REMEDIATION OF RADIOACTIVE WASTE BY STIMULATED RADIOACTIVE DECAY

Abstract: An apparatus and method for treating long-lived radioisotopes and transmuting them into short-lived radioisotopes through applied nuclear physics. Nuclear reactions, specifically of the ( gamma , n) type, also known as photodisintegration, are utilized to accomplish this transmutation from a radioisotope of given atomic mass to that of lower atomic mass. A radioactive element (1) is irradiated by high-energy photons, preferably in the form of gamma rays. The gamma rays are absorbed by the nucleus of the radioactive element placing it in an excited state. Upon relaxation from the excited state, the nucleus ejects a neutron, thereby transmuting the element to an isotope of lower atomic mass and shorter half-life.

Description: REMEDIATION OF RADIOACTIVE WASTE BY STIMULATED RADIOACTIVE DECAY DESCRIPTION RACKGROIIND OF THE INVENTION Field of the Invention. Embodiments of the present invention relate to a method for accelerating the decay of radioactive waste products, and more particularly they relate to nuclear transmutation of heavy radioactive elements into lighter ones with shorter half-lives. The invented process therefore relates to reducing long-term toxicity of radioactive waste and to an economic and effective process facility for doing so. Related Art. The present invention relates to a method for photon excitation of nuclear reaction/transmutation processes, thereby accelerating the decay of radioactive waste products. More particularly, the present invention relates to a method of accelerating the decay of radioactive isotopes, which method comprises bombarding radioactive atoms with X-rays, gamma rays, electrons or high-energy photons, and thereby assisting in the deactivating or neutralizing of radioactive waste. U.S. Pat. No. 3,974,390 entitled "Method of Producing Excited States of Atomic Nuclei" issued to Masato Morita on Aug. 10, 1976, discloses a method of producing excited atoms by using x-rays to knock electrons from the K shell. When another electron falls into the vacant K shell, the energy is imparted to the nucleus thereby exciting the nucleus which relaxes by gamma or beta emission. This is an indirect method of imparting energy to the nucleus of an atom. U.S. Pat. No. 4,961,880 entitled "Electrostatic Voltage Excitation Process and Apparatus" issued to William Barker on Oct. 9, 1990, discloses a process and apparatus for utilizing electrostatic charge to lower the Coulomb barrier thus reducing the energy required for the nucleus to expel an alpha particle, thereby accelerating the decay rate of the radioisotope. Electrostatic fields are generally not of sufficient energy to effect nuclear reactions as they are defined herein. U.S. Pat. No. 5,076,971 entitled "Method of Enhancing Alpha Decay in Radioactive Materials" issued to William Barker on Dec. 31, 1991, discloses a method and apparatus for decontaminating radioactive materials by accelerating the decay rate by electrostatic means. Electrostatic fields are generally not of sufficient energy to effect nuclear reactions as they are defined herein. Each of the above cited U.S. patents describe a method for effecting the decay rate of radioisotopes, yet none of them have seen practical application. At present, there are four industrially demonstrated separations processes that are applicable in the present invention to separate radioactive isotopes from nuclear waste. These processes are designed primarily for the concentration and purification of plutonium, but only the PUREX™ process is well established in current worldwide use. In the past, the British have used a solvent extraction process called BUTEX™, the French have used ion exchange, and there have been a number of ion exchange processes that have had limited production use in the isolation of minor actinides. Several potentially applicable separations processes based on new solvents, such as the TRUEX-CMPO™ process, and new ion exchange materials are in various conceptual or laboratory scale development stages. Such advanced aqueous processes have been proposed to achieve high decontamination factors but have not been demonstrated at the full engineering pilot- plant level. What is still needed, then, is an effective method for remediation of radioactive waste by stimulated radioactive decay. What is still needed is a process facility for effectively performing this remediation, with efficient use of the by-products of the remediation. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of transmuting radioactive atomic nuclei through a nuclear process in a comparatively simple manner. The present invention may be used with high-toxicity, radioisotopes with atomic Z number greater than 50, which are the isotopes of principal concern in waste disposal. It is another object of the present invention to provide a method of transmuting radioactive isotopes by bombarding atoms with X-rays, gamma rays, electrons or high- energy photons. It is still another object of the present invention to provide a method and apparatus to stimulate decay of radioactive materials rapidly enough for it to be of practical value in disposing of radioactive wastes in storage. The invented process may be used for treating long-lived radioisotopes and transmuting them into short-lived radioisotopes through applied nuclear physics. Nuclear reactions, specifically of the gamma, neutron type, written as (γ, n), which are also known as photodisintegration, are utilized to accomplish this transmutation from a radioisotope of given atomic mass to that of lower atomic mass. Photodisintegration usually gives rise to neutron emission, i.e., to a gamma, neutron (γ, n) reaction, by the nuclei which have been raised to excited states by the absorption of photons. In the preferred embodiment of the invention, this neutron emission may be used for bombardment of other radioisotopes. Generally speaking, the target nucleus of the radioisotope to be treated is irradiated by gamma photons of an energy greater than the binding energy of the neutron in the target nucleus, thereby causing the ejection of said neutron through the (γ, n) reaction. That is to say, a radioactive element is exposed to high-energy photons preferably in the form of gamma rays. These gamma rays are absorbed by the nucleus, placing it in an excited state. Upon relaxation, the nucleus ejects the neutron, thereby transmuting the element to an isotope of lower atomic mass. The processes of the present invention may be performed in a process facility including an accelerator for producing the desired flux of photons, a reactor system for containing the radioactive isotopes to be gamma-treated and preferably also the radioactive isotopes to be treated by the neutron emissions from the transmuting gamma- treated isotopes. In addition, appropriate controllers are preferably included, such as those that monitor progress of the transmutation, recharge the reactor system, and control utilization of the heat produced from the transmutation. Other objects, features and advantages of the present invention will become apparent from the following description. BRTEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph which summarizes experimental data on photonuclear cross- sections integrated to 30 MeV as a function of Z. Figure 2 is a schematic diagram representing a commercial waste transmutation process according to an embodiment of the invention. Figure 3 is a detail schematic diagram representing one embodiment of the chemical separation section of Figure 2. DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, therefore, there is provided a method of producing unstable atomic nuclides, which method comprises the bombarding of atoms with high energy, such as X-rays, gamma rays, electrons, or high-energy photons. When the energy of bombarding gamma rays, for example, is greater than the binding energy of a neutron to the target nucleus, and the nucleus is excited by this energy absorption from its ground state to an excited state, then a neutron is ejected from the nucleus upon its relaxation from the excited state. This process is called "stimulated radioactive decay." The process transmutes unstable isotopes by using photons to release neutrons from atomic nuclei, thus transmuting isotopes to isotopes of less atomic mass. When neutrons are removed, the resulting isotopes have a considerably shorter half-life and then decay to stable forms in shorter amounts of time. Steps involved in the preferred process of stimulated radioactive decay are as follows: First, the radioactive isotopes are separated from radioactive waste by well known chemical processes. Then, electrons are accelerated in an accelerator, such as a linear accelerator or a betatron, to impact a high-Z target, such as a tungsten or titanium target. This impact generates the high energy gamma rays, which are then directed at the nucleus of the radioactive isotope to be reduced. This gamma ray bombardment knocks a neutron from the nucleus of the radioactive isotope atom, as described above. In the prior art, a bombardment process has been used to produce neutrons, but using different materials and resulting in different products than the invented process. In the prior art bombardment process, a stable, non-radioactive atom is typically subjected to bombardment, and a neutron is ejected from the nucleus. The neutron is the desired product and the atom minus the neutron is a radioactive waste product. In contrast to the prior art bombardment process, the method of the present invention subjects an unstable, radioactive atom to bombardment, resulting in a neutron being ejected from the nucleus. The resulting atom minus the neutron is more radioactive product, but has less half-life, and the neutron is a by-product. An example of half-life reduction that may be accomplished according to the invention is cesium-137 with a half-life of 30.17 years being transformed into cesium-136 with a half-life of 13 days. Long-lived daughter products may be treated in the same manner. These and other examples of processes according to the current invention are listed in the following Table I: Table I H 3 (γ,n) H 2 (stable) C 14 (γ, n) C 13 (stable) Y 90 (γ, ή ) Y 89 (stable) Ni 63 (γ, ) Ni 62 (stable) Kr 85 (γ,n) Kr 84 (stable) Co 60 (γ, 7i) Co 59 (stable) Tl 204 (γ, n) Tl 203 (stable) Sr 90 (γ, In) Sr 88 (stable) Bι 210 (γ,n) Bi 209 (stable) Tl 204 (γ, ) Tl 203 (stable) Ba 133 (γ, ) Ba 132 (stable) Pb 210 (γ, In) Pb 208 (stable) Sr 90 (γ, 7 ) Sr 89 (50 day half-life) - β " - Y 89 (stable) Tc 99 (γ, 3n) Tc 96 (4.3 day half-life) - β " -> Ru 96 (stable) I 129 (γ, ) I 128 (25 minute half-life) - β " -» Xe 128 (stable) Cs 137 (γ, 77) Cs 136 (13 day half-life) - β " -> Ba 136 (stable) Fe 60 (γ, ) Fe 59 (44.5 day half-life) - β " -> Co 59 (stable) Pb 210 (γ,n) Pb 209 (3.25 hour half-life) - β ' -> Bi 209 (stable) Examples of sources of high energy photons for photodisintegration of atomic nuclei include gamma rays of one energy, or gamma rays from a source which yields a continuous spectrum of energies, such as Bremsstrahlung, including a high-voltage X-ray tube or a betatron. For example, the best known gamma, neutron reaction is the photodisintegration of the de ' uteron, ι H 2 +γ→ lP 1 + 0 nl In the short-hand typically applied to reactions, if the "Q" of a reaction is negative, kinetic energy is converted to mass in an endothermic reaction. A negative Q value therefore means that kinetic energy must be brought into the nucleus to make the reaction proceed. An example is a reaction that cannot proceed until a photon brings in enough energy to a nucleus to satisfy conservation of energy. This means, in layman's terms, that the extent of the gamma reaction, or, in the terms of the art, that the "cross section" for a gamma reaction is 0 until the energy of the projectile is at least equal to Q. The energy of the projectile at which the reaction first has a non-zero cross section is called the "threshold " energy for the reaction. The threshold of the reaction is that energy of the gamma ray which is just sufficient to break the proton-neutron bond; i.e., the gamma ray must deliver an energy equal to or greater than the binding energy of the nucleon to affect the photodisintegration reaction. When describing the reactions of the instant invention, one may use the equation R =σφN, in which R is the reaction field, σ is the nuclear cross-section, φ is a constant characteristic of the nuclear reaction in question, and N is the number of target atoms, σ has the dimensions of an area, and is therefore usually called the "cross-section" of the reaction. The number of photons per square centimeter per second incident on a target is called the "flux." When a target is exposed to a flux of photons, the number of nuclei reacting is proportional to the flux, and to the number, N, of target atoms. We can picture each target atom as having a disk of area σ, with reaction occurring every time an incident photon strikes the disc. In some circumstances, σ is indeed equal to the physical cross- section of the nucleus. Tables and graphs of photonuclear cross-sections exist and may be used to calculate reaction yields. In such tables σ is usually expressed in barns or millibarns, one barn being X0 ~ ^° m 2 . Cross-sections vary with the energy of the incident photon, and the tables usually indicate this variation. Where there is a threshold energy, σ is 0 below the threshold but σ rises to positive values above the threshold. The relation between cross-section and energy is called the "excitation function" of the process. Therefore, the energy of the gamma ray for which neutrons are first observed to be ejected is the binding energy of the neutron. Baldwin and Koch (1945) were able to determine the threshold for photodisintegration of several different stable nuclei in the range of atomic numbers Z=6 to Z=47. Sher, Halpern, and Mann determined the threshold of many (γ, n) reactions. The threshold value of the (γ, n) reaction with any isotope of mass number A corresponds to the binding energy of the neutron in the nucleus of the isotope of mass number (A-l). The (γ, n) cross-section is very large for most nuclei for gamma energies between 10 and 20 MeV. This effect, called the "giant resonance," is responsible for much of the neutron background of high-energy gamma ray machines. The giant resonance occurs in all nuclei and is viewed as a general property of nuclei. Its width is 3-10 MeV and it is located between 13 and 18 MeV for medium and heavy stable elements, namely Z > 16, and near 20 MeV for light stable elements, namely Z < 16. Figure 1 summarizes experimental data on the photonuclear cross-sections integrated to 30 MeV as a function of Z for stable isotopes. Figure 1 shows that nuclei with atomic number Z greater than 50 have the largest absorption cross-sections. Figure 1 indicates the minimum energy needed to eject a neutron is less than about 30 MeV. Therefore, this is the minimum amount of energy required to use the preferred method of the subject invention. /// /// /// /// /// /// /// /// /// /// /// /// /// /// Table II shows a list of useful neutron capture gamma rays and their relative intensities. Table II *The errors on these intensities are of the order of ± 30 per cent. Not present in Table II are the useful neutron capture gamma rays and their relative intensities for the radioactive elements of the present invention. This is because, to the knowledge of the instant inventor, such experimental work has not yet been done. However, the inventor envisions that such experimentation may be done for the radioactive elements in the same general way it was done for the listed non-radioactive elements. The inventor expects the amounts for the radioactive elements of the present invention to be about 5 to 14 MeV. The core excitation model of the nucleus, or the "weak-coupling model," is a model involving electromagnetic properties of the nucleus. This is a model devised for the description of low lying states of odd- A nuclei, which tries to relate such properties to those of the odd particle and the even-even core. In other words, a state of an odd-A nucleus with an angular momentum J is written as: Ψ (J) = Σ A JcJ φ (J c , j; J) Here φ (J c , j; J) is a state in which the core carries an angular momentum J c , and the odd particle is in the state j . It is important to note that, formulated in this "core excitation model" way, there is no assumption about the mechanism that leads to the various core-states. These could be collective vibrations, or single particle excitations, or quasi-particle excitations, or anything else. The essential ingredient that goes into this model is the assumption of a weak coupling between the odd particle and the rest of the nucleus. Weak, that is, in comparison with the interactions involved in the core itself. For a photonuclear reaction in which a species A is converted into a species B (A- B), if the cross-section of this reaction is σ, then nuclei of A are destroyed at a rate of σφN A , while those of B are produced at this same rate: R = -dN A /dt = dN B /dt =σφN A This equation is of the same form as that for radioactive decay of A to B, but with σφ, in place of the disintegration constant λ. There is indeed an extensive analogy between the kinetics of a radioactive decay and kinetics in a constant flux of nuclear photons, and the equations concerned are closely similar. If the target species A is radioactive, then both nuclear reaction and decay contribute to its disappearance. The rate of loss is the sum of the two terms: -dN A /dt = σφN A + λ A N A = (σφ + λ A )N A If φ is constant, there will be a corresponding effective half-life of 0.693/(σφ + λ A ). Again, if the product B is radioactive, and we neglect the loss of B by further nuclear reaction, it will be produced at a net rate: dN B /dt = σφN A - λ B N B The application of the invented method toward treating fission waste products is useful in several ways. Fission products of principal concern because of their substantial thermal impact on waste repositories, as opposed to posing health risks, are strontium 90 and cesium 137 . These two radionuclides are dominant contributors to the heat released by spent fuel at least for the first several decades. Cesium 137 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 risks are technicium" and iodine 129 . They 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 neptunium 237 , uranium 234 , uranium 236 , plutonium 239 , plutonium 240 and plutonium 242 . However, the long-term risk is dominated in most scenarios by iodinel 29 and technicium 99 because they are typically water soluble and mobile in groundwater pathways. Photon transmutation of nuclear waste yields exceptionally high fractional rates for transmuting the transuranic and long-lived fission products. The method of the present invention reduces the transuranic in waste to such an extent that all waste containing residual transuranic will be Class C or less which are suitable for shallow land burial. The goal of photon transmutation for waste management purposes is to convert a long-lived radionuclides that is potentially troublesome at a waste disposal site to a shorter- lived or stable nuclide by exposing the troublesome nuclide to a high gamma flux for a sustained time. This has the effect of reducing the long-term toxicity of the waste because most of the waste constituents then decay to a non-radioactive nuclide in a short time. The application of the processes of this invention into a waste treatment facility will provide a boost to the nuclear power industry by providing a cheap, effective method for disposal of reactor waste products. Figure 2 shows a schematic of one embodiment of a commercial fission-waste transmutation system of the present invention. The various steps and equipment involved are outlined below. To obtain high purity fractions that can be made into transmutation targets, chemical processes such as those described in the Related Art section are preferably employed to separate the chosen radioactive components from the wastes. Figure 3 illustrates a detail schematic for a chemical separation section such as may be employed in the overall process flow scheme of Figure 2. Generally, the operating techniques and conditions for the systems shown in Figure 3 may be derived from conventional chemical processes. The preferred commercial waste transmutation facility requires head-end treatment of spent reactor fuel to chop and dissolve the fuel, followed by separation of the transuranic and selected fission products. Either aqueous or non-aqueous processes may be used for the initial separations. The well-established PUREX process may be used for this separations step. This is followed by an aqueous separations process using an advanced technology such as the TRUEX process. A full scale separations system may be designed with high confidence, and present engineering capabilities, for overall separations process losses of less than 0.1 %. Solid metals may be separated by pyrochemical processes. Pyrochemical processes may require less capital expense than aqueous processes because the volume of shielded space may be smaller as well as the reduction in the size of the plant and equipment needed. The radioactive materials to be transmuted are in either an aqueous slurry or in solid form when fed into the system. Some compounds as well as individual atoms may be used as charge for the transmutation system. After the chemical separation and containment of the radioactive isotopes into a target structure or area, the targets are irradiated in a flux having sufficient intensity and energy such that preferably substantially all of the radionuclides in the targets will either be transmuted or fissioned into stable elements or isotopes with substantially shorter half- lives at an acceptable rate. Many different methods for generating gamma rays can be utilized in the facility of the present invention. The betatron, for one, is very efficient, simple in construction, and compact enough to be portable. The gamma source need produce a beam of gamma rays with an energy greater than the binding energy of the isotope being treated. That is, the preferred photon beam has an adjustable beam energy up to a maximum of about 15 MeV per photon. The photonuclear reactions within the gamma-bombarded isotope 1 target release neutrons which are then utilized to treat other radioactive waste products by neutron absoφtion. This neutron flux by-product, on the order of 10 15 n/cm 2 sec, may be used for activation as well as neutron transmutation of other radioactive waste products, such as: Tc" + n → Tc 100 (16 second half-life)→ β + Ru 100 (stable) I 129 + n → I 130 (12.4 hour half-life) - β + Xe 130 (stable) τl27 + n → ιl28 (25 m i nu te half-life) → β + Xe 128 (stable) The waste products treated according to the present invention become heat sources due to their inherently short half-lives. The overall invented process therefore generates heat, which is recovered to produce the power for the gamma source, to be utilized in conversion systems for producing electrical power, to powering the treatment equipment itself, or to produce excess power for sale to the grid. As schematically depicted in Figure 2, the radioactive isotopes to be gamma- treated may be confined in a cylindrical area positioned appropriately to be bombarded by the photon beam. Other radioactive waste/isotopes 2 may be positioned as an annular cylinder concentric to the gamma treatment isotopes 1, so that the neutrons emitted by isotopes 1 may bombard the other isotopes 2. For example, charges of gamma treatment radioisotopes 1 and neutron treatment isotopes 2 may be added to a reactor system 10 that receives the gamma radiation (photon beam). The charge of isotopes 1 may be in solid form and added batch-wise to the reactor system 10, and the charge of isotopes 2 may be in aqueous form and added batch- wise to the reactor system 10. After the gamma treatment of isotopes 1 and the resulting neutron treatment of isotopes 2, the transmuted products may be removed from the reactor system and recycled to the chemical separations plant for separation of the stable and short-lived products from the radioactive isotopes that are to be recycled back to the reactor system. Heat from the photon and neutron treatment reactions may be recovered by conventional means to produce electric power. Various controllers may be utilized to control the photon beam and to control the process in general. Once the invented process and control needs are understood, one skilled in the art of process control may install various systems for efficient operation. A conventional controller may be used to control the gamma ray flux imparted to the gamma treatment isotopes 1. This may be done by varying/adjusting the beam current within the accelerator. The transmutation facility may include a control system for controlling the duration of gamma exposure and photodisintegration. For example, the neutron flux from isotopes 1 may be measured to monitor the progress of transmutation. When the neutron flux has dropped to a level indicating that substantial transmutation of isotopes 1 has been completed, the transmuted products may be discharged from the reactor system and recharging may commence. Alternatively, other control systems for controlling duration of gamma exposure are envisioned, such as monitoring the heat evolved in the reactor system. Thus, the duration control system may comprises monitoring the progress of the transmutation process and recharging the reactor system appropriately. In summary, the important industrial products of the instant reactions are: less dangerous radioactive materials in the long run, produced neutrons, radioactive materials with increased specific activity and increased specific energy, and, depending on the amount of energy imparted to the subject nuclei, also produced protons and alpha particles. 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: CLAIMS I claim: 1) A process for treating long-lived radioisotopes by transmuting them into short- lived radioisotopes or non-radioactive isotopes, comprising: a) separating long-lived radioisotopes from radioactive waste by chemical processes; b) accelerating electrons in an accelerator; c) impacting a target of high atomic number with said electrons; d) generating, on said impact, a flux of giant resonant gamma rays from said target; e) bombarding nuclei of said long-lived radioisotopes with said gamma ray flux; f) knocking a neutron from said nuclei by photodisintegration according to the process excitation function and threshold energy; wherein the long-lived radioisotopes are transmuted into short-lived isotopes or non- radioisotopes at a rate defined by the reaction field equation R=σφN. 2) The process of claim 1 wherein the long-lived radioisotopes includes radioactive atoms selected from the group consisting of Cs 137 , Sr 90 , 1 129 , and Tc 99 . 3) The process of claim 1 wherein said long-lived radioisotopes include radioactive atoms with atomic numbers exceeding 50. 4) The process of claim 1 wherein the flux of giant resonant gamma rays is sufficient to ensure that substantially all of the nuclei receive imparted energy sufficient to cause photodisintegration. 5) A method for reducing the long-term toxicity of radioactive waste comprising: a) using a chemical separation process to separate radioactive isotopes from radioactive waste; b) using a linear accelerator to accelerate electrons; c) using said accelerated electrons to impact a high Z target; d) using said high Z target to generate a flux of giant resonant gamma rays to knock a neutron from nuclei of said separated radioactive isotopes by photodisintegration; wherein long-term toxicity of the radioactive waste is thereby reduced. 6) The method of claim 5 wherein said radioactive isotopes include radioactive atoms selected from the group consisting of Cs 137 , Sr 90 , 1 129 , and Tc 99 . 7) The method of claim 5 wherein said radioisotopes include radioactive atoms with atomic numbers exceeding 50. 8) A radioactive waste transmutation facility comprising: a) a chemical separation plant for separating radioactive isotopes from radioactive waste; b) an accelerator for electron acceleration; c) a high atomic number target for receiving the impact of accelerated electrons; d) an adjustable flux controller for controlling the flux of giant resonant gamma rays emerging from said target throughout the duration of said impact; e) a reactor system for subjecting a quantity of said separated radioactive isotopes to bombardment of the controlled gamma ray flux to eject a neutron from nuclei of said separated radioactive isotopes by photodisintegration; f) a duration control system for controlling the duration of photodisintegration and the corresponding transmutation of said separated radioactive isotopes; wherein the transmutation facility is thereby adapted to reduce the long-term radioactivity of the radioactive waste. 9. The commercial radioactive waste transmutation facility of claim 8 wherein the separated radioactive isotopes includes atoms from the group consisting of Cs 137 , Sr 90 , I 129 , and Tc 99 . 10. The commercial radioactive waste transmutation facility of claim 8 wherein the separated radioactive isotopes include atoms whose atomic number exceeds 50.

Inventors: BROWN PAUL M, BROWN, PAUL, M.
Applicants: BROWN PAUL M, BROWN, PAUL, M.
Application year: 1999
Main classification: G21G1
IPC codes: G21F9, G21G1