One (the) big worry in nuclear power from fission processes is the creation of very heavy atoms, those more heavy than the uranium used for fuel – such as curium and actinium. The group is called “actinides,” the heavy elements that can take millions of years to get through half-lives to less radioactive states. The idea of containing such things for very long periods sets even the most callous thoughts on edge as millions of years of storage managed by mankind isn’t an idea that builds confidence. Especially as these elements would be a dirty bomb maker’s dream component.
To “eliminate” these materials requires a transmutation into elements of either non-radioactive or much shorter half-lives. Helmut Leeb and Erwin Jericha at the Vienna University of Technology with collaboration at CERN’s n TOF Facility will continue their work with a restart of the facility’s neutron reactor, called the Accelerator Driven System. The new concept is an “undercritical reactor” meaning it can’t run so that the reaction is self-sustaining.
The neutrons necessary for an undercritical reactor operation are supplied by a proton accelerator with a spallation target located in the reactor core. “During the spallation, the atomic nuclei of the target (mainly lead) are broken with high-energy protons, while a large number of neutrons are normally released, neutrons which are necessary for the stationary operation of the reactor. If the accelerator is turned off, the chain reaction ceases,” Leeb explains. Worldwide, studies are based on the assumption that at least two decades will be necessary to transfer this concept to the industrial level, a concept that is fully understood at the scientific level. But, that may change as the collaborators proceed.
The prerequisite for this development is a thorough knowledge of the neutrons’ interaction and reactions with other materials as developed so far. In 2000, the n Tof facility became operative at CERN, which is a unique facility in the world, especially suitable for measuring the reactions of radioactive materials when bombarded with neutrons. Between 2002 and 2005, a large number of radiative captures and fission reactions, previously insufficiently known, were measured as part of a European Union project, in which nuclear physicists from TU Vienna were deeply involved. Due to an operational pause for the construction of the Large Hadron Collider at CERN, now the consortium can restart the operations at the upgraded n_TOF facility with a new target. The first series of experiments are neutron radiative captures on iron and nickel, which are to be analyzed by the Viennese nuclear physicists.
“The core concept of transmutation – which was formulated as early as mid 20th century – consists of irradiating the actinides by fast neutrons. The highly stimulated nuclei that are generated this way suffer a fission, which leads to relatively short-lived nuclei, which in turn rapidly disintegrate into stable isotopes. Then, they cease to be radioactive,” explained Professor Leeb. Thus, the required radioactive waste isolation time of several millions years could be decreased to 300 and up to 500 years. The technological progress made in the last decades has made the transmutation possible at the industrial level.
Another way to burn the heavy actinides is in research by a consortium between Americans and Russians where the heavy atoms are added to a thorium fuel reactor and burned.
Leeb points out another path, the “thorium-uranium cycle,” an alternative nuclear fuel, which leads to a reduced incidence of radioactive waste. “Thorium is a potential nuclear fuel, which may be incubated into a light uranium isotope, whose fission generates basically no actinide. Furthermore, thorium can be found approximately five times more often than uranium. However, special reactors must be still developed for this, reactors that would be appropriate for the reaction pattern and for the somewhat harder gamma radiation. India is one of the countries that already host experiments with thorium in reactor cores,” said Leeb.
That adds another way or discussion point to offer in the effort to increase fission based power production. There is good reason to have concern for the isolation of the actinide share of nuclear waste and its safekeeping. Better still will be the methods that use the materials in a fuel burning leaving nothing at all of concern behind. Even as the Viennese physicist’s effort might come up not necessary if burning in thorium reactors comes of age, the research is worthwhile. Driving to more transmutation skills, processes and results could offer many other potential paths to produce materials of great use.