WO2002043076A2

WO2002043076A2 - Fission-voltaic reactor

Info

Publication number: WO2002043076A2
Application number: PCT/US2001/044666
Authority: WO
Inventors: James D. Cramer
Original assignee: Nuclear Energy R & D Partnership
Priority date: 2000-11-20
Filing date: 2001-11-19
Publication date: 2002-05-30
Also published as: AU2002217933A1; CA2431372A1; EP1350255A4; EP1350255A2; WO2002043076A3

Abstract

A nuclear reactor including a core having nuclear fuel material, moderator material and a semiconductor device, the core being subcritical (see Fig.5). The reactor also includes a fission-inducing source which is movable from a first position outside the core to a second position inside the core, whereby the core becomes subcritical. The reactor also includes a neutron reflector which surrounds the core and includes an opening to permit insertion and removal of the fission-inducing source. Preferably, the core consists of concentric cylinders of the semiconductor device, the nuclear fuel material, and the semiconductor device, whereby the nuclear fuel material is sandwiched between layers of the semiconductor device. The semiconductor device is selected from a group including p-n junction diodes and Schottky barier diodes.

Description

FISSION-VOLTAIC REACTOR Field of the Invention [0001] This invention relates to the practical and efficient production of electrical energy directly from the nuclear fission reaction. Particularly, this invention relates to the direct production of electrical energy by what I have termed the fission-voltaic process. More particularly, this invention relates to the direct production of electrical energy in a nuclear reactor utilizing fission-voltaic technology. The reactor incorporates a semiconductor device having a high electrical collection efficiency and which is able to perform in both high temperatures and high radiation environments. Examples include amorphous semiconductor material incorporated in p-n junction diodes and Schottky barrier diodes. The nuclear fission conversion method described herein is safer, more secure, and more compact than conventional nuclear-electric conversion technology provides. Thus, the invention is useful in both space electric power generation and terrestrial replacement of conventional nuclear-electric power reactors.

Background of the Invention [0002] The fission-voltaic process utilized in this invention is based on the application of a well understood principle used to detect high energy charged-particles in nuclear research. Charge collection in single-crystal silicon diode devices has been used in experiments to measure the fission reaction rate in both nuclear reactors and fission experiments. See Charged Particle Detection, Silicon Charged Particle Detectors, www.canaberra.com / literature / basic-principles / charged.htm, 06/14/00.

However, the lifetime of the delicate devices designed for this measurement application is limited because of the damage suffered when they are exposed to high temperatures and high radiation levels.

[0003] Numerous articles have been published and several U.S. patents have issued in the last 25 years describing various direct electrical-energy conversion methods for nuclear fission. U.S. patent No. 4,415,526, issued to D.L. Garrett on

November 15, 1983 (based, in part, on application filed in 1971) relates to a nuclear fission electrical generator for directly extracting electrical power from fissionable material having semiconductor characteristics, without the necessity of any intervening heat conversion step. In the Garrett fission-voltaic device, the fissionable material having semiconductor characteristics can be either: (1) a single chemical substance that has semiconductor characteristics and includes a fissionable constituent; or (2) a material which is not a single substance but two distinct chemical substances that comprise, respectively, a fissionable material and a semiconductor material. Uranium 235 phthalocyanine is illustrative of a material that is a single chemical compound having both semiconductor characteristics and also a fissionable constituent. For the second case Garrett states that the semiconductor material must retain at least some semiconductor characteristics in the presence of the fissionable material. A specific example of the laminated structure is a high resistivity oxide of uranium 235 and silicon. Garrett also discloses a fuel element composed of a semiconductor material having finely divided particles of fissionable material dispersed therein.

[0004] Regardless of the form of the material capable of nuclear fission and having semiconductor characteristics, the fissionable material is located so that fission fragments from nuclear fission collide with the semiconductor material. These collisions, in turn, produce electron-hole pairs in the semiconductor material. More specifically, Garrett states that electrons resulting from the collision of fission fragments with the semiconductor material are separated from holes resulting from such fission fragment collisions, and these electrons and holes are moved, respectively, in generally opposite directions within the semiconductor material.

Groups of electrons and groups of holes resulting from the fission fragment collisions are conducted to respective output terminals in operative electrical connection with the semiconductor material. The output terminals are connected with each other through a circuit that includes an external load. This produces an electromotive force across the output terminals and a resulting electrical current through the load. The depleting, separating, moving apart and conducting of electrons and holes are effected through at least one charge-affecting field of force within the semiconductor material. The rate of production of electrons and holes from the collision of fission fragments with the semiconductor material is maintained at a sufficiently high level to deliver to the external load a net positive yield of electrical power over the total input of any electrical power used to produce the charge-affecting field or fields of force by means of which Garrett's method is carried out. With reference to Figure 2 of Garrett, the depletion, separating, moving apart and conducting steps of the method of this invention are all accompanied by a single charge-affectmg field of force (i.e., the field provided by p-n junction 212).

[0005] Notwithstanding the advantages claimed by Garrett, there is no known commercialization of his invention. This fact is largely due to the undocumented performance of the postulated semiconductor material. Furthermore, there is not enough quantitative description relative to the design of the proximity of the nuclear fuel material with the semiconductor material to provide a workable, efficient energy conversion device. Moreover, Gaπett postulates a very limiting configuration of the neutron source with his conversion device, that requires his device(s) to operate in a conventional reactor to provide the necessary radiation fluences to generate the fission reaction. This provides little, if any, economy in the generation of electrical energy over conventional nuclear reactor technology.

Objects of the Invention

[0006] It is an object of the present invention to provide for the direct production of electricity utilizing improved semiconductor technology coupled with the fission- voltaic process to dramatically improve electrical power conversion performance efficiencies.

[0007] More particularly, it is an object of the present invention to overcome the deficiencies of Garrett and apply the fission voltaic process in an efficient and practical manner to generate electricity.

[0008] It is another object of the present invention to provide for the direct (i.e., without the need for heat exchange) energy conversion of nuclear fission to electricity in a nuclear reactor configuration that will operate at or near a critical mode (depending in the orientation of an independent neutron or other fission inducing source), making such a system inherently safer than conventional nuclear reactors.

[0009] It is a further object of the present invention to provide for the direct and efficient conversion of nuclear fission to electricity in a system which is modular, compact, requires minimum shielding, and is easily transportable. [0010] It is still a further object of the present invention to provide a safe, secure, and compact energy source for use in space applications.

[0011] It is further an object of the present invention to directly convert the ionizing energy of fission-fragments into electrical energy utilizing semiconductor devices which have high electrical collection efficiency and have the ability to withstand both the high temperatures and radiation fluences inherent in nuclear reactor.

[0012] It is still another object of the present invention to provide for a semiconductor device which can: (1) operate at high temperatures; (2) perform electronically and survive in the harsh radiation environment of the fission process;

(3) is available in large quantities; and (4) is relatively inexpensive.

[0013] It is yet still another object of the present invention to provide a fission- voltaic system incorporating semiconductor devices that are shallow depletion region devices and suitable for the energy deposition profile of fission fragments. [0014] It is yet still another object of the present invention to utilize fission-fuel foils or depositions thin enough to minimize energy loss of the energetic escaping fission-fragments.

[0015] It is yet still another object of the present invention to utilize p-n junction semiconductor diodes. [0016] It is yet still another object of the present invention to utilize Schottky- barrier diodes.

[0017] These and other objectives will be apparent from the detailed description set forth below. Summary of the Invention

[0018] The nuclear reactor of the present invention includes a core having nuclear fuel material, moderator material and semiconductor device material. The core by itself has a subcritical mass. The reactor also includes a fission inducing source which is movable from a first position outside the core to a second position inside the core, whereby the core becomes critical. Finally, the reactor includes a neutron reflector which suπounds the core and includes an opening to permit the insertion and removal of the fission inducing source. Preferably, the core consists of alternate layers of the semiconductor device, the nuclear fuel material, and the semiconductor device, whereby the nuclear fuel material is sandwiched between layers of the semiconductor device. The nuclear fuel material is in the form of a thin foil or deposit which has a thickness in the range of 1.5 mg/cm² to 10mg/cm², which thickness depends on the particular type of fuel used (e.g., uranium 235, plutonium 239, reactor grade uranium, and oxides of uranium). The reactor has a cross-sectional configuration selected from the group including concentric cylinders and spirals. With such a design: the core consists of alternate layers of moderator, semiconductor device, nuclear fuel material, semiconductor device and moderator; and the electrodes are sandwiched between the nuclear fuel material and the semiconductor devices. The semiconductor device includes a semiconductor material that has a high electrical collection efficiency and is able to survive both the high temperatures and high particle fluences generated by the nuclear fuel material. Preferably, the semiconductor material is amorphous. Also, preferably, the semiconductor device is selected from the group including p-n junction diodes and Schottky barrier diodes. Where a Schottky barrier diode is used, the fuel foil or deposit can be bonded directly to the semiconductor material. The fission- inducing source is selected from the group including neutron generators, gamma ray sources, high energy charged particles and electron beam sources. Finally, for teπestrial applications, the reactor includes a shield suπounding the neutron reflector. [0020] The method of the present invention for generating electricity via the fission- voltaic process, includes the steps of providing a core having a subcritical mass; providing a fission inducing source; inserting the fission inducing source into the core whereby the core becomes critical; and generating electricity by bombarding the semiconductor device with fission fragments while minimizing the generation of heat.

Brief Description of the Drawings

[0021] Figure 1 is a diagram comparing the fission-voltaic process of the present invention with the well known and defined photo-voltaic and beta-voltaic processes;

[0022] Figure 2 is the energy band diagram of a p-n junction fission- voltaic cell of the present invention under fragment iπadiation;

[0023] Figure 3 is a schematic diagram of a Schottky-barrier fission-voltaic cell of the present invention;

[0024] Figure 4 is a schematic of an equivalent circuit the fission-voltaic cell (e.g., p-n junction, Schottky-barrier) of the present invention; and [0025] Figure 5 is a perspective view, partially broken out, of the fission-voltaic reactor of the present invention. Description of the Preferred Embodiment

[0026] The fission-voltaic electrical-energy process of the present invention can best be defined in terms analogous to the well-known and documented solar photovoltaic process and the less well known beta-voltaic process. A graphic comparison of these two technologies and the technology of the present invention is illustrated in

Figure 1. Relative to photo-voltaic and beta- voltaic devices, the intrinsic efficiency and energy density of the fission-voltaic device of the present invention is much larger. Energy density is governed by the available source energy that can be made incident on the surface of the collecting material and the intrinsic efficiency of the energy conversion device. For solar photo-voltaics, that limitation is the maximum solar radiation at the earth's surface of 0.1 watt/cm². In the case of beta-voltaic conversion, where a radioactive nuclide is the energy source, the available power of the beta particle fluence at the surface of the conversion material is limited by radioactive decay rates of specific radioisotopes. For ¹⁴⁷Pm, a beta emitting source previously used in beta-voltaic devices, the upper power limit is approximately,

0.0025 watts/cm². For fission-voltaics, the criticality calculations described below produce an incident fission power of 0.085 watts/cm² from the fuel foil or deposit on the surface of the semiconductor materials; an incident energy fluence approximately equal that of solar energy fluence. [0027] While the incident power of solar energy on photo-voltaics and that produced in the reactor described below are similar, because of the energy losses from the respective sources and the intrinsic efficiencies of the respective devices, the overall efficiency of the fission- voltaic device of the present invention is much greater than that of photo-voltaic devices. The overall efficiency of the device of the present invention can be greater than 50%, while that of photo-voltaic devices is on the order of 15%. Beta-voltaic devices are even less efficient, having an overall efficiency on the order of 4-5%. [0028] The energy available for solar photo-voltaics is that part of the solar spectrum where solar energy photons have energy greater than the band-gap energy of the semiconductor used. For silicon that fraction is approximately 75%. Thus, 25% of the solar radiation at normal incidence on a photo- voltaic device surface is of an energy level below that necessary to excite an electron in the semiconductor to the conduction band. [0029] In the situation where a thin lawyer of radioisotope or nuclear fuel material is the power source (i.e., for beta- voltaics and fission-voltaics), some of the energy of the ionizing charged particle is lost to self-absorption in escaping the source material. Furthermore, as the energy is transferred from source to device via energetic particles, the particles experience energy loss as they traverse and emerge from even a very thin layer of energetic fuel material (i.e. beta emitter or fission-fragment source). When the source particles are beta-particles of energy less than lOOkeV, their range is much less than the range of fission-fragments in similar fuel materials. For example, more than 80% of the energy from the beta emitter ¹⁴⁷Pm is lost emerging from the 0.0025 watt cm source referenced above, compared to less than 5% of the fission-fragment energy emerging from the 0.085 watt/cm² (1.5 mg/cm²) fission source incorporated in the reactor calculation of the present invention.

[0030] The energy exchange in the conversion device itself further reduces energy efficiency. The incremental kinetic energy exchange process between incident photons and electrons resident in the deposition region of the semiconductor must produce electrons with energy greater than the band-gap energy of the semiconductor. Furthermore, the energy transfer must occur in a thin depletion layer at or near the surface of the semiconductor device to produce an electrical current. The thickness of this layer depends on specific device design parameters and material properties. For most common p-n semiconductor diodes (e.g., silicon), this thickness is, approximately, 10 micrometers at zero bias voltage. This fact limits the usefulness of the entire solar spectrum for photo-voltaic conversion since most photons of energy significantly greater than the band-gap are not absorbed in this thin depletion layer but pass through, to be absorbed in the remaining non essential thickness of the semiconductor material. This phenomenon further reduces the efficiency of solar cells resulting in a practical conversion efficiency from them of not more than 15%. On the other hand, in the case of fission-voltaics, more than 80% of the incident energy from fission-fragments is deposited within 10 micrometers of the surface of most semiconductor materials considered in the present invention. Their energy loss as a function of penetration depth for these materials has been well determined by experiment and calculation. Thus, the theoretical maximum efficiency achievable for photo-voltaic conversion is nearly a factor of three less than that for fission- voltaic conversion. Additionally, energy fluxes incident on the fission-voltaic device can be better controlled (i.e., increased or decreased) to suit a particular mode of operation. Together, these qualities make fission-voltaic technology superior to photo- voltaic and beta-voltaic technologies.

[0031] In addition to the foregoing comparison to photo-voltaics and beta- voltaics, the fission voltaics used to directly generate electrical energy in the materials of the present invention yields far greater efficiencies than can be achieved in electric power conversion with conventional nuclear reactors. This more efficient reactor will operate with less heat generation (because the use of thin fuel foils or deposits permit the fission fragments to escape the fuel) and require less cooling, thus creating a less stressful environment on parts and materials in the reactor system, all of which makes the reactor of the present invention inherently more safe and reliable. Depending on the type of fuel used (metals having a higher electron density than oxides), the fuel foil or deposit can be up to lOmg/cm² thick. Further, the layered geometry of fuel and semiconductor material, as set forth below, provides for a more complete "burn up" of the fuel than is presently achieved in conventional nuclear reactors, thus extending the lifetime of the reactor. Moreover, the efficient design can be made to achieve greater electric power from smaller volumes than conventional reactors providing for a : smaller packaging of the system for portable power supplies. [0032] The proper choice of semiconductor material, conversion device design, and it's geometric configuration in relation to the nuclear fuel is critical to achieving the optimum performance (e.g., efficiency, longevity) of the present invention. For the ionization energy of fission-fragments to be used to effectively generate electrical energy directly in the semiconductor devices of the present invention, the fuel must be adjacent to the thin depletion layer of the conversion device (i.e., within the typical range of the fragments (typically, 5-25 micrometers)). Additionally, the semiconductor device material must: operate in high temperatures; perform electronically in an intense thermal neutron environment; and be capable of surviving in the harsh radiation environment of the fission process. Amorphous semiconductor materials are prefeπed over single crystal materials. [0033] I have determined that the configuration of the semiconductor device is, preferably, either a p-n junction semiconductor diode or a Schottky barrier diode. Both are shallow depletion region devices that are suitable for the energy deposition profile of the fission-fragments utilized in the present invention. The p-n diode utilizes a variety of doping compounds diffused into the selected semiconductor materials to create opposite carrier types (i.e., p-type carriers in an n-type material or visa- versa) at the junction. The Schotty-barrier diode utilizes a metal-semiconductor interface to create the voltage potential across the junction. [0034] Figure 2 shows an energy band diagram for an ideal semiconducting p-n junction of the present invention. The energy level of the conduction band and valence band are designated E_c and E_v, respectively. The high-energy fission- fragments penetrate the layer and pass into the p material, creating a large number of electron-hole pairs. Depending on the energy partition, ΔE , of the incident fission- fragments, the thickness of the n-layer, and the lifetime of minority carriers in both the p and n regions (as well as other pertinent semiconductor parameters such as energy gap Eg, mobility, diffusion coefficient, etc.), the flow of created ion pairs of charge q is collected across the junction and produces an open circuit voltage N_oc. Generally, the larger the energy gap, the higher the incident fragment fluence, the larger the resultant electrical power produced. [0035] When a metal is brought into contact with a semiconductor, it forms two types of junctions according to the relative energy level of the metal and the semiconductor. In n-type semiconductors, if the electron affinity of the semiconductor is greater than the work functions of the metal, the electrons flow to the semiconductor and make it more strongly n-type. This is called an ohmic contact. The voltage and cuπent characteristics of ohmic contacts exhibit linear relationships. If the work function of the metal is greater than the electron affinity of the n-type semiconductor, then a Schottky barrier diode is formed. The height of this barrier depends on the difference in the energy levels of the semiconductor and the metal. The Schottky barrier gives a nonlinear rectifying current-voltage characteristic similar to that of a p-n junction, which causes cuπent to flow when it is excited with photons or ionizing charged particles. Figure 3 is a schematic representation of the basic configuration of a Schottky cell fission-voltaic energy conversion system, wherein II is the load current. [0036] The advantages of Schottky barrier diodes over p-n junction devices are: low-temperature fabrication because no high-temperature diffusion is required; adaptability to polycrystalline materials; high radiation resistance due to a high electric field near the fuel-semiconductor surface; and high cuπent output because the close proximity of the depletion region to the surface of the semiconductor can reduce the effects of low carrier life-times and recombination. An important parameter for the proper selection of the metal-semiconductor interface used in Schottky-barrier diodes is the barrier height for each individual combination of material. Theoretically, this height approaches 2/3 of the band-gap of the semiconductor. These parameters, known for several semiconductor materials, are shown in Table 1, along with other important semiconductor material properties. Table 1. Band Gaps and Barrier Heights

[0037] The Schottky barrier diode used with the fission- voltaic power conversion process can best be described by considering the case of an ideal Schottky barrier diode with a constant-cuπent source resulting from the excitation of excess carriers by fission-fragment irradiation. Referring to the idealized equivalent circuit of Figure 4, which neglects series and shunt resistive losses, the fission-fragment ionization thus causes an electric cuπent, I, to flow in the load. The magnitude of this cuπent is the difference between the generated short-circuit cuπent and the cuπent flowing in the diode. Thus,

I=I_s[exp(qV/kT)-l]-I_L) (Eq. 1)

[0038] Where

I = cuπent flow in the load

Is reverse saturation in the diode

V voltage across the diode q electronic charge k Boltzmann constant

T absolute temperature

II cuπent generated by fission-fragments [0039] For Schottky diodes, the saturation cuπent is given by

I_s= AA*T²exp(-q 0_B kT), (Eq. 2) where = cross-sectional area of the junction

A* = effective Richardson constant

0_B = barrier height of the Schottky diode. Thus, at a fixed temperature, the larger the barrier height, the lower the saturation cuπent.

[0040] The short-circuit cuπent, open-circuit voltage, and maximum power output can be calculated from Eq.(l) as follows. The short-circuit cuπent, Isc, can be obtained by setting V=O, thus,

Isc = -I

(Eq. 3)

Similarly, the open-circuit voltage, Voc_. can be obtained by setting I=O,

Voc=(kT/q) In [I_I7I_S)+1]

(Eq. 4) [0041] The power output, P, is the product of the voltage and the cuπent at a particular operating point. Thus,

P=VI=VI_s[exp(qN/kT)-l]NlL

(Eq. 5)

[0042] The voltage coπesponding to maximum power output, Nmp can be calculated in the usual way by maximizing the power. Thus, setting the partial derivative of the power P with respect to the voltage N to zero, we obtain,

exp(qNm_P / kT) = (i/Ts)±l 1+ q Vmp kT (Eq. 6) The cuπent at maximum power output is then given by

I_mp=I_s[exp(qN_mp/kT)-l] -I_L (Eq. 7) [0042] Finally, the maximum power output, Pmax, is equal to the product of Vmp and Imp. We therefore obtain

(Eq. 8) [0043] Since the power output is negative in sign, this relationship states that the maximum power output increases as the reverse saturation cuπent (IS) decreases.

[0044] Substituting Eq. (6) into Eq. (8), we have

[0045] Therefore, since IS decreases IB gets larger, the greater the barrier height, the higher the maximum possible power.

The Fission-voltaic Reactor

[0046] To demonstrate the feasibility of a fission-voltaic nuclear power reactor a detailed calculation using the state-of-the-art MCΝP v. 4.2 reactor criticality computer codes (See "MCMP-A General Monte Carlo Code for Neutron and Photon Transport", Los Alamos National Laboratory Report, LA-7639-M, REV.) were utilized for a specific electric power generator to serve as an example of how this invention functions. Figure 5 is a perspective view, with a section broken out and a basic reactor unit enlarged for clarity, of the fission-voltaic reactor of the present invention. Reactor 11 includes a fuel-moderator-semiconductor device core 13, a removable neutron source 15, and a beryllium reflector 17. The neutron source 15 is a line source of thermal neutrons in this example. This fission-inducing source could be any number of energetic particles or radiation. Also, preferably, the reflector 17 is 20 cm thick in both the radial and axial directions in this example. [0048] The core 13, which in this example is lm long, lm in diameter and has a volume of 0.785 m3, includes 10kg of fuel (i.e., uranium 235 distributed throughout in the form of 1.5 mg/cm2 foil), 260 kg of hydrogenous moderator (e.g., polyethylene) and 1300 kg of semiconductor boron carbide (B4C). With neutron source 15 removed, the mass of core 13 is sub-critical. As those skilled in the art will appreciate: alternate fuels include reactor grade uranium (e.g., 7% uranium 235 and

93% uranium 238, and plutonium isotopes; alternate moderators include water and pure carbon. The alternatives to the B4C semi-conductor used in the example calculation are set forth above in Table 1. Core 13 includes cooling channels 19, as illustrated in the broken out section. [0049] With reference to the enlarged section, each basic reactor element 21 includes a first p-n semiconductor device layer 23, an electrode 25, a thin layer of uranium fuel foil 27, a second electrode 29, and a second p-n semiconductor device layer 31. The reactor element 21 is sandwiched between first 33 and second 35 moderator layers. The electrical contacts shown schematically could be, as those skilled in the art would appreciate, combined with the cooling channels. Where a

Schottky barrier diode is used, the fuel foil or deposit can be brought into contact with the semiconductor material to also function as the metal component of the diode. In such an aπangement, the metal fuel foil or deposit can, additionally, serve as the electrodes. In cross-section, core 13 is composed of a series of concentric cylindrical elements increasing in diameter. Alternately, core 13 can have a spiral configuration. One third of the core volume is occupied by the moderator which has a density of 1.Og/cc. The rest of the core is occupied by the boron carbide semiconductor with a theoretical density of 2.5 g/cc. [0050] Based on the design information set forth above, the known densities of the materials used, and for an assumed 1 IB enrichment of 99.9975%, the computer code was used in KCODE mode to calculate Keff as well as the volumetric fission density. The cylindrical core illustrated in Figure 5 was divided into 10 annular regions of thickness 5 cm, to accurately determine neutron flux variation in the radial direction. Each of these regions were assigned a homogeneous mixture of the materials listed above, with an average atom density of 1.2247 x 10-1 . Required cross-sections were obtained from the Evaluated Nuclear Data File (ENDF-V), available in the open literature through the Radiation Shielding Information Center, Oak Ridge National Laboratory, Oak Ridge, TN. [0051] Results of this MCNP calculation indicate that the reactor core has a

E _ff of 1.01030 with the neutron source inserted, indicating that the core has reached criticality, thus generating a self sustaining nuclear reaction generating the fission densities listed in Table 2. Coπesponding neutron absorption and fission densities are summarized in Table 2 for the calculated reactor thermal power of 1MW. Given an assumed electrical conversion efficiency of 50%, this fission- voltaic reactor would produce 500kW of continuous electric power. These values can be linearly scaled for other reactor powers. As evident from Table 2, the fission density varied from 6.87xl0¹⁰ fissions/cm³ /sec to 3.76xl0¹⁰ fissions/cmVsec; coπesponding to a volume weighted average fission density of 4.76xl0¹⁰ fissions/cm³/sec. Table 2. MCNP predictions for Neutron Absorption and Fission Densities for the Solid-Sate Reactor Core. Power = 1MW

Cell Inside Outside Cell Volume Absorption Fission Density Radius(cm) Radius(cm) (cm³) (neutrons/cmVsec) fissions/cm³/sec) 1 0.0 5.0 7.85E+03 7.89E+10 6.87E+10

2 5.0 10.0 2.36E+04 7.16E+10 6.24E+10

3 10.0 15.0 3.93E+04 6.98E+10 6.08E+10

4 15.0 20.0 5.50E+04 6.57E+10 5.72e+10

5 20.0 25.0 7.07E+04 6.46E+10 5.63E+10

6 25.0 30.0 8.64E+04 6.12E 10 5.34E+10

7 30.0 35.0 1.02E+05 5.77E+10 5.03E+10

8 35.0 40.0 1.18E+05 5.15E+10 4.49E+10

9 40.0 45.0 1.34E+05 4.50E+10 3.92E+10

10 45.0 50.0 1.49E+05 4.30E+10 3.76E+10

[0051] As part of the criticality study, variations of K_eff were determined for variations in the material composition as well as reflector design (no radiation shielding was provided for in this calculation). Calculated variations indicate that the initial reflector is an optimum thickness (i.e., increasing its thickness does not contribute to any considerable increase in the K_eff.)

[0052] In summary, from the foregoing calculated Keff and the fission densities for the example fission- voltaic reactor core, the fission density in this 1 MWt reactor will vary from 6.87x1010 fissions/cm3/sec at the center to 3.76x1010 fissions/cm3/sec at the radial boundary, with a volume averaged fission density of 4.76xl010fissions/cm3/sec. These values can be scaled to obtain fission densities at other reactor powers.

[0053] Whereas the drawings and accompanying description have shown and described the prefeπed embodiment of the present invention, it should be apparent to those skilled in the art that various changes may be made in the form of the invention without affecting the scope thereof.

Claims

I Claim:

1. A nuclear reactor comprising:

(a) a core including nuclear fuel material, moderator material and semiconductor device, said core having a sub-critical mass; (b) a fission inducing source, said source being movable from a first position outside said core to a second position inside said core, whereby said core becomes critical; and (c) a neutron reflector, said reflector suπounding said core, said reflector further including an opening to permit the insertion and removal of said fission inducing source.

2. The reactor of claim 1 , wherein said core consists of alternate layers of said semiconductor device, said nuclear fuel material, and said semiconductor device, whereby said nuclear fuel material is sandwiched between layers of said semiconductor device.

3. The reactor of claim 2, wherein said nuclear fuel material is in the form of a thin layer of material.

4. The reactor of claim 3, wherein said layer of material has a thickness in the range of 1.5 mg/cm² to 10mg/cm².

5. The reactor of claim 4, wherein said layer is selected from the group including foils and deposits.

6. The reactor of claim 3, wherein said nuclear fuel material is selected from the group including uranium 235, plutonium 239, reactor grade uranium, and oxides of uranium.

7. The reactor of claim 2, wherein said core has a cross-sectional configuration selected from the group including concentric cylinders and spirals.

8. The reactor of claim 7, wherein said core consists of alternate layers of said moderator, said semiconductor device, said nuclear fuel material, said semiconductor device and said moderator.

9. The reactor of claim 2, further including electrodes, said electrodes being sandwiched between said nuclear fuel and said semiconductor device.

10. The reactor of claim 1, wherein said semiconductor device includes a semiconductor material that has a high electrical collection efficiency and is able to survive both the high temperatures and high particle fluences generated by said nuclear fuel material.

11. The reactor of claim 10, wherein said semiconductor material is amorphous.

12. The reactor of claim 10, wherein said semiconductor device is selected from the group including p-n junction diodes and Schottky barrier diodes.

13. The reactor of claim 12, wherein said nuclear fuel material is the form of a thin layer, said layer selected from the group including foils or deposits.

14. The reactor of claim 13, wherein said thin layer is bonded to said semiconductor material to form said Schottky barrier diode.

15. The reactor of claim 1 , wherein said moderator material is selected from the group of hydrogenous materials including polypropylene and carbonaceous materials including graphite.

16. The reactor of claim 1, wherein said fission inducing source is selected from the group including neutron generators, gamma ray sources, and high energy charged particles.

17. The reactor of claim 1, wherein said core includes cooling channels.

18. The reactor of claim 1 , further including a shield, said shield suπounding said neutron reflector.

19. A method of generating electricity via the fission- voltaic process, said method including the steps of:

(a) providing a core having a sub-critical mass, said core including nuclear material, moderator material and semiconductor device;

(b) providing a fission inducing source;

(c) inserting said fission inducing source into said core whereby said core becomes critical; and

(d) generating electricity by bombarding said semiconductor device with fission fragments while minimizing the generation of heat