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Moderators for Pulsed Neutron Source

#2787


Research and Development of an Optimal Neutron Moderators Configuration for IBR-2M Pulsed Neutron Source

Tech Area / Field

  • FIR-REA/Reactor Concept/Fission Reactors
  • FIR-MAT/Materials/Fission Reactors
  • PHY-ANU/Atomic and Nuclear Physics/Physics

Status
3 Approved without Funding

Registration date
10.06.2003

Leading Institute
Joint Institute of Nuclear Research, Russia, Moscow reg., Dubna

Supporting institutes

  • FEI (IPPE), Russia, Kaluga reg., Obninsk\nFederal State Unitary Enterprise Research and Development Institute of Power Engineering named after N.A.Dollezhal, Russia, Moscow

Collaborators

  • JAERI / Tokai Research Establishment, Japan, Tokai Mura\nOsaka University, Japan, Osaka\nUniversity of Tennessee / College of Arts and Sciences / Department of Physics and Astronomy, USA, TN, Knoxville\nIndiana University / Cyclotron Facility, USA, IN, Bloomington\nHokkaido University / Faculty of Engineering, Japan, Sapporo\nArgonne National Laboratory (ANL) / Intense Pulsed Neutron Source Division, USA, IL, Argonne\nScience and Technology Facilities Council, UK, Didcot\nForschungszentrum Jülich GmbH / Institut für Festkörperforschung, Germany, Jülich

Project summary

The main content of the project is the development of moderators for currently operating and future pulsed neutron sources (PNS). Moderator is a device, which contains moderating hydrogenous substance, and has a technologically sophisticated design to provide neutron beams of different spectra in different directions for effective operation of respective neutron spectrometers. Most of the new projects require "cold" moderators with temperature of moderating substance in the range 20-100 K, because they are effective for production of neutrons with wavelength longer than 4 Angstrom. To ensure the stable and sufficiently long operation of a moderator at such temperatures and at high radiation background is not a simple task.

The cold moderators are required for new coming sources like ESS (power 5 MWt), NSNS (1 - 4 MWt), and by the currently operating source IBR-2 that at 2007-2010 is planned for modernization with majority of spectrometers installed around it. In the development of moderators for all these sources at present stage it is possible to consider the complex “source-moderator-spectrometer” as a whole unit to meet the needs of users. To take these needs into account completely is possible only in tight collaboration of moderator designers and scientists - consumers of the neutrons.

In paper [3] the list of inquiries from consumers on a type of moderator for the future sources ESS and NSNS is given. In between of 40 neutron spectrometers only 13 require room temperature moderator. The others need neutrons of lower energies.

In the Joint Institute for Nuclear Research a decision was made to modernize or to create anew 20-24 spectrometers at the reactor IBR-2. They include 8 diffractometers (against 6 operating now), 3-4 small-angle scattering spectrometers (1 now), 3-5 reflectometers, including 3 with polarized neutrons (2 now), 3 inelastic scattering instruments (3), and facilities for transmission spectrometry, material irradiation, neutron activation analysis, and for nuclear physics.

The basic goal of the present project is the development of a concept of composition and arrangement of neutron moderators around the modernized research reactor IBR-2M, in such a way as to provide the best opportunities for the effective use of both the modernized, and the new spectrometers. At the same time it is necessary to take into account that the existing spectrometers with quite contradictory requirements to spectra should remain at their current positions. It is this condition that makes mandatory the development of a new moderator for IBR-2M. In other case it could be sufficient to concentrate the spectrometers with similar requirements to neutron spectra in a single direction. For solution of this problem we need to elaborate a moderator of complex composition, which provides different spectra at different positions of outgoing neutrons and in different directions of their flight. Development of such a moderator is undertaken for the first time. At first sight this purpose may look narrow and appropriate only for the IBR-2 reactor, however, solution of the task may appear to be a breakthrough in the neutron engineering as a whole and in engineering of high-intense neutron sources, in particular. For instance, it can be useful for target stations of the Spallation Neutron Sources (SNS).

Another particular purpose of the project is the development and manufacturing of a cold neutron moderator with frozen hydrocarbons or water ice operating in a wide interval of temperatures - from 20 K up to 100 K, which has long operation time in radiation fields up to 0,3-0,5 kGy/s and provides high efficiency for attached spectrometers. This is an actual task for the new SNS, and it can be solved during solution of the main problem, because IBR-2 reactor also needs such a cold moderator.

Besides this direct application of the project, results of study of radiation effects in hydrogenous materials at low temperatures which is proposed to be done in the course of elaboration of the project, especially results on accumulation and release of the hidden chemical energy of radicals, can be effectively used in astrophysics and astrobiology because of the fact that the blend of water ice, methane and other simplest chemical compounds is an essential constituent of small and large bodies of the Universe. Radiogenic chemical processes going in this blend under action of protons may play an important role in evolution of cold bodies and in evolution of organic matter.

For solution of the main problem related to IBR-2 reactor, where spectrometers arranged in all three directions have contradictory demands to neutron spectra, the following measures improving their efficiency are suggested:

- By implementation of a combined moderator. “Combi-moderator” is a combination of several chambers (modules) each containing either a hydrogenous substance cooled down to 20 K - 100 K, or light water at ambient temperature, or void, or some non-absorbing material reflecting neutrons. Such a moderator gives neutron beams with different neutron energy in different direction and/or at different positions of outflow; this would be a good solution when several instruments view the same moderator.

- Using moderating materials for a cold moderator that are more resistant to radiation damage than methane (mixtures of solid methane with inhibitors of molecular hydrogen, light water ice doped with an atomic hydrogen scavenger, 1,3,5-trimethylbenzene or another aromatic hydrocarbon, et al.) with periodic change of the moderating materials.

- Using beryllium reflectors.

- Using ‘neutron gun’ effect.

For optimization of the "reactor-moderator-neutron guide-sample-detector" system, the multilevel approach is offered which includes the research on moderator materials properties, investigation of the heat transfer processes, a detailed Monte Carlo modeling of the neutron transport through the whole system, and manufacturing of the pre-production model of the cold moderator. The pre-production model allows to find solution for the basic constructive, technological and scientific problems and to investigate spectral and time characteristics of the emitted neutrons.

Successful performance of the project will be promoted by the following factors:


- experience of development and realization of a cold neutron moderator based on solid methane, in Frank Laboratory of Neutron Physics, JINR, and in RDIPE;
- operation of an irradiation facility URAM-2 at the IBR-2 reactor;
- experimental and theoretical study on radiation effects in frozen hydrogenous substances (solid methane, ice of water) under fast neutron radiation, made up, first of all, in Russia (the group of theorists headed by academician V.I. Gol'dansky), and also in Japan and USA;
- long-term experience of calculations of neutron transport in IPPE and FLNP, JINR, database on slow neutron scattering in the Centre of Nuclear Data IPPE;
- participation of JINR in the international collaboration of experts on cold neutron moderators ACoM.


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