Optics 101

Transmuting Nuclear Waste with Laser Driven Gamma Rays

During nuclear reactions, radioactive isotopes become transmuted (changed) as they decay into another element, or a stable isotope of the same element. This process generates a lot of hazardous nuclear waste that is notoriously difficult to dispose of. Whilst there are processes already in place to deal with nuclear waste, there has been a lot of work into further transmuting this nuclear waste using gamma rays from a laser to speed its decay, so that it no longer poses a risk to health or the environment. In this article, we look at how laser driven gamma rays are used to transmute nuclear waste.

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What is Nuclear Transmutation?

Transmutation is the process of changing one substance into another. Within the nuclear sector, nuclear transmutation is a process of changing one chemical element (or the specific isotope of an element) into a different chemical element or isotope. Transmutation often occurs with radioactive elements as their unstable nature means that will eventually change into a different, more stable element. When this occurs depends on whether the transmutation is a natural process, or whether an experimental approach helps it along.

One example of a natural process is seen within celestial bodies, such as the sun, where radioactive elements naturally decay and release radiation. The time it takes for a radioactive element to decay ultimately depends on its half-life.

However, transmutation can occur experimentally, such as within nuclear power plants where nuclear fission occurs by either a nuclear reaction or radioactive decay. However, when the radioactive isotopes have been reacted in these chambers, they leave a large amount of nuclear waste. Nuclear waste still contains radioactive material and is hazardous to all forms of life, as well as the environment. There aren’t many ways to deal with nuclear waste, but the most common approach has been to bury it deep underground for long periods of time where it poses no risk to people (and environment) above ground.

However, this is a huge logistic task that requires a significant amount of regulations, careful handling and careful transport to move the nuclear waste from the reactor to the “refuse site”. In the last decade or so, there has been a few examples of how lasers that produce high energy gamma rays can be used to transmute nuclear waste and accelerate the radioactive decay process.

How Lasers Can Be Used to Transmute Nuclear Waste

There have been reports of using gamma rays to break down various types of radioactive waste, including 137Cs, 135Cs, 129I, 126Sn, 107Pd, 99Tc, 93Zr and 90Sr. Although, some of these are still postulated at this stage (i.e. through computational experiments), whilst others, namely the heavier isotopes, have been transmuted experimentally.

These heavy isotopes can be transmuted into less hazardous elements/isotopes using high power pulsed gamma rays. The process of using these gamma rays helps to transform long-lived nuclei into short-lived or stable elemental isotopes through a photo-transmutation mechanism.

The most common process to create these high-powered gamma rays is achieved by using a non-conventional mode of accelerating electrons. This mechanism uses relativistic electrons which0 are focused from a laser-plasma-driven electron beam onto a high-Z metallic (thin) target that produces gamma rays through a bremsstrahlung mechanism– i.e. when a one electron collides with another electron, and in turn, causes the electrons to slow down and emit a gamma ray. The second, but less common, mechanism to produce gamma rays is through Compton scattering. This is when a photon is scattered by an electron, which decreases the energy of the photon and triggers the release of a gamma ray.

There has been a lot of advancements in laser technologies within recent years, which alongside high brightness linear accelerators, has enabled the production of Compton-X and Gamma-photon beams which possess a high brightness and energy. Some of the lasers employed to create these high energy gamma rays include femto- and petawatt lasers, alongside various acceleration principles– such as laser wake-field acceleration (LWFA) and direct laser acceleration (DLA) methods.

Regardless of whether the high energy gamma rays are produced through Compton scattering or the bremsstrahlung mechanism, they induce photo-transmutation within the nuclear waste via photonuclear reactions. How effective the photonuclear reactions are is dependent on the various factors, but the most important are the intensity of the laser, the irradiation time and repetition rate of the laser. The resulting products depends entirely on the radioactive elements present, e.g. 107Pd turns into the stable 106Pd isotope; but they not only offer a way of making various types of nuclear waste safe, some of the resulting isotopes can be further used in other applications– one example of this is in the production of heavy metal isotopes for medical applications.

Sources:

  1. Stanford University: http://large.stanford.edu/courses/2011/ph241/noll1/
  2. Photo-transmutation of long-lived radionuclide 135Cs by laser-plasma driven electron source”- Luo W. et al, Laser and Particle Beams, 2016, DOI: 10.1017/S0263034616000318
  3. “Gamma rays transmutation of Palladium by bremsstrahlung and laser inverse Compton scattering”- Sadighi-Bonabi R. et al, Energy Conversion and Management, 2014, DOI: 10.1016/j.enconman.2013.09.029
  4. “Laser-driven photo-transmutation of 129I—a long-lived nuclear waste product”- Ledingham K. et al, Journal of Physics D: Applied Physics¸ 2003, DOI: 10.1088/0022-3727/36/18/L01
  5. “Transmutation prospect of long-lived nuclear waste induced by high-charge electron beam from laser plasma accelerator”- Wang X. L. et al, Physics of Plasmas, 2017, DOI: 10.1063/1.4998470

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