31. January 2007

Neutrons for research and nuclear waste disposal

Key experiment for nuclear technology successfully completed

Megapie is an international pioneering experiment at the Paul Scherrer Institute (PSI) in Villigen, Switzerland, the goal of which is to produce neutrons from a liquid metal target when hit from a proton beam. In a world first, a high power neutron source was produced from about one megawatt of proton input. Neutrons of high initial energy are used in many research fields and could also be used to incinerate nuclear waste. The first phase of the experiment was recently completed and to the great satisfaction of the international scientific community.

Due to their unique properties neutrons are indispensable particles for research. Atomic structures and their dynamics as well as biological substances can be investigated with these atomic building blocks. For instance they help us to understand how superconductors work or the molecular origins of illnesses. In order to obtain neutrons they must be released from the atomic nucleus, as they are at the Spallation Neutron Source (SINQ) at PSI. Here a high energy proton beam is directed at a metal target and the protons knock or “spallate” the neutrons off the metal's atoms. Up until now the target has been solid, but theoretical calculations predicted that a liquid metal target would produce a higher neutron flow.

Thus Megapie (Megawatt Pilot Experiment) was set up to demonstrate whether this was so, and the feasibility of running long term a 920 kg liquid lead-bismuth target with the proton beam from the ring cyclotron at PSI, which could provide a power output of one megawatt. Using this, the world's most powerful proton beam, is like heating a tea kettle with the power of 500 electric stoves.

Valuable experience for the conversion of nuclear waste

Neutrons with high energy can also be used for feeding a subcritical reactor system in which highly radioactive substances such as neptunium, plutonium, americium and curium, found in long lived waste from nuclear power plants can, in principle, be transmuted into short lived or even stable elements. Megapie has provided valuable information for developing this technique, even if according to experts, there is still a long way to go on this road.

Yet the prospect of being able to incinerate radioactive waste caused considerable international interest in Megapie. The project costs of 50 Million Euros were shared by many partners, including the EU. Since 2000, an interdisciplinary team has assiduously set up the experiment and conducted countless tests. The 170 strong group consists of scientists, engineers and technicians from nine research institutes from Europe (CEA, CNRS, ENEA, FZK, PSI, SCK·CEN), Japan (JAEA), Korea (KAERI) and the USA (DOE). The experiment required extremely high standards of safety and put high demands on the technology and materials used.

PSI itself was responsible for the installation and assembly of the target and for the operation of the complex system. Extensive safety controls were involved which were overseen by the Swiss Federal Office of Public Health (FOPH).

Megapie leads by a nose

As everywhere in the research world, there is stiff competition in the field of liquid metal targets. In the race to develop this technology there are also projects in the USA and Japan. During the operation of the Megapie experiment which ran between August and the end of 2006, results obtained show that there was an 80% higher neutron flux as compared with a solid metal target, greatly exceeding predictions. Many congratulations were received from scientists around the world. After a cooling off period, the examination of the now frozen target will continue for the next couple of years and will deliver invaluable information about the composition and behaviour of materials used in the experiment. This feedback will flow into the design and operation of new spallation neutron sources like SINQ. New accelerator driven systems will also benefit enormously from the experience gained from Megapie, which is a key experiment in the technological drive to such industrial facilities.