3. DESCRIPTION OF BENCHMARK
Most transients in a VVER reactor can be properly analyzed with a system thermal-hydraulics code, with simplified neutron kinetics models (point kinetics). A few specific transients require more advanced modeling for neutron kinetics for a proper description. A coupled thermal-hydraulics 3D neutron kinetics code would be the right tool for such tasks.
The proposed benchmark problem has already been analyzed by the coupled system code ATHLET-BIPR-VVER. This allowed a better fixing of the Benchmark Specifications. However, within the present context the results of participants will be compared against the measurements. Interesting additional problems have to be solved in order to perform correctly the comparisons. This experience is incorporated in the text of the specification.
The reference problem chosen for simulation is the MCP #1 switching off at nominal power when the other three main coolant pumps are in operation, which is a real transient of an operating VVER-1000 power plant. This event is characterized by rapid rearrangement of the coolant flow through the reactor pressure vessel resulting in a coolant temperature change, which is spatially dependent. This leads to insertion of spatially distributed positive reactivity due to the modeled feedback mechanisms and a non-symmetric power distribution. Simulation of the transient requires evaluation of core response from a multi-dimensional perspective (coupled 3D neutronics/core thermal-hydraulics) supplemented by a one-dimensional (1D) simulation of the remainder of the reactor coolant system. The purpose of this benchmark is four-fold:
- To verify the capability of system codes to analyze complex transients with coupled core-plant interactions and complicated fluid mixing phenomena.
- To fully test the 3D neutronics/thermal-hydraulic coupling.
- To evaluate discrepancies between predictions of the coupled codes in best-estimate transient simulations with measured data.
- To perform uncertainty analysis having at disposal not only the measured values but also their accuracy
The benchmark includes a set of input data for the NPP Kalinin-3 and consists of four exercises:
Exercise 1 - Point kinetics plant simulation
The purpose of this exercise is to test the primary and secondary system model responses. Provided are compatible point kinetics model inputs, which preserve the axial, and radial power distribution, and CR #10 and #9 reactivity obtained using a 3D code neutronics model and a complete system description.
Exercise 2 - Coupled 3-D neutronics/core T-H response evaluation
The purpose of this exercise is to model the core and the vessel only. Inlet and outlet core transient boundary conditions are provided by the benchmark team on the basis of calculations performed with the ATHLET-BIPR-VVER coupled code system: alternatively the participants can apply the measured data. HFP state (Exercise #2a) of the core is required for comparison.
Exercise 3 - Best-estimate coupled code plant transient modeling
This exercise combines elements of the first two exercises of this benchmark and represents an analysis of the transient in its entirety. For participants that have already taken part in the Kozloduy-6 NEA/OECD Benchmark , it is suggested to start directly with this exercise. As a preliminary step for these latter participants it is recommended to perform steady state core calculations at HZP state (Exercise #3a), HFP (Exercise #3b) and deliver the results for comparisons. Exercise #3a and Exercise #3b will ensure and check out the correct application of the cross section libraries, the core loading and the core design geometry.
Exercise 4 - Performing of uncertainty analysis for the purpose of Phase-III
(System Phase) of the OECD Benchmark for Uncertainty Analysis in Best -Estimate Modelling (UAM) for Design, Operation and Safety Analysis of LWRs.
The aim and the specification of this exercise will be described in a separate volume, which will depict the state of the art of the results and requirements identified after performing the UAM Phases I and II.
The specification document (Edition 1) that covers Exercises 1-3 of the OECD Kalinin-3 VVER-1000 Coupled Code Benchmark and the corresponding experimental database and report is available to participants.