oo A. R. E. A. E. A. / Rep. 318

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ARAB REPUBLIC OF EGYPT ATOMIC AUTHORITY REACTOR AND PHYSICS DEPART

SLOW AND SECONDARY RAY DISTRIBUTIONS IN SHIELDS FOLLOWED BY REFLECTING LAYERS

BY A.S. MAKARI.OUS, Y;I. SWILEM, Z. AWWAD

AND T. BAYOMY

1993 INFORMATION AND DOCUMENTATION CENTER ATOMIC ENERGY POST OFFICE CAIRO, A.R;I:.

VOL 2 7 id Q 7 AREAEA/Rep.318

ARAB REPUBLIC OF EGYPT ATOMIC ENERGY AUTHORITY REACTOR AND NEUTRON PHYSICS DEPART,

SLOW NEUTRONS AND SECONDARY DISTRIBUTIONS IN CONCRETE SHIELDS FOLLOWED BY REFLECTING LAYERS

BY A,S.M\KARIOUS, Y.I.SWILEM, 2.AWWAD AND T.BAYOMY

INFORMATION AND DUCUMENTATICN CENTER ATOMIC ENERGY POST OFFICE CAIRO, A.R«E. CONTENTS x

Page

ABSTRACT *<,..».»••... i INTRODUCTION . . *« . *.*,...... 1 EXPERIMENTAL DETAILS ••»*«•««»« • • a • » « •»»«*««* *»»*«•»»»«»<>• — RESULTS AND DISCUSSION ..,.••+ .*.....•...•.. 4 ACKNOWLEDGEMENTS ...,...... •..<...»,..>...... 10 REFERENCES „...»,«.**»» 11 ABSTRACT

Slow neutrons and secondary gamma r>ay distributions in concrete shields with and without a reflecting layer behind the concrete shield have been investigated first in case of' using a bare reactor beam and then on using & B.C filtered beam. The total and capture secondary gair-m-a ray coefficient (B^and B^) , the ratio of the reflected (Thermal neutron (S ) the ratio of the secondary gamma rays caused by reflected neutrons to those caused by transmitted neutrons ( and the effect of inserting a blocking l&yer (a B^C layer) between the concrete shield and the reflector on the sup- pression of the produced secondary gamma rays have been investigated, It was found that the presence of the reflector layer behind the concrete shield reflects sor/so thermal neutrons back to the concrete shields end so it increases the number of thermal neutrons at the interface between the concrete shield and the reflector. Also the capture secondary gamma rays was increased at the interface between the two medii due to the capture of the reflected therwal neutrons in the concrete; shields. c It was shown that B^ is higher than and that B. B^and 1^th / I'f for the differen°t concrete types it; higher in case 0 0 of using the graphite-reflector than that in using cither water or paraffin reflectors., Putting ft blocking layer (B.C layer) between the concrete shield and the reflector decreases the produced secondary gamma rays due to the absorption of the reflected thermal neutrons, INTRODUCTION

The weight and dimensions of the reactor ?f'-i-33.d ars determined by the hard gamma arising from in the shield. It is therefore extremely important to find ways of reducing this gamma ray f-'.ux* The most intense secondary gamma radiation arises.1! from the radiative capture of thermal neutrons which have been slowed down in the materials of the shield,. If the reflected thermal neutrons are captured in a rcaterial which has a high capture gamma ray cross«section such as iron which to the production of hard capture gamma radiation. The decrease of the production of the hard capture gamma .radiation in the shield was achieved by placing a carbide layer between the concrete shU-ld and the reflector layer which has a high absorption cross-sect to thermal neutrons giving rise to soft weakly ponp-trating gamma - iation with an energy E^O.,5 Mev, and the provision of /*' shielding against these is very much easier,

Many investigations have been can-led out on the production of secondary gamma rays and tho methods for its suppression^ ~ '„ In the present work the increase in the production of the capture secondary gamma rays due to i:he increase of the reflected thermal neutrons from the reflectors have been studied. Also the effect of introducing a blocking layer (B.C layer) between the concrete and t!>.« reflector layer on the suppression of the secondary gmma rays was investigat-eti EXPERIMENTAL DETAILS

In the present work three types of Generate have been used (ordinary concrete, ilmenite-lirr.unite concrete and ilmenite concrete) whose composition is given elso where" ' ., The concrete shield is composed of two blocks each has the dimension 120 x:.l20 x 40cirl which are wet together to have 3 the dimension of 120 x 120X,80.» The concrete shield is placed in front of one of the horizontal chanrifcls of the ET-RR-1 reactor. The reactor beam is 10 cmad.la'metflr'"the-"-ET-RR-l ''reactor is a water moderated water reflected reactor operating at maximum power (2MW), The sample center must be on the same horizontal line. The sample height ivac1 adjusted by the four screws of the legs of the sample holder, Each concrete sample has two holes separated by 20 om for inserting the sample holders,. For each concrete rypts, the following runs have been carried out once with a bare reactor beam and the other with a boron carbide filtered reactor beam a) Measurements of gamma rays and slow neutron distribu- tions for the concrete assembly alone (i«e without any , r:. reflector.or. blocking, layer) b) Measurements of gamma rays and Glow neutron distributers for the concrete assembly after adding behind it a reflector layer from each of graphite^,. paraffin and water with dimension 120 x 120 x 20 cm0, c) Measurements of gamma rays and slow neutron distributions for the concrete assembly after putting a' blocking' (B,C layer between the concrete shield and the reflecting layer having the dimensions of 120 x 120 x 1 cm , The measurements are carried out both ^lor;:; t-he beam direc- tion (Z direction) and perpendicular to it (R direction)*

The measurements of the secondary yamsna rays and slow neutrons were carried out using Harsho-A- I.iF-700 and LiF-600 TLD ribbons, respectively having dimensions' of 3,1 x 3,1 x 0,89m.m,For the measurements of slew nsutror.s. LiF~700 ribbona have been used together with LiF-600 ;•!!:>!:ono in order to correct for the response of LiF-600 o^ ^mMa rays to obtain the net neutron response.. To obtain the response of both LiF-700 and LiF-600 ribbons to gamma rays, they have exposed together to a standard 60Co gamma source,, The used TLD ribbons have a main glow peak at 210°C* The were annealed at 400 C for two hours then at 100°G for 1 hour then there was a post anneal at 100°C for 10 minutes*

A ZnS(Ag) was used as a monitor to check the reactor power variation during the wcsie-urements,

For each dose the secondary gamma ray coefficient (B ) o Qv the capture secondary gamma ray coeff.ici.ent (B^), the ratio of the reflected thermal neutrons ('£ )s ^'n& la'cio of the secondary gamma rays obtained from the reflected thermal neutrons to those obtained from the transmitted neutrons tri f (I~, / I^y) and at last the blocking factor (F) have been investigated.. - 4 -

RESULTS AND DISCUSSION

The slow neutrons and secondary ga'»nia ray distributions have been carried out (for each of the concrete and reflector type) at first for a bare reactor beam and then for a boron carbide filtered reactor beam.. Figures Cl»5) show the slow neutrons and total secondary gamma ray distributions for the ilmenite concrete with and without a graphite reflector as an example in case of using a bare reactor beam. By subtracting secondary gamma ray distribution for a B.C filtered reactor beam from that for a bare roactor beam, the capture secondary gamma ray distributions for neutrons with below 10 keV have been obtained. It is known that the measurments with a B C filtered reactor beam gives the secondary gamma rays for neutrons with energies ^10 keV. Figures (3,7) show the slow neutrons and the capture secondary gamma rays distributions for llmeriite concrete and graphite reflector as an example in case of (bare reactor beanuB.C filtered reactor beam) i.e for neutrons with energy S 10 keV, with and without using the reflector layer. The subtraction of the B.C filtered reactor beam run from the bare reactor beam run eliminates the primary gamma ray contribution.

Figures (2,6) show the slow neutrons and total secondary gamma ray distributions for ilmenite concrete on using a graphite reflector layer as an example also then on introduc- ing a B.C blocking layer between the concrete and the reflector.

Figures (4,8) show the slow neutrons and capture second- ary gamma ray distributions for ilmeriito concrete also in.c«s - 5 - of (bare reactor beam - B.C filtered reactor beam) on using a graphite reflector then on insertl.no % blocking layer between the concrete and the reflector layers,

It was found that the slow neutrony and the secondary gamma rays decrease almost exponential]'/ with the- increase of Z.From the displayed figures it is clear that the presence of a reflector layer behind the concrete shield leads to the increase in the intensity of U;s thermal neu- trons around the interface between the two r^odii (i..c at 80 cm) as the reflector reflects soine thermal neutrons back to the concrete shield. Introducing a blocking layer (B,C) between the concrete and ref lectcx- layers loads to the decrease in the intensity of the reflcscted thermal neutrons As the B C layer has a high absorption cross.,section for thermal neutrons so it absorbes n?ost: of the reflected thermal neutrons preventing them to be captured in the concrete to ;' f-1 x, produce capture secondary gamma rays*,, Sc;me parameters car. ''•' be estimated from the results (i) Determination of the total and capture secondary gamma c ray coefficients (B^ and B^ respectively) ** tv It is known that B ,= Bv + B;l % * * i.e total secondary gamma ray coef vicient —capture secondary gamma ray coefficient -i in&lastic scattering secondary gamma ray coefficient

where q^ is the number of capture secondary gamma rays with.in o " interval &E escaping from concrete shield, q|T is the number of the inelastic scattering secondary - 6 - gamma rays within interval^E escaping from the concrete shield and a . is the total number of secondary gamma rays within interval ^ E escaping frotfi the concrete shield, ,..' and Q is the total number of neutrons escaping from the shield,Figures (9-11) show both the total and capture secondary gamma radiation coefficients (Bv and EL. ) and & against (2) for different concrete^ . different reflectors It can be seen that the values of Bw and sS increase with the the increase of Z values till it reaches its maximum at thv. interface between the two medii (.1*3 at 80 cm). The tote,l secondary gamma radiation coefficient (B) is higher than the capture secondary gamma radiation coefficient accord- ing to the relation B-sB*.C + B^-^ThYl e bapture secondary gamma radiation coefficient (BS ) for ilmenite is higher and increase sharply with the increase of Z as it contains iron punching and ilmenite aggregate (an iron ore) in its constituents who'se capture cross.-section for thermal neutrons is high/^^0.19 barnsV, 8^ decreases sharply with the decrease of Z in ilmenite concrete also due to its higher to gain ma rays and its smaller- relaxation length/8 cm) compared with^lO cm and 11 cnft for ilmenite limonite and ordinary cncrefe respectively,,. Q From the figures it can bo noticed that Bv have the highest O (g} values in case of graphite refleotcr as the moderatingv ; ratio of graphite is higher than that for water and paraffin Also£di^£, and ^^ for graphite are very small compared with paraffin and water. So it is expectsd that graphite reflects thermal neutrons more than the other two reflact- ors which are then captured in the concrete generating hard capture secondary gamma rays. (2) Determination of the ratio of the capture secondary - 7 - secondary gamma rays obtained from reflected nsutrons to that obtained from transmitted neutrons (I.,t h/J ^f ) fur both bare reactor • bearrr •'••• and (bare reactor beam - E^C Mlterecl rs^ctcv beatn)

T_.th '//T_f ' a iCaptur ii 1 • ie I illsecondar •!- T INI- 111 I iiy T i iufclgammn m a rayff si oiln l uGln\~'q ia ii-- " reflecto• • 1 n • I ill r ~- . - K0 ^ X° Capture secondary gamma rays without using a reflector Figures (12-14) show iJ^/lL for different and 15 O reflectors for both bare reactor beam and (hare reactor beam- B.C filtered reactor beam)* Form the figures it is clear that •t" K -P Ibv / L*o. increases with the increase of 7, tii?t it reaches a , maximum value at the interface between the two ruedii (at 80cm).; It also decreases with the increase of R values, I~ '/ I reachrva 9 Ov a value of 11 for ordinary concrete at R-r £orc and Z~ 80 cmt 1^1~h / Iyf has higher values on using graphite reflector also. This because of the high moderating ratio of graphite and its low abs.orption for thermal neutrons as compared with water and paraffin which leads to the of most of thermal neutrons back to the concrete where they were captured producing qapture secondary gamma rays. Also from the figures it is clear that I / I for ordinary concrete is higher than the other two concrete types due to its higher relaxation length for gamma rays (Nsll cm and consequently its low attenuation coefficient, (3) Determination of the fraction of the reflected thermal neutrons (g ) which are reflected from the reflector layer back to the concrete where they are captured in it at thickness of the order of the diffusion length (Lsr7,4 cm) for ordinary concrete. - 8 -

neutron flux with reflector-neutron flux without refletor

neutron flux with a reflector has a maximum vlue at the interface between the two medii where the reflected thermal neutrons are maximum. It depen- ds slightly on the shield thickness. The values of g is higher for graphite reflector due to it. s higher reflecting power and its lower obsorption cross section for thermal neutrons. $ depends also on the thermal neutrons capture cross sec- tion of the medium where the thermal neutrons are being cap- tured. Thus S^^-S for ilmonite concrete (whose thermal neutrons capture cross section is high) while §5; 0.5 and 0.4 for ilmenite limonite concrete and ordinary concrete respectively. (4) Calculation of the blocking factor (F),

Capture secondary gamma rays without an absorbing layer

Capture secondary gamma rays with an absorbing layer

The intensity of the caputre secondary gamma rays in the concrete shield generated from the capture of the refl- ected thermal neutrons can be suppressed by either adding to the concrete shield a material which have large absorption cross section for thermal peutrons producing no hard gamma radiation e,.g He , Li and B . The other method is to insert a blocking layer (B.C layer) between the concrete and the reflector. The blocking layer absorbs most of the reflected thermal neutrons due to its high absorption cross section to thermal neutrons and consequently decreases . - 9 -

the capture secondary gamma rays in concrete. Figures (15-17) show the blocking factor for different concrete types and different reflectors for the hare reactor beam. From the figures it is clear that the blocking factor (F) decreases with the increase of R while it increases with the increase of Z reaching a maximum at the interface between the two medii also. It was found that (F) depends on the compost ion and location of the Vieavy and light shields, their thickness, the spectrum of incorcp.i.-ig neutrons and the total thickness behind which the factor is svieasured. - 10 -

ACKNOWLEDGEMENTS .

The authors like to express their deepest gratitude to professor Dr.M.K.M.Fayek Head of fhs Reactor and Neutron physics Department, Nuclear Research Center, Atomic Energy Authority, for his continuous support and encouragement during the fulfillment of the work.. - II -

REFERENCES

1- H. Goldstein "Fundamental aspects of Radiation shielding" Pergamon Press. Inc. New York (1957),

2- E.T Price, C.C.Horton and K.T.Spinney "Radiation Shield;;.!-;.;" Pergamon Press, Inc. New York (1957)

3- D.L.Broder et al. Atomnya Energio !U'.3L.<35 (1952),,

4- 0. Bozyap et al. 28, 10,1 (1975).

5- L.Harris et al. Trans Am. Nucl. Soc. 1-2, 459 (1969)

6- P.G. Gromov. Oournal of Nuclear Energy 20, 178 (1966).

7- A.S.Makarious et al. Internation^'Journal of Applied Radiation and 33,559 (1932),

8- YU.A.Kazansky "physical Investigations of Reactor Shieldiriu" Atomizdat Moscow (1966).,

9- Reactor Hand Book Vol. 1 Materials Edifed by:: C.R.Tipton Or. Page 836 (1960). Interscience Publishers Inc.,New York, i-1 M

With graphite reflector \ ^< g,-aphite reflector: W;tnout graphite reflector graphite refiec*or on!y

10

30 50 70 Z.cm Fig I 1 ):Neutron distributions in ilmenite concrete shield* Fig.( 2)'Neutron distributions in ilmenite concrete shield* graphite reflector in case of bare reactor beam. B^C layer* graphite reflector in case of bar* reactor beam. With graphite reflector B4C lever*graphite reflectorV ' «.: . Without graphite? reflects .- %*•.•** 7? t IT \. 'f 1 i i ] ? t 1 ? 30 50 70 TO 30 SO 70 Z,cm Z.cm )=N*uiron distributions in itmenrte concrete shield * Fig. { 4 ):Neutron distributions in ilmemte concrete shseltU graphite reflector incase of bare reactor beam B^C layer* graphite reflector in case of bore reactor filtered beam ~ b~e'arh •• 84C filtered beam 10" "R=0c With graphite retijctor With B^C layer*graphite reflector Wrfh graphiie reflector only Without graphite reflector

10*

•5 10 a. "5 o o

10

10

10 30 50 70 Z,cm Fig.( 5 ); Total gamma ray distributions in ilmenite concrete Fig.{ 6 ):Total gamma ray distributions in ilmeniteconcrete shield * graphite reflector due to bare reactor shield* BAG layer* graphite reflector due to bare beam. reactor beam. with graphite reflector With B4C layer*graphite refl •r. - Without graphite reflector With graphite reflector only I

01 t

10 50 70 Z.cm Z.cm Fig. (7 ):Secondary gamma ray distributions in ilmenite Fig.( 8 .^Secondary gamma ray distributions in ilmenite concrete shield * graphite reflector due to concrete shield+f&C layer »graphite reflector neutrons with energies< 10 KeV . due to neutrons with energies <10 KeV. -1 X10 '3 4 Graphite 3 1 Paraffin 2 0

1 10 0 7 6 fo1 6 5 5

L 3 Water 3 2 2 1 0 i_ 60 65 70 75 80 60 55 70 75 80 •z,cm Fig.i 9) Z.cro Total secondary gamma ray coefficient in case of Capture secondary gamma ray coefficient ordinary concrete using a' bare reactor in case of ordinary concrete using beam and'different reflectors. ( bare- B^C filter )' beam and different reflectors. xib Graphite

Paraffin 8 Graphite 2 Water i 6 Paraffin B : ° < 4 Water

2 101 0 o: i 10 x101 8 6 Graphite 6 Paraffin sj 4 I i Water 2 Water 0 60 70 75 80 60 65 70 75 80 Z,cm 2 .cm Fig. I 10 } Totai scondory gamma stay coefficient in case Capture secondary gam me fey coefficient of ilmenite lirnonite concrete us it. g a bare in case of iimenite limonite cor»r.rete using reactor beam and differeni; reflectors. (bare-B4C filter I beam and different reflectors. ,16' -j Graphite Graphite 6 5 r ') Paraffin L j Paraffin 3 3 2 Water 2 1 Water 1 0

'0 By -1 0 xlO CO 7 Graphite Graphite i 6

Paraffin

1 0 60 65 70 80 60 65 70 75 60 ZjCm Fig. (n ) Total secondary gamma ray coefficient in case of Capture secondary gamma ray coefficient in case ilmenite concrete using a bare reactor beam and of ilmenite concrete using ( bare- B^C fitter ) different reflectors* beam and different reflectors. 10 Graphite. 8 Paraffin Water 6 L Graphite 3 Paraffin Water

12 *-o Gr ophite 10

8 Paraffin Water 6

K

50 65 70 75 80 60 65 70 75 80 2 ,cm Fig. 2,, ctn The ratio between the capture secondary The ratio between the total secondary gamma gamma ray due to reflected thermal neutrons ray due to reflected thermal neutrons and and that due to transmitted neutrons for that due to transmitted neutrons for ordinary ordinary concrete concrete. 1.8 2.0 1.6 Graphite 1.8 1:4 Pa raff in Water 1.6 1.2 1.4 1.0 1.2 0.8 1.0

2.2 *0 2.0 \ 2.0 1.8 ^ 1.8 Graphite o 1.6 1.6 i Pdraffin Graphite 1.4 Paraffin Water 1.2 Water 1.2 1.0 1.0 1 0.8 60 55 70 75 80 60 65 70 75 80 Z,cm Fig. (13) Z.cm The ratio between the capture secondary • The ratio between the total secondary gamma ray due to reflected thermal neutrons gamma ray due to reflected thermal neutrons and that due to transmitted neutrons for and that due to transmitted neutrons for Hmenite limonite concrete. ilmenite limonite concrete. 1

X I JC . ro

The ratio between the capture secondary gamma The ratio between the total secondary gamma ray due to reflected thermal, neutrons and that due ray due to reflected thermal neutrons and that to transmitted neutrons for ilmenite concrete. due io transmitted neutrons for ilmenite concrete. 22

Fig.l 15 !•• Blocking factor for ordinary concrete using different reflectors. - 23 -

1.5 1.4

1.3 o 1.2 ft. 1.1

1.0 60cm •£ 0.

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1.4 1.3 1.2 1.1

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10 15 20 25 R ,

Fig. ( 16 hBlocking factor for ilmenite limoniic- concrete using different reflectors. Fig. I 17): Blocking factor for ilmenife concrete osing different reflectors.