ML20054H880
| ML20054H880 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 05/26/1982 |
| From: | YANKEE ATOMIC ELECTRIC CO. |
| To: | |
| Shared Package | |
| ML20054H878 | List: |
| References | |
| RTR-NUREG-0737, RTR-NUREG-737, TASK-2.B.1, TASK-TM PROC-820526, NUDOCS 8206250130 | |
| Download: ML20054H880 (16) | |
Text
.
RCS VENT SYSTEM OPERATIONAL GUIDELINES Revision 1 May 26, 1982 I
l l
l l
l l
l l
l l
8206250130 820615 s
I PDR ADOCK 05000029 P
TABLE OF CONTENTS Page
1.0 INTRODUCTION
1 2.0 SYSTEM DESCRIPTION AND DESIGN BASES.............................
2 3.0 REACTOR COOLANT SYSTEM COMPONENT CONSIDERATION..................
4 3.1 RCS........................................................
4 3.2 RCP's......................................................
5 3.3 LPST.......................................................
5 3.4 containment................................................
6 3.5 Pressurizer................................................
6 3.6 Hydrogen Analyzers.........................................
6 3.7 Charging /ECCS..............................................
7 4.0 RCS VENT SYSTEM GUIDELINE F RAMEW0RK............................... 8 APPENDIX A - RCS CASEOUS VOID DETECTION AND SIZING...............
A-1 l
r I (
N
i I
RCS HEAD VENT OPERATIONAL GUIDELINES
1.0 INTRODUCTION
l This document describes the design basis, system description and the recommended guideline for operation of the Reactor Coolant System Vent System. Guidance as to when the operator should and should not manually vent the reactor coolant system is suggested.
The guideline is based on the following assumptions. The first venting of the reactor vessel head region will begin when the RCS has been returned to a stable condition and end when there is adequate assurance that the head is vented or that containment hydrogen levels warrant termination of the venting operation.
The venting operation removes any non-condensible (NC) gases which collect in the upper head region. For most events, negligible amounts of NC's will be present following an accident, making the use of this system unnecessary. The venting of the head assures removal of NC's, should they be present.
The components of the system are examined which play an active role in the venting process and classified as to their active or passive role on the operation of the vent system. These include the reactor vessel, the pressurizer, the vent system, the LPST, the containment, the reactor coolant pumps and the associated instrumentation, and the H 2
analyzers. Indigenous to the components role in the venting process is the operational f ramework in which the venting is carried out. The guideline presented ascribes to the philosophy presented above.
(
_1_
s
2.0 SYSTEM DESCRIPTION AND DESIGN BASES The Reactor Coolant System Vent System is designed to remotely vent gases from the reactor vessel head or pressurizer steam space to containment during post-accident situations if non-condensible gases collect in these high points. If the non-condensible gases accumulate in these locations, the gas could potentially interfere with core cooling or reactor coolant system pressure control.
The system is not intended for use during normal power operation, and administrative controls will be provided to minimize the possibility of inadvertent operation.
The purpose of the vent system is to remove non-condensible gases from the RCS in a timely manner. A system' flow diagram is shown in Figure 1.
Since the system may be required to operate under a variety of post-accident conditions to remove gases from the RCS, the system is designed to vent non-condensible gas from the RCS in a reasonable period of time without reference to a specific bubble size or reactor coolant temperature and pressure condition.
For the reactor vessel head vent, the chosen design point was taken as H2 gas at 2200 psig and 650 F.
The resulting design hydrogen flowrate under this condition was calculated to be approximately 150,000 SCFH (14.7 psia and 60 F).
Similarly, the H2 "*"E **E' through the pressurizer gas vent under similar conditions is also on the order of 150,000 SCFH.
The reactor vessel head vent path has the capability to vent all the potential hydrogen from 100% of the Zr-H O reaction in approximately 2
one-half hour under the above stated design conditions. Similarly, the time for the pressurizer vent path is about one-half hour.
If the vent system is inadvertently operated during normal operation, the design of the system is such that the resulting liquid or vapor loss through either the reactor head vent path on the pressurizer path would not exceed charging capability. The system is designed to permit
( %
the operator to vent the reactor vessel head ~or pressurizer steam space from the control room under post-accident conditions, and is operable following most events except those requiring evacuation of the control room or a complete loss of all ac power.
The vent path from either the pressurizer or reactor vessel head is single active failure proof with active components powered from two emergency power sources. Two series valves powered off alternate power sources are provided in each vent path. A common cross-connect is provided to assure a vent path exists in the event of a single failure of a power source before venting commences.
The system is designed to limit flow such that the mass flow rate of g
reactor coolant system fluid out of the vent is less than the makeup f
capacity of the charging pumps. This effectively / limits the flow to less than the LOCA definition of 10CFR50, Appendix A.
The vent rate
/
limitation also assures that RCS pressure control is not compromised by venting operations.
I l
l l
I
f g
6
/.
d 4
A M
,/
( s i
3.0 REACTOR COOLANT SYSTEM COMPONENT CONSIDERATION The following discussion focuses on the components that will affect or be affected by the use of the RCS Vent System. Particular attention is paid to the role a component will play during post-accident venting.
31 Reactor Coolant System (Passive)
The source of the non-condensible gases to be vented are all within the RCS. Briefly, these sources are:
1.
Normally Entrained Gases - Nitrogen and Hydregen 2.
Entrained Cases in SI Water 3.
Fuel Rod Fill Gas (Helium) 5.
H fr m cladding Zr-Water Reaction 2
6.
Radiolysis of RCS Water All these RCS sources can simultaneously add up to a very large amount of non-condensible gases. However, for the scenario assumed for this document, the oxidation of the zircaloy cladding is the most important source. For example, the upper head, the outlet plenum and the upper downcomer volume down to the upper lip of the RV nozzles is approximately 360 cu. ft.
At 2200 psig and 650 F, if this volume contained hydroger. it would represent 4
the oxidation of approximately 30% of the zircaloy in the core.
The oxidation of all the zircaloy in the core represents approximately 1230 ft at the above conditions. The entire volume of the reactor vessel is approximately 1400 ft
( s
To keep things in perspective, the current design basis H 2
generation is to be kept below 1% on a core wide basis.
3.2 Reactor Coolant Pumps (Passive)
It is assumed that' reactor coolant pump status during venting operations need not change. Tripping an operating reactor coolant pump could result in gases in the reactor coolant loops collecting in the steam generator U-tubes and may disturb natural circulation and primary-to-secondary heat transfer. Starting reactor coolant pumps would disperse any gases already collected in the vessel head and make their removal more difficult.
Therefore, the existing status of the reactor coolant pumps should be maintained during the venting operation.
3.3 LPST (Passive)
Any hydrogen being vented from the reactor vessel head or pressurizer steam space is discharged directly to the LPST. If a substantial hydrogen release rate is encountered, the safety valve discharge piping rupture disc, set at 240 psig will rupture or the LPST reliefs will blow within several minutes or less, after which the hydrogen is discharged directly to the containment.
If steam is flowing through the vent system, it will take approximately one hour or so to exceed the LPST relief set point. For liquid venting, the time to exceed the LPST relief setpoint would be slightly more than an hour. These estimates have been made on the assumption of the LPST at normal conditions with no cooling or draining operations taking place.
( '
3.4 Containment (Passive)
With regard to venting, the containment plays no active role, merely serving as a repository of the vented gases in the event of a LPST relief or rupture disc failure and as a repository of vented gases through the break. The containment hydrogen level should be maintained below 4% by volume during venting operations. No purging operations should be taking place. All containment air handling equipment should be on to assure mixing of released hydrogen with the containment atmosphere.
l l
The 4% hydrogen level above is based on the lower flammability limit for hydrogen / air mixtures. At no time should containment integrity be compromised due to potentially dangerous hydrogen levels. Containment integrity takes precedence over venting.
Venting must be terminated prior to reaching the 4% containment i
hydrogen level. (Note that it is quite possible for non-condensible gases to enter the containment via the break.
In such a situation, keeping the vent system closed when high containment hydrogen levels are encountered helps minimize the severity of the release.)
3.5 Pressurizer (Passive)
During venting operations, the pressurizer is assumed to be bottled up with minimum spray.
3.6 Hydrogen Analyzers (Active)
There are two containment hydrogen analyzers, each having ranges of 0 - 5% and 0 - 20% with recording capability. The source of the reading in the containment is a sampling point on Broadway and one at the top of the biological shield. Sampling lines from these points meet in a common line to provide a mixed sample to l
the analyzer. There is approximately a five minute transit delay from the sample points to the analyzer plus a one minute response time on the analyzer itself for a six minute total delay on the
( '
reading at normal containment conditions. If the containment pressure is much higher due to the accident, the total reading delay could be as long as twenty minutes.
In order to determine the current containment hydrogen level, the above delay times must be accounted for in order to arrive at the correct hydrogen concentration.
Assuming a conservative maximum allowable hydrogen level of 3%
and hydrogen vent rates based upon design flow conditions at different pressures, a table of vent duration times and H2 level at vent termination is given in Table 1.
The rationale for i
l the 3% number is that it allows ample margin to the 4% hydrogen deflagration level. The times given in Table 1 are based solely on hydrogen vent rates and assumes unlimited hydrogen supply. No attempt has been made to quantify the amount of hydrogen produced by the LOCA event. Thus, the duration of the venting following an accident is assumed to be limited to 10 minutes (at 2200 psig RCS pressure) unless the trend seen on the hydrogen analyzers indicates a smaller hydrogen release rate. If the reactor coolant system pressure is less, longer vent duration times are suggested as shown in Table 1.
3.7 Charging System /ECCS ( Active)
Some form of overpressure capability for the RCS is required during venting operation. Either the charging system via the normal charging path or the emergency core cooling system (HPSIs) l is acceptable. The reason for this is that the venting procedure results in a small, controllable leak from the reactor coolant system. Having either charging or HPSI available helps maintain system pressure, helps vent steam or gas, and causes the upper j
head region to be filled with liquid quickly. By helping to
)
maintain system pressure, higher subcooling margins result.
l
( s l
1 I
a 1
4.0 RCS HEAD VENT PROCEDURE FRAMEWORK i
A flow chart of a suggested RCS head vent procedure is shown on Figure 2.
Inherent to this suggested procedure is that the first
" venting" following an event occurs af ter the system has been stabilized.* Additional ventings, if necessary, must be coordinated with OP-2658, Vapor Container Hydrogen Purge.
l Since capability exists for venting both the upper head and the pressurizer, venting via the reactor vessel head vent path should be i
viewed as the primary means of venting with the pressurizer vent path as a backup. Simultaneously, venting from both sources should not be allowed due to the potentially high H release rates to containment 2
and RCS depressurization. During the venting operation, containment H level, RCS pressure and subcooling should be closely monitored.
2 1
- RCS stabilized - RCS pressure control and means of decay heat removal established.
b
( s
PKESS UR/Z ER
\\/ SAJ7' +- Bloc K val.VES X / S T/N G'LhL t
,(_,
?ANVAL
\\
f
.sc-2 g2 y
2
' A LVES Y3 V'
SC -2 NWS s otestoso
[
0
& Ol'EAATED
'fg" RUrruRE DISL <
L K
p B L O C K VB t VE N
(EXIS T/MG) f C E$lSr/NG)
ESSURIE EM K1 BYPASS L /NE l
guPrattG P!APHRAGM l
E XIS TING NEv!
Q
\\
LFS T l i L a L a I
(
EX /S T/NG ll VM Y'
s lNb' ANUllL VAL VE Sc,
3( _7 Sc.g gg$
.C l
V C.
IfEA C TOR V E A/T + B L O C. It
[ke^cT &\\
VA LVE S F.IG. I R EAC TO R C00LAN T' S>' STEM \\/EN T S >' S TE M
/
ABNORNAL EVE N T A
,s
"* / SIAS
/
U i
1
/a>
~
.- sng
/
14 l
GECOMOAky MAL Evr1Lun TE NEED FCA VEN T I N #1 8N 5%
pa susurs or B
ey ft c S l --
l Halo
\\'STA6Lt/
yth i
o
'/ \\
i earn
' & RE(,. lRC i
k WAL1tcts '
vnt l..
W F/GURE 2 v
v
- )PPEND /x "6 b"O
- l #qg
% NOT
)
g gaseous votD YES,g 3
swr ve ncres cyng n..
v w
e
-w a-p
- - m
VE NT A
> REAGrod
//SA D
.. =
C.ost r tN u e V E N T / N Gi UNT/L AHy of riyt CONDO noms BElow EXtsT:
Co n r Hz } 5*/,
RCS PREssv&G Drops noorse b
- TA blE.I Tine L/tstr AEACMSD s uecoot trof estow /0
.*f I
t PRE Level. Saou ao%,
RV HEAD REFILLE D r\\
JVO Cou'C %
3%
YES wr 1r l
VEN7~
EV ALVATE
/=/Gu RE 2
\\
age 2 of 2
TABLE 1 CONTAINMENT HYDROGEN LEVEL FOR HEAD VENTING TERMINATION RCS Hydrogen Approximate Pressure Level Change Vent Duration (PSIG)
(%/ Min)
(Min) 2200 0.3 10 1500 0.25 12 1000 0.2 15 500 0.14 21 S
4 s
APPENDIX "A" RV READ VENT GUIDELINE RCS GASEOUS VOID DETECTION AND SIZING 1.
Achieve a constant pressurizer level and pressure condition.
2.
Record the following parameters:
PSI RCS Pressure inches PZR Level
=
CPM Charging Rate
=
=
Time 3.
Isolate the RCS letdown flow.
4.
Allow the RCS charging flow to either increase RCS pressure 100 psi or increase pressurizer level 20 inches of span.
5.
Record the RCS pressure, pressurizer level and time:
=
RCS Pressure inches PZR Level
=
=
Time 6.
Reinitiate RCS letdown flow and restore normal pressurizer pressure and level control.
7.
Obtain the initial and final pressurizer vapor space volumes from the attached Figure Al (to be supplied) at the above initial and final pressurizer levels.
Initial Vapor Volume GAL *l/7.5 =
FT3 Final Vapor Volume 3
CAL *l/7.5 =
FT '
=
8.
Determine the total charged volume into the RCS.
Charged Volume - (Charging GPM) X 1
(Time) X ( 7.5 GAL)
FTJ 3
FT
=
A-1
)
s
9.
Determine the expected pressurizer level change.
3 Expected level = (Charging Volume FT )
X 1"*
0.8 FtJ inches
=
- 10. If the actual pressurizer level change is'less than the expected level change then a gaseous void exists in the reactor coolant system.
i l
l l
l l
- Based on one inch of pressurizer level equal to 6 gallons A-2 h
s
,.... _ _ _ __ _ _ _