ML19317F887
| ML19317F887 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 03/23/1977 |
| From: | Mattimoe J SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| To: | Engelken R NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V) |
| Shared Package | |
| ML19317F888 | List: |
| References | |
| NUDOCS 8002210616 | |
| Download: ML19317F887 (3) | |
Text
- l y.
a skuo og' ' SACRAMENTO MUNICIPAL UTILITY DISTRICT C 6201 95813; (916) 452 3211 March 23, 1977 1
'sk
~
%p/
Mr. R. H. Engelken
=
ggL Director of Regulatory Operations e
NRC Operations Office
,g 1990 N. California Boulevard Y
Walnut Creek Plaza, Suite 2027gE C0Q d
Walnut Creek, California 94596
~
g P
Re: Operating License DPR-54 Docket No. 50-312 Reportable occurrence 77-3
[
Dear Mr. Engelken:
In accordance with Technical Specifications for Rancho Seco, Nuclear
[,f Generating Station, Section 6.9.1, and Regulatory Guide 1.16, Revision 4, Section C.2.b.4, the Sacramento Municipal Utility District is hereby reporting
~
the following Reportable Occurrence as RO 77-3.
(S On February 16, 1977, it was reported that the Reactor Coolant System
- f t,
Drain Tank (RCSDT) had partially collapsed and in doing so had' cracked one t
L inlet line attached to the tank. The first indications of abnormal conditions were high reading spikes on the Ventilation Systems Gas Radiation Monitor sampling-the decay heat pump room that adjoins the RCSDT room. The source of the spikes.
was finally isolated to the drain tank room, which is a closed cell requiring removal of a block wall for access. The subsequent inspection of the 34,000 gj gallon tank revealed several 5' to 6' wide areas on the walls that had been o
compressed inward approximately one foot. Ribs across the upper portion of the' tank provide support for the attached piping. As the tank collapsed, the ribs pulled away from the piping causing cracks which were the source of the radiation release.
In the original plant conception.this tank was only to be.used when the RCS was being drained. However, later operation showed the need to handle the 3.5 gpm flow of water from the RCP ' seal leakoff. This flow is directed to the Pressurizer Relief Tank, then drained by gravity to the RCSDT through the RCS drain header. The water can then be recycled through the Coolant Radwaste System.
Because of this operating requirement, a temporary repair had to be made. A non-conforming report was dispoattioned to drain the PRT through a temporary line into the Reactor B silding Drain Accumulator Tank, which then channels the water into the Miscellaneous Radwaste System. The temporary line went from a valve between the safety features valves on the RCS drain header to a valve beyond the safety features valves on line to the RB Drain Accumulator Tank. This fix was originally deemed appropriate as. satisfying General Design Criterion 57 in regards to contain-ment integrity. This criterion requires one containment isolation valve on a line enetrating containment from a closed system and this requirement was satisfied.
(_ 8.. :
t
,y a E
,I
'_ ' ) - '# '
- d l
?
U-Q f y..' = Q
. w. ::
8.0.02.210 Qg
.g 2
m : s m s..
rn
^
q; y
.rN b)
(
Mr. R. H. Engelken March 23, 1977 4
However, it was later determined that the Pressuriser Relief Tank is open to the Reactor Building atmosphere during accident conditions, in which case Criterion 56 applies. When this error was recognized steps were taken to achieve proper containment isolation.
j
- s The Plant Review Committee recommended that the bypass line be routed around the RCSDT, beyond both safety. features valves on the RC Drain Header.
This change would permit recycling the seal leakoff while maintaining contain-ment integrity. While this change was being made, the Committee recommended chat the Safety Features Valve before the existing bypass line be closed if the PRT was not being drained. While the PRT was being drained, an Operator was required to be stationed next to the manual valve to permit closure if the safety features valve failed. *
~
The second bypass line was installed the day following the PRC meeting gf (March 5) and will remain in use until the permanent repair is completed. The 'V damaged sections of the tank will be cut out and replaced with new ' sections of identical material. The sections will receive radiographic and hydrostatic testing prior to returning to service. The tank collapse was caused by improper venting creating a vacuum. Operator error was present in that the vent line
[
\\
co the miscellaneous waste tank was not. opened as called for by the operating j
(,_,)
procedure; however, it is possible that the system design is also faulty and use of ths vent line could create a dangerous situation. The vent line from the oxygen free RCSDT containing hydrogen gas connects to the miscellaneous waste tank which is oxygenated. Opening this line could possibly result in an explosive mixture. The District is reviewing this venting problem and will.
resolve the question prior to returning the repaired tank to service.
j By reviewing radiation monitor records, it is possible to determine that gas was being released from the tank between January 12 and February 16, 1977.
The amount of activity released was calculated by assuming that all of the strippable gas activity of the reactor coolant going to the drain tank was vented through the pipe cracks and released through the Auxiliary Building Vent 114 tion System. This assumption is conservative since some of the gas remains in solution. Only xenon and krypton isotopes were involved in the offsite exposure calculations since the Auxiliary Building Filter System removes any particulate or iodine isotopes.
This calculation showed that as a maximum a total of 8.99 curries were
- ~
_ f released by the tank failure. Personnel working on the tank received a total of 0.296 man rem exposure, with a maximum exposure per man of 20 m rem. The 1
activity released resulted in a maximum of 50 mile population dose of 0.0035 j
man rem. These totals are conservative; however, an estimate was also made i
based on a gas sample taken from the release point. This estimate, which is possibly nonconservative, calculated release totals on order of magnitude less than the estimate based on RCS gas activity.
d('~'N j
-J O
_~. _ - - ~....
t i
i
~
l 1'
+,
I t
6 4
Mr. R. H. Engelken March 23, 1977 4
L
~
\\
t No plant shutdown or transient resulted from this occurrence.
l Respectfully submitted, F!
t I
fl. %&
1 J. J. Mattimos
~
Assistant General Manager and Chief Engineer i.
N
~ h
(
l
/f i
e f'/
2 t
i 1
i 8
t
+
1 i
tf t.
t
?
4 i.
'JJM:RWC: sal Encl.
i t
l cs: Director, MIPC (3) a-Director IE (30) 8
- 4 e
h "
4 e
i 4..
y-4 4
4 4
p 4
2
-,..--,,e
,,-,,wy.
,,. _.,.,,~.,-,,,
__.y..r.,,_,.,,,,
,,.__..,my,,,_,,w.,_,_.m,m.,,,w,,,.-.e-wn,=,