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VVER Vessel in Severe Accident

#3635


Scale Experimental Investigation of the Thermal and Structural Integrity of the VVER Pressure Vessel Lower Head in Severe Accident

Tech Area / Field

  • FIR-NSS/Nuclear Safety and Safeguarding/Fission Reactors
  • FIR-EXP/Experiments/Fission Reactors
  • FIR-MAT/Materials/Fission Reactors
  • FIR-MOD/Modelling/Fission Reactors
  • FIR-REA/Reactor Concept/Fission Reactors
  • PHY-STM/Structural Mechanics/Physics

Status
8 Project completed

Registration date
16.10.2006

Completion date
13.08.2012

Senior Project Manager
Tocheny L V

Leading Institute
Moscow Power Engineering Institute, Russia, Moscow

Supporting institutes

  • OKB Gidropress, Russia, Moscow reg., Podolsk\nSiberian Branch of RAS / Lavrent'ev Institute of Hydrodynamics, Russia, Novosibirsk reg., Novosibirsk

Collaborators

  • Institute of Nuclear Safety System, Incorporated, Japan, Fukui\nForschungszentrum Rossendorf, Germany, Rossendorf\nCEA / DEN / DDIN / CEA Centre de Saclay, France, Gif-sur-Yvette Cedex\nForschungszentrum Karlsruhe GmbH, Germany, Karlsruhe\nEuropean Commission / Joint Research Center / Institute for Transuranium Elements, Germany, Karlsruhe\nUniversity of Wisconsin-Madison / College of Engineering, Nuclear and Engineering Physics, USA, WI, Madison\nPennsylvania State University / Department of Mechanical and Nuclear Engineering, USA, PA, University Park\nRoyal Institute of Technology / School of Engineering Sciences / Department of Physics, Sweden, Stockholm

Project summary

Present project is associated with the peaceful uses of atomic energy and it is aimed on the considerable enhancement safety of the nuclear power plants with the VVER reactors due to the unique experimental findings on the heating, structural behaviour and rupture of the VVER vessel scale models in the severe accident (SA) conditions. It is well known, the safety assessment of the operating and new generation reactors is based on the computer simulation of the SA phenomena by means of the numerical SA codes. The verification and validation of them against the corresponding experimental data is the indispensable condition for such work. Analysis of the reactor vessel integrity is the key moment in the overall safety analysis of the reactor installations.

It is known, that exists a possibility of the melt relocation and accumulation in the lower head (LH) of the reactor vessel during SA for the vessel-type reactors (VVER, LWR, PWR). In that case the vessel plays the role of the main barrier to prevent the radioactive materials release to environment. Interaction of the corium and vessel steel is accompanied by heating and melting of the vessel wall, as well as by formation of the low-melting eutectics in the contact zone. Beside that, heating of the vessel steel to more than 500 C leads to the essential decreasing of its strength properties. Analysis of the native and foreign investigations on the vessel behaviour and failure during SA revealed the high-temperature creep of the vessel steel is the main cause of their deformation and failure.

Comprehensive study of several aspects (heating, deformation, failure etc.) of the reactor vessels behaviour in SA was the principal item of many foreign research programs in particular, VIP (Vessel Investigation Project, OECD), LHF and OLHF Projects (Sandia Lab., USA), CORVIS (PSI, Switzerland), FOREVER-experiments (the Royal Institute of Technology, Sweden), MOSES etc. Experimental results received in the course of those investigations, are very important from the view-point of the possibility of validation and verification of the computer SA codes and mathematical models of the studied phenomena (creep, failure of the vessel steel etc). The basic element of the researchs mentioned before was the experimental study of the reactor pressure vessel (RPV) scale models behaviour during its heating and creep deformation. Never before similar combined experimental and analytical works on VVER vessel, concerned with its heating, creep and failure in the SA were done in Russia.

The overall objective of this project is the experimental and numerical investigation of the scaled VVER LH reactor vessel models within transient thermal and its overpressure loading which correspond to realistic SA scenarios accompanied by the high-temperature heating, creep deformation of the reactor vessel.

More specifically, major project tasks are:

  1. to design and build up the experimental test facility and to carry out the tests on the scale (up to ~1:5) models of the VVER vessel lower head in SA conditions;
  2. to carry out the material property creep tests with the samples from the 15Kh2NMFA vessel steel on more than 30 hours time range and temperature above 700 C. These experiments will allow to receive the creep diagrams and refine the mechanical properties of VVER steel including the high-temperature creep parameters at 750-1050 C and the constitutive creep model of this steel;
  3. to carry out the numerical pre- and post-test calculations of the scale experiments with VVER vessel models.

The urgency and necessity of the suggested project is determined by: a) a lack of the experimental data on the VVER vessel deformation and failure in SA because of the high-temperature creep phenomenon. These experimental data are necessary to straighten out the real behaviour of the VVER vessel during SA, as well as for the verification of the computer SA codes (native and western commercial). The given results will allow more adequately estimate the influence of the associated processes (the vessel creep effect, thermal and mechanical loads etc.) to a lifetime and VVER RPV behaviour during SA; b) achievement of the project’s tasks will permit to reduce noticeably the conservatism of the numerical simulations associated with VVER safety, and also to elaborate and to carry out all necessary actions to control the SA; c) the creep test results of the samples from 15Kh2NMFA vessel steel shall permit to define more precisely and to complete available data on creep and failure, and also to clarify the creep model of this steel; d) the results of this project will become the base of the next stage of the investigations associated with development of the core-catcher constructions for localization of the core material both inside the VVER vessel and beyond it. Experimental control of the work efficiency of the different core-catcher constructions would be possible test by using of experimental equipment of suggested project.

The leading participant of the project is the team from MPEI-TU (NPP Department). This team for more than 10 years carried out the complex of works concerned with different thermo physical aspects of the events obtained during the design-basis and beyond the design-basis accidents in NPP. For realization of those tasks the special computational programs were designed for numerical simulations of studied phenomena, in particular, the thermomechanical codes ASHTER-VVER and ATM-VVR, as well as NARAL and NARAL/FEM codes. The numerical investigations of the reactor pressure vessels behaviour of the VVER-440, VVER-640 and VVER-1000 in SA were carried out by means of these computer codes. It is supposed to use ATM-VVR code as the basic code for pre- and post-test thermomechanical numerical simulations in present project. The members of the team also have essential experience in the preparatory and pursuance of the high-temperature research and experiments.

Experimental and Design Organization "GIDROPRESS" (EDO “GP”) as a work co-executor is the representative of the organization, that is the head designer of the VVER reactors. The primary task of this team is the analysis and choosing the severe accident scenarios and tests conditions of the scale experiments with VVER vessel models. Members of this team are highly skilled and have the long-term experience of the carrying out the design-analytical works on the SA for the NPP with VVER by means of domestic (BISTRO, RATEG/SVECHA/GEFEST) and commercial SA codes (MELCOR and RELAP/SCDAP). The experience of the EDO "GIDROPRESS" staff will give the possibility to choose substantially the accident scenarios, to carry out pre- and post-test simulations and to develop the tasks of the planned scale experiments.

Static Strength laboratory of Lavrentyev Institute of Hydrodynamics (“IGiL") of the Siberian Branch of the Russian Academy of Sciences (SO RAN), as a second co-executor of this work, is one of the oldest labs in Russia which are working in the field of high-temperature testing of the metals and metal alloys. During the last five years lab staff together with MPEI received fundamentally important results on the high temperature creep testing of the VVER vessel steel. Some of group’s investigations are concerned with construction of the adequate creep models of testing materials, as well as with high-temperature testing of the machine-building and special items’ fragments.

Suggested project fully conforms the principal purposes of the ISTC and will permit especially to integrate former weapons scientists from Russia into the international scientific community. It will also make its contribution to self-sustaining civilian activities.


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