ML19322B740
| ML19322B740 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 03/01/1977 |
| From: | Parker W DUKE POWER CO. |
| To: | Rusche B, Schwencer A Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 7912050740 | |
| Download: ML19322B740 (11) | |
Text
-
NRC r onM'195 U.s. NUCLE AR REGULATORY COMMIESION DOC 8ER n-75 8 '
m_2 e 270 /pA7 NRC DISTRIBUTION FOR PART 50 DOCKET MATERIAL FROM:
DATE OF DOCUMENT TO:
Mr. Benard C. Rusche Charlotte, N.C.
282h2 09-03-77 ATE RECerveo William O. Parker, Jr.
09 1h_77 OLETTER O N OTORIZE O PROP INPUT FORM NUMBE R OF COPtES HECEsVED O ORIGIN AL Q U NC LASSIFIE D 8
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DESCAIPTION EN CLOSU R E Ltr. Eef their 01-13-77 ltr...Tran s
Consists of Proposed revision to-F.'.W The Following; 1009h transmitted by B&W Ltr. dated Jan. 2h, 1977 to Mr. S. A. Varga, superseded the earlier revision....
( l page )
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( 9 pages)
DO NOT REMOVE g" NT NAME:
OCONEE UNITS 1-3 AcIGOM-G7M SAFETY FGR ACTION /INFORMATION pipTynn ASSIGNED AD:
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DUKE POWER COMPANY Powen Ut:n.oixo 422 Socrn Cucucn Srnrer. CuantoTTE. N. C. cc 4a anuau o a*anca.an.
March 1, 1977 ReguIatcry DocketfA_-=a
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m Mr. Benard C. Rusche, Director
\\4! l I.Tf f Office of Nuclear Reactor Regulation 89 pM/o U. S. Nuclear Regulatory Commission p
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.my v Attention:
Mr. A. Schwencer, Chief Lf g
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Reference:
Oconee Nuclear Station 5, qNI g Docket Nos. 50-269, -270, -287
Dear Sir:
My letter of January 13, 1977 provided a proposed revision to the Babcock and Wilcox Company; THETA computer code as described in Topical Report BAW 10094, which was submitted by B&W letter dated December 20, 1976 to Mr. S. A. Varga. The attached proposed revision to BAW 10094 was trans-mitted by B&W letter dated January 24, 1977 to Mr. S. A. Varga and super-sedes the earlier revision. LOCA calculations performed for the Oconee class reactors at the two and six foot elevations indicate that the Oconee Nuclear Station Technical Specifications remain valid to assure that the requirements of Appendix K to 10CFR50.46 are met.
Very truly yours,
,n G. [
l n,.
William 0. Parker, Jr.
MST:ge Attachment 2350 OM7So 3t&t
1 1.
Introduction It has been determined by 'llF.C Staff that the post-CllF heat transfer calculations performed in the B/J1 THETA 6F computer code may not be consistent with the requirements set forth in Appendix K of 10 CFR 50.
In particular, the calculation of local heat transfer by nucleate boiling subsequent to the occurrence of CHF as is performed in THETA 6F was considered questionable. An investigation was undertaken at Bfal to establish an alternate post-CHF heat transfer model for implementation in the THETA code.
The scope of resulting modifications to be proposed includes both revisions of the post-CHF switching logic and elimination of the post-CHF return to nucleate boiling.
2.
THETA Code liodification According to BAW-10094 (p. 26-4):
"If departure from nucleate boiling (DNB) has been calculated to have occurred for a particular axial node, both transition flow boiling and nucleate i
boiling will be calculated; the lower heat flux is used." Examination of the THETA 6F switching logic showed that the comparison of transition boiling heat flux to nucleate boiling heat flux was not made.
Rather, referring to Figure 1, a trial value of the heat flux was calculated for a particular axial node according to the fluid void fraction.
for: 0 1 a 1 80 nucleate boiling (mode 2)
. 80 < a <. 90 interpolation between nucleate boiling (mode 2) and forced convection vaporization (mode 3)
.90 1 a < l.0 forced convection vaporization If the trial heat flux was less than CHF, the trial value was taken as the local heat flux.
If the trial value exceeded CHF, transition boiling (mode 4) heat flux was used.
To correct this, the post-CHF switching logic was modified so that, subsequent to CliF at a particular axial node, regression on the transition boiling curve is restricted to heat fluxes (1) less-than CHF for local fluid void fractions less than 80 percent, (2) less than the heat flux calculated by interpolation between nucleate boilina (mode 2) and forced i
convection vaporization (mode 3) for local void fractions aetween 80 percent i
l
and 90 percent or (3) :ess then the heat flux t.alculated by forced convection vaporization (mode 3) for local void fractions greater than 90 percent. That is, transition boiling heat fluxes are restricted to values less than those calculated using pre-CliF correlations approcriate to local fluid conditions.
In the event that the local trcnsition boiling (mode 4) heat flux is 3
calculated to exceed the heat flux calculated using pre-CHF correlations according to local fluid conditions, the local heat flux is determined by switching to film boiling (modes 5 and 7). Thus, with this modification, the return to nucleate boiling is replaced by a temporary switch to film boiling.
3.
Case Studies One additional version of the THETA code was preoared to investigate the effects of the modification presented in Section 2.
Hot channel thermal analysis was perfomred with this modified THETA version for an 8.55 f t2 split break (CD = 0.8) on a 177 FA lowered loop plant. This split break case is the benchmark used in Section 3 of Reference 1.
This benchmark case is not part of any evaluation model. Rather, this case was chosen for its particular sensitivity to the post-CHF switching logic in the THETA code. As such the magnitude of the peak clad temperature differences are not applicable to cases reported in References 2,3, and 4.
These case studies and code versions are described below and summarized in Table 1.
Case 1 Case 1 was the benchmark case. This was the same case as Case 1 reported in Section 3 of Reference 1.
The peak clad temperatures calculated for the unruptured node was 1992 F and 1746 F for the ruptured node.
Case 2*
Case 2* was executed on the modified (as presented in Section 2 herein)
THETA code.
The peak clad temperatures for the ruptured and unruptured nodes 0
0 increased 67 F and 31 F respectively over the base case, Case 1.
The clad temperature results are presented in Table 1.
n.
4.
Spectrum Studies The validity of the Spectrum Analysis results and trends reported in References 3 and 4 was demonstrated by re-analyzing the worst spectrum case for each of these two plant categories. The results of this re-analysis are presented in Table 2.
For both cases, the hot spot peak clad temperature 0
increased less than 20 F.
Since the worst spectrum cases were reanalyzed, the peak clad temperature increases should provide an upper bound for all other spectrum cases. Hence, the spectrum results and trends in References 3 and 4 are still valid.
Furthermore, since the spectrum trends in Reference 2 are basically the same as those in P.eferences 3 and 4 and since the THETA analyses for the worst spectrum cases in References 2,3, and 4 all proceed in a similar manner, the soectrum results and trends presented in Reference 2 should also still be valid.
5.
LOCA Limits Studies The final modification (Case 5) presented in Reference 1 showed no significant impact on the LOCA limits presented in References 2,3, and 4.
This lack of impact is due to the fact than in these base LOCA limit cases (those presented in References 2,3, and 4) minimal time is soent in transition boiling and oscillating between transition and nucleate boiling type heat fluxes. By approximately 0.8s the ruptured and the hot spot nodes are locked into film and/or film pool boiling by virtue of the 300 F temperature difference criteria.
Hence, for the THETA code modification presented herein. (Section 2),
no impact on the LOCA limits should be evidenced.
During the course of this THETA code modification program it was determined that by evaluating the ruptured and unruptured node temperature differe'nces (Tmodified - Tbase case) once the forced convection to superheated steam cooling mode (i.10de 8) was established (sl3s) an accurate determination of the final effect on peak clad temperatures can be made. In fact, the ats at aporoximately 13s translate one to one relative to the peak clad temperatures of the base cases. Use of this method allows an accurate determination of the ruptured end unruptured node peak clad temperatures by executing cases on modified THETA versions to only 15s. This method is restricted by assuring that significant l
l
i 4
chang:s in the volume tverage fuel temperatures, af ter establishing Lode 0 heat transfer, do not exist.
j The two foot elevation (ruptured node limited) and the six foot elevaticn (unruptured node limited) LOCA limits presented in References 3 and 4 were l
reanalyzed using the THETA code modification in Section 2.
These cases were run to 15s, at which time mode 8 heat transfer was established in all nodes.
At 14.5s the clad and fuel temperatures were compared to the base case results in References 3 and 4.
These comparisons at: shown in Table 3.
Since no significant differences between the modified and the base cases exist at 14.5s, it can be concluded that, as anticipated, the THETA code modification presented herein does not impact the existing LOCA limits in References 3 and i
4.
Hence, the LOCA limits in References 3 and 4 are still valid. In the course of the analysis presented in Reference 1, Section 4, it was determined that the base LOCA limit cases reported in Reference 2 racidly locked into aode 5 and/or mode 7 type heat transfer - thereby spending minimal time in transition boiling.
In view of this fact and verification of its implications herein for the LOCA limits reported in References 3 and 4, it must be concluded that the LOCA j
limits reported in Reference 2 are also still valid.
1 4
I 1
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REFEREf!CES 1.
Letter, K.E. Suhrke to S. A. Varga. (flRC), dated 12/20/76,
Subject:
TIIETA Code CHF Modifications.
2.
BAW-10102, Rev. 02 3.
BAW-10103, Rev. 02 4.
BAW-10105, Rev. 01 1
TABLE 1 CASE C0 W.kIS0*l Case 1
2*
Version Original 6F THETA 6 CY=1 Post-CHF Old New Switch Logic (Mode 2 or 3)
(Mode 4)
Returnable Modes from Mode 4 Mode 2 and 3 Mode 5 or 7 O
Peak T F 1746 181 Ruptured Node (Base)
(+67]F)
O Peak T F 1992 202g Uniuptured Node (Base)
(+31 F) 1 4
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2 TABLE 2 SPECTRU! STUDIES j
Topical
. Break Rupture Temp.gredNode UnrupguredNode Rupt Rtport Type Case Time, s F/ Time,s Temp. F/ Time,s 2
BAW-10103, 8.55 ft double Base 13.8 1916/43.5 2079/61.5 Rev. 02 ended, pump (177-FA Low discharge, Loop)
CD=1 Modified 14.0 1936/43.5 2091/61.5 l
BAW-10105, 8.55 ft.2 double Base 16.3 1914/43 2066/67 Rev. 01 ended, ptmp (177-FA discharge, RaisedLoop) CD=1.
Modified 16.2 1932/43 2072/67 i
i i
1 i
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,e T?t,LE 3 LOCA LIMITS STUDIES t = 14.Es f
Tt perature, 'F 1
Toafcal Elevation Geport_
Ft.
Node Base Modified a
e
)
1 1316 1317
+1 2
1531 1531 0
3 1645 1645 0
23'J-10103 2
Clad 4(ruptured) 1530 1530 0
Rev. 02 5(hotspot) 1670 1670 0
i 3
6 1645 1645 0
7 1544 1547
+3 1
1402 1402 0
t 2
1667 1667 0
1 I
3 1824 1824 0
Fuel 4
1950 1950 0
5 1872 1871
-1 6
1847 1847 0
7 1757 1761
+4 1
1658 1658 0
2 1687 1687 0
1 3(ruptured) 1698 1698 0
6 Clad 4(hot spot) 1698 1699
+1 i
5 1691 1692
+1 C
1675 1676
+1 7
1644 1644 0
i j
1 1855 1855 0
1 2
1885 1885 0
4 3
1893 1893 0
Fuel 4
1886 1888
+2 5
1871 1872
+1 6
1843 1844
+1 7
1792 1792 0
1 1291 1291 0
2 1591 1591 0
3 1763 1764
+1 i
8A6!-10105, 2
Clad 4(ruptured) 1848 1847
-1 Rev. 01 5(hotspot) 1832 1832 0
6 1751 1752
+1 7
1686 1689
+3 1
1373 1373 0
2 1688 1688 0
3 1869 1869 0
Fuel 4
1968 1968 0
^
4 5
1930 1930 0
6 1854 1854 0
7 1786 1789
+3 1
1553 1560
+7 i
2 1651 1651 0
3(hot spot) 1667 1669
+2 6
Clad 4(ruptured) 1674 1676
+2 i
5 1666 1668
+2 6
1652 1654
+2 7
1625 1625 0
1 1716 1725
+9 2
1840 1841
+1 3
1855 1857
- +2 Fuel 4
1857 1860
+3 5
1840 1842
+2 6
1813 1816
+3 7
1765 1766
+1-t i
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e Temp. D W 4 7 = % - Ty,.is Mode 1: Forced Convection to Liquid
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Hode 2: Nucleate Boiling Mode 3: Forced Convection Vaporization Mode 4; Flow Transition Boiling Mode 5:
Flow Film Boiling Mode 7: Pool Film Boiling Mode 8:
Forced Convection to Gas Mode 10: Reflood Cooling e
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