Materials science: Plastic beads may provide a way to mop up radiation in nuclear powerstations and reduce the amount of radioactive waste
NUCLEAR power does not emit greenhouse gases, but the technology does have another rather nasty byproduct: radioactive waste. One big source of low-level waste is the water used to cool the core in the most common form of reactor, the pressurised-water reactor. A team of researchers led by Börje Sellergren of the University of Dortmund in Germany, and Sevilimedu Narasimhan of the Bhabha Atomic Research Centre in Kalpakkam, India, think they have found a new way to deal with it. Their solution is to mop up the radioactivity in the water with plastic.
In a pressurised-water reactor, hot water circulates at high pressure through steel piping, dissolving metal ions from the walls of the pipes. When the water is pumped through the reactor’s core, these ions are bombarded by neutrons and some of them become radioactive. The ions then either settle back into the walls of the pipes, making the pipes themselves radioactive, or continue to circulate, making the water radioactive. Either way, a waste-disposal problem is created.
Because the pipes are steel, most of the ions are iron. When the commonest isotope of iron (56Fe) absorbs a neutron, the result is not radioactive. The steel used in the pipes, however, is usually alloyed with cobalt to make it stronger. When common cobalt (59Co) absorbs a neutron the result is 60Co, which is radioactive and has a half-life of more than five years.
At present, nuclear engineers clean cobalt from the system by trapping it in what are known as ionexchange resins. These swap bits of themselves for ions in the water flowing over them. Unfortunately, the ion-exchange technique traps many more non-radioactive iron ions than radioactive cobalt ones.
To overcome that problem Drs Sellergren and Narasimhan have developed a polymer that binds to cobalt while ignoring iron. They made the material using a technique called molecular imprinting, which involves making the polymer in the presence of cobalt ions, and then extracting those ions by dissolving them in hydrochloric acid. The resulting cobalt-sized holes tend to trap any cobalt ions that blunder into them, with the result that a small amount of the polymer can mop up a lot of radioactive cobalt.
The team is now forming the new polymer into small beads that can pass through the cooling systems of nuclear power-stations. Concentrating radioactivity into such beads for disposal would be cheaper than trying to get rid of large volumes of low-level radioactive waste, according to Dr Sevilimedu. He thinks that the new polymer could also be used to decontaminate decommissioned nuclear power-stations where residual radioactive cobalt in pipes remains a problem.
Nuclear power is undergoing a renaissance. Some 40 new nuclear power-stations are being built around the world. The International Atomic Energy Agency estimates that a further 70 will be built over the next 15 years, most of them in Asia. That is in addition to the 439 reactors which are already operating. So there will be plenty of work for the plastic beads, if Drs Sellergren and Narasimhan can industrialise their process.
Source of Information : The Economist 2009-09-05