Reactivity Burst in VVER Reactor
Study of the Conditions for the Reactivity Burst Occurrence at the Reflood Phase of a Loss-of-Coolant-Accident in VVER-type Reactors
Tech Area / Field
- FIR-NSS/Nuclear Safety and Safeguarding/Fission Reactors
3 Approved without Funding
Kurchatov Research Center, Russia, Moscow
- VNIITF, Russia, Chelyabinsk reg., Snezhinsk\nVNIIEF, Russia, N. Novgorod reg., Sarov
Project summaryAims of the Project
- The project aims at the elaboration of the best-estimate mathematical model for the effective correlated neutron-physical and thermal-hydrodynamic calculations of the nuclear reactor core under reactivity initiated accident (RIA) conditions.
- Current project supposes investigations based on a comprehensive approach to RIA in a time moment after the beginning of reflood or in the cold reload state, contrary to the common analysis of accidents with water loss in WER cores, when a lot of efforts in studies are devoted to melting processes and corium-vessel interactions.
- The main attention is given to the creation of such mathematical models of the processes in the nuclear reactor, where the calculation accuracy is limited only by the uncertainty in knowledge of material properties (in the absence of other uncertainties). These models must approach in the calculation accuracy to the precise models and in the computing costs - to the engineering ones.
One of the mathematical models satisfying these requirements is the unique approach developed in the Kurchatov institute and known as a Surface Harmonics Method (SHM). The SHM permits to thoroughly take into account reactor heterogeneities, to analyze main calculation stages and on the basis of this analysis to choose a reasonable approximation at each stage. The SHM has a possibility of the sequential refinement of the approximation.
- The direct purpose of the primary application of the created tool is the analysis of the most dangerous accidents connected with reactivity burst at the reflood of the core in WER-type reactors.
The created tool could be useful also for the more wide range of accident types and for the PWR-type reactors that may be of interest for the scientists from other countries.
- On the basis of calculation results, suggestions for the performance of experiments enabling to verify the calculation codes and to simulate the accident situations in nuclear reactors will be developed.
Technical approach and methodology
1. The existent calculation tool is rather poorly suitable for the analysis of RIA of the mentioned type, because each code complex contains as a part exploitation or engineering codes using certain adjustment parameters obtained for a certain reactor and, moreover, in the vicinity of its exploitation range (where the corresponding experimental data can be found). But for the description of accident conditions such reactor states should be also described which differ a lot from the basic ones (e.g. the great change in the neutron spectrum hardness under LOCA conditions).
2. The Surface Harmonics and Surface Pseudo-Sources Methods, suggested for the neutron-physical reactor calculations are aimed at the elaboration of the finite-difference equations derived directly from the transport equation without the usual homogenization stages and without use of adjustment parameters.
Stationary calculation and experimental benchmarks verify the Surface Harmonics Method (SHM). It is shown that in suitable approximations an essential improvement of the calculation accuracy is obtained practically without increase in the expenditures.
3. It is expected that enhancement could be reached also at the analysis of accident conditions extending the SHM to the nonstationary neutron-physics and thermal-hydrodynamic problems.
4. It is necessary to carry out the neutron-physical calculations together with the thermal-hydrodynamic and other calculations. One of the weak points of existent various models and codes for the thermal-hydrodynamic calculations of the different reactors are the insufficiently exact account of the heat and mass transfer between the cells. It is possible to overcome these deficiencies of the thermal-hydrodynamic calculations by an application of the main ideas of the SHM.
5. Besides the elaboration of the calculation tool (with a maximum usage of existing codes) it is supposed that suggestions on the verification experiments simulating accident conditions will be prepared.
Due to the complexity of the verification of the codes describing the accidents, it is extremely difficult to carry out an experiment simulating certain accident accurately enough. Therefore, the requirements to the verification of the separate modules of the calculation code usually are very rigid. Though even after the completion of such a procedure it is desirable to carry out the experiments testing the workability of the code as a whole.
The staff of our researcher group accumulates the excellent knowledge in the field of reactor analysis and the highest mathematical qualification required for the fulfillment of all the listed project steps.
1. Analysis of possible scenarios for the accidents of the mentioned type and primary substantiation of the present interest of their consideration for the reactor design and operation.
2. Deepening of the analysis of the existing calculation tools, specification of the items where the mathematical models for neutron-physical, thermal-hydrodynamic and other processes must be essentially improved.
3. Development of the separate code modules required for the more exact calculations. Elaboration of the initial variant of the code system intended for the description of the accidents of the considered type with the extensive use of existing and verified codes.
4. Execution of the first improved calculations of the neutron-physical and thermal-hydrodynamic processes at the reflood phase of a LOCA.
5. Preparation of the suggestions on the experiments necessary for the verification of the separate calculation modules of the complex code.
6. Conceptual development of the informative experiment required for the verification of the whole complex code and the elaboration of an international standard problem on its basis.
7. Preparation of the program for future studies.
The work results will be the mathematical model and the experimental version of a code package for the effective neutron-physical and thermal-hydrodynamic calculations of the nuclear reactor core approaching in the calculation accuracy to the precise models and in the computing costs - to the engineering ones.
In addition to the direct purpose of the primary application for the analysis of the most dangerous accidents connected with reactivity burst at the reflood of the core in the WER-type reactors, the created tool could be useful also for the more wide range of accident types and for the PWR-type reactor, that may be of interest for the scientists from other countries.
The suggestions for the performance of experiments enabling to verify the calculation codes and to simulate the accident situations in nuclear reactors will be developed.
Potential role of foreign collaborators
Evidently, the fulfillment of the described studies in the full volume requires efforts of a significant number of scientific collaborators from many countries and an extensive financing. So, our researcher group considers that within the temporal and financial frameworks, possibly available from the ISTC support, the works in the context of the current project will be carried out as the preparatory studies.
We expect that a number of foreign scientific and regulatory organizations will be interested in the improvement of existing calculation tool for RIA studies, both from the theoretical and from practical (implementation for analysis) points of view. This includes discussions of approaches, definition of specific cases and verification of codes.
Among these organizations could be, e.g., Westinghouse El. Corp. in USA, FRAMATOME in France, Siemens and GRS in Germany, PNC in Japan.
The International Science and Technology Center (ISTC) is an intergovernmental organization connecting scientists from Kazakhstan, Armenia, Tajikistan, Kyrgyzstan, and Georgia with their peers and research organizations in the EU, Japan, Republic of Korea, Norway and the United States.
ISTC facilitates international science projects and assists the global scientific and business community to source and engage with CIS and Georgian institutes that develop or possess an excellence of scientific know-how.