AN INTEGRATED APPROACH TO ACCIDENT ANALYSIS IN PWR

The purpose of the present doctoral research is to provide a contribute to the TH-SYS codes assessment for nuclear reactor applications, and particularly for the predictive analysis of PWR transient behavior, such as MSLB accident scenario, in which strong coolant flow asymmetries and multi-dimensional turbulent mixing effects strongly influence two relevant safety issues: recriticality and PTS. The contribution consists in proposing and developing of an integrated analytical approach for TH-SYS code (TRACE-V5) assessment in relation to the investigation of the coolant transient flow processes in the reactor coolant system. The developed approach is focused on set up a methodology able to assess the accuracy of the numerical predictions based on the use of reliable experimental database that covers all relevant thermal hydraulic processes observed to occur simultaneously at system and component phenomenological levels during a selected accident scenario in a PWR system. To achieve this goal the first phase was to perform an independent assessment through the formulation of an independent assessment matrix for two classes of tests (basic test and integral effect test). The aim was to evaluate the capabilities of the code and models in reproducing the relevant thermal-hydraulic phenomena which characterize the simulated experiments. The second phase has concerned the development of the integrated methodology by defining a specific transient scenario important for PWR system safety (namely MSLB). Once the specific scenario has been identified, the methodology was oriented to define the relevant phenomena and processes that drive the system response. After the definition of all phenomena and interactions during the selected scenario, a corresponding process for establishing a test matrix was developed. The construction of the test matrix was carried out identifying a set of tests performed in integral and separate effects tests facilities achieved in a complementary way. Finally, the suitability of code in predicting the results of the complementary tests was obtained splitting the quantification of the accuracy in two phases. The first phase concerned the evaluation of the accuracy in a integral sense that is assess the code results at the system level (analysis of the overall thermal hydraulic response of the PWR system) against experimental data of the integral test performed in PKL-III facility. The second phase was oriented to measure the code discrepancies focusing the attention to the component level phenomena identified in the PWR system during the accident scenario under investigation and not experimental captured by the integral test. This last phase is connected, from the experimental point of view, with the tests carried out in the ROCOM test facility. In this way it is possible to cover experimentally the overall spectrum of phenomena expected to occur during the MSLB transient and assess the computational results using the same code (TRACE-V5 TH-SYS code) to simulate both tests: integral and separate effects tests. Therefore, the methodology addresses also the issue of the validation of the 3-dimensional modules existing as an option in the codes like TRACE-V5 in simulating complex multi-dimensional flow patterns, such as mixing flow present in the RPV during the transient scenario (MSLB). In view of the methodology goals, the work is supported by code validation and application results obtained in the frame of OECD/NEA CSNI PKL-2 project. It consists of an experimental program of eight tests (G series) carried out in integral test facility. The third test, identified as G3.1, has been selected for the application of the integrated approach, since the results of this PKL test, which is oriented PWR system behavior, also provide the boundary conditions for complementary tests in the ROCOM facility on mixing cold and hot water in the RPV downcomer as well as in the lower plenum and for determining the fluid state at the core inlet. Within the proposed approach, the relevant modelling issues are identified and discussed, so as to point out the main capabilities and limitations in the present state-of-the-art tools and methods.