WO2024155695A1

WO2024155695A1 - Lithium flouride powered nuclear fission subcritical reactor

Info

Publication number: WO2024155695A1
Application number: PCT/US2024/011813
Authority: WO
Inventors: Matthew SZYDAGIS
Original assignee: The Research Foundation For The State University Of New York
Priority date: 2023-01-17
Filing date: 2024-01-17
Publication date: 2024-07-25

Abstract

A subcritical nuclear fission reactor having a Lithium Fluoride core that is selectively bombarded with neutrons to create a subcritical nuclear reaction. The reactor has a sealable vessel with an interior thereof containing an aqueous liquid, and the reactor further has a sealed passage into its interior and an external vent for steam from the interior. The core has a core cavity therein and is held within the interior of the sealable vessel and is at least partially immersed in the liquid. A neutron source selectively directs neutrons through the sealed passage and into the core cavity such that a subcritical fission reaction is created within the core which heats the liquid in the interior of the sealable vessel; steam is created and vented through the external vent.

Description

LITHIUM FLOURIDE POWERED NUCLEAR FISSION SUBCRITICAL REACTOR

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of US Provisional Patent Application No. 63/439,406, filed January 17, 2023, the entirety of which is hereby incorporated herein by this reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to nuclear fission reactors. More particularly, the present invention relates to a nuclear reactor using Lithium Fluoride to fuel a subcritical fission reaction.

[0004] 2. Description of the Related Art

[0005] A nuclear fission subcritical reactor produces fission without achieving or potentially achieving criticality. Instead of producing a sustaining chain reaction, a subcritical reactor must use neutrons from an outside source to maintain the fission reaction of a fissile material. There are two general classes of such devices, one uses neutrons provided by a nuclear fusion machine, a concept known as a fusionfission hybrid, and the other uses neutrons created by an external source.

[0006] A subcritical reactor normally is used to destroy heavy isotopes contained in the used fuel from a conventional nuclear reactor or other spent nuclear materials, while at the same time producing electricity. For example, the long-lived transuranic elements in nuclear waste can be fissioned, releasing energy in the process and leaving behind fission products that are shorter-lived. However, some isotopes have high-energy-threshold fission cross sections and therefore require a fast reactor for being fissioned. Also, the used fuel releases, on average, too few new neutrons per fission, so that meaningful subcriticality cannot be reached. The three main longterm radioactive isotopes that are normally considered for a subcritical fission reaction are neptunium-237, americium-241 and americium-243, and thorium (to breed fissile uranium-233). The nuclear weapon material plutonium-239 may also suitable. But it is very difficult to handle and utilize these heavily radioactive materials.

[0007] Another advantage of a subcritical fission reactor is that it is inherently safe, unlike a conventional fission reactor. In most types of critical fission reactors, the rate of fission can increase rapidly, damaging or destroying the reactor and allowing the escape of radioactive material, such as with the Chernobyl disaster. With a subcritical reactor, the reaction will cease unless continually fed neutrons from an outside source. However, lingering heat generation after ending the subcritical chain reaction can still be problematic; in many instances the continuous cooling of such a subcritical reactor for a considerable period after shut-down remains vital in order to avoid overheating and damage to the core. The addition of the cooling system and its operation therefore add significant cost but mitigate risk from a superheating subcritical fission reactor.

[0008] It is known to use isotopes of Lithium as an element in nuclear fission reactions. Lithium-6 is valuable as the source material for the production of tritium (hydrogen-3) and as an absorber of neutrons in nuclear fusion reactions. Between 1.9% and 7.8% of terrestrial lithium in normal materials consists of lithium-6, with the remainder being Lithium-7. Lithium-7 is by far the most abundant isotope of lithium, making up between 92.2% and 98.1 % of all terrestrial lithium. A Lithium-7 atom contains three protons, four neutrons, and three electrons. Industrial production of Lithium-6 results in a waste product which is enriched in Lithium-7, so it is easily accessible and fairly inexpensive.

[0009] Lithium-7 has been used as molten lithium fluoride in molten salt fission reactors. Lithium-7 has a very small thermal neutron cross section (about 45 millibarns) and is used for alkalizing of the coolant in pressurized water reactors. Molten salt reactors (MSRs) use molten fluoride salts as primary coolant, at low pressure. MSRs may operate with epithermal or fast neutron spectrums, and with a variety of fuels.

[0010] The lithium salts in the MSR are a primary coolant, mostly lithium-beryllium fluoride and lithium fluoride, which remain liquid without pressurization from about 500°C up to about 1400°C. In an MSR, the nuclear fuel is dissolved in the coolant as fuel salts ultimately are reprocessed. Thorium, uranium, and plutonium can all form suitable fluoride salts that readily dissolve in the LiF-BeF2 (FLiBe) mixture, and thorium and uranium can be easily separated from one another in fluoride form. However, Lithium-7 has not in itself been considered as a subcritical fissile material. [0011 ] It would thus be advantageous to create a subcritical fission reactor that can maintain a sufficient subcritical reaction from neutron bombardment without core superheating that would require a cooling system. It would also be advantageous for such a reactor to use a safe and abundant fissile material that is normally stable, such as Lithium-7. Further, such a reactor will have industrial applicability in that it can produce sufficient heat for efficient steam production and power generation that allows the system to be net positive in energy production. It is thus to address the problems with previous subcritical fission reactors that the present invention is primarily directed.

BRIEF SUMMARY OF THE INVENTION

[0012] Briefly described, the present invention is a subcritical nuclear fission reactor having a Lithium Fluoride compound core with the natural abundance of Lithium-7 or enriched in it that is selectively bombarded with neutrons to create a subcritical nuclear reaction. The reactor has a sealable vessel with an interior that contains an aqueous liquid, and the reactor further has a sealed passage into the interior and an external vent for steam that can be used to motivate an electric generator. The core is held within the interior of the sealable vessel and at least partially immersed in the liquid, and the core further has a core cavity therein. A neutron source selectively directs neutrons through the sealed passage and into the core cavity such that a subcritical fission reaction is created within the core such that the subcritical fission reaction heats the liquid in the interior of the sealable vessel such that steam is created and is vented through the external vent.

[0013] The invention advantageously utilizes the nuclear fissions of Lithium-7 and Fluorine-19 instead of standard fissile material, like uranium, plutonium, thorium, cobalt, etc., as the basis for a subcritical nuclear fission. The subcritical reaction is driven by an external radioactive source that, when removed, would cause fission to cease. In the preferred embodiment, the primary reactions used are ⁷Li + n -> ⁴He + ³H + n + y rays + kinetic energy and ¹⁹F + n -> ¹⁸F + 2n + y rays + kinetic energy, with subsequent reactions stimulated on both Lithium-7 and Fluorine-19, stimulated by neutron activation from a neutron source, like ²⁵²Cf or a neutron beam/generator/gun including DD I DT, or any other means of producing neutrons, such as protons, deuterium or other ions, or a laser beam, as three examples, on lithium fluoride or another neutron-producing target.

[0014] The Lithium in the present invention is preferably in a solid form, either crystalline or powder, such as solid Lithium Fluoride, with a passage to the center of the core, that is contained in a vessel with a concentration of water, and can include a concentration of NaCI. The subcritical fission is caused by an external neutron source bombarding the center with neutrons from a neutron source. This is very different from "normal" nuclear fission which requires (epi-)thermal neutrons, such as in the fission of Uranium(U)-235. The neutrons sent from the neutron source must have a minimum kinetic energy of 2.5 MeV to start the subcritical process, which would then produce less net energy output than II fission, but with sufficiently high- energy neutrons bombarding Li-7 sufficiently quickly one creates a net power output. [0015] In one embodiment, the present invention includes a subcritical nuclear fission reactor that includes a sealable vessel having an interior thereof containing an aqueous liquid, which can include a mixture of normal water and/or heavy (deuterated) water. The vessel further has at least one sealed passage into the interior and an external vent for steam from the interior. The reactor also has a core containing at least Lithium Fluoride and is held within the interior of the sealable vessel and is at least partially immersed in the liquid therein, the core further including a core cavity therein. There is a neutron source external to the vessel that selectively directs neutrons through the sealed passage and into the core cavity such that a subcritical fission reaction is created within the core which heats the liquid in the interior of the sealable vessel such that steam is created and is vented through the external vent. It is preferable that the neutron source generates neutrons at an energy greater than 2.5 MeV, with an optimal energy of 17 MeV, at least for Lithium Fluoride (LiF).

[0016] In one embodiment, the core is comprised of LiF and is greater than 400 kg in mass, and the neutron source can be a neutron beam or a natural source selectively placed through the sealed passage and into the core cavity. The liquid can be water in a concentration range of 20-60% NaCI, combined with one of H2O or D2O.

[0017] The invention also includes, in one embodiment, a method of generating steam in a subcritical nuclear fission reactor by the steps of providing a sealable vessel having an interior thereof containing an aqueous liquid, where the vessel further has at least one sealed passage into the interior and one external vent for steam from the interior. The method then continues by placing a core containing at least Lithium Fluoride within the interior of the sealable vessel and at least partially immersed in the liquid therein, with the core further including a core cavity therein. Then directing, from a neutron source, neutrons through the sealed passage and into the core cavity, a subcritical fission reaction is created within the core, creating heat within the interior and the liquid, and venting steam through the external vent.

[0018] There are numerous potential applications of the present invention, to include clean (zero-Carbon-footprint) energy. Renewable energy sources such as solar, wind, and geothermal, have still failed to replace coal, oil, natural gas, and other fossil fuels, still making up only a tiny market share, despite decades of trying, with government subsidies in many cases. Their cost and their intermittency combine to make them at least temporarily non-viable for addressing current energy needs. The present invention provides a major advantage over existing uranium/plutonium-based reactor alternatives in terms of being safer, as well as cheaper, but still using only proven physics and engineering.

[0019] Another advantage of the present invention is that it addresses the environmental crisis of an excess of Lithium batteries being thrown out annually to allow for recycling (the infrastructure is simply not there). As a result, Li batteries are piling up dangerously in landfills and leading to explosions and other hazards. The core of the present reactor can "recycle" old Li batteries with fluoride or another element for fission.

[0020] Another significant advantage of the present invention is that the byproducts of the subcritical reactions, even if ultimately radioactive, have much shorter halflives than the dangerous byproducts of the fission of uranium and plutonium-based reactors. Both with and without the inclusion of D2O, one major byproduct of the reactor is H-3 (tritium), which is considered a precious national resource of which there is very little available on Earth. H-3 can be used for nuclear fusion. Another byproduct of the subcritical reaction here is He-4, which is most often used in party balloons and blimps, but also in airbags, hard drives, and countless other critical usages.

[0021] A further advantage of the present invention is that, by lowering the neutron energy, one can produce a different reaction and create Li-8 that then beta-decays, thereby releasing neutrinos. They can be used for neutrino physics, for example, to make better measurements perhaps of Coherent Elastic Neutrino Nucleus Scattering (CEvNS) with table-top neutrino detectors. Neutrinos can also be useful, like neutrons, especially when capitalizing on the CEvNS channel, for calibrating experiments that look for dark matter. Neutrinos may further be useful, when coupled to proper neutrino detectors, for communication, useable in space, underwater, and even through masses of planets, without needing to go first up to a satellite or utilize the atmosphere.

[0022] The above and other advantages of the present invention will become apparent to one of skill in the art after review of the Drawings, Specifications, and Claims herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Fig.1 is a side cross-sectional view of one embodiment of the subcritical reactor with the core in situ within the NaCI liquid in the outer vessel’s interior. [0024] Fig. 2 is a graph of simulated power output for a Li-7 F embodiment as different-shaped plotlines, as a function of incoming neutron rate along the lower x- axis and input neutron beam power on the upper x-axis, with an exemplary output extrapolated based on real data as a point.

[0025] Fig. 3 is a graph of a test of the output of excess neutrons from LiF solid when hit by a neutron source.

DETAILED DESCRIPTION OF THE INVENTION

[0026] With reference to the figures in which like numerals represent like elements throughout the several views, Fig .1 is a side cross-sectional view of one embodiment of the subcritical reactor 10 with the core 12 in situ within the NaCI liquid 14 in the reactor interior 14. The subcritical nuclear fission reactor 10 includes a sealable vessel 18 having an interior 14 thereof containing a liquid 16 comprised of, at least, a predetermined concentration of NaCI, preferably in a mixture with water and/or deuterated water. The liquid 16 in the interior 14 can be maintained at a concentration of 20-60% NaCI, with a mixture of either pure water (H2O) or heavy water (D2O), with that ratio maintained as steam 20 is produced from the heat of the subcritical reaction. The vessel 18 further has at least one sealed passage 22 into the interior 14 and an external vent 24 for steam from the interior 14. The steam 20 can be used to power electric generators as is well known in the art of utilizing nuclear reactors for electric power production.

[0027] The reactor 10 also has a core 12 containing at least Lithium Fluoride (LiF) that is held within the interior 14 of the sealable vessel 18 and is at least partially immersed in an aqueous liquid 16 therein, which can be pure water, or include other isotopes, elements, and compounds, such as NaCI, deuterium, or other nuclides / nuclei or atoms as described herein. The core 12 further includes a core cavity 26 therein.

[0028] There is a neutron source 28 external to the vessel 18 that selectively directs neutrons through the sealed passage 22 and into the core cavity 26 such that a subcritical fission reaction is created within the core 12 which heats the liquid 16 in the interior 14 of the sealable vessel 18 such that steam 20 is created and is vented through the external vent 24. It is preferable that the neutron source generates neutron energy greater than 2.5 MeV.

[0029] In one embodiment, the core 12 is comprised of LiF, is greater than 400 kg in mass, and is preferably in a range of 400-1 ,000 kg. Furthermore, the neutron source 22 can be a neutron beam, or a natural source selectively placed through the sealed passage and into the core cavity such as a piece of a radioactive material such as Uranium or Plutonium or Cf-252. The core 12 is preferably kept at least 10 cm (distance A) from the walls of the vessel 18. Further, the core 12 can be in a range of 10-30 cm from the walls of the vessel 18.

[0030] In one embodiment, the lithium (in a salt) would be surrounded by a shell of a neutron reflector (for example, Beryllium or Tungsten) which would keep the neutrons released coming back again and again, in order to stimulate further reactions, making one design a sphere within a sphere, with neutron reflector as the outer sphere, and the lithium salt/compound as the inner sphere. In the cylindrical embodiment, if present, wetted NaCI serves the same purpose. Computer simulations (Geant4 or G4 software) were used to determine the optimal masses and thicknesses, for near-critical ity (but not super-criticality, which should not even be possible as explained above due to the need for neutron activation as the first step). As in a standard nuclear reactor, the core 12 boils steam and drives a turbine, strictly using the heat output of the nuclear reactions taking place. However, given that the present reactor 10 only maintains a subcritical reaction with the neutron source 28 actively providing neutrons, it is quite safe compared to those based on Uranium and Plutonium, with a dangerous meltdown not possible. The salt chosen to contain the Lithium can also be chosen to be inherently safe prior to neutron activation and also include other atoms to help stimulate the subcritical reaction, keeping it going, without thermalizing the neutrons significantly, since this is a high-energy reaction -- for instance, the non-reactive Li compound or salt LiF, i.e. Lithium Fluoride.

[0031] Despite the drawback of less energy (more than an order of magnitude smaller than Uranium's 200+ MeV) released per energetic reaction and smaller cross sections (i.e., probabilities of interactions), the present subcritical reaction can provide between 1 .01-1 .15x plus energy gain compared to the energy of the neutrons in the neutron source. The net positive output of energy is possible because of both fission, of both the Lithium-7 and Fluorine-19, as well as incoming neutrons being captured by Lithium-7, turning it into Lithium-8, which then emits heat because of beta decay.

[0032] It can be seen that, in one embodiment, the invention includes a method of generating steam 20 in a subcritical nuclear fission reactor 10 by the steps of providing a sealable vessel 18 having an interior 14 thereof containing a liquid 16 comprised of, at least, a predetermined concentration of NaCI, where the vessel 18 further has at least one sealed passage 22 into the interior 14 and an external vent 24 for steam 20 from the interior 14. The method then continues by placing a core 12 containing at least Lithum-7 within the interior 14 of the sealable vessel 18 and at least partially immersed in the liquid 16 therein, with the core 12 further including a core cavity 26 therein. Then directing, from a neutron source 28, neutrons through the sealed passage 22 and into the core cavity 26, a subcritical fission reaction is created within the core 12, creating heat within the interior 14 and the liquid 16, and venting steam 20 through the external vent 24.

[0033] To produce neutrons one can accelerate ds (i.e., deuterium nuclei or deuteron particles, which are just heavy hydrogen or hydrogen-2 i.e. one proton + one neutron). This is is much easier than making high-intensity, high-energy neutron beams directly, since ds are positively charged instead of neutral like ns. Thus, d responds to electric fields, another massive benefit. In the same vein, protons (a symbol of p or p+) can also be used to make neutrons, d and/or p+ can be used indirectly to make neutrons (ns) or be directed at the Lithium, Li-7, LiF, or Li-7 Fluoride. Other ions, or lasers, also work.

[0034] Fig. 2 is a graph 30 of the simulated power output for a Li-7 F embodiment with different-shaped plotlines, as a function of the incoming neutron rate along the x axis 32, at an exemplary neutron energy of 17 MeV. The upper x axis 34 has corresponding incoming neutron power. All plotlines, which illustrate the power coming out, demonstrate a net gain: non-zero power on the y axis 36, in Watts. The power output of Fig. 2 has been experimentally verified with bombardment of a Li F (92% Li-7) sample with a neutron source, yielding the power range 38, extrapolating from the 1 -kg scale (which was tested) to 1 ,000 kg (unshielded, without a salt water shield or any other kind of shield, still), and extrapolating from one detector of power placed at one point to a sphere encompassing the entire sample. Therefore, one of skill in the art would regard this as verification of the reliability of the simulation used and of the energetic operation of the reactor 10 of the present invention. The extrapolations are linear and thus conservative, as non-linearities may exist, even in a subcritical nuclear fission reactor.

[0035] It should be noted that, at 10^A13 Bq for example, the input neutron power is [10^A13 * 17 MeV (1e6 eV/MeV) (1.602e-19 J/eV)] I (1 second) = 27 Watts, so 4 Watts on the graph translates to 31 Watts in total. Thus, in this embodiment, 27 Watts went in, 27+4 came out. However, significant further gains are possible. For example, deuterons (d) of 1.5 MeV can make 17 MeV neutrons by bombarding a tritiated target such as Titanium infused with H-3. This could result in over ten times more power output than input. The reaction 1 .5 MeV d + H-3 -> n + He-4 produces 17 MeV neutrons (n). In such embodiment, 1.5 MeV, not 17 MeV, is the input; this means, using the earlier equation: In the 10^A13 (10 trillion) Bq (ns per second) example, [10^A13 * 1 .5 MeV (1 e6 eV/MeV) (1 ,602e-19 J/eV)] I (1 second) = 2.403 Watts (replacing 17 with 1.5 in the equation) input. So, with 31 Watts of output, as an example, this is approximately 12.9 times the power, instead of only 1.15x (31/27 in the other embodiment).

[0036] Fig. 3 is a graph 40 of a test of the output of excess neutrons from Li F solid when hit by a neutron source. For the control 42, a 2.5 MeV d beam was always hitting a square of LiF, 1 cm x 1 cm x 1 mm thickness, properly centered on the beam, to produce neutrons. The control output is illustrated with fewer neutrons output than input, due to scattering and absorption in the LiF control square and/or surrounding materials.

[0037] A crystal rod (Crystal 44) of 620 g of LiF. The crystal rod was a cylinder 5.5 cm in diameter by 9.9 cm long. The Crystal 44 data represented herein were taken only with the crystal rod inside the deuterium beam tube next to the control square. The Crystal 44 rod of LiF output illustrates a gain in neutrons over control of typically 1 ,2x. Despite this value >1 , the fission reactions are still subcritical due to the fact that ns are not produced naturally by LiF, but required an external source of neutrons, or neutron-producing particles such as deuterons, and thus additional neutrons will lose energy, making them incapable on their own of maintaining a self- sustaining nuclear fission reaction at the high energy required for the fast-neutron fission of Lithium-7 and/or Fluorine-19. The inherent safety of subcriticality remains, despite the high n multiplicity, which extrapolates to 4x at 1 ,000 kg utilizing Geant4 sim.

[0038] Pressed LiF powder (1 ,35kg) inside a steel cylinder was also used, along side of the crystal rod (Crystal+powder 46), with the former outside of the deuterium beam pipe and the latter inside still, and a significant gain in neutrons was realized once again as shown in Crystal+powder 46 plot. The neutron gain is typically 1.2x. Therefore, the experiment shows that a consistent range of neutron output for a LiF core is about 1.2x for neutron input, which corresponds to a range of 1 .01-1 .15x in the power gained based on reasonable extrapolations and on Geant4 simulations of the nuclear physics.

[0039] The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims below, if any, are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of one or more aspects of the invention, and the practical applications, and to enable others of ordinary skill in the art to understand one or more aspects of the invention, for various embodiments with various modifications, as are suited to the particular use contemplated.

Claims

CLAIMS What is claimed is:

1 . A subcritical nuclear fission reactor, comprising: a sealable vessel having an interior thereof containing a liquid, the vessel further having at least one sealed passage into the interior and an external vent for steam from the interior; a core containing at least Lithium Fluoride (LiF) having a concentration of Li-7 at least at 92% or higher, the core held within the interior of the sealable vessel and at least partially immersed in the liquid therein, the core further including a core cavity therein; a neutron source that selectively directs neutrons through the sealed passage and into the core cavity such that a subcritical fission reaction is created within the core; and wherein the subcritical fission reaction heats the liquid in the interior of the sealable vessel such that steam is created and is vented through the external vent.

2. The reactor of claim 1 , wherein the liquid includes NaCI.

3. The reactor of claim 1 , wherein the neutron source is a neutron beam.

4. The reactor of claim 1 , wherein the neutron source is a natural source selectively placed through the sealed passage and into the core cavity.

5. The reactor of claim 1 , wherein the neutron source has a predetermined neutron energy, and the subcritical fission reaction produces at least greater than 101 % of the predetermined neutron energy.

6. The reactor of claim 2, wherein the core is greater than 400 kg in mass.

7. The reactor of claim 1 , wherein the neutron source generates neutrons of energy greater than 2.5 MeV in the core.

8. A method of generating steam in a subcritical nuclear fission reactor, comprising: providing a sealable vessel having an interior thereof containing an aqueous liquid, the vessel further having at least one sealed passage into the interior and an external vent for steam from the interior; placing a core containing at least Lithium Fluoride (Li F) within the interior of the sealable vessel and at least partially immersed in the liquid therein, the core further including a core cavity therein; directing, from a neutron source, neutrons through the sealed passage and into the core cavity such that a subcritical fission reaction is created within the core, creating heat within the interior and liquid; and venting steam through the external vent.

9. The method of claim 8, wherein placing a core is placing a core comprised of either natural LiF, or LiF enriched in Li-7.

10. The method of claim 8, wherein directing neutrons is directing a neutron beam.

11 . The method of claim 8, wherein directing neutrons is placing a natural neutron source through the sealed passage and into the core cavity.

12. The method of claim 8, wherein the neutron source has a predetermined neutron energy, and the subcritical fission reaction produces at least greater than 101 % of the predetermined neutron energy.

13. The method of claim 8, wherein directing neutrons is directing neutrons having an energy greater than 2.5 MeV.

14. A subcritical nuclear fission reactor, comprising: a sealable vessel means for holding an aqueous liquid in an interior thereof, the vessel means further including at least one sealed passage into the interior and a venting means for venting steam from the interior; a fission means for selectively creating a subcritical nuclear fission reaction, the fission means containing at least Lithium Fluoride (Li F), the fission means held within the interior of the sealable vessel means and at least partially immersed within the liquid therein, the fission means further including a cavity therein; and a neutron-emitting means for selectively directing neutrons through the sealed passage and into the fission means cavity such that a subcritical fission reaction is created within the fission means such that the subcritical fission reaction heats the liquid in the interior of the sealable vessel such that steam is created and is vented through the venting means.

15. The reactor of claim 14, wherein the liquid includes NaCI.

16. The reactor of claim 14, wherein the neutron-emitting means is a neutron beam.

17. The reactor of claim 14, wherein the neutron-emitting means is a natural source selectively placed through the sealed passage and into the fissionmeans cavity.

18. The reactor of claim 14, wherein the directed neutrons have a predetermined neutron energy, and the subcritical fission reaction produces at least greater than 101 % of the predetermined neutron energy.

19. The reactor of claim 15, wherein the fission means is greater than 400 kg in mass.

20. The reactor of claim 14, wherein the neutron-emitting means further generates neutron energies greater than 2.5 MeV.