Conference Agenda

Severe accidents (SA) in nuclear power plants (NPPs) pose critical safety challenges due to the potential release of radioactive aerosols, particularly various iodine species. This study aims to develop advanced filtration technology to reduce the release of fission particles into the atmosphere during SA. Experimental tests were performed to measure the electrostatic precipitator’s (ESP) reduction efficiencies using caesium iodide (CsI, 5 g/l) particles, generated via the droplet-to-particle method. Varying electric voltages were applied across the ESP electrodes depending on the total flow rate to optimize the reduction efficiency. After optimising the operating parameters, the ESP achieved a mass and number filtration efficiency of ~90% for CsI particles, effectively capturing particles in the size range of 0.04–0.5 µm. These findings demonstrate the potential of optimised ESP technology in significantly enhancing NPP safety by effectively capturing radioactive aerosols during SA scenarios.

Estimates from the measurements after Fukushima accidents suggest that substantial part of the
total radioiodine release to environment was formed by volatile compounds, I2 and organic iodides.
The computational efforts to guess on the iodine behavior at an accident are very difficult to carry
out. For example, serious attempts to do such calculations integrally -like those performed in the
OECD BSAF-2 Project for Fukushima- were all limited to modeling of just aerosol forms of iodine.
As “almost anything” can influence the volatile iodine behavior at an accident, we now would like to
know which processes and parameters are really crucial and what might be possibly abandoned. This
cannot be resolved through thousands of runs of one particular containment iodine code (with a
different code, the results will be different). In this work we use engineering estimates, iodine codes,
and methods from the PRA.
We try to identify the processes and circumstances which do matter at the prototypical conditions.
We employ some dedicated Fukushima analyses, both the already published ones and also our own,
new calculations. Secondly, selected experiments and their simulations are analyzed, including
PHEBUS FTP-1 test which is an illustrative example of how the huge uncertainties in the iodine
modeling can influence the interpretation of an experiment.
The findings clearly indicate that the modeling of the volatile iodine formation and behavior is
needed for the basic understanding of what is happening with iodine. However, the focus on really
relevant processes and parameters is necessary. Otherwise we are lost in the plethora of
phenomena, the effects of many are easily smeared out given the significant uncertainties linked to
the iodine analysis.

During nuclear power plant accidents, substantial radioactive materials accumulated in the reactor core may release into the containment or environment, posing radiological hazards. Notably, 60–70% of the core-accumulated iodine inventory is released, with iodine-based aerosols and vapor representing dominant forms of fission products during such events. As a critical severe accident mitigation measure, nuclear power plants employ containment spray systems to depressurize the containment atmosphere and remove suspended radioactive fission products. Investigating the removal characteristics during spray processes is of significant importance for understanding the elimination mechanisms of radioactive iodine-based fission products under accident conditions. This study conducted a series of spray removal experiments using independently developed aerosol and iodine behavior experimental platforms. Results demonstrate that the removal processes of aerosols and iodine vapor generally follow an exponential decay pattern. The spray system rapidly removes aerosols and iodine vapor, with aerosol removal efficiency increasing proportionally to spray flow rate. Smaller droplet sizes resulted in higher removal efficiencies. Additionally, this work analyzed the iodine removal dynamics during spray processes and the speciation of iodine within the containment sump.

As the deployment of Small Modular Reactors (SMRs), particularly integral Pressurized Water Reactors (iPWRs) advances, understanding the behavior of radioactive releases under severe accident (SA) conditions is critical to ensure safety. This study investigates the source term during a selected set of SA scenarios, involving LOCA and non-LOCA type events, occurring in a submerged containment type of iPWR and evaluates the implications for Emergency Planning Zones (EPZs). Unlike traditional reactors, iPWR SMRs feature integrated primary systems, reduced reactor size, and passive safety mechanisms, which impact source term behavior and, subsequently, EPZ requirements. MELCOR is used in this study to develop conservative source terms estimates and MACCS is used to calculate radiological consequences. This research examines potential release pathways, the performance of passive safety systems, and containment responses. The study quantifies key parameters such as fission product release rates, containment retention effectiveness, and potential environmental impact, focusing on how these factors influence EPZ size and scope. It is observed that, with the Swedish dose criterion of the current regulatory framework, iPWRs do not necessitate a precautionary action zone (PAZ), but an urgent protective action planning zone (UPZ). Even in the most severe accident scenario, the UPZ is around 16.1km. Comparative analysis with conventional reactors identifies enhanced containment capabilities of iPWR SMRs, suggesting the potential for smaller EPZs.

Accurate assessment of radioactive source terms following severe accidents of nuclear power plant is critical for off-site consequence analysis and calculation of radioactive release frequencies. Currently, worldwide research institutions have conducted plenty of studies on uncertainty and sensitivities of nuclear power plant radioactive source terms based on Best Estimate Plus Uncertainty (BEPU) analysis method; however, studies on the impacts caused by actions from the Severe Accident Management Guidelines (SAMG) are limited. This paper first develops a procedure for uncertainty and sensitivity analysis of radioactive source terms for nuclear power plant (NPP) based on BEPU method, especially considering inclusion of SAMG actions. Secondly, integrated severe accident analysis code MAAP was used to build a model of China typical two-loop nuclear power plant and 200 MAAP input cases using Latin hypercube sampling method were generated through own developed sampling code. Then 200 cases were run by MAAP and statistical evaluated by self written code, and uncertainty and sensitivity analysis was performed. Uncertainty analysis results indicate that all selected parameters have certain influences on release source terms in containment intact release category; and sensitivity analysis shows that the impact of SAMG actions may far larger than those from severe accident phenomena or model uncertainties. This study highlights the importance of selecting accident sequences for release categories with particular attention to SAMG action uncertainties during source term calculation. The research provides valuable references for selecting representative accident sequences and conducting uncertainty/sensitivity analysis of radioactive releases following severe accidents of nuclear power plants.