ML20197G368
| ML20197G368 | |
| Person / Time | |
|---|---|
| Site: | Columbia  |
| Issue date: | 04/17/1984 |
| From: | Willis C Office of Nuclear Reactor Regulation |
| To: | Rajender Auluck Office of Nuclear Reactor Regulation |
| References | |
| CON-WNP-0751, CON-WNP-751 NUDOCS 8405010027 | |
| Download: ML20197G368 (9) | |
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APR 1 *' 984 aEckWMT@s0'.39f METB Docket File METB Reading File CAWillis Docket No. 50-3G7 ETSS Staff tiEMORAflDi?4 FOR: Rajender Auluck, Project Manager Licensing Branch M. 2, OL FRON:
Charlss A. Willis, leader Effluent Treatnent Systens Section Meteorology and Effluent Treatment Branch, DSI
SUBJECT:
SETPOINT FOR HTE REACTOR SUILDING HVAC EXHAUST PELNUf1 RADIATION tdONITOR PLAtlT NAt:E: Washington Nuclear Public Power Siipply Systen, Unit No. 2 LICEMSitXi STAfiE: 01, DOCKET NUMBER:
S0-397 RESPO451P,LE BRANCH: L362; R. Auluck, LPM REVIEu STATUS: Contimiing As e result of a recent inspecticn, I was asked to review the set point Calculation for the WNP-2 reactor building W/AC exhaust plenum (References 1 and 2).
I conclude that while the WPPSS setpoi.nt value (S rR/hr) is only slightly hisner than seens approcriate (3 mR/hr), the calculations contain substantive deficiencies and should be redone.
The principal snortceaing in the WPPSS calculation is the model used to calculate tte of fsite t;amru dose froin the passing cloud. A volone source fron a ground level release st.culd be used rather than an elevated line source.
This error could result in a substantial underestinate of the dose rate. Or lesser ir:portarce is the use c.f cones to approxinate the plenun volum; this error so@what overestinates detector response. Da the other har.d. the assumption that the site boundary is only 0.5 miles ariey seeus unnecessarily conservative.
1 copy of ny analysis is enclosed.
It night be of interest to the UFOSS staff.
CMehM.d Wd hir w
h % i. u. ua Charles A. Hillis, leader 5C05010027 840417 y h Effluent Treatr.ent Systens Section M
ADCCK 05000397 Peteoroingy and Effluent TreatJ,ent drar.ch k
h Division of Syst.cns Integratics h
Enclosure:
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's REFERENCES 1.
Owen, F. E., " Radiation Monitors - Control Room Intake and Reactor Building Plenum," WPPSS Interoffice Memorandum to R. G. Graybeal, March 9, 1983.
2.
Owen, F. E., WPSS Calculation Number NE-02-83-06, 4/5/83, t
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N SET-POINT ANALYSIS C. Willis Step 1.
Determine Dose Rate at the Detector Detector is at the center of the largest surface of a 7' x 12' x 16' plenum.
For simplicity, neglect shielding and approximate the plenum with a h, emisphere, o volume = 7 x 12 x 16 = 1344 ft o equal volume hemisphere:
hR 1344 =
R=
8.625 ft S
o dose rate
- from the hemisphere:
d0 =
dv a
4WX dy = 2 WX dX R
0=q a
dX o
O= hepar d
1' Step 2.
Relate to the Semi-Infinite Cloud Dose Rate In an infinite medium, energy released must equal energy absorbed so the dose rate at the surface of a semi-infinite cloud is Os=fqE The ratio of the semi-infinite cloud dose rate to the dose rate from the plenum is 0,/0=1/ par
- O is dose rate, q is activity concentration, E is effective gamma energy, pa is energy absorption coefficient, v is volume, and X is distance from the detector.
. The energy absorption coefficient (pa) varies with gamma energy (E) as fol lows *:
I E(MeV) pa/P(cm /gm) uaR OJO
.08
.0234
.008022 124.7
.1
.0231
.007852 127.3
.5
.0297
.01010 99.0 1.
.0280
.009518 105.1 2.
.0234
.007954 125.7 Assuming a hemisphere overestimates detector response as does neglecting the shielding provided by the plenum. These effects should be small so it seems realistic to assume 9 /9 = 150.
Step 3.
Estimate Atmospheric Dilution The flow rate is 95,000 cfm, equivalent to a dilution factor of X/Q = [95,000 (ft / min) x (min /s) x.02832 (m /ft )]~I
= [44.84]~ =.0223 (sec/m )
Atmospheric dilution will be much greater:
annual average: X/Q = 2 x 10-6 (sec/m )
3 accident (F-1): X/Q = 2 x 10 be @ 3)
-4 Thus, after release, concentrations will be reduced by factors of:
annual average:.0023/2 x 10-6 = 1.115 x 104 2
accident:
= 1.115 x 10
- Air density, P, is taken as 1.293 x 10-3 (g,je,3),
Step 4.
Estimate the Set Point For annual average dilution, the dose rate will be essentially equal to the semi-infinite cloud dose rate and the criterion dose rate is 500 mR/yr =
.057 mR/hr. Therefore, the setpoint should be:
S1 =.057 x 11150/150 = 4.3 mR/hr For accident (F-1) dilution, the dose will be about 25% of the semi-infinite cloud dose
- and the criterion dose rate is 2 mR/hr. Thus, the setpoint should be S2 = 2 x 111.5/( 24 x 150) = 5.9 mR/hr Clearly the 500 mR/yr criterion is limiting.
Step 5.
Decay Correction The site boundary is about 2 km from the release point so it is reasonable to recognize that some decay will occur during transit. The amount of decay will depend on the age of the fission products when they reach the plenum and the wind speed. For present purposes, it is reasonable to assume that 10 minutes elapse between passing the detector and passing the site boundary.
The composition 'of the radioactive material passing the detector is not known but, for most situations where this instrument is used, the release will occur at least 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> after shutdown.
In these situations, the reduction in radioactivity provided by an additional 10 minutes decay will be negligible.
Where this instrument is used during normal operations, it is appropriate to assume the noble gas composition given in NUREG-0133 (Page 40) with a 10-minute delay before reaching the detector. An additional.10 minutes decay during transit will provide a 40% reduction, (Figure 1) which increases the setpoint to:
S1 = 6 mR/hr
...during power opweration.
- C. A. Willis, G. W. Spangler and W. A. Rhoades, "A New Technique for Reactor Siting Dose Calculations," Health Physics,19,,1, (July 1970).
. Step 6.
Elevated Release Correction Releasing radioactivity via a tall stack can substantially reduce the offsite doses (Figure 2). However, to be considered an elevated release, the release point must be much higher (nominally 2.5 times) than the
, buildings.
Releases at intermediate hights are evaluated as " mixed mode" releases and the appropriate X/Q values from the safety evaluation report should be used.
Releases from points near the top of a building are evaluated as ground level releases. At WNP-2 the release is near the top of the reactor building so no correction was made in the SAR or SER.
Step 8.
Instrument Considerations The set point should be adjusted downward to account for instrument accuracy and calibration accuracy.
If they are assumed to be 10% and 2%, respectively, the adjustment factor is i=(1-2}/1 f
+.02
) =.898 A correction should be made for possible drif t.
Taking this as 10% gives an adjustment f actor of f2 = 1
.1 =.9 With these corrections, the setpoint becomes S = 4.3 x.9 x 8.98 = 3.5 mR/hr
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