A neutron radiography unit based on a (D, T) neutron generator

International Journal of Applied Radiation and Isotopes, 1969,Vol. 20, pp. 423-428. Pergamon Press. Printed in Northern Ireland

A Neutron Radiography Unit based a (D, T) Neutron Generator

on

I. C. H E N D R Y Department of Industrial Science, University of Stirling, Scotland

(Received 13 December 1968) A description is given of the Dounreay neutron radiography unit based on the use of a generator producing 101: neutrons per second at 14 MeV. The available flux and shielding requirements are discussed and the main development problems highlighted. U N UNITI~ DE R A D I O G R A P H I E A N E U T R O N S BASt~ SUR U N GI~NI~RATEUR DE N E U T R O N S (D, T) On d~crit l'unitfi de radiographie ~ neutrons situ6 ~ Dounreay, lequel est basil sur l'emploi d'un gdn6rateur produisant 101: neutrons par seconde ~ 14 MeV. On discute le flux dont on peut disposer, et on souligne les principaux probl~mes de ddveloppement. I1PHBOP ~JIfI ItE17ITPOHOFPAq)HH, OCHOBAHttbI1;I HA F E H E P A T O P E HEFITPOHOB (D, T) 3~ecb )IaeTc~ om4cauHe npnSopa Dounreay (~ayup~n) ~Ian HefiTpoHorpa~n~, OCHOBaHHt,Ifl Ha npllMeHeHnII renepHpymmero 10 u He~ITpOHOB B ceRyH~Iy reHepaTopa oHeprnei~ B 14 Mera~aeHwpOHBOa~,V. 05Cym~amTCH nMemnIneeu rIOTOHH H wpe6ymmnecn 3aIKI4WHt~Ie aKpanm a TaHme OCBematOTCH OCHOBHLIenpo6neMH pa3pa6oTKn. EINE N E U T R O N E N - R A D I O G R A P H I E A N L A G E A U F DER G R U N D L A G E EINES (D, T) N E U T R O N E N G E N E R A T O R S Es wird eine Beschreibung der Dounreay Neutronen-Radiographieanlage gegeben, die auf der Verwendung eines 101: Neutronen je sec erzeugenden Generators mit 14 MeV beruht. Die bestehenden Fluss- und Abschirmungsanforderungen werden besprochen und die Hauptentwicklungsprobleme beleuchtet. INTRODUCTION r a d i o g r a p h y is well established at D o u n r e a y as a tool for use in the n o n - d e s t r u c t i v e testing of fuels. (1) T h e m e t h o d uses a well c o l l i m a t e d n e u t r o n b e a m from the D o u n r e a y Materials Testing Reactor (D.M.T.R.). Unf o r t u n a t e l y all specimens m u s t be b r o u g h t to t h e r e a c t o r for e x a m i n a t i o n , b u t a m o r e flexible r a d i o g r a p h y unit can be b u i l t using a neutron generator.* This generator produces 1011 nsec -1 (14 M e V ) energy) a n d a unit has b e e n designed a r o u n d this for use in the D o u n r e a y R a d i o a c t i v e E x a m i n a t i o n Caves. I t NEUTRON

* Incorporating an Elliot P-tube. 423

is i n t e n d e d to p e r m i t n e u t r o n r a d i o g r p a h y to b e m a d e a v a i l a b l e to research organisations outside the a t o m i c energy field. T h e cost is slightly g r e a t e r t h a n a 300 k e V X - r a y m a c h i n e , b e i n g a p p r o x i m a t e l y £ 9 0 0 0 . This p a p e r summarises the feasibility s t u d y u n d e r t a k e n d u r i n g the design of the unit. MODERATING TANK T h e basic design is illustrated in Fig. 1. I t includes a m i l d steel t a n k 3 ft dia. 3} ft h i g h filled to a d e p t h of 3 ft with oil. O i l (C7H18 , density 0.87 g c m -a) was selected in preference to w a t e r since its m o d e r a t i n g p r o p e r t i e s a r e t h e same a n d its use replaces t h e (n, y) r e a c t i o n

I. C. Hendry

424

Filling/emfying pipe '\

Oil level indicator // ~

Tank 3ft dia x 3¼ ft high

Detector

~

Neutrongenerator / /

oI Specimen / If-: : Collimator i " "

' l~Driptray

5!-

FIO. 1. Basic components of neutron radiography unit.

Tank \

Neutron source ~

convertor 7cmron thickm .-.

system

F m

1 1

I Adaptorst, for collimators f /'\

1

~

"~Collimators \ oo/adjusta"'eradia"

z

\ \Extrapolated f

!

IO I Fro. 2. Collimator positions on tank. between fast n e u t r o n s a n d oxygen with a weaker c a r b o n reaction, t h e r e b y r e d u c i n g the shielding problems. T h e n e u t r o n generator is a cylinder 12 in. dia., 3 ft long with the n e u t r o n source at one e n d , this generator is m o u n t e d horizontally so that the source is at the centre of the oil in the tank. T h e tank is capable of a c c o m m o d a t i n g five horizontal n e u t r o n collimators of up to 3 in. dia. O n l y one collimator is illustrated in Fig. 1 a n d it is envisaged that only one will be in use at a time, the o t h e r s - - t w o at 30 ° a n d two at 90 ° to the " h e a d o n " position (see Fig. 2 ) - - a r e to be used to d e m o n s t r a t e the flexibility of the system since it will be possible to extract a b e a m i n a n y required direction. All collimators are adjustable a n d can be m o v e d i n a n d out to assist i n selecting the best flux a n d n e u t r o n spectrum. FLUX LEVELS T h e best estimates of flux have b e e n o b t a i n e d from calculations performed at A . W . R . E . Cm using the s n p r o g r a m m e , S N A T C H . T h e r m a l flux boosting will be possible on the u n i t a n d

\ \using \ ~ar-Zexp(-~r Tank wall \ ,/ 2'0 ~o / o ~ ~) bO Distance from point source, cm

FIG. 3. Thermal flux distribution expected in oil. calculations are i n c l u d e d for the case of a 7-cm thick n a t u r a l u r a n i u m " c o n v e r t e r " s u r r o u n d i n g the source of 14 M e V n e u t r o n s ; this " c o n v e r t e r " utilises fast n e u t r o n fission in 2asu to e n h a n c e the neutron population. T h e t h e r m a l fluxes are illustrated i n Fig. 3 which indicates that the useful region for b e a m extraction is at a b o u t 10 c m radius where the flux level is 10 s n c m -2 sec -1 w i t h o u t the converter a n d a b o u t 4.7 × l 0 s n c m -2 sec t with the converter. SHIELDING

Dose ratefrom gamma radiation U s i n g the S N A T C H data, S I D D O N S (2} estim a t e d the g a m m a intensities i n the two systems. T h e results for the 1011 nsec -1 source have b e e n used to calculate the g a m m a dose rate at the surface of the m o d e r a t i n g t a n k (radius 47.75 cm) T h e results are given i n T a b l e 1. I n the worst case, with a u r a n i u m booster, the dose rate is a b o u t 15.5 rh -1 at the t a n k surface. T h e n o r m a l working level should be

A neutron radiography unit based on a (D, T) neutron generator 2"5 m r h -1 so t h a t c o n s i d e r a b l e shielding is r e q u i r e d (see below) ; in p r a c t i c e a 3 ft b l a n k e t o f w a t e r is t h e cheapest shield. T h e a c t u a l location of t h e u n i t will d e t e r m i n e t h e shielding r e q u i r e m e n t s outside t h e t a n k a n d in fact t h e D o u n r e a y e x p e r i m e n t s a r e b e i n g p e r f o r m e d in a " c a v e " w i t h 5 ft thick walls. T a b l e 4 is i n c l u d e d to allow users to assess the v a l u e of o t h e r shielding materials, a l l o w a n c e h a v i n g b e e n m a d e for m o d e r a t i o n of the original g a m m a rays in the oil.

425

T h e r e should be no a d d i t i o n a l p r o b l e m associa t e d w i t h this because t h e r m a l n e u t r o n abs o r p t i o n rates in m a t e r i a l s a r e g e n e r a l l y h i g h e r t h a n for fast n e u t r o n s ; a check can b e m a d e , however, b y a p p l y i n g the " r -2 exp (--Y~ar)" law using ~]a(cm -1) as t h e t h e r m a l n e u t r o n a b s o r p tion cross-section. T h e p r o b l e m c a n also be t a c k l e d b y using B o r a l ; a ~ in. thick a l u m i n i u m clad s a n d w i c h of b o r o n c a r b i d e in a l u m i n i u m is c a p a b l e of r e d u c i n g the t h e r m a l n e u t r o n c u r r e n t b y a factor o f 1000.

Dose rate from fast neutrons

SIZE OF S H I E L D E D U N I T

F i g u r e 3 shows t h a t n e a r t h e t a n k wall the fluxes in p u r e oil a r e h i g h e r t h a n in t h e u r a n i u m / oil system. F o r t h e shielding assessment t h e flux d a t a for oil h a v e b e e n used b y c o n v e r t i n g to dose rates using t h e conversion factors shown in T a b l e 2 covering all t h e e n e r g y groups. As expected, the greatest h a z a r d is from the most energetic neutrons. T h e c o m b i n e d dose r a t e from neutrons in groups 1-15 is 32,200 m.p.1, or 80.5 r h -1. T h e choice of shield for the t a n k c a n b e m a d e b y reference to T a b l e 3 w h i c h uses t h e A r g o n n e N a t i o n a l L a b o r a t o r y c o r r e l a t i o n b e t w e e n r e m o v a l cross section a n d a t o m i c w e i g h t ; TM t h e r e m o v a l cross-sections h a v e been c a l c u l a t e d to c o r r e s p o n d to t h e a t o m i c constituents of t h e different materials. T o assist p o t e n t i a l users in assessing t h e i r own shielding r e q u i r e m e n t s the d a t a has also been s u m m a r i s e d in a suitable form in T a b l e 4. T h e dose r a t e will fall a c c o r d i n g to " r -2 exp ( - - Y / r ) " w h e r e r is d i s t a n c e from n e u t r o n source a n d Y/ is the r e m o v a l cross section.

Dose rate from thermal neutrons T h e t h e r m a l n e u t r o n dose r a t e at the t a n k wall is q u o t e d below as 1870 m.p.1, i.e. 4.7 r h -1.

I t is obvious t h a t the shielding p r o b l e m is d o m i n a n t l y a fast n e u t r o n p r o b l e m a n d calls for provision of e n o u g h m a t e r i a l to p r o d u c e 2.5 m r h -1 or 1 m a x i m u m permissible level of d e p o s i t e d energy. T h e cheapest shield is w a t e r a n d calculations using t h e d a t a in T a b l e 4 show t h a t a 3 ft t h i c k b l a n k e t will be satisfactory. I n o r d e r to ensure t h a t the t h e r m a l n e u t r o n level is also at 1 m.p.1, the w a t e r should be l o a d e d w i t h b o r o n to the extent of 1 m g n a t u r a l b o r o n p e r m l T h e 3 ft shield o f w a t e r will p r o v i d e a d e q u a t e shielding against the g a m m a rays. T h e smallest p r a c t i c a l unit envisaged is thus a 3 ft m o d e r a t i n g t a n k s u r r o u n d e d b y a 3 ft w a t e r b l a n k e t i.e. a 9 ft dia. assembly. Because t h e highest fluxes a r e nearest the source (Fig. 3) it can be a r g u e d t h a t the m o d e r a t i n g t a n k need o n l y e x t e n d for, say, 6 - 8 in. a r o u n d t h e source. H o w e v e r , this w o u l d not basically r e d u c e the overall size of t h e assembly since t h e w a t e r b l a n k e t w o u l d h a v e to be increased to c o m p e n sate for loss of shielding. T h e m a i n cost of the unit is the n e u t r o n t u b e w h i c h is a b o u t £ 8 0 0 0 ; the m o d e r a t i n g t a n k costs a few h u n d r e d p o u n d s I n t h e D o u n r e a y trials the n e e d for a w a t e r t a n k

TABLE 1. System Reaction Gamma energy per event (MeV No. of events per see Dose rate at tank surface (rh -1)

Pure oil

Pure oil ÷ uranium

H(n,y)

C(n, n'y)

H(n,y)

C(n, n'y)

z35U fission

2zsu fission

2.23

4.4

2.23

4.4

8 = 4 × 2

8 = 4 × 2

7.55 × 101°

2.30 .'< 1010

19.4 x 1010

0.85 × 101°

1.75 × 101°

11.7

1.02

1-97

2.16

2-33

Note: most of the fission is in z35U so use of enriched U should give better boosting. ,

0.7 × 101° 0.79

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426

TABLE 2.

Neutron gp.

1

2

3

4

5

6

7

8

U p p e r energy, M e V

14.6

13.5

11

6.5

2.4

1.1

0.55

0.26

1.01

0.577

1.26

1.08

0-685

0.395

0.306

0.205

Flux at tank wall 105 ncm -2 sec-1 Flux which gives lm.p.1 (2.5 mrh 1) ncm -2 sec -1 .'. m.p.ls, at tank surface

16

16

16

18

18

20

22

42

6310

3600

7890

6010

3810

1975

1390

488

Neutron gp.

9

10

11

U p p e r energy, M e V

0.13

0-043

0.010

12 1.6 × 10 -a

13 2-6 × 10 -~

14 4.2 × 10.5

15 5.5 × 10.6

16 4 × 10-7

0.234

0-244

0.284

0.28

0.28

0.313

0.385

12.5

70

170

420

670

670

670

670

670

355

144

68

42

42

47

57

1870

Flux at tank wall 105 ncm -2 sec-1 Flux which gives lm.p.1 (2'5 m r ~ 1) ncm -2 sec-1 .'. m.p.ls, at tank surface

N.B. Group 16 is the thermal neutron group. "FABLE 3.

Material

Oil

Water

Polythene

Jabroc (masonite wood)

Density, gcm -a Removal cross-section, cm -1

0.965

I-0

1-4

1.33

3'35

8.0

0.075

0.11

0-18

0'128

0'067

0.16

has b e e n e l i m i n a t e d b y using a l a r g e " c a v e " w i t h 5 ft t h i c k c o n c r e t e walls, b u t t h e cost of this t a n k is a few h u n d r e d p o u n d s . COLLIMATED

FLUX

EXPECTED F o r n e u t r o n r a d i o g r a p h y t h e c r i t i c a l factors a r e flux l e v e l at t h e d e t e c t o r a n d t h e d e g r e e o f collimation. For the Dounreay radiography u n i t t h e r a t i o o f d i a m e t e r / l e n g t h , (d/l) of t h e c o l l i m a t o r is a p p r o x i m a t e l y 1 : 80. I t p r o d u c e s a b e a m c a p a b l e o f r e s o l v i n g cracks in i r r a d i a t e d fuel a n d , in g e n e r a l , c a n resolve cracks as n a r r o w as 0.001 in. or less. A l t h o u g h this d e g r e e o f c o l l i m a t i o n is desirable, a c o m p r o m i s e m a y

Concrete (barytes)

Iron

b e n e c e s s a r y since t h e b e t t e r t h e c o l l i m a t i o n (d/1 small) t h e l o w e r t h e c o l l i m a t e d flux (see T a b l e 5). F r o m Fig. 3, a r e a s o n a b l e v a l u e for t h e m a x i m u m a v a i l a b l e t h e r m a l flux at t h e i n n e r e n d o f a c o n v e n i e n t l y p o s i t i o n e d c o l l i m a t o r will b e l 0 s n c m - 2 sec - 2 (or 4-7 × l0 s n c i n -2 sec -1 in t h e b o o s t e d system). T h e flux e m e r g i n g f r o m t h e c o l l i m a t o r is g i v e n b y :

w h e r e ~ b ' = t h e a n g u l a r flux, n c m -2 sec -1 p e r 2~" s t e r a d i a n s 0 = m a x i m u m a n g l e possible in collimator between neutron direction and radius vector.

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A neutron radiography unit based on a (D, T) neutron generator

TABLE 4. Summary of data relevant to the operation of unit described Radiation doses at tank wall H(n,y) 2-16

Gamma, rh -1 Fast n's, rh -1 Thermal n's, rh -1

Pure oil C(n, n~v) 2-13 80-5 4.7

1 Gamma attenuation data, 6 ~ Material Density gem-a Attenuation coefft, for gammas from H and from U, cm-1 Attenuation coefft, for gammas from C, cna-1 10-folding length for attenuation of H and U gammas, in. 10-folding length for attenuation for C gammas, in.

Lead 11-4

Total 4.49

Oil and uranium (natural) H(n,y) C(n, n~v) U Total 11.7 i '02 2.76 15.48

required i.e. 10-3'8 required Jabroc 1.33

Concrete 3.35

Water 1.0

Iron 8.0

1.06

0.124

0.311

0.093

0.744

0.524

0-061

0.154

0.046

0-368

0.85

7.3

2.9

9.7

1.22

1-7

14.8

5.9

19.7

2-46

1 Fast neutron attenuation data, - required i.e. 10-4.2 required 32,200 Material Attenuation coefft, for fast neutrons, cm-1 10-folding length for attenuation of fast neutrons, in.

Lead

Jabroc

Concrete

Water

Iron

0.114

0.128

0-067

0.I 1

0.16

7.9

7-1

8.3

5-7

FIELDHOUSE a n d SIDDONS(2) showed that, for the t h e r m a l group, q~' is just over half the scalar flux therefore i n pure oil ~' will be a b o u t 0"5 × l0 s n c m -~ sec 1. It was also shown that, ~ ' is a b o u t ½ the scalar flux i n the boosted system so i n that case, q~' will be a b o u t i N 4.7 × 10s = 1.6 × 108 n c m -2 sec -1. I t r e m a i n s to select d/1 a n d this will d e t e r m i n e the flux at the sample. T h e mobile u n i t is being fitted with collimators 36 in. long, the m a x i m u m possible d i a m e t e r being 3 in. N o r m a l l y a space of (approximately) 6 in. will be needed between detector a n d collimator for insertion of specim e n s ; with this in m i n d T a b l e 5 has been constructed to illustrate the i n t e r p l a y of b e a m size at detector, collimator length a n d collimator d i a m e t e r with flux a t t e n u a t i o n i n t r o d u c e d by the collimator. T o achieve collimation similar to the D o u n -

13.5

reay U n i t , i.e. say 1/72, a collimation factor of 10 . 4 will have to be tolerated. This m e a n s that, i n the boosted system the n e u t r o n flux will be 1.6 × 104 n c m -2 sec -1. However, i f a value for d/1 of 1/36 could be tolerated the collimation factor would be 0.38 × 10 .3 a n d the operating fluxes a b o u t 2 × 10 a n c m -2 see -1 a n d 6 × 104 TABLE 5. Beam

d in.

1 in.

size at detector in.

1 1 2 2 3 3

36 72 36 72 36 72

la]3 11/6 22/3 21/3 4 31/2

d i-

Collimation factor ~-(d/l) 3 ( × 10-3)

1/36 1/72 1/18 1/36 1/12 1/24

0.38 × 10-3 0.095 x 10.3 1-5 × 10-a 0.38 ~'< 10--a 3.45 x 10-a 0.88 x 10_3

L C. Hendry

428

n c m -2 sec -1 for the p u r e a n d boosted systems, respectively. T a b l e 5 shows t h a t it is b e t t e r to h a v e a 2 in. dia. 72 in. long c o l l i m a t o r r a t h e r t h a n one 1 in. dia. 36 in. long since the collim a t i o n factors are the same b u t the b e a m d i a m e t e r is d o u b l e d . RADIOGRAPHY

REQUIREMENTS

I t is p l a n n e d to use two techniques to o b t a i n radiographs. (a) the transfer m e t h o d using a d y s p r o s i u m foil d e t e c t o r to p r o d u c e a r a d i o g r a p h . (b) I m m e d i a t e r a d i o g r a p h y using a n e u t r o n scintillator in the b e a m a n d v i e w i n g the scintillation p a t t e r n w i t h a n i m a g e intensifier. (4)

Transfer method T h e activity b u i l t - u p in a d y s p r o s i u m foil is given b y : A oc 411 - - exp ( - - t 2 ) ] (2) where, 4 = t h e r m a l flux, ;t = d e c a y constant = 5 x 10 .3 m i n -1, t = i r r a d i a t i o n t i m e (min). T h e r a d i a t i o n dose given to the r a d i o g r a p h i c film is given b y : A D oc A exp ( - - 2 t ) dt = 0 2

f

~

c o u p l e d b y a relatively inefficient optical system to a n e u t r o n scintillator (NE 421) gave a visible p i c t u r e in a t h e r m a l flux of 3 X 106 n c m -2 sec -1 a c c o m p a n i e d by a g a m m a flux of 7 " 5 r h -1 ( I M W r e a c t o r power). A t 2 0 M W r e a c t o r power, i.e. in a flux of 6 × 107 n c m -2 sec -1 a n d in a g a m m a flux of 150 r h -1 the p i c t u r e was just as clear even t h o u g h s o m e w h a t brighter. F o r the p r o t o t y p e e x p e r i m e n t s a new i m a g e intensifier o f g a i n l0 s will be available. This should be c a p a b l e of v i e w i n g the scintillation p a t t e r n p r o d u c e d b y a flux o f 3 × 10 a n c m -2 sec -1 with ease, so the flux o f 2 × 1 0 4 n c m -2 sec -1 q u o t e d for t h e p r o t o t y p e unit will p r o d u c e a visible picture. T h e resolution possible with this flux, however, will not be g o o d since there will be only 800 neutrons passing t h r o u g h 1 c m 2 every 1/25 sec, i.e. one n e u t r o n event can occur on a v e r a g e every 1/25 sec in a n a r e a e q u a l to 1/28 c m a n d 1/28 c m a p p r o x i m a t e l y . T h e eye accepts television pictures at a rate of one every 1/25 sec so b y a n a l o g y the low flux is expected to resolve sizes d o w n to 1/28 cm. REFERENCES 1. HENDRY I. C. N e u t r o n R a d i o g r a p h y at Dounreay,

(3)

2.

Thus, D is c o m p l e t e l y d e t e r m i n e d b y A as long as the d y s p r o s i u m is allowed to d e c a y completely. A is k n o w n from c u r r e n t experience because, for a flux of 10 v n c m -2 sec -1, the i r r a d i a t i o n time is 1 rain. T h e limit of flux for t h e t e c h n i q u e is t h a t for w h i c h an infinite i r r a d i a t i o n will p r o d u c e a c t i v i t y A. L e t the present D . M . T . R . flux be 41, a n d the l i m i t i n g flux be 42 then, using e q u a t i o n (2):

3.

41{(1 - - e x p ( - - 5

4.

5. 6.

× l 0 - 3 X 1)} = 42{1 - -

exp ( - - co) } i.e. 42 = 5 × 10 a × 10 v = 5 × 1 0 4 n c m -2 see-1 T h e D . M . T . R . i m a g e intensifier experiment(4) showed t h a t an i m a g e intensifier of g a i n 10 a

7. 8. 9.

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