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Investigation of the Delayed Neutron Characteristics

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Investigation of the Delayed Neutron Characteristics from the Fission of Compound Nuclei Th-233, U-234, U-235, Am-244, Np-238, Cm-246, Pa-233, Pa-234, Np-239, Np-240 at the Excitation Energies from 5 to 20 МeV

Tech Area / Field

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

Status
8 Project completed

Registration date
11.07.2001

Completion date
22.06.2006

Senior Project Manager
Kulikov G G

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

Supporting institutes

  • FEI (IPPE), Russia, Kaluga reg., Obninsk\nVNIIEF, Russia, N. Novgorod reg., Sarov

Collaborators

  • Los-Alamos National Laboratory / Primary Design & Assessment Group X-4, USA, NM, Los-Alamos\nCEA / DSM / DAPNIA/CEN Saclay, France, Saclay\nENEA, Italy, Rome\nJAERI / Department of Nuclear Energy System, Japan, Tokai Mura

Project summary

The management of radioactive waste is one of the key issues in today’s political and public discussions on nuclear energy, especially the long term disposal of high level radioactive wastes. In this case it is supposed that for a long time the wastes will be stored in specially equipped burials, until activity is lowered to a safety level. Rather than waiting for their radioactive decay, it is principally possible to reduce the period of toxicity of the actinides and long lived fission products through transmutation of these isotopes in fission reactor or accelerator driven system (ADS). Main source of the dominant long-lived activity of nuclear waste obtained after the extraction of uranium and plutonium isotopes from the spent nuclear fuel is related to minor actinides (MA). Therefore any concept of the nuclear waste transmutation or disposal will to a large extent depend on the accuracy of our knowledge of the basic nuclear data for MA.

The fundamental role of delayed neutrons in the safety operation and time-dependant behavior of nuclear reactors has been well known and is now a matter of practical experience in hundreds of nuclear installations around the world. A satisfactory evaluation of the macroscopic effects of the delayed neutrons following fission in a nuclear reactor and ADS requires, among other data, an accurate knowledge of the delayed neutron data. Nevertheless the delayed neutron data for MA cannot be considered today as a totally reliable basis for the design and understanding of the behavior both the nuclear reactors and ADS systems intended for the transmutation of nuclear waste.

Main objective of the Project is the measurements of the total delayed neutron yields, the relative yields and half-lives of separate delayed neutron groups from the fast neutron induced fission of 232Th, 233U, 234U, 243Am (SSC RF–IPPE), the total delayed neutron yields from the thermal neutron induced fission of 237Np and 245Cm (JINR), the total delayed neutron yields from the proton and deuteron induced fission of 232Th and 238U in the energy range of incident particles 10-12 MeV (RFNC–VNIIEF), and the energy spectrum of delayed neutrons from the fast neutron induced fission of 237Np (SSC RF–IPPE).

In spite of the great efforts devoted to the investigation of delayed neutron physics the fundamental delayed neutron characteristics of even the most common fissionable isotopes encountered in reactor systems are still poorly known and are now under investigation. For example, the experiments conducted at IPPE accelerators have shown that the relative abundances and half-lives of DN obtained on the basis of the summation techniques and incorporated into ENDF/B-VI library systematically deviate from appropriate experimental data. Reactor experiments have shown that ENDF/B-VI group parameters for 235U underestimate the reactivity by 2 to 47% in the range from +0.80 to –0.80 $ [3]. Especially large discrepancies are found between the DN data obtained by experimental and summation methods for plutonium and americium isotopes. There is no experimental information on the delayed neutron parameters for curium isotopes. Such results point out on the need for careful checking and improvement of the summation method used as an alternative to the experimental approach for deriving the DN group parameters and DN energy spectra.

The energy spectrum of the delayed neutrons is probably the poorest known of all input data required in the calculation of the effective delayed neutron fractions which largely determines the kinetic behavior and control margins of any fission chain reactor.

Such a state of DN data points out, on the one hand, to the necessity for a careful check-up of a microscopic approach used as an alternative method for acquiring DN data, and on the other hand, to the necessity for performing additional experiments.

The measurements of the total delayed neutron yields, the relative yields and half-lives of separate delayed neutron groups from the fast neutron induced fission of 232Th, 233U, 234U, 243Am will be made on the set-up installed at the high current electrostatic accelerator CG-2.5 (SSC RF–IPPE). This set-up and the method of the DN data measurements have already successfully been used for the measurements of the absolute total DN yields, relative yields and half-lives of DN groups from neutron induced fission of 237Np in the energy range 0.3-5 MeV. The DN data measurements on 232Th, 233U nuclei will be made using the same method and equipment with several developments and modifications of its components. The main features of the planned experiments on 243Am and 234U nuclides as compared with previous one will be the development of a new high intensity neutron target based on the 9Be(d,n) nuclear reaction. The reason for such modification of the DN set-up is related to considerably less amount of high purity 243Am and 234U samples as compared with other isotopes.

The measurements of the energy distribution of delayed neutrons emitted from neutron induced fission of 237Np will be made using the newly developed installation on the base of accelerator CG-2.5 and the fast neutrons 3He-spectrometer (FNS-1). The experiment will be done at different irradiation of 237Np sample and measuring of neutron spectra within different time intervals after irradiation is over.

The measurements of the total delayed neutron yields in the thermal neutron induced fission of 237Np and 245Cm will be made at the experimental facility based on the IBR-2 pulsed reactor (JINR). Such a powerful pulsed neutron source coupled with bent reflector neutron guide and a slow neutron chopper synchronized with the reactor bursts makes it possible the variation of the exposure duration and effective suppression of the fast neutron background originated from the reactor core. To decrease the proper background arising due to (a,n)-reaction on light elements contained in a-active samples instead of usually applied actinide oxides and fluorides the samples will be manufactured of platinum triad metals intermetallic compounds. Advantage of the set-up based on the IBR-2 reactor is that the irradiation of the samples and detection of their delayed neutron activities will be done at the same location without transferring the samples. The experimental set-up at JINR has been already successfully used for the investigations of the total delayed neutron yields from the fission of main fuel isotopes – 233U, 235U, 239Pu.

The measurements of the total delayed neutron yields from the proton and deuteron induced fission of 232Th and 238U will be made at the tandem accelerator EGP-10 (RFNC–VNIIEF) in a pulsed beam operation mode accomplished by an electrostatic deflection system. Delayed neutrons will be registered in the time intervals between the neutron pulses by 4p-neutron detector comprising of 40 3He-counters of SNM-18 type embedded in the moderator. Thus the planned experiments will allow obtaining the unique information on both the parameters of delayed neutrons and properties of their precursors for 233,234Pa and 239,240Np fissionable systems.

The highly qualified personnel experienced in the nuclear data measurements will work for the Project using the unique installations and equipment of JINR, RFNC–VNIIEF, and SSC RF–IPPE. A large group of the experts (mainly from RFNC–VNIIEF) in the field of nuclear weapon testing and development will be employed in the Project ensuring the Russian programs for creating the safe atomic power engineering and solving the related problems of the transmutation of the long-lived highly radioactive actinides in nuclear reactors and high flux facilities based on ASD (accelerator driven systems).

At the first stage of the Project the work will be done on the preparation of the highly enriched fissionable materials, and developments, modernization and testing of the equipment planed to be employed in the planned experiments. During the second stage of the Project the work will be directed to conducting the experiments in which the measurements of the delayed neutron characteristics for the indicated above isotopes will be made. Finally all experimental information will be processed on the basis of up-to-date mathematical methods. The obtained delayed neutron data will be analyzed in the frame of new systematics of delayed neutron characteristics that will ensure more general approach in the development delayed neutron data base for the wide range of fissile systems at different excitation energies.

As a result of the Project new experimental data will be obtained on the total delayed neutron yields, delayed neutron energy spectrum, relative yields and half-lives of separate delayed neutron groups for a wide range of excitation energy of compound nuclei in the neutron induced fission of 232Th, 233U, 234U, 243Am, 237Np, 245Cm and the proton and deuteron induced fission of 232Th and 238U. The obtained data will allow to make essential improvements of the delayed neutron data base for up-to-date reactor applications including ADS systems. One of the most important benefits of the project will be a more accurate value of effective delayed neutron fractions and a more reliable characterization of the relationship between reactivity and asymptotic period of a nuclear reactor. A better knowledge of this relationship will undoubtedly enhance the capability to predict more accurately the dynamic behavior of a nuclear reactor over a wide range of reactivities.


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