ML20215N141
| ML20215N141 | |
| Person / Time | |
|---|---|
| Issue date: | 10/29/1986 |
| From: | Block S NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V) |
| To: | Erickson P Office of Nuclear Reactor Regulation |
| References | |
| RTR-REGGD-01.086, RTR-REGGD-1.086 NUDOCS 8611040351 | |
| Download: ML20215N141 (10) | |
Text
,
OCT 291986 MEMORANDUM FOR:
Peter B. Erickson, Special and Standardization Project, NRR FROM:
S. Block, Health Physicist, Region V
SUBJECT:
CONVERSION OF REGULATORY GUIDE 1.86 SURFACE CONTAMINATION LIMITS INTO EXPOSURE RATE FOR RELEASE FOR UNRESTRICTED USE My response to your request of '.ugust 18, 1986, to send you the informal calculationsImadeatN$$,88087mersion of Regulatory Guide 1.86 surface contamination limits of into 5 prem/hr at 1 meter, is attached to g
this memo. This exposure rad cEiteria is being used, to some limited degree, for research reactor license termination surveys, in parallel with Regulatory Guide 1.86 recommendation, among other criteria.
Please note that staff peer review of these calculations and conclusions has not been thoroughly made at NRR, and might be warranted. Also, Region V management does not consider this submittal to be under their auspices.
OklgnN.D msb S. Blbck v
Health Physicist, Region V
Enclosure:
As stated.
cc w/ enclosure:
F. Wenslawski, RV G. Yuhas, RV R. Fish, RV E. Garcia, RV bec w/ enclosure:
RSB/ Document Control Desk (RIDS)
G. Cook B. Faulkenberry J. Martin Region V:
SB ' / norma 10/J9/86 e61029 0
9 mg
Conversion of Regulatory Guide 1.86 Surface Contamination Limits into Exposure Rate Introduction Based on Regulatory Guide 1.86, " Termination of Operating Licenses for Nuclear Reactors," surface contamination limits given in Table 1 of that report, the staff has been using, among other criteria, the value of 15,000 m as the maximum beta / gamma surface radioactivity for research reactor license termination. To convert this surface activity into equivalent exposure rate (pR/hr) a mathematical model is used which provides a conversion factor that-is in good agreement with some Oak Ridge experimental data.( }
Experimental measurements vs Theoretical Model The Oak Ridge measurements show that a uniform deposition ~of 1.44 x 104 2
Ci/m of 131 I over a 20 meter diameter plane, provides a dose rate measurement of 400 pR/hr at 1 meter using a Reuters-Stokes pressurized ionization chamber.
It can be shown that a good approximation of the above measurement (400 pR/hr) can be determined theoretically by the following mathematical model proposed by Blizzard and Foderaro( ) for a disc source of 10 meter radius:
2
$ =
In sec 0 photons /cm -sec (1) where A
& = particle fluence rate (ph/cm -sec) at height h a' Q a
2 SA = surface source strength in ph/cm -sec h
0 = is defined in the figure R = radius of disc
(
R
)
h = height above disc as shown in figure (1) Private communication from Carl Distenfeld - Teledyne Isotopes.
(2) Engineering Compendium on Radiation Shielding, Volume 1, page 368.
[
tyy 8
r y
\\ [,
. [i-1,_+
.4.$ '
- .g
.g
{'
i;
~ L '; _
,2 5
~
- 4
~t.~
.3
- c
'y.*-
n For our'.ex'amplet' a
~
p.
~
.h
= 1 meter R
='10 meters (R2.+h)(
2 a =
sec0=(.=a.,
~
From the Oak Ridge data-we get, for.h = 1 meter, 6
Sa=1.44-x107.4kx 10 -pci/Ci 2.2 x 10s d/m 3.17 xi 10 _
4
=
m 105 cm /m pC1 cm z
Converting this-activity into fluence rate:
4kx 1 Ph*
I
= 5 - x 10 ph 2
Sa = 3.17 x 10 x
d cm d
60 sec/ min-ca -sec From the above figure, with R = 10 meters sec 0 = 10.0498-in sec 0 = 2.307
~
2 and & = 5 x 10 x12.307 = 577 ph/cm -sec a
2 131 From the decay scheme of
-I,-it can.be shown that there are 1.4 x 10 for the 364~ Kev gamma ra s of 131 2
6 Ph/cm -see 1.(3) Therefore, the exposur ate is D181I-1.4 x p
ec/R/hr-= 412 mR/hr. This
-exposure' rate agrees ~ reasonably.well>with the Oak-Ridge measured value.of i
'400 pR/hr. Consequently,.theLaodel appears to be acceptable.
f~
Now, assuming a larger area of surface contamination (N30 meter radius-Cs,'with.15 00 m
disc) of 137 the dose rate at l' meter above the l
t t.
- 131I has a complex gamma emission with the 364 Kev ~ gamma representing 82% of-the_ photons /d. We will assume 100% of the gammas are emitted at 364 Kev.
(3) Radiological Health Handbook, 1970, page 132.
e T
?
4 l'
L
'ma
F I
3 surface, taken at the center of the disc, can be computed, using equation (1), as follows:
/
1 (S.0 0
A0 1 meter N
Sa
= 3 n sec 0 photons /cm -sec d
I 2
(
3 0 -->
- _ 15,000 d/m 0.85 ph*
1 100 cm3 d
60 sec/ min Sa = 2.~125 ph surface activity d-sec sec.0 = 30.01666 In sec 0 = 3.4 '
& = 2.125 x'3.4 =,3.6 ph z
2 4cm -sec From Reference'(3)'there are, for 137 Cs 660 Kev photons, 7.8 x.105 ph /R/hr, so that z
cm -see s
3.6 R/hr D137Cs 7.8 x 10b
= 4.6 pR/hr Comparison with other Theoretical Models The attached Figure 1, taken from Reference (4),.gives the calculated and 137 measured exposure rate for Cs at h = 1 meter for various radii. The data shows good agreement with the calculations shown above.
4 10 Isotope '"Cs' f
~
v E
i 3ge---
L_-
r E
-Cateutated' M
o Measured 10' 10' 10' 10' Radius. Rim)
Fig. 1 Calculated and measured reference exposure rate.
(4) Health Physics Journal, December 1983, 1041.
3
- For 1 7Cs, the decay scheme shows 0.85 ph/d.
4 The figure shows that for a 30 meter radius, 137 10 Cs gives about 2 x 10
! 5. Converting to our conventional units:
2 B
2 x 10-10R/hr 3.7 x 10 d/sec 4
4 2 10 cm -
.4 x 102 R/hr Bqm"Z
- pCi/1 d/sec/Bq
- 2 e
m Ci/cm 7.4 x 10-2R/hr 15,000 d/m 7.4 x 1.5 x 106 R/hr pCi/cm. x 2.2 x 106d/m/pCi
- 100 cmz 2.2 d
= 5 pR/hr This agrees well with the calculation of 4.6 pR/hr using equation (1).
Comparison of 137Cs with Other Radionuclide Contamination It should be noted that if the contaminated surface has activity of other radionuclides such as 60Co or 152Eu, the value of Sa will change accordingly.
For 60Co the correlation can be found as follows, using Reference (3) for 60C0 gamma ray fluence rate to dose rate conversion factor.
Sa-(60Co)
_'7.8 x 10 ph/cm -sec/R/hr 2 ph/d 5
a ph/cm -sec/R/hr
- 0.85 ph/d - 4*07 Sa (13#Cs)
_4.5 x 10b d
Thus 60Co yill provide 4.07 greater dose rate, as compared to 137Cs, for the same contamination level.
For 152Eu, the fluence rate per' unit exposure rate, from its complex decay-6 scheme, provides a value of 1.9 x 10 sec /hr (see Appendix A) and 1.5 R
photons / disintegration. Consequently, SA(152Eu) 5 Sa(137Cs)
- 7.8 x 10 1~ 5
- 0.85 0.72 6
1.9 x 10 Table 1 provides respective dose rates in pR/hr at 1 meter from a surface I
152 contamination level of fr 137Cs, 60Co, and Eu of radius from 1 O
to 500 meters using equation (1).
n
.g j
- s.,
I. -
-5,,
'y.
r i.
s.
Table 1 5 000 m
152Eu Exposure Rate o'f Surface Contamination of 137Cs, 60Co, and 9
as a Function of Contamination Field Radius pR/hr R (Meters)-
&-(Photons /cm -sec) 137Cs 60Co
'152Eu 2
1 0.-368 0.47 1.9 0.34.
5 1.730 2.20 8.9 1.6 10 2.450 3.14 12.8 2.2 40 3.920 5.00 20.0 3.6 100.
4.890 6.27 25.5 4.5 300 6.060 7.80 31.7 5.6
-500 6.600 8.45 34.0 6.1 -
Figure 1 is a plot of exposure rate as-a function of radius of a 15,00 m 137 contamination field of Cs activity'at h = 1 meter.. Corrections 0c for'60Co'or 152 137 Eu can be made by multiplying the Cs rate by 4.07 or 0.72, respectively.
Effects of Activation of Concrete Structures-As a final ~ note, if thick concrete structures (walls, floor, etc.) activated for fission neutrons (i.e., from 'research reactors) follow the dose rate from 15 000'd/,m activation levels, as shown in Table 1, at one meter from their 100 cm surface, then it'is unlikely that the subsurfaces would show an increase in activation, and commensurate dose rate, due to build-up of activity fEom lower energies as the fast neutrons are moderated. Figure-2 shows a curve of attenuation of fission neutrons in concrete. Note that the curve shows no build-up of relative: neutron intensity with depth. It is assumed, that non-concrete materials (e.g., Lebar), within the concrete structure, would~
.not activate t'o a degree greater than the surrounding concrete, to provide a
^
dose rate in excess of the 5_pR/hr at I meter limit.
8-i e
~
3 i
6 i
Discussion and Conclusions From the data in Table 1 and Figure 1, it appears that the exposure rate, at 1 15 000 d/m meter from a uniformly contaminated surface of c ntaminated with 100 cmz 137Cs or 152Eu, is 6 5 pR/hr up to a radius of about 40 meters. For a surface
> 40 meter radius,.the. 5 pR/hr would provide a conservative (high) estimate of the contamination level.
It should be noted that for radii smaller than 40 meters, a 5 pR/hr reading could underestimate the surface contamination by a large factor. For example, from Table 1 data, a contaminated surface of 1 5
15 000 mete'r radius, that shows SS pR/hr exposure rate, could have 0.47 150 160 0 f 137Cs or more of 152Eu. However, this surface would not be'in 0
compliance with' Regulatory Guide 1.86 limits. Therefore, the criteria for release for unrestricted use would not be satisfied.
Consequently, the surveyor should use appropriate technical judgement in his evaluation for compliance with Regulatory Guide' l.86 recommendations when using the 5 pR/hr value for release for unrestricted use for'137Cs or 152Eu activity.
A note of cautionlis worthy of mention. By use of the technique discussed in this report, specific local hot spots, if any, that exceed the allowable limits of Regulatory Guide 1.86, may not be located unless several measurements are made within the area of concern. Measurements made using instruments making surface-~ contact could suffer a similar fault. This is because for large contaminated a'eas, random sampling of measurement is r
made that is to.be representative of the entire area. Thus, there exists a probabilitiy of missing a local hot spot, even using this technique since inference could be made, by this sampling technique, that no hot spot exists, or the probability of its existence is small.
( :-
r.
APPENDIX A is2 The decay scheme of Eu is~ complex *'and was used to determine the fluence rate per unit exposure rate asffollows:
k**
(n)-
Photon /cm -sec 2
E Number of Photons / Dis.
R/hr (n) x'(k) 0.122 6
1.9200 x 106 0.245 H0.37-5.20 x 10 -
~0.1800 x 106 O.08 2.25 x 106
.0.344 0.27 1.50 x 10s-
-0.4050 x 106
~
5 6
0.779 0.14 6.70 x 10 L 0.0940 x 10 '
O.965 0.15 5.50 x 10s.
0.0825 x 106~
-1.087 0.12 5.20 x 105 0.0620 x-106 2
1.113 0.14 4.95 x 105 0.0680 x 106 -
1.408 0.22 4.20 x 105
'0.0920 x 106 In = 1.49 Ink = 2.9000 x 10" 6
2 ggInkg2.9x10 - 1.9 106 Ph/cm -sec In 1.49'
.R/hr
^
s s
4
~
3 e
.3
'n
- Table of Isotopes - C. M. Lederer, et al., 1967.
- Reference (3)
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6.
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