rexresearch.com
Claus ROLFS, et
al.
Nuclear Waste Decay Acceleration
Nuclear Waste Decay Acceleration
http://physicsworld.com/cws/article/news/2006/jul/31/a-cool-solution-to-waste-disposal
A cool solution to waste disposal
Jul 31, 2006A group of physicists in Germany claims to have discovered a way of speeding up radioactive decay that could render nuclear waste harmless on timescales of just a few tens of years. Their proposed technique – which involves slashing the half-life of an alpha emitter by embedding it in a metal and cooling the metal to a few degrees kelvin – could therefore avoid the need to bury nuclear waste in deep repositories, a hugely expensive and politically difficult process. But other researchers are sceptical and believe that the technique contradicts well-established theory as well as experiment.
The leader of the German-based group, Claus Rolfs of Ruhr University in Bochum, is an astrophysicist and made the discovery about alpha decay after replicating the fusion reactions that take place in the centre of stars. Using the university’s particle accelerator he fired protons and deuterons (nuclei containing a proton and a neutron) at various light nuclei. He noticed that the rate of fusion reactions was significantly greater when the nuclei were encased in metals than when they were inserted into insulators. He also observed that the effect is enhanced at lower temperatures (J. Phys. G: Nucl. Part. Phys. 32 489).
Rolfs believed this effect could be explained in simple terms by assuming that the free electrons in a metal act like the electrons in a plasma, as described in a model by Dutch physicist Peter Debye. The lower the temperature of the metal, the closer the free electrons get to the radioactive nuclei. These electrons accelerate positively charged particles towards the nuclei, thereby increasing the probability of fusion reactions.
But Rolfs realized that the reverse reaction might also occur and that free electrons could enhance the ejection of positively charged particles from a nucleus. This would reduce the half-lives of α-decay or β+-decay, and increase half-lives for processes involving electrons (which are repelled by the free electrons within the metal), i.e. β–-decay and electron capture.
The group has investigated this hypothesis by embedding a number of radioactive nuclei inside metals and then cooling the metal to a few degrees kelvin. As expected, they observed a longer half-life for the electron capture of beryllium-7 and shorter half-lives for β+-decay in sodium-22 (Eur. Phys. J. A 28 251) and α-decay in polonium-210. They are now investigating the α-decay of radium-226, a hazardous component of spent nuclear fuel with a half-life of 1600 years. Rolfs calculates that this half-life could be reduced to as little as a year and at the very least to 100 years, and believes that the half-lives of all other hazardous alpha emitters within nuclear waste could be shortened by similar amounts.
"This means that nuclear waste could probably be dealt with entirely within the lifetimes of the people that produce it," he says. "We would not have to put it underground and let our great-great-grandchildren pay the price for our high standard of living."
Rolfs admits that much engineering research needs to be done to convert his idea into practice, but he believes there are probably no insurmountable technical barriers. Other physicists, however, think that the basic idea may be flawed. According to Nick Stone, a nuclear physicist recently retired from Oxford University, physicists have already carried out experiments in which they cooled alpha emitters to 4 K and below, but found no significant changes in their half-lives.
Meanwhile, Hubert Flocard, director of the CSNSM nuclear-physics lab near Paris, believes that Rolfs' model contradicts standard solid-state physics, although he admits that he cannot explain the group's data himself. Rolfs concedes that he needs a more sophisticated theory, but stands by his results. "Nature decides what is right," he says.
US5978432
METHOD OF TREATING NUCLEAR WASTE
METHOD OF TREATING NUCLEAR WASTE
Inventor(s): ROLFS SIGRID [DE]; KETTNER KARL-ULRICH [DE]; KETTNER MONIKA [DE]; ROLFS CLAUS [DE] +
A method of treating a-emitting nuclear waste, wherein the a-emitting nuclear waste is embedded in a matrix of metal atoms, said matrix being selected for providing an electron spatial probability near the a-emitting nuclei which has an influence on the a-decay of these nuclei, and keeping the matrix with the embedded waste to allow the a-emitting waste to decay with a reduced half-life.
Recent investigations, for example F. Raiola et al., Eur. Phys. J. A 13 (2002), p. 377; F. Raiola et al., Eur. Phys. J. A 19 (2004), p. 283; D. Zahnow et al., Z. Phys. A 359 (1997), p. 21 1 , have shown that a rate of fusion reactions in metals is drastically enhanced compared to fusion reactions in gases or insulators. The data of the fusion reactions are quantitatively explained using the plasma model of Debye to describe the quasi-free metal electrons.
Further investigations of the screening effect on the reactions of various nuclides across the periodic system, using metallic environments, show that fusion or decay rates depend on the density of quasi-free electrons near the in- volved nucleus.
It has been found that the influence of the electron probability density on nuclear processes may be purposefully used to control a nuclear process on a commercial or industrial scale.
The subject matter of the invention is a method of treating [alpha]-emitting nuclear waste, wherein the [alpha]-emitting nuclear waste is embedded in a matrix of metal atoms, said matrix being selected for providing an electron spatial probability near the [alpha]-emitting nuclei which has an influence on the [alpha]-decay of these nu- clei, and keeping the matrix with the embedded waste to allow the [alpha]-emitting waste to decay with a reduced half-life.
The matrix is, for example, provided being in one of the solid, the liquid, and the gaseous state. However, the substance may be introduced into the matrix at an introduction state of the matrix which may differ from the state that is acquired afterwards. For example, the matrix may be formed by a solid metal, the substance having been introduced into the metal at a melted state of the metal.
Useful details of the invention are indicated in the dependent claims. For example, the substance containing said [alpha]-emitting nuclei is introduced into said matrix to a selected mass ratio. For example, the substance may be introduced into the matrix to a mass ratio of less than 10%. However, the mass ratio may be optimized in view of the kind of substance, nuclear proc- ess and the matrix used.
Experiments show that the influence of the electron spatial probability on the nuclear process may depend on the temperature. Accordingly, the nuclear process preferably is controlled by maintaining the matrix in a selected tem- perature range. The matrix with the embedded substance may be cooled to a cooling temperature which is preferably below 100 K and may be, for example, approximately the temperature of liquid helium, i.e. 4 K.
In one embodiment, the matrix provides quasi-free electrons having a non- vanishing spatial probability near said nuclei.
For example, the matrix is formed by one of the group of a metal, alloy, compound containing a metal, and mixture containing a metal. The electron density can be controlled by selecting suitable metals, alloys, components, mix- tures or compounds. At present, metals like Palladium (Pd) or Indium (In) seem to show the largest free-electron plasma effect. A suitable material has to be selected depending also on its melting properties, its worldwide availability, and its price. The material may contain more than one metal. The mixture or the compound may also contain other elements than metals.
In nuclear fission reactors, radioactive waste including fission products and transuranic isotopes is produced. These are described by L. C. Hebel et al., Rev. Mod. Phys. 50 (1978), p. 1 , as the most important components of radioactive waste. One of the most dangerous components of nuclear waste is the radium isotope Ra-226. The radioactive isotope Ra-226 is an alpha emitter and has a half- life of approximately Ti/2 = 1600 years. It has a high solubility and is readily incorporated in biological organisms. Because of the high radioactivity and/or the comparatively long half-life of Ra-226 or other components of radioactive waste, parts of the radioactive waste may have to be safely stored for thousands of years. The method of the invention facilitates the elimination of radioactive waste, especially high level radioactive waste and transuranic radioactive waste. This is achieved by the following steps: radioactive waste is embedded in a matrix providing quasi-free electrons having a non-vanishing spatial prob- ability near the radioactive nuclei of the radioactive waste, is cooled to a cooling temperature, and is kept at the cooling temperature or below to let the radioactivity of the radioactive waste decay. Depending on the chosen cooling temperature and on the matrix, the decay time and thus the half-life of radioactive elements contained in the radioactive waste is drastically reduced due to an accelerating effect of the quasi-free metal electrons on the nuclear decay.
This embodiment of the method is especially useful for the removal of high level radioactive waste or transuranic radioactive waste. However, the method may also be applied to intermediate level radioactive waste or to low level radioactive waste.
This embodiment of the method is also especially useful when the radioactive waste comprises an isotope having a medium-long radioactive half-life of, for example, more than 25 years and less than 35000 years. For example, the radioactive waste comprises Ra-226 having a half-life of approximately 1600 years. Preferably, the matrix with the embedded radioactive waste is then kept at a cooling temperature of, for example, T=4 K for at least one year. However, the cooling may be interrupted and later continued. Thus, the time during which the matrix with the embedded radioactive waste is kept at the cooling temperature preferably accumulates to at least one year.
For example, the used-up rods of a reactor containing the remaining fissible nuclides together with the transuranic waste may be added directly to the metallic melt so as to be melted therein. When the transuranic waste has been removed (after, say, one year), the rod material still contains fissible nuclides and the fission-product waste, notably the nuclides <129>I (Ti/2 = 15 x 10<6> y). Since the rods have now a relatively low radioactivity, one can separate the less problematic nuclides <129>I from the fissible nuclides using meth- ods of chemistry or mass separation or the like. The separated <129>I nuclides can then be exposed to a thermal neutron flux, whereby <129>I is transformed by neutron capture in a short time (days) into <130>I, which in turn decays within minutes to the stable isotope <130>Xe. Thus all long-living waste is removed.
Preferred embodiments of the invention will now be described in conjunction with the drawings, in which:
Fig. 1 schematically shows a method of controlling a nuclear process; and
Fig. 2 schematically shows a method of eliminating radioactive waste by controlling nuclear decay.
In Fig. 1 , in a first step Sl of the method of controlling a nuclear process, a substance is provided which contains nuclei that are to undergo said nuclear process.
In a second step S2, a matrix of atoms is selected which provides, when the substance is embedded in the matrix in a step S3, an electron spatial probability near the nuclei which has an influence on said nuclear process.
The steps Sl and S2 may be performed in any order.
In a step S4, the nuclei undergo said nuclear process, the process being influenced by the electron spatial probability of the matrix. This situation may be maintained as desired, and the nuclear process may take place at a number of nuclei, its rate being determined by the electron spatial probability of the selected matrix.
Fig. 2 shows a more specific embodiment of the method of Fig. 1.
In the first step Sl ', radioactive waste, for example, high level radioactive waste is obtained by extracting it from spent nuclear fuel. The extraction may be done, for example, using methods which are known from the reprocessing of nuclear fuel for separating the components of spent nuclear fuel. Option- ally, used-up fuel rods of a fission reactor are provided in a step Sl " and are directly subjected to the subsequent process steps. The high level radioactive waste thus obtained comprises, for example, ra- dium-226 having a half-time of approximately Ti/2 = 1600 years. Radium-226 undergoes an alpha decay emitting alpha particles of an energy of 4.78 MeV.
In the step S2', a matrix is selected having quasi-free electrons providing, when the radioactive waste or the rods are embedded in the matrix in the step S3', a non-vanishing electron spatial probability near the radioactive nuclei of the waste. This has an accelerating influence on their nuclear decay rate.
In step S3', the radioactive waste and/or the rods are introduced into a melted metal or alloy to a mass ratio of less than about 10%. For example, the high level radioactive waste containing radium-226 is introduced into palladium (Pd) with a mass ratio of several percent.
In step S4', the obtained product is cooled to a cooling temperature T which is, for example, approximately the temperature of liquid helium, e.g. T = 4 K.
The obtained product is then kept at the cooling temperature of, for example, T = 4 K in step S5'. Surrounded by the cooled palladium, the radioactive isotopes experience a drastic shortening of their half-life. For example, the half- life of radium-226 may be reduced by a factor of 4500. Thus, under the described conditions, the half-life of radium-226 is only 0.36 - 2 years . Thereby, the radioactivity has completely decayed in a few years.
A similar reduction of the half-life is expected for all transuranic elements in the radioactive waste produced by nuclear fission reactors.
When in a step S6<1>, for example, after several years, the radioactive content of the product is small enough, the method is brought to an end. In the step S6', for example, the [alpha]-decaying waste has disappeared almost completely and thus, the product is practically free of transuranic isotopes. The method may also be finished when the radioactivity is low enough to allow a different handling or storage of the radioactive waste. Thus, the radioactive waste is eliminated in an efficient manner in a time which is drastically shorter than the decay time of the radioactive waste when left unattended.
Instead of the metal palladium, also indium (In) or tin (Sn) or other materials showing the described effect may be used. Moreover, a melted mixture containing a metal or a melted compound containing a metal may be used instead of the metal or alloy.
Instead of extracting the radioactive waste from spent nuclear fuel in step Sl<1>, the radioactive waste may alternatively be obtained in a different way and may be introduced into the melted metal or alloy or mixture or compound, as is indicated by a dashed arrow in Fig. 2.