nELBE: High brilliance pulses of fast neutrons enable novel research on transmutation of radioactive waste
A.R. Junghans, A. Wagner, F.P. Weiss, E. Grosse
One - for many even the strongest - of the arguments against a long-lasting commitment to nuclear power as an energy source is the need to permanently dispose of the long-lived radioactive waste produced in nuclear reactors. Significant efforts are thus being made worldwide in order to minimize, manage, and dispose of highly radioactive nuclear waste. Partitioning of nuclear waste and transmutation of long-lived isotopes into nuclides with a shorter lifetime are being investigated in the EURATOM FP6 program. Several transmutation schemes have been proposed and for an optimum solution detailed numerical simulations are under way. Regarding the development of new concepts to produce less waste via very high burn-up, different designs involving critical reactors or sub-critical accelerator-driven systems (ADS) are being studied in view of their transmutation capabilities. The Generation IV (Gen-IV) International Forum (GIF) has selected six nuclear energy systems which call for research and development in order to confirm their viability. Also, their expected performance is to be demonstrated which, amongst others, aims at producing less waste.
Different schemes have been proposed which may considerably reduce the radioactivity of the spent fuel after burn-up. Studies choosing the best options make extensive use of simulation methods in order to predict the system behavior in a great variety of possible configurations and running conditions. A fundamental prerequisite for these Monte Carlo computing techniques is the availability of reliable cross section data. These are needed for processes and operating parameters which significantly differ from those of currently operating systems.
Neutron time-of-flight at ELBE
In particular, accurate knowledge of neutron induced nuclear reactions at appropriate energies is crucially important for predicting the capabilities of new systems. This means that for detailed waste transmutation research and design work on Gen-IV systems energy dispersive studies are needed. Among the ways to determine neutron energy the time-of-flight method can be applied for a wide range of energies: starting with a broad spectrum, the neutrons are tagged according to their energy by measuring their velocity. The FZD radiation source ELBE with its ultra-short electron bunches is suited especially well for this method and time-of-flight measurements with good resolution can be performed here even for fast neutrons.
Regarding waste reduction, the possible use of fast (i.e. un-moderated) neutrons as they directly come from the fission process is highly important. The strong processes induced by these fast neutrons are known in principle, but reliable predictions of the relevant physical processes and phenomena depend on the availability of high-quality nuclear data. As the fission neutron spectrum bears great resemblance to the neutron distribution originating from the nuclear photo effect, a high intensity electron beam like the one at ELBE allows suitable measurements in the fast neutron domain. The neutrons are generated by hitting high-Z material with the electrons and thus producing bremsstrahlung which in turn causes the very same material to emit neutrons.
Obviously the neutron flux, which determines the statistical accuracy of a cross section measurement carried out in a given time, depends on the primary beam intensity and on the amount of converter target material put in the beam. At ELBE flux as a limiting factor is restricted by the maximum beam power accepted on the neutron producing target; however, it is not limited by the available beam current from the accelerator. Based on a molten lead circuit a technologically innovative solution for neutron converters suited for very high beam power deposition (≈ 5 kW/g) was designed in a collaborative effort of the FZD Institutes of Radiation Physics and Safety Research.
The most important feature of this source, which is advantageous for transmutation-related measurements, is its extremely high flux at a still reasonable time resolution: The extremely high neutron density of more than 107 n/cm³ produced in the radiator by each micro-pulse (at ≈1 MHz) results in nearly 107 n/(s·cm²) at a flight path of roughly 5 m. By making use of the new superconducting RF-photo-gun at ELBE, the repetition rate can be adjusted to the neutron energy range studied at the given flight path. Due to the uniquely high bunch charge of up to 2nC of this electron gun the full neutron flux is available for neutron energies above 20 keV.
ELBE is the first superconducting electron linac combined with a neutron time-of-flight facility. A big advantage is that radiofrequency is permanently present which allows accelerating nearly any pulse repetition rate delivered by the electron gun. At ≈ 1MeV a resolution ΔE/E of ≈ 2 % may be reached with detectors of ≈ 1ns resolution. The time resolution of the e-beam is much better. Due to its small dimension of ≈ 1cm, the radiator generates n-bunches shorter than 1ns. The set-up is devoted to measurements of transmutation-relevant data for actinides as well as for fission fragments. Measurements with targets of only 10 mg are planned.
The Rossendorf neutron time-of-flight set-up entitled nELBE is part of the EU-funded Integrated Infrastructure Initiative (I3). This is also called "European Facilities for Nuclear Data Measurements" (EFNUDAT) and has been created by a consortium of European experimental facilities for nuclear data measurements. Joint Research Activities (JRA) within I3 are concerned with the completeness, comparability and quality assurance of the nuclear data produced by the ten participating institutions in seven European countries. The FZD participates in three JRAs and heads one which is dedicated to neutron generators and targets. A major task of this JRA is to test and optimize the molten metal radiator in beam and to investigate how this design can eventually improve neutron production at the other facilities of I3.
Transnational access to nELBE (as well as to the other neutron facilities) is supported by I3 and some of the partners will perform experiments there. The only two other neutron time-of-flight facilities within EFNUDAT have concentrated in the past on slower neutrons (including moderated, i.e. thermal ones). The installation at the European Commission Institute of Reference Materials and Measurements (IRMM) in Geel has considerably less primary beam power and thus less neutron flux. At the proposed short (20 m) flight path of CERN/n_TOF the flux will be somewhat larger as compared to nELBE in the energy range accessible here, but the energy resolution is predicted to be superior at FZD.
The FZD activities in the field of molten metal neutron converters are paralleled by similar attempts in the USA and Japan, which are carried out in view of the upcoming Spallation Neutron Source (SNS) and the Japanese Proton Accelerator Research Complex J-PARC, respectively. Both are built to deliver significantly higher particle fluxes than available today. The research performed at FZD will thus not only represent a significant step forward to upgrade the nuclear data measurements within the EFNUDAT initiative, but it may also lead to results which can possibly be used in the future European spallation source ESS, which many European scientists in the field of materials research desire for.
 E. Altstadt, C. Beckert, H. Freiesleben, V. Galindo, M. Greschner, E. Grosse, A.R. Junghans, B. Naumann, S. Schneider, K. Seidel and F.-P. Weiß, Energiedispersive Untersuchung der Wechselwirkung schneller Neutronen mit Materie; Scientific Technical Report, FZD-426, April 2005; Abschlussbericht DFG-Projekt Gr 1674/2.
 M. Salvatores, State of the art and perspectives in radioactive waste transmutation; CEA Cadarache 2005.
 “A photo-neutron source for time-of-flight measurements at the radiation source ELBE”, E. Altstadt, C. Beckert, E. Grosse, H. Freiesleben, J. Klug, A.R. Junghans, R. Schlenk, F.-P. Weiss et al., Annals of Nuclear Energy 34, 36 – 50 (2007).
 “Development of a neutron time-of-flight source at the ELBE accelerator”, J. Klug, E. Altstadt, C. Beckert, R. Beyer, H. Freiesleben, V. Galindo, E. Grosse, A.R. Junghans, D. Legrady, B. Naumann, K. Noack, G. Rusev, K.D. Schilling, R. Schlenk, S. Schneider, A. Wagner, F.-P. Weiß, Nuclear Instruments and Methods in Physics Research A (2007), accepted for publication.
 “Proton-recoil detectors for time-of-flight measurements of neutrons with kinetic energies from some tens of keV to a few MeV”, R. Beyer, E. Grosse, K. Heidel, J. Hutsch, A.R. Junghans, J. Klug, D. Legrady, R. Nolte, S. Röttger, M. Sobiella, A. Wagner, Nuclear Instruments and Methods in Physics Research A 575, 449 – 455 (2007).
Fig. 1: The chart demonstrates how transmuting the waste from a traditional fission reactor may reduce radioactivity by more than 100. For three possible transmutation schemes the dose is plotted vs. the time after fuel removal from the reactor :
Storage of spent fuel without treatment
+ Fast neutron reactors with recycling of the transuranium elements
o Transmutation of actinides in a separate compact fuel cycle
Light water reactors with full recycling
Fig. 2: The picture shows the set-up for the measurements at nELBE. The neutrons are generated by the electron beam and are transferred through a massive concrete radiation shield (needed to warrant sufficient shielding between neutron radiator and experiment). The good emittance of ELBE in combination with an especially designed collimator results in a narrow neutron beam allowing the use of small targets and compact detector set-ups.
The measurements allow for photon detection following radiative neutron capture (4π-array of BaF2 scintillators) and for the registration of neutrons scattered from the target (in specially equipped plastic scintillators). By using the photon and neutron detectors in coincidence inelastic neutron scattering can be identified. Both detector types reach time resolutions clearly below 1ns and are thus well suited for a proper time-of-flight tagging.