ML20198F010
| ML20198F010 | |
| Person / Time | |
|---|---|
| Site: | Washington Public Power Supply System |
| Issue date: | 12/20/1976 |
| From: | Suhrke K BABCOCK & WILCOX CO. |
| To: | Varga S Office of Nuclear Reactor Regulation |
| References | |
| CON-WNP-1140 NUDOCS 8605280388 | |
| Download: ML20198F010 (9) | |
Text
-
uvW-Babcock &Wilcox ? 3
- aa 2e p
,,,,,,,n,,,,,,,,,u, y e v n.,
P.o. Box 1260. Lynent:urg, Va. 24505 Tele;:nene:(8041384 5111 December 20, 1976 Mr. S. A. Varga, Chief LWR Branch #4 Division of Project Management Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, D.C.
20555
Dear Mr. Varga:
B6W has been advised by the NRC Staff (letter from D. F.
Ross to K. E. Suhrke 'of December 2,1976), that certain portions of our ECCS Evaluation Model concerning the use of a nucleate boiling heat transfer correlation during blowdown after critical heat flu:< (CHF) is first predicted, may not conform to the requirements of Appendix K to 10CFR50.
A revisien to the B6W ECCS Evaluation Model is propos-id horcin which should alloviate the Staff's concerns on the above matter.
This revision is not a significant ECCS Evaluation Model change, since results show peak cladding temperature changes of less than 20 F.
36W requests approval of this change and, following approval, ue will incorporare the chnce into To,,ical
... port LJ.:- 10
- W l,
- 9. : v.
1, "S f,l's ECCS F. c al.ua ncn ;::d el. '
ilc wotd d ha willing to n at with you to discuss thic chenge at yo;.r conre.nica:?.
'f you huso c.2ditionc.1 c.ucst m u, ple ase cc.m u rc !1. t. laile) of v e taf; ery truly vour.
$4)cn ) '.
Kenneth E. Suhrke Manager, Licensing KES:dsf Enclosure cc:
Zoltan R. Rosttoc:y (NRC)
R. B. Borsum (B6W) 8605280388 770118 PDR ADOCK 05000460 A
PDR Tbo e:cccck & Wdcot Comcany / Estselisned 1867
t 1.
Introduction It has been determined by NRC Staff that the post-CEF heat transfer calculations performed in the B&W 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 CEF as is performed in THETA 6F was considered questionable. An investigation was undertaken at B&W to establish an alternate post-CHF heat transfer model for implementation in the THETA code. The scope of the resulting modifications to be proposed includes both revisions of the DNB switching logic and elimination of the post-CHF return to nucleate boiling.
2.
THETA Code Modification According to BAW-10094 (p 26-4):
"If departure f rom nucleate boiling (DNB) has been calculated to have occurred for a particular axial node, both transition flow boiling and nucleate 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 < a $,.80 nucleate boiling (mode 2)
.80
<a<
.90 interpolation between nucleate boiling (= ode 2) and forced convection vaporization (mode 3)
.90 1,a < l.0 forced convection vapori:stion f
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 (= ode 4) heat flux was used. To correct this, the post-DNB switching logic was modified so that, subsequent to CHF at a particular node, rcgression on the transition boiling curve is restricted by the critical heat flux.
In the event that the transition boiling (mode 4) heat flux is calculated to be greater than the critical heat flux, two modes were considered:
1.
forced convection vaporization (mode 3) 2.
film boiling (mode 5,7).
As a result of the case studies described in the next section, it was concluded that, with the corrected switching logic, the return to nucleate boiling could be replaced by a temporary switch to film boiling.
.3;', Casa Studies Fiva v;rsion3 cf th3 THETA codo c ro prapsred to ir.vectignt3 tha ecperat?
cod combined effects of modifications to.the post-CH7 switching logic and clinination of " return to nucleate boiling" calculations. Hot channel thermal 2
analyses were perfor=ed using sach of ghe five THEIA versions for an 6.55 f t split break (Cp = 0.8) en a 177 FA lowered loop plant.
These case studies and code versions are described below and summarized in Table 1.
Case 1 Thts case was the benchmark case, based upcn the present recognized evaluation model THETA 67 version. A return to pre--CHF heat transfer regimes, cceleste boiling (mode 2) or forced convection vaporization (mode 3), was permitted when the heat flux calculated for these modes was less than CHF.
Both the ruptured and unruptured peak temperature nodes showed a rapid return from transition boiling (mode 4) to forced convection vaporization (mode 3) subsequent to initial CRF. Heat transf er at these locations remained in mode 3, switching to film boiling at about 1 second into the transient.
Elevations
- below the peak temperature nodes shoved a custained return to nucleate boiling (mode 2) during the same period.
The peak clad te=perature calculated for the unruptured node was 1992 F and for the ruptured node was 1746 F.
Crse 2
~~ ~ ~-For'this case study,' the switching logic of the base version (case 1) was ratained. However, no access to nucleate boiling (mode 2) was perz1tted sub-saquent to initial CHF. As in case 1, " switching back" from transition boiling w s based en comparing a trial heat flux using modes 2 and 3 to the CHF value; however, the return to mode 2 was replaced by a switch to film boiling. Re turn
' o seda 3 was still permitted for void fractions arcater than 90 percent.
The c
THEIA calculations for case 2 showed essentially the same mode-switching behavior the pesk ccmperature nodes as case 1, i.e., a sustained return to forced at convectiDu vaporitation (mode 3) from transition boiling (mode 4) subsequent to initial CEF.
However, at the lower elevations, the return to nucleate boiling vts replaced by switching to fils boiling. The reduced heat transfer at these lower elerations produced two eff ects:
(1) lower void fraction fluid at the downstream high temperature nodes and (2) increased cladding and fuel temperatures ce the low elevations.
The first effect produced a reduction in forced convection v:porization heat transfer at both high temperature elevations and, consecuently, slightly higher clad temperatures af ter the first 1 second than were observed in crse 1.
The second effect resulted in higher superheat temperatures later in the transient.
For this case the peak clad temperature of the unruptured node was 2018 F, 26 F higher than case 1.
At the ruptured elevation, the peak clad
. s temperature was 1782 F, 36 F higher than that calculated for case 1.
~
~
Cases 3, 4, 5 The THETA versions utilized in cases 3, 4, and 5 contained modified switching logic such that the " return" from transition boiling (mode 4) was possible only if the transition boning heat flux exceeded CEF. The versions differe'd in the heat transfer modes applied subsequent to the
" return" condition:
Case 3: nucleate boiling Imode 2) and forced convection vaporization (mode 3)
Case 4:
forced convection vaporization (mode 3) and film boiling (modes 5, 7)
Case 5:
film boiling (modes 5, 7).
The analyses for all three cases showed that ene modification to the switching logic was the overriding factor in the heat transfer calculations.
The rapid return from tr'ansition boiling at the high temperature elevations which had been observed in case 1 and case 2 was not evident in cases 3, 4 cud 5.
Rather the initial CRF was followed by a longer period of transition boiling.
During this perio,d, the clad temperatures at the peak locations showed an initial increase over those calculated in case 1 for the same time Sowever, with decreasing surface temperature, the transition boiling interval.
h:at flux was observed to be greater than that obtainable by forced convection vrporization (mode 3).
Consequently, the peak locations in cases 3, 4 and 5 showed a regression up the transition curve to the CHF point followed by inter-nittent switching between high transition boiling heat transfer and lower heat fluxes produced by forced convection vaporization or film boiling. At 3 seconds, che clad temperatures at the peak locations for these were nearly equal to those cciculated in case 1.
From 3 seconds on, the heat transfer calculations for the prak temperature locations for cases 3, 4 and 5 were consistent with those of cese'1.
In all three cases, the peak ruptured node clad temperature was 1745 F, l
1 degree lower than case 1, and the unruptured node peak clad temperature was 1988 F, 4 degrees lower than case 1.
On the basis of these case studies, it was decided that the THETA version prepared for case 5 would best meet the acceptance criteria requirements.
I l
.._-...--._e,
. ~. -....
..-._.....-.,-_-..____.,,,_,.-y
-,...v.---
- m__,
PIGURE 1 HEAT TRANSFER MODES Y
4 k3 2
+
8 IL
-F r
d w7 e1
- Temp, D iW 4 T =
L, - Ty,_,4 Mode 1:
Forced Com[ection to Liquid Mode 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
9
TABLE 1 CASE COMPARISON CASE 1
2 3
4 5
VERSION ORIGINAL 6F Ch=1 CY=2 CY=3 CY=5 Post-CHF Old Old New New
.New Switch Logic (Mode 2 or 3) Mode 2 or 3 (Mode 4)
(Mode 4)
(Mode 4)
Returnable Modes Mode 2 Mode 3 Mode 2 Mode 3 Mode 5 or 7 from Mode 4 and 3 5 or 7 and 3 5 or 7 Peak T
.F 1746 1732 1745
.1745 1745 Ruptured Node (Base)
(+360F)
(-10F)
(-l F)
(-10F)
F Peak T 1992 2018 1988 1988 1988 Unruptured Nude (Base)
(+26cF)
(-4 F)
(-4 F)
(-40F) 6
F.
4 LOCA Limit Studies The final step in the study was to determine the impact of the THETA code changes on the LOCA limits reported in reference 4, 5 and 6, (BAW-10102, BAW-10103, and BAW-10105). A studf was conducted for the two foot elevation, which is ruptured noda limited, and for the six foot elevation, which is unruptured node limited, for 205 FA type plants,177 FA lowered loop type plants, and 177 FA raised loop type plants.
Based on the case studies described in Section 3, no change (greater than 20 F) in the peak clad temperatures (PCT) for the LOCA limits reported in references 4, 5 and 6 is expected. This has been confirmed by LOCA limits studies sunmarized in Table 2.
From Table 2 it can be seen that the ef fects of the THETA program changes on PCT and rupture time (and consequently metal-water reaction and blockage) are minimal.
In view of this lack of change at the two and six foot elevations, it was concluded that significant deviations would not occur at the four, eight, and ten foo t elevations.
Therefore, it has been determined that the LOCA limits reported in BAW-10102, BAW-10103, and BAW-10105 remain valid and conservative.
e
~
6.
References 1.
B. M. Dunn, et. al., B&W's ECCS Evaluation Model, BAV-10104, Rav.1, Babcock & Wilcox, December,1975.
2.
R. H. Stoudt and K. C. Heck, THETAl-B Computer Ccde for Nuclear Reactor Core Thermal Analysis - B&W Revisions to IN-1445, (Idaho Nuclear, C. J. Hocever and T. W. Wineiger), BAW-10094, Rev. 1, Babcock & Wilcox, April, 1975.
3.
Letter from D. F. Ross, Jr. to K. E. Suhrke, Dec. 2,1976.
4.
BAW-10102, Rev. 02 5.
BAW-10103, Rev. 02 6.
BAW-10105, Rev. 1 e
t i
1 I
9 t
1 i-ik
!!( )
1 U::,
14o 1.
1 4, g 4{
o I
f{i !
I l e
w i
b hk l h
) hl
$*b l
s.
lN!,9;it S. n id g, di c d
! ~e 2
dt i
L t1 cf* } ' 0; )g) h I
j j ). 23 I Fijyh: 31 2
e gn a
h F1
.1 d i J.h,J E V t
en2 ik d
bn!ti2>l n ni n De b e., S e.> r a ~s I
- h.
h ( l 'a y hM h(
D f'2 h=
l
{'%.4 Y TY l
ti v
y Ja 4
ia n L. 4 u
N y
2..
mk E
La R
$ E l ; $'
D[
},,
l 48 D
4 j
e.
di l'
Es C! !!
Yif 1
$d N $-
L N ! a a, n h %
5 4 hj j
- 1
@n l 's. S t. D 4Ji.
d.i.
d.i.
4 i,
1A 3$
$1 I
I!
, mw u
u p
'A Mh g4 3 i, ga l(
,3 c.] 3-N t
si 1,.
t
.1
==
4 4 't I'e e,,,
- t..
'd.
?,.
- t. n.
N 5
$A l
i I
t' 5
'e 1,
2 e'
,o.
e, *
'l 4
Mi oi u.
a.
oe
'a v{
J,.,:
t
<d.,.1~
- a.,e j%j ftj 4
{
$0 ib' i$b h$d