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Uranium Enrichment Measurements by Neutron Irradiation


Development of the Active Neutron Method for Control over Enrichment Degree of Uranium in Closed Shielding Containers

Tech Area / Field

  • FIR-NSS/Nuclear Safety and Safeguarding/Fission Reactors
  • INS-MEA/Measuring Instruments/Instrumentation

3 Approved without Funding

Registration date

Leading Institute
VNIIEF, Russia, N. Novgorod reg., Sarov


  • Lawrence Livermore National Laboratory, USA, CA, Livermore

Project summary

The system of measures limiting unauthorised utilization of weapon-grade nuclear fissile materials (FM), i.e. plutonium and highly enriched uranium released during scheduled nuclear weapons reduction, include technical control over storage and transportation of FM in shielding containers.
The proposed active neutron control method is based on the process of induced nuclei fission under action of stationary neutron flux from an external source. During this process, secondary neutrons, i.e. prompt fission neutrons, are emitted from fissioned nuclei fragments within 10-14s. Relevant physical constants (fission, capture and scattering cross sections), prompt neutron yield, and spectral distribution are quite adequately studied FM characteristics.
The method is highly efficient and expressive and most promising at control over weapons parts made of HEU, which (unlike plutonium) have essentially no natural neutron activity.
In the general case the active neutron method complements the passive ones at expeditious control over contents of internal enclosure of containers without their opening, especially when presence of FM natural radiation imitators and FM replacement with inert material or HEU substitution for low-enrichment uranium are suspected. A distinctive feature of the method as compared to other nuclear control methods (for example, gamma spectrometry) is the three-dimensional induced radiation nature due to a high penetrability both of primary and secondary neutrons.
The purpose of this Project is practical development of the active neutron control method and computational and experimental study of its possibilities for remote estimation of the enrichment degree of uranium in the massive AT-400 type shielding container developed jointly by Russia and US. Solution of this problem will allow expeditious determination, whether uranium is weapon-grade or reactor-grade, which, in its turn, will considerably facilitate the FM control and accounting procedure.
The Project provides for using selective neutron detectors recording neutrons of energies above some threshold agreed on in advance, depending on spectral characteristics of the probing source. Thus, for example, when using a source of ~2.5 MeV neutron energy (dd reaction), an appropriate neutron recording threshold is above 2.5 MeV. In this case the detector will not record external source neutrons and the whole recorded effect will be mainly due to fission processes in uranium.
It is planned to study non-weapons components made of uranium of various enrichment degree placed in the AT-400 type container. It is suggested that specimens made of materials, other than uranium, both inert and apt to spontaneous fission, would be also studied in order to assess the contribution of neutron scattering and generation in the container.
It is proposed that two versions of the active neutron method for control of uranium enrichment degree would be considered within the Project:
· Version 1 uses one external probing neutron source. The spectral distribution of prompt fission neutrons of energy higher than external source neutron energy is selectively recorded in a measurement geometry fixed relative to the source and the container. Subsequently using special methods for processing of experimental data and source power data allows estimation with an acceptable accuracy of the enrichment degree of the uranium specimen under examination with mass specified in advance.
· Version 2 uses a similar recording system and a fixed measurement geometry, but two external probing sources with neutron energy both higher and lower than the natural uranium fission threshold. Subsequently using special methods for processing of experimental data and source power data allows estimation with an acceptable accuracy of the enrichment degree of the uranium specimen under examination without a priory information about its mass.
The two method versions in combination allow control of the uranium enrichment degree in the range from 10% to 100%.
When verifying the method for induced fission excitation, radionuclide neutron sources of various energy or a compact electrostatic deuteron accelerator with changeable targets will be used.

The Project provides for:
§ theoretical and computational feasibility studies for remote estimation of uranium enrichment degree with using external probing sources of various spectral characteristics (selection of optimal parameters and characteristics of neutron sources; selection of an optimal measurement geometry and method for secondary radiation recording);
§ selection of hardware for the method;
§ verification and testing of the selective neutron recording detectors adapted to specific conditions of the measurements;
§ development of specialized software for expeditious experimental data processing;
§ experimental studies to validate various types of the active neutron control method employment.
At RFNC-VNIIEF there are the experimental base and highly skilled former weapons experts needed for successful accomplishment of the Project. The Project participants have gained many-years experience in research into radioactive material properties, neutron recording methods, neutron detector development and designing.

It is suggested that the Project would result in:
§ developed measuring technique;
§ theoretical and computational justification of the technique, including:
a) optimal geometry for setting up the experiment;
b) neutron detector characteristics;
c) selection of external neutron sources with optimal parameters (power, neutron energy, irradiation duration);
§ experimental demonstration of using the developed technique for specimens made of low and high enrichment uranium.
The results of the studies will persuasively demonstrate practical abilities of the proposed method for expeditious control over enrichment degree of uranium in closed shielding containers.
The Project suggests higher objectiveness and reliability of the control system ruling out potential unauthorized utilization of weapon-grade FM, higher efficiency of the procedure of control over safety of components made of various enrichment degree uranium in nuclear material storage facilities, and higher reliability in assessment of utilized FM quality, which can be achieved using the proposed active neutron method in combination with other nuclear or non-nuclear control methods.

The proposed Project completely meets the ISTC goals:
1. Provides RFNC-VNIIEF weapons scientists and experts with an opportunity to reorient their abilities and talents to peaceful activities;
2. Promotes integration of RFNC-VNIIEF scientists to the international scientific community;
3. Encourages application studies and development of technologies for peaceful purposes, particularly in the area of FM safe storage system;
4. The Project results will promote solution of technical problems in the area of FM control.
5. The economical benefit from introduction of the method consists in improvement of the FM control and safe storage systems with potential automation of the measuring process and enhancement of measured data reliability, avoidance or prevention of nuclear material thievery.


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