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Reactivity Evaluation Device


Development and Testing of the Reactivity Evaluation System to Provide Nuclear Safety of Subcritical Systems and Non-destructive Control of Nuclear Materials Inside Technological Equipment and Containers at Nuclear Industry Enterprises

Tech Area / Field

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

8 Project completed

Registration date

Completion date

Senior Project Manager
Yakusheva A A

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

Supporting institutes

  • NPO Mayak, Russia, Chelyabinsk reg., Oziorsk\nVNIITF, Russia, Chelyabinsk reg., Snezhinsk


  • Oak Ridge National Laboratory, USA, TN, Oak Ridge

Project summary

It is proposed to develop and test a system (reactimeter) to control subcritical states of nuclear materials (NM) containing systems. A reactimeter is being developed by RFNC-VNIIEF specialists. Reactimeters will be tested on RFNC-VNIIEF solution core reactor, two RFNC-VNIITF solution core reactors and using a technological device available at RT-1 plant of Production Association “Mayak” (hereinafter, PA “Mayak”).

It is universally recognized that in many cases nuclear safety of multiplying systems is assured by continued or periodical control over their reactivity.

A reactimeter includes two separate channels for recording neutrons and also a channel for remote control of a neutron source movement. Preprocessing of detector signals and their digitization, as well as output of control signals to perform remote control of a neutron source movement is performed using two multifunction boards, such as AT-MIO-16E-2 by “National Instruments”. Such type boards are mounted on mother boards of IBM-type PCs using standard connectors. Software is operated under OS Windows 95 (Windows NT).

The idea of the reactimeter operation is based on finding solution to the inverted reactor kinetics equation in one-point approximation. Algorithms used for processing signals from detectors, when controlling subcritical states, allow consideration of factors related to possible presence of the background of neutrons in a detector signal and also the fact that fission processes in reactor are of statistical nature. Without consideration of these factors a systematic error of subcritical states control may become intolerable and data on the multiplying system status may become invalid. By the background of neutrons we principally mean neutrons passed straightly and scattered from an outer source that didn’t participate in fission chains but were recorded by a neutron detector. It is evident that when controlling a reactor in a near delayed criticality, the background of neutrons can be neglected because of its insignificant value.

The reactimeter can provide on-line control of a system reactivity that implies knowledge of both the effective neutron source value and the background of neutrons.

The reactimeter can also operate in off-line mode. In this mode of operation data is resultant from analysis of transient processes, i.e. processes of a multiplying system relaxation to form a new equilibrium state after the exciting factors have ceased to affect the system (for example, changes of reactivity or power of an outer neutron source). Off-line data allows reactimeter adjustment and its commutation to on-line operation.

Simple design of the reactimeter and its relatively low cost (~US$10,000) are able to provide general and reliable quantitative control over the chain uranium (plutonium) fission reaction during technological processes at nuclear industry plants.

Introduction of a neutron source of power ~107neutron/s into the technological chain causes no effects on the radiation background at the site of uranium (plutonium) reprocessing, and for reactor it is an obligatory requirement.

The above-described method for diagnosing reactivities of multiplying systems would be applicable at nuclear industry enterprises engaged in reprocessing nuclear fissile materials (NFM) as a physical barrier to prevent nuclear accidents caused by violations in technological cycles. The recent nuclear accident of such a kind took place at the Japanese plant producing fuel for power reactors on September 30, 1999. The power outbreak happened, when the “dry” technological material, containing uranium with ~20% 235U enrichment, was uncontrolled poured out into the reservoir for their dissolution holding in a nitric acid. Availability of a reactimeter in the technological chain would allow good-time locking (before critical configuration occurrence) of access to NFM load in the reservoir and personnel warning.

The second area of activities under the project is the use of the system under development for non-destructive control of NFM when performing inventory inspections at nuclear industry enterprises. The method is based on measurements of multiplication factors, when a neutron source moves near a NFM container within a strictly fixed geometry. A reactimeter records transient processes and data are written into a DB of PC. Thus, a kind of “neutron portrait” is formed for each NFM container. If a NFM container was not opened between inventory inspections, then comparison of a newly measured “neutron portrait” with an old one provides NM identification inside the unsealed container.

The method should be used in addition to already available methods of inventory inspections.

The second project task supposes studies to be performed at VNIIEF and VNIITF NM storage facilities using the reactimeter developed to confirm NFM presence inside containers in bulk-form and give recommendations on application of reactimeters to control NFM in containers at PA “Mayak”.


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