ML20056B751

Ao:On 700921,localized Fire in Portion of SGTS Train a Charcoal Filter Bank Rendered Unit Inoperable.Caused by Overheating One of Four Strip Heaters.Corrective Actions: Place Temp Switch on Filter

ML20056B751
Person / Time
Site:	Monticello
Issue date:	09/29/1970
From:	Anthony D, Duncanson R NORTHERN STATES POWER CO.
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 9102080495
Download: ML20056B751 (5)

Text

.

I NORTHERN STATES POWER COMPANY Minneapolis, Minnesota 55401 gY< s1 Q s f DOCKETED September 29, 1970 USAEC (3 _

)

_ ocT mo > r ~

J+ i trmm 13 ,;w t>

e ffy s

:,:4 2 g yif,r i s ' "' Q _ C .,

Dr. Peter A. Morris, Director /g [4B h.; .  ;' p j Division of Feactor Licensing '

W,g United States Atomic Energy domission

.9  !

Washington, D.C. 20545 ,

"S g

-C V

\

Dear Dr. Morris:

I MONTICELLO NUCLEAR GENEFATING PLANT 1 Docket No. 50-263 License No. DPR-22 Reporting of Abnormal Occurrences A condition has occurred at the Monticello Nuclear Generating Plant which requires reporting to your office in accordance with provisions of A ppendix A, Technical Specifications, of the Provisional Operating License DPR-22. The {

condition was cir.ssified as an Abnormal Occurrence as defined in Section 1. A.4.

of the Technical Specifications and requires reporting in accordance with Section 6.6.B.3 of the Speci fications. The Region lil Compliance Office has been {

notified in accordance with the requirements of Section 6.6. A of the Technical l Speci fi cati ons.

The occurrence involved a failure in the Standby Gas Treatment System which resulted in the burning of a portion of the charcoal filter in the A circuit of the system. The cause of the problem has been determined and the necessary repairs have been completed. T he details of tFe occurrence are described l in detail in the attached Abnormal Occurrence Report No. 4.

Yours ver truly, hPGA R.O. Duncanson, Jr., P. .

Gen. Supt. of Power Plants-Mechanical Chairman - Monticello Safety Audit Committee ROD / car 9102000495 700929 317va/

CF ADOCK 05000263 CF .

, September 259 1970 MONTICELLO NUCLEAR GENERATING PLANT Abnormal Occurrence Repori No. 4 1

1. Sumarv Descriotion of Occurren~

At approximately 1440 hours0.0167 days <br />0.4 hours <br />0.00238 weeks <br />5.4792e-4 months <br />, September 21, 1970, an abnormal occurrence, consisting of a localized fire in a portion of the Standby Gas Treatment Sy stem train A charcoal filter bank, rendered the unit inoperable. An investigation of the occurrence was begun immediately and was concluded on September 22, 1970.  !

l

2. Detailed Descriotion of the Occurrence

a. Summarv of Condi tions  ;

At the time of the occurrence there were 406 fuel elements in the core and conditions were in accordance with Technical Specification 3.7 C such that Secondary Containment integrity was not required. ]

i

b. Account and Ara!vsis of the Occurrenca l

.At approximately 1440 hours0.0167 days <br />0.4 hours <br />0.00238 weeks <br />5.4792e-4 months <br />, September 21, 1970, Standby 3as {

Treatnent System train A was started for the purpose of taking air '

temperature data at the inlet of the air heater and at the inlet of the charcoal filter bank. When the unii was siaried, sparks were' noticed goirig past an inspection port downstream of the charcoal fil ter bank. The unit was immediately shutdown from the control room j and power to SGTS train A was interrupted from a local power panel. l 1 Immediate investigation revealed that three of the charcoal filter '

4 j

uryits had ignited (there are twelve charcoal filter units per filter )

bank). A CO 2 fi re xtinguisher was obtained and C0 w s inje ted upstream' ar.d downstream of the filter bank. 2

' Approximately one hour later the three filter units were removed from the filter bank and i cooled wi th water.

Investigation revealed that the fire was caused by overheating i

one of the four strip heaters (Chr omalox Type SE-48) of which the i

charcoal blanket heater is comprised. The four strip heaters are wired in a series / parallel arrangenent (see the basic elenentary

, . diagram attached). An electrical short across one of the strip heaters

' resulted in a higher voltage drop across the second strip heater which was wired in series with the shorted heater ce.using it to overheat.

The short was cauced by deierioration of the plastic tape applied to the strip heater 1 cads.

! c. Resu14 s of the Occur renca Section 4.7.B.1.b.(2) of the Technical Sp ecification reg; ires the removal efficiency of the charcoal filters to be og;al to or greater than 99 percent. As a result of the fire three of the charcoal filter units needed replacement and train A was c,onsidered inoperable.

3179

I l

' I

~ '

. .. \

.A.W :. . l l

l l 3

3. Corrective Actions  ; j i On September 21, 1970, all of the SGTS train B strip heater leads were )

inspected and where the possibility of a short existed, the leads were j l taped with asbestos tape. B train heater resistances were measured before and after the leads were inspected and no shorts were found to ,

j exist.  !

1

On September 24, 1970, six of the twelve tra;n A charcoal filter units )

were replaced. The filter bank is comprised of two stacks of six filter l units each. The six new filter units replaced the three units which burned 4

and the three remainin i which had overheatedsee (g uniis attached in thefilter stack and adjacent to the stripThe heater diagram). heater lead

. wires to the overheated strip heater were replaced and the strip heater leads were inspected and taped with asbestos tape where possibility of i a short existed. Four temperature switches, each sensing the surface temperature of one of the four train A strip heaters, were installed and will interrupt a control signal to the gewer control unit if any of the strip heater temperatures exceed 400 f. Similar temperature j switches will be installed on train B when received. l On September 25, 1970, Nuclear Containment System (NCS) Incorporated i personnel performed a Freon test on the train A charcoal filter bank and i 1

found the efficiency to be preater than 99.9 percent. NCS also l perforced DOP tests on the $GTS train A HEPA filters at this firm and found their efficiencies to be greater than 99.9 percent.

. Prepared by:

o ka$$ ,

D. D. Antor'iy Engineer -

)

Occurrence , Investigator l

l 1

i 4

i 1

t. #

I

. i

. ,- l 4;  ;

1 I

9

, . , . . , . . , ~ - - ~ . . , , . ~ . --

1

+- '1'80 VOLT PowR SUPPL y-~ l l

fro rs iceus , SC R - 2 R

, POWER CONTROL IAN/T ,

l l

I i

R 8 l I

2 I I f

~

I . . I J s a f I i

[ _. _ _ _ _ j OVEP HP Alib SHDfGED EL E M E NT ELEWGNT C D

~

I R, B , C nuo D eyaasunt ras route STRIP HERTER SEcil0?J5 of V!if t'C H Ch ARCcAL BLnNKET HE RTE R E- 3W9 - 2 IS Ca th PRISE D.

I 1

l l

8 6 5 , c. E u ia rirne y Din un is a

- - . .v . .

I i 1 l

k p (suonec si.weur) u-;- . ////////////;/ /////s l

7 -

i P r

2 -

0

/

. D 3 '9 0 B 3 (orre engo

/ -

' E L E Mc NT) f /0 3 1

f // 3 -

a _

, /

t.

6 /2 -

I __

t- - Ex, -/xx//x-7-/<w - -- J C

i i

( R , Ei, C cou{ , D re p c e se u r -t he fourz steue Hearse

-se c1t o ns of ' witic a c une con c tit nitx57 Henren

& E-3yA-2 is co rn pres se D.

i W m kces ] theousn lR refnMer1 t he. Clut t Ve C H RfcCML

$1LTG R VIVIT3 Corn PRISINc- SG13 inau% 14 c H A R COA t flL TE R, l? A N if. Fu.ra uuurs ?, 9 Ano 9 burneo, [Anas 7 u ovgk Q wect neewdv. -

FIL TE R AND HERTER r3RI?ANG EMENT