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We focus our attention on the numerical simulations of compressible flows obtained by using Finite Difference in time /Finite Element in space approximation. In particular, we determine optimal explicit Runge-Kutta methods capable to maximize the stability features of the resulting numerical scheme. Two different regimes characterized by low and moderate Mach numbers have been taken into account. In the former regime, we have determined an explicit Runge-Kutta method of fourth order that is approximately 15% more efficient than classical ERK(4 , 4) schemes. For moderate Mach numbers, M a ≈ 0.4 , and transitional Reynolds numbers we have determined ERK schemes that outperform classic ERK (3 , 3) or ERK (4 , 4). Optimal ERK have a reduced CFL approximatively four or five times larger than classical ones. These optimized ERK schemes are then promising for the study of transitional flows for global stability or transient growth analyses. [ABSTRACT FROM AUTHOR]

Stereotactic body radiotherapy (SBRT) is a form of radiotherapy (RT) classically delivered in ≤ 10 fractions. In last years, SBRT has evolved to have a wide range of clinical applications, from curative-intent in the primary disease to the control of oligometastatic disease. Mediastinal and hilar lymphadenopathies (MHL) are a frequent pattern of cancer spread, especially in the lung malignancies. Recently, there has been interest in the application of SBRT for MHL in order to improve Local Control (LC) and achieve shorter treatment durations to minimize systemic treatment breaks. Aim of this study is to evaluate LC, Loco Regional Nodal Control (LRNC), Distant Nodal Control (DNC), Distant Metastasis Free Survival (DMFS), Progression Free Survival (PFS) and Overall Survival (OS) of patients treated on MHL by SBRT and predictive factors of response. This retrospective multi-institutional study analyzes a group of patients treated by SBRT on MHL. From to 2007 to 2023 we have evaluated 127 patients with 159 MHL. The median age was 67 years. The single fraction was administered in the 16% of cases and multiple fractions in the 84 % of cases. The schedule most represented was 48 Gy in 6 fractions (range 23-60 Gy in 1-10 fractions). The medium BED(10) was 75 Gy (range: 37.5-120 Gy). The primary most represented was lung in 57%. Thirty-eight percent of MHL were classified as oligorecurrent lesions, 35% oligoprogressive, 14% oligometastatic, 13% oligopersistent. The LC was reached in 81% of MHL treated by SBRT with rates at 1-3- and 5- years of 80%, 75% and 75% respectively. At univariate analysis we have appreciated a statistically significant correlation with LC and the number of fractions, with better results whit the single fraction (p: 0.0018). The rates of LRNC at 1-3- and 5- years were 72%, 66% and 50% respectively, whit a statistically significant correlation with BED(10) >75 Gy and the state of metastasis (oligorecurrent disease have better results), p: 0.0001 and 0.0011 respectively. The rates of DNC at 1-3- and 5- years were 82%, 75% and 68% respectively, whit a statistically significant correlation with BED(10) >75 Gy and the single fraction, p: 0.0001 and 0.0396 respectively. The rates of DMFS at 1-3- and 5- years were 50%, 40% and 24% respectively, and the rates of PFS at 1-3- and 5- years were 32%, 19% and 9% respectively. At univariate analysis we have found a statistically correlation between the state of metastasis (oligorecurrent disease have better results) DMFS and PFS with a p value of 0.0022 and 0.0058 respectively. The rates of OS at 1-3- and 5- years of 66%, 41% and 28% respectively. The best response in terms of complete, partial and stable response was maintained in a medium of 9 months (range: 1-68 months). SBRT is an option for the treatment of MHL, with high rates of LC and LRNC able to improve the rates of progression free survival and survival. More studies need to better understand the predictive factors of response. [ABSTRACT FROM AUTHOR]

The OECD/NEA proposes a new international benchmark based on data collected during an instability transient occurred at the Oskarshamn-2 NPP. This benchmark is aimed at testing the coupled 3D Neutron Kinetic–Thermal Hydraulic (3D NK–TH) codes on challenging situations. The ENEA “Casaccia” Research Center, is participating to this benchmark, developing a computational model using the RELAP5-3D© code. The 3DNK model has been already developed from the cross sections dataset calculated by OKG, the Oskarshamn-2 licensee, through the CASMO lattice code. In order to use this neutron cross sections database in RELAP5-3D© a n -dimensional polynomials data fitting and base cross sections values calculations are required. An ad-hoc tool, named PROMETHEUS, has been developed for automatically generating RELAP5-3D©-compatible cross sections libraries. This tool allows at easily visualizing the complex structure of the neutronic datasets; moreover it is exploited for deriving different cross sections libraries needed to evaluate neutronic parameters effects on the reactor instability prediction. Thus, the effects of the fuel temperature and control rod histories, of the discontinuity factors (averaged/not averaged) and of the neutron poisons has been assessed. A ranking table has been produced, demonstrating the relevance of the not-averaged discontinuity factors and of the on-transient neutron poisons calculations for the correct prediction of the Oskarshamn-2 event. [ABSTRACT FROM AUTHOR]

• A DET analysis has been applied to a BWR during a hot shutdown phase. • RAVEN acts as a logic driver while MELCOR simulates the thermal–hydraulic response. • Cyberattack alters plant state variable processed by I&C disabling coolant injection systems. • DET focuses on cyberattack and recovery time that would compromise core integrity. Over the last decade, probabilistic risk assessment (PRA) for nuclear power plants shifted its focus toward simulation-based methods of evaluating safety and risk aspects. Classical reliability modeling based on static Boolean structures does not capture the dynamic nature of plants experiencing accident sequences. Dynamic PRA extends conventional reliability modeling, adding a higher degree of variability to events occuring during accident sequences. This paper demonstrates the capability of a new dynamic event tree (DET) analysis tool applicable to severe accident sequences. A cyberattack was assumed to penetrate the instrumentation system of a boiling water reactor during its hot shutdown phase, altering the thermal–hydraulic variables being processed. The analysis focused on scenarios that lead to the core being uncovered, in which the peak cladding temperature is reached due to the disabling of the cooling systems. A limiting surface is defined, associating the cyberattack timing to the recovery time limit that would compromise the core integrity. [ABSTRACT FROM AUTHOR]

The Management and Uncertainties of Severe Accidents (MUSA) project, funded in HORIZON 2020 and coordinated by CIEMAT (Spain), aims at consolidating a harmonized approach for the analysis of uncertainties and sensitivities associated with Severe Accidents (SAs) focusing on Source Term (ST). In this framework, the objectives of the Innovative Management of Spent Fuel Pool Accidents (IMSFP – WP6), led by IRSN (France), are to quantify and rank the uncertainties affecting accident analyses in a Spent Fuel Pool (SFP), to review existing and contemplated SA management measures and systems and to assess their possible benefits in terms of reduction of radiological consequences. To quantify the propagation of the uncertainties of the input parameters to the output uncertainties of severe accident codes (ASTEC, MELCOR, RELAP/SCDAP), a diverse set of uncertainty quantification (UQ) tools (DAKOTA, RAVEN, SUNSET, SUSA) are used. The statistical framework used by the different UQ-tools is similar e.g. pure random (Monte Carlo) and Latin hypercube sampling (LHS). Fourteen partners from three different world regions are involved in the WP6 activities. The target of this paper is to describe the achievements during the first three years of the project. In a first part, a description is given of the SFP accidental scenario, of the key target variables and radionuclides chosen as ST Figures of Merit (FoM) and of the identified uncertainty sources in models and input parameters. A key element when defining the SFP scenario has been the consideration (or not) of the reactor building, as it is expected to significantly affect analyses. In a second part, the first insights coming out from the calculation phase of the project are presented. The review of existing SA management measures is also exposed, as well as systems whose benefits will be assessed in the second phase of the project. Finally, challenges that arise from such an exercise are discussed, as well as major difficulties found when applying UQ methodologies to SFP scenarios and solutions adopted. [ABSTRACT FROM AUTHOR]

The U.S. Nuclear Regulatory Commission is currently proposing rulemaking to revise the Loss Of Coolant Accident (LOCA) and therefore the Emergency Core Cooling System (ECCS) acceptance criteria, to include the effects of higher burnup on cladding performance as well as to address other technical issues. As motivated by the new rule, the need to use advanced cladding designs may be a result. A loss of operational margin may result due to the more restrictive cladding embrittlement criteria. Initial and future compliance with the rule may significantly increase vendor workload and licensee cost, as a spectrum of fuel rod initial burnup states may need to be analyzed to demonstrate compliance. Consequently, there will be an increased focus on licensee decision making related to LOCA analysis to minimize cost and impact, and to manage margin. The study here presented has been part of a big project used to investigate technical issues and approaches for future industrial applications within the Risk-Informed Safety Margin Characterization (RISMC) Pathway. Specifically, the primary aim of this study is to lay out a roadmap to demonstrate the application of the new methodology. The present analysis shows a simplified version of the methodology of an industrial application on the Core Design and the Multi-Cycle Analysis. [ABSTRACT FROM AUTHOR]

• Experimental data on Heavy Liquid Metal flow in the CIRCE-HERO facility. • Numerical benchmark activity on Heavy Liquid Metal with SYS-TH codes and SYS-TH/CFD codes. • HERO double wall bayonet tube steam genarator thermal–hydraulic characterization. • CIRCE-HERO experimental campaign on protected loss of flow accident scenarios. In the framework of the HORIZON2020 SESAME European project, an experimental campaign has been carried out on the large Lead-Bismuth Eutectic pool integral effect CIRCE facility at ENEA Brasimone Research Center, implementing the HERO test section. This test section is aimed at supporting the development of the ALFRED design. Within the SESAME Project, three protected loss of flow accident tests have been designed and executed, identified as SE-Test1, SE-Test2, and SE-Test3. The third test (SE-Test3) has been selected by the project participants for a benchmark activity involving system thermal-hydraulic codes and coupled system thermalhydraulic/ CFD. This activity has been divided into two phases: a blind phase, and a post-test phase. The present paper illustrates the results achieved during the post-test phase of the CIRCE-HERO benchmark activity. Four participants simulated the experimental test. Three (ENEA, SCKCEN and UNIROMA1) used different versions of RELAP5 code, i.e. RELAP5-3D and RELAP5Mod3.3, and one (NRG) used a coupled approach based on an in-house system thermal-hydraulic code (SPECTRA) and the commercial code ANSYS CFX. The benchmark activity hereafter presented is a relevant exercise for evaluating and comparing the predictive capabilities of system thermal-hydraulic codes, CFD codes and coupled technics, in relation to phenomena occurring during the transition between forced and natural circulation (e.g. loss of flow) in a heavy liquid metals Generation IV system. The results of the numerical exercise showed an adequate capability of the codes to reproduce the relevant phenomena involved during the experiment. The SYS-TH codes have been more accurate than SYS-TH/CFD in predicting the trend of the main parameters during the transient, reproducing them with quite satisfactory accuracy. SYS-TH/CFD simulation, instead, provided a more satisfactory representation of the pool thermal stratification. The results highlighted that the planning and the execution of experiments fully devoted for the code V&V process is needed for the further development of such numerical tools, in particular for SYS-TH/CFD coupled approaches, whose use is more recent respect to SYS-TH codes. [ABSTRACT FROM AUTHOR]