specification change) will not be required prior to plant startup for the
'next cycle of operation (Reference (2)) as a result of removing the thermal shield.
The slight resultant heating effect due to Gamma radiation is actually beneficial to PTS considerations, and the normal approach of neglecting Gamma heating is thereby a conservative simplification.
The Gamma heating within the vessel wall introduces a slight thermal gradient stress which initially adds to a PTS cooldown transient, however, the te'mperature.increase also improves the material ductility more than enough to offset the slight increase in thermal stresses.
Since the reactor would be tripped during any transient severe enough to be of PTS concern, the Gamma heating question only affects the initial conditions and the very early.
portion of a transient.
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Office of Nuclear Reactor Regulation Mr. Robert A. Clark, Chief The temperature gradient across the vessel wall would be about 5oF with the thermal shield in place, and would increase by about 22oF with the thermal shield removed.
The temperature gradient is not linear, but instead would achieve about 80% of the maximum temperature differential at I/4 of the wall depth from the inner surface.
~ In order to consider the effect of Gamma heating of the reactor vessel wall on the PTS integrity evaluations, two fracture mechanics analyses were performed.
- First, a calculation which evaluated a hypothetical high-copper weld reactor vessel subject to a large steamline break was performed without any Gamma heating.
This calculation was performed with the QCA-I program which is typically used for PTS evaluations.
The results of this calculation indicated that crack initiation might occur after a peak fluence of 4.6 x IOI> n/cm2 has been attained.
(Note that a hypothetical set of high-copper vessel parameters were chosen, which were more severe than those for St. Lucie Unit I, in order to force crack initiation to be calculated for the sake of quantifying Gamma heating effects).
A second analysis considering Gamma heating was performed.
An initial temperature gradient from OoF on the inside of the vessel wall to 22oF on the
- outside, was presumed to be superimposed on the initial vessel temperature conditions.
An analysis, similar to that described above was performed.
Crack initiation was found to require a peak fluence of 4.8 x IOI> n/cm2, all other things being equal.
Comparing the results of the two analyses, if is concluded that Gamma heating is a slight benefit to vessel integrity.
The detrimental effect of the additional thermal gradient stress is outweighed by the beneficial effect of the increased temperature in the vessel wall.
Therefore, ignoring Gamma heating becomes a
slight conservatism in the PTS 'vessel integrity evaluations.
We have previously concluded that PTS was not a concern for the expected life of St. Lucie Unit I, even without a thermal shield (Reference (I)).
We have also previously concluded that the pressure-temperature limit in the technical specifications is applicable for the next cycle of operation (Reference (2)).
In view of the above assessment of Gamma heating, we therefore confirm that our previous conclusions remain valid and that no license amendment (i.e., technical specification change) will be needed prior to plant startup for Cycle 6, as a result of removing the thermal shield.
Should you have any questions on the
- above, please call me at your earliest convenience.
Very truly yours, Robert E. Uhrig Vice President Advanced Systems 8 Technology REU/DAC/cab cc: Harold F. Reis