ML20085M594
| ML20085M594 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 09/30/1975 |
| From: | Hillyard H, Ninschl D, Swart F PUBLIC SERVICE CO. OF COLORADO |
| To: | Howard E NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| Shared Package | |
| ML20085M589 | List: |
| References | |
| RO-50-267-75-18, NUDOCS 8311090037 | |
| Download: ML20085M594 (7) | |
Text
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P. O. Box 361, Platteville, Colorado 80651 I
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September 30, 1975 4
Hr. E. Morris lloward, Director
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Nuclear Regulatory Commission Region IV 09fico Of Inspection and Enforcement
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Suite 1000 Arlington, Texas 76012 J
IE FILE COPY REF:
Facility Operating License No. DPR-34 Docket No. 50-267
Dear Mr. Howard:
Enclosed please find a copy of Unusual Event Report !!o. 50-267/75/18, Preliminary, submitted per the requirements of the Technical Specifi-cations.
Very truly yours, Frederic E. Swart Superintendent, Nuc1 car Production Fort St. Vrain Nuclear Generating Station FES/alk cca Mr. Roger S. Boyd em e
8311090037 75'1215 PDR ADOCK 05000267 S
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REPORT DATP.:
St:p tembe r 30, 1975 UNUSUAL EVENT 75/18 Page 1 of 6 s,
OCCURRENCE DATE:
Sep tember 24, 1975 FORT ST. Vi!AIN NUCLEAR CENERATINC STATION PUBLIC SERVICE COMPANY OF COLORADO P.O. BOX 361 PLATTEVILLC, COLORADO 80651 REPORT NO.
50-267/75/18' Preliminary IDENTIFIhT10:4 0F OCCURRENCE:
While removing the borou carbide balks from the Reserve Shutdown hopper assembly j
of CRD SN 28 from core region 13, it a noted that there van a light deposit of while crystals on the surface of severe:. of the balls.
Since the presents of the crystals did not affect the operahility of the Reserve Shutdown Syste, this was defined as an unusual event.
CONDITIONS PRIOR
.TO OCCURRENCE:
Steady State Power Routine Shutdown llot Shutdown Routino Load Change 0
X Cold Shutdown Other (specify)
Refueling Shutdown Routine Startup Thp major plant parameters at the time of the event vere as follows:
Power RTR 0
MWth
-ELECT 0
HWe Secondary Coolant Pressure N/A Paig Temperature
_N/A
- F Flow' N/A f/hr.
Primary Coolant Pressure O
psig Temperature ^
120
- F Core Inlet 120
- F Core Ont1ct-Flow 1 Circ. 0 4000 'RPr'
- /hr W
^
Am=._
Page 2 o f 6 i
3.
Determine tre effect, if any, they have on rne performance of the Reserve Shutdown System.
)
4.
Determine the effect, if any, they have on the core components and core performance i f they are transported f rom the hopper assembly.
Not all of the analysis and tests have been completed, but there is sufficient evidence to indicate that the crystals are a boric acid residue, that they originated in the Reserve Shutdown hopper and the majority are still retained there, and that they ulli not af fect the satisfactory operation and performance of the Reserve Shutdown System.
'1he following is a summary of the tests and inspections performed to date and some of the detailed results obtained.
Identification o f Crystals Chemical analysis at th ree different laboratories have indicated the crys.talt, to be distilled boric acid.
X-ray dif fraction has shown then to be either hydrogen borate, IIB 0 r boric acid crystal, ll M.
The crystals on the surface of the 2
3 3 hopper vere all 11 B03" U#"
3 both.
Trace anounts of other material, Fe, Si and Mg uere also found.
Cause and Origin
'Ih e maj o ri t.y, greater than 99 percent, of the boron in the reserve shutdown balls is contained as B C, but trace amounts are present as B23 The measured anoont 4
of B 0 in the as l>unt 1 aus uas daemined h batch sampling to be so.15 v/o 2 3 o f t he ball, weight.
Some time in January of this year, water uas inadvertantly admitted into the PCRV (Ahnormal Occurrence Report #50-26 7/ 75/ 7-A).
Subsequent to the unter ingress, water was removed f rom the PCRV through the heliu:a purificat. ion coolers of the helium purification systen from January 23 through February 15.
During' the process, the moisture level in the PCRV was nearly saturated, in excess of 10,000 ppmv.
'lhe reactor pressure was cycled hetueen 50 and 250 psia a number of times essentially creating a pumping action in order to pull or squeeze the water from the PCRV liner insulation.
The reactor was then pnnped down and evacuated to less than 10 mm of Ilg to complete the water removal.
This was completed on March JO and the reactor taken critical on l! arch 17 to establish the integuity of the fuel and core components and insure that the water had heen completely removed.
It has been postulated that while the PCRV waa being dried out, particclarly during the 11r~ t phase when the moluture level was high and the reactor pressure s
cycled, that a significant amount of water saturated helium was introduced int o the Reserve Shutdown hopper through the vent to the reactor coolant.
During this time, the gas t emperature was maintained between 3 50 and 250*F.
At these temperat-ures, the water vapor leached n2 3 0
ut f the Reserve Shutdown halls in the form of boric acid.
3!! 0 i B 0 + 2H 10 W 2
23 3 3
Pcg3 3 of @
N_)
c,)
G DESCRIPTION OF OCCURRENCE:
During the modification of the 37 Control Rod Assemblics to minimize the bypass of primary coolant flow from the orifice valve, CRD SN 028 was being converted for use in an outer core. region.
This conversion requires replacing the control rode and the reserve shutdown halls contained in the RSD hopper.
In the outer 18 core regions, reserve shutdowa balls containing 40 w/o horon are used instead of those contalutng 20 w/o boron.
Af ter the lower weight boron balls had been r,emoved from the hopper, it was noted that several of these had a white crystal deposit.
This white crystal appeared to be firmly attached to the boron graphite balls and varied significantly in the quantity deposited on each of the balls.
In the initial analysis which was later supported by results from a random sample of 500 balls, less than 5 percent had the majority of their surface covered, about 25 percent had rioticeable traces, and the remainder had no apparent traces.
These white crystals were water soluble.
Later inspection of the reserve shutdown hopper and balls of four other CRD assemblics also showed similar deposits.
Not only uas the deposit found on the reserve shut-down balls, but the top surface of the hopper had a similar crystalline coating.
One of the five hoppers inspected showed traces of the deposit on the side vall of the hopper tangential to the CRD wall.
The deposit appears similar in quantity and characteristics for those presently inspected.
CRD SN 028 from core re*gion 13 CRD Stj 037 from core region 2 CRD SN 011 from core region 21 CRD SN 021 from core region 30 CRD SN 027 from core regio ~n 22 APPARENT CAUSL OF OCCURRENCE:
DESIGN Unusual Service Cond.
Including Environ.
MANUFACTURE Component Failure Ins talla tion / Cons t.
Other (specify)
Operator procedure
- See Analysis Section for Detailed description of cause.
ANALYSIS OF OCCURRENCE:
A thorough investigation has been initiated since first observing the crystals to:
1.
Identify the chemical composition, form, and quantity of crystals found in the'RSD hoppers.
2.
Determine the cause and origin of the c ystals.
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Page 4 of 6
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b In the saturated environment, some liquid 11 B0 settled with water in the bottom 3
of the hopper, while the vapor permeated the cr} tire hopper. Some vapor condensed on the top of the hopper or at the cold wall on the side tangential to the CitD wall.
Liquid drops of 11 B0 and II O fell upon the upper layers of the balls in the hopper and depending upotl t2mperatbre crystallized at H B0 rH0' 3 3 2
H B0 + HB02 + U 0 at T > 300*F 3
2 As the reactor coolant was dried out, the crystals remained on the reserva shutdown balls and the hopper wall.
During the last stage of PCRV dryout, evacuation to 10 mm of Hg, some ll B0 was comeM m Ge anWmns Mrk acM, HB0 -
2 3 3 This postulation agrees with the distribution of crystalline deposit as sean in the reserve shutdown hoppers on four inspected. But it has been further observed by a laboratory experiment in which a reserve shutdown hopper and the respective conditions were simulated.
The results were identical to those already described.
In this experiment, distilled 110 was vaporized at 250*F below a bed of reserve 2
shutdown balls.
The water saturated vapor in the closed system condensed on the top surface (150* to 200*F) and dripped back onto the walls.
Af ter about 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> the balls were examined and found to have white crystalling deposits spattered on the surface.
The system was dried out by venting to the atmosphere.
The white spots on the ball surfaces remained' and were identified to be 11 n0
- 3 3 Quantitative Evaluation The simulation experiment demonstrates that horon can be leached by water vapor that w s lresent in the balls after manufacture.
The total amount of from B 0 i
23 B203 present in'the PSC reserve shutdown balls (as Manufactured) averaged to be:
Boron prese~nt as B 023 - 0.15% of the compact weight.
This amounts to 60 grams of leachable boron per hoppgr.
A quantitative analysis was made on the random sample of balls taken from the first reserve shutdown hopper inspected.
The results of that analysis are shown below:
Total Boron 20.8 % of ball wgt.
B as llB0 /H BU n surface 0.1 ut % of ball ugt.
2 3
E as B 02 3 remaining in ball 0.1 wt % of ball ugt.
B as IIB 0 n surface of " worst" ball 0.5 wt % of ball ugt.
2 B as B 0 rem in in " clean" ball 0.1 wt % of ball wat.
23
'the quantitative analysis showed that most of the B 0 uitMn the balls umained 23 within the balls, thus: only a fraction of the B 0 was leac W out to form &
23 crystals observed.
In addition, the total boron concentration and analysis of cican balls taken f rom the same hopper show that no B C was converted to a 4
leachable form. ' At the temperature of interest, no B C reaction was expected to 4
take placc.
If we assume that boron remaining as B 0 insid the ball was 0.1% of ball weight 23 for all balls in the hopper, and that the original concentration of B as B 023 in the' ballu was 0.15%, the boron leached out would bc 0.057, of 40 kg or 20 gm of e
Page 5 of 6 p3 r3 V
D boron per hopper
- llowever, clean balls from the middle of the hopper showed boron remaining as B 023 inside the balls to be 0.15% indicating that boron may not have been leached out of all balls in the hopper.
'lhe re is alno uncertainty in the boron present and H O in the original balls because of the variation between manufactured lotn. 2'lile data definitely show that boron wan ~ leached out of the upper and lower levels of balla in the hopper and that the limit of thlu leached boron was 1/3 of the total leachable boron.
It in entimated t. hat approximately 2-5 grams of boron may be present as crystals on balla and that another 2-5 grams may be deposited as crystalp in the hopper.
'lh e upper limit of boron present as boric acid cryntals in any one hopper in estimated to be 15 grams boron (natural enrichment).
Ef fect on performance of RSD System Since the majority of boron in the balls is present as B C (>99%) and it was 4
con'firwell in' the quadtltative analysis that thin dan not af fected, very little of the boron has been removed f rom the balls.
'Jhe min! mum baron loadint; in any hopper is 5400 gma and it han been estimated that only 15 gms has been leached out.
With thtn heavy baron bonding, the reserve shutdown column is neutronically black to thermal neutrons and consequent.ly even a 1% lona in boron content would not change the reactivity worth.
'lhe re fo re, f rom the reactivi ty control consideration, no change is expected.
Some testa vere done in the laboratory to determine if bonding was possible.
Boric acid crystals were grown over a stack of balls by dripping boric acid solut ion onto them and heating in a furnace. A significant amount of crystals were formed between adjacent balls, but the crystals were so weak that the load was broken with only the sljghtest nuvement of the speciuan dish.
Every attempt to form a strong
- bond between the balls proved to be negative.
As a final t.ent, it is planned to deliberately drop a loaded bonded with reserve shutdown balls containing the crystals to insure that the performance in not adversely' a f fected.
An attempt is being made to select a hopper containing the maximum amount o f crystals.
ljf fect on Care Coinponen_ts and performance "there is a substantial anount of evidence to believe that the crystals have been pretty much contained to the reserve shutdown hopper.
1.
In the quan titative analysis, the baron deposited and prenent on the surface of the balls agrees closely to that expected from the before and af ter quantity of B 02 3 present in the balls.
2.
In working on the CRD assenibly modification, the only significant amount of cryntala noted han been either on the balla inside the RSD hopper, or in the colder portions of the line leading from the hopper.
3.
The reactor was taken critical on Narch 17, 1975 af ter the pCav had been dried out and no nignificant abnormalities were noted.
'ihe expected critical position was within 0.001 Ak of the actural critical position.
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hiowever, more tests are being made to investigate any potential transport mechanism from the RSD hopper.
In addition, further experiments arc in progress to determine the stability of these c.rystals for a viriety of reactor core environments should the balls be dropped in the core for an,cmergency shutdown.
CORRECTIVE ACTION:
In addition to demonstrating that the presence of the boric acid crystals.in the reserve shutdown hopper does not adversely affect either the RSD system performance or the core performance, action has been taken to prevent further formation of the boric acid. crystals.
Further formation is prevented by limiting the water content in the PCRV atmosphere.
(Sec corrective action for Abnormal-Occurrence Report #50-267/ 75/ 7-A).
FAILURE DATA /SIMILAR REPORTED OCCURRENCES:
None PROGRAMMATIC IMPACT:
None CODE IMPACT:
None Submitted By: N c7+v U. 7de.-[-[^- A +
liarvey W. llillyard, 'Jr.
Technical Services Supervisor Reviewed By:"
u,f N M td
_c Di k Nirr/clil g' &
GAC Fuels Div. Reg resentative
(
s Approved By:
' n,fd 4 d, ' _ d, r, n, <
Fr6deric E. Swart Superintendent, Nuclear Production I
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