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Control of subcriticality of neutron multiplying systems


Development of experimental Prototype of Equipment and Measuring Technique for Control of Subcriticality of Neutron Multiplying Systems

Tech Area / Field

  • FIR-NSS/Nuclear Safety and Safeguarding/Fission Reactors
  • ENV-WPC/Water Pollution and Control/Environment
  • FIR-EXP/Experiments/Fission Reactors
  • FIR-INS/Nuclear Instrumentation/Fission Reactors

8 Project completed

Registration date

Completion date

Senior Project Manager
Tocheny L V

Leading Institute
All-Russian Research Institute of Automatics, Russia, Moscow

Supporting institutes

  • FEI (IPPE), Russia, Kaluga reg., Obninsk


  • Lawrence Livermore National Laboratory, USA, CA, Livermore\nPacific Northwest National Laboratory, USA, WA, Richland\nNuclear Safety Solutions Limited, Canada, ON, Toronto

Project summary

The goal of the project is development of methods and hardware for control of subcriticality of multiplying media in storages of spent fuel (SF) of nuclear power plants (NPP) and experimental reactors, receiving tanks of liquid radioactive wastes (LRW) storages, various process vessels. Different methodical and experimental approaches to solution of subcriticality control problems will be considered, theoretical studies and model experiments will be carried out, proposals on optimal measuring technique and construction of equipment with the use of modified neutron generators developed in VNIIA will be elaborated.
As a result of the project implementation the experimental prototype of equipment will be created and measuring technique for control of subcriticality of neutron multiplying media will be developed as applied to measurements of subcriticality of SF storages (SFS) and LRW receiving tanks.
About 750 tons of SNF are unloaded annually from cores of reactors under operation. SF of WWER and FBR reactors, precooled in reactor storages, is transported to PA “Mayak” for reprocessing (VVER-440 and BN-600) and to Krasnoyarsk Mining and Chemical Plant for long-term storage (VVER-1000) in special storage facility. Before transportation SF is kept in NPP storage ponds for at least three years in order to reduce energy release of fuel assemblies and to provide the decay of short-lived radionuclides. Nowadays about 10 thousand tons of SF (90% - RBMK SF) are kept in NPP storage facilities.
Unloaded SF of RBMK type reactors (this type reactors are used at three RF NPPs – Kursk NPP, Leningrad NPP and Smolensk NPP) is kept in storage facilities located just in NPP area, since SF of RBMK reactors is not subjected to reprocessing. By the end of 90-s of the last century it has really turned out that all NPP storage facilities were almost filled up. It was decided to compact them because construction of the central storage facility is a matter of future activities. Such action provides about two times increase of storage capacity. Reconstruction of SFS has been accomplished at Leningrad and Kursk NPPs. It resulted to increase of storage capacities 2 and 1.67 times respectively. Work on compacting SFS at Smolensk NPP is scheduled to start in the nearest future, as well as to introduce a unified scheme of compact storage in reactor cooling ponds (CP) at RBMK NPP.
Compacting of storage facilities consists in reduction of a pitch of fuel assembly (FA) arrangement in CP. Selection of a pitch of FA arrangement is limited by nuclear safety requirements according to which the effective multiplication factor Keff in SFS shouldn’t exceed 0.95 under normal and emergency conditions. Nuclear safety is substantiated with the use of calculations considered to be reliable enough. But since the knowledge of fuel burnup in cassettes is rather uncertain, direct verification measurements of multiplying properties in various CP points should be performed regularly. If measured Keff value is less than 0.73, Keff value of the system under any kind emergency situations, related with change of water density, water evaporation from cans, cooling pond, etc., will not exceed 0.95. This is regulated by the Nuclear Safety Rules(NSR).
Besides this the problem exists of control of subcriticality in SFS of research reactors. Control of subcriticality is also required when reactors and especially research ones are put out of operation.
Practically no study was made on the problem of occurrence of spontaneous chain reaction (SCR) in receiving tanks (reservoirs) of LRW storages and collecting systems. In the course of long-term operation both wastes containing various isotopes (including fission fragments) and wastes containing fissile materials including uranium and plutonium are accumulated on the bottoms of such reservoirs. The same thing is with process vessels where solutions containing fissile materials are kept continually or temporarily. Calculations for substantiation of nuclear safety of such systems are sufficiently complicated by unknown deposit composition on the one hand and by geometry and density of bottom deposits on the other hand. In the situation like this experimental determination of subcriticality and measurements of multiplying properties of the system are essential.
Pulse reactivity measurement method using pulsed neutron generator (ING) is most suited for measuring deep subcriticalities (Keff is less than 0.95). There is a large number of method modifications (e.g. Sjostrand, Gozani methods), based on determining ratios of areas of delayed and prompt neutron distributions. Subcriticality can be also determined by Simmons-King method, based on determining the constant of decay of prompt neutron flux density. Subcriticality of LRW tanks can be also determined by the modified Rossi-alpha method. There are some difficulties in applying these methods, such as the necessity of account of spatial effects, determination of some parameters on the basis of calculations etc. So the combination of several methods of reactivity measurement excluding systematic errors and supplementing each other is required to obtain the most reliable results.
To solve the problem of determining degree of subcriticality of neutron multiplying media it is suggested to develop the equipment on the basis of ING. The equipment will enable to perform measurements of multiplying characteristics of different media in items of random geometry. It is also proposed to develop the complex software as applied to measurements of subcriticality in SFS and LRW receiving tanks.

To meet the project objectives it is necessary to:
- perform methodical studies on evaluation of subcriticalities of neutron multiplying media as applied to LRW tanks and SFS;
- specify technical requirements for pulsed neutron generator (or modifications of pulsed neutron generators);
- develop and manufacture a specialized pulsed neutron generator;
- develop and manufacture the experimental prototype of measuring equipment;
- carry out the complex of experiments with critical assemblies and LRW tanks on measuring effective multiplication factor using different methods;
- perform analysis of multiplying properties of selected experimental models;
- evaluate methodical errors.
To realize the project high-skilled scientific personnel of VNIIA and IPPE will be employed.

All-Russian Research Institute of Automatics is a leading Russian institution engaged in development and manufacture of pulsed neutron generators. About 20 types of industrial and research generators (12 types are series-produced) have been developed in VNIIA over 45 year activity in the field of creation of portable neutron generators on the basis of sealed neutron tubes and plasma focus chamber. VNIIA activities are not limited to creation of neutron generators. Neutron technologies on their basis and measuring equipment for scientific and technical applications are being intensively developed.
State Scientific Centre of Russian Federation - Institute for Physics and Power Engineering is involved in complex studies in the field of physics of different type nuclear reactors and nuclear safety. It possesses a unique experimental base and many year experience in work with pulsed neutron sources, carries out examination of facilities from the viewpoint of nuclear safety. The equipment for measuring NPP nuclear reactivity has been developed and certified in IPPE. It has an appropriate technical base for development of techniques for measuring subcriticality of neutron multiplying media using both experimental means (for example, critical assemblies) and precision model calculations.
Expected results:
The following main results will be achieved under the project:
- technique for evaluating subcriticality of neutron multiplying media as applied to tanks of liquid radioactive wastes and storages of spent nuclear fuel;
- experimental prototype of measuring equipment complex;
- modified pulsed neutron generator;
- experimental data on measurements of subcriticality in LRW tanks and simulation of different multiplying systems on critical assemblies;
- results of calculation analysis;
- evaluation of methodical errors;
- optimal technical and methodical solutions enabling to control nuclear safety parameters of different types of LRW and SF storages with the use of pulsed neutron generators.
Results of the project will contribute to solution of the problem of control for nuclear reactors put out of operation.
Results of the project can be used for evaluation and substantiation of nuclear safety of LRW and SF storages at NPPs, enterprises and research institutes of nuclear fuel cycle.
Foreign collaborators
The Pacific North-West National Laboratory and Oak-Ridge National Laboratory (USA) proposed to carry out the project activities under the General ISTC Programs program. In the framework of scientific and technical collaboration with foreign partners it is planned to exchange the relevant information, to discuss the achieved results and develop proposals on introduction. When working under the project it is planned to issue joint publications and make joint presentations at seminars and conferences.
Meeting ISTC goals and objectives
The project implementation meets ISTC goals and objectives, supports military conversion and involvement of the Russian scientists and specialists in the international scientific and technical community. The activities under the project will be carried out by VNIIA and IPPE specialists who were involved in the past in development of nuclear weapons test equipment.
The project is aimed at support of fundamental and applied research and development of peaceful technologies in the area of nuclear safety and environment control.


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