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Fission Cross Sections for Minor Actinides

#1749


Measurements of the Cross Sections of Fast and Resonance Neutrons Induced Fission of Minor Actinides for Their Transmutation with Accelerator-Driven Systems

Tech Area / Field

  • FIR-NOT/Nuclear and Other Technical Data/Fission Reactors
  • PHY-ANU/Atomic and Nuclear Physics/Physics

Status
3 Approved without Funding

Registration date
22.12.1999

Leading Institute
FEI (IPPE), Russia, Kaluga reg., Obninsk

Supporting institutes

  • VNIIEF, Russia, N. Novgorod reg., Sarov\nRussian Academy of Sciences / Institute of Nuclear Research, Russia, Moscow

Collaborators

  • Tokyo Institute of Technology / Research Laboratory for Nuclear Reactors, Japan, Tokyo\nEURATOM-Ciemat, Spain, Madrid\nJAERI / Tokai Research Establishment / Center for Proton Accelerator Facilities, Japan, Tokai Mura\nEuropean Commission / Joint Research Center / Institute for Reference Materials and Measurements, Belgium, Geel

Project summary

Main objective of the Project is measurement of the fission cross sections of 241Am, 242mAm, 243Am, 243Cm, 244Cm, 245Cm, 246Cm, 247Cm, 248Cm by fast neutron in the energy range from 5 MeV to 30 MeV on the Tandem Generator EGP-15 (SSC of RF IPPE, Obninsk) and by resonance neutrons in the energy range from 1 eV to 30 keV on the Lead Slowing Down Spectrometer (LSDS) based on the 350 MeV proton accelerator beam of Moscow Meson Facy (INR RAS, Troitsk).

Minor acitnides (MA) of which Am and Cm are most important are key factor determining the complex problems related to the storage and processing of long-lived high level waste of nuclear power industry. The development of the industrial methods of MA transmutation based on reactor technology and especially on the technology of accelerator-driven systems generates a task of essential improvements of the nuclear data foe minor actinides in a broad energy range to insure accuracy comparable with the accuracy achieved for main components of nuclear fuel – U and Pu isotopes. In the assessment of various approaches to minor actinides transmutation problem the use for this aim of accelerator-driven subcritical blankets multiplying neutrons generated in a massive target by 1 - 1.5 GeV protons is considered most promising. Neutron spectrum in such facilities reaches hundreds of MeV with the region of dozens of MeV making principal contribution. That’s why “the tails” of high energy neutrons should be accounted for in the calculations of the intensity of nuclear reactions involving the nuclides to be transmuted. Though average share of those super - fast neutrons is rather low, their role in the region of “the halo” around the target is quite important. Neutronics, nuclear physics and material science in this halo region are quite different from their “reactor sisters” and should be specially investigated.

Fission is by far the most important of relevant nuclear reactions achieving the final goal of the transmutation – transformation of the long-lived highly radiotoxic and long-lived alpha-emitting MA into less hazardous fission products which also decay generally faster. This factor is decisive in determining the importance and timeliness of broadening neutron energy interval where MA fission cross sections are known in detail.

In the framework of ISTC Project N304 (1995-97) the measurements were made in SSC of RF IPPE of MA fission cross sections in the energy range 150 keV - 6.5 MeV. The results improved the situation considerably and to obtain the data for the fast reactor energy range accurate enough for practical purposes. At the same time the status of the data in broader energy range remains quite unsatisfactory. There is practically no data above 15 MeV, especially for Cm isotopes. In the resonance neutron region (1 eV-10 keV) the discrepancies between the results obtained by the time of flight method (with nuclear explosion as neutron source) and in the measurements on a lead slowing-down spectrometer are in hundreds per cent.

The evaluations based on theoretical models and calculations are unreliable if not supported by experiments, especially in higher energy region where pre-equilibrium and direct processes dominate. Here the influence of the details of the nuclear shell structure and of the process going through the compound nucleus gets weaker giving place to the statistical and thermodynamic laws determining nuclear level density. That’s why the measurements in the 5-30 MeV energy range are important not only for getting nuclear data for this range itself but for significant improvements of theoretical models, of their parameter values and for creation methodical base for the evaluations of neutron cross sections at higher neutron energies.

Highly enriched nuclides 243Cm, 244Cm, 245Cm, 246Cm, 247Cm, 248Cm, 241Am, 242mAm, 243Am as well as 239Pu and 235U (used as standards in relative measurements of the cross sections) were produced by double electromagnetic separation of Am, Cm and Pu isotopes on the electromagnetic mass separator S-2 (RFNC VNIIEF, Sarov). Fissioning samples will be prepared in IPPE and VNIIEF in the form of thin oxide films on metallic backings placed in high efficiency fission detectors – fast (~1 ns) ionization chambers.

The measurements in 5-30 MeV range will be carried out on the electrostatic accelerator EG-1 and on the tandem generator EGP-15 (SSC IPPE). Pulsed proton and deutron beams (t=1 ns, f=2 MHz, <I>=1––8 mcA) and time of flight technique (pulse synchronization) for suppression of spontaneous fission will be used. The use of T(p,n)-, D(d,n)- and T(d,n)-reactions as neutron sources is planned. Energy resolution will be 3 – 5 %, achievable statistical accuracy is 1-3 %.

In the resonance region (1 eV-30 keV the measurements will be made on the Lead Slowing Down Spectrometers (“PITON” and Large LSDS) with the neutrons generated by the interaction of the pulsed beam of protons (Ep=200–450 MeV, t=0.15 mcs, f=50 Hz, <I>=0.1–1 A) of the Moscow Meson Factory (Troitsk). Lead Slowing Down Spectrometer PITON is a massive (15 t) cube of super-pure lead (99.99%). Controlled lead target serves as a neutron source placed in the centre of the cube. The tareget is put in place before the measurements by remote control through a vacuum port. Air cooling system allows to take up to 400 W of heat. Pulsed proton beam generates the spallation neutrons with high efficiency (6-10 neutrons per proton). Neutrons slow down scattering on the lead nuclei so their mean energy E at the moment t (mcs after arrival of the pulse) is given by the expression E = 180/(t+0.5)2. The spectrometer is equipped by channels where the investigated samples are placed. Neutron energy range is from 1 eV to 50 KeV. Measerd energy resolution of 37-40% is typical for the spectrometer of this kind. Achievable statistical accuracy is 0.5-1%. Intensity of the neutron source with the parameters indicated is 7ґ1013 n/s. Lead slowing down spectrometers are known to provide neutron flaux on the sample 102-104 times higher than time-of-flight facilities at the same source intensity. Thus the LSDS with the spallation neutron source provides super high neutron fluxes and allows to measure the cross sections of neutron induced reactions on the small or radioactive targets.

Large LSDS differs from the PITON facility mainly due to a larger mass of lead (100 t) which allows to increase the neutron flux densities below 100 eV. The large LSDS, besides the measurment channels, has technological shafts to accomodate massive samples transported by the remote control. This facility will be used, in particular, to optimize the samples geometry and study the influence of the sample on the neutron spectra in the modderator which is essential for the development of ADS technology. The facility is designed to use in futureneutron source of 30 KW thermal power when linked to the proton storage ring.

To use the expensive beam time efficiently the measurements will be carried out for 3-4 investigated nuclides simultaneously in the multi-stop mode on the time-analyzers ORTEC (Model 9308).

These results will be important for fission physics first of all since they allow to enlarge substantially the domain of “chance analyses” and thus to investigate fission of more and more neutron deficient compound nuclides formed after preliminary emission of a few neutrons. The longer the chain of investigated nuclides the more accurate is the description of the experimental curve of total fission cross section. The majority of calculations describing the energy dependence of fission cross section and reproducing its “chance structure” were made for the incident neutron energies of 15 - 20 MeV when maximum number of pre-fission neutrons is two which makes the analyses simpler than at higher energies. This description allows to evaluate such important parameters of fission theory as barrier heights and level density parameters for highly excited states of fissioning nuclei. Some of those parameters might be obtained in a most reliable way from the averaged fission cross sections in the resonance region where available information, as was mentioned before, is highly controversial and needs new experimental data to be improved.

The description of fission cross sections covering their chance structure for the isotopic chains of Am and Cm in a consistent theoretical approach supported by new experimental information in the energy region 5 – 30 MeV combined with the earlier results will enable to test the parameters of theoretical models and improve the calculations of fission cross sections up to 100 MeV and beyond.

Highly qualified personnel with a lot of experience in actinide handling and nuclear data measurement will work for the Project and use unique equipment of SSC IPPE, RFNC VNIIEF and INR RAS. A large group of ex-weapon scientists and engineers will be employed in an important domain of peaceful R&D – obtaining of nuclear data for the development of nuclear facilities transmuting high level nuclear waste.

Preparation of experimental equipment, accumulation of highly enriched materials and testing of the equipment will take the first year of the Project. During the second and third year the work will be done in following order (including parallel activities): cleaning of the samples from decay products by groups of the nuclides, reprocessing of samples and preparation of fissioning targets; the measurements on the electrostatic accelerators and on LSD-spectrometers; processing and analyses of the experimental data and theoretical calculations.

As a result of the Project new experimental data will be obtained on the fission cross sections for 243, 244, 245, 246, 247, 248Cm an 241, 242m, 243Am at 5-30 MeV and new data on averaged fission cross sections of the same nuclides in the resonance region from 1 eV to 30 keV. The results will enhance and make more accurate the key data needed for the development and designing of ADS based transmutation facilities, for selection of optimal technical decisions ensuring their efficient and safe operation. In some cases these improvements will be of principal importance.

Besides, the analyses of the data will supply new information on the parameters of fission probabilities of heavy nuclei in a broad energy range (fission barriers, nuclear level densities, the ratios of fission and neutron widths, Гnf), needed for verification of the theoretical models evaluating MA fission cross sections in the energy range from dozens to hundreds MeV. New modern evaluation of fission cross sections for Am and Cm isotopes will be made on the base of new experimental data and theoretical calculations.

The participation of foreign collaborators will unable systematic discussions, at least twice a year, of the Projects progress on the international level and make corrections necessary to make the results to correspond completely to the declared objectives of the Project.


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