- Email: [email protected]
Measurement of fission fragment induced fluorescence in xenon
Nuclear Instruments and Methods in Physics Research B 103 (1995) 1-14
B
N
Beam Interactions with Materials & Atoms
ELSEVIER
Measurement of fission fragment induced fluorescence in xenon G.R. Imel a,,, W.P. Poenitz a, A.M. Snyder b a Argonne NationaILaboratory, P.O. Box 2528, Idaho Falls, ID 83403-2528, USA b LockheedIdaho Technologies, Inc., P.O. Box 1625, Idaho Falls, ID 83415, USA
Received 9 January 1995; revised form received 10 April 1995 Abstract A research program was conducted at Argonne National Laboratory-West (ANL-W) to measure the 172 nm fluorescence in xenon induced by fission fragments. Additionally, the survivability of uranium dioxide coatings under high fission power conditions was investigated, and the fission fragment escape energy from coatings of different thicknesses was directly measured. A definite rearrangement of the uranium dioxide coating was observed after high power irradiations, but this rearrangement did not affect the fission fragment energy escape fraction. The measurement of fluorescence induced by fission fragments was done in the Neutron Radiography Reactor (NRAD); this measurement was complicated by the presence of the gamma field and by gas impurity effects. Compensations for gamma and impurity effects were made during the program. Additionally, signal degradations during system pressure changes were observed; however, the signal recovered after several minutes, so this phenomenon did not adversely affect the measurements. The effficiencies of fission fragment and gamma induced fluorescence were measured to be 0.37 and 0.44 respectively.
1. Introduction
A research program was conducted at Argonne National Laboratory-West (ANL-W) to measure the efficiency of fluorescence induced by fission fragments in xenon; specifically, the 172 nm Xe~ line was measured. The high power densities potentially available from fission fragment energy deposition has stimulated research in nuclear pumped lasers since the 1960s [1], and more recently, concepts are being developed to use ground-based nuclear pumped lasers to beam power into space [2]. The research described in this paper provides data of use to a photolytically driven excimer laser in which the fission fragment induced fluorescence pumps a lasant gas. A nuclear pumped laser or nuclear driven flashlamp uses the energetic reaction products of nuclear reactions to drive a laser or excite fluorescence in materials. The reaction products used can be alpha particles from certain neutron absorption reactions (e.g., l°B(n,a)7Li), fission fragments, or gamma rays from thermonuclear explosions.
~"Work supported by the U.S. Department of Energy under Contract W-31-108-ENG-38. * Corresponding author. Tel. + 1 208 533 7941, fax + 1 208 533 7471.
Besides a weapons-driven gamma source, which is essentially uncontrollable, fission fragments from nuclear fission provide the most energy available for excitation into the medium for a given energy input. This has led to studies of nuclear reactor driven lasers and flashlamps in which a reactor provides an intense source of neutrons that can induce fissions on a target of fissile material (e.g., 235U or 239pu). The target is designed so a large number of the fission fragments can escape and lose their energy through excitation of the lasing or fluorescence medium; this is accomplished by using very thin films of fissile material, using a gas of fissile material (e.g., UF6), or creating an aerosol of fissile material particles. For this work, the use of thin films was used, specifically uranium dioxide plated on aluminum. A photolytically driven laser uses fluorescence energy to pump the laser; the rare gases (e.g., Xe) are efficient fluorescent media. As an example, the fluorescence from Xe~ at 172 nm can be used to dissociate XeF2 and drive a XeF* laser photolytically. Further details of nuclear pumped lasers can be found in several overview articles E3,4]. Energy efflciencies of certain processes must be known to design a photolytically driven excimer laser. These are fission fragment energy deposition into the fluorescent, conversion to fluorescence energy, coupling of this energy to the lasant, and subsequent laser energy efficiency. This
0168-583X/95/$09.50 © 1995 EIsevier Science B.V. All rights reserved SSDI 0168-583X(95)00564-1
2
G.R. Imel et al./Nucl. Instr. andMeth, ht Phys. Res. B 103 (1995) 1-14
paper will describe the measurement of the first two stages; i.e., the efficiency of fission fragment energy deposition (defined as "Off) in 2 v geometry, and fluorescence efficiency (defined as "Ouv) which is total fluorescence energy divided by fission fragment energy deposited in the gas (xenon). Additionally, the survivability of fissile coatings on aluminum fuel plates was studied as the plates were subjected to high neutron fluxes; the concern was that "Off would be adversely affected by rearrangement of the fissile coating due to the high fission power achieved in the fuel plate. Three reactor facilities at ANL-W were used for these measurements: The Argonne Fast Source Reactor (AFSR), the Transient Test Reactor (TREAT), and the Neutron Radiography Reactor (NRAD). Several issues were addressed in the measurement of "Off. Flat aluminum plates, 5 cm × 5 cm × 6.4 mm were coated with uranium dioxide (UO 2) by the Reuter-Stokes Corporation to nominal loadings of 0.5, 1.0, 1.5, 2.0, and 2.5 mg UO2/cm 2. The dependency of coating thickness on fission fragment escape energy from a flat plate source has been reported [5]. It was wanted to investigate possible differences compared with the results of Ref. [5] for the specific coatings obtained from Reuter-Stokes, which were produced by electroplating uranium metal followed by oxidation. It was important to investigate if the effects of irradiation of the fuel plates would adversely affect the energy escape fraction. This issue is particularly significant in pulsed reactor-pumped laser systems in which the power densities in the fissile material are quite high. The plates from Reuter-Stokes were subjected to high power transients in the TREAT reactor, achieving power densities about 2 M W / g 235U. Survivability of the coating and changes in the fission fragment energy escape fraction were issues studied. Precise measurement of the total mass of the coatings before and after irradiation was required to detect if any fissile material was lost. Direct measurements of the fission fragment escape energy were made using an ionization fission chamber; Monte Carlo calculations yielded adjusted results for the efficiency. These values were then used in unfolding the measurements of the fluorescence efficiency. Ranges of fluorescence efficiencies from 39 to 68% for "O~,, have been reported [6,7]. As lasing depends on achieving threshold power densities, the above spread of efficiencies is not acceptable to design a system. This is particularly true since most reactors available for nuclear pumped laser systems have marginal neutron fluxes [8]. The measurements of "Ouv in xenon described in this paper were performed using a calibrated photodiode in a special test assembly in which the gas pressure could be varied. The test assembly loaded with a fuel plate was placed in the beam of the NRAD reactor. Controlled steady state measurements of vacuum ultraviolet (VUV) output under variations in pressure, neutron beam, and gamma beam parameters were done. These measurements
were complicated by the gamma background of the beam, and by gas impurity problems. The following sections describe this measurement program in detail.
2. Measurement of r l ~ - fission fragment energy depo-
sition efficiency This phase of the research dealt with characterization of the UO 2 coated fuel plates as obtained from Reuter-Stokes and their stability under high power irradiations. This included visual inspection, scanning electron microscopy (SEM), Auger electron, and electron dispersive spectroscopy (EDS) examinations of unirradiated and irradiated plates, measurements of fissile masses via c~ particle counting, and direct measurement of the fission fragment escape fraction. Destructive chemistry was also done on some samples to obtain isotopic information. Initially, calorimetric measurements of total fission energy were done in the TREAT reactor to determine the survivability of the UO 2 coating. This issue is important for any nuclear pumped laser system that uses fuel coating subjected to high neutron fluxes during reactor pulses. Two coated plates were irradiated in the TREAT reactor in successive transients of up to 16000 MW peak power (pulse widths of 100 ms FWHM). Temperatures attained in the aluminum plates were monitored with embedded thermocouples. Examination of these data suggested that the peak temperatures did not decrease from transient to transient, which in turn shows that most of the coating survives the high power densities necessary for reactor pumped laser systems. However, these calorimetric measurements do not have the precision necessary to measure either fission fragment energy escape or total mass on the plates. It was thus decided to do more precise measures of these parameters, and further investigate the characteristics of the coating before and after irradiation. Alpha counting was done on one plate before and after irradiation in TREAT. These results showed that little, if any, fissile material was lost from the plate after irradiation, at least within the precision of the analysis. This result was consistent with the calorimetric measurement done in TREAT during the irradiations. Subsequent measurements of other plates described later in this paper show this plate was an anomaly in that so little material was lost. However, the c~ energy spectrum taken after irradiation showed a substantial increase of the width of the distribution when compared with the spectrum taken before the irradiation, showing the possibility of fuel rearrangement on the plate. Visual examinations of both samples showed that the coverage of the coating was not uniform after irradiation. Additionally, the color of the coating was distinctly brighter than found on unirradiated samples from the same batch. Because of the visual difference of unirradiated fuel plates
G.R. Imel et al. /Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 1-24
3
Fig. 1. Unirradiated fuel plate SEM (1000 × ).
versus irradiated fuel plates, it was decided to do a series of examinations using SEM, Auger, and EDS. The Auger and EDS examinations did not reveal any significant difference between the irradiated and unirradiated plates. However, the SEM examinations did reveal some very interesting differences in the coating characteristics. Examples of the SEM results at magnifications of 1000 × are shown in Fig. 1 (unirradiated plate) and Fig. 2 (irradiated
plate that had received 5 × 1015 total fissions). The coating of the unirradiated plate in Fig. 1, shows a fairly flat, but cracked, coating, similar to a di:ied mud flat. In contrast, the coating of the irradiated plate shows a distinct "clumping" effect; particles about 10 ~m appear on the surface with large areas of bare aluminum between-fuel particles. It is felt that this explains the brighter coloration of the irradiated plates, i.e., aluminum ig~simply showing through.
Fig. 2. Irradiated fuel plate SEM (1000 × ).
4
G.R. Imel et al./Nucl. Instr. andMeth, in Phys. Res. B 103 (1995) 1-14
Table 1 Masses from alpha counting (c~-counting, mg of total U) A~ 1 2 3 4 5 7 9 10
Sum
Mass before irradiation [mg]
Mass after irradiation [mg]
No. of fissions
% change
86.9 58.1 57.3 56.0 58.2 58.3 58.2 55.5 58.5 547.0
85.1 57.1 55.4 55.4 55.0 57.0 57.0 54.7 53.3 530.0
5 X 10 ~s 1.6 × 10 ~s 1.6 × 10 ~s 1.6 × 10 ~s 1.6 X 1015 1.6 X 10 ~s 1.6 X 10 ~s 1.6 X 10 t5 1.6 X 10is 1.6 X 10 ~5
2.1 1.7 3.3 1.1 5.5 2.2 2.1 1.4 8.9 3.1
a The A plates were counted as a pair.
experimental series. It was especially important to know whether the fuel redistribution or "clumping" would cause a decrease in the escape fractions reported in Ref. [5]. The measurement of pre-and post-irradiation masses nondestructively was crucial to the measurement of r/u,,. It was decided to use c~-counting (primarily from the 234U in the UO z) as the preferred method, particularly since it yields a highly accurate relative measure of changes in total fuel mass, and it is a nondestructive assay technique. Additionally, a counting has been shown to yield accurate measurements of absolute masses of uranium in coatings, as shown by comparisons with destructive wet chemistry and fission counting [9,10]. Destructive chemical analysis, in which the coating is leached off the plate, was also done on some samples to obtain isotopic information.
It is assumed that high temperatures in the fuel coating caused that clumping effect (recall that power densities about 2 M W / g were achieved in the coating). Cross-sectional examinations revealed that only minor amounts of fuel could have been driven into the aluminum plate, which is consistent with the lack of low energy tail in the observed a energy spectra. Despite the exact mechanism, it was clear from the SEM examinations that there had been rearrangement of the fuel coating after irradiation in TREAT. Besides measuring the relevant efficiencies r/el and rh,, in unirradiated plates, it was wanted to learn if high power irradiations typical of nuclear pumped laser systems would adversely affect these efficiencies. Consequently, it was decided to do precise measurements on each plate to be used in the
90
80
70
-Flat Plane - - - Restructured • Unirradiated 0 Irradiated • U-235 Reference (1.3 mg total U) ........... Kahn, et al. (Reference 5)
".~ "...~ ~ , , , ".. \ ~ "'-,~
........~ "'".
- - . ~ - - ~
60
t-, ILl 50
......... .........> . .
""..... ~
............O....~
40
30
I
2 Nominal Uranium Thickness (mg UO2 / cm2) Fig. 3. Average kinetic energy of fission fragments leaving deposit.
-
G.R. Imel et al. / Nucl. hlstr, and Meth. in Phys. Res. B 103 (1995) 1-14
The results of the c~ counting of a sampling of the plates before and after irradiation are summarized in Table 1. As shown in Table 1, there is about an average 3% loss of mass after irradiation; material lost during handling cannot be ruled out as a cause. For the measurement of efficiency, the mechanism does not matter if a precise measurement of total mass is made of the plate to be used before the actual efficiency measurements. Because of the fuel coating rearrangement that occurs during irradiation as showed by the SEM examination, it was decided to do a direct measurement of the fission fragment energy escape. This was needed to determine if this rearrangement would adversely affect the escape energy (over that assumed for a flat coating as in Ref. [5]). A parallel plate ionization chamber was constructed with the collector plate-fissile deposit spacing such that all fission fragment energy escaping from the coating was deposited in the counting gas. The counting gas was P10 (90% argon, 10% CH4). Fission fragment energy spectra were obtained in 2'rr geometry using the thermal column of the AFSR reactor. Unirradiated plates of various coatings and irradiated plates of relatively high loadings (up to 2.5 m g / c m 2) were measured. The spectra of the fission fragments were recorded with a multi-channel pulse height analyzer. The calibration of the energy deposition was achieved with a very thin deposit (61.6 ~ g / c m 2) of known mass, which has a 99.5% fission fragment escape fraction into 4'rr. A Monte Carlo simulation of the energy escape fraction of fission fragments from a UO 2 deposit was done, taking into account the energy and mass distri-
butions in thermal fissions. Energy loss and range parameters were taken from the literature [11,12]. Calculations were made for a homogeneous fiat plane deposit and for a restructured clumped deposit. The size of the clumps was chosen to reproduce the increase of the c~-peak energy width observed with the low-geometry counter after irradiation. These results are compared in Fig. 3 with the measured values (also shown are values from Ref. [5]). As can be seen from the figure, the escape energy of the irradiated plates is in the range of 39-42 MeV. This yields a nominal energy escape fraction of 0.24 (assuming 168 MeV per fission) for irradiated plates of 2.5 mg UO2/cm; loadings. (It should be noted that in 2'n" geometry, 0.5 is the maximum escape fraction.) There is essentially no difference between the irradiated and unirradiated samples that cannot be explained simply by the difference of the amount of material on the various samples. The Monte Carlo simulation shows that the restructuring during irradiation should have increased the energy loss in the deposit. That this was not so is probably due to compensating effects (e.g., a change of the oxidation state of the deposit, or smoothing of the backing, etc.). The differences between the current measurements and the calculations based on parameters from the literature on the one hand and the experimental data from Ref. [5] on the other hand are probably due to effects that are difficult to define (structure of the backing, structure and chemical composition of the fissile deposit). The conclusion of this phase of the research is that the plates do not have the energy escape fraction that would be predicted for a perfectly flat
REMOTE :SOLENOID V A L V E B
GAS BOTTLE
TA TEMP TA PRESS
~/
5
DIODE SIGNAL
Fig. 4. Simplified schematicof system for measurementof ~v.
6
G.R. Imel et al,/NucI, hzstr, and Meth. bz Phys. Res. B 103 (1995) 1-14
deposit, but that fact is true for both irradiated and unirradiated samples. Additionally, the rearrangement that occurs during irradiation does not produce further degradation of the energy escape fraction.
3. Measurement of r / u v - fission fragment induced fluorescence efficiency Methods described in the previous section allowed accurate measurement of fission fragment escape energy fractions for UO e coated plates, whether irradiated or not. Upon completion of that phase of the project, equipment was designed and built to measure the efficiency of fluorescence induced by the fission fragments in xenon (r/uv). The NRAD reactor at ANL-W was chosen for this phase of the experiment because of its neutron beam and shielded beam room (normally used for radiography). Neutron fluxes in the beam room are about 10 s n/cmZs. Total power densities available for the efficiency measurement are quite low in the NRAD beam: about 4 × 10 .3 W / g a35U (contrast with the 2 M W / g in TREAT) for fission power and about 2 × 10 .5 W / g for gamma power (in xenon). The equipment was designed to allow 0-3 atm xenon gas to be remotely pumped into a 1.5 1 test assembly.
On-line calibrated monitors of neutron flux, gamma dose, and test assembly temperature and pressure were placed in the system. A simplified schematic of the system is shown in Fig. 4 and the test assembly is shown in Fig. 5. The photodiode used for measurement of the VUV output was an SAI ultraviolet standard photodiode (EVSD model 540-000), with a magnesium fluoride window, and a cesium telluride photocathode of 1 cm diameter uniform active area; it was calibrated by the National Bureau of Standards (now NIST). In the original system, a shutter was placed inside the test assembly to absorb the fission fragment energy; when closed, the shutter provided a flat plane parallel and very close to the fuel coating such that fission fragments would lose their energy in the shutter instead of the xenon. This was to allow direct measurement of the background gamma induced fluorescence. Through the initial testing phase, it was discovered that operation of the shutter was causing degradation of the signal. This degradation was possibly caused by air infiltration through the shutter shaft seals (i.e., movement of the shaft would cause a temporary break in the shaft seal). Consequently, improvements were made to the system to remove the internal shutter by replacing it with an external neutron absorbing shutter (1 mm of cadmium). Additionally, modifications were made
Diode assembly
Fuel p l a t e . Vessel ....
/
Chopper assembly
t9
.....
Vacuum/gas
connection
Fig. 5. Conversionefficiencytest assembly.
G.R. Imel et al. / Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 1-14
7
800 700 600 500
g 400 o
300
5
200 100 1
10b00
20L00
30~00
4000
5000
Time (Seconds)
Fig. 6. Photodiodesignal(mV)fortest UF5A.
to allow a cleanup (through bakeout and simultaneous vacuum pumping) and a refill of the test assembly with fresh gas without opening any gas lines to the atmosphere. As part of the impurity control, a nickel-plated test assembly was used exclusively for this test series. Data during the tests were logged every 10 s. The photodiode digital voltmeter (DVM signal), the gamma ion
chamber current, the test assembly pressure, and the test assembly temperature were logged. In addition, the 235U and 237Np (a threshold fission reaction with Eth ~ 0.6 MeV) fissions in calibrated chambers were monitored via a multi-channel analyzer. The calibration of the ion chamber was accomplished using a string of 35 6-mm CaF Thermoluminescent Dosimeters (TLDs) inside the test assembly
0.6
0.5
0.4
0.3
0.2
0.1
0.0
T
r
1
T
1000
2000
3000
4000
Time (Seconds)
Fig. 7. Fluorescence efficiency(Etauv) for test UF5A.
5000
8
G.R. Imel et al./Nucl. Instr. andMeth, in Phys. Res. B 103 (1995) 1-14
(yielding the axial profile of the gamma field inside the test assembly), while the ion chamber was placed in a fixed position external to the test assembly. Calibrations were done for both the cadmium shutter open and closed cases, as the cadmium shutter does affect the gamma field. The use of the TLD measurements to estimate the gamma induced fluorescence is described in the Appendix of this paper. The neutron flux was calibrated using gold foils and a calibrated fission chamber, as described in the Appendix. The special fission chamber used is described in the literature [13]. Corrections for the gold/235U reaction rates were made. Several types of experiments were done in this program. Unfueled experiments (test assembly full of gas, but no fuel plate) using the reactor determined the efficiency of pumping by gammas alone (with a component from neutron heating). In addition, several experiments were performed using a pure gamma source (decay gammas from a spent reactor subassembly). The fueled experiment reported in this paper utilized plate 10 as shown in Table 1. A typical diode signal from this test series is shown in Fig. 6, which is due to gamma and neutron heating. The data were taken every 10 s; since the diode and electronics were susceptible to noise, noise spikes were removed from the signal presented in Fig. 6, but no data smoothing was done. The reactor was constant at 250 kW, and the system was initially filled to 1.89 bar. To start the test, the large biological shield in the NRAD beam was opened at 0 + s. At 2000 + s, the cadmium shutter was closed, and opened again at 2500 s. At approximately 2700 s, pressure was decreased to 0.92 bar, and at 4000 s, the reactor was
scrammed. To calculate an efficiency from this diode signal, the calibrations of the diode, gamma ion chamber, and fission counter were used as described in the Appendix. The gamma efficiency as calculated for this test is shown in Fig. 7, along with horizontal lines representing the first and third quartile of the data from the period 280-2950 s. Because the measurements taken during these experiments typically did not exhibit the characteristics of stationary time series, confidence intervals using conventional statistical distributions (e.g., normal) could not be set. After performing the usual time series data analysis, it was determined that the median of the series provides the most representative value for the efficiency, while the first and third quartiles yield the error bars (without a statistical measure of confidence). For the UFfA test, the median is 44%, consistently predicted at open and closed shutter position, at the two pressures, and even somewhat during the reactor scram. The first quartile is 0.4401 and the third quartile is 0.4448; half the measurements for the period are within these values. It is significant that the efficiency dropped during the pressure change around 3000 s but did recover. This behavior was consistent throughout all the series in that a pressure change would temporarily degrade the signal. The pressure decrease was accomplished in about 160 s. During this period, the efficiency as shown in Fig. 7 steadily decreased, followed by a recovery of the efficiency that tookabout an equivalent amount of time, for a total of 330 s of degraded efficiency. It is feasible that gas dynamic phenomena could cause the degradation and subsequent recovery in the signal. Variations in density can cause changes in local fluorescent energy, or can cause changes in the gas refractive index [14]. It should be
1000
S
f-._.~
800
G" 600
g 400 O
2O0
-200
I 500
10JO0
15100
I 2000
Time (Seconds)
Fig. 8. Photodiodesignal for test UF2D,
I
2500
3000
G.R. hnel et al. / Nucl. bzstr, and Meth. in Phys. Res. B 103 (1995) 1-14
0.6
0.5
0.4
0.3 ILl
0.2
0.1
0.0
i 500
10lO0
15100
i 2000
2500
Time (Seconds)
Fig. 9. Fluorescence efficiency (Etauv) for test UF2D.
noted that this test involved only gamma and neutron heating, which is quite uniform across the test assembly; there clearly would be non-uniform heating for fission fragment pumping. No attempt was made in these experiments to model quantitatively the effects of gas dynamics; compensation for the effects was made by allowing enough time for the measurements so that the system could come to equilibrium after pressure changes.
The diode signal for another unfueled test (UF2D) is shown in Fig. 8. In this test, the changes in the diode signal are driven by pressure changes: at the start of the test, the pressure is 1.86 bars, it is increased to 2.43 bars at 640 s, decreased back to 1.86 bar at 1140 s, then stepped through 1.32 bar, 1.1 bar, and 0.81 bar. The efficiency as measured for the this test is shown in Fig. 9. As seen, it is consistent with the test UF5A; the median value for all of
500
400 G" :~ 300
g
r-"
._m 200 ._o Q
100
500
1000
2000 Time (Seconds)
Fig. 10. Photodiodesignalfortest G1A.
2 O0
3000
10
G.R. Imel et al. /Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 1-14
5 < ,= YJ
500
1000 '
1500 ~
2 0 O0
25~O0
3000
Time (Seconds)
Fig. 11. Fluorescence efficiencyfor test G1A (arbitrary units).
the test period (including the periods of frequent pressure changes) is 0.434, while from 210-1540 s (before the rapid and frequent pressure changes) the median is 0.438. The first and third quartiles shown in Fig. 9 are at 0.4335 and 0.4423 respectively from 210-1540 s. Clear perturbations in the inferred efficiency are seen during the pressure changes at 640 s, 1140 s, and 1500 s. After 1500 s, the
overall efficiency declines, probably because of subsequent rapid pressure decreases such that the system does not achieve equilibrium. Impurity control was a problem in this test series. After doing the UF2D test, the test assembly was evacuated and refilled with fresh gas. It was left overnight, and the next test (UF3) was done. The gamma efficiency dropped to
1000
800
600 ¢.
O3 "o o
400
200
I
50O
10'00
15'00
20JO0
25100
Time (Seconds) Fig. i2. PhotodiodesignaI for test F533B.
30'00 35~00
4000
11
G.R. Imel et al. / Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 1-14 800
700
....
gamma/neutron diode signal fission fr;~gment diode sigrial
6O0
500
g 400 300 a
/
2OO
500
1000
1500
2000
2500
3000
3500
4000
Time (Seconds)
Fig. 13. Photodiode signal due to gamma/neutron heating and fission fragment heating for test F533B.
ciency does not recover significantly during the irradiation period, as it did in Fig. 7. This does suggest the existence of two mechanisms for degradation: a long term, irrecoverable impurity contamination, and a short-term degradation, possibly due to gas motion during pressure changes. It was wanted to determine if the effect of the pressure change seen in Figs. 7 and 9 was in any way dependent on
15%, a factor of three decrease, which suggests impurity contamination either through outgassing of the test assembly overnight, or air infiltration. A pumpdown and refill did bring the efficiency immediately back to 445. It is also significant that the overnight contamination effect is different from a pressure change effect during a run. The effects of overnight contamination are such that the effi-
0.6
I I lt
ILl
twTI
t
0.2
0.1
0.0
,~ .... 0
,
500
1000
1500
2000
,
i
2500
3000
3500
Time (Seconds)
Fig. 14. Fluorescence efficiency due to fission fragment heating for test F533B.
4000
12
G.R. Imel et aL / Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 1-14
the neutron flux through some unspecified activation reaction (e.g., introduction of impurities during a pressure change followed by burnout of the impurities through some neutron reaction). To remove neutron activation effects, a spent EBR-II driver subassembly was placed in the radiography specimen tube of the NRAD facility, yielding a mostly pure gamma field (except some spontaneous fission neutrons). This gamma field could not be calibrated adequately because of the complicated shielding caused by the gas system hardware between the source and test assembly. The diode signal is shown in Fig. 10. The pressure was increased from 0 to 1.89 bar in 120 s, increased to 3.3 bar at 1200 s, bled to 1.89 bar at 1800 s, and bled to 0.92 bar at approximately 2200 s. The efficiency (in arbitrary units) is shown in Fig. 11. As seen, the degradation and short term recovery due to the pressure change are quite evident; this eliminates any neutron phenomena as the cause. In the last phase of the experiments, a coated fuel plate was placed in the test assembly. A typical diode signal is shown in Fig. 12. Here, the test assembly was initially filled to 0.92 bar, the cadmium shutter cycled twice, the pressure increased to 1.89 bar at about 2000 s and the shutter cycled twice. To calculate the efficiency due to fission fragments alone, it was assumed that the efficiency due to gamma pumping was known (from the unfueled tests), as described in the Appendix. A gamma and neutron induced diode signal was inferred, and subtracted from the total diode signal to yield that due to fission fragments alone, which was then used to calculate the efficiency. The adequacy of this method was checked by calculating a predicted gamma/neutron induced diode signal for the fueled test via the methods described in the Appendix. The predicted signal was compared with the signal measured in an unfueled test that immediately preceded the fueled test. The agreement was within a few percent, showing that the background gamma/neutron signal was being properly subtracted from the fueled test. The diode signals due to fission fragment and combined gamma/neutron pumping as predicted by these methods are shown in Fig. 13. It is important to note in Fig. 13 that the fission fragment contribution is a small fraction of the gamma/neutron pumping, even at the low pressure of 0.92 bar. At higher pressures, the fission fragment contribution is less than 20% of the gamma contribution. Improvements to these measurements would require a smaller volume for the test assembly, or a larger neutron flux compared with the gamma field. The efficiency due to fission fragments as calculated from these results is shown in Fig. 14. When the cadmium shutter is closed, there is essentially no fission fragment signal, as is seen by looking at Fig. 13; this leads to the extremely noisy areas shown in Fig. 14. The median efficiency is 0.366, the first quartile is 0.3544 and the third quartile is 0.3731. The median and quartiles were obtained utilizing data only when the shutter was open. Again, the
degradation due to a pressure change is clearly seen at about 2000 s.
4.
Summary
An experimental program has been described in this paper in which measurements of fission fragment escape energy and fission fragment induced fluorescence utilizing UO a coated aluminum plates were done. For these data to be of interest to nuclear pumped laser programs, it was necessary to learn if the measurements would also be valid for plates subjected to high neutron fluxes. Therefore, a series of experiments and examinations were conducted to study the effects of irradiation on the coatings. From transients conducted utilizing the TREAT reactor, it was found that the coating definitely changed after irradiation, probably due to high internal temperatures achieved. Additionally, small amounts of fissile material were lost between the pre-irradiation and post-irradiation alpha counting; losses simply due to handling could not be ruled out as the cause. However, subsequent direct measurements of the fission fragment energy showed that the coating changes did not change the escape fraction, probably due to compensating effects. Given these measurements, it was determined that the coated plates could be used in a nuclear pumped laser system if wanted; a series of measurements was subsequently done on the fission fragment induced fluorescence in xenon. Controlling the effect of impurities was necessary while doing the fluorescence measurement. This was done by designing the system so that it was possible to bakeout the system while simultaneously vacuum pumping, followed by fresh gas refill, without opening any of the system to the atmosphere. Even with this impurity control, a temporary degradation of the diode signal was observed due to gas pressure changes; however, the efficiency recovers within 5-10 min. This temporary degradation is possibly due to gas dynamic effects. The best estimates of efficiencies based on these tests are: "quv= 0.44
(y pumping),
r/u,, = 0.37
(fission fragment pumping).
The thermal neutron contribution is 15%, and no correlation with fast neutrons (E > 0.6 MeV) as monitored by the 237Np reaction was found. Improvements could be made to the hardware to yield a better measurement of "quv due to fission fragments. Testing a lightly loaded plate to compare differences was not possible because of the decreased signal. Also, testing at higher pressures was not possible because of the increase of the gamma signal (with no corresponding rise in the fission fragment signal). Significant improvements could be made by incorporating a clad moderator inside the test
G.R. Imel et aL /Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 1-14 assembly (as a backstop), shrinking the volume of the test assembly, using a boral shutter (which would be a 1/v absorber, closer to the ~SU fission reaction rate than cadmium), and the use of a lead filter. These changes would increase the fission fragment signal over the gamma signal. Due to cancellation of the program, pursuing these issues further was not possible.
Acknowledgements The support of the following people is gratefully acknowledged: E. Wood for the SEM analysis, D. Maddison for the alpha counting, J. Fincke, K. Watts, P. Dickson (now of Westinghouse Savannah River) and D. Nigg from Lockheed Idaho Technologies for photodiode calibration, test assembly design, and other technical support, W. Murphy and the late R. McConnell for gas supply system design, and the NRAD Operations crew for reactor support.
Appendix A
13
inferred gamma dose. These measurements showed an average gamma dose in the test assembly of 6367 r a d / h and 5667 r a d / h for the shutter open and closed cases respectively. The effective heating volume of the test assembly (inside volume minus fuel plate and holder volume) was calculated to be 1.35 × 10 3 c m 3. From this, constants of 2.391 X 10 .5 W / k g m s and 2.127 X 10 .5 W / k g m 3 were obtained for the shutter open and closed respectively. To obtain total heating, the density was calculated to be:
p ( P ) = 5.488P - 0.603 k g / m 3 where P is the pressure in bar. The above density/pressure relation is valid at 305 K and at low pressures ( < 10 bar). At the same time that the TLDs were being irradiated, the voltage (proportional to current) from the external ion chamber was monitored. A correlation between the absorbed gamma dose inside the test assembly as obtained by the TLDs and that as monitored by the external ion chamber was obtained. A calibration value was obtained for chamber voltage vs. gamma power per gas density (in kg/m3). This value was: C'.y = 3.90 × 10 .5 W/kgm3/V~
Calculation of fluorescence efficiency An extensive series of calibrations was done to enable the calculation of fluorescence efficiency from monitored signals (photodiode signal, gamma ion chamber current, test assembly temperature and pressure, and ~5U fission rate from an external fission chamber). These calibrations are described in this appendix. The first set describes the methods used to calculate the induced fluorescence from gamma pumping alone. This was necessary because most of the fluorescence is due to gamma pumping, particularly at the higher pressures. The second set of calibrations addresses the inference of Z3SU fission reaction rate on the coated fuel plate. Measurements described in Section 2.0 of this paper were of mass and energy escape fraction of a given fuel plate, so a monitor of specific fission rates (fissions/s/g 23SU) is required. Both the gamma measurement and the fission rate measurement used external monitors to infer parameters within the test assembly. Thirty-five 6-mm CaF Thermoluminescence Dosimeters (TLDs) were placed inside the test assembly in a line, to obtain the axial profile and absorbed dose of the gamma field. They were irradiated for 10 min, and separate calibrations were made for the shutter open and shutter closed cases. The dose rates obtained were axially integrated yielding 1.337 × 105 r a d c m / h and 1.19 × 105 r a d c m / h for the shutter open and closed respectively (that is, the shutter degrades the gamma field about 11%). The dosimeters were calibrated to yield the absorbed dose in iron. At an average energy of 1 MeV, the dose absorbed in xenon is 0.967 of that of iron, and this factor was applied to the
and C.~ = 4.09 × 10 .5 W / k g m 3 / V v where V~ and V.~ are the voltages produced by the ion chamber for the shutter open and closed respectively. With ion chamber voltage and pressure monitored, the total heating power to the gas due to gammas can be calculated. Therefore, the power due to gammas is: C =
c pv,
It was postulated that there would be some contribution due to direct neutron heating and/or activation. The basis for this postulate was that the diode signal decreases by some 20% when the cadmium shutter was closed, but the gamma signal only decreases by 11% (cf., Fig. 5 of the diode signal for test UF5A). To calculate a neutron contribution, it was assumed that the total power to the diode consists of a neutron and a gamma component, which is written as ,
Puv = n,P +Pn. No attempt is made to derive a neutron induced efficiency in the above equation. After a change (i.e., shutter opened)
P;v: n P; +Pn', where it is assumed that the gamma efficiency is constant. If it is assumed that
P;;a'Pn,
14
G.R. Imel et al. / NucI. Instr. and Meth. in Phys. Res. B 103 (1995) 1-14
and A is represented by the change seen by the fission chamber, then an effective % and Pn can be calculated. Puv-A'Puv
Pn = PuvThe gamma efficiency is then calculated continuously as in Fig. 6 by
in the external fission chamber and the fission fragment power deposited in the gas (Pff) is obtained. The calibration factor was (3.83 × 10 -9) W/cps for the plate used in this series. It was further assumed that the gamma efficiency and the ratio of neutron heating to gamma plus neutron heating was known from the previous unfueled tests. With fuel in the test assembly, the total uv power is Puv
P,v-Pn
+
The gamma power is =
where P.~ is monitored as described earlier, and % is assumed to be known and constant. The neutron power is It is assumed that the ratio PJP~v is a linear function of A, the monitor of the neutron flux obtained with the shutter open and closed, such that Pn/Puv=K • A + C .
Given the measured ratios of Pn/Puv for the shutter opened and closed cases, the constants K and C are obtained. For the test UF5A, the slope K was 0.14776, and the constant C was 0.00386. The fact that C is so small suggests the fact that the inferred neutron contribution to the diode signal is nearly directly proportional to the change in A. The UV power (Puv) as monitored by the DVM voltage of the photodiode depends on window transmission, instrumentation settings such as sensitivity and preamplifier gain, and the effective view angle, which was calculated using Monte Carlo techniques for gamma and fission fragment pumping. The constants are 1.885 X 10 -7
W / m V DVM signal
and 1.442 × 10-7 W / m V DVM signal for gamma pumping and fission fragment pumping respectively. For the fueled tests, calibrations were done to determine the fissions/gram-Z35u per counts per second of a 235U fission chamber placed externally to the test assembly. This was done by placing gold foils on an aluminum dummy plate, while monitoring the external fission counter. Additionally, the fission counter was placed inside the test assembly while gold foils were irradiated externally (in the fission counter location). In this manner, the gold (n,'y) and 235U (n,f) reaction rates were cross-correlated, and the albedo due to the fission plate backing and holder properly treated. Given that the number of fissions/s in the fueled plate can thus be measured, the escape efficiency as given in Section 2.0 is combined with the fission rate to yield an energy deposition rate in the gas from the fission fragments. A calibration factor relating the measured counts/s
K.A+C
1 -x
-c
where it is assumed that the constants K and C are known from the unfueled tests. The fission fragment induced uv power is then e
if_
1 v-Puv-
I_K.
c ,
and the efficiency is
References [1] A.M. Voinov, Laser and Particle Beams 11 (1993) 635. [2] R.J. Lipinski, D.K. Monroe and P.S. Piekard, 11th Symp. on Space Nuclear Power systems , Albuquerque, NM, USA, 1994. [3] R.T. Schneider and F. Hohl, in: Advances in Nuclear Science and Technology, J. Lewins and M. Beeker, eds. (Plenum, New York, 1984). [4] M.A. Prelas, F.P. Boody, G.H. Miley and J. Kunze, Lasers and Particle Beams 6 (1988) 25. [5] S. Kahn, R. Harman and V. Forque, Nucl. Sci. Eng. 23 (1965) 8. [6] M.A. Prelas et al., 1988, op. cit. [7] R.A. Waiters, J.D. Cox and R.T. Schneider, Trans. Am. NucI. Soc. 34 (1980) 810. [8] A.M. Voinov (1993), op. cit. [9] W.P. Poenitz and J.W. Meadows, Nuclear Data Measurements Series, Argonne National Laboratory,ANL/NDM-84, Nov. 1983. [10] W.P. Poenitz, D.W. Maddison, J.M. Gasidlo, S.G. Carpenter and R.J. Armani, Argonne National Laboratory, ANL-87-5, Jan. 1987. [11] Y.M. Alexander and M.F. Gagdek, Phys. Ref. 120 (1960) 874. [12] Y.B. Niday, Phys. Ref. 121 (1961) 1471. [13] W.G. Davey and P.I. Amundson, Nucl. Sci. Eng. 28 (1967) 111. [i4] J.R. Torczynski et aL, NueI. Sci. Eng. 101 (1989) 280.
B
N
Beam Interactions with Materials & Atoms
ELSEVIER
Measurement of fission fragment induced fluorescence in xenon G.R. Imel a,,, W.P. Poenitz a, A.M. Snyder b a Argonne NationaILaboratory, P.O. Box 2528, Idaho Falls, ID 83403-2528, USA b LockheedIdaho Technologies, Inc., P.O. Box 1625, Idaho Falls, ID 83415, USA
Received 9 January 1995; revised form received 10 April 1995 Abstract A research program was conducted at Argonne National Laboratory-West (ANL-W) to measure the 172 nm fluorescence in xenon induced by fission fragments. Additionally, the survivability of uranium dioxide coatings under high fission power conditions was investigated, and the fission fragment escape energy from coatings of different thicknesses was directly measured. A definite rearrangement of the uranium dioxide coating was observed after high power irradiations, but this rearrangement did not affect the fission fragment energy escape fraction. The measurement of fluorescence induced by fission fragments was done in the Neutron Radiography Reactor (NRAD); this measurement was complicated by the presence of the gamma field and by gas impurity effects. Compensations for gamma and impurity effects were made during the program. Additionally, signal degradations during system pressure changes were observed; however, the signal recovered after several minutes, so this phenomenon did not adversely affect the measurements. The effficiencies of fission fragment and gamma induced fluorescence were measured to be 0.37 and 0.44 respectively.
1. Introduction
A research program was conducted at Argonne National Laboratory-West (ANL-W) to measure the efficiency of fluorescence induced by fission fragments in xenon; specifically, the 172 nm Xe~ line was measured. The high power densities potentially available from fission fragment energy deposition has stimulated research in nuclear pumped lasers since the 1960s [1], and more recently, concepts are being developed to use ground-based nuclear pumped lasers to beam power into space [2]. The research described in this paper provides data of use to a photolytically driven excimer laser in which the fission fragment induced fluorescence pumps a lasant gas. A nuclear pumped laser or nuclear driven flashlamp uses the energetic reaction products of nuclear reactions to drive a laser or excite fluorescence in materials. The reaction products used can be alpha particles from certain neutron absorption reactions (e.g., l°B(n,a)7Li), fission fragments, or gamma rays from thermonuclear explosions.
~"Work supported by the U.S. Department of Energy under Contract W-31-108-ENG-38. * Corresponding author. Tel. + 1 208 533 7941, fax + 1 208 533 7471.
Besides a weapons-driven gamma source, which is essentially uncontrollable, fission fragments from nuclear fission provide the most energy available for excitation into the medium for a given energy input. This has led to studies of nuclear reactor driven lasers and flashlamps in which a reactor provides an intense source of neutrons that can induce fissions on a target of fissile material (e.g., 235U or 239pu). The target is designed so a large number of the fission fragments can escape and lose their energy through excitation of the lasing or fluorescence medium; this is accomplished by using very thin films of fissile material, using a gas of fissile material (e.g., UF6), or creating an aerosol of fissile material particles. For this work, the use of thin films was used, specifically uranium dioxide plated on aluminum. A photolytically driven laser uses fluorescence energy to pump the laser; the rare gases (e.g., Xe) are efficient fluorescent media. As an example, the fluorescence from Xe~ at 172 nm can be used to dissociate XeF2 and drive a XeF* laser photolytically. Further details of nuclear pumped lasers can be found in several overview articles E3,4]. Energy efflciencies of certain processes must be known to design a photolytically driven excimer laser. These are fission fragment energy deposition into the fluorescent, conversion to fluorescence energy, coupling of this energy to the lasant, and subsequent laser energy efficiency. This
0168-583X/95/$09.50 © 1995 EIsevier Science B.V. All rights reserved SSDI 0168-583X(95)00564-1
2
G.R. Imel et al./Nucl. Instr. andMeth, ht Phys. Res. B 103 (1995) 1-14
paper will describe the measurement of the first two stages; i.e., the efficiency of fission fragment energy deposition (defined as "Off) in 2 v geometry, and fluorescence efficiency (defined as "Ouv) which is total fluorescence energy divided by fission fragment energy deposited in the gas (xenon). Additionally, the survivability of fissile coatings on aluminum fuel plates was studied as the plates were subjected to high neutron fluxes; the concern was that "Off would be adversely affected by rearrangement of the fissile coating due to the high fission power achieved in the fuel plate. Three reactor facilities at ANL-W were used for these measurements: The Argonne Fast Source Reactor (AFSR), the Transient Test Reactor (TREAT), and the Neutron Radiography Reactor (NRAD). Several issues were addressed in the measurement of "Off. Flat aluminum plates, 5 cm × 5 cm × 6.4 mm were coated with uranium dioxide (UO 2) by the Reuter-Stokes Corporation to nominal loadings of 0.5, 1.0, 1.5, 2.0, and 2.5 mg UO2/cm 2. The dependency of coating thickness on fission fragment escape energy from a flat plate source has been reported [5]. It was wanted to investigate possible differences compared with the results of Ref. [5] for the specific coatings obtained from Reuter-Stokes, which were produced by electroplating uranium metal followed by oxidation. It was important to investigate if the effects of irradiation of the fuel plates would adversely affect the energy escape fraction. This issue is particularly significant in pulsed reactor-pumped laser systems in which the power densities in the fissile material are quite high. The plates from Reuter-Stokes were subjected to high power transients in the TREAT reactor, achieving power densities about 2 M W / g 235U. Survivability of the coating and changes in the fission fragment energy escape fraction were issues studied. Precise measurement of the total mass of the coatings before and after irradiation was required to detect if any fissile material was lost. Direct measurements of the fission fragment escape energy were made using an ionization fission chamber; Monte Carlo calculations yielded adjusted results for the efficiency. These values were then used in unfolding the measurements of the fluorescence efficiency. Ranges of fluorescence efficiencies from 39 to 68% for "O~,, have been reported [6,7]. As lasing depends on achieving threshold power densities, the above spread of efficiencies is not acceptable to design a system. This is particularly true since most reactors available for nuclear pumped laser systems have marginal neutron fluxes [8]. The measurements of "Ouv in xenon described in this paper were performed using a calibrated photodiode in a special test assembly in which the gas pressure could be varied. The test assembly loaded with a fuel plate was placed in the beam of the NRAD reactor. Controlled steady state measurements of vacuum ultraviolet (VUV) output under variations in pressure, neutron beam, and gamma beam parameters were done. These measurements
were complicated by the gamma background of the beam, and by gas impurity problems. The following sections describe this measurement program in detail.
2. Measurement of r l ~ - fission fragment energy depo-
sition efficiency This phase of the research dealt with characterization of the UO 2 coated fuel plates as obtained from Reuter-Stokes and their stability under high power irradiations. This included visual inspection, scanning electron microscopy (SEM), Auger electron, and electron dispersive spectroscopy (EDS) examinations of unirradiated and irradiated plates, measurements of fissile masses via c~ particle counting, and direct measurement of the fission fragment escape fraction. Destructive chemistry was also done on some samples to obtain isotopic information. Initially, calorimetric measurements of total fission energy were done in the TREAT reactor to determine the survivability of the UO 2 coating. This issue is important for any nuclear pumped laser system that uses fuel coating subjected to high neutron fluxes during reactor pulses. Two coated plates were irradiated in the TREAT reactor in successive transients of up to 16000 MW peak power (pulse widths of 100 ms FWHM). Temperatures attained in the aluminum plates were monitored with embedded thermocouples. Examination of these data suggested that the peak temperatures did not decrease from transient to transient, which in turn shows that most of the coating survives the high power densities necessary for reactor pumped laser systems. However, these calorimetric measurements do not have the precision necessary to measure either fission fragment energy escape or total mass on the plates. It was thus decided to do more precise measures of these parameters, and further investigate the characteristics of the coating before and after irradiation. Alpha counting was done on one plate before and after irradiation in TREAT. These results showed that little, if any, fissile material was lost from the plate after irradiation, at least within the precision of the analysis. This result was consistent with the calorimetric measurement done in TREAT during the irradiations. Subsequent measurements of other plates described later in this paper show this plate was an anomaly in that so little material was lost. However, the c~ energy spectrum taken after irradiation showed a substantial increase of the width of the distribution when compared with the spectrum taken before the irradiation, showing the possibility of fuel rearrangement on the plate. Visual examinations of both samples showed that the coverage of the coating was not uniform after irradiation. Additionally, the color of the coating was distinctly brighter than found on unirradiated samples from the same batch. Because of the visual difference of unirradiated fuel plates
G.R. Imel et al. /Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 1-24
3
Fig. 1. Unirradiated fuel plate SEM (1000 × ).
versus irradiated fuel plates, it was decided to do a series of examinations using SEM, Auger, and EDS. The Auger and EDS examinations did not reveal any significant difference between the irradiated and unirradiated plates. However, the SEM examinations did reveal some very interesting differences in the coating characteristics. Examples of the SEM results at magnifications of 1000 × are shown in Fig. 1 (unirradiated plate) and Fig. 2 (irradiated
plate that had received 5 × 1015 total fissions). The coating of the unirradiated plate in Fig. 1, shows a fairly flat, but cracked, coating, similar to a di:ied mud flat. In contrast, the coating of the irradiated plate shows a distinct "clumping" effect; particles about 10 ~m appear on the surface with large areas of bare aluminum between-fuel particles. It is felt that this explains the brighter coloration of the irradiated plates, i.e., aluminum ig~simply showing through.
Fig. 2. Irradiated fuel plate SEM (1000 × ).
4
G.R. Imel et al./Nucl. Instr. andMeth, in Phys. Res. B 103 (1995) 1-14
Table 1 Masses from alpha counting (c~-counting, mg of total U) A~ 1 2 3 4 5 7 9 10
Sum
Mass before irradiation [mg]
Mass after irradiation [mg]
No. of fissions
% change
86.9 58.1 57.3 56.0 58.2 58.3 58.2 55.5 58.5 547.0
85.1 57.1 55.4 55.4 55.0 57.0 57.0 54.7 53.3 530.0
5 X 10 ~s 1.6 × 10 ~s 1.6 × 10 ~s 1.6 × 10 ~s 1.6 X 1015 1.6 X 10 ~s 1.6 X 10 ~s 1.6 X 10 t5 1.6 X 10is 1.6 X 10 ~5
2.1 1.7 3.3 1.1 5.5 2.2 2.1 1.4 8.9 3.1
a The A plates were counted as a pair.
experimental series. It was especially important to know whether the fuel redistribution or "clumping" would cause a decrease in the escape fractions reported in Ref. [5]. The measurement of pre-and post-irradiation masses nondestructively was crucial to the measurement of r/u,,. It was decided to use c~-counting (primarily from the 234U in the UO z) as the preferred method, particularly since it yields a highly accurate relative measure of changes in total fuel mass, and it is a nondestructive assay technique. Additionally, a counting has been shown to yield accurate measurements of absolute masses of uranium in coatings, as shown by comparisons with destructive wet chemistry and fission counting [9,10]. Destructive chemical analysis, in which the coating is leached off the plate, was also done on some samples to obtain isotopic information.
It is assumed that high temperatures in the fuel coating caused that clumping effect (recall that power densities about 2 M W / g were achieved in the coating). Cross-sectional examinations revealed that only minor amounts of fuel could have been driven into the aluminum plate, which is consistent with the lack of low energy tail in the observed a energy spectra. Despite the exact mechanism, it was clear from the SEM examinations that there had been rearrangement of the fuel coating after irradiation in TREAT. Besides measuring the relevant efficiencies r/el and rh,, in unirradiated plates, it was wanted to learn if high power irradiations typical of nuclear pumped laser systems would adversely affect these efficiencies. Consequently, it was decided to do precise measurements on each plate to be used in the
90
80
70
-Flat Plane - - - Restructured • Unirradiated 0 Irradiated • U-235 Reference (1.3 mg total U) ........... Kahn, et al. (Reference 5)
".~ "...~ ~ , , , ".. \ ~ "'-,~
........~ "'".
- - . ~ - - ~
60
t-, ILl 50
......... .........> . .
""..... ~
............O....~
40
30
I
2 Nominal Uranium Thickness (mg UO2 / cm2) Fig. 3. Average kinetic energy of fission fragments leaving deposit.
-
G.R. Imel et al. / Nucl. hlstr, and Meth. in Phys. Res. B 103 (1995) 1-14
The results of the c~ counting of a sampling of the plates before and after irradiation are summarized in Table 1. As shown in Table 1, there is about an average 3% loss of mass after irradiation; material lost during handling cannot be ruled out as a cause. For the measurement of efficiency, the mechanism does not matter if a precise measurement of total mass is made of the plate to be used before the actual efficiency measurements. Because of the fuel coating rearrangement that occurs during irradiation as showed by the SEM examination, it was decided to do a direct measurement of the fission fragment energy escape. This was needed to determine if this rearrangement would adversely affect the escape energy (over that assumed for a flat coating as in Ref. [5]). A parallel plate ionization chamber was constructed with the collector plate-fissile deposit spacing such that all fission fragment energy escaping from the coating was deposited in the counting gas. The counting gas was P10 (90% argon, 10% CH4). Fission fragment energy spectra were obtained in 2'rr geometry using the thermal column of the AFSR reactor. Unirradiated plates of various coatings and irradiated plates of relatively high loadings (up to 2.5 m g / c m 2) were measured. The spectra of the fission fragments were recorded with a multi-channel pulse height analyzer. The calibration of the energy deposition was achieved with a very thin deposit (61.6 ~ g / c m 2) of known mass, which has a 99.5% fission fragment escape fraction into 4'rr. A Monte Carlo simulation of the energy escape fraction of fission fragments from a UO 2 deposit was done, taking into account the energy and mass distri-
butions in thermal fissions. Energy loss and range parameters were taken from the literature [11,12]. Calculations were made for a homogeneous fiat plane deposit and for a restructured clumped deposit. The size of the clumps was chosen to reproduce the increase of the c~-peak energy width observed with the low-geometry counter after irradiation. These results are compared in Fig. 3 with the measured values (also shown are values from Ref. [5]). As can be seen from the figure, the escape energy of the irradiated plates is in the range of 39-42 MeV. This yields a nominal energy escape fraction of 0.24 (assuming 168 MeV per fission) for irradiated plates of 2.5 mg UO2/cm; loadings. (It should be noted that in 2'n" geometry, 0.5 is the maximum escape fraction.) There is essentially no difference between the irradiated and unirradiated samples that cannot be explained simply by the difference of the amount of material on the various samples. The Monte Carlo simulation shows that the restructuring during irradiation should have increased the energy loss in the deposit. That this was not so is probably due to compensating effects (e.g., a change of the oxidation state of the deposit, or smoothing of the backing, etc.). The differences between the current measurements and the calculations based on parameters from the literature on the one hand and the experimental data from Ref. [5] on the other hand are probably due to effects that are difficult to define (structure of the backing, structure and chemical composition of the fissile deposit). The conclusion of this phase of the research is that the plates do not have the energy escape fraction that would be predicted for a perfectly flat
REMOTE :SOLENOID V A L V E B
GAS BOTTLE
TA TEMP TA PRESS
~/
5
DIODE SIGNAL
Fig. 4. Simplified schematicof system for measurementof ~v.
6
G.R. Imel et al,/NucI, hzstr, and Meth. bz Phys. Res. B 103 (1995) 1-14
deposit, but that fact is true for both irradiated and unirradiated samples. Additionally, the rearrangement that occurs during irradiation does not produce further degradation of the energy escape fraction.
3. Measurement of r / u v - fission fragment induced fluorescence efficiency Methods described in the previous section allowed accurate measurement of fission fragment escape energy fractions for UO e coated plates, whether irradiated or not. Upon completion of that phase of the project, equipment was designed and built to measure the efficiency of fluorescence induced by the fission fragments in xenon (r/uv). The NRAD reactor at ANL-W was chosen for this phase of the experiment because of its neutron beam and shielded beam room (normally used for radiography). Neutron fluxes in the beam room are about 10 s n/cmZs. Total power densities available for the efficiency measurement are quite low in the NRAD beam: about 4 × 10 .3 W / g a35U (contrast with the 2 M W / g in TREAT) for fission power and about 2 × 10 .5 W / g for gamma power (in xenon). The equipment was designed to allow 0-3 atm xenon gas to be remotely pumped into a 1.5 1 test assembly.
On-line calibrated monitors of neutron flux, gamma dose, and test assembly temperature and pressure were placed in the system. A simplified schematic of the system is shown in Fig. 4 and the test assembly is shown in Fig. 5. The photodiode used for measurement of the VUV output was an SAI ultraviolet standard photodiode (EVSD model 540-000), with a magnesium fluoride window, and a cesium telluride photocathode of 1 cm diameter uniform active area; it was calibrated by the National Bureau of Standards (now NIST). In the original system, a shutter was placed inside the test assembly to absorb the fission fragment energy; when closed, the shutter provided a flat plane parallel and very close to the fuel coating such that fission fragments would lose their energy in the shutter instead of the xenon. This was to allow direct measurement of the background gamma induced fluorescence. Through the initial testing phase, it was discovered that operation of the shutter was causing degradation of the signal. This degradation was possibly caused by air infiltration through the shutter shaft seals (i.e., movement of the shaft would cause a temporary break in the shaft seal). Consequently, improvements were made to the system to remove the internal shutter by replacing it with an external neutron absorbing shutter (1 mm of cadmium). Additionally, modifications were made
Diode assembly
Fuel p l a t e . Vessel ....
/
Chopper assembly
t9
.....
Vacuum/gas
connection
Fig. 5. Conversionefficiencytest assembly.
G.R. Imel et al. / Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 1-14
7
800 700 600 500
g 400 o
300
5
200 100 1
10b00
20L00
30~00
4000
5000
Time (Seconds)
Fig. 6. Photodiodesignal(mV)fortest UF5A.
to allow a cleanup (through bakeout and simultaneous vacuum pumping) and a refill of the test assembly with fresh gas without opening any gas lines to the atmosphere. As part of the impurity control, a nickel-plated test assembly was used exclusively for this test series. Data during the tests were logged every 10 s. The photodiode digital voltmeter (DVM signal), the gamma ion
chamber current, the test assembly pressure, and the test assembly temperature were logged. In addition, the 235U and 237Np (a threshold fission reaction with Eth ~ 0.6 MeV) fissions in calibrated chambers were monitored via a multi-channel analyzer. The calibration of the ion chamber was accomplished using a string of 35 6-mm CaF Thermoluminescent Dosimeters (TLDs) inside the test assembly
0.6
0.5
0.4
0.3
0.2
0.1
0.0
T
r
1
T
1000
2000
3000
4000
Time (Seconds)
Fig. 7. Fluorescence efficiency(Etauv) for test UF5A.
5000
8
G.R. Imel et al./Nucl. Instr. andMeth, in Phys. Res. B 103 (1995) 1-14
(yielding the axial profile of the gamma field inside the test assembly), while the ion chamber was placed in a fixed position external to the test assembly. Calibrations were done for both the cadmium shutter open and closed cases, as the cadmium shutter does affect the gamma field. The use of the TLD measurements to estimate the gamma induced fluorescence is described in the Appendix of this paper. The neutron flux was calibrated using gold foils and a calibrated fission chamber, as described in the Appendix. The special fission chamber used is described in the literature [13]. Corrections for the gold/235U reaction rates were made. Several types of experiments were done in this program. Unfueled experiments (test assembly full of gas, but no fuel plate) using the reactor determined the efficiency of pumping by gammas alone (with a component from neutron heating). In addition, several experiments were performed using a pure gamma source (decay gammas from a spent reactor subassembly). The fueled experiment reported in this paper utilized plate 10 as shown in Table 1. A typical diode signal from this test series is shown in Fig. 6, which is due to gamma and neutron heating. The data were taken every 10 s; since the diode and electronics were susceptible to noise, noise spikes were removed from the signal presented in Fig. 6, but no data smoothing was done. The reactor was constant at 250 kW, and the system was initially filled to 1.89 bar. To start the test, the large biological shield in the NRAD beam was opened at 0 + s. At 2000 + s, the cadmium shutter was closed, and opened again at 2500 s. At approximately 2700 s, pressure was decreased to 0.92 bar, and at 4000 s, the reactor was
scrammed. To calculate an efficiency from this diode signal, the calibrations of the diode, gamma ion chamber, and fission counter were used as described in the Appendix. The gamma efficiency as calculated for this test is shown in Fig. 7, along with horizontal lines representing the first and third quartile of the data from the period 280-2950 s. Because the measurements taken during these experiments typically did not exhibit the characteristics of stationary time series, confidence intervals using conventional statistical distributions (e.g., normal) could not be set. After performing the usual time series data analysis, it was determined that the median of the series provides the most representative value for the efficiency, while the first and third quartiles yield the error bars (without a statistical measure of confidence). For the UFfA test, the median is 44%, consistently predicted at open and closed shutter position, at the two pressures, and even somewhat during the reactor scram. The first quartile is 0.4401 and the third quartile is 0.4448; half the measurements for the period are within these values. It is significant that the efficiency dropped during the pressure change around 3000 s but did recover. This behavior was consistent throughout all the series in that a pressure change would temporarily degrade the signal. The pressure decrease was accomplished in about 160 s. During this period, the efficiency as shown in Fig. 7 steadily decreased, followed by a recovery of the efficiency that tookabout an equivalent amount of time, for a total of 330 s of degraded efficiency. It is feasible that gas dynamic phenomena could cause the degradation and subsequent recovery in the signal. Variations in density can cause changes in local fluorescent energy, or can cause changes in the gas refractive index [14]. It should be
1000
S
f-._.~
800
G" 600
g 400 O
2O0
-200
I 500
10JO0
15100
I 2000
Time (Seconds)
Fig. 8. Photodiodesignal for test UF2D,
I
2500
3000
G.R. hnel et al. / Nucl. bzstr, and Meth. in Phys. Res. B 103 (1995) 1-14
0.6
0.5
0.4
0.3 ILl
0.2
0.1
0.0
i 500
10lO0
15100
i 2000
2500
Time (Seconds)
Fig. 9. Fluorescence efficiency (Etauv) for test UF2D.
noted that this test involved only gamma and neutron heating, which is quite uniform across the test assembly; there clearly would be non-uniform heating for fission fragment pumping. No attempt was made in these experiments to model quantitatively the effects of gas dynamics; compensation for the effects was made by allowing enough time for the measurements so that the system could come to equilibrium after pressure changes.
The diode signal for another unfueled test (UF2D) is shown in Fig. 8. In this test, the changes in the diode signal are driven by pressure changes: at the start of the test, the pressure is 1.86 bars, it is increased to 2.43 bars at 640 s, decreased back to 1.86 bar at 1140 s, then stepped through 1.32 bar, 1.1 bar, and 0.81 bar. The efficiency as measured for the this test is shown in Fig. 9. As seen, it is consistent with the test UF5A; the median value for all of
500
400 G" :~ 300
g
r-"
._m 200 ._o Q
100
500
1000
2000 Time (Seconds)
Fig. 10. Photodiodesignalfortest G1A.
2 O0
3000
10
G.R. Imel et al. /Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 1-14
5 < ,= YJ
500
1000 '
1500 ~
2 0 O0
25~O0
3000
Time (Seconds)
Fig. 11. Fluorescence efficiencyfor test G1A (arbitrary units).
the test period (including the periods of frequent pressure changes) is 0.434, while from 210-1540 s (before the rapid and frequent pressure changes) the median is 0.438. The first and third quartiles shown in Fig. 9 are at 0.4335 and 0.4423 respectively from 210-1540 s. Clear perturbations in the inferred efficiency are seen during the pressure changes at 640 s, 1140 s, and 1500 s. After 1500 s, the
overall efficiency declines, probably because of subsequent rapid pressure decreases such that the system does not achieve equilibrium. Impurity control was a problem in this test series. After doing the UF2D test, the test assembly was evacuated and refilled with fresh gas. It was left overnight, and the next test (UF3) was done. The gamma efficiency dropped to
1000
800
600 ¢.
O3 "o o
400
200
I
50O
10'00
15'00
20JO0
25100
Time (Seconds) Fig. i2. PhotodiodesignaI for test F533B.
30'00 35~00
4000
11
G.R. Imel et al. / Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 1-14 800
700
....
gamma/neutron diode signal fission fr;~gment diode sigrial
6O0
500
g 400 300 a
/
2OO
500
1000
1500
2000
2500
3000
3500
4000
Time (Seconds)
Fig. 13. Photodiode signal due to gamma/neutron heating and fission fragment heating for test F533B.
ciency does not recover significantly during the irradiation period, as it did in Fig. 7. This does suggest the existence of two mechanisms for degradation: a long term, irrecoverable impurity contamination, and a short-term degradation, possibly due to gas motion during pressure changes. It was wanted to determine if the effect of the pressure change seen in Figs. 7 and 9 was in any way dependent on
15%, a factor of three decrease, which suggests impurity contamination either through outgassing of the test assembly overnight, or air infiltration. A pumpdown and refill did bring the efficiency immediately back to 445. It is also significant that the overnight contamination effect is different from a pressure change effect during a run. The effects of overnight contamination are such that the effi-
0.6
I I lt
ILl
twTI
t
0.2
0.1
0.0
,~ .... 0
,
500
1000
1500
2000
,
i
2500
3000
3500
Time (Seconds)
Fig. 14. Fluorescence efficiency due to fission fragment heating for test F533B.
4000
12
G.R. Imel et aL / Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 1-14
the neutron flux through some unspecified activation reaction (e.g., introduction of impurities during a pressure change followed by burnout of the impurities through some neutron reaction). To remove neutron activation effects, a spent EBR-II driver subassembly was placed in the radiography specimen tube of the NRAD facility, yielding a mostly pure gamma field (except some spontaneous fission neutrons). This gamma field could not be calibrated adequately because of the complicated shielding caused by the gas system hardware between the source and test assembly. The diode signal is shown in Fig. 10. The pressure was increased from 0 to 1.89 bar in 120 s, increased to 3.3 bar at 1200 s, bled to 1.89 bar at 1800 s, and bled to 0.92 bar at approximately 2200 s. The efficiency (in arbitrary units) is shown in Fig. 11. As seen, the degradation and short term recovery due to the pressure change are quite evident; this eliminates any neutron phenomena as the cause. In the last phase of the experiments, a coated fuel plate was placed in the test assembly. A typical diode signal is shown in Fig. 12. Here, the test assembly was initially filled to 0.92 bar, the cadmium shutter cycled twice, the pressure increased to 1.89 bar at about 2000 s and the shutter cycled twice. To calculate the efficiency due to fission fragments alone, it was assumed that the efficiency due to gamma pumping was known (from the unfueled tests), as described in the Appendix. A gamma and neutron induced diode signal was inferred, and subtracted from the total diode signal to yield that due to fission fragments alone, which was then used to calculate the efficiency. The adequacy of this method was checked by calculating a predicted gamma/neutron induced diode signal for the fueled test via the methods described in the Appendix. The predicted signal was compared with the signal measured in an unfueled test that immediately preceded the fueled test. The agreement was within a few percent, showing that the background gamma/neutron signal was being properly subtracted from the fueled test. The diode signals due to fission fragment and combined gamma/neutron pumping as predicted by these methods are shown in Fig. 13. It is important to note in Fig. 13 that the fission fragment contribution is a small fraction of the gamma/neutron pumping, even at the low pressure of 0.92 bar. At higher pressures, the fission fragment contribution is less than 20% of the gamma contribution. Improvements to these measurements would require a smaller volume for the test assembly, or a larger neutron flux compared with the gamma field. The efficiency due to fission fragments as calculated from these results is shown in Fig. 14. When the cadmium shutter is closed, there is essentially no fission fragment signal, as is seen by looking at Fig. 13; this leads to the extremely noisy areas shown in Fig. 14. The median efficiency is 0.366, the first quartile is 0.3544 and the third quartile is 0.3731. The median and quartiles were obtained utilizing data only when the shutter was open. Again, the
degradation due to a pressure change is clearly seen at about 2000 s.
4.
Summary
An experimental program has been described in this paper in which measurements of fission fragment escape energy and fission fragment induced fluorescence utilizing UO a coated aluminum plates were done. For these data to be of interest to nuclear pumped laser programs, it was necessary to learn if the measurements would also be valid for plates subjected to high neutron fluxes. Therefore, a series of experiments and examinations were conducted to study the effects of irradiation on the coatings. From transients conducted utilizing the TREAT reactor, it was found that the coating definitely changed after irradiation, probably due to high internal temperatures achieved. Additionally, small amounts of fissile material were lost between the pre-irradiation and post-irradiation alpha counting; losses simply due to handling could not be ruled out as the cause. However, subsequent direct measurements of the fission fragment energy showed that the coating changes did not change the escape fraction, probably due to compensating effects. Given these measurements, it was determined that the coated plates could be used in a nuclear pumped laser system if wanted; a series of measurements was subsequently done on the fission fragment induced fluorescence in xenon. Controlling the effect of impurities was necessary while doing the fluorescence measurement. This was done by designing the system so that it was possible to bakeout the system while simultaneously vacuum pumping, followed by fresh gas refill, without opening any of the system to the atmosphere. Even with this impurity control, a temporary degradation of the diode signal was observed due to gas pressure changes; however, the efficiency recovers within 5-10 min. This temporary degradation is possibly due to gas dynamic effects. The best estimates of efficiencies based on these tests are: "quv= 0.44
(y pumping),
r/u,, = 0.37
(fission fragment pumping).
The thermal neutron contribution is 15%, and no correlation with fast neutrons (E > 0.6 MeV) as monitored by the 237Np reaction was found. Improvements could be made to the hardware to yield a better measurement of "quv due to fission fragments. Testing a lightly loaded plate to compare differences was not possible because of the decreased signal. Also, testing at higher pressures was not possible because of the increase of the gamma signal (with no corresponding rise in the fission fragment signal). Significant improvements could be made by incorporating a clad moderator inside the test
G.R. Imel et aL /Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 1-14 assembly (as a backstop), shrinking the volume of the test assembly, using a boral shutter (which would be a 1/v absorber, closer to the ~SU fission reaction rate than cadmium), and the use of a lead filter. These changes would increase the fission fragment signal over the gamma signal. Due to cancellation of the program, pursuing these issues further was not possible.
Acknowledgements The support of the following people is gratefully acknowledged: E. Wood for the SEM analysis, D. Maddison for the alpha counting, J. Fincke, K. Watts, P. Dickson (now of Westinghouse Savannah River) and D. Nigg from Lockheed Idaho Technologies for photodiode calibration, test assembly design, and other technical support, W. Murphy and the late R. McConnell for gas supply system design, and the NRAD Operations crew for reactor support.
Appendix A
13
inferred gamma dose. These measurements showed an average gamma dose in the test assembly of 6367 r a d / h and 5667 r a d / h for the shutter open and closed cases respectively. The effective heating volume of the test assembly (inside volume minus fuel plate and holder volume) was calculated to be 1.35 × 10 3 c m 3. From this, constants of 2.391 X 10 .5 W / k g m s and 2.127 X 10 .5 W / k g m 3 were obtained for the shutter open and closed respectively. To obtain total heating, the density was calculated to be:
p ( P ) = 5.488P - 0.603 k g / m 3 where P is the pressure in bar. The above density/pressure relation is valid at 305 K and at low pressures ( < 10 bar). At the same time that the TLDs were being irradiated, the voltage (proportional to current) from the external ion chamber was monitored. A correlation between the absorbed gamma dose inside the test assembly as obtained by the TLDs and that as monitored by the external ion chamber was obtained. A calibration value was obtained for chamber voltage vs. gamma power per gas density (in kg/m3). This value was: C'.y = 3.90 × 10 .5 W/kgm3/V~
Calculation of fluorescence efficiency An extensive series of calibrations was done to enable the calculation of fluorescence efficiency from monitored signals (photodiode signal, gamma ion chamber current, test assembly temperature and pressure, and ~5U fission rate from an external fission chamber). These calibrations are described in this appendix. The first set describes the methods used to calculate the induced fluorescence from gamma pumping alone. This was necessary because most of the fluorescence is due to gamma pumping, particularly at the higher pressures. The second set of calibrations addresses the inference of Z3SU fission reaction rate on the coated fuel plate. Measurements described in Section 2.0 of this paper were of mass and energy escape fraction of a given fuel plate, so a monitor of specific fission rates (fissions/s/g 23SU) is required. Both the gamma measurement and the fission rate measurement used external monitors to infer parameters within the test assembly. Thirty-five 6-mm CaF Thermoluminescence Dosimeters (TLDs) were placed inside the test assembly in a line, to obtain the axial profile and absorbed dose of the gamma field. They were irradiated for 10 min, and separate calibrations were made for the shutter open and shutter closed cases. The dose rates obtained were axially integrated yielding 1.337 × 105 r a d c m / h and 1.19 × 105 r a d c m / h for the shutter open and closed respectively (that is, the shutter degrades the gamma field about 11%). The dosimeters were calibrated to yield the absorbed dose in iron. At an average energy of 1 MeV, the dose absorbed in xenon is 0.967 of that of iron, and this factor was applied to the
and C.~ = 4.09 × 10 .5 W / k g m 3 / V v where V~ and V.~ are the voltages produced by the ion chamber for the shutter open and closed respectively. With ion chamber voltage and pressure monitored, the total heating power to the gas due to gammas can be calculated. Therefore, the power due to gammas is: C =
c pv,
It was postulated that there would be some contribution due to direct neutron heating and/or activation. The basis for this postulate was that the diode signal decreases by some 20% when the cadmium shutter was closed, but the gamma signal only decreases by 11% (cf., Fig. 5 of the diode signal for test UF5A). To calculate a neutron contribution, it was assumed that the total power to the diode consists of a neutron and a gamma component, which is written as ,
Puv = n,P +Pn. No attempt is made to derive a neutron induced efficiency in the above equation. After a change (i.e., shutter opened)
P;v: n P; +Pn', where it is assumed that the gamma efficiency is constant. If it is assumed that
P;;a'Pn,
14
G.R. Imel et al. / NucI. Instr. and Meth. in Phys. Res. B 103 (1995) 1-14
and A is represented by the change seen by the fission chamber, then an effective % and Pn can be calculated. Puv-A'Puv
Pn = PuvThe gamma efficiency is then calculated continuously as in Fig. 6 by
in the external fission chamber and the fission fragment power deposited in the gas (Pff) is obtained. The calibration factor was (3.83 × 10 -9) W/cps for the plate used in this series. It was further assumed that the gamma efficiency and the ratio of neutron heating to gamma plus neutron heating was known from the previous unfueled tests. With fuel in the test assembly, the total uv power is Puv
P,v-Pn
+
The gamma power is =
where P.~ is monitored as described earlier, and % is assumed to be known and constant. The neutron power is It is assumed that the ratio PJP~v is a linear function of A, the monitor of the neutron flux obtained with the shutter open and closed, such that Pn/Puv=K • A + C .
Given the measured ratios of Pn/Puv for the shutter opened and closed cases, the constants K and C are obtained. For the test UF5A, the slope K was 0.14776, and the constant C was 0.00386. The fact that C is so small suggests the fact that the inferred neutron contribution to the diode signal is nearly directly proportional to the change in A. The UV power (Puv) as monitored by the DVM voltage of the photodiode depends on window transmission, instrumentation settings such as sensitivity and preamplifier gain, and the effective view angle, which was calculated using Monte Carlo techniques for gamma and fission fragment pumping. The constants are 1.885 X 10 -7
W / m V DVM signal
and 1.442 × 10-7 W / m V DVM signal for gamma pumping and fission fragment pumping respectively. For the fueled tests, calibrations were done to determine the fissions/gram-Z35u per counts per second of a 235U fission chamber placed externally to the test assembly. This was done by placing gold foils on an aluminum dummy plate, while monitoring the external fission counter. Additionally, the fission counter was placed inside the test assembly while gold foils were irradiated externally (in the fission counter location). In this manner, the gold (n,'y) and 235U (n,f) reaction rates were cross-correlated, and the albedo due to the fission plate backing and holder properly treated. Given that the number of fissions/s in the fueled plate can thus be measured, the escape efficiency as given in Section 2.0 is combined with the fission rate to yield an energy deposition rate in the gas from the fission fragments. A calibration factor relating the measured counts/s
K.A+C
1 -x
-c
where it is assumed that the constants K and C are known from the unfueled tests. The fission fragment induced uv power is then e
if_
1 v-Puv-
I_K.
c ,
and the efficiency is
References [1] A.M. Voinov, Laser and Particle Beams 11 (1993) 635. [2] R.J. Lipinski, D.K. Monroe and P.S. Piekard, 11th Symp. on Space Nuclear Power systems , Albuquerque, NM, USA, 1994. [3] R.T. Schneider and F. Hohl, in: Advances in Nuclear Science and Technology, J. Lewins and M. Beeker, eds. (Plenum, New York, 1984). [4] M.A. Prelas, F.P. Boody, G.H. Miley and J. Kunze, Lasers and Particle Beams 6 (1988) 25. [5] S. Kahn, R. Harman and V. Forque, Nucl. Sci. Eng. 23 (1965) 8. [6] M.A. Prelas et al., 1988, op. cit. [7] R.A. Waiters, J.D. Cox and R.T. Schneider, Trans. Am. NucI. Soc. 34 (1980) 810. [8] A.M. Voinov (1993), op. cit. [9] W.P. Poenitz and J.W. Meadows, Nuclear Data Measurements Series, Argonne National Laboratory,ANL/NDM-84, Nov. 1983. [10] W.P. Poenitz, D.W. Maddison, J.M. Gasidlo, S.G. Carpenter and R.J. Armani, Argonne National Laboratory, ANL-87-5, Jan. 1987. [11] Y.M. Alexander and M.F. Gagdek, Phys. Ref. 120 (1960) 874. [12] Y.B. Niday, Phys. Ref. 121 (1961) 1471. [13] W.G. Davey and P.I. Amundson, Nucl. Sci. Eng. 28 (1967) 111. [i4] J.R. Torczynski et aL, NueI. Sci. Eng. 101 (1989) 280.