1
ADS experiments in the JINR Dubna
M. Majerle1,2
for the “Energy Plus Transmutation” collaboration
1) Nuclear Physics Institute of ASCR PRI, 250 68 Řež near Prague, The Czech Republic
2) FNSPE of CTU, 115 19 Prague, The Czech Republic
Rich tradition of experiments connected with Accelerator Driven Systems (ADS) exists in the Joint Institute for Nuclear Research (JINR) in Dubna, Russia. The focus is on experiments where relativistic protons and deuterons (<2 GeV) are directed to thick, lead targets (in some experiments surrounded by uranium blanket or graphite moderator). The produced spectra of secondary particles are measured with several types of detectors: activation detectors, solid state nuclear track detectors, nuclear emulsions, 3He counters and others. The data from these experiments are useful in the design of further experimental and real ADS, as well as for benchmark tests of Monte Carlo codes. To improve the accuracy of the codes, extensive sets of cross-section data are needed. Most of these cross-sections are not experimentally measured and evaluated data are used in simulations. Some necessary measurements were already started by our group, but more detailed and independent data are needed.
Introduction
Accelerator-driven systems (ADS) are considered to be a promising option for future nuclear energy production and nuclear waste incineration. In ADS spallation neutrons (produced via interactions of high energy ions with heavy nuclide targets – lead, tungsten …) are added to subcricital reactor core to sustain the fission chain reaction. ADS are so far in the early experimental and design stage.
One of the requirements in the design of future ADS is the ability to predict the behaviour of such systems with the Monte Carlo (MC) codes. Experiments on smaller scale ADS facilities are therefore performed and the results are being reproduced with MC codes. Most nuclear processes and necessary cross-sections in such experiments are qualitatively known and simulations are able to reproduce experiments inside the accuracy of tens of percents. One way to increase the accuracy of simulations is to improve the precision of the cross-section data.
JINR ADS research
Several groups in the world scale perform ADS relating experiments (MUSE, TRADE, MEGAPIE [1-3]). Among them is the Joint Institute for Nuclear Research (JINR), where new series of such experiments started in the 90’s with the cross-section measurements for the ADS construction material in the high energy proton beams, and were continued with the studies of neutron production on thick targets.
Gamma-2 to Gamma-MD
Gamma-2 was the experiment focused to the studies of production of neutrons in the spallation process and their moderation/transport in the neutron moderator. The setup consisted of a thick, lead target (diameter 8 cm, length 20 cm) surrounded with 6 cm thick paraffin moderator which slowed spallation neutrons to resonance energies. Neutrons were detected by radiochemical detectors placed on top of the polyethylene along the setup length. One experiment with the 660 MeV protons directed to the target was performed. Gamma-2 was a simple setup providing results which are very useful for comparison with computer codes predictions [4].
Extended target of lead (diameter 8 cm, length 60 cm) is nowadays used in another setup with similar name, Gamma-MD, in which graphite is used as the moderator (1.1x1.1x0.6m3). The experiments are focused on the studies of fast neutron moderation in the graphite. The setup was by now irradiated with 2.33 GeV deuterons, more irradiations are planned. Activation detectors, solid state nuclear track detectors and transmutation samples are used to measure the spectral fluences of the neutron field and its transmutation properties.
1
Figure 1 Photo of the Gamma-MD setup. Lead target is in the centre of the graphite cube. Detectors and transmutation samples can be placed inside the graphite, place for them is reserved in removable cylinders.
Figure 2 Schematic drawing of the Phasotron setup. Activation detectors and two transmutation samples are placed on top of the target along its length.
3
Phasotron experiment
The focus of the Phasotron experiment was on the spatial distribution of high energy spallation neutrons (>10 MeV) along the target. Transmutation properties of radioactive iodine 129I in fast neutron spectrum were also measured. A bare, lead target (diameter 9.6 cm, length 45.2 cm) was irradiated with 660 MeV protons from the Phasotron accelerator. Activation detectors were placed on top of the target along its whole length together with the iodine transmutation samples. This setup was successfully simulated with several codes [5].
3
Figure 3 Comparison of experiment and FLUKA simulation. The ratios between the numbers of produced isotopes in Al and Au activation detectors measured experimentally and simulated are shown.
3
Energy Plus Transmutation
The production of neutrons and their transport in the uranium “core” became possible with the "Energy plus Transmutation" (EPT) setup. It consists of a thick, lead target (diameter 8.4 cm, length 48 cm) surrounded with the uranium blanket (206.4 kg) and placed in a polyethylene box. In series of experiments, relativistic protons and deuterons of energies from 0.7 to 2.52 GeV were directed to the target. Produced neutron flux and its transmutation capabilities were studied at different places of the setup with activation, solid state nuclear track, 3He and other detectors. The EPT setup consists of several parts, which all influence the produced neutron field. MC codes are describing successfully the experiments with this complicated setup, with the exception of the distribution of 10-100 MeV neutrons at the beam energies higher than 1 GeV [6]. It seems that distribution of neutrons with either higher energies (>100 MeV) or lower energies (<1 MeV) are predicted well at the same experiments [7].
3
Figure 4 The EPT setup. Lead target is surrounded with uranium blanket. Polyethylene moderator is not shown in the figure. The detectors are placed in the gaps between the target/blanket sections, transmutation samples are placed on top of the setup.
Figure 5 Comparison of the experiment and MCNPX simulation. The 197Au detectors were placed at different radial distances from the central axis in the first gap. The experimental/calculation ratios clearly increase with the radial distance.
3
Measurement methods
Activation detectors and solid state nuclear track detectors
Both types of the detectors are mainly used for their small size (less than 1g of material is sufficient). They can be placed almost anywhere inside the setup without significant influence to the produced neutron field. During the irradiation in the neutron field, the neutrons interact with the detector material, which is analyzed offline for the traces that the neutrons left in the material.
In the case of the activation detectors, the neutrons interact with the detector material via reactions of type (n,g), (n,a), and (n,xn), exciting a small part of the nuclei in the detector. Providing that the decay times of the excited isotopes are in orders of minutes-days and that during the de-excitation they emit photons (>100 keV), their number can be determined with gamma-spectroscopy methods. Activation detectors cover a wide range of neutron spectrum: eV-MeV neutrons via (n,g) reaction, 1 MeV - 100 MeV neutrons via (n,a) and (n,xn) reactions. Materials that were used in mentioned experiments were Au, Al, Bi, Y, In, Ta, Co, Cu, and others.
Transmutation samples consist of small amount of the material from the nuclear waste (Am, Np, Pu, I, ..), which is sealed in a safety container. These samples are as well studied by the gamma-spectrometry methods after the irradiation, and transmutation characteristics of nuclear waste materials in the spallation neutron field are determined.
Solid state nuclear track (SSNT) detectors consist of two parts: of the heavy metal that interacts with neutrons via nuclear fission (irradiator) and of the material in which fission fragments leave tracks. The second material (plastic or mica foil) is chemically processed, and the tracks are counted with optical microscope. Materials that were used in the mentioned experiments as iiradiators are U, Pb, W, Au, and others. While different isotopes of U are fissioned by neutrons of a wide range of energies (thermal-fast), Pb, W, Au ... irradiators are sensible mostly to neutrons with energies >100 MeV.
Monte Carlo simulations
Neutron spectral fluence
All mentioned setups were implemented in Monte Carlo codes MCNPX and FLUKA (KASKAD MC code was also used some calculations). The codes simulate the spallation reactions of primary ions with the target material, all subsequent neutron reactions, and tally the spectral fluences of secondary neutrons, protons, pions, photons … at the places where the detectors were placed at the experiment. Tallied spectral fluences are folded with appropriate cross-sections to obtain the numbers of activated nuclei/fissions in activations/SSNT detectors.
Cross-sections
Cross-sections implemented in the MC codes are taken from evaluated cross-sections databases (ENDF, JEFF …), which usually extend up to 20 MeV (150 MeV in LA150 libraries), above which the cross-sections are calculated by the nuclear model currently used by the code.
The folding of the spectral fluences with the cross-sections is performed manually with the cross-section calculated with TALYS (<150 MeV) and MCNPX (>150 MeV) codes. These cross-sections are additionally checked against the experimental values found in EXFOR and commonly the values for the same cross-sections differ for tens of percents. With few exceptions, there is a lack of experimentally measured cross-sections for activation and SSNT detectors, especially at energies above 20 MeV.
Comparison with the experiment
With some exceptions (see Figure 5) MC codes are able to reproduce the experimental results qualitatively well (Figure 3). There are however differences on quantitative scale in the range of tens of percents [7, 5]. These differences can be due to calculated neutron distribution or wrong cross-sections. For better accuracy of the ADS simulations, the MC codes will have to be improved, and the wide range of missing cross-sections will have to be experimentally measured.
Conclusion
At the ADS connected experiments in the JINR Dubna, the neutron distributions are measured with activation and SSNT detectors. For the reproduction of experimental values with the MC codes, good cross-sections data is needed, especially at energies higher than 20 MeV. The accuracy of today cross-sections in this energy region is not satisfactory for accurate simulations of the mentioned experiments or future ADS systems.
Acknowledgements
Authors are grateful to the METACentrum (the Czech Republic) for offering computers for the calculations. This work was carried out partly under support of the Grant Agency of the Czech Republic (grant No. 202/03/H043) and IRP AV0Z10480505 (the Czech Republic).
References
[1] RUGAMA, et al., Some Experimental Results from the Last Phases of the MUSE Program, Proceedings of Physor 2004, Chicago, Ill., 2004.
[2] M. SALVATORES, et al., TRADE (TRIGA Accelerator Driven Experiment): A Full Experimental Validation of the ADS Concept in a European Perspective, Proc. of the Sixth International Meeting on Nucl. Appl. of Accelerator Tech., AccApp’03, American Nuclear Society, 2004, p. 8-16.
[3] Juanita Schlaepfer-Miller, MEGAPIE leads the way to waste transmutation, CERN Courier, (April 2007), 29.
[4] Westmeier W. et al., Transmutation experiments on 129I, 139La and 237Np using the Nuclotron accelerator, Radiochimica Acta 93 (2/2005) 65-75.
[5] M. Majerle, et al. JINR Preprint E15-2008-94.
[6] F. Křížek, et al, Czech. J. of Phys. 56 (2006) 243.
[7] S.R. Hashemi-Nezhad, et al., NIM A, Volume 591, Issue 3, 1 July 2008, Pages 517-529