Lasers spark new paths in radio-isotope transmutation

Scientific breakthrough in the transmutation of isotopes

Collaboration between the European Commission’s Joint Research Centre (JRC) DG, the University of Jena (Germany), the University of Strathclyde (UK), Imperial College (UK), and the Rutherford Appleton Laboratory (UK) has led to the transmutation of long-lived radioactive iodine-129 into short-lived iodine-128 using very high intensity laser radiation. Until recently, transmutation could only be achieved in nuclear reactors or particle accelerators.

Transmutation – making use of nuclear reactions that will change very long-lived radioactive elements into less radioactive or shorter-lived products – is a concept for nuclear waste management under development in several countries. Very long-lived iodine-129 has a half-life of 15.7 million years, high radiotoxicity and mobility, and is an important constituent of nuclear waste – making it one of the primary risk considerations in the nuclear industry. It currently has to be sheathed in glass and buried deep underground. Handling of iodine is also difficult as it is corrosive and volatile. Through the laser-induced photo-transmutation process, this long-lived isotope was transmuted first to the short-lived isotope iodine-128, which then decays with a half-life of 25 minutes to the stable inert gas xenon-128. The experiments demonstrate the feasibility of transmuting radioactive iodine-129; limitations to scaling up this technique may be the high energy consumption of the laser and the low cross sections of the elements in question, resulting in low transmutation efficiencies.

The JRC Institute for Transuranium Elements in Karlsruhe, Germany first proposed use of laser radiation to split radioactive elements in 1990 but lasers of sufficient power were not available. Now a novel amplification technique (chirped pulse amplification) has boosted intensities to some 1020 W/cm2 – the equivalent of focusing the entire energy output of the sun onto an area of just 0.1 mm2. By focusing such a laser onto a tantalum metal target, the beam generates a plasma with temperatures of ten billion degrees (1010K) – comparable to those that occurred one second after the ‘big bang’ believed to have created the universe. The electrons in the plasma generate gamma radiation intense enough to induce nuclear reactions in the iodine target.

Through collaboration with the University of Strathclyde, experiments were performed with the giant pulse VULCAN laser at the Rutherford Appleton Laboratory. And, in collaboration with the University of Jena, the experiment was performed with a high repetition rate tabletop laser. This work opens the way to transmutation experiments on a laboratory scale – rather than at large-scale facilities – using much cheaper and more accessible instrumentation.

Further research is necessary to investigate the potential for scaling up the process to deal with the volumes of iodine-129 produced by the nuclear industry. From the present experiments, much useful basic information on transmutation reactions can be obtained. Nuclear cross section data on iodine was obtained for the first time for the photonuclear reaction described here. Laser induced nuclear reactions may also be used to transmute other elements. Indeed, the laser-induced fission of uranium-238 and thorium-232 had been demonstrated earlier through the above collaboration.