ML20091A918
| ML20091A918 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 04/09/1976 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Stello V Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20091A917 | List: |
| References | |
| NUDOCS 9105220317 | |
| Download: ML20091A918 (8) | |
Text
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NSF J
NOMTHEMN STATES POWEM COMPANY M O N N E A POLI S. MIN N E G OTA 3940%
April 9, 1976 (July 21, 1976 Revisions Incorporated)
Mr. Victor Stello, Director Divisica of Operating Reactors U.S. Nuclear Regulatory Comission Washington, DC 20555
Dear Mr. Stello:
MONTICELLO NUCLF.AR GENERATING PIANT Docket No. 50-263 License No. DYR-22 Failed Fuel Detection Report This report is sutunitted in compliance with Technical Specification 6.7.C.2.3.
During early licensing discussions there was an interest on the part of AEC per-sonnel to see confirmatory evidence of certain analytical predictions presented by the licensee.
One such area of interest was the sensitivity of the main steamline (MSL) radiation monitor and its ability to isolate the reactor in the eveut of a postulated control rod drop accident (RDA).
Technical Specification 6.7.C.2.3 was therefore issued, requiring a sumary technical report on " Failed Fuel Detection" to be submitted within five years of the initial ecumnercial ser-vice date. This report concludes that, based on past operating data, the isolation signal will be generated as designed in the event of the highly unlikely event of an RDA!
The MSL monitor system consists of four area radiation monitors sensing radiation in the main steamlines.
The four main steamlines go from the reactor vessel, through containment into the steam chase and to the turbine.
In each MSL are two main steamline isolation valves (MSIV) in series, one immediately inside and the 91052203.17 760721 PDR ADOCK 05000263 P
Mr. Victor Stello April 9, 1976 other innediately outside of the primary containment boundary.
We four outboard MSIV's are located in the enclosed steam chase room.
W e MSL radiation monitors are mounted in the steam chase near the ceiling so as to have approximately equal sensitivity to all steamlines.
Were is a significant background radiation level 16 in the steam chase due to the short-lived N mixed with the steam.
Section 14-6.2 of the Monticello FSAR discusses the postulated RDA event and its consequences. An extremely unlikely combination of plant conditions, operator errors and equipment failures are assumed. Under these conditions, the analysis shows that limited local fuel damage results and the offsite radiclogical conse-quence are determined.
It is assumed that the plant has operated for a long period of time prior to being shutdown.
Wirty minutes af ter the shutdown, the plant is assumed to have returned to hot standby and is passing 57. of rated steam flow to the condenser.
We mechanical vacuum pump is assumed to be in service, transferring gases from the condenser to the plant stack through a short holdup line.
In the 16 contribution to the MSL monitor reading will be event of such an RDA, the N negligible. However, if fuel failures exist,it is assumed that noble gas fission products will pass through the steam lines, tripping the MSL monitors, resulting in a reactor isolation.
Technical Specification 3.2.A requires the trip setting to be less than or equal to ten times normal background at rated power, he FSAR analysis of the RDA is a unique analysis for the initial Monticello core.
It later became apparent that a generic, bounding treatment of the RDA was appropriate.
Through parametric studies of the parameters involved, bounds were defined such that if each parameter falls within the bounds, the RDA will be less severe than calcula-ted.
W e dose consequence calculation assumes all parameters are at the bounding value; in reality, those conditions are not expected to occur simultaneously and l
l l
l l
l t
Mr. Victor Stollo April 9, 1976 therefore the RDA results are overly conservative.
In adopting the bounding value concept, the source tem and the resulting dose are double those reported in the FSAR. Wie was acceptable because of the relatively minor impact of the postulated RDA. W e maximum offsite dose based on bounding values is less than 0.27. of the whole body dose limit and less than 107, of the thyroid dose limit of 10 CFR Part 100.
Do,
..td information on the bounding value concept can be found in a March 2, 1973 lette from L. O. Mayer (NSP) to D. L. Ziemann (USAEC) and references stated therein.
We initial and modified source tems are tabulated below:
Bounding FSAR Value Concept Number of Failed Fuel Pins (7x7) 330 660 Noble Cases Released to the Coolant 6.2 x 104 C1 1.24 x 10 Ci 3
4 Noble Gases Carried to the Condenser 5.5 x 10 Ci 1.1 x 10 Ci Operation at Monticello over the past years has provided sufficient data to calibrate the MSL radiation monitors.
During the warranty run at the end of the startup test program in 1971, the average reading of the four MSL radiation monitors was 654 mr/hr.
We air ejector offgas Icvel was negligible during this time, indicating that essentially no fission gas was contributing to the MSL monitor reading.
We 654 mr/hr was at-10 tributed to the N background.
We background was conservatively defined as a nominal 500 mr/hr and Technical Specification 3.2.A interpreted as requiring the setpoint for isolation as 5000 mr/hr.
In early 1975 the MSL radiation monitor reading showed a noticable increase due to failed fuel rods in the reactor.
(Since then the defective fuel has been replaced i
and the MSL monitor has shown a corresponding decrease.) We noole Eas fission products, the significant contributors to the increased readings, are measured at the air ejector.
When at rated power in early 1975, the average of the four MSL radiation l
4 Mr. Victor Stello April 9, 1976 monitors vr.s 808 mr/hr.
We corresponding air ejector offgas monitor reading showed basically a recoil mixture, which, for the sum of the 15 principal long-lived noble gas fission products, corresponds to 20.7 Ci/sec at the reactor core, ne volume of steam in the reactor vessel head and the steam lines involves a 6.7 second transit time f rom the core to the MSL monitors at rated steam flow, at which time there is a significant contribution to the MSL monitor reading f rom short lived nobic gases.
We 21 principal noble gas isotopes af ter 6.7 seconds of decay represent 50.8 Ci/sec, his source term is responsible for the MSL monitor increase from 654 to 808 mr/hr.
W e sensitivity of the monitor is therefore 5640 {/g.
ne nobla gases considered are listed in Table I.
The perfomance of the MSL radiation monitor can be detemined using the empirically established sensitivity.
Two aspects must be considered; first, to verify that the censor trip vill occur as assumed in the analysis for the vorst case release and second, if a smaller release is involved such that an automatic isolation does not occur, that the offsite dose is acceptable assuming sufficient time for manual isolation, he activity in the reactor is expressed as:
( h + L ) Nr,i (1) dN r,i
= -
t dt
(
i + L)t (2)
N,t e N,i
=
o r
and the activity transferred to the condenser is expressed as:
-(ki + L)t (3) d N,i L
Nr,i L = No,i e t
=
dt No,1 L i., - ( h + L) t (4) i Ng,i
= h+L t
4 Mr. Victor Stallo April 9, 1976 where activity in the reactor vessel (curies)
N, e
steam flow rate from the reactor vessel livided by the vapor volume L
=
of the reactor vessel (sec~l)
)(
radioactivity decay constant (sec"l) a activity released to the reactor vessel from the perforsted rods (curies)
No a
activity transferred to the condenser (curies)
Ng a
i a identifier of ith isotope.
The activity flow rate past the MSL monitor is given by equation 3.
This is at its maximum at time zero; the decay in transit to the MSL monitors is negligible. For the bounding value source term, 5
j[No,i 1.24 x 10 Cu rie,
=
i dNt =
924 Ci/sec.
dt Multiplying.by the MSL monitor sensitivity and dividing by the steam flow rate (5% of rated) gives 56,000 mr/hr.
MSL monitor reading
=
This is well in excess of the Technical Specification trip setting of 5000 mr/hr, verifying that a trip will occur as designed. Since for the isotopes of significance af ter 30 minutes j
)( t
<K L, equations 3 and 4 can be written
- Lt
($)
dNt y,
dt
-Lt (6) t No 1-e N
=
4 Mr. Victor Stello April 9, 1976 Revised July 21, 1976 in the event that the number of failed fuel pins during the postulated RDA is suf ficiently small such tie t the MSL nonitor trip setting for isolation is not exceeded, the source term at 5% of rated steam flow would be 4
No 1.24 x 105 5,000 1.11 x 10 ci.
56,000 Using maximum steam flow (57, of rated) in the calculation is conservative because the activity passing the monitors is in its most diluted form. Greater than 5% steam flow while operating the mechanical vacuum pump is not a realistic plant condition.
Using the above source term and assuming a manual isolation in 10 minutes, equation 6 indicates that N
1.1 x 10 Ci.
e No decay has been assumed. At 57. of rated steam flow,7.53 minutes are required to reach the condenser which will allow a measurable decay.
In the solution for Nt at 10 minutes, it should be noted that 997, of the source term, No, passes to the condenser. This means t't will not be af fected significantly if more than 10 minutes is required for operator action.
Prompt operator action is expected, however, because the control room operator will receive alarms at 507, of the MSL monitor isolation setting and from a high neutrom flux scram.
The operator has controls on the bench panel before him with which he can manually close the MSIV's and trip the mechanical vacuum pump.
In conclusion, a review of operation over the past years has confirmed the design and function of the MSL radiation monitor system.
The bounding value analysis of the RDA shows the offsite dose consequence to be conservatively low with respect to 4
.0 CFR Part 100 limits for a maximum transfer of 1.1 x 10 Curies of noble gases to the condenser.
This report shows the ability to isolate under these maximum conditions.
It also shows that for an RDA release source term too small to trip the
Hr. Victor Stello 7
April 9, 1976 MSt. annitor, the noble gases transf erred to the condenser will not exceed that of the the autcastic isolation for a 10 minute manual isolation.
Yours very truly, cQ h-jv L. O. Maye r, PE Manager, Nuclear Support Services LQi/MllV/ deb cci J. G. Keppler C. Chamoti MPCA Attn:
J. W. Fe rman At taciunent l
l t
l 1
4 Table 1 Principal Noble Cases Used in Analysis Isotope Ng - Yield 7.
Tg t h (sec'D kg Ng kgNe-180k**
t g
Kr 0.10 1s 6.93 x 10-1 6.93 x 10-2 94
)
Kr 0.48 1.3s 3.33 x 10-1 2.56 x 10 93
-1 l
~I Xe '1 1.33 1.72s 4.03 x 10'I 5.36 x 10 i
2.78x10}I Kr 1.87 1.84s 3.77 x 10~
7.04 x 10 Kr 3.45 8.6s 8.06 x 10-0 Xe 3.80 13.6s 5.10 x 10 1.94 x 10, 90
-2
-1 Ir 5.00 32.3s 2.15 x 10 1.07 x 10 139
-2
-2 Xe 5.40 40s 1.73 x 10 9.36 x 10 9
-3
-2 5
2.50x10~j Kr 4.59 3.2m 3.61 x 10 1.66 x 10-2 7.85 x 10 Xe 6.00 3.82m 3.02 x 10 1.81 x 10 3 Xe 4
~
138
-4
-3
-3 Xe 5.90 14.2m 8.13 x 10,4 4.80 x 10,4 1.11 x 10,4 87 2.92 x 10 I
Kr 2.53 76ei 1.52 x 10 '
3.84 x 10-4.47 x 10-5 1.03 x 10'5 5.38 x 10 '3 8*
J Kr 0.52 1.86h i
88 2.45 x 10-2.16 x 10'5 Kr 3.56 2.8h 6.88 x 10 5.69 x 10 '5 0
-5 Kr 1.30 4.4h 4.38 x 10 Xe 6.30 9.16h 2.10 x 10 1.32 x 10~
5.26 x 10 135
-5 1.27 x 10 133c'
-3 l
Xe 6.69 2.26d 3.55 x 10-6 2.37 x 10-3
- 2. 98 x 10 Xe 0.16 5.27d 1.52 x 10-0 2.44 x 10-7 2.43 x 10 133
-7 Xe 0.44 12.Od 6.68 x 10-7 2.94 x 10-7 2.94 x 10 131m
-7 85 2.03 x 10-9 5.51 x 10-10 5.51 x 10-10 Kr 0.27 10.87
- k N represents the relative isotopic distribution at time zero.
t i
- k N
-1800h t ie i represents the relative isotopic distribution after 30 minutes of decay.
j i
1
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