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Beryllium-filtered neutron beam quality improvement at a pool reactor through core element rearrangements
NUCLEAR
INSTRUMENTS
AND METHODS
65 ( I 9 6 8 ) I 2 5 - [ 3 [ ;
© NORTH-HOLLAND
PUBLISHING
CO.
B E R Y L L I U M - F I L T E R E D N E U T R O N BEAM QUALITY I M P R O V E M E N T
AT A P O O L R E A C T O R T H R O U G H CORE E L E M E N T R E A R R A N G E M E N T S J. J. A N T A L a n d A. A. W A R N A S
Army Materials and Mechanics Research Center, Watertown, Massachusetts 02172, U.S.A. Received 18 J u n e 1968
N e u t r o n spectra in the region o f 4 to 7.5 ~ f r o m a radial b e a m tube have been obtained for various fuel a n d BeO reflector a r r a n g e m e n t s in the A r m y Materials Research Reactor. Intensity a n d signal-to-noise ratio variations in the range of 15 % to 2 0 ~ have been obtained t h r o u g h r e a r r a n g e m e n t o f core elements
immediately adjacent to the b e a m tube termination. In general, the results indicate that the experiment external to the reactor s h o u l d view reflector element material only at the core face, a n d this reflector material s h o u l d be s u r r o t m d e d by fuel for the m o s t favorable intensity a n d signal-to-noise ratio combination.
1. Introduction Although the distribution of neutrons in the vicinity of the reactor core is well known for the purposes of reactor operation, the details of flux distribution as it affects the available neutron beam at a particular experimental port are known usually only in a general way. At the time of installation of the first experimental equipment at a beam port of the Army Materials Research Reactor (AMRR)I), considerable discussion ensued regarding the optimum arrangement of fuel and reflector elements for a particular experiment. It became apparent that some quantitative measurements were desirable to settle the matter. The core of most water pool research reactors is constructed with fuel and reflector elements which plug into interchangeable positions on a grid plate. It is possible thereby to arrange the fuel and reflector elements in a variety of configurations to provide for more uniform burn-up of the fuel and possibly to provide special flux and spectral distributions for particular beam experiments. It is this latter possibility which is the subject of this report.
core face to an area approximately 9 cm horizontally and 23 cm vertically. The beam tube employed, HU-1 in fig. 1, was not ideal in that it did not extend to the reactor core face. A volume of water having a 15.2 cm dimension along the beam path separated the end of the beam tube from the core. The water was displaced by the insertion of an aluminum can which served as a void, but the additional aluminum windows produced strong "absorption" profiles in the intensity spectra measured. The experimental data consists primarily of intensity spectra for neutrons issuing from the beam port in the HU-13
9
NOVEMBER 1968
7
FuelFuel
HU-I
2. Experimental arrangement The beam issuing from radial beam tube HU-I of the A M R R was examined with the neutron analyzer previously installed. This instrument is a crystal spectrometer employing a 20.3 cm bismuth filter and a 25.4 cm beryllium filter for the suppression of fast neutrons, gamma rays and second-order thermal neutrons. The filters remove essentially all of the thermal neutrons with wavelengths less than 4 • (5 meV), which allows the crystal spectrometer to analyze the neutron beam through a range of 4-8 /k with the use of a magnetite monochromator. This is a region which is important to many recent scattering experiments on solids and liquids. The collimator limited the view of the reactor
8
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C6
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3 2
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B
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D
E
F
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Fig. 1. Location of core elements a n d b e a m tube HU-1 in the A M R R (top view). Only elements in the n e i g h b o r h o o d o f HU-1 have been designated. C4, C5 a n d C6 are element positions of greatest interest. Fuel element R R contains a regulating rod. T h e core elements have 7.62 c m x 7.62 cm d i m e n s i o n s in this view. 125
126
J. J. A N T A L
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A. A. W A R N A S
3. Data analysis
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Fig. 2. Typical strip chart record of the monitor count rate showing the average "one M W " power level, line A-A, as determined from the average monitor count rate in the region of the chart record where the gross count was taken. The prominent discontinuities in the record are traceable to manual manipulation of the reactor controls.
wavelength region from 3.4-7.4 A. Obtaining spectra allowed a search for possible effects as a function of wavelength. Spectra were obtained for various fuel and BeO reflector element arrangements in the immediate vicinity of the beam tube termination, especially for those positions designated as C4, C5 and C6 in fig. 1. The elements were arranged in a particular configuration each morning before reactor startup, and reactor operation was not disturbed during its normal 14-h schedule. The monitor was a parallel plate BF 3 detector which intercepted a constant fraction of the whole beam area incident upon the monochromator. The analyzer detector received neutrons of wavelength 2 = 9.665 sin 0 from the F%O4 (111) monochromator, and background spectra were obtained with this crystal rocked off by 3° in 0. Four items of data were recorded during each spectral determination: 1. The analyzer gross count accumulated during a fixed time T (usually 100 sec) at a particular angle 0. 2. The analyzer background count accumulated during a time T at the same angle 0. 3. The monitor count accumulated during each gross and background counting period. 4. A continuous record of the monitor count rate.
A particular difficulty with this type of measurement is the lack of a suitable neutron intensity reference level or an associated precisely reproducible reactor power level to which data taken on various days can be normalized. The steady-state power level for the reactor is determined by reactor operations personnel from a measurement of the rate of removal of heat from the system (thermal power). This data is presented to the reactor operator on a strip chart recorder which he uses as a guide to place the reactor at the proper level for the day. The ability of various operators to reproduce this power level from day to day is the limiting accuracy of our measurements. A study of the continuous reactor power level records over a period of several weeks indicates that the power deviates from an average level considered to be one megawatt by ___2% maximum. The daily record of monitor count rate showed significant variations and the recorded monitor count was used to correct the analyzer counter data collected during a single day's run. A typical record of monitor count rate is shown in fig. 2. The abrupt changes in count rate were in all cases related to manual operation of the reactor control. They were accentuated by the presence of a regulating control rod in a fuel element near the beam tube termination. The position of this rod is noted as " R R " in fig. 1. The region of the monitor count rate chart record which contained a steady level over the longest period of time was considered to be the best value for the average power level on a particular day. Although this level was selected by visual examination, the actual correction applied to all analyzer count data was determined from the digital record of monitor count accumulated during the corresponding period. The individual analyzer gross and background intensities were multiplied by the fraction: Steady level monitor count f' =
Individual monitor count
to correct for daily variations in reactor power level. The resulting normalized background count was subtracted point for point from the normalized gross count to obtain the net count which is plotted in figs. 3, 4 and 7. The signal-to-noise ratio was computed at each point as defined by the following expression: signal-to-noise ratio = S I N = (analyzer net count)/(unittime) (analyzer background count)/(unit time) This data is plotted in figs. 5, 6 and 7.
BERYLLIUM-FILTERED
NEUTRON
QUALITY
127
IMPROVEMENT
desire, and the intensity of t h e ~ b e a m is usually of primary importance. On the other hand, experiments involving several scatterings, or w o r k with neutrons of exceptionally long wavelength, usually have such low intensities at the detector that the background intensity is comparable or larger in magnitude than the signal. The signal-to-noise ratio n o w becomes an important factor since it will determine if the effect to be measured is in fact measurable. For these reasons both signal and signal-to-noise data are presented here. It should be noted that the detector e m p l o y e d in these studies was a 1 m m thick 6Li-loaded glass scintillator NE905, attached to an E M I 9356B photomultiplier tube. This neutron detector is very efficient in the wavelength region of these studies, but it is also s o m e w h a t sensitive
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Fig. 3. Net intensity spectra for four different core element arrangements which give almost identical intensity magnitudes. The point symbols are identified with their associated core element arrangements in the inset, and points relating to a particular spectrum are interconnected with straight lines for visual clarity. The standard deviation of the ordinate at the points is less than one-half the symbol height. F = fuel element, R = reflector element.
The chart record of m o n i t o r count rate was carefully m o n i t o r e d each day, and any unusual steady-state level reached by the reactor was cause for deleting f r o m consideration all data taken during that day's operation. It is expected therefore, that the several spectra presented in the figures can be compared on an absolute intensity basis within the accuracy of the reactor power level setting described earlier. 4. Discussion of data The b e a m intensity and signal-to-noise ratio are two inter-related factors in neutron b e a m detection which are of primary importance to the performance ot a neutron scattering experiment. In general, neutron beams are never as intense as experimenters would
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Fig. 4. Net intensity spectra for three core element arrangements which show significant differences from those of fig. 3, with one of the spectra of fig. 3, F-R-F-, repeated here for comparison. The point symbols are identified with their associated core element arrangements in the inset, and points relating to a particular spectrum are interconnected with straight lines for visual clarity. The standard deviation of each point is less than one-half the symbol height. F = fuel element, R = reflector element.
128
J. J. A N T A L A N D A. A. W A R N A S
Integral intensity and Arrangement
SIN
Integral net intensity (arbitrary units)
TABLE 1 ratio values for the core element arrangements investigated. Fractional diff.* relative to R-R-R
SIN ratio
Integral
Fractional diff.* relative to R-R-R
F-R-F R-R-F F-F-R R-F-F F-F-F R-F-R R-R-R
445 440 439 436 420 401 391
331 900 296 246 088 570 393
0,138 0.126 0.122 0. l 15 0.073 0.026 0.000
596.33 586.40 533.27 532.30 534.67 526.65 627.61
~- 0.050 - 0.066 -0.150 - 0.152 - 0.148 -0.161 - 0.000
F-F-Ft R-R-Rt
586 830 519 639
0.129 0.000
325.19 419.65
-0.255 - 0.000
* Fractional diff. = [ f ( x x x ) - - f ( R R R ) ] / f ( R R R ) , wheref(xxx) is the integral intensity or SIN ratio for a particular arrangement x-x-x. t Values for a different experimental setup (see text). Fractional diff. is c o m p u t e d relative to R R R t .
to gamma radiation and thereby operates with lower SIN ratios than would be expected with the more usual BF 3 counter. The intensity data presented in figs. 3 and 4 are the most reliable runs obtained and represent a wide variation in fuel and reflector element arrangements in the immediate vicinity of the beam tube termination. It was learned early in the experiment that the variations in neutron intensity were not large even for the most extreme change in core arrangement, and that core element changes more distant from the beam tube termination than the first row were relatively unimportant.The study was therefore narrowed to the three core element positions labelled C4, C5 and C6 in fig. 1. Because the measured beam intensity changes were small and the problem of normalization to a constant reactor power level was present, another experiment was desired to establish a higher level of confidence in the data obtained. Two additional spectra were obtained some time later after several elements of the spectrometer arrangement had been altered sufficiently that a totally new region of beam and background parameters would be investigated with a higher degree of statistical accuracy. The alterations resulted in a higher beam intensity since the reactor power had been increased to 2 MW and the beryllium filter reduced in length from 25.4 to 15.2 cm, and the background was significantly higher (as reflected in the smaller SIN ratios) because of these changes and alterations in the shielding of the spectrometer. The spectra in fig. 7 are those obtained under these conditions. They show significantly larger changes in intensity and SIN ratio than the equivalent rearrangement in the earlier data. It would appear that the presence of more neutrons in
the spectrometer environment of energies greater than those in the beryllium-filtered region make attention to core element rearrangements even more important. This data is of sufficiently high statistical accuracy to allow an attempt to study the wavelength dependence of the intensity changes, as discussed later. In all spectra shown in the figures, the individual points have been connected with straight lines to provide a visual guide. Many deviations from a smooth curve are seen in the spectra which lepresent the abrupt change in scattering cross section at the Bragg cutoffs in the material of the aluminum windows, the BeO reflector elements, and particularly in the large polycrystalline bismuth filter. The point-to-point resolution is not great enough in these data to outline the cutoffs clearly. A more precise quantitative measure of the intensity and SIN ratio changes found is obtained from the integral data presented in table t. This data was obtained from a summation of the individual ordinates between 4.1 ~ and 6.8 ~. The net intensity spectra for four of the core element arrangements was practically coincident, as detailed in fig. 3, although the signal-to-noise ratios for the same arrangement were not coincident, fig. 5. Superior SIN ratios were obtained with a BeO reflector element in the central position, C5, with an F-R-F arrangement being slightly better than an R-R-F arrangement. Three other core arrangements gave varying intensities at a particular wavelength and, as shown in fig. 4, each was inferior to those of fig. 3. The SIN ratios for these arrangements are shown in fig. 6, where again an arrangement with a central element of BeO was superior, although the R-R-R arrangement was slightly better than the F-R-F arrangement.
BERYLLIUM-FILTERED
NEUTRON
129
IMPROVEMENT
5. Optimum element arrangements
80
It is possible to obtain high intensity combined with high SIN ratio through the use of the F-R-F arrangement. The epithermal and fast neutron component of the beam which contributes to background appears to be reduced by the shielding effect of moderating material in the central-element position. At the same time, the moderating efficiency of the central element appears to be high when closely coupled to the surrounding fuel. One can go a step further in light of the results above and replace with fuel the BeO in the upper and lower portions of the central reflector element which are not viewed by a beam hole experiment to further feed the central element "source" and increase the intensity. This particular arrangement has been investigated by Eliot et al. 2) at the Naval Research Laboratory
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F i g . 5. Signal-to-noise ratio spectra for the four core element arrangements o f fig. 3. The statistical error is almost identical for
each spectrum, but is indicated for only one spectrum. The data points for a particular spectrum have been interconnected with straight lines for visual clarity. F = fuel element, R = reflector element.
It was not unexpected to find that the use of three moderator elements as a source gave the best SIN ratio and the lowest beam intensity. It is interesting to note, however, that the opposite arrangement did not produce the opposite effect. The beam intensity from the three fuel element arrangement was only moderately high, although the SIN ratio was one of the poorest. With a pool reactor where interchange of core elements is not allowed either because of reactivity requirements or as a matter of policy, the beam source situation is represented by these two arrangements. The R-R-R arrangement represents that of a fully reflected core, while the F-F-F arrangement represents that of a core reflected only by pool water. Neither situation appears to be optimum for subthermal neutron beams. In this and the following discussions, one should remember that the spectrometer viewed the central element primarily, as noted in section 2 above. Application of these results to other situations may require modification of the comments listed here.
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Fig. 6. Signal-to-noise ratio spectra for the cove element arrangements of fig. 4. T h e F - R - F data is that of fig. 5 and is repeated here for comparison. The statistical error along the ordinate is the same as that of fig. 5. The data points for a particular spectrum have been interconnected with straight lines for visual clarity. F = fuel element, R = reflector element.
130
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Fig. 7. Net intensity and SIN ratio spectra for two core element arrangements of fig. 3 taken under a new set of experimental conditions. The standard deviation of each point is less than the symbol height, and the points have been interconnected with straight lines for visual clarity. The strong fluctuations in the spectra are the result of Bragg scattering in the filters and appear to be emphasized in these curves relative to the earlier data because of the lesser Be filter length employed. The two lower curves are the signal-to-noise ratio spectra for the same core element arrangements. Reactor. They have employed a "split" fuel element, which simply had no fuel in the center region of an otherwise n o r m a l fuel element. W h e n they replaced a s t a n d a r d fuel element in the central element location with a "split" fuel element, a 13% increase in intensity of thermal n e u t r o n s in the range 0.40 A to 2.9 ,~ was observed. A similar increase should have been observed in the s u b t h e r m a l region. It is difficult to make a direct c o m p a r i s o n since their "source" was a b o u t ½ A1 and 2 H / O by v o l u m e a n d ours was wholly BeO, b u t we estimate that a further increase in intensity of a b o u t 16% would be expected for the F - R - F a r r a n g e m e n t if a " s p l i t " fuel element were placed in our central position. They also f o u n d a decrease in fast n e u t r o n flux (greater t h a n 10 eV) of a b o u t a factor of 2 when a n o r m a l
A. A. W A R N A S
element was replaced by a "split" element. O u r results indicate a m u c h smaller decrease in fast n e u t r o n b e a m intensity when a fuel element is replaced by a BeO reflector element, but the scattering power and absorption in BeO is roughly ¼ that of H 2 0 for equal volumes in this region of n e u t r o n energies, a n d water would be expected to have a better fast n e u t r o n shielding effect. U n f o r t u n a t e l y , we were unable to o b t a i n a reliable r u n with water in the central element position to check this before the experimental system had to be disassembled. It is interesting to note we have also confirmed the Naval Research L a b o r a t o r y finding, as m e n t i o n e d earlier, that the first element adjacent to a beam tube t e r m i n a t i o n is by far the most significant in affecting the intensity at that beam port. The data of fig. 8 shows the difference in beam intensity spectra to a sufficiently high degree of accuracy that it is possible to determine a difference between the two spectra as a function of wavelength. W h e n this is done, a fractional increase in net analyzer c o u n t for a change in a r r a n g e m e n t R - R - R to F - F - F is f o u n d which increases from 0.110 at 3.5 /~ to 0.151 at 7.0 /~. This would indicate that the three BeO elements represents too " t h i c k " a source for long wavelength n e u t r o n s which are expected to travel down the b e a m tube. The inelastic scattering cross section increases with increasing wavelength and the BeO is scattering n e u t r o n s back to the core as is expected of reflector elements. This further confirms that an arr a n g e m e n t with more leakage such as F - R - F is required for high b e a m intensities.
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WAVELENGTH (Angstroms)
Fig. 8. A plot of the fractional difference in the net intensity data of fig. 7 as a function of wavelength. An average straight line has been drawn through the data points to indicate the degree of wavelength dependence. Some structure outside the statistical error appears to be present in these data, but is not worthy of analysis from this single experiment.
BERYLLIUM-FILTERED
N E U T R O N BEAM Q U A L I T Y I M P R O V E M E N T
Note that the later, higher intensity, experimental arrangement provided a SIN ratio only one-half that for the earlier setup. This was the result of an additional thermal and epithermal neutron scattering in the vicinity of the analyzer counter. These data show an increase in SIN ratio averaging 22.5% over the spectrum when reflector elements are interposed between the beam tube termination and the fuel elements, and this increase is obtained with only a 12.9% loss in the beam intensity. These results would suggest that in experimental situations where the background is high, core element rearrangement or special core elements might be particularly desirable. Even withdrawal of the beam tube termination from the face of the reactor core to interpose about 3 cm of pool water would be beneficial. We have verified this in other experiments on the removal of water from the gap between the end of the beam tube and the face of the reactor corea). Apparently the peaking of the thermal flux very near the core edge in pool type reactors 4) is responsible for the non-linear relationship between net intensity and background intensity as viewed by experimental apparatus at a beam port.
6. Summary We have shown that it is possible to attain increases in both beam intensity and beam-to-background ratio in the region of beryllium-filtered neutron energies amounting to 15 to 20% by taking special care to arrange the fuel and/or reflector elements at the termination of beam tubes in a pool reactor. In reactors
131
such as the A M R R where the reflector and fuel element positions are physically identical, this is a simple matter, although it supposes sufficient excess reactivity in the core to make up for any losses. It may be practicable at times to have special core elements manufactured which will be placed adjacent to beam tube terminations. The interposition of a water gap at the core face may also be beneficial in high background situations. Although the improvements in beam quality discussed here are not very large, they can often be gained with little effort and they represent one of many small gains which can add up to very worthwhile improvements in the experimental situation at a low-power reactor. We wish to acknowledge the generous cooperation of the reactor operations personnel in the Nuclear Research Laboratory of the A M M R C , who provided the many core configurations studied.
References 1) For a description of the reactor, see REIC Report no. 16, Survey of Irradiation Facilities, Battelle Memorial Institute, Columbus, Ohio (1961). 2) j. O. Elliot, Programming and utilization o f research reactors (Academic Press, New York, 1962) p. 395. a) j. j. Antal and A. A. Warnas, A M R A TR 67-05 (February, 1967) unpublished. 4) Chart issued as O R N L 2518; see also U. S. reactor operation and use (ed. J. W. Chastain, Jr.; Addison-Wesley Press, Reading, Mass., 1958) p. 69.
INSTRUMENTS
AND METHODS
65 ( I 9 6 8 ) I 2 5 - [ 3 [ ;
© NORTH-HOLLAND
PUBLISHING
CO.
B E R Y L L I U M - F I L T E R E D N E U T R O N BEAM QUALITY I M P R O V E M E N T
AT A P O O L R E A C T O R T H R O U G H CORE E L E M E N T R E A R R A N G E M E N T S J. J. A N T A L a n d A. A. W A R N A S
Army Materials and Mechanics Research Center, Watertown, Massachusetts 02172, U.S.A. Received 18 J u n e 1968
N e u t r o n spectra in the region o f 4 to 7.5 ~ f r o m a radial b e a m tube have been obtained for various fuel a n d BeO reflector a r r a n g e m e n t s in the A r m y Materials Research Reactor. Intensity a n d signal-to-noise ratio variations in the range of 15 % to 2 0 ~ have been obtained t h r o u g h r e a r r a n g e m e n t o f core elements
immediately adjacent to the b e a m tube termination. In general, the results indicate that the experiment external to the reactor s h o u l d view reflector element material only at the core face, a n d this reflector material s h o u l d be s u r r o t m d e d by fuel for the m o s t favorable intensity a n d signal-to-noise ratio combination.
1. Introduction Although the distribution of neutrons in the vicinity of the reactor core is well known for the purposes of reactor operation, the details of flux distribution as it affects the available neutron beam at a particular experimental port are known usually only in a general way. At the time of installation of the first experimental equipment at a beam port of the Army Materials Research Reactor (AMRR)I), considerable discussion ensued regarding the optimum arrangement of fuel and reflector elements for a particular experiment. It became apparent that some quantitative measurements were desirable to settle the matter. The core of most water pool research reactors is constructed with fuel and reflector elements which plug into interchangeable positions on a grid plate. It is possible thereby to arrange the fuel and reflector elements in a variety of configurations to provide for more uniform burn-up of the fuel and possibly to provide special flux and spectral distributions for particular beam experiments. It is this latter possibility which is the subject of this report.
core face to an area approximately 9 cm horizontally and 23 cm vertically. The beam tube employed, HU-1 in fig. 1, was not ideal in that it did not extend to the reactor core face. A volume of water having a 15.2 cm dimension along the beam path separated the end of the beam tube from the core. The water was displaced by the insertion of an aluminum can which served as a void, but the additional aluminum windows produced strong "absorption" profiles in the intensity spectra measured. The experimental data consists primarily of intensity spectra for neutrons issuing from the beam port in the HU-13
9
NOVEMBER 1968
7
FuelFuel
HU-I
2. Experimental arrangement The beam issuing from radial beam tube HU-I of the A M R R was examined with the neutron analyzer previously installed. This instrument is a crystal spectrometer employing a 20.3 cm bismuth filter and a 25.4 cm beryllium filter for the suppression of fast neutrons, gamma rays and second-order thermal neutrons. The filters remove essentially all of the thermal neutrons with wavelengths less than 4 • (5 meV), which allows the crystal spectrometer to analyze the neutron beam through a range of 4-8 /k with the use of a magnetite monochromator. This is a region which is important to many recent scattering experiments on solids and liquids. The collimator limited the view of the reactor
8
Water
C6
Fuel
6 5 q.
BeO FuelFuel
3 2
Water
I A
B
C
D
E
F
G
Fig. 1. Location of core elements a n d b e a m tube HU-1 in the A M R R (top view). Only elements in the n e i g h b o r h o o d o f HU-1 have been designated. C4, C5 a n d C6 are element positions of greatest interest. Fuel element R R contains a regulating rod. T h e core elements have 7.62 c m x 7.62 cm d i m e n s i o n s in this view. 125
126
J. J. A N T A L
.
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llo
A
[-8G~cou.; L"Lss.couNT---[---~ '
REGION
~EGIOt
g
,.a
o
IE
71710 o
IME OF DAY (houri
A. A. W A R N A S
3. Data analysis
o
--
I
AND
0840 Ot
Fig. 2. Typical strip chart record of the monitor count rate showing the average "one M W " power level, line A-A, as determined from the average monitor count rate in the region of the chart record where the gross count was taken. The prominent discontinuities in the record are traceable to manual manipulation of the reactor controls.
wavelength region from 3.4-7.4 A. Obtaining spectra allowed a search for possible effects as a function of wavelength. Spectra were obtained for various fuel and BeO reflector element arrangements in the immediate vicinity of the beam tube termination, especially for those positions designated as C4, C5 and C6 in fig. 1. The elements were arranged in a particular configuration each morning before reactor startup, and reactor operation was not disturbed during its normal 14-h schedule. The monitor was a parallel plate BF 3 detector which intercepted a constant fraction of the whole beam area incident upon the monochromator. The analyzer detector received neutrons of wavelength 2 = 9.665 sin 0 from the F%O4 (111) monochromator, and background spectra were obtained with this crystal rocked off by 3° in 0. Four items of data were recorded during each spectral determination: 1. The analyzer gross count accumulated during a fixed time T (usually 100 sec) at a particular angle 0. 2. The analyzer background count accumulated during a time T at the same angle 0. 3. The monitor count accumulated during each gross and background counting period. 4. A continuous record of the monitor count rate.
A particular difficulty with this type of measurement is the lack of a suitable neutron intensity reference level or an associated precisely reproducible reactor power level to which data taken on various days can be normalized. The steady-state power level for the reactor is determined by reactor operations personnel from a measurement of the rate of removal of heat from the system (thermal power). This data is presented to the reactor operator on a strip chart recorder which he uses as a guide to place the reactor at the proper level for the day. The ability of various operators to reproduce this power level from day to day is the limiting accuracy of our measurements. A study of the continuous reactor power level records over a period of several weeks indicates that the power deviates from an average level considered to be one megawatt by ___2% maximum. The daily record of monitor count rate showed significant variations and the recorded monitor count was used to correct the analyzer counter data collected during a single day's run. A typical record of monitor count rate is shown in fig. 2. The abrupt changes in count rate were in all cases related to manual operation of the reactor control. They were accentuated by the presence of a regulating control rod in a fuel element near the beam tube termination. The position of this rod is noted as " R R " in fig. 1. The region of the monitor count rate chart record which contained a steady level over the longest period of time was considered to be the best value for the average power level on a particular day. Although this level was selected by visual examination, the actual correction applied to all analyzer count data was determined from the digital record of monitor count accumulated during the corresponding period. The individual analyzer gross and background intensities were multiplied by the fraction: Steady level monitor count f' =
Individual monitor count
to correct for daily variations in reactor power level. The resulting normalized background count was subtracted point for point from the normalized gross count to obtain the net count which is plotted in figs. 3, 4 and 7. The signal-to-noise ratio was computed at each point as defined by the following expression: signal-to-noise ratio = S I N = (analyzer net count)/(unittime) (analyzer background count)/(unit time) This data is plotted in figs. 5, 6 and 7.
BERYLLIUM-FILTERED
NEUTRON
QUALITY
127
IMPROVEMENT
desire, and the intensity of t h e ~ b e a m is usually of primary importance. On the other hand, experiments involving several scatterings, or w o r k with neutrons of exceptionally long wavelength, usually have such low intensities at the detector that the background intensity is comparable or larger in magnitude than the signal. The signal-to-noise ratio n o w becomes an important factor since it will determine if the effect to be measured is in fact measurable. For these reasons both signal and signal-to-noise data are presented here. It should be noted that the detector e m p l o y e d in these studies was a 1 m m thick 6Li-loaded glass scintillator NE905, attached to an E M I 9356B photomultiplier tube. This neutron detector is very efficient in the wavelength region of these studies, but it is also s o m e w h a t sensitive
600
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Fig. 3. Net intensity spectra for four different core element arrangements which give almost identical intensity magnitudes. The point symbols are identified with their associated core element arrangements in the inset, and points relating to a particular spectrum are interconnected with straight lines for visual clarity. The standard deviation of the ordinate at the points is less than one-half the symbol height. F = fuel element, R = reflector element.
The chart record of m o n i t o r count rate was carefully m o n i t o r e d each day, and any unusual steady-state level reached by the reactor was cause for deleting f r o m consideration all data taken during that day's operation. It is expected therefore, that the several spectra presented in the figures can be compared on an absolute intensity basis within the accuracy of the reactor power level setting described earlier. 4. Discussion of data The b e a m intensity and signal-to-noise ratio are two inter-related factors in neutron b e a m detection which are of primary importance to the performance ot a neutron scattering experiment. In general, neutron beams are never as intense as experimenters would
)b-Z
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Fig. 4. Net intensity spectra for three core element arrangements which show significant differences from those of fig. 3, with one of the spectra of fig. 3, F-R-F-, repeated here for comparison. The point symbols are identified with their associated core element arrangements in the inset, and points relating to a particular spectrum are interconnected with straight lines for visual clarity. The standard deviation of each point is less than one-half the symbol height. F = fuel element, R = reflector element.
128
J. J. A N T A L A N D A. A. W A R N A S
Integral intensity and Arrangement
SIN
Integral net intensity (arbitrary units)
TABLE 1 ratio values for the core element arrangements investigated. Fractional diff.* relative to R-R-R
SIN ratio
Integral
Fractional diff.* relative to R-R-R
F-R-F R-R-F F-F-R R-F-F F-F-F R-F-R R-R-R
445 440 439 436 420 401 391
331 900 296 246 088 570 393
0,138 0.126 0.122 0. l 15 0.073 0.026 0.000
596.33 586.40 533.27 532.30 534.67 526.65 627.61
~- 0.050 - 0.066 -0.150 - 0.152 - 0.148 -0.161 - 0.000
F-F-Ft R-R-Rt
586 830 519 639
0.129 0.000
325.19 419.65
-0.255 - 0.000
* Fractional diff. = [ f ( x x x ) - - f ( R R R ) ] / f ( R R R ) , wheref(xxx) is the integral intensity or SIN ratio for a particular arrangement x-x-x. t Values for a different experimental setup (see text). Fractional diff. is c o m p u t e d relative to R R R t .
to gamma radiation and thereby operates with lower SIN ratios than would be expected with the more usual BF 3 counter. The intensity data presented in figs. 3 and 4 are the most reliable runs obtained and represent a wide variation in fuel and reflector element arrangements in the immediate vicinity of the beam tube termination. It was learned early in the experiment that the variations in neutron intensity were not large even for the most extreme change in core arrangement, and that core element changes more distant from the beam tube termination than the first row were relatively unimportant.The study was therefore narrowed to the three core element positions labelled C4, C5 and C6 in fig. 1. Because the measured beam intensity changes were small and the problem of normalization to a constant reactor power level was present, another experiment was desired to establish a higher level of confidence in the data obtained. Two additional spectra were obtained some time later after several elements of the spectrometer arrangement had been altered sufficiently that a totally new region of beam and background parameters would be investigated with a higher degree of statistical accuracy. The alterations resulted in a higher beam intensity since the reactor power had been increased to 2 MW and the beryllium filter reduced in length from 25.4 to 15.2 cm, and the background was significantly higher (as reflected in the smaller SIN ratios) because of these changes and alterations in the shielding of the spectrometer. The spectra in fig. 7 are those obtained under these conditions. They show significantly larger changes in intensity and SIN ratio than the equivalent rearrangement in the earlier data. It would appear that the presence of more neutrons in
the spectrometer environment of energies greater than those in the beryllium-filtered region make attention to core element rearrangements even more important. This data is of sufficiently high statistical accuracy to allow an attempt to study the wavelength dependence of the intensity changes, as discussed later. In all spectra shown in the figures, the individual points have been connected with straight lines to provide a visual guide. Many deviations from a smooth curve are seen in the spectra which lepresent the abrupt change in scattering cross section at the Bragg cutoffs in the material of the aluminum windows, the BeO reflector elements, and particularly in the large polycrystalline bismuth filter. The point-to-point resolution is not great enough in these data to outline the cutoffs clearly. A more precise quantitative measure of the intensity and SIN ratio changes found is obtained from the integral data presented in table t. This data was obtained from a summation of the individual ordinates between 4.1 ~ and 6.8 ~. The net intensity spectra for four of the core element arrangements was practically coincident, as detailed in fig. 3, although the signal-to-noise ratios for the same arrangement were not coincident, fig. 5. Superior SIN ratios were obtained with a BeO reflector element in the central position, C5, with an F-R-F arrangement being slightly better than an R-R-F arrangement. Three other core arrangements gave varying intensities at a particular wavelength and, as shown in fig. 4, each was inferior to those of fig. 3. The SIN ratios for these arrangements are shown in fig. 6, where again an arrangement with a central element of BeO was superior, although the R-R-R arrangement was slightly better than the F-R-F arrangement.
BERYLLIUM-FILTERED
NEUTRON
129
IMPROVEMENT
5. Optimum element arrangements
80
It is possible to obtain high intensity combined with high SIN ratio through the use of the F-R-F arrangement. The epithermal and fast neutron component of the beam which contributes to background appears to be reduced by the shielding effect of moderating material in the central-element position. At the same time, the moderating efficiency of the central element appears to be high when closely coupled to the surrounding fuel. One can go a step further in light of the results above and replace with fuel the BeO in the upper and lower portions of the central reflector element which are not viewed by a beam hole experiment to further feed the central element "source" and increase the intensity. This particular arrangement has been investigated by Eliot et al. 2) at the Naval Research Laboratory
70
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each spectrum, but is indicated for only one spectrum. The data points for a particular spectrum have been interconnected with straight lines for visual clarity. F = fuel element, R = reflector element.
It was not unexpected to find that the use of three moderator elements as a source gave the best SIN ratio and the lowest beam intensity. It is interesting to note, however, that the opposite arrangement did not produce the opposite effect. The beam intensity from the three fuel element arrangement was only moderately high, although the SIN ratio was one of the poorest. With a pool reactor where interchange of core elements is not allowed either because of reactivity requirements or as a matter of policy, the beam source situation is represented by these two arrangements. The R-R-R arrangement represents that of a fully reflected core, while the F-F-F arrangement represents that of a core reflected only by pool water. Neither situation appears to be optimum for subthermal neutron beams. In this and the following discussions, one should remember that the spectrometer viewed the central element primarily, as noted in section 2 above. Application of these results to other situations may require modification of the comments listed here.
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130
J. J. A N T A L
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Fig. 7. Net intensity and SIN ratio spectra for two core element arrangements of fig. 3 taken under a new set of experimental conditions. The standard deviation of each point is less than the symbol height, and the points have been interconnected with straight lines for visual clarity. The strong fluctuations in the spectra are the result of Bragg scattering in the filters and appear to be emphasized in these curves relative to the earlier data because of the lesser Be filter length employed. The two lower curves are the signal-to-noise ratio spectra for the same core element arrangements. Reactor. They have employed a "split" fuel element, which simply had no fuel in the center region of an otherwise n o r m a l fuel element. W h e n they replaced a s t a n d a r d fuel element in the central element location with a "split" fuel element, a 13% increase in intensity of thermal n e u t r o n s in the range 0.40 A to 2.9 ,~ was observed. A similar increase should have been observed in the s u b t h e r m a l region. It is difficult to make a direct c o m p a r i s o n since their "source" was a b o u t ½ A1 and 2 H / O by v o l u m e a n d ours was wholly BeO, b u t we estimate that a further increase in intensity of a b o u t 16% would be expected for the F - R - F a r r a n g e m e n t if a " s p l i t " fuel element were placed in our central position. They also f o u n d a decrease in fast n e u t r o n flux (greater t h a n 10 eV) of a b o u t a factor of 2 when a n o r m a l
A. A. W A R N A S
element was replaced by a "split" element. O u r results indicate a m u c h smaller decrease in fast n e u t r o n b e a m intensity when a fuel element is replaced by a BeO reflector element, but the scattering power and absorption in BeO is roughly ¼ that of H 2 0 for equal volumes in this region of n e u t r o n energies, a n d water would be expected to have a better fast n e u t r o n shielding effect. U n f o r t u n a t e l y , we were unable to o b t a i n a reliable r u n with water in the central element position to check this before the experimental system had to be disassembled. It is interesting to note we have also confirmed the Naval Research L a b o r a t o r y finding, as m e n t i o n e d earlier, that the first element adjacent to a beam tube t e r m i n a t i o n is by far the most significant in affecting the intensity at that beam port. The data of fig. 8 shows the difference in beam intensity spectra to a sufficiently high degree of accuracy that it is possible to determine a difference between the two spectra as a function of wavelength. W h e n this is done, a fractional increase in net analyzer c o u n t for a change in a r r a n g e m e n t R - R - R to F - F - F is f o u n d which increases from 0.110 at 3.5 /~ to 0.151 at 7.0 /~. This would indicate that the three BeO elements represents too " t h i c k " a source for long wavelength n e u t r o n s which are expected to travel down the b e a m tube. The inelastic scattering cross section increases with increasing wavelength and the BeO is scattering n e u t r o n s back to the core as is expected of reflector elements. This further confirms that an arr a n g e m e n t with more leakage such as F - R - F is required for high b e a m intensities.
_ Z~FFF)- Z{RRR) f I(RAR)
I
f
0.1--
3
I 4
I 5
E 6
1 7
WAVELENGTH (Angstroms)
Fig. 8. A plot of the fractional difference in the net intensity data of fig. 7 as a function of wavelength. An average straight line has been drawn through the data points to indicate the degree of wavelength dependence. Some structure outside the statistical error appears to be present in these data, but is not worthy of analysis from this single experiment.
BERYLLIUM-FILTERED
N E U T R O N BEAM Q U A L I T Y I M P R O V E M E N T
Note that the later, higher intensity, experimental arrangement provided a SIN ratio only one-half that for the earlier setup. This was the result of an additional thermal and epithermal neutron scattering in the vicinity of the analyzer counter. These data show an increase in SIN ratio averaging 22.5% over the spectrum when reflector elements are interposed between the beam tube termination and the fuel elements, and this increase is obtained with only a 12.9% loss in the beam intensity. These results would suggest that in experimental situations where the background is high, core element rearrangement or special core elements might be particularly desirable. Even withdrawal of the beam tube termination from the face of the reactor core to interpose about 3 cm of pool water would be beneficial. We have verified this in other experiments on the removal of water from the gap between the end of the beam tube and the face of the reactor corea). Apparently the peaking of the thermal flux very near the core edge in pool type reactors 4) is responsible for the non-linear relationship between net intensity and background intensity as viewed by experimental apparatus at a beam port.
6. Summary We have shown that it is possible to attain increases in both beam intensity and beam-to-background ratio in the region of beryllium-filtered neutron energies amounting to 15 to 20% by taking special care to arrange the fuel and/or reflector elements at the termination of beam tubes in a pool reactor. In reactors
131
such as the A M R R where the reflector and fuel element positions are physically identical, this is a simple matter, although it supposes sufficient excess reactivity in the core to make up for any losses. It may be practicable at times to have special core elements manufactured which will be placed adjacent to beam tube terminations. The interposition of a water gap at the core face may also be beneficial in high background situations. Although the improvements in beam quality discussed here are not very large, they can often be gained with little effort and they represent one of many small gains which can add up to very worthwhile improvements in the experimental situation at a low-power reactor. We wish to acknowledge the generous cooperation of the reactor operations personnel in the Nuclear Research Laboratory of the A M M R C , who provided the many core configurations studied.
References 1) For a description of the reactor, see REIC Report no. 16, Survey of Irradiation Facilities, Battelle Memorial Institute, Columbus, Ohio (1961). 2) j. O. Elliot, Programming and utilization o f research reactors (Academic Press, New York, 1962) p. 395. a) j. j. Antal and A. A. Warnas, A M R A TR 67-05 (February, 1967) unpublished. 4) Chart issued as O R N L 2518; see also U. S. reactor operation and use (ed. J. W. Chastain, Jr.; Addison-Wesley Press, Reading, Mass., 1958) p. 69.