Gateway for:

Member Countries

Brickwork Materials for Operational and New NPP's

#1974


Perspective Materials for Brickwork Elements of External Catchers of Operating and Design NPP. Methods of Severe Accidents Consequences Mitigation

Tech Area / Field

  • FIR-MAT/Materials and Materials Conversion/Fission Reactors
  • FIR-NSS/Nuclear Safety and Safeguarding/Fission Reactors

Status
3 Approved without Funding

Registration date
16.08.2000

Leading Institute
Joint Institute for High Temperatures RAS / High Energy Density Research Center, Russia, Moscow

Supporting institutes

  • All-Russian Scientific Research Institute of Non-Organic Materials named after A. Bochvar, Russia, Moscow\nVNIIEF, Russia, N. Novgorod reg., Sarov\nKurchatov Research Center / Institute of Nuclear Reactors, Russia, Moscow

Collaborators

  • University of Michigan / College of Engineering / Department of Aerospace Engineering, USA, MI, Ann Arbor\nTampere University of Technology / Institute of Materials Science, Finland, Tampere\nSiemens Nuclear Power NDSI, Germany, Erlangen\nInstitut für Kern und Energietechnik, Germany, Karlsruhe\nArgonne National Laboratory (ANL) / West, USA, ID, Idaho Falls\nLos-Alamos National Laboratory / Detonation Science and Technology Group (MS P952), USA, NM, Los-Alamos

Project summary

The purpose of the project is to develop advanced materials for brickwork elements of ex-vessel catchers of existing and projected nuclear power plants (NPP), which are intended for localization and retention of core melt (corium) when it escapes from the reactor vessel under a severe accident. The catcher is to guarantee the localization of*- core melt, its cooling down and retention for an infinitely long time without serious damage to the containment structure and environmental effects.

The persity of possible scenarios of a severe accident, which are considered at present, causes uncertainty in estimating the parameters of core melt ejected from the reactor vessel. Lack of precise data on important parameters such as the moment and duration of the core melt escape from the reactor vessel, its composition (for example, the amount of unoxidized zirconium), the temperature, the geometry of destruction of the vessel, and a number of others give rise to serious difficulties in practical realization of systems of localization of core melt (LS) inside the containment.

Note that, at present, none of the VVER (PWR) type reactors in the world has a catcher. Especially urgent is the problem of catcher for reactors of electric power of the order of 1,000 MW. Of 13 VVERs existing in Russia, seven are VVER-1000. The need for a catcher is well realized by all NPP development engineers. Various schemes of ex-vessel catchers are presently under development, both for projected and for existing NPPs. The problem of completion of the existing project involving a VVER-1000 for Lianyungang NPP (China) is under study in Russia. A European pressurized-water reactor (EPR) with ex-vessel catcher has been designed in the European Union.

Note that the localization schemes for the currently existing nuclear reactors of the PWR type differ from those for advanced reactors such as EPR. The main difference is that, in the existing reactors, it is virtually impossible to provide for the spreading of the melt over a large surface after it escapes from the reactor vessel; therefore, all of the melt will accumulate in the concrete shaft of the reactor. In light of this, the conditions of cooling of the core melt in the existing and projected reactors will differ considerably. However, the choice of catcher materials is a common problem of fundamental importance in designing a catcher for the existing and projected NPPs.

This proposal is associated with the use of advanced materials for elimination of nonstandard situations in the core melt catcher of reactors of the VVER type.

Proposals for the use of advanced materials to eliminate nonstandard situations in the core melt catcher of reactor of the EPR type.

From the standpoint of advanced nuclear reactors, the most advanced is the catcher for EPR, developed in the EU countries. It consists of two main parts, namely, a pit for receiving and conditioning the core melt and a spreading compartment of the melt with its subsequent water cooling. In the pit, provision is made for the use of sacrificial concrete (SC) based on iron, silicon, and calcium oxides for conditioning the melt (complete dissolution of uranium oxides, oxidation of zirconium, reduction of the density and viscosity of the oxide melt, etc.). The pit exterior is made of a refractory material based on zirconia concrete.

The concrete foundation of the spreading compartment is insulated by a protective layer of a zirconia-based ramming compound. The integrity of this layer in the course of continuous operation under extreme conditions will presumably be ensured by ruling out the possibility of its contact with the core melt. For this purpose, it is suggested that an additional layer of SC on the basis of borosilicate glass be provided on top of the refractory material. The melting of borosilicate glass and the dissolution of its components in the core melt must be accompanied by the inversion of the latter (the oxide layer in the core melt will float up and position itself above the metallic layer). A steel sheet with a calibrated content of chrome and nickel is placed between the refractory material and SC for guaranteed protection of the refractory layer.

The results of analysis of heat- and mass transfer processes occurring in the catcher of EPR reactor enable one to identify several unlikely events which can result in deviations from the preassigned operating mode (nonstandard situations). Such situations may arise both in the pit and in the spreading compartment.

A nonstandard situation in the pit may be caused by local melting through the sacrificial layer (SL) of SC due to additional heat release upon oxidation of zirconium by SC components and focusing of the heat flux in the steel "knife" region. The steel knife may form because of the stratification of the melt. This effect will lead to a higher rate of SC erosion and to a longer duration of contact between the melt and refractory material.

The following is suggested to eliminate this situation:

1. The use of a high-density ZrO2 ceramics as the material for the protective refractory layer of the pit. The results of preliminary experiments have demonstrated that the rate of erosion of dense ZrO2 ceramics (porosity of about 1%), according to the results of experiments with model corium (UO2 + ZrO2 + FeO), is 0.2 mm/min with a temperature of 2,273 K, while the rate of erosion of zirconia concrete (porosity of about 20%) is in the range from 2.2 to 3 mm/min (S.V. Bechta et al, 1999).

2. The use of titanate concrete for the SL, which must guarantee the desired parameters of core melt at the outlet from the pit, the reduction of heat release at the stage of oxidation of zirconium, and the binding of radionuclides for their subsequent burial.

For solving these problems, it is suggested to use titanate concrete with a filler in the form of "synrock" whose composition includes the minerals of rutile, perovskite, zirconolite, hollandite. This material is actually free of iron oxide. Perovskite and hollandite included in the composition of synrock immobilize 90Sr, U, and rare earths. One of the conditions to be borne in mind in developing such an SC is the possibility of using it to form a crystallized structure in the core melt at a relatively high temperature. This will enable one to retain the melt in the intercrystalline space and, thereby, preclude its penetration deep into the sacrificial layer and then into the refractory one.

A nonstandard situation may arise in the spreading compartment as a result of disturbance of the physicochemical and physical stability of the steel melt-refractory interface.

The layer of metal which separates the oxide melt from the refractory material contains Fe, 3–7% Cr, and 7% Ni. The presence of chromium in steel is necessary to reduce the FeO activity to a level at which no dissolution of ZrO2 in FeO will occur at the steel-refractory interface (S. Hellmann et al, 1999).

The physicochemical stability at the steel-refractory interface may be disturbed as a result of interaction of the molten layer of chrome-containing steel with the underlying layer of ZrO2 refractory. The results of our preliminary experiments involving the interaction of stainless steel with different samples of ZrO2-based ceramics and concretes in Ar with an oxygen impurity under isothermal conditions at 2,273 K have revealed that, in the course of interaction, the samples are impregnated with metal to form a cermet (metal-ceramics) layer. The overall metal content in this layer may reach 23 wt.%. In view of this, the efficiency of refractory protection by the layer of molten steel calls for further assessment.

The loss of physical stability may be associated with the destruction of the refractory material due to thermomechanical stresses and subsequent floating up of a part of the refractory material to the melt surface. The results of analysis of the thermomechanical stability of zirconia-based refractory demonstrate that, under conditions of thermal stimulation by the melt, the thermoelastic stresses within the refractory may be comparable with or exceed its strength. The use of refractory in the form of rammed mass (characterized by low durability) may result in the chipping of refractory and in the floating of its fragments prior to the beginning of hardening of the surface layer of the material as a result of sintering. It is sound practice to make the refractory layer of concrete or ceramics. If the protective layer is made as a multilayer brickwork of ZrO2 bricks, the thermomechanical stresses may be reduced. The results of preliminary testing of bricks of different refractory materials under conditions close to those of initial contact with the melt (average heating rate of about 80 K/min) revealed no formation of surface cracks. The most heat-resistant refractory on the basis of zirconia stabilized with yttria exhibited no damage on the surface even under more intense thermal stimulation under conditions of cooling.

A further study into the thermomechanical stability of refractory layer involves the reproduction of the real geometry of the catcher and of the conditions of pouring out of the melt.

Proposals for the use of advanced materials in the core melt catchers of existing nuclear reactors.

As was mentioned above, projected melt catchers for existing nuclear reactors of the PWR type are directed mainly towards the utilization of the limited subreactor space inside the reactor shaft. In light of this, the volume of the catcher proper designed to contain all of the core melt, will likewise be limited.

It is suggested to protect the walls of these catchers by zirconia-based refractories which, as was already mentioned, must be isolated from contact with iron oxides.

The results of our experiments have demonstrated that, with the ratio of the mass of iron oxides to the mass of ceramics (porosity of 20%) ranging from 1:5 to 1:7, iron oxide is fully absorbed by the ceramics without destruction of the latter. It is possible to realize this method of absorption of the entire mass of iron oxides from the core melt, but this is hardly practical in view of the need to use large amounts of ceramics.

The problem of protection of refractory zirconia ceramics may be solved, for example, by disposing on its surface a layer of sacrificial material (for example, involving the use of Nd2O3) which will bind a part of iron oxide in the melt and, thereby, reduce its aggressive effect on the ceramics. This suggestion is based on the comparison of the results of our preliminary experiments involving the interaction of zirconia-based ceramics with melts of different oxide compositions, including a model corium. The mechanism of binding of iron oxides in the core melt needs to be studied.

In addition, titanate ceramics may be introduced into such a catcher for retention of the fission products with a view to subsequent burial of radioactive waste.

The assessment of the possibility of using natural minerals as sacrificial material presents an attractive avenue for research.

Sacrificial and refractory materials will be developed within the framework of this project, whose properties compare well with those of the materials for the EPR catcher presently under development. These materials will possess additional properties capable of ruling out the possibility of unlikely nonstandard emergency situations in the projected EPR catcher. In addition, the sacrificial materials will serve as immobilizers of radioactive materials.

Methods of preparing the protective and refractory layers of the catcher will also be developed, such as the concrete-pouring technology and the technology of brickwork and block masonry. Appropriate binders and cements will be developed for this purpose. The available experimental data indicate that such materials may be developed on the basis of ultradisperse zirconia powders.

Physical and mathematical simulation of methods for easing the consequences of unlikely events in catchers. Development of a data base of physicochemical properties of proposed materials.

One of the main objectives of the project is the assessment of the thermal conditions of the catcher made of zirconium oxide in the case of severe accident. For this, it is proposed to use the physical and 1D–3D mathematical models and computer codes developed by us. In so doing, a data base of the thermal properties of the proposed sacrificial and refractory materials, as well as of melts, will be developed.

Special attention will be given to computer simulation of the processes of thermal and chemical effects of the core melt on the pit, especially in the zone of its contact with the metallic layer of the melt, as well as of the effect of the jets of melt escaping from the destroyed reactor vessel on the pit walls. The impregnation of the ceramics by metal oxides will also be taken into account. In addition, the calculation results will enable one to formulate the requirements of the choice of the optimum ceramics structure (porosity, pore size).

It is further proposed to perform estimation of the heating of the reactor shaft structures and the elements of destroyed reactor vessel due to radiation from the core melt surface.

The effect of atmosphere on the process of heat transfer in the reactor shaft will be included in the calculations. For this purpose, it is planned to perform experimental investigations of the spectral and temperature dependence of the effective absorptivity, the diffusion coefficient of radiation, and the temperature dependence of true (phonon) thermal conductivity.

Note that, in the case of a severe accident, a situation is possible that is characterized by the presence of water in the catcher. This may cause a steam explosion of varying rate and amount of energy release. One can use experiments with explosive materials to estimate the resistance of elements of the catcher and reactor shaft to dynamic loads and to design explosion-proof structures. In particular, porous refractory materials are capable of effectively suppressing dynamic loads. Within the framework of this project, it is planned to study the damping properties of porous materials on the basis of titanate and zirconia ceramics.

A joint effort of specialists from IHED IVTAN SA RAS, RFNC-VNIIEF, INR Kurchatov Inst. RSC, and SSC RF VNIINM leads one to expect a successful completion of this project and acquisition of a large body of unique experimental and prediction data which will be of use to specialists concerned with the problem of retention of core melt during severe accidents at NPPs.

The participants of the project were previously involved in a number of State-run scientific and technical programs. The techniques and methodology employed in these programs will be applied to investigations under this project.

1. Development of sacrificial materials.

2. Development of the technology of brickwork masonry of sacrificial and refractory materials.

3. Physical and mathematical simulation of methods for easing the consequences of unlikely events in catchers. Development of a database of the physicochemical properties of proposed materials

The collaborators in this work will, in particular, discuss and coordinate the investigation program and apply the theoretical prediction procedures developed by us, as well as computer programs and experimental data, to the LSs they are developing. As a result, a comparison of all available data will help considerably expand joint scientific and technical data, obtain more reliable results and, in the end, advance further toward the development of reliable and efficient systems for localization of core melt under conditions of severe accidents at NPPs.

For permanent working contact with the collaborators, a Steering Committee will be set up, formed of representatives of the project executors and collaborators.


Back