A high-power proton beam and a special target have produced a high-power neutron source at PSI, with potential for the transmutation of radioactive waste.
Megawatt-class beam targets are nowadays attracting attention from a wide variety of users, for investigations that span the spectrum from the transmutation of long-lived radioactive waste, through material research, to radioactive beams and neutrino factories. At the Paul Scherrer Institute (PSI), the Megawatt Pilot Experiment (MEGAPIE) has recently demonstrated the feasibility of safely running a liquid heavy-metal target in the world’s most powerful DC proton beam. The experiment is particularly important for the development of an accelerator driven system (ADS) for the transmutation of long-lived radioactive waste. It serves to demonstrate the feasibility, potential for licensing, and long-term operation under realistic conditions, of a high-power spallation target, which could later provide the high-energy neutrons required to induce fission in waste atoms.
Spallation neutrons are produced efficiently by firing a proton beam at a heavy-metal target such as lead where the reactions of the protons with nuclei literally knock out or “spallate” neutrons, while further neutrons are evaporated. On average each proton produces about 11 neutrons. Up until now spallation targets have always been solid, but MEGAPIE has demonstrated the advantages of a liquid target, namely an increase in neutron flux and convectional cooling of the target window. The second advantage gives the liquid target potential for higher power, in contrast to a solid target, which cannot be cooled sufficiently. In MEGAPIE, the use of a liquid target with the 1 MW beam at the Swiss Spallation Neutron Source (SINQ) increased the neutron flux by about 80% compared with the previous solid-lead target.
A powerful alliance
MEGAPIE is a collaboration of nine research institutes in Europe, Japan, Korea and the US, which have agreed to design and build a liquid-metal spallation target suitable for 1 MW beam power, and to license and operate it at PSI, where SINQ is the world’s only spallation neutron facility with a sufficiently powerful proton driver. The present 1.1 MW proton beam from PSI’s 590 MeV ring cyclotron delivers, after passing two secondary-beam production targets, a continuous proton-beam current of up to 1.4 mA (about 800 kW) at an energy of 575 MeV to the SINQ spallation source. For MEGAPIE, the collaboration decided that the liquid-metal target must be irradiated for a minimum of three months, both to achieve a sufficiently high irradiation dose on the component materials and to demonstrate that the system could operate reliably. During its operation, the target served as the source for the neutron-scattering programme at PSI, which involves some 260 experiments.
The MEGAPIE target consists of 920 kg of liquid lead-bismuth eutectic (LBE), contained in a steel casing. On impact, the 800 kW proton beam deposits about 580 kW of heat in the target material. The heat is removed by circulating the lead-bismuth in forced convection through a heat exchanger. The proton beam penetrates the lead-bismuth to a depth of 27 cm and generates an integrated flux of 1017 neutrons a second.
During the four months of operation, the target operated very satisfactorily and according to predictions. It triggered only a small number of unscheduled beam shutdowns and experienced more than 8000 beam interrupts of different durations without damage. Its availability reached 95%, with an accumulated proton charge amounting to 2.8 Ah.
Earlier Monte Carlo simulations had indicated that the liquid-metal target should provide a 40% increase in neutron flux (at identical current) compared with a solid target. However, initial measurements at selected instruments confirmed an increase in neutron flux, which the collaboration met at first glance with some scepticism: instruments at the cold guide gave a flux increase as high as 70–80%. However, gold-foil activation measurements have confirmed flux increases of 80–90% at both a thermal and a cold beam port. New calculations with more detailed target and moderator geometry now reproduce these results.
The higher flux means that it will be possible to carry out more experiments within the same time frame, a definite benefit for the over-booked beam lines. With a flux gain of this magnitude, operation with a permanent liquid-metal target at SINQ has become a priority and PSI has launched a new project to pursue this goal.
From nuclear waste to beta-beams
The success of MEGAPIE is particularly important for research into ADS transmutation of radioactive waste. The long-lived minor actinides (neptunium, americium and curium) are the main contributors to the long-term radio-toxicity of nuclear wastes. However, it should be possible to transmute them into short-lived or stable elements using a sub-critical ADS equipped with an internal neutron source and driven by a high-energy proton beam. CERN has made major contributions to this concept with the experiments FEAT and TARC (CERN Courier April 1997 p8). In 1998, a technical working group headed by Carlo Rubbia established a roadmap to achieve ADS transmutation. The group considered the development of a high-power spallation target and the demonstration of its reliable operation to be vital steps en route.
Researchers are now also considering ADS scenarios based on megawatt spallation neutron targets for the next-generation European Radioactive Ion Beam Facility, EURISOL. Here a 1 GeV superconducting linear proton driver with separate post-acceleration capabilities will allow low-, intermediate- and high-energy, very intense radioactive ion-beams to probe fundamental questions in nuclear structure, nuclear astrophysics and fundamental symmetries and interactions. Another use of an ADS based on a spallation source would be the production of a neutrino beam in a “beta beam”. In this case, radioactive ions circulating in a storage ring beta-decay to produce a pure beam of electron-neutrinos/antineutrinos; the ions themselves are produced in a two-step process from the interaction of spallation neutrons in a suitable secondary target.
The pie is opened
The accelerator shutdown at the end of 2006 marked the end of the irradiation phase for MEGAPIE. The final phase of the experiment – the post-irradiation examination of the target components – will start after the target, which is now solidified, has been stored for two years. The analysis will provide information about corrosion effects on structural materials and allow the validation of various models. The state of the beam window will allow the combined effect of LBE and proton irradiation to be assessed and provide information on the potential lifetime of such a beam window. The analysis of the LBE will also furnish information on the spallation products and their chemistry, so validating neutronic and radiochemical models. This information will feed back into the design and operation of new spallation sources. New versions of ADS will also benefit enormously from the experience gained from MEGAPIE, which has also proved to be a key experiment for future industrial projects involving the transmutation of nuclear waste.
