Investigation of the Delayed Neutron Characteristics
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 MeV
Tech Area / Field
- FIR-NOT/Nuclear and Other Technical Data/Fission Reactors
- PHY-ANU/Atomic and Nuclear Physics/Physics
8 Project completed
Senior Project Manager
Kulikov G G
Joint Institute of Nuclear Research, Russia, Moscow reg., Dubna
- FEI (IPPE), Russia, Kaluga reg., Obninsk\nVNIIEF, Russia, N. Novgorod reg., Sarov
- Lawrence Livermore National Laboratory, USA, CA, Livermore\nCEA / DSM / DAPNIA/CEN Saclay, France, Saclay\nENEA, Italy, Rome
Project summaryThe 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. 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.
There are several nuclear processes under investigation, which could transmute a radio-nuclide into a less toxic one. Amongst them the most promising processes are those related to nuclear reactors and ADS with intense neutron fluxes generated by charged particles accelerators using nuclear reaction, for example, on a lead – bismuth target. The later concepts do not significantly differ from that of nuclear reactor, except that they can use thermal or fast neutron sub-critical arrangements driven by accelerators.
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 (DN) 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 requires, among other data, an accurate knowledge of the delayed neutron data.
Those data that of great importance to the kinetics and safety operations of nuclear reactors (including the ADS) are the absolute yield of DN, relative abundances of DN, half-lives of their precursors, and energy spectra of DN. In spite of great efforts devoted to the investigation of delayed neutron physics, these 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 the IPPE accelerators have shown that the relative abundances and half-lives of DN incorporated into ENDF/B-VI library and obtained on the basis of the summation techniques 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$  as compared to experimental data. Especially large discrepancies are found for plutonium and americium isotopes. Such results point out to the need for careful checking and improvement of the microscopic method used as an alternative to experimental one to derive the DN data as well as corresponding data obtained by this method. Namely these questions are being studied by Subgroup 6 on Delayed Neutrons (NEA/OECD) aimed at the working-out of recommended delayed neutron data for basic fuel isotopes .
The experimental studies of the total delayed neutron yields from neutron induced fission of 237Np and 238U made at the IPPE show the prominent energy dependence of this value in the energy range of primary neutrons above the threshold of the (n, f) reaction . This feature gives the definite indication that the constant value of the total DN yield accepted for the energy range from thermal to 4 MeV in the ENDF/B-VI data base for all elements must be carefully tested.
The energy spectrum of the delayed neutrons is probably the poorest known of all input data required in the calculation of effective delayed neutron fractions which largely determines the kinetic behavior and control margins of any fission chain reactor. The energy spectrum of delayed neutrons incorporated in ENDF/B-VI data library is calculated using known fission fragment yields and the information on neutron emission probabilities, and energy spectra from inpidual DN precursors. However, as it was indicated above, the summation method used in deriving the DN spectra needs in serious testing. Therefore at present the most reliable data related to the DN spectra for any fissionable spices including MA nuclides are connected with experimental determination.
In the framework of the International Delayed Neutron Data Revision program made up to date by Subgroup SG6 (NEA/OECD), the only data having been revised are data for the basic fuel isotopes – 235U, 238U, 239Pu and it should be noted that the revision resulted in significant corrections of the data .
Thus it can be concluded that the DN 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 present 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, 239Pu, 241Am, 243Am (SSC RF–IPPE), the total delayed neutron yields from the thermal neutron induced fission of 237Np,
241Am 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 10-10.5 MeV (RFNC–VNIIEF), and the energy spectrum of delayed neutrons from the fast neutron induced fission of 237Np and thermal neutron induced fission of 235U (SSC RF–IPPE).
As a result of the whole project implementation the following experimental data should have to be obtained:
- the total delayed neutron yields from the thermal neutron induced fission of 237Np, 241Am and 245Cm;
- the absolute total delayed neutron yields, relative yields and half-lives of delayed neutron groups from neutron induced fission of 233U, 241Am and 232Th in the energy range from 0.35 MeV (or threshold one) up to 5 MeV;
- the absolute total delayed neutron yields, relative yields and half-lives of delayed neutron groups from fast neutron induced fission of 234U, 243Am;
- the absolute total delayed neutron yields from proton and deuteron induced fission of 232Th and 238U in the energy range from 10 MeV up to 10.5 MeV;
- the absolute total delayed neutron yields, relative yields and half-lives of delayed neutron groups from neutron induced fission of 239Pu in the energy range from
0.35 MeV up to 17.5 MeV;
- the integral energy spectrum and spectra of separate delayed neutron groups from thermal neutron induced fission of 235U;
- the integral energy spectrum and spectra of separate delayed neutron groups from fast neutron induced fission of 237Np.
But due to the account restriction of the project in comparison with initially declared (140,000 Euro instead of $ 360,000), we pided the project implementation into two stages. The first stage indicated below will be assumed to be performed in four quarter. According to the Governing Board recommendations the decision on the second stage financing should be additionally made: “EU projects funded at 50% are approved for 100% funding upon a progress review conducted at the end of the first funding phase. The progress review meeting will involve both CIS representatives and EU representatives, the latter being nominated by the EU.”
If a continuation of the project for next four quarters will be approved then the rest part of the initial proposal (with some additional tasks) will be completed in the framework of the second stage of the project.