January 10, 2007

storage of nuclear waste

Scientists commented on a paper published in Nature which looked at the durability of materials which could potentially be used to contain nuclear waste.

Prof. Robin Grimes, Professor of Materials Physics, Imperial College London, said:

“This paper describes an exciting new approach that will enable scientists to better monitor how a material responds to radiation damage. The authors apply the technique to the mineral phase zircon. As the authors admit, however, zircon was never going to provide the durable material needed for nuclear waste immobilization. Significantly, other mineral phases, particularly those based on the fluorite structure, are already known to be significantly more radiation tolerant than zircon. The fluorite family of materials therefore make much more attractive radioactive atom hosts and provide a better focus for our pursuit of an effective durable mineral-based waste form.”

Dr Sue Ion, Vice President of The Royal Academy of Engineering, said:

“Firstly it is good research which will help inform. The headline which may lead you to be concerned and potentially rule out use of ceramic encapsulants is not necessarily the right one to draw from the research itself. Radiation damage in itself does not necessarily mean a material would be inadequate from a durability and ‘fit for purpose’ standpoint. If you lose crystallinity then a more ‘glassy’ material is likely to form. The current internationally accepted method of encapsulation is to incorporate the waste products into specially formulated glass which solidifies in metal containers in a process known as vitfrification.”

Dr Neil Hyatt, from the Immobilisation Science Laboratory at the University of Sheffield, said:

“Plutonium undergoes radioactive decay by emission of fast moving alpha particles and in so doing transforms to a more stable isotope of uranium. This process is rather like firing a shotgun. The alpha particles are like fast moving bullets, but just as a gun recoils on firing, so the uranium atoms produced by this process also recoil. Recoiling in this way, the uranium atoms collide with other atoms in a solid causing their regular arrangement to be destroyed. This process is called amorphisation. Now, Cambridge scientist Ian Farnan and colleagues at the US Pacific Northwest National Laboratory, have shown the accumulation of radiation damage in solids can be measured using a technique called NMR spectroscopy. Farnan and colleagues use this method to examine how radiation damage destroys the regular arrangement of Si atoms in (Zr,Pu)SiO4 as Pu atoms undergo radioactive decay. From their data, Farnan and co-workers suggest that the regular crystalline arrangement of atoms in zircon, ZrSiO4, would be destroyed after only 1,400 years – a short time in comparison to the desired lifetime of a radioactive waste repository (some 250,000 years). Is this reason to be alarmed? Unlikely – as Farnan and colleagues point out naturally occurring zircons, almost as old as the Earth itself, have retained radioactive elements like uranium despite complete destruction of the regular internal arrangement of atoms.”

Dr Neil Milestone, Director of the Immobilisation Science Laboratory, University of Sheffield, said:

“This is the first time where actual rates of breakdown of the structure of crystalline ceramics containing synthetic radioisotopes have been quantified and shown to be higher than expected. It highlights the need to conduct experiments with these highly radioactive elements. These experiments are difficult to carry out and these types of material are only now being prepared in specialised laboratories in Russia and USA able to handle the highly toxic and radioactive elements. Such a laboratory will be available soon in UK. For this reason, most estimates of stability of potential wasteforms have relied on modelling studies or the use of surrogates such as those containing uranium being conducted in the ISL at the University of Sheffield. Experiments to determine whether this greater than expected damage will mean ceramics are unsuitable as wasteforms still have to be made.”

Prof. Charles Curtis, University of Manchester and Head of Research and Development Strategy UK Nirex Ltd., said:

“The relevance (of this paper) for long-term radioactive waste management is that radiation damaged ceramics and minerals would be more soluble in groundwater than crystalline equivalents therefore might release radionuclides such as 239Pu to groundwater more quickly. But long-term radioactive waste management in deep geological repositories, the preferred choice of international waste management organisations, relies on several different barriers, not just the inertness of the synthetic waste form itself. Collectively, these either contain the radioactive nuclides completely or delay their release and migration until such time as their radioactivity has decayed to extremely low levels.

“Farnan and colleagues rightly focus on actinides because of their very long decay half lives and cite the example of 239Pu (t ½ = 24100 years). Where these have been isolated it makes very good sense to seek stable immobilisation matrices. A useful outcome from the research will be an enhanced capability to predict future damage in materials used for isolating key radionuclides.

“Radiation from most radioactive wastes decays much more quickly. 90Sr and 137Cs, for example, have half-lives of around 30 years and will almost have disappeared after 300 years. The safety regulators require that there shall be only insignificant release of radioactivity from all sources for up to a million years into the future.”