WO2000072329A2

WO2000072329A2 - Apparatus for generating power from fission of spent nuclear waste

Info

Publication number: WO2000072329A2
Application number: PCT/US2000/013922
Authority: WO
Inventors: Paul M. Brown
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-05-21
Filing date: 2000-05-19
Publication date: 2000-11-30
Anticipated expiration: 2001-11-21
Also published as: HUP0301807A2; BR0011595A; PL364809A1; CA2371484A1; SK16862001A3; JP2003525424A; BG106217A; EP1234309A2; EA200101225A1; CN1421037A; WO2000072329A3; NO20015656D0; CZ20014161A3; IL146657A0; AU5149600A; FI20012249A7; MXPA01011907A; NO20015656L; KR20020043456A

Abstract

A linear accelerator, preferably of the monochromatic type, accelerates electrons to an energy of about 10 MeV which are directed onto a high Z target such as tungsten to generate gamma rays which are directed onto the fuel material such as U238 which results in the (ηf) reaction, thus releasing about 200 MeV. A reactor built according to this principle requiring an accelerator driven by 1 MW will develop about 20 MW of power. The reaction is not self-sustaining and stops when the beam is turned off. This accelerator driven reactor may be used to 'burn-up' spent fuel from fission reactors, if simply operated at 10 MeV. The photo-fission results in typical spent fuel waste products such as Cs?137 and Sr90¿ which undergo photodisintegration by the (η,n) reaction resulting in short lived or stable products.

Description

Title of Invention: POWER FROM FISSION OF SPENT NUCLEAR WASTER

DESCRIPTION

BACKGROUND OF THE INVENTION Field of the Invention.

This invention relates generally to treatment of spent nuclear wastes. More specifically, this invention relates to electrical power generation from the heat of fission reactions in spent nuclear waste, and to effective disposal of nuclear wastes.

Related Art.

A typical 1000 megawatt-electric (Mwe) pressurized water (PWR) nuclear reactor operating at 75% capacity generates about 21 tons of spent fuel at a bum-up of 43 gigawatt per ton (GWe/t). The 21 tons of spent fuel (contained inside 42 PWR fuel elements with a total volume of about 11 m³) will have produced an electric energy of about 6.6 TWh (6.6 billion kWh). This same energy output corresponds to the burning of 2 million tons of coal in a conventional power plant giving rise to 120,000 tons of ashes, 5.4 million tons of CO and 50,000 tons of SO..

Spent nuclear reactor fuel comprises uranium (U²³⁸) which accounts for about 96% by weight of the spent fuel removed from commercial reactors. In the case of light water reactors (LWR-the type most commonly used, that includes PWR types) the spent fuel contains about 0.90% U²³⁵ whereas natural uranium contains only about 0.70% of this isotope. Plutonium constitutes about

1% of the weight of spent fuel. The plutonium is fissile which means that it can be used as fuel in nuclear reactors. The minor actinides constitute about 0.1% of the weight of spent fuel. They comprise about 50% Np, 47% Am and 3% Cm which are very radiotoxic. The fission products (iodine, technetium, neodymium, zirconium, molybdenum, cerium, cesium, ruthenium, palladium, etc.) constitute about 2.9% of the weight of spent fuel.

The two fission products of principal concern because of their substantial thermal impact on the repository as opposed to posing a health risk are Sr⁹⁰ and Cs¹³⁷ . These two radionuclides are dominant contributors to the heat released by spent fuel at least for the first several decades. Cs¹³⁷ is also a major source of penetrating radiation emitted by spent fuel. The two fission products of principal concern because of their potential contribution to health risk are Tc" and I¹²⁹. These are of principal concern because they are long-lived, produced in significant amounts in the fission process, generally soluble under geologic conditions, and migrate relatively quickly under common ground water conditions.

The long-term toxicity of spent fuel is dominated by the actinides such as Np²³⁷, U²³⁴, U²³⁶, and Pu²³⁹,Pu²⁴⁰, Pu²⁴² . The transmutation of long-lived nuclides in high level waste to stable or short-lived nuclides by stimulating nuclear reactions is a desirable alternative approach for the reduction of high level waste.

There are about 300 different radioactive species generated by the operation of a nuclear reactor, primarily as a result of neutron capture and neutron-induced fission. The adverse impact of the various radionuclides varies because of the differences in the chemical behavior in the body of, and the radiation emitted by, the radionuclides. The risk focus of the radionuclides is related to waste disposal in a geologic repository. The most common release and exposure mechanisms from a repository involve ground water contacting the waste form followed by slow dissolution, transport of radionuclides to the accessible environment, distribution in the biosphere, and eventual uptake from food and water. Although hundreds of isotopes are present in spent fuel or wastes derived from them, only a few of them are important for disposal. These four isotopes Cs¹³⁷, Sr⁹⁰, 1¹²⁹ and Tc⁹⁹ are the primary focus of concern for light-water reactor spent fuel, i.e., nuclear waste, due to their excess heat, groundwater solubility, or health risk.

The management of spent fuel should ensure that the biosphere is protected under economically acceptable conditions without entailing unfavorable short-term consequences and the public must be convinced of the effectiveness of the methods. Since the spent fuel contains very long-lived radionuclides, some protection is required for at least 100,000 years. Two means are possible:

1. We can wait for the natural decay of the radioactive elements by isolating them physically from the biosphere by installing successive barriers at a suitable depth in the ground. This strategy is called deep geological disposal; 2. We can make use of nuclear reactions that will transmute the very long-lived wastes into less radioactive or shorter-lived products. This strategy is called transmutation.

The problem with storing nuclear waste below ground is that there is no material that will outlast its radioactive contents and radioactive wastes continuously produce heat, hydrogen and helium outgassing, as well as other labile products.

Defense-generated transuranic waste is temporarily stored at 23 sites nationwide, while mixed radioactive and hazardous wastes are currently stored at 40 sites around the country.

There are some 114 nuclear reactors in the United States and about 400 commercial nuclear power plants in operation around the world including about 120 GWe nuclear electric capacity in Western Europe and 45 GWe operational in the ex-USSR and East European countries. In the US alone, we have accumulated 34,000 tons of nuclear waste. The current US production rate of high-level waste (primarily spent fuel) is 3,000 tons per year. The average commercial power plant puts 60 used fuel assemblies into "temporary" storage each year and is expected to do so until the year 2000 when the waste is to be transferred to DOE. This does not include low-level wastes such as gloves, filters, tools, clothing, etc., that come from nuclear power plants, research centers, and hospitals that use radioactive materials. There are about 100,000 US facilities that use radioactive materials. They produce 1.6 million cubic feet of low-level waste each year.

Current projected costs of the US Environmental Management program are about $7.5 billion per year. Paper studies currently account for about 20% of the Environmental Restoration budget. According to the Baseline Environmental Management Report, the total clean-up cost of the nuclear weapons program is $230 billion over a 75 year period, including the $50 billion projected Hanford clean-up.

Most nuclear reactors commercially operating presently are light-water nuclear reactors of large capacities having electric power output of an order of 400 MWe. A boiling water reactor, which is known as a kind of light-water reactor, has a pressure vessel and a reactor core disposed in the pressure vessel. The reactor core includes a multiplicity of fuel assemblies. Control rods for controlling the power of the reactor are adapted to be inserted into the reactor core. The boiling water reactor has also a recycling system for recycling a coolant through the reactor core and serving also as means for effecting a fine adjustment of the power of the nuclear reactor. The steam generated in the pressure vessel of the nuclear reactor is introduced into a steam turbine to drive the latter and is then condensed in a condenser. The condensate is then recycled as the coolant into the pressure vessel.

Another typical example of a light-water reactor is a pressurized water reactor which is constituted by a pressure vessel containing a reactor core having a multiplicity of fuel assemblies, a steam generator and a primary cooling system which forms a closed loop including the pressure vessel and the steam generator. The hot coolant after being heated in the reactor core is introduced into the steam generator through the pipe of the primary cooling system to make a heat exchange with feed water fed into the steam generator. The coolant, the temperature of which has been lowered as a result of the heat exchange, is returned from the steam generator into the pressure vessel through the pipe of the primary system. On the other hand, the feed water is evaporated to become steam as a result of the heat exchange. The steam is introduced into a turbine to drive the latter and, thereafter, condensed in a condenser. The condensate is returned as the feed water to the steam generator.

The nuclear fission of heavy elements following the absorption of electromagnetic radiation (photofission) was first predicted by Bohr and Wheeler in their famous 1939 paper.

Haxby, Shoupp, Stephens, and Wells (1941) were the first to produce fission with gamma rays. A survey of the literature indicates that photonuclear reaction studies in actinide nuclei have been the pursuit of several laboratories during the last 40 years, using several types of gamma sources. The main objective of these studies has been to obtain nuclear information at excitation energies in the region of the giant dipole resonance and in the region of low energy, near the photofission and photoneutron thresholds. Giant dipole resonance (GDR) has been found to dramatically improve the effective cross section presented to incident photons impinging upon target nuclei.

Bowman, using a quasi-monochromatic photon beam obtained from the anihilation in flight of monochromatic positrons, was the first to observe the characteristic splitting of the giant dipole resonance of a fissile nucleus into two components, a phenomenon observed for other permanently deformed nuclei as well. However, it was found that the photon-induced r„ / I ratio was strongly energy dependent, a result in complete disagreement with data obtained from neutron-induced fission, bremsstrahlung-induced fission and charged-particle-induced fission. The systematics of the giant dipole resonance, which characterizes the absorption of electromagnetic radiation by nuclei in the energy range from about 5 to 30 MeV, have been of interest since the discovery of the giant resonance itself. Over the years, the photoneutron cross sections for many nuclei have been measured with monoenergetic photons in numerous laboratories. All these data are presented in the Atomic Data Nuclear Data Tables. For most cases studied, the agreement is remarkably good.

The classical description of the dipole photon absorption process predicts that for spherical nuclei the total photon-absorption cross section is characterized by the Lorentz line shape, σ (E_γ) = σ_m / [1+(E -E_m ²)²/E_γ ² T ²] (Eq.l) where O is the peak cross section, E_m is the resonance energy, and T is the full width at half maximum. For deformed (spheroidal) nuclei, the collective picture predicts a splitting of the giant resonance into two components, corresponding to oscillations parallel and perpendicular to the nuclear axis of symmetry. For medium and heavy nuclei, the Coulomb barrier inhibits the emission of charged particles at giant-resonance energies, and the photon-scattering cross section is always small above the (γ,n) threshold; therefore, the total photoneutron cross section is a good approximation to the total photon-absorption cross section.

The intrinsic quadrupole moment Q₀ for a deformed nucleus can be computed from the expression,

Q₀=2/5 ZR²ε=2/5 ZR² (η-1) η ^{2 3} (Eq. 2)

Where the nuclear radius R=R₀A^{1 3} , Z and A are the atomic number and atomic weight, respectively, e is the nuclear eccentricity, and the parameter η is the ratio of the mayor axis to the minor axis (for the prolate nucleus) given by the formula, E_m (2)/E_m (l) = 0.911 η + 0.089 (Eq. 3)

Where E_m (1) and E_m (2) are the lower and higher resonance energies of a two-component Lorentz-curve fit to the giant resonance.

The energy of the dipole resonance is so low that mostly rather simple processes-such as (γ,n), (y,p), (γ,2n), and photofission reactions-take place in the giant-resonance region. The competition between these processes is governed by the usual statistical considerations of compound-nucleus de-excitation, so that neutron emission usually dominates.

The characteristics of the giant dipole resonance for the actinide nuclei are of particular interest. For such high-Z, high-Coulomb barrier nuclei, the total photon-absorption cross section is equal to the sum of the photoneutron and photofission cross sections. The total photoneutron cross section is the sum of the following reaction cross sections. σ (γ,n_tol) = σ (γ,n) +2σ (γ,2n)+vσ (γ,f) (Eq. 4) where v is the neutron multiplicity of a fission event. The total neutron production cross section is then, σ_γ,_N = σ_γ,„^{+ V(}V (Eq- ⁵)

The competition between neutron emission and fission may be expressed,

σ (γ,n)/σ (γ,f)(E) (Eq. 6)

The value for _n/T₍ decreases exponentially with the fissility of the nuclei. The theoretical expression for r_n/T_f which explains this behavior for the neutron emission and fission competition is derived from the Constant Nuclear Temperature for the level density, and is expressed. r_n/T_f = 2 TA^{2 3}/l 0 exp {(Ef - Bn')/T} (Eq. 7) where (Ef - Bn') are the effective thresholds for the respective photonuclear processes and T is the nuclear temperature.

The fact that more than one neutron is emitted per fission in the fission of such isotopes as Th²³², U²³³, U²³⁵, U²³⁸, and Pu²³⁹ leads to the possibility of a chain reaction in a mass of fissionable material. Whether the chain reaction remains steady, builds up, or dies down depends upon the competition between the production of neutrons through fission and the loss of neutrons through a variety of processes such as non-fission capture of neutrons, primarily (n, γ) reactions in the system, and the leakage of neutrons through the surface of the system.

Energy is released at the rate of 200 MeV per fission of one atom or about 23 X 10⁶ kw-hr per fission of one kilogram of U²³⁵. The fission fragments carry off 82% of the energy in the form of kinetic energy. Prompt neutrons carry off another 2.5%, prompt gammas carry off 3.5%, beta decay accounts for 4%, delayed gammas account for 3%, and neutrinos carry off the remaining 5%. The neutrinos and their energy are lost, since the probability of interaction with neutrinos is so small. Some fission also occurs as a fast neutron strikes a U²³⁸ atom. Also, as the fuel is burned, plutonium is produced, and a large percentage of the energy is actually coming from the fission of Pu²³⁹ atoms. About 80% of the neutron absorption in U²³⁵ results in fission; the other 20% are (n,γ) reactions. The average kinetic energy released in the photofission of Th²³² is about 0.8 of that released in the slow-neutron fission of U²³⁵, or about 160 MeV.

SUMMARY OF THE TNVRNTTON Accordingly, an object of the present invention is to provide a nuclear reactor fueled with depleted or non-fissile radioactive material such as spent fuel from a conventional nuclear reactor.

Another object of the invention is to provide a nuclear reactor of increased safety by using a sub-critical fuel mass.

Still another object of the present invention is to provide a nuclear reactor of small size and capacity. According to this invention, which has been made to attain the above objects, there is provided a system in which electrons are accelerated to an energy of at least 6 MeV by an accelerator and the accelerated electrons or gamma photons caused by the accelerated electrons hitting a target are introduced to the fuel of the nuclear reactor thereby producing fission of the fuel by the nuclear reaction process known as photofission. The photofission of the fuel produces heat and neutrons as in a conventional nuclear reactor, although the fission is not self- sustaining due to the use of sub-critical mass and or non-fissile fuel elements. Thus, the photofission process stops immediately upon stopping the electron beam. The heat and neutrons produced may be utilized as in a conventional nuclear reactor, such as for the production of electrical power. The disclosed invention is a method and means for producing nuclear energy from heavy elements, but not fissile elements. The reaction is not driven by the well known self-sustained, chain-reaction, of U²³⁵, rather by an accelerator. The fuel for this type of accelerator driven reactor may be the spent fuel from fission reactors. The mechanism by which nuclear energy is released from non-fissile material is known as photofission, wherein a photon or gamma is introduced greater than the photofission threshold energy resulting in fission of the target nucleus. For instance, with U²³⁸, the threshold of photofission is about 6 MeV and results in fission of the U²³⁸ nucleus releasing about 200 MeV. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a flow diagram of the invention.

FIG. 2 is a schematic representation of the preferred embodiment of the electron accelerator of the invention.

FIG. 3 is a chart of the partial and total photonuclear cross sections for U²³⁸ showing the (γ,n), (γ,2n), ( f), reactions and the (γ, total) cross section.

FIG. 4 is a chart of the photonuclear cross sections of Th²³², U²³⁸, Np²³⁷.

FIG. 5 is a chart of the photonuclear cross section of Pu²³⁹.

FIG. 6 is a chart of the gamma spectoscopic analysis of the resulting product from 30 MeV photofission of U²³⁸.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an embodiment of this invention wherein a linear accelerator 1 , preferably of the monochromatic type, accelerates electrons to an energy of between about 5-30 MeV, preferably about 10 MeV, which impact a high Z converter such as tungsten to generate gamma rays, which are directed onto the fuel material such as U²³⁸ within the reactor 2 which results in the (Y_J ) reaction, also known as photofission, thus releasing about 200 MeV. Coolant is pumped through the reactor 2 by the primary coolant pump 3 to carry heat out of the reactor and into the heat exchanger 4 where the heat is conveyed to the secondary coolant system driven by the secondary coolant pump 5. The heat carried away by the secondary coolant system is used to drive a turbine 6 before the coolant passes through the condenser 8 and is re-circulated. Turbine 6 drives a generator 7 to produce electrical power. A reactor built according to this invention requiring an accelerator 1 with a beam power of 1 MW will develop about 20 MW of power. The photofission reaction occurring within reactor 2 is not self-sustaining and stops when the accelerator 1 beam is turned off. This invention may be used to "bum-up" spent fuel from conventional light-water fission reactors, if simply operated from 10 to 20 MeV. Operation of the invention results in typical spent fuel waste products such as Cs¹³⁷ and Sr⁹⁰ which undergo photodisintegration by the (γ,n) reaction within the reactor 2, resulting in short lived or stable products. Chemical separations of the spent fuel isotopes is not necessary. Of course, more than one accelerator 1 may be used to drive the reactor 2 to higher power levels, and speed-up the bum-up process. Ideally, four spaced accelerators would require about 4.8 MW of power to run resulting in about 100 MW from the reactor. The fact that the reaction is not self-sustaining is a safety feature allowing immediate shut-down in the event of a problem.

The primary nuclear reactions of importance that occur within the reactor 2 are listed in Table I.

The photofission and neutron fission thresholds of relevant isotopes are listed in Table II. Table fl-Fission Threshold Energy of Select Isotooes

PHOTOFISSION NEUTRON-FISSION

The invention requires a high-power, low-energy (10 MeV) electron accelerator 1 or linac to produce the gamma rays to drive the reactions in the reactor 2, the preferred embodiment of which is shown in FIG. 2. Current technology suggests the use of a traveling wave resonant ring (TWRR) type electron accelerator 1 energized by two 1.2 MW continuous wave (CW) L-band klystrons 11 preferably operating at 1249 MHz RF to produce an electron beam with an energy of 10 MeV and a current of 100 mA.

The TWRR was selected to enhance the threshold current of beam break-up and to get high accelerator efficiency that results from the low value of attenuation constant and high field multiplication factor which are permitted only with TWRR. The advantages of using TWRR rather than a standing wave accelerator guide are: simplicity of cavity structure, larger aperture size, ease of fabrication, and easy mechanical separation from the recirculating wave guide, all these things make it easy to handle under a high radiation field.

The klystrons 11 are preferably driven by a 90 KVDC power supply to produce 1.2 MW RF. The 1.2 MW RF power is fed into the TWRR through the directional couplers 12. The injector consists of a 200 KVDC electron gun 13, two magnetic lenses, an RF chopper 14, a prebuncher 15 and a buncher 16. A peak current of 400 mA with beam energy of 200^'K.eV is required for the electron gun from the chopper 14 and the buncher 16 system design. The accelerator 1 consists of seven accelerator guides 18. Each unit of accelerator section forms a TWRR. Each of the accelerator guides 18 of which the length is 1.2 m, contains 13 2π/3 mode cavities and two coupling cavities. All accelerator guides 18 are constant gradient structure types under the condition of 100 mA beam loading. The first klystron energizes a buncher and three accelerator guides while the second klystron energizes the remaining four accelerator guides. The RF power fed into the buncher and each accelerator guide are 220 to 250 KW, respectively.

The U²³⁸ itself may be used as both the gamma converter and the photofission target, that is eliminate a separate electron to gamma converter such as tungsten and use the U²³⁸ target material itself as the x-ray source. The advantage here is the recovery of the heat normally dissipated in the converter, which is on the order of 70% of the beam energy.

It is important to note that although the reactor is sub-critical and driven by gamma rays, the neutrons produced still induce both fast and slow neutron fission just as in any conventional reactor. These neutron reactions result in additional energy output thereby increasing the input/output ratio from 1/20 to a value determined by the design.

FIG. 3 is the photonuclear cross sections of U²³⁸ and the competing reactions (γ,n), (γ,2n), (Y_J ), and (γ,total). Note that at the preferred energy level, 10 MeV, the two photonuclear reactions are (γ,n) and (γ,/).

FIG. 4 shows the total photonuclear cross sections of Np²³⁷ , U²³⁸ and Th²³² while FIG. 5 shows the total photonuclear cross section of Pu²³⁹. Compare the cross section shown in both

FIG. 4 and FIG. 5, and note that the cross sections are almost identical; specifically with a variation of only about 10%. This is important because the photon source can not differentiate between any of these four fuel sources. That is, performance is the same whether fueled with U²³⁸, Th²³², Np²³⁷ ,Pu²³⁹ or any other photofissile material. FIG. 6 is the gamma spectroscopic analysis of the products produced by the actual photofission of U²³⁸. The only long-lived products are Na²², Kr⁸⁵ and Cs¹³⁵, which when exposed to the continuous 10 MeV photon flux are photodisintegrated.

Calculations show that efficient (γ,n) incineration of the fission waste products requires a gamma flux of 10¹⁸ γ/cm²sec to accelerate the time decay by 180 times. The number of nuclei (γ,n) reacting during the irradiation can be determined by the following differential equation:

dN/dt = -(λ, + σ,φ) N+∑(λ ., + σ„φ)N., (Eq. 8) j≠ i

F=l,2...-N_a where

N. = number of the /th nucleus, λ, = decay constant of the /th nucleus, σ₍ = total photonuclear cross section of the z^'th nucleus λ., = decay constant from they^'th nucleus transmuting to the /th one, φ = γ-ray flux,

N_a = number of nuclei considered in the model.

Using the matrix representation, eq. (8) is written as follows: dΝ/dt = A'Ν, (Eq. 9) where

-(λ. + σ.φ) (/ = ), A,, = {λ.-+ σ,. φ (i≠j).

The matrix of the nuclei Ν at the time t=Δt can be obtained by the Taylor's expansion:

Ν (t+Δt) = Ν(t) +Σ (Δt)Vr! dN⁽ⁿ⁾ (t)/dt, (Eq. 10) r=l where dN⁽ⁿ⁾ (t)/dt is the rth derivative of N(t).

Combining equations (9) and (10), we can obtain N (t+Δt) as follows:

N (t+Δt) = N(t) +Σ (Δt)7r! A^rN(t). (Eq. 11 ) r=l

The matrix A contains two kinds of data: the decay constants and the photonuclear cross sections.

Also, fast neutrons exposed to nickel result in Compton scattered gamma rays from the

Ni⁵⁸ (n,γ) Ni⁵⁹ reaction, producing gamma rays with energies of 5 to 9 MeV. In our application, if the reactor contains a converter material like nickel, this is like placing a "repeater station" every time nickel is encountered. For example, if the rector contain layers of nickel and U²³⁸: 1. Accelerator produces gamma rays of 10 MeV exposed to uranium target to produce photofission resulting in fast neutrons;

2. Attenuation of the gamma rays as they pass through the first uranium layer reduces the energy of the gamma rays to below the threshold for the (γ,n) reaction; and,

3. Absorption of fast neutrons in the first nickel layer produces 5 to 9 MeV gamma rays which will effect photofission in the next layer of uranium.

Using this layered approach, there is no limit on how thick the reactor core can be, since the accelerator beam is continually amplified in each nickel layer.

In addition or instead of nickel as the "repeater station" described above, other elements or compounds of elements or mixtures thereof may also be used, for example, sulfur, dysprosium, yttrium, calcium, titanium, beryllium, manganese, lead, iron, aluminum, and copper. Although this invention has been described above with reference to particular means, materials and embodiments, it is to be understood that the invention is not limited to these disclosed particulars, but extends instead to all equivalents within the scope of the following claims.

Claims

CT .ATMSI claim:

1. A heat source which comprises an accelerator driven nuclear reactor, said accelerator accelerating electrons to an energy of from about 5 to 30 MeV, and said nuclear reactor containing spent nuclear fuel.

2. The heat source of Claim 1 wherein said accelerator is operated in continuous wave manner.

3. The heat source of Claim 1 wherein said accelerator is a traveling wave resonant ring type accelerator.

4. The heat source of Claim 1 wherein said accelerator is energized by L-band klystrons.

5. The heat source of Claim 4 wherein said klystrons operate at a frequency of 1249 MHz.

6. The heat source of Claim 1 wherein the nuclear reactor contains a converter material for conversion of fast neutrons to gamma rays by the (n,γ) reaction.

7. The heat source of Claim 6 wherein the converter material is nickel.

8. The heat source of Claim 6 wherein the converter material is sulfur.

9. The heat source of Claim 6 wherein the converter material is dysprosium.

10. The heat source of Claim 6 wherein the converter material is yttrium.

11. The heat source of Claim 6 wherein the converter material is calcium.

12. The heat source of Claim 6 wherein the converter material is titanium.

13. The heat source of Claim 6 wherein the converter material is beryllium.

14. The heat source of Claim 6 wherein the converter material is manganese.

15. The heat source of Claim 6 wherein the converter material is lead.

16. The heat source of Claim 6 wherein the converter material is iron.

17. The heat source of Claim 6 wherein the converter material is aluminum.

18. The heat source of Claim 6 wherein the converter material is copper.