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Angular Gamma Dose Rate Distribution at the Surface of Injected Ducted Concrete Shield.

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Annalen drr Physik. 7. Folge, Band 46, Heft 2, 1988, S. 117-123
VEB J. A. Barth, Leipzig
Angular Gamma Dose Rate Distribution at the Surface
of Injected Ducted Concrete Shield
By E. &I. SAYEDAHMEDand A. ABBOUU
Nuclear Research Centre, Cairo, Egypt
A b s t r a c t . The shielding problems that arise due to the irregular penetrations such as neutral
beam injection ducts should be treated carefully t o aid in the shield design. The present work was
imdrrtaken‘ t o describe the effects arising due t o radiation streaming through the neutral be,Lm injector ducts (NBID) on the angular distribution of the total gamma ray doses a t the outer surface
of illmenite concrete shield (e = 4.6 g/cm3). The shield is pierced with NBID of different diameters
and lrngths.
The measurements were performed using a collimated beam of both gamma rJys and neutrons
emitted from one of the horizontal channels of the ET-RR-1reactor. The measurements were carried
out using 7.LiF teflon thermoliminescrnt dosimeters. Generally the obtained data reveal that the
prest’nce of the total dose increase a t the centerline of NBID and which in turn tends to decrease
with the increase of scattered angle. An empirical formula describing the differential dose rate ratio
is predicted. The experimental data obtained reveal good agreement with the calculated ones.
Die rmliale Verteilung der y-Dosisrate auf der Oberflache
einer durchliieherten Betonabschirmung
L n h a l t s u b e r s i c h t . Abschirmprobleme, die ihrcn Ursprung in irregularem UurchlaBvermBgen
habon, sollten sorgfiiltig untersucht werden, um die Konstruktion von Abschirmnngen zu unterstiitzen. I n der vorliegenden Arbeit wird versucht, den Effekt von ausgetretener Strahlung (nich
dem Mechanismus der neutralen Strahlinjektordurchfuhrung(NBID))auf die radiale Verteilung der
totslen ;>-Strahlendosisauf der auBeren Oberflache ,einer Illmenitbetonabschirmung (e = 4,G g/cm’)
nufzuzeigen. Der Schild ist mit NBID’s verschiedener LBngen und Durchmesser versehen. Die experimentcllen Werte stimnien gut mit berechneten uberein. Eine empirische Formel fur die radiale
Verteilang wird angegeben.
Introduction
As previously reported by Wade et al. [I] that most of the shield designed for nuclear
reactors contain airfilled holes. These irregularities represent serious problems for shield
designers.
The most effective method available for reducing the amount of gamma ray and
neutrons that travel through such holes is to design the ducts or the pathways 50 that
these radiation do not penetrate straight forward through the shield similar to the cases
of bend ducts, and neutral beam injector ducts. So, the radiation transmitted through
the holes may constitute the major fraction of the dose penetrating through the shield.
Thus, in order to avoid the costs increase associated with the ducts construction the
shicltl designer must predict the effect of various types of holes for a given set of conditions.
118
Ann. Physik Leipzig 45 (1988) 2
Raso [a,31 had calculated differential backscattered dose rate from a semi-infinite
medium of a concrete shield using the Monte Carlo technique. Experiments for measuring the differential dose albedo for thick slabs of concrete and several other materials
had been previously performed by Clifford [4] using 13'Cs, source and by Barrett, et al.
[5] using 60Co source. I n their work the gamma radiation incident on of the material
slabs was collimated and detected using uncollimated isotropic detectors.
The radiation streaming through the neutral beam injector (NBI) has been studied
by several authors [6, 71.
I n the present work, the gamma ray angular dependence is discussed and a n approximate formula describing the differential dose-rate measurements of the back scattered
gamma rays from neutral beam injector duct is obtained.
Experimental Details
The angular gamma ray dose distribution through the NBID illmenite concrete
shield blocks were measured using a three large concrete samples in the form of rectangu~
em3. Each block contains a cylindrical hole.
lar blocks each of dimension 1 2 0 120x40
The first two blocks include holes with 10 cm diameter while the third one is changed
with diameter ranging from 2.9 t o 10 cm. This construction enables us to design a
neutral beam of diameter 10 cm and length 80 cm connected with injector duct of length
40 cm and diameters 2.9, 5.8, and 10 cm. The NBID illmenite concrete shield blocks
were a r r a n g d in front of one of the horizontal channels of the ET-RR-1 reactor. A
schematic diagram of the experimental layout is shown in Fig. l a , b.
Y
.5cm.
Fig. l a . Block Diagram for Experimental Layout
Fig. l b . Coordinate System
Special care is done to coincide the ducted axis with the beam axis in order to
minimize the effect of scattering from the front face of the concrete blocks. The chemical
and elemental composition of the illmenite concrete were reported previously by Megahid et al. [ 8 ] . A perspex sample holder with special openings were arranged to fix the
detector a t the desired positions. The measurements were performed in two 1directions
with respect to the duct center line a t the end surface of the shield. Since the gamma doses are proportional to the neutron fluxes, these doses are measured using small discs of
7LiF teflon thermoluminescent dosimeters each of 12.7 mm diameter and 0.4 mm thick-
119
F. M. 8. AHMED,A. ABBOUD,Gamma Dose Rate Distribution
ness. They have the advantage of accumulating the incident doses for any irradiation
time which in turn allow the measurements of very low intensity gamma doses.
The 7LiF dosimeters were first calibrated atainst gamma doses using a 137Cs standard gamma source and the obtained calibration coefficients were used to transform
the responses of 7LiP dosimeters t o absolute gamma doses.
The 7LiF dosimeters are annealed before each measurement in a proheated oven a t
temperature 300OC for 3 hr and then transferred to another preheated oven a t 80°C
for 24 hr and then left t o cool gradually. Using TL read out system model Teledyne
Isotopes 7300 the TLD response was measured. Before each measurement the system was
calibrated using 14C standard source.
Each measurement was repeated a t least twice in order to reduce the statistical uncertanties as well as the f1uctuation”of the reactor power. The accuracy of the measurements were found t o lie within statistical accuracy of 11%.
Results and Discussion
I n the present investigation the differential doserate ratio was taken as the ratio
between the scattered dose rate Do, a t any scattering angle 0, to the incident dose
rate Dee a t the front face of the neutral beam injector duct a t its centre position. Fig. 2
represents the variation of both Do, and the duct radius r as a function of scattering
angle 0,(in degrees), along the perpendicular direction of the NBlD axis in the form
of isodose curves.
25
20
15
10
5
0
5
10 15
20
25 8(deg)
Fig. 2. Angular ?-Dose Rate Distribution as a Function of Scattering Angle 0,
The figure revealed also that the total doses intensity have pronouiiced maxima fo
0,equals zero and a t the three injector duct diameters. The isodose curves, have shown
also that the doses decrease with the increase of the scattering angle 0,. This decrease
was noticed to be fast for the two injector ducts of diameters 2.9, 5.8 ern compared with
the corresponding values for the injector duct diameter of 10 em. This was attributed
to the fact that in the last case the neutral beam injector duct behave as a straight duct
of diameter 10 cm.
Ann. Physik Leipzig 45 (1988) 2
1%
From the figure, one can see also that the total doses a t any angle have high values
a t large injector diameters. It is well known also that the total, doses consist mainly of
components of unscattered dose streaming directly as well as the albedo dose scattered
and reflected, at the duct walls and inside the shield. Consequently one could expect
that the value of close albedo would increase with the decrease of the injector diameter
T'Chich in turn affects the total dose 1-alue a t the end of the concrete shield block.
Raso [2, 31 and Haggmark et al. [9] had calculated the differential dose of the backscattered rays; using a semiinfinitd medium of concrete. They used the following formula in their calculations
where
Ad(f2) the differential dose rate ratio, Q, the effective detection solid angle, Do the incident dose rate a t the center of the slab,
therefore
Ad(f2)Q
n the detected dose rate,
D
--
-Do'
Raso [2, 31 as well as Haggmark et al. [9] tried to find a relation between Ad(S1)and the
They found that all the data points for both the same source energy
scattering angle 0,.
01
02
03
04
0 (Radians)
Fig. 3. Variation of Parameter A as a Function of Scattering Anglp.
r = 2.9 cm, X r = 1.15 cm
Injector duct A r = 5 cm.
P. &I.8. AHMEI),A. A 4 ~ ~ o uGamma
u,
Dose Rate Distribution
121
and incident angle, 0,)lie near a single smooth curve. Following that, a n empirical
formula was found in the following form :
A,(Q)
= A exp (-m
0,) + b ,
where c , 112, b were constants to be evaluated for each set of the data.
In the present work, it is found that the forementioned formulae were not able t o
reproduce the present experimental data. This may be attributed to the difference in
the geometrical factors (source, geometry, type of shield) applied in the present work.
So the best fit for the geometry used in the present work was found to be matched by
the following formula :
Ad(Q)Q,
where
G
=
L
Z
2 __
rm,
( 4i L )
f34-B
=
G A exp (-m 0,)
+0,
, L, the channel length = 375 cm, I, the neutral beam length + the
+
injector duct length = 80
40 = 120 cm, r,,, the injector duct radius = 5, 2.9, and
1.45 cm, rAyBthe neutral beam radius = 5 cm, A function of scattering angle and d u c t
radius as seen in Fig. 3, m TC (Haggmark et al. [9]), b function of source energy E , incident angle 0,, and the type of shielding material,
2
C
6
8
10
12
1C
16
18 20
22 2 L
26
Scattering angle O(deg)
Fig. 1. ('omparison 13cta een Experimental and Calcrilnted Differential y-Dose Ratio D,,/D,
-C~ilcubtedValues,
Expcrimcntal Values
0
122
Ann. Physik Leipzig 46 (1988) 8
where Lo, L,rNBand m are constants all over the calculations. rini was found to be
changing from one curve to another. Attempts have been performed to obtain the best
value of the parameter b which fits the present experimental data.
For the case of:
r N , = rini
(straight duct),
b = 6.471 x
0, + 8.333 x lop3,
for the case of:
rim?
<~ N B ,
7.927 x lo3
+ 1.101x
+ (1.08~10-2- 5 . 1 0 3 5 ~ 1 0 Consequently the final empirical formula could be given as
Aa(Sa)Qe= GAexp(--n@,)
+
7 . 9 2 7 ~ 1 0 - ~ +l . l O l ~ l O - ~ 5 ) @ ,
~ N B
By substituting the parameters values in the above mentioned formula, a good agreement is obtained between the experimental and calculated differential y-dose ratio which
could be shown in Fig. 4.
A c k n o w l e d g e m e n t . The authors wish to express their appreciation to Dr. A. S.
Makarious for his cooperation and kind assistance during the experimental work.
References
[l] WADE,E.; CLABIBORNE,
H. C.: Methods for Calculating Effects of Ducts, Access Ways and Holes
in Radiation Shields, QRNL-RSIC-SO, (DASA-1892-1), January 1968.
[2] RASO,D. J.: Monte Carlo Calculations on the Reflection and Transmission of Scattered Gamma
Radiation Technical Operations. Inc. Report t o B61-39 (Revised) 1961.
[3] RASO,D. J.: Monte Carlo Calculations on the Reflection and Transmission of Scattered Gamma
Rays. Nucl. Sci. Eng. 17 (1963) 411.
[4] CLIFFORD,C. E.: Differential Dose Albedo Measurements for 0.66 MeV X’S Incident on Concrete
Tron and Lead ( U ) .Def. Res. Chem. Lab. Ottawa. Canada Report 412 (1963) and Can. J. Phys. 42
(1964) 967.
[5] BARRETT,
M. J.; WADMANN,
J.: Experimental Gamma Ray Back Scattering by Varions Materials. Technical Operations Research, Report No. TO-P 6448 (1964).
[GI IDE,T.; SEKI, Y.; IIDA, H.: Effects of Neutrons Streaming Through Injector Ports on Neutonics
Characteristics of Fusion Reactor. Proc. Second TopL; Mtg. Technology of Controlled Nuclear
Fusion, Conf. 760935-P2, p. 395 (1976).
F. M. S. AHMED,A. ABBOUD,
Gamma Dose Rate Distribution
123
[7] SANTORE,
R. T.; LILLIE,R. A.; ALSMILLBR,R. 0.; BARNES,
J. M.: Two and Three Dimensional
Neutronics Calculations for the TFTR Neutral Beam Injectors. ORXL/TM-6354, Oak Ridge
National Lab. 1978.
[8] MEGAHID,R. Y.;
MAKARIOIS, A. S.; EL-KOLALY,M. A.: Egypt J. Phys. 12 (1981) 1.
[9] HAWMARK,
L. G.; JONES,T. H.; SCOFIELD,N.; EAND
GURNEY,
W. J.: ,,Differential Dose-Rste
Measurments of Backscattered Gamma Rays from Concrete, Aluminium and Steel". Nuclear
Science and Engineering 23 (1965) 138.
Bei der Redaktion eingegangen am 27. Febniar 1986.
Anschr. d. Verf.: Dr. FIKRIA
M. SAYED
AHMED
Dr. AIDAABBOUD
Reactor and Neutron Physics Department
Xuclear Research Cenhre
Atomic Energy Authority
Cairo, Egypt
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