ML20198G720
| ML20198G720 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 01/05/1998 |
| From: | Sorensen J NORTHERN STATES POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GL-97-04, GL-97-4, NUDOCS 9801130103 | |
| Download: ML20198G720 (17) | |
Text
_ _ _ _ _ - _ - _ _
~
Northern States Power Company NAB Prairie Island Nuclear oenerating Plant nn sota 5 i
January 5,1998 10 CFR 50.54(f)
Generic Letter 97-04 U S Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket Nos. 50 282 License Nos. DPR-42 50 306 DPR-60 Response to Generic Letter 97 04:
Assurance of Sufficient Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal Pumps On October 7,1997, the Nuclear Regulatory Commission issued the referenced Generic Letter regarding an issue which may have generic implications for Emergency Core Cooling System pumps. By this letter, Northern States Power Co. is providing the required 90 day response for the Prairie Island Nuclear Generating Plant and is making no new NRC commitments. Please contact John Stanton (612-388-1121) if you have questions regarding this issue.
W Joel P Sorensen Plant Manager Prairie Island Nuclear Generating Plant c:
Regional Administrator - Region lil, NRC NRR Project Manager, NRC g
Sontor Resident inspector, NRC Kris Sanda, State of Minnesota i
l i
Affidavit Attachment
'1 ii i i ] i ' '
- 9901130103 9 5
PDR ADOCK 0500o282 P
pog
t 4
e UNITED STATES NUCLEAR REGULATORY COMMISSION NORTHERN STATES POWER CCMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT DOCKET NO.
50 282 50-3C6 GENERIC LETTER 97-04: Assurance of Sufficient Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal Pumps Northern States Power Company, a Minnesota corporation, with this letter is submitting information requested by NRC Generic Letter 97-04.
This letter contains no restricted or other defense information.
NORTHERN STATES POWER COMPANY BY w
[/Joei P Sorensen Plant Manager Prairie Island Nuclear Generating Plant On this ay of d42WM6L before me a notary public in and for said County, pers ' ally apps Jobl P S'orensen, Plant Manager, Prairie Island Nuclear Generati Plant; and g first duly sworn acknowledged that he is authorized to execute thi document o ehalf of Northern States Power Company, that he knows the contents thereof, and that to the best of his knowledge, inforrnation, and belief the statements made in it re t d that it is ot interposed for delay.
/64
,e &
~-
g,,
- MNwwwwwwm imRCIA K. LaCORE NOTARf PUBUQ4mMNESOTA '
HENNEMNCQUNTI
?W N
4 Response to Generic letter 97-04 This summary report is submittea in response to NRC Ceneric Letter 97-04,
" Assurance of Sufficient Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal Pumps." This report includes a description and summary of operation during post-accident mitigation of the various Emergency Core Cooling Systems and responses to the various requests for information in the Generic Letter.
l.
System Description
The Emergency Core Cooling Systems (ECCS) are primarily comprised of the Safety injection (SI) and the Residual Heat Removal (RHR) Pumps. Dur!ng post acddent mitigation, the pumps initially draw suction from the Refueling Water Stridge Tank (RWST). When specific liquid levels are reached in the RWST, the trancier to recirculation of the containment sump liquid is initiated. This socikM orlofly desenoes the Si and the RHR Systems. Containment Spray is not Aldiessed in this evaluation es the Containment Spray System is not used
'fering tecirculation operation.
A.
Safety injection The primary purpose of the SI System is to automatically deliver cooling water to the reactor core in the event of a loss of coolant accident. This protection is afforded for all Reactor Coolant System (RCS) pipe break sizes up to and including the hypothetical instantaneous circumferential rupture of a reactor coc! ant loop with unobstructed discharge from both ends.
The SI System consists of two high head pumps, either of which is capable of satisfying post-accident requirements. Initially, the SI Pumps draw suction from a Boric Acid Storage Tank (BAST) to supply highly concentrated boric acid. When a low level condition is reached in tne BAST, the SI pump suction is automaticaily transferred to the RWST. If, during recirculation operation, RCS pressure is above the RHR Pump discharge pressure, the RHR Pump (s) are aligned to provide' suction to the Sl Pump (s) for high head recirculation.
The SI Pumps discharge into both cold legs. Throttle valves are provided in the lines to balance the flow rates between the two lines to ensure that adequate flow is provided to the intact loop should the other loop be ruptured.
Design flow rate for the S1 Pumps is 700 gpm.
Runout flow rate for the Sl Pumps is 835 gpm.
1/198 NSP 90DYRSPN DOC 1
i 1
B.
Residual Heat Removal The Residual Heat Removal (RHR) Pumps serve dual functions. The normal function of the RHR Pumps is performed during periods of reactor shutdown. During normal power operation the P.HR pumps are aligned to perform the low head safety injection function.
During post accident mitigation, the RHR Pumps are used to inject borated water at low pressure to the Reactor Coolant System through nozzles in the Reactor Vessel. The RHR Pumps are also used to recirculate liquid from the containment sump and send it back to the reactor or the suction of the high head Sl Pumps.
The RHR System consists of two low head pumps, either of which is capable of satisfying post accident requirements.
During the injection phase of post accident mitigation, the RHR Pumps draw suction from the RWST.
Should RCS pressure be above the RHR Pump discharge pressure, the pumps would initially be discharging through the minimum flow bypass line during the injection phase, then to the suction of the Si Pumps (" piggy-back" mode) during the recirculation phase of post-accident mitigation.
Design flow rate of the RHR Pumps is 2000 gpm.
Runout flow rate of the RHR Pumps is 2600 gpm.
C.
Containment Sump B Containment Sump B is located in the basement elevation of containment to provide a water collection source for the suction of the RHR Pumps.
During recirculation, both RHR Pumps draw suction from Sump B. Figures 6.2 3 and 6.2-4 from the Sefety Analysis Report are attached as Figures 1 and 2 for additional information. During mitigation of an RCS pipe break (LOCA), Sump B will quickly fill and a liquid level will be established on the basement floor. The height of the liquid levelis a function'of the size of the RCS break. That is, for the large break LOCA, a higher water level will be established due to injection of the SI Accumulators and volding in the RCS. For a small break LOCA (depending on the RCS break size),
the Accumulators may be isolated prior to injection and the RCS may remain full resulting in less liquid accumulation on the containment basement floor. However, for a small break LOCA, the RHR Pump flow and associated NPSH requirements would be much less.
I!3S8 NSP 90DYRSPN D(x:
2
9 l
~
Sump B is completely covered by a standard floor grating with 3/4 x 3-11/16 inch openings. The grating is designed with two inclined surfaces forming a triangular right pyramid. The ends of the pyramid are enclosed with standard floor grating. The approximate total surface area of the strainer is 60 square feet. The base of the debris strainers are elevated by approximately six inches for.ning a debris curb to protect the RHR Pump suction from debris located on the containment basement floor.
Regarding the sizing of the grating, the USAR, Section 6.2.2.1.2 states:
"The size of the opening in the grating was based on precluding entrance of any large pieces into the sump. Gravity separation of any entering debris is facilitated by the use of the elevated side-
{
wall inlets to the residual heat removal pumps. Minimization of screen clogging debris is accomplished by the use of metallic reflective insulation, specially qualified fiberglass blankets, and protective coatings conforming to ANSI Standard N101.5 (October, 1970) Inside the primary containment."
II.
System Response Figures 3 through 6 are provided to show simplified diagrams for each mode of system operation described below. Recirculation is only used during post LOCA mitigation and not during Main Steam Line Break mitigation. Therefore, this evaluation only reviews post LOCA mitigation.
The SI Pumps are automatically started by the Si signal and initially draw suction from a BAST. After the low level condition in the BAST is reached, the Sl Pump suction is automatically transferred to the RWST. The RHR Pumps are automatically started by the SI signal and draw suction from the RWST. The discharge rate of both the Si and the RHR Pumps is a function of how fast the RCS depressurizes.
The RWST has both a low level and a low-low level alarm. As the RWST is drained during the injection phase, one SI, one RHR and one CS pump are stopped upon reaching the low level alarm setpoint. This action is taken to slow the RWST depletion rate while the shift to recirculation (aligning the RHR pumps to take suction from containment sump B) is made. The first train is transferred to recirculation at this point. At the low-low RWST level alarm point the operator is directed to complete the transfer to the recirculation phase following plant
. emergency operating procedures.
9 1/$98 NSP 90DYRSPN. DOC 3
. ~,. -., -
..rr n..-,r.---..
n-,
,,r.
e,
5 The sequence, from the time of the safety injection signal, for the changeover from the injection to the recirculation is as follows (assuming both trains of ECCS are available),
a.
Sufficient water is del:vered to the containment floor to provide the required net positive suction head (NPSH) of the residual heat removal pumps to change to recirculation. This is accounted for by the setpoint for the RWST low level alarm.
b.
When the low level alarm setpoint on the refueling water storage tank is reached, the operator performs the switchover to the recirculation modo for one train of ECCS.
c.
When the low low level alarm on the refueling water storage tank is reached, the operator performs the switchover operation to the recirculation mode for the second train of ECCS.
Ill.
Pump Available NPSH Analysis A.
Methodology The licensing basis information relative to the available NPSH to the RHR Pumps from the Containment Sump B is contained in the Final Safety Analysis Report (FSAR). This is the most recent information regarding RHR Pump available NPSH frorr the Containment Sump for which a Safety Evaluation Report was issued. The FSAR, Table 6.2-5, indicates that the minimum available NPSH for the RHR Pumps during the recirculation phase is 31 feet. Regarding the determination of this value, the FSAR, Section 6.2.3, states:
' Recirculation operation gives the limiting NPSH requirement. The available NPSH H determined from the containment water level, and the pressure drop in the suction piping from the sump to the pumps.
Based on this limited information, two conclusions are made: (1) The calculated available NPSH from the containment sump was 31 feet and (2) this determination took no credit for containment overpressure; where overpressure is defined as containment pressure above the vapor pressure corresponding to the containment sump fluid temperature.
Searches through available documentation could not recover the analysis substantiating the analysis inputs, assumptions and results, in addition to the absence of the documented analysis, other recent issues (e.g.,
1/198 NSP 90DYRSPN. DOC 4
consideration of small break LOCA, etc.) have prompted the need to generate a new calculation. This new calculation has been prepared and is currently being reviewed.
The methodology for the licensing basis analysis is described in the FSAR as discussed above. The general methodology used to calculate the available NPSH to the RHR Pump suction from the containment sump in the new draft calculation is as follows:
The basic equation for calculating available NPSH is:
NPSH4vut = HA - Hvex + Har - He wher.
NPSHAvut = Available NPSH at Pump Suction H = Absolute pressure en the surface of the liquid supply A
level HypA = Head corresponding to vapor pressure of the liquid Hsr = Static head the liquid supply level is above the pump centerline He = Suction line losses including entrance and friction losses Additional detall on each cf the terms is provided below:
HA = Absolute pressure on the surface of the liquid supply level This term is the containment pressure that is taken uedit for in the analysis.
For the purposes of this calculation, no credit is taken for containment overpressure.
Cnntainment overpressure is containment pressure above the vapor pressure corresponding to the sump liquid temperature. That is to say, that for this calculation, the containment pressure is equal to the vapor prr.4sure of the sump liquid.
HveA = Head corresponding to vapor pressure of the liquid As discussed above, HA is equal to and canceled by HvPA.
Har = Static head the liquid supply level is aboa the pump centerline This term is simply the minimum static height of fluid above the pump centerline. The static height of the liquid inside of containment is determined based on the following:
un NSP 90M'RSPN. Doc 5
I t
Quantity of liquid spilled to the sump is determined based on minimum e
RWST and Accumulator levels, No credit is taken for the Boric Acid Storage Tank or the Caustic Standpipe, Not all of the liquid which is spiled to the containment will collect at e
the basement floor. Some smaller quantitles will colled in other holding areas. These are accounted for, then subtracted from the volume of liquid available to determine the liquid spilHxi to the basement floor, The not floor area at the basement level of containment is determined f
e by calculating the gross floor area (based on containment radius) then i
subtracting the areas occupied by structures and components inside of containment, Based on the liquid spilled to the basement floor (above), and the net e
floor area, the resultant liquid height is determined.
H = Suction line losses including entrance and friction losses t
This term involves suction line losses associated with the RHR Pump.
The following parameters are considered in the calculation of this term:
i 1.
Suction line friction losses The important parameters considered here are:
Pipe Roughness e
Piping Length e
Number and Types of Valves and Fittings e
Fluid Velocity 2.
Heaa loss associated with the open area of the strainer that is free of debris, Consistent with the Safety Analysis-Report, Section 6.2.2.1.2, any debris loading on the stralners is considered to be minimal and will not effect the pressure drop between the containment basement and the pump suction. In addition, due to the screen design (grating with 3/4 x 3-11/16 inch openings per USAR, Section 6.2.2.1.2) and relatively low fluid velocity (0.1 to 0.2 ft/sec) the head loss through the clean strainer will be negligible.
3.
- Entrance losses The head loss associated with the pipe entrance in the containment sump.
?
i 1/598 NSF 90DYRSPNDOC 6
4.
The head loss associated with the expected post LOCA debris loading on the strainer.
As described above from the Safety Analysis Report, the potential for any detsris loading on the strainer will be minimal and is not considered in the evaluation.
l With HA = Hm,.he available NPSH equation is reduced to:
i NPSHwa = Hsr - Ho This is consistent with the Safety Analysis Report, Section 6.2.3; which states that 'the available NPSH is determined from the containment water level, and the pressure drop in the suction piping from the sump to the pumps."
8.
Required vs. Available NPSH For the RHR Pumps, the required NPSH vs. various flow rates is:
14 feet at runout flow (2600 gpm) 8 feet at design flow (2000 gpm)
The minimum available NPSH is from the containment sump during the recirculation mode of operation. From the FSAR, the minimum available NPSH during recirculation is 31 feet. Without the supporting calculation, several of the key inputs cannot be verified; for example, credited RWST volume spilled to containment.
Pisliminary results from the new calculation generally nnfirms this number for both the small and large break LOCA scer,arios si the entire contents of the RWST are considered.
Based on the transfer to recirculation procedure, the first RHR Pump could be aligned to the Sump with the RWST level as high as the low i
level alarm. This would be the minimum available NPSH case, in this case, the minimum available NPSH is approximately 27 to 30 feet, depending on the LOCA scenario. Therefore, the available NPSV is greater than the required NPSH for all conditions.
For the St Pumps, the required NPSH vs. various flow rates is:
21 feet at runout flow (835 gpm) 4 17 feet at design flow (700 gpm)
The minimum available NPSH occurs when drawing suction _ from the RWST (26 feet per FSAR, Table 6.2-5). As noted in the Safety Analysis 1 Report, during high head recirculation, the suction of the St Pumps is boosted by the RHR Pumps. A calculation does not exist to specifically U$38 NSP 90DYRSPN. DOC -
7 r++
'r r -
4yr w--
v
+w---
t pwr
e e
r -
a-
-. _ =.
verify thE ulere is sufficient available suction head for the Sl Pumps during ' piggy back" mode of operation.
However, by engineering ludgment, this is not a concern. As noted above, the Sl Pump required NPSH at runout flow is 21 feet. At 835 gpm discharge flow rate, the RHR Pump discharge head is approximately 335 feet. The piping between the RHR Pump and the Sl Pump is eight inch nominal inside diameter reducing to six inch nominal inside diameter. In the six inch piping, at 835 gpm, the flow velocity is approximately 9.5 feet per second. From Crane Technical Paper No. 410 (Flow of Fluids), this corresponds to a pressure 4
drop of approximately 2 psi for every or.c 5undred feet of six inch pipe.
After accounting for pressure losses through fittings, heat exchanger and height differences, the available margin would be more than sufficient to ensure that adequate NPSH was available to the Si Pump during the
" piggy back" mode of operation.
C.
Current Design Basis NPSH Analysis vs. Licensing Basis Analysis t
As previously discussed above, the licensing basis analysis is summarized in the FSAR.
Due to the inability to confirm the inputs, ast umption and results for the calculation which formed the basis of the FSAR, a new revision to the current calculation is being issued. As described above, preliminary results from this new calculation are consistent with the FSAR results.
D.
Credit for Containment Overpressure Containment overpre ;sure la defined as containment pressure above the vapor pressure corresponding to the sump fluid temperature. Based on this definition, the latest calculation takes no credit for containment overpressure. As discussed above, this is consistent with the current licensing basis analysis as described in the FSAR.
E.
Containment Overpressure Calculation As discussed above, credit for containment pressure is not taken in the analysis of RHR Pump available NPSH.
Therefore, no minimum containment pressure analysis is necessary for this case.
1/198 NSP 900YRSPRDOC 8
)
IV, Conclusions In conclusion, odequate NPSH is provided from Containment Sump B to the suction of the RHH Pumps for post-accident recirculation for all conditions.
j i
i l
t i
b i
e t
i
- t i
b J
+
S It198'NSF 90DYRSPN. Doc 9
3
-e+
e-m. -& 4 -ww.
,e y,+,-
- as-m
--w4-w.4+re---,--m, er m v-
,-cot '=
n u-
- veu
-w-e-c-
.-,=ws,%es<n m-ew-'
4 l'
PRAIRIE ISLAND A'
O WW
-.5S
=W 5*
dhe k
5 MofdE.. O Y
I.T p,.g l,h,__g*r,fI-
/
(#
m W
j?g
- f 9..
s o
.et.
g 5:
z I
O Ys3 F
t b
?
.e E
1
... r y
b a
w v
a I
m
- g a
. ~. *.
E
..ee;i N
m 0
g l
a
=
z m
m i
=g 2
o i
.< v w
E -E =
3
~
d
=
m n
m f
f_ 3 M h '55!
$ $,",=
h 0
u u3 t
/
5 Y
O d
h a
8 g S
=
es
=
4 act
=
=
g
\\
=,l" g,
d C
o
()
"3 M
W Q
5.
g 5
~
s
'e e
o N
C I
~
m1*
dd s
=
(
/
e=5 si i
- o.'
/s 1
y
=
S C
U M
4
e PRAlgjg ;SLANO a
C 5
o A'
e N
a l-
=
and R
.E*
W 5
K 5
g.
a<
8 i
g&
Wo l
f E
5
,s WS *Mi.f*4lho I
/l 2
e-~
$39.#$;g$$$$
oo
\\
/
o 3.'.
~
mo g
- .3 g@
I
/
2 e
gx d'
x w
Q&
I L
a 3
R.* <
=T k
',W N
~wt g
\\
\\
-s
}
e.
\\
i
-.N v
D
\\/
i -
FROM Ur5T 2 -- -
-H 11 $3 REC 3RC PMP 3r st453DE OUTSIDE
~
W-32202 I
12 11 Cot 4TAINMD(T COf41AlHMENT gj1 11 ACCUM ACCUM
[.
g g
m m
W-32203_
C w-I 32 m susT
^
5r M
I M W-s a,
1 '
'L 32083 g
TO Ut6T 2 * >4-->4 W-l W-32073 H 32082 qj W-32072 W-32071 I
y -NZ. J l :
1r 32079 W-32162 n W,
f.
I 4-s H
[-
r 96_
{4
(
g W-32074 V
W-32163 W~
'l
'l 32000 l
I
~W-32068 g
I I'
g W-32207 @-)[)(I W-32206 3
J.OOP 8 ICT I
1 W-32097 LOOP TO G N 4
32070 g
{
g W-32006 IIC g
I W-32067 l
3 g g,g g 3g W-32004 Wr -,
h 3r (
J 7
'd
g 2
H W-32064
]
h I r
-Qy.
I
?'-
=
J i
r 06_
I
~
W-32075 12 mm W W-32085 W-32069 l
12 Rim PMP p,1 T W-32077 l
3 W-32075 p
IL W-32076 Fiqure 3 ECCS INJECTION FROM BAST 4
vt
4 e
~
i.,
m 3
E.
j n
8-i.}-g i 2
g
.g )r z
n-i o
n E
5@44g*48 0d v
0i il a-2 @-2 wi
=
=I
~
i si s
9
@-:::HE g a
gOlBOft b Y
~~
5 3
3 e E
E
/
/
E o
- c g
' H a o I
3
)S
= E i
I
{
$m E
j
=
, m
~
n a
o D
O N
N 6
<N0 0
3 5 9 3
5 n
+
-6. ;:
l5 k &IUM )
iI 8 2 2
i il H
E 8
w y
t._
w 2
is O
la O
u W
s!
=
=
t o
1 i
h N
h D
- l(- h- '
i X-@
i yi s_em<
vCM :
h.
s e
E n
. q om g
li n
se,
~
o 7
so n
3 8d al(_
M
{-
g i
a-3
Otcu UtMT 2 - e
->4 11 O RECIRC PMP.
,r g
n
+:
It610E OUTSIDE e3 W-32202 12 11 CONTANWOli g CONTAlteWENT
'121 11 ACCUM ACCUM
[
[.
BAST BAST
' i.
O
^
I g'( #S
%s g.
3 gr M
MT I
M W-TO Utai 2 - - >4 - - >4 l C.
~
W-32072 W-32071 l W-32073 W-H 32082 q ;-
J l:W-32162 H W-l_
l_
l
[
Ws g
32079 1'
E p
.-(>< g
(
g W-32074 W-32163 Wg W-32068 g
pyp i
LOOP 8 ItOT g
W-32207 h[][2 W-32208 RX I
H p
w_gy 070 0GN
[$
l it0T ac m
s Coto W-32067 l
11 RHR llX 11 W-32004 RHR PMP H
g
( 'E j --
?
,, C A
/
7 13 W-32064 I
ir b
- l Z
~J a
W-2065~
~
MV-32075 12 RHR lix W -32085
~
p W-32069 I2 W-32077
$'yp#d"UN U
,3.. _32078 W
- ---.- w.
.- ~
v W-32076 1
Fiqure 5 ECCS RECIRCULATION PHASE
e 4
A a -
O N
4 FROW LPet 2 OO p
3: $s RECIRC PMP I
2 g,
pim Or, asE NT i
U 11
-s,
-s, L
m e
w.322, g,'
I C
- Efe, m,
I 32E85 l
i 10 usel 2 + H-'M h
us-320?)
w -32ct2 w -32o7 H 2002
%J g
do-id s)
" " n
. w-32ss2 m w-
=
'YS
,ht
-s2ias g;,o w
I jd I
w-32207 h tooe s E W-3M D
8" toop w-32oer g
2070
{
10 C5 PUMPS M0f w -32096 LEC W
I coto Ltc u I
a w.32067 I
w-320e4
~
lf'.J
~U y
~ ~ ' ' '
!>=
T,,'; ' I %T
- g,,,[,,.
c w.32.,3 c
s w-320es t
uI
,2 -
I
. 3 CoNTAdesENT Qg :w-32078 _
I
-e Ib w-32076 t
l
' Fiaure 6 ECCS HIGH HEAD RECIRCULATION i
P'
,