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UNSOLICITED PROPOSAL SUBMITTED TO THE DEPARTMENT OF ENERGY'S DIVISION OF ADVANCED ENERGY PROJECTS R. GAJEWSKI, DIRECTOR BY Nuclear-Pumped Laser Corporation P.O. Box 214 Kingston, NJ 08528 FOR STUDY OF THE BASIC TRANSPORT PROPERTIES (CHARGED PARTICLE TRANSPORT, FLUORESCENCE TRANSPORT AND COUPLING EFFICIENCY) OF THE PHOTON INTERMEDIATE DIRECT ENERGY CONVERSION TECHNIQUE Proposed Duration: 3 Years Amount Requested:

$811,402

Requested Starting Date: September, 1984 Small Business.Jl Minority_ Profit_ Nonprofit~

[

Educational_.

Other

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Principle Investigator/a: Mark A. Prelas Frederick P. Boody Mark S. Zediker

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Phone: '

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M. A. Prelas -314-882-8201 F. P. Boody -609-683-2831 M. s. Zediker-314-232-6470

Business Contact: F. P. Boody Phone: 609-683-2831 Date of Submission: March 9, 1984 .~may

be subjected to

This Proposal _may not

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external review.

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Signiture of Principal Investigator

~-i?~cf Authorized Officer

NUCLEAR-PUMPED LASER CORPORATION PO BOX 214 KINGSTON NJ 08528-0214

STUDY OF THE BASIC TRANSPORT PROPERTIES (CHARGED PARTICLE TRANSPORT, FLUORESCENCE TRANSPORT AND COUPLING EFFICIENCY) OF THE PHOTON INTERMEDIATE DIRECT ENERGY CONVERSION TECHNIQUE

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Mark A. Prelas

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Frederick P. Boody and Mark S. Zediker .

Figure Captions Figure 1. Comparison of the Two-Step Photon Intermediate Energy Conversion Process used in the Aerosol Reactor Energy Conversion System (ARECS) with a hypothetical One-Step Direct Energy Convertor and a Thermal Conversion·System producing the same output as the ARECS. Figu,re 2. Non-Concentrating aerosol reactor/energy conversion system configuration. Figure 3. Concentrator design. Figure 4. A diagram of an Alkali Metal Thermoelectric Converter (AMTEC). Figure 5. A multicycle energy conversion scheme.

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Figure 6. Nuclear surface created or B) a used to

pumping sources are of two forms; A) a source where the high energy ions are in a coating on the walls of the cell, volume source where a gas additive is create the high energy ions in the plasma.

Figure 7. UF6 photoabsorption spectrum [24,25]. Figure 8. Fission fragment transport efficiency in microspheres and slabs of various thichnesses. Figure 9a. Comparison of reflectance by U02 to MgO. Figure 9b, Calorimetric Measurements of UV-visible absorption in various oxides.

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Figure 11. A general concentrating cell designed to the photon mean free path.·

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Figure 10. An illustration of the path an ion travels to escape the micropellet. Figure 15. Surface barrier detector setup to measure the fission fragment spectrum from a micropellet. Figure 16. Schematic drawing of the nuclear-pumping facility developed for use at the University of Missouri Research Reactor. Figure 17. Energy current calibration device for measurement of energy current from a surface source. Results from this measurement can be compared to theoretical values generated in the cylindrical geometry code developed by Chung and Prelas. Figure 13. Nonimaging solar concentrator. Figure 14. Nonimaging volume source concentrator. Note that the source is internal to the concentrator cell:

a major difference from the nonimaging solar concentrator. Figure 19. Micropellet coated with a reflective material. Figure 18. A test cell for absorption measurements of the ARECS medium. Figure 21. Experimental setup for coupling research 'at Texas A&M University. Figure 20. Design of the coupling experiment cell. An ablility to look at side light, reactant product and aerosol density was incorporated in this design. Figure 22. A diagram of the fluorescence absorber which will be used to measure coupling efficiencies. Figure 23. Experimental positions in the University of Missouri Research Reactor. Figure 24. Diagram of the unique track system incorporated in the Texas A&M University research reactor.

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List of Tables Table 1. Excimer fluorescence efficiencies. Table 2: Critical dimensions for the ARECS. Table 3. Comparison of nuclear-pumping sources. Table 4. A comparison of mean free paths for UF6 and micropellets. TABLE

s.

Excimer fluorescence efficiency (%).

Table 6. Beam characteristics after collision with microspheres. Table 7. Micropellet efficiency tasks. Table 8. Parameters of reactors typically used in research today.

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Table 9. Steady-state fluorescence tasks. Table 10. Pulsed fluorescence program tasks. Table 11. Coupling experiment tasks. Table 12. ARECS proposal timetable.

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Table 13. Pay scale.

TABLE OF CONTENTS 1.0 Summary •••••••••

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2. 0 Introduction.. . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3. 0 Technical Discussion ...........•............................. 17

4.0 Proposed Work • .•....••...· .•......•.

.......................... 39

4.1 Microsphere Efficiency ••••••••••••

....................... 40

4.2 Fluorescence Efficiency •••••••••••••

..................... 44

4.3 Coupling Efficiency •••••••••••••••••••••••••••••••••••••• 52 5. 0 References. . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 6.0 Personnel .................................................... 67 7.0 Facilities. •••••••••••••••••••••••••••••••••••••••••••••••••• 69

8.0 Timetable and Budget ••

....................................... 73

Appendicies A. ARECS Patent Application •••••

.......................... . 87

B. Aerosol Core Reactor Design ••••••••••••••••••••••••••••• l04

c.

Charged Particle Spectra From Fissile Aerosols •••••••••• l35

D. UMC

Nuclear~Pumping

Facility •••••••••••••••••••••••••••• l62

E. Energy Current and Power Deposition in Cylinders •••••••• l71 F. Res'llilles • •••••••••••••••••••••••••••••••••••••••••••••••• 180 (

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We propose a three year research program to investigate the feasibility of the ARECS concept at a cost of $811,402. This proposal can be divided into three subproposals, including Micropellet Efficiency at $142,010 for one year, Fluorescence Efficiency at $360,864 for two years, and Coupling Efficiency at $308,528 for two years (+ $85,400 of equipment that would be purchased as part of the Fluorescence Efficiency subproposal).

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INTRODUCTION The Aerosol Reactor Energy Conversion System (ARECS) utilizes a new two-step method for directly converting the energy of the charged particles from nuclear fission into a usable energy form (eg. electricity, chemicals, or coherent light). In the first step the fragments produced by nuclear fission reactions: (1) Pu(n;vn,ff )ffh 1 transfer their energy to an intermediate photon generator (eg. a fluorescer medium). In the second step the intermediate photons are absorbed by a reactant material producing excited states which result in the useful energy form. Figure 1 compares the two-step Photon-Intermediate Direct Energy Conversion (PIDEC) process used for the ARECS with one-step direct energy conversion and thermal energy conversion, which is a many step process. The advantages of the PIDEC process over thermal conversion are 1) that it is a direct process producing a useful energy from high grade energy and thus avoiding the Carnot cycle efficiency limits imposed by thermalization and 2) that it is much simpler, potentially leading to more compact, more reliable, and less expensive energy conversion systems.

The advantage of the PIDEC process over a one-step direct energy conversion process is the matching of scale lengths (energy transport distance). The scale length for the transport of the primary high grade energy must match the geometrical scale of the energy converter. Fission fragments have a transport length of micrometers while useful energy converters, on the other hand, have a scale length of fractions of meters. For this reason direct conversion of fission fragment energy has not previously been possible. What was required was the concept of an intermediate high-level energy converter that can be intermingled with fissioning material on a micrometer scale-length but produces an energy form that can be transported to meter scale-length direct converters producing useful output. A sort of "impedance matching" for scale length of energy forms. With PIDEC that scale length matching medium is a fluorescing gas. The photons it produces can be transported great distances, making them easy to couple to various energy conversion processes. Also, some conversion processes require greater power densities than primary energy sources can provide. A PIDEC's intermediate photon flux can be concentrated, enabling achievement of the high threshold power density for such conversion processes.

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Evaluating the ARECS Concept

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In evaluating the ARECS concept, two criteria must be met: 1) efficiency and 2) feasibility. The estimated efficiency of the ARECS process must be high compared to alternatives and, whatever the efficiency level estimated, it must be obtainable in real systems. To evaluate the efficiency of an ARECS, the efficiency of the several steps indicated in Figure 1 must be determined. The steps to be considered are: 1) the efficiency of the transport of the fission fragment energy from the microsphere into the fluorescer gas; 2) the fluorescence generation efficiency; 2

3) (since the photons are an intermediate step in this process) the photon coupling efficiency to a reactant; and 4) (with the interaction of reactant and photon) the useful product generation efficiency from the reactant. Hence, the system efficiency for the entire ARECS will be: (2)

ns = nt nf nc np nex

Since some second step energy converters have thresholds, the intensity of the fluorescence flux on (or the density of fluorescence power deposition in) the reactant material must be determined to assure that threshold is achieved. The threshold for pumping excimer lasers for example, is greater by an order of magnitude than the power density which can be provided by the in situ U-235 micropellet density that allows the laser gas to remain optically thin. The governing equation for power deposition in the reactant material isr PDr

= PDfnfnc Vf/V f

(3)

where PDf is the fission fragment power deposition in the fluorescer region (W/cc), Vf is the volume of the fluorescer (cc), and Vris the volume of the reactant. Similarly the fluorescence intensity on the surface of the reactant is:

where n.is the efficiency for coupling the fluorescence to the surface of reactant material, and A5 is the area of the reactant material (cm 2 ). As shown in Figure 1, there are four significant parts of the ARECS: 1) the fuel; 2) the fluorescer; 3) coupling of the fluorescence to the reactant; and 4) the reactant material. The major research problems in the determination of the factors which effect the above parts of the ARECS are experimental. The models of these ARECS parts generated to date [1-3] have yet to be tested and, as indicated in the references, many approximations which could lead to errors were utilized. Experimental measurement of all of the essential factors affecting the ARECS are contained in this proposal; including the size distribution, particle density, and energy transport characteristics of an aerosol fuel, the efficiency and spectra of the fluorescer, the geometrical coupling of the fluorescence to the reactant, and the effects of materials on the coupling efficiency of the ARECS cell. Next, each of these parts will be addressed in more detail followed by discussions of the ARECS criticality and potential ARECS applications. The Fuel The ARECS fuel is fissile micropellets mixed with a fluorescer medium to form an aerosol. The transport efficiency of the radiation from the nuclear fuel to the fluorescer medium varies with the micropellet size distribution, the chemical form of the fuel, and the uniformity of the pellet density. For example, it can be seen that as the radius of the micropellets increase, the transport efficiency decreases. In addition, as the atomic density of the fuel increases, the transport efficiency decreases. Nothing is known about the behavior of a fissioning aerosol in a strong flow situation at high temperatures, however, a few important things can be said. Clearly, it is

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Comparison of the Two-Step Photon Intermediate Energy Conversion Process used in the Aerosol Reactor Energy Conversion System (ARECS) with a hypothetical One-Step Direct Energy Convertor and a Thermal Conversion System producing the same output as the ARECS. ': ·~

4

important to show that aerosols can have relatively high particle densities and a good size distribution, which averages in the 5 to 20 micrometer diameter range, at temperatures of around 1500 K. There are some studies which show that the particle densities and the size distributions needed by the ARECS can be achieved. Specifically, in the so called continuum regime of aerosols (paticles with diameters greater then 1 micrometer) particle densities of 1 x lOG particles/co are routinely observed in the laboratory [4]. Second, aerosols at higher temperatures with steep gradients, such as those observed in cigarette smoke, volcanic dust, and in the NASA aerosol solar collector design have particle densities of about 1 x 109 , 1 x lOG , and 1 x 10 9 particles/co respectively [5-7]. Based upon these observations it is reasonable to expect that the ARECS can maintain the proper aerosol density and size distributions at about a 1500 K operating temperature to be self critical (a discussion of criticality follows the analysis of the ARECS-factors) and to haven values of the order of 50 to 80%. t The Fluorescer The fluorescer medium must be able to channel the energy absorbed from the nuclear radiation to an excited state rather than to gas heating. As described previously, the lack of an ability to do this has been the major weakness in previous nuclear energy conversion systems. There exist excited states in fluorescence media which can take advantage of the processes involved with the interaction of nuclear radiation with matter. It is well known that, when nuclear radiation interacts with rare gases, ions and metastable states are generated at very high efficiencies. Consequently, rare gas, and rare gas halide excimer mixtures, which efficiently generate the excimer state through mechanisms dependent upon the rare gas ions and metastable states, are able to take advantage of the products of the interaction of nuclear radiation with matter. As shown in Table 1, the rare gas and rare gas halide excimers are efficient nuclear radiation driven fluorescers.

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There are some research problems yet to be solved. First, the experimental data generated thus far is only preliminary and was plagued with power deposition and absolute intensity calibration problems. Reliable calibration methods must be developed and implemented in the studies of the rare gas and rare gas halide excimer systems. In this document, procedures to directly calibrate the power deposition and to calibrate the fluorescence intensities for direct measurement of efficiencies (nf) are proposed. Coupling

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Coupling the fluorescence to the reactant is the major factor in the design of the ARECS. It is a combined problem of materials and geometry. The materials affect the absorption of the fluorescence as it is transported while the geometry of the fluorescer region affects the transport distance of the fluorescence to the reactant. This problem becomes more complex if the reactant has a high reaction threshold for either the intensity of the fluorescence or for the power density produced by the absorption of the fluorescence (such problems exist with a laser medium for example). As described in appendix A and also in a later section of the main text, the aerosol fuel will conservatively be two orders of magnitude more effective for photon energy transport than any fuel commonly used in current design studies and could be convieniently used in a non-concentrating configuration as shown in Figure 2. The major design feature in this case is to keep the average distance a photon travels in the

5

Table 1. Excimer Fluorescence Efficiencies Excimer i

Wavelength

Theoretical Maximum Efficiency

nm

%

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415

16.8

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172

47.7

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particular geometry less than the mean free path. In general, the average distance a photon will travel in a given geometry is, d

= 4V/S

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where V is the volume of the geometry, and S is the surface area of the geometry. With a system that requires a concentrating geometry, the average distance traveled by a photon is more difficult to calculate. Some important work on non-imaging concentrators for solar energy has been reported [8]. Although similar, the ARECS problem is much more complex in that the source is internal to the geometry, the source exists in a large region, and the many point sources within the geometry are isotropic. We have developed. a volume concent~r~a~t~o~----~ theory for specific geometries as previously has been described [ ) • The result of this theory is that a relatively efficient conc.entrator for the ARECS can be developed with designs which are long on one axis and thin on another as shown in Figure 3. The volume source concentrator theory needs to be expanded for a more general set of geometries to help design more efficient concentrators. ·rn addition, the theory needs to be tested by examining a specific geometry and a fuel-fluorescer mixture. A proposal to update the "volume source" concentrator theory plus perform tests on a geometrical configuration with an aerosol fuel rare gas excimer mixture is discussed in this document. The Reactant There are many possible reactant materials which have natural resonances with light that will create useful energy. The types of reactions which can occur are: 1) photoelectric and photoelctrochemical and 2) photochemical. Photochemical reactions can lead to the production of excited atomic and molecular states which can lead to coherent photons from photolytically driven lasers [9] or to the production of useful stable chemicals [10]. Photochemical reactions are well researched with most of the useful light wavelenghts. Semiconductors can be used for photovoltaic generators as well as photoelectochemical generators [11]. The efficiency of a photovoltaic generator can be described by the following equation after the derivation of S. Angrist [12]:

nmax = exp[eVmp /(kT)]Vmp J s I [l+eVmp /(kT)]NphEr q where nmax is the maximum theoretical efficiency, e is the charge of an electron, Vmp is the maximum bias potential and is calculated from V =exp[eV /(kT)][l+eV /(kT)] = l+J /J , mp mp mp . s o Js is the short circuit current density and is calculated from Js

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To calculate a transport efficiency, a series of grids are set up in the cell which correspond to one volume element. The emission of photons from this J ---element is divided into a series of differential solid angles dQ •.. The__phot.ons·~ from each solid angle are then tracked using equations ~ana-jiJ.f. · Absorption in any specific region in the cell is calculated from the' distance traveled, the number density, and the absorption cross section. Thus far only simple geometries have been studied (eg. simple forms of 57

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(z'-z) = f(y'-y)). Consequently, the geometry which has given the best coupling efficiency thus far is a parabolic geometry ~l;- With a coricelitra:ting celi o:f 7.62 em major radius, 3.81 em minor radius, and a length of 100 em, it was determined that a concentration ratio of 20 and a coupling efficiency of about. 40% was possible. Other geometries must be examined as well as absorptive properties of the cell media: some cells with concentration ratios of 4 and coupling efficiencies of 60% have already been designed. Techniques for incorporating other geometries are being explored, and will be studied further along with the maximization of the coupling efficiency at any given concentration ratio. It is important to note that in nonconcentrating geometries, coupling efficiencies of 90% can routinely be achieved for reactants /lhich do not have photolytic thresholds. ' ,'-,)

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1) describing the geometry with a general three dimensional function; the current program assumes a linear x and either a circle or a parabola for z = f(y), and

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2) developing a routine to calculate dy/dz and dx/dz (for calculation of the angle of reflectance) at any point on the surface of the cell. Once this program is upgraded, will be initiated using the fission the Microsphere Efficiency research fluorescence data to _i::le~rovided by des:ribed in Section!J£.2·; Based on . desl.gned and tested. '';2,/'

a study of concentrating cell geometries fragment transport data to be~!.lE!r~t,~d.-in program described in Sectiollr~;-1 \and tJ;le the Fluorescence Efficiency ~rch program this study, an experimental cell will be

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The experimental cell will be tested at the Texas A&M University Research !~ Reactor (TAMURR). This reactor is capable of generating a thermal neutrQ!Lflux ...--->L-of 3x1016 over a 1 meter length in a 10 ms pulse (S'e~efl:d:ix~ecell will be submerged in the TAMURR "swimming pool .. next to the core (See Figures The cell will have a waveguide on the outer portion of the reflector to measure side light intensity. This fluorescence will be picked up by an Optical Multichannel Analyzer (OMA) at the reactor bridge area. This fluorescence will be spectrally analyzed by the OMA. The solid angle (dn) of the volume fluorescence source can be found from the above program. From the intensity measurement, the fluorescence efficiency can be calculated directly I

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The data from the OMA will be fed into a data aquisition and analysis computer system (LSI 11/23). With the aid of a calibration procedure using a "known source," this data will provide information on the coupling of nuclear energy to a fluorescence medium. Strong and Weak Absorber Experiments A second crucial parameter which must be measured is the coupling of the fluorescer material to the reactant. Coupling is m~terial dependent. Materials which have high absorption coefficients, such as ~~solid state photoelectric cell cH;~p.t.e'!'=-r, will absorb the fluorescence primarily on the !'\ surface. Materials which are gaseous and have a weak absorption coefficient, ~/ · such as the photochemical reactions or photolytic lasers di-s-cussed in cfta-p-'&e:r-.Z,""'-will absorb the fluorescence throughout the volume of the material. One must be careful in adjusting the reactant cell size so that the fluorescence mean free path is on the order of 4V/S (where V is the volume of the reactant and S is the surface area of the reactant). This type of adjustment will eliminate spatial inhomogeneities in the power density of the reactant. · Strong Absorbers

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Strong absorber materials can be directly modelled with an experiment and the fluorescence energy current can be measured in the experiment. This measurement can be made directly. A reactant material which efficiently absorbs the fluorescence, such as a solid state material, will be placed in the absorbing cell and it's temperature rise measured in order to find the intensity of the fluorescence impinging upon it. This experiment will of course be refined as the fluorescence efficiency, micropellet efficiency, transport .~ efficiency, and concentrating cell design progress. The absorber cell wilL---c1 1 allow direct measurement of the energy impinging upon it (see Figure...M"): From the temperature differential, the coupling of the fluorescence to the ~~t nt can be calculated from ff" ~

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