Laser Transmutation of Nuclear Waste

The issue of radioactive waste is a major challenge in the widespread acceptance of a nuclear energy industry. The issues of where to store the nuclear waste and the possibility of radioactive materials leaching into the under-ground water supply seriously undermine the potential of nuclear energy.  However, with the ongoing development of ultra-intense laser techniques, researchers are exploring the possibility of laser transmutation of radioactive materials into stable isotopes.  Sadighi-Bonabi et al. (2009) analyzed the opportunity for transmutation of 93Zr, a highly radioactive nuclear waste with a half-life of 1.53 million years.  The authors suggest that through ultra-intense laser transmutation, 93Zr can be converted to 92Zr, its stable isotope.  High-energy electron generation, Bremsstrahlung, and photonuclear reactions were observed and the number of reactions that produced 92Zr calculated.  It was found that the laser intensity, irradiation time, and repetition rate of laser have strong and direct effects on the yield of 92Zr and the number of reactions. —Carolyn Campbell
Sadighi-Bonabi, R., Irani, E., Safaie, B., Imani, Kh., Silatani, M., Zare, S., 2010. Possibility of ultra-intense laser transmutation of 93Zr (γ, n) 92Zr a long-lived nuclear waste into a stable isotope. Energy Conservation and Management 51, 636–639.

 
          The disposal of long-lived radioactive waste is a significant challenge for the nuclear industry.  Through the development of ultra-intense laser technologies, the possibility of photonuclear transmutation of nuclear waste to more stable isotopes has offered new solutions to solving this problem.  When an ultra-intense laser pulse interacts with the radioactive waste, gamma radiations induce nuclear reactions for transmutation of the waste into a stable isotope.  Sadighi-Bonabi et al. focused their study on Zirconium, particularly 93Zr, a fission product in nuclear reactors with a half-life of 1.53 million years.
          In order to assess the number of reactions and laser-induced photonuclear activation of 93Zr, the authors analyzed available experimental data from focusing intensities onto a solid target.  For this study, the laser intensity was assumed to be 1020W/cm2 with a repetition rate of 10 Hz.  Calculations were also extended to higher intensities of 5 x 1020 W/cm2, 1021 W/cm2, 5 x 1021 W/cm2, and 1022 W/cm2.  The number of reactions was calculated by evaluating the energy spectrum of the laser and a cross section of the photonuclear reaction between threshold and cut-off energy.  It was found that the Bremsstrahlug spectrum, relating to the deceleration of electrons, depended on the intensity of the laser, with higher intensities increasing the number of reactions.  Additionally, irradiation time and repetition rate were found to have substantial effects on the yield of 93Zr (γ, n) 92Zr and the number of reactions.  If the target is irradiated for an hour by a laser light of 1020 W/cm2 at a repetition rate of 10 Hz, approximately 2.7 x 107 reactions will occur.  By increasing the repetition rate, yield would also increase.  However, achieving higher rates also means using more power, and more advanced lasers.  Additionally, although increased intensity of the laser would lead to a higher number of reactions and activity, there is an optimum intensity at which a maximum number of reactions is reached and, beyond that point, the overlap between reaction cross-section and the number of photons disappears.  For 93Zr (γ, n) 92Zr, this optimum intensity was calculated at 3 x 1021 W/cm2
          Through this study, it was found that laser intensity, irradiation time, and repetition rate of the laser have a significant, direct effect on the yield of 92Zr and the number of reactions.  Maximizing the efficiency of laser technology for the transmutation of radioactive isotopes will prove valuable in future efforts to solve the problem of long-lived nuclear waste. 

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