Conference Agenda

Swiss NPPs undergo a Periodic Safety Review (PSR) every ten years. In this framework, the deterministic safety analyses must be updated following the requirements of the regulatory body ENSI. In September 2018 ENSI put a revision of the guideline for technical safety analyses (ENSI-A01) into effect. As a result, new events must be analyzed at DBA level (safety level 3) and BDBA level (safety level 4a). Besides, the fulfilment of the safety goals (acceptance criteria) must be proved also for operating conditions other than full power (such as zero-power or start and shutting-down conditions).

In preparation of the next PSR, the Gösgen NPP, a 3-Loop PWR; is working in tight collaboration with its vendor Framatome GmbH to evaluate and update the existing accident analyses. The present paper reports on the main findings from the new safety analyses, which are being carried out with the proprietary system code S-RELAP5. Attention is given on the update of the plant operational procedures, which are optimized for increasing safety margins and reducing the radiological impact of the accident.

A SBLOCA (break of a measuring line connected to main coolant line or pressurizer) is analyzed implementing a new fast secondary-side cooldown, which allows the stable shutdown without core uncovering (DBA). Operational procedures have been optimized in case of SGTR, preventing the interruption of natural circulation in the affected loop (DBA). New analyses have been performed for ATWS sequences (BDBA). The importance of operator measures is highlighted in the accident mitigation to reach the cold-shutdown state.

Helical coil steam generators play a critical role in the operation of small modular reactors, primarily due to their superior heat transfer capabilities. However, the intricate design of these systems makes it challenging to fully understand the boiling phenomena occurring within the helical coil tube, which is crucial for reactor safety. During boiling, it is common to observe oscillatory flows, which can have a substantial impact on the operating conditions of the reactor and introduce potential safety risks. While thermal-hydraulic system codes have been widely used over the past decades, they often fall short of accurately capturing reverse flows, staggered grid issues, and simplistic spatial discretization. These limitations might result in discrepancies between experimental data and code predictions.

In response to these challenges, a new system code (in-house code for helical coil steam generator) is currently under development, designed to bridge the gap between theory and experimental observation. The goal of this enhanced approach is to provide a more accurate representation of the oscillatory movements induced by two-phase flow within the helical coil tube. By improving the depiction of complex bubble dynamics, this new system code aims to advance the understanding of these oscillations, ultimately contributing to more effective reactor safety analysis and providing a solid foundation for the validation of reactor designs.

The NOUS is a Python™ based software which is capable of setting the initial plant data for the VVER-1200 type reactor for coupled thermo-hydraulic and neutron kinetic (TH-NK) calculation. The program can define the thermo-hydraulic (TH) parameters both for the primary and secondary circuit and can vary the spatial discretization.

The neutron kinetic coupling can be selected between point kinetic, 3D Fuel Assembly (FA) or even pin-wise resolution.

A further option is that the user can choose the availability of the safety and non-safety system trains during an initiating event and the corresponding safety functions.

The software can be used for testing of maneuvering modes, for deterministic safety analysis and for functionality analysis, as well.

Airborne radioactive materials are inevitably released through stacks during nuclear power plant operation. Accurate monitoring is crucial to assess environmental impact and ensure regulatory compliance, as it depends on samples that accurately represent stack radioactivity.

According to the ISO 2889-2023 standard, the study simulated the flow field of a 5:1 scaled model of a stack. The diameter and the height of the scaled model are of 0.6 meters and 12 meters. The gas carrying aerosols in the scaled stack model enters through a horizontal mixing pipeline, with the Reynolds number ranges from 400,000 to 700,000 in the vertical main stack. The Standard k-epsilon Model was employed to calculate the flow field, while the Species Transport Model was used to simulate the mixing processes of tracer gas and air. After achieving convergence, the Discrete Phase Model was employed to compute the trajectories of aerosol particles, thereby obtaining the characteristics of the motion and distribution.

By analyzing the airflow streamlines, aerosol particle trajectories and contour plots at different sampling sections, the study reveals the flow characteristics, the concentration distribution patterns of tracer gases and aerosol particles. The results were further processed to evaluate whether the sampling sections at different elevations meet the well-mixed criteria based on five parameters: average resultant angle, velocity variation coefficient, maximum tracer gas concentration deviation, tracer gas concentration variation coefficient and aerosol particle concentration variation coefficient. The findings of this study serve as a reference for selecting sampling sections and evaluate the mixing performance of the model.