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Applications of solid state track recorder neutron dosimetry for fuel debris location in the three mile island unit 2 makeup and purification demineralizers
Nuclear Tracks a n d Radiation Measurements, Vol. 8, Nos. I-4, pp. 473-481, 1984. Printed in Great Britain.
0191-278X/84 $3.00 + .00 c 1984 Pergamon Press Ltd.
APPLICATIONS OF SOLID SLATE TRACK RECORDER NEUTRON DOSIMETRY FOR FUEL DEBRIS LOCATION IN THE IHREE MILE ISLAND UNIT 2 MAKEUP AND PURIFICATION DEMINERALIZERS F. H. Ruddy, J. H. Roberts, R. Gold, and C. C. Preston
Westinghouse Hanford Company Hanford Engineering Development Laboratory Richland, Washington 99352, U.S.A.
ABSTRACI As a result of the Three Mile Island Unit 2 (TMI-2) accident on March 28, 1979, fuel debris wa! dispersed into the primary coolant and a u x i l l i a r y systems of the reactor. The presence of fuel may be traced by using the neutron a c t i v i t y which is associated with the burn-in of higher actinides (about 300 neutrons/sec/kgU). Solid state track recorder (SSTR) neutron dosimetry is the most sensitive technique for measuring low neutron fluxes. Hence, neutron dosimetry is being performed at TMI-2 to locate fuel debris and subsequently aid the reactor recovery e f f o r t . Herein, the results of a scoping measurement on the fuel content of TMI-2 Makeup Demineralizer A are reported along with relevant c a l i b r a t i o n measurements. The t o t a l amount of fuel estimated in Demineraizer A, 1.7 kg, corresponds to a t o t a l neutron source of about 500 neutrons/sec. At the detector positions, data were obtained with neutron fluxes as low as TO-3 n/sec/cm2, demonstrating the extreme s e n s i t i v i t y of the SSTR method. KEYWORDS Solid State Irack Recorders, Mica, Reactor Dosimetry, Neutron Dosimetry, Fuel Tracing. INTRODUCIION As a part of the TMI-2 reactor recovery program, the quantities and locations of fuel debris that has been dispersed into the primary coolant and a u x i l l i a r y systems of the reactor must be determined. Two makeup and p u r i f i c a t i o n demineralizers, A and B, which maintain coolant water p u r i t y , were in operation at the time of the accident. Due to the high gammaray intensities in the location of these demineralizers, fuel was presumed to be located in the demineralizers As part of the Westinghouse Hanford Company TMI-2 Demineralizer Resin Removal Program, a measure of the amount of fuel debris in Demineralizer A was sought so that other phases of the ion exchange resin removal could proceed. This report describes the SSTR neutron dosimetry measurements that were carried out in the IMI-2 Demineralizer A Cubicle and the relevant c a l i b r a t i o n measurements that were made at the Hanford Engineering Development Laboratory. On the basis of these measurements and c a l i b r a tions, the amount of fuel present in TMI-2 Demineralizer A was estimated. TMI-2 MEASUREMENTS TMI-2 fuel has an estimaged average neutron specific a c t i v i t y of about 300 n/sec/kg (Vinjamuri et a l . , 1981; Gold et aT., 1983) which originates mainly from Pu buildup. Assuming that Pu is a good tracer for uranium, SSIR neutron dosimetry can be used to assess the location and quantity of fuel present. lhe use of SSTRs for fuel detection has been described previously (Gold et a l . , 1983). B r i e f l y , enriched 235U f o i l s are placed in firm contact with mica SSTRs and the neutroninduced fission fragments from 235U register a tracks in the mica. The track density in the mica can be used to deduce the fission density in the adjacent uranium f o i l and, with 473
F . H . RUDDY eta/.
474
a p p r o p r i a t e c a l i b r a t i o n d a t a , t h i s f i s s i o n r a t e can be used to determine the neutron f l u e n c e . The neutron f l u e n c e and d u r a t i o n o f the exposure of the SSTRs can then be used t o a s c e r t a i n the amount of f u e l p r e s e n t . SSTR neutron dosimeters were c o n s t r u c t e d as shown in Fiqure I . Two 3" x l " sheets of 0.004" t h i c k 93% enriched 235U were sandwiched between two pieces of mica and pressed in f i r m c o n t a c t a g a i n s t an aluminum support p l a t e between two 0.25" t h i c k pieces o f l u c i t e . The l u c i t e was used to enhance the neutron s i g n a l v i a the albedo e f f e c t which has been r e p o r t e d p r e v i o u s l y (Gold et a l . , 1983). The t o t a l SSTR area of t h i s neutron dosimeter is a p p r o x i m a t e l y 85 cm2.
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SSTR Neutron Dosimetry Holder f o r TMI-2 D e m i n e r a l i z e r A Measurements.
These dosimeters were assembled at TMI-2 i m m e d i a t e l y p r i o r to the exposure (Figure 2) t o reduce background due to cosmic ray neutron induced f i s s i o n and from spontaneous f i s s i o n o f t h e 238U in the uranium. Also c i r c u l a r ( l " d i a m e t e r ) CR-39 SSTRs were attached t o the o u t e r surface o f each d o s i m e t e r (see Figure 2) t o measure the high energy (E > ~0.5 MeV) neutron flux. (The CR-39 SSTRs have not y e t been a n a l y z e d . ) The arrangement of the TMI-2 D e m i n e r a l i z e r A and B Cubicles i s shown in Figure 3. Due t o t h e i n t e n s e gamma ray f i e l d s present near the d e m i n e r a l i z e r s , neutron dosimeters had to be placed r e m o t e l y from o u t s i d e the c u b i c l e . The D e m i n e r a l i z e r A Cubicle was accessible through penet r a t i o n #891 shown in Figure 3. A v e r t i c a l s t r i n g e r was prepared by f a s t e n i n g t o g e t h e r SSTR dosimeters at measured i n t e r v a l s using f i s h i n g l i n e . This s t r i n g e r was enclosed in p l a s t i c t u b i n g to p r o t e c t the dosimeters from c o n t a m i n a t i o n i n s i d e the d e m i n e r a l i z e r c u b i c l e . A h o r i z o n t a l set of dosimeters was prepared by a t t a c h i n g the dosimeters t o a pipe which was then enclosed in p l a s t i c t u b i n a . Both the h o r i z o n t a l and v e r t i c a l s t r i n g e r s were i n s e r t e d i n t o the c u b i c l e through p e n e t r a t i o n #891 as shown in Figure 4. The dosimeters were i n s e r t e d at I I : 5 5 p.m. on September 14, 1982, and occupied the p o s i t i o n s shown in Figure 5. The l o c a t i o n of the v e r t i c a l s t r i n g e r was confirmed d u r i n g a r o b o t e n t r y o f the c u b i c l e . The dosimeters were l e f t in place f o r t w e n t y nine days and removed on October 13, 1982, at 6:20 p.m. Some d i f f i c u l t i e s were encountered d u r i n g removal, r e s u l t i n g in a maximum u n c e r t a i n t y of 3% in the v e r t i c a l s t r i n g e r exposure time. In o r d e r t o measure the d e t e c t o r background, c o n t r o l dosimeters were assembled at the same time and exposed in a d e m i n e r a l i z e r c u b i c l e (D) where f u e l was not p r e s e n t . Assembly and disassembly of a l l of the dosimeters r e q u i r e d a few hours so t h a t background cosmic ray neutron exposure i s a p p r o x i m a t e l y the same f o r a l l d e t e c t o r s . A f t e r exposure, the SSTRs were t r a n s p o r t e d to HEDL where they were processed by etching w i t h 49% HF at room temperature f o r 90 minutes. The developed t r a c k s from s e l e c t e d dosimeters were manually counted w i t h the a i d o f a microscope. The measured 235U t r a c k d e n s i t i e s f o r t h i s exposure are aiven in Table I .
SOLID STATE TRACK RECORDER NEUTRON DOSIMETRY
475
= MH4
Fig. 2.
SSTR Neutron Dosimeters During Assembly. Mica and 235U f o i l s are wrapped in polyethylene bags in foreground. F u l l y assembled dosimeters are in background.
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2
MAKE-UP Et PURIFICATION DEMINERALIZERS
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RESULTS The background corrected SSTR 235U track densities from Table 1 are plotted in Figure 6. Shown f o r compar~§gn is a curve r e s u l t i n g from gamma scanning (McNeece et a l . , 1983) of the f i s s i o n product J~4Ce a c t i v i t y present in the demineralizer tank. Within experimental l i m i t a t i o n s , the positions of the two peaks are the same. i n d i c a t i n g the presence of fuel near the 309' e l e v a t i o n . The assumptions are made here that 144Ce and Pu are both c l o s e l y associated with the f u e l , r e s u l t i n g in fuel traceable gamma rays and neutrons, respectively. Although the background measurements give a track density of about 5 tracks/cm 2 due to cosmic r a d i a t i o n , the baseline f o r the measurements in the demineralizer cubicle is about I0 tracks/cm 2. The 5 tracks/cm 2 difference is due to room return neutrons r e s u l t i n g from thermalization of source neutrons in the walls of the cubicle. The use of t h i s room return signal to q u a n t i f y the t o t a l neutron source ( f u e l ) in the cubicle is described below. SSTR ROOMRETURN NEUTRONRESPONSE Whenever neutron dosimetry is conducted in a laboratory bounded by walls containing moderator materials (hydrogeneous concrete in the present case), the source neutrons w i l l be transported, scattered, and absorbed throughout the environment and p a r t i c u l a r l y in the walls i f
476
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Emplacement of SSTR Neutron Dosimeters--Measurement of Reference Point f o r Stringer Location.
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the dimensions of the laboratory are small. Room return neutrons are the last vestiges of neutrons o r i g i n a l l y emitted by the source, and, indeed, these neutrons have been scattered so often t h a t they have a t t a i n e d thermal e q u i l i b r i u m with t h e i r environment. They pervade the e n t i r e laboratory space l i k e a uniform homogeneous mist or fog. They r e t a i n no knowledqe of t h e i r o r i g i n with the exception of t h e i r i n t e n s i t y , which is proportional to the t o t a l emission rate of the source. A d e t a i l e d analysis of the room phenomenon has been given by White, 1966, who used d i f f u s i o n theory to describe experimental data obtained in a laboratory bounded by concrete w a l l s . According to t h i s analysis, the thermal neutron f l u x can be expressed as = C,SnlR 2
,
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SOLID STATE TRACK RECORDER NEUTRON DOSIMETRY
477
TABLE 1 Measured SSTR Track Densities at selected Locations Alon9 lhe Vertical Traverse (Data Taken in TMI-2 Demineralizer a Cubicle)
Measured Tracks/cm 2 Obs. 1 Obs. 2
Elevation* (Ft.)
Measured Tracks/cm 2 Avg.**- B. G.
317
II.I
6.1 + 2.0
312
26.1
21.1 + 2.8
311
25.2
27.0
21.1 + 2.7
310
30.I
30.6
25.3 + 3.2
309
36.8
36.1
31.5 + 4.2
308
13.3
12.4
7.9 + 1.7
307
I0.2
5.2 + 1.9
Demineralizer D Measured Background (B. G.)
4.8 5.3 (B. G. Average = 5.0 + I . I * * )
*The elevation corresponding to the bottom of the demineralizer tank is 307 f t . **Average of Observers l and 2.
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Track Density as a Function of A x i a l Location f o r SSTR Neutron Dosimeters Exposed in TMI-2 Demineralizer A Cubicle in Comparison w i t h 144Ce Gamma A c t i v i t y Obtained from Continuous Gamma Ray Spectrometry (See McNeece et a l . , 1983)
where Sn is the neutron emission o f the source in neutron/sec, C is a constant, and R is the e f f e c t i v e radius of the l a b o r a t o r y given by R-2 : I/6
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SSTR Neutron Dosimeter A x i a l Responses at a Radial Distance of 4' from a 252Cf Source. X Source Height 2' 0 Source Height 3 . 5 '
In a separate measurement, the i n t e n s i t y of the 252Cf source was c a l i b r a t e d and found to be (3.08 + 0.42) x 108 n/sec by measuring the induced f i s s i o n r a t e in a 238U-SSTR d o s i m e t e r . Since 7 3 8 U ( n , f ) is a t h r e s h o l d r e a c t i o n , the f i s s i o n r a t e w i l l c l o s e l y f o l l o w an inverse square law w i t h d i s t a n c e from the source. The response t o t h i s source o f an a l b e d o - t y p e dosimeter i d e n t i c a l t o t h a t used at IMI-2 D e m i n e r a l i z e r A is 74.8 + 7.5 tracks/cm2min. In an exposure of s l i g h t l y less than 29 days, a room r e t u r n response o f 5.2 + 1.9 t r a c k s / c m 2 was o b t a i n e d o r 1.26 x 10 -4 t r a c k s / c m 2 / m i n . This response corresponds to a T o t a l neutron source in the D e m i n e r a l i z e r A Cubicle of 517 n e u t r o n s / s e c o n d . I f an average neutron s p e c i f i c a c t i v i t y o f 300 n / s e c / k g i s assumed f o r the TMI-2 f u e l , an SSTR neutron source i n t e n s i t y d e r i v e d value o f f u e l d e b r i s of 1.72 + 0.63 kg is o b t a i n e d . l h i s SSTR e s t i m a t e is s u b j e c t to e r r o r from a number o f sources. The most i m p o r t a n t e f f e c t s t h a t have not been taken i n t o account are the a b s o r p t i o n and moderation of neutrons in the l a b o r a t o r y . These e f f e c t s are p a r t i c u l a r l y s i g n i f i c a n t when hydrogenous media e x i s t in the l a b o r a t o r y , as is the case f o r the D e m i n e r a l i z e r C u b i c l e . Neutron a b s o r p t i o n decreases the number of neutrons which a t t a i n thermal e q u i l i b r i u m in the c u b i c l e . On the o t h e r hand, moderation by a hydrogeneous medium produces a s o f t e r neutron spectrum i n c i d e n t upon the l a b o r a t o r y w a l l s , t h e r e b y i n c r e a s i n g neutron r e f l e c t i o n and the thermal neutron room r e t u r n flux. Consequently, these two e f f e c t s work in o p p o s i t e d i r e c t i o n s and tend t o cancel each o t h e r . Obviously, the s p e c i f i c l a b o r a t o r y environment and geometry w i l l p l a y a dominant r o l e in the r e l a t i v e w e i g h t i n g of these e f f e c t s . In o r d e r t o e x p l o r e the e f f e c t of having hydrogeneous media p r e s e n t , SSTR c a l i b r a t i o n measurements were done w i t h the 252Cf source suspended in the c u b i c l e described p r e v i o u s l y at a h e i g h t o f 2' at the surface of the water in a 2' high by 4' diameter t a n k . The r a d i a l d o s i m e t e r response data are shown in Figure I0. The response is less by a f a c t o r o f about two from t h a t shown in Figures 7 and 8, but i t is s t i l l f l a t . The decrease in i n t e n s i t y is due to t h e r m a l i z a t i o n and a b s o r p t i o n o f source neutrons in the water which subtends about 50% of the geometry o f the source. The a x i a l response d i s t r i b u t i o n at a d i s t a n c e o f 4' is shown in Figure I I . As in the corresponding a x i a l d i s t r i b u t i o n s o f Figure 9, no albedo devices were used. Below the l e v e l o f the w a t e r , the response is q u i t e low due t o neutron a b s o r p t i o n in the w a t e r . From 2' t o 5' the response i s reasonably f l a t and above 5' increases s l i g h t l y as the 8' c e i l i n g o f the c u b i c l e i s approached. This measurement was repeated using albedo dosimeters i d e n t i c a l t o those used in the measurements in the D e m i n e r a l i z e r A C u b i c l e , y i e l d i n g the data p l o t t e d in Figure 12. The shape o f t h i s albedo response is q u a l i t a t i v e l y s i m i l a r t o the a x i a l d i s t r i b u t i o n o f Figure 6 o b t a i n e d f o r D e m i n e r a l i z e r A. The peak t o room r e t u r n r a t i o o b t a i n e d in the d e m i n e r a l i z e r c u b i c l e i s much h i g h e r than the r a t i o o b t a i n e d in Figure 12, i n d i c a t i n g t h a t the HEDL c a l i b r a t i o n experiment o n l y q u a l i t a t i v e l y mocks up the d e m i n e r a l i z e r measurements. A d i r e c t c a l i b r a t i o n o f the peak dosimeter response i s not attempted here because o f u n c e r t a i n t i e s in the neutron spectrum and corresponding spectrum averaged cross s e c t i o n f o r the f i s s i o n response o f the d o s i m e t e r . On
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As in Figure I I but with Albedo Dosimeters of the Design Shown in Figure I.
479
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the basis of the c a l i b r a t i o n data obtained, the room return neutron estimate of the amount of fuel debris in TMI-2 Demineralizer A should be regarded as a lower l i m i t , with an upper l i m i t t h a t does not exceed t h i s value by more than a f a c t o r of two. SUMMARY lhe SSTR i n t e n s i t y d i s t r i b u t i o n in Figure 6 is asymmetric, decreasing less r a p i d l y above the 309' e l e v a t i o n than below. The explanation is t h a t the tank is dry above the 309' e l e v a t i o n and contains degraded resin and p o s s i b l y boronated water below t h i s l e v e l . The resin attenuates fuel neutrons below 309', whereas the slow f a l l o f f in i n t e n s i t y above 309' is due to the increase in distance between the SSTR neutron dosimeters and the neutron source. Independent Compton Recoil Gamma Ray Spectrometry measurements (McNeece et a l . , 1983) r e s u l t e d in a fuel estimate of 1.3 + 0.6 kg. This r e s u l t is in e x c e l l e n t agreement with the SSTR e s t i mate. Whereas SSTR evidence t h a t the d e m i n e r a l i z e r tank is dry above the 309' level provided guidance f o r reduction of the Compton-recoil spectrometer data, source d i s t r i b u t i o n data from these gamma ray measurements were used to guide the d i r e c t i o n and planning of the SSTR c a l i b r a t i o n measurement. Thus, the two methods provided independent but complimentary data. lhe track d e n s i t i e s observed in the SSTR neutron dosimeters correspond to extremely low neutron f l u x e s . The t o t a l neutron emission rate in the Demineralizer A Cubicle is about 500 neutrons per second, corresponding to a f l u x of less than 3 x 10-3 n/cm2/sec at the p o s i t i o n of the SSTR neutron dosimeter with the maximum s i g n a l . To our knowledge, SSTR neutron dosimetry is the only known method t h a t could be successfully applied in such f u e l debris q u a n t i f i c a t i o n experiments. Our conclusion t h a t the Demineralizer A tank was dry has been substantiated over six months l a t e r by gathering of samples from the d e m i n e r a l i z e r tank which showed t h a t the tank was dry. ACKNOWLEDGMENTS These measurements were undertaken as a r e s u l t of the i n t e r a c t i o n and exchange of ideas with a number of o r g a n i z a t i o n s ; among these are DOE, EPRI, GPU, NRC, EG&G, HEDL, and the Technical Assistance Advisory Group (TAAG) Committee f o r TMI-2 Recovery. Support and f u r t h e r encouragement f o r t h i s work was received through the WHC TMI-2 Demineralizer Resin Removal Program, under the d i r e c t i o n of Mike Mahaffey, of WHC. The assistance of Ken Lionarons and Ted Rekart of GPU, Herb Sanchez, of EG&G, and Bruce Kaiser, Jim McNeece, and B i l l Jenkins, of WHC, during placement and removal of the dosimeters at TMI-2 is g r a t e f u l l y acknowledged. L e s l i e Dunn, Jess Smart, Cesar Icayan, and Tom Cross of WHC provided track recorder scanning assistance. The a d d i t i o n a l support, encouragement, and/or t e c h n i c a l assistance provided by Fred L e i t z , Ben Hayward, Wally Sheely, B i l l McElroy, Parvin L i p p i n c o t t , F. Schmittroth, Don Doran, Herb Yoshikawa, and Ersel Evans of WHC, and Geoff Quinn of EG&G, are g r a t e f u l l y acknowledged. REFERENCES Gold R., Ruddy F. H., Roberts J. H., Preston C. C., Ulseth J. A., McElroy W. N., L e i t z F. J . , Hayward B. R., and Schmittroth F. A. (1983) A p p l i c a t i o n of s o l i d state track recorder neutron dosimetry f o r Three Mile Island Unit 2 r e a c t o r recovery. Nuclear Tracks, pp. 13-30, Pergamon, Oxford. McNeece J. P., Kaiser B. J . , Gold R., and Jenkins W. W. (1983) Fuel content of the Three Mile Island Unit 2 makeup d e m i n e r a l i z e r s . HEDL-7285. Vinjamuri K., Mclsaac C. V., B e l l e r L. S., Isaacson L . , Mandler J. W., and Hobbins R. R. Jr. (1981) Nondestructive techniques f o r assaying f u e l debris in piping at Three Mile Island Unit 2. General Public U t i l i t i e s , E l e c t r i c Power Research I n s t i t u t e , U.S. Nuclear Regulatory Commission and U.S. Department of Energy Report GEND-OI8. White P. H. (1966) The room scattered background produced by a neutron source in a l a b o r a t o r y with concrete w a l l s . Nucl. I n s t . Methods 39, pp. 256-
SOLID STATE TRACK RECORDER NEUTRON DOMIMETRY
481
where Di, i = 1 . . . . 6 are t h e s i x d i s t a n c e s from t h e source t o each w a l l o f t h e l a b o r a t o r y . A v a l u e o f R t h a t i s p e r t i n e n t t o t h e TMI-2 D e m i n e r a l i z e r A C u b i c l e can be d e r i v e d from t h e known dimensions o f the c u b i c l e and t h e a p p r o x i m a t e l o c a t i o n o f t h e f u e l as d e r i v e d from the Compton gamma r a y s p e c t r o m e t e r d a t a (McNeece e t a l . , 1983). These gamma-ray r e s u l t s show t h a t the source can be a p p r o x i m a t e l y r e p r e s e n t e d by a p o i n t source which l i e s two f e e t from the bottom o f the d e m i n e r a l i z e r t a n k and v e r y c l o s e t o t h e n o r t h s i d e o f the t a n k . On t h e b a s i s , one f i n d s a v a l u e o f a p p r o x i m a t e l y 4 . 2 7 ' f o r R. In o r d e r t o e x p e r i m e n t a l l y d e t e r m i n e the room r e t u r n response o f t h e SSTR d o s i m e t e r s , c a l i b r a t i o n s were c a r r i e d out in a c u b i c l e where s i m i l a r v a l u e s o f R would be o b t a i n e d . With a 252Cf source suspended a t a h e i g h t o f 2' in t h i s c u b i c l e , t h e c o r r e s p o n d i n g v a l u e o f R is 3 . 5 9 ' and w i t h t h e source a t 3 . 5 ' , the c o r r e s p o n d i n g v a l u e o f R i s 4 . 3 3 ' These two h e i g h t s b r a c k e t t h e e f f e c t i v e r a d i u s v a l u e f o r t h e TMI-2 D e m i n e r a l i z e r A C u b i c l e n e u t r o n s o u r c e . The r a d i a l response r e s u l t s f o r source h e i g h t s o f 2' and 3 . 5 ' are shown in F i g u r e s 7 and 8. Here, 235U d o s i m e t e r s w i t h no albedo d e v i c e s were used, and t h e response is seen t o be r o u g h l y c o n s t a n t f o r both cases w i t h the e x c e p t i o n o f the 2' r a d i a l measurement w i t h the source at 2' which corresponds t o t h e s m a l l e s t source t o d o s i m e t e r d i s t a n c e and t h e h i g h e s t response. A x i a l response d i s t r i b u t i o n s at a r a d i a l d i s t a n c e o f 4' are shown f o r t h e 2' and 3 . 5 ' source h e i g h t s in F i g u r e 9. In both cases, t h e response is f l a t and c o n s t a n t f o r both source h e i g h t s . A p p a r e n t l y , Equation I l l is not v a l i d f o r the small room dimensions addressed h e r e , and the c o n s t a n t response o f the two c a l i b r a t i o n cases must be a p p l i e d t o t h e aemineralizer data. D~IM£~R k~'SPONSE RAD]F4_ 0[STRIBUTION WITH T ~ 5 0 ~ E R't R ~ E ~ OF TV,IO ~ E T
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Response as a F u n c t i o n o f Radial L o c a t i o n f o r SSTR Neutron Dosimeters Exposed t o a 252Cf Source Suspended at 2' from t h e F l o o r in the Center o f a Concrete C u b i c l e w i t h O v e r a l l Dimensions 1 1 . 5 ' x 8' and a H e i g h t o f 8 ' .
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APPLICATIONS OF SOLID SLATE TRACK RECORDER NEUTRON DOSIMETRY FOR FUEL DEBRIS LOCATION IN THE IHREE MILE ISLAND UNIT 2 MAKEUP AND PURIFICATION DEMINERALIZERS F. H. Ruddy, J. H. Roberts, R. Gold, and C. C. Preston
Westinghouse Hanford Company Hanford Engineering Development Laboratory Richland, Washington 99352, U.S.A.
ABSTRACI As a result of the Three Mile Island Unit 2 (TMI-2) accident on March 28, 1979, fuel debris wa! dispersed into the primary coolant and a u x i l l i a r y systems of the reactor. The presence of fuel may be traced by using the neutron a c t i v i t y which is associated with the burn-in of higher actinides (about 300 neutrons/sec/kgU). Solid state track recorder (SSTR) neutron dosimetry is the most sensitive technique for measuring low neutron fluxes. Hence, neutron dosimetry is being performed at TMI-2 to locate fuel debris and subsequently aid the reactor recovery e f f o r t . Herein, the results of a scoping measurement on the fuel content of TMI-2 Makeup Demineralizer A are reported along with relevant c a l i b r a t i o n measurements. The t o t a l amount of fuel estimated in Demineraizer A, 1.7 kg, corresponds to a t o t a l neutron source of about 500 neutrons/sec. At the detector positions, data were obtained with neutron fluxes as low as TO-3 n/sec/cm2, demonstrating the extreme s e n s i t i v i t y of the SSTR method. KEYWORDS Solid State Irack Recorders, Mica, Reactor Dosimetry, Neutron Dosimetry, Fuel Tracing. INTRODUCIION As a part of the TMI-2 reactor recovery program, the quantities and locations of fuel debris that has been dispersed into the primary coolant and a u x i l l i a r y systems of the reactor must be determined. Two makeup and p u r i f i c a t i o n demineralizers, A and B, which maintain coolant water p u r i t y , were in operation at the time of the accident. Due to the high gammaray intensities in the location of these demineralizers, fuel was presumed to be located in the demineralizers As part of the Westinghouse Hanford Company TMI-2 Demineralizer Resin Removal Program, a measure of the amount of fuel debris in Demineralizer A was sought so that other phases of the ion exchange resin removal could proceed. This report describes the SSTR neutron dosimetry measurements that were carried out in the IMI-2 Demineralizer A Cubicle and the relevant c a l i b r a t i o n measurements that were made at the Hanford Engineering Development Laboratory. On the basis of these measurements and c a l i b r a tions, the amount of fuel present in TMI-2 Demineralizer A was estimated. TMI-2 MEASUREMENTS TMI-2 fuel has an estimaged average neutron specific a c t i v i t y of about 300 n/sec/kg (Vinjamuri et a l . , 1981; Gold et aT., 1983) which originates mainly from Pu buildup. Assuming that Pu is a good tracer for uranium, SSIR neutron dosimetry can be used to assess the location and quantity of fuel present. lhe use of SSTRs for fuel detection has been described previously (Gold et a l . , 1983). B r i e f l y , enriched 235U f o i l s are placed in firm contact with mica SSTRs and the neutroninduced fission fragments from 235U register a tracks in the mica. The track density in the mica can be used to deduce the fission density in the adjacent uranium f o i l and, with 473
F . H . RUDDY eta/.
474
a p p r o p r i a t e c a l i b r a t i o n d a t a , t h i s f i s s i o n r a t e can be used to determine the neutron f l u e n c e . The neutron f l u e n c e and d u r a t i o n o f the exposure of the SSTRs can then be used t o a s c e r t a i n the amount of f u e l p r e s e n t . SSTR neutron dosimeters were c o n s t r u c t e d as shown in Fiqure I . Two 3" x l " sheets of 0.004" t h i c k 93% enriched 235U were sandwiched between two pieces of mica and pressed in f i r m c o n t a c t a g a i n s t an aluminum support p l a t e between two 0.25" t h i c k pieces o f l u c i t e . The l u c i t e was used to enhance the neutron s i g n a l v i a the albedo e f f e c t which has been r e p o r t e d p r e v i o u s l y (Gold et a l . , 1983). The t o t a l SSTR area of t h i s neutron dosimeter is a p p r o x i m a t e l y 85 cm2.
× 1.25 × o.o6o}
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SSTR Neutron Dosimetry Holder f o r TMI-2 D e m i n e r a l i z e r A Measurements.
These dosimeters were assembled at TMI-2 i m m e d i a t e l y p r i o r to the exposure (Figure 2) t o reduce background due to cosmic ray neutron induced f i s s i o n and from spontaneous f i s s i o n o f t h e 238U in the uranium. Also c i r c u l a r ( l " d i a m e t e r ) CR-39 SSTRs were attached t o the o u t e r surface o f each d o s i m e t e r (see Figure 2) t o measure the high energy (E > ~0.5 MeV) neutron flux. (The CR-39 SSTRs have not y e t been a n a l y z e d . ) The arrangement of the TMI-2 D e m i n e r a l i z e r A and B Cubicles i s shown in Figure 3. Due t o t h e i n t e n s e gamma ray f i e l d s present near the d e m i n e r a l i z e r s , neutron dosimeters had to be placed r e m o t e l y from o u t s i d e the c u b i c l e . The D e m i n e r a l i z e r A Cubicle was accessible through penet r a t i o n #891 shown in Figure 3. A v e r t i c a l s t r i n g e r was prepared by f a s t e n i n g t o g e t h e r SSTR dosimeters at measured i n t e r v a l s using f i s h i n g l i n e . This s t r i n g e r was enclosed in p l a s t i c t u b i n g to p r o t e c t the dosimeters from c o n t a m i n a t i o n i n s i d e the d e m i n e r a l i z e r c u b i c l e . A h o r i z o n t a l set of dosimeters was prepared by a t t a c h i n g the dosimeters t o a pipe which was then enclosed in p l a s t i c t u b i n a . Both the h o r i z o n t a l and v e r t i c a l s t r i n g e r s were i n s e r t e d i n t o the c u b i c l e through p e n e t r a t i o n #891 as shown in Figure 4. The dosimeters were i n s e r t e d at I I : 5 5 p.m. on September 14, 1982, and occupied the p o s i t i o n s shown in Figure 5. The l o c a t i o n of the v e r t i c a l s t r i n g e r was confirmed d u r i n g a r o b o t e n t r y o f the c u b i c l e . The dosimeters were l e f t in place f o r t w e n t y nine days and removed on October 13, 1982, at 6:20 p.m. Some d i f f i c u l t i e s were encountered d u r i n g removal, r e s u l t i n g in a maximum u n c e r t a i n t y of 3% in the v e r t i c a l s t r i n g e r exposure time. In o r d e r t o measure the d e t e c t o r background, c o n t r o l dosimeters were assembled at the same time and exposed in a d e m i n e r a l i z e r c u b i c l e (D) where f u e l was not p r e s e n t . Assembly and disassembly of a l l of the dosimeters r e q u i r e d a few hours so t h a t background cosmic ray neutron exposure i s a p p r o x i m a t e l y the same f o r a l l d e t e c t o r s . A f t e r exposure, the SSTRs were t r a n s p o r t e d to HEDL where they were processed by etching w i t h 49% HF at room temperature f o r 90 minutes. The developed t r a c k s from s e l e c t e d dosimeters were manually counted w i t h the a i d o f a microscope. The measured 235U t r a c k d e n s i t i e s f o r t h i s exposure are aiven in Table I .
SOLID STATE TRACK RECORDER NEUTRON DOSIMETRY
475
= MH4
Fig. 2.
SSTR Neutron Dosimeters During Assembly. Mica and 235U f o i l s are wrapped in polyethylene bags in foreground. F u l l y assembled dosimeters are in background.
TMI
2
MAKE-UP Et PURIFICATION DEMINERALIZERS
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Isometric View of Makeup Demineralizer Cells A and B.
RESULTS The background corrected SSTR 235U track densities from Table 1 are plotted in Figure 6. Shown f o r compar~§gn is a curve r e s u l t i n g from gamma scanning (McNeece et a l . , 1983) of the f i s s i o n product J~4Ce a c t i v i t y present in the demineralizer tank. Within experimental l i m i t a t i o n s , the positions of the two peaks are the same. i n d i c a t i n g the presence of fuel near the 309' e l e v a t i o n . The assumptions are made here that 144Ce and Pu are both c l o s e l y associated with the f u e l , r e s u l t i n g in fuel traceable gamma rays and neutrons, respectively. Although the background measurements give a track density of about 5 tracks/cm 2 due to cosmic r a d i a t i o n , the baseline f o r the measurements in the demineralizer cubicle is about I0 tracks/cm 2. The 5 tracks/cm 2 difference is due to room return neutrons r e s u l t i n g from thermalization of source neutrons in the walls of the cubicle. The use of t h i s room return signal to q u a n t i f y the t o t a l neutron source ( f u e l ) in the cubicle is described below. SSTR ROOMRETURN NEUTRONRESPONSE Whenever neutron dosimetry is conducted in a laboratory bounded by walls containing moderator materials (hydrogeneous concrete in the present case), the source neutrons w i l l be transported, scattered, and absorbed throughout the environment and p a r t i c u l a r l y in the walls i f
476
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Fig. 4.
Emplacement of SSTR Neutron Dosimeters--Measurement of Reference Point f o r Stringer Location.
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the dimensions of the laboratory are small. Room return neutrons are the last vestiges of neutrons o r i g i n a l l y emitted by the source, and, indeed, these neutrons have been scattered so often t h a t they have a t t a i n e d thermal e q u i l i b r i u m with t h e i r environment. They pervade the e n t i r e laboratory space l i k e a uniform homogeneous mist or fog. They r e t a i n no knowledqe of t h e i r o r i g i n with the exception of t h e i r i n t e n s i t y , which is proportional to the t o t a l emission rate of the source. A d e t a i l e d analysis of the room phenomenon has been given by White, 1966, who used d i f f u s i o n theory to describe experimental data obtained in a laboratory bounded by concrete w a l l s . According to t h i s analysis, the thermal neutron f l u x can be expressed as = C,SnlR 2
,
(I)
SOLID STATE TRACK RECORDER NEUTRON DOSIMETRY
477
TABLE 1 Measured SSTR Track Densities at selected Locations Alon9 lhe Vertical Traverse (Data Taken in TMI-2 Demineralizer a Cubicle)
Measured Tracks/cm 2 Obs. 1 Obs. 2
Elevation* (Ft.)
Measured Tracks/cm 2 Avg.**- B. G.
317
II.I
6.1 + 2.0
312
26.1
21.1 + 2.8
311
25.2
27.0
21.1 + 2.7
310
30.I
30.6
25.3 + 3.2
309
36.8
36.1
31.5 + 4.2
308
13.3
12.4
7.9 + 1.7
307
I0.2
5.2 + 1.9
Demineralizer D Measured Background (B. G.)
4.8 5.3 (B. G. Average = 5.0 + I . I * * )
*The elevation corresponding to the bottom of the demineralizer tank is 307 f t . **Average of Observers l and 2.
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Track Density as a Function of A x i a l Location f o r SSTR Neutron Dosimeters Exposed in TMI-2 Demineralizer A Cubicle in Comparison w i t h 144Ce Gamma A c t i v i t y Obtained from Continuous Gamma Ray Spectrometry (See McNeece et a l . , 1983)
where Sn is the neutron emission o f the source in neutron/sec, C is a constant, and R is the e f f e c t i v e radius of the l a b o r a t o r y given by R-2 : I/6
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In a separate measurement, the i n t e n s i t y of the 252Cf source was c a l i b r a t e d and found to be (3.08 + 0.42) x 108 n/sec by measuring the induced f i s s i o n r a t e in a 238U-SSTR d o s i m e t e r . Since 7 3 8 U ( n , f ) is a t h r e s h o l d r e a c t i o n , the f i s s i o n r a t e w i l l c l o s e l y f o l l o w an inverse square law w i t h d i s t a n c e from the source. The response t o t h i s source o f an a l b e d o - t y p e dosimeter i d e n t i c a l t o t h a t used at IMI-2 D e m i n e r a l i z e r A is 74.8 + 7.5 tracks/cm2min. In an exposure of s l i g h t l y less than 29 days, a room r e t u r n response o f 5.2 + 1.9 t r a c k s / c m 2 was o b t a i n e d o r 1.26 x 10 -4 t r a c k s / c m 2 / m i n . This response corresponds to a T o t a l neutron source in the D e m i n e r a l i z e r A Cubicle of 517 n e u t r o n s / s e c o n d . I f an average neutron s p e c i f i c a c t i v i t y o f 300 n / s e c / k g i s assumed f o r the TMI-2 f u e l , an SSTR neutron source i n t e n s i t y d e r i v e d value o f f u e l d e b r i s of 1.72 + 0.63 kg is o b t a i n e d . l h i s SSTR e s t i m a t e is s u b j e c t to e r r o r from a number o f sources. The most i m p o r t a n t e f f e c t s t h a t have not been taken i n t o account are the a b s o r p t i o n and moderation of neutrons in the l a b o r a t o r y . These e f f e c t s are p a r t i c u l a r l y s i g n i f i c a n t when hydrogenous media e x i s t in the l a b o r a t o r y , as is the case f o r the D e m i n e r a l i z e r C u b i c l e . Neutron a b s o r p t i o n decreases the number of neutrons which a t t a i n thermal e q u i l i b r i u m in the c u b i c l e . On the o t h e r hand, moderation by a hydrogeneous medium produces a s o f t e r neutron spectrum i n c i d e n t upon the l a b o r a t o r y w a l l s , t h e r e b y i n c r e a s i n g neutron r e f l e c t i o n and the thermal neutron room r e t u r n flux. Consequently, these two e f f e c t s work in o p p o s i t e d i r e c t i o n s and tend t o cancel each o t h e r . Obviously, the s p e c i f i c l a b o r a t o r y environment and geometry w i l l p l a y a dominant r o l e in the r e l a t i v e w e i g h t i n g of these e f f e c t s . In o r d e r t o e x p l o r e the e f f e c t of having hydrogeneous media p r e s e n t , SSTR c a l i b r a t i o n measurements were done w i t h the 252Cf source suspended in the c u b i c l e described p r e v i o u s l y at a h e i g h t o f 2' at the surface of the water in a 2' high by 4' diameter t a n k . The r a d i a l d o s i m e t e r response data are shown in Figure I0. The response is less by a f a c t o r o f about two from t h a t shown in Figures 7 and 8, but i t is s t i l l f l a t . The decrease in i n t e n s i t y is due to t h e r m a l i z a t i o n and a b s o r p t i o n o f source neutrons in the water which subtends about 50% of the geometry o f the source. The a x i a l response d i s t r i b u t i o n at a d i s t a n c e o f 4' is shown in Figure I I . As in the corresponding a x i a l d i s t r i b u t i o n s o f Figure 9, no albedo devices were used. Below the l e v e l o f the w a t e r , the response is q u i t e low due t o neutron a b s o r p t i o n in the w a t e r . From 2' t o 5' the response i s reasonably f l a t and above 5' increases s l i g h t l y as the 8' c e i l i n g o f the c u b i c l e i s approached. This measurement was repeated using albedo dosimeters i d e n t i c a l t o those used in the measurements in the D e m i n e r a l i z e r A C u b i c l e , y i e l d i n g the data p l o t t e d in Figure 12. The shape o f t h i s albedo response is q u a l i t a t i v e l y s i m i l a r t o the a x i a l d i s t r i b u t i o n o f Figure 6 o b t a i n e d f o r D e m i n e r a l i z e r A. The peak t o room r e t u r n r a t i o o b t a i n e d in the d e m i n e r a l i z e r c u b i c l e i s much h i g h e r than the r a t i o o b t a i n e d in Figure 12, i n d i c a t i n g t h a t the HEDL c a l i b r a t i o n experiment o n l y q u a l i t a t i v e l y mocks up the d e m i n e r a l i z e r measurements. A d i r e c t c a l i b r a t i o n o f the peak dosimeter response i s not attempted here because o f u n c e r t a i n t i e s in the neutron spectrum and corresponding spectrum averaged cross s e c t i o n f o r the f i s s i o n response o f the d o s i m e t e r . On
SOLID STATE TRACK R E C O R D E R N E U T R O N D O M I M E T R Y
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AXIAL r I ; G I O N ~ DI£1R[eUTION f l ; f l RRDII~I- DlSTflNC~ OF 4 r~v ~ m c~-zs~ SOLmC~ eT TW[ eU~e~:E o f 2 r t E x o r W~T~R
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SSTR Neutron Dosimeter Axial Response at a Radial Distance of 4' to a 252Cf Source Suspended at the Center of the Surface of a 2' High by 4' Diameter Tank of Water.
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As in Figure I I but with Albedo Dosimeters of the Design Shown in Figure I.
479
480
F . H . R U D D Y et al.
the basis of the c a l i b r a t i o n data obtained, the room return neutron estimate of the amount of fuel debris in TMI-2 Demineralizer A should be regarded as a lower l i m i t , with an upper l i m i t t h a t does not exceed t h i s value by more than a f a c t o r of two. SUMMARY lhe SSTR i n t e n s i t y d i s t r i b u t i o n in Figure 6 is asymmetric, decreasing less r a p i d l y above the 309' e l e v a t i o n than below. The explanation is t h a t the tank is dry above the 309' e l e v a t i o n and contains degraded resin and p o s s i b l y boronated water below t h i s l e v e l . The resin attenuates fuel neutrons below 309', whereas the slow f a l l o f f in i n t e n s i t y above 309' is due to the increase in distance between the SSTR neutron dosimeters and the neutron source. Independent Compton Recoil Gamma Ray Spectrometry measurements (McNeece et a l . , 1983) r e s u l t e d in a fuel estimate of 1.3 + 0.6 kg. This r e s u l t is in e x c e l l e n t agreement with the SSTR e s t i mate. Whereas SSTR evidence t h a t the d e m i n e r a l i z e r tank is dry above the 309' level provided guidance f o r reduction of the Compton-recoil spectrometer data, source d i s t r i b u t i o n data from these gamma ray measurements were used to guide the d i r e c t i o n and planning of the SSTR c a l i b r a t i o n measurement. Thus, the two methods provided independent but complimentary data. lhe track d e n s i t i e s observed in the SSTR neutron dosimeters correspond to extremely low neutron f l u x e s . The t o t a l neutron emission rate in the Demineralizer A Cubicle is about 500 neutrons per second, corresponding to a f l u x of less than 3 x 10-3 n/cm2/sec at the p o s i t i o n of the SSTR neutron dosimeter with the maximum s i g n a l . To our knowledge, SSTR neutron dosimetry is the only known method t h a t could be successfully applied in such f u e l debris q u a n t i f i c a t i o n experiments. Our conclusion t h a t the Demineralizer A tank was dry has been substantiated over six months l a t e r by gathering of samples from the d e m i n e r a l i z e r tank which showed t h a t the tank was dry. ACKNOWLEDGMENTS These measurements were undertaken as a r e s u l t of the i n t e r a c t i o n and exchange of ideas with a number of o r g a n i z a t i o n s ; among these are DOE, EPRI, GPU, NRC, EG&G, HEDL, and the Technical Assistance Advisory Group (TAAG) Committee f o r TMI-2 Recovery. Support and f u r t h e r encouragement f o r t h i s work was received through the WHC TMI-2 Demineralizer Resin Removal Program, under the d i r e c t i o n of Mike Mahaffey, of WHC. The assistance of Ken Lionarons and Ted Rekart of GPU, Herb Sanchez, of EG&G, and Bruce Kaiser, Jim McNeece, and B i l l Jenkins, of WHC, during placement and removal of the dosimeters at TMI-2 is g r a t e f u l l y acknowledged. L e s l i e Dunn, Jess Smart, Cesar Icayan, and Tom Cross of WHC provided track recorder scanning assistance. The a d d i t i o n a l support, encouragement, and/or t e c h n i c a l assistance provided by Fred L e i t z , Ben Hayward, Wally Sheely, B i l l McElroy, Parvin L i p p i n c o t t , F. Schmittroth, Don Doran, Herb Yoshikawa, and Ersel Evans of WHC, and Geoff Quinn of EG&G, are g r a t e f u l l y acknowledged. REFERENCES Gold R., Ruddy F. H., Roberts J. H., Preston C. C., Ulseth J. A., McElroy W. N., L e i t z F. J . , Hayward B. R., and Schmittroth F. A. (1983) A p p l i c a t i o n of s o l i d state track recorder neutron dosimetry f o r Three Mile Island Unit 2 r e a c t o r recovery. Nuclear Tracks, pp. 13-30, Pergamon, Oxford. McNeece J. P., Kaiser B. J . , Gold R., and Jenkins W. W. (1983) Fuel content of the Three Mile Island Unit 2 makeup d e m i n e r a l i z e r s . HEDL-7285. Vinjamuri K., Mclsaac C. V., B e l l e r L. S., Isaacson L . , Mandler J. W., and Hobbins R. R. Jr. (1981) Nondestructive techniques f o r assaying f u e l debris in piping at Three Mile Island Unit 2. General Public U t i l i t i e s , E l e c t r i c Power Research I n s t i t u t e , U.S. Nuclear Regulatory Commission and U.S. Department of Energy Report GEND-OI8. White P. H. (1966) The room scattered background produced by a neutron source in a l a b o r a t o r y with concrete w a l l s . Nucl. I n s t . Methods 39, pp. 256-
SOLID STATE TRACK RECORDER NEUTRON DOMIMETRY
481
where Di, i = 1 . . . . 6 are t h e s i x d i s t a n c e s from t h e source t o each w a l l o f t h e l a b o r a t o r y . A v a l u e o f R t h a t i s p e r t i n e n t t o t h e TMI-2 D e m i n e r a l i z e r A C u b i c l e can be d e r i v e d from t h e known dimensions o f the c u b i c l e and t h e a p p r o x i m a t e l o c a t i o n o f t h e f u e l as d e r i v e d from the Compton gamma r a y s p e c t r o m e t e r d a t a (McNeece e t a l . , 1983). These gamma-ray r e s u l t s show t h a t the source can be a p p r o x i m a t e l y r e p r e s e n t e d by a p o i n t source which l i e s two f e e t from the bottom o f the d e m i n e r a l i z e r t a n k and v e r y c l o s e t o t h e n o r t h s i d e o f the t a n k . On t h e b a s i s , one f i n d s a v a l u e o f a p p r o x i m a t e l y 4 . 2 7 ' f o r R. In o r d e r t o e x p e r i m e n t a l l y d e t e r m i n e the room r e t u r n response o f t h e SSTR d o s i m e t e r s , c a l i b r a t i o n s were c a r r i e d out in a c u b i c l e where s i m i l a r v a l u e s o f R would be o b t a i n e d . With a 252Cf source suspended a t a h e i g h t o f 2' in t h i s c u b i c l e , t h e c o r r e s p o n d i n g v a l u e o f R is 3 . 5 9 ' and w i t h t h e source a t 3 . 5 ' , the c o r r e s p o n d i n g v a l u e o f R i s 4 . 3 3 ' These two h e i g h t s b r a c k e t t h e e f f e c t i v e r a d i u s v a l u e f o r t h e TMI-2 D e m i n e r a l i z e r A C u b i c l e n e u t r o n s o u r c e . The r a d i a l response r e s u l t s f o r source h e i g h t s o f 2' and 3 . 5 ' are shown in F i g u r e s 7 and 8. Here, 235U d o s i m e t e r s w i t h no albedo d e v i c e s were used, and t h e response is seen t o be r o u g h l y c o n s t a n t f o r both cases w i t h the e x c e p t i o n o f the 2' r a d i a l measurement w i t h the source at 2' which corresponds t o t h e s m a l l e s t source t o d o s i m e t e r d i s t a n c e and t h e h i g h e s t response. A x i a l response d i s t r i b u t i o n s at a r a d i a l d i s t a n c e o f 4' are shown f o r t h e 2' and 3 . 5 ' source h e i g h t s in F i g u r e 9. In both cases, t h e response is f l a t and c o n s t a n t f o r both source h e i g h t s . A p p a r e n t l y , Equation I l l is not v a l i d f o r the small room dimensions addressed h e r e , and the c o n s t a n t response o f the two c a l i b r a t i o n cases must be a p p l i e d t o t h e aemineralizer data. D~IM£~R k~'SPONSE RAD]F4_ 0[STRIBUTION WITH T ~ 5 0 ~ E R't R ~ E ~ OF TV,IO ~ E T
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