Safety assessment of next generation nuclear systems: methodology development and case studies on fission and fusion devices

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CFD modelling of pressurized gas releases: sensitivity analysis of driving parameters

The consequences analysis is a crucial aspect of the Risk Assessment, especially for Oil & Gas structures, where hundreds of accidental scenarios must be simulated. This work investigates the accidental release of high-pressure flammable gas in a congested offshore environment considering multi-physics and multi-scale nature of the phenomenon. Initially, the flow results supersonic with compressible effects and then it evolves in a subsonic dispersion. To handle this change of physics, a Computational Fluid Dynamics (CFD) two steps approach is developed at the SEADOG laboratory in Politecnico di Torino. This approach imposes two simulations: the first one considers the compressible phenomena in a small domain called Source Box (SB), the second one considers the gas dispersion in the platform. The advantage is to use the results of the first simulation as an input for several dispersion simulations. The aim is to compile a library of plausible SB and to evaluate the consequences of an accidental scenario selecting the proper SB for the dispersion simulation, allowing a timesaving. This work is focused on the optimization of the number of SB to construct a SB library. The objective of this work is to achieve a sensitivity analysis on the input parameters (release pressure, hole diameter, distance and dimension of an obstacle inside the SB) in order to optimize the number of SB to be simulated reducing the computational effort

Italian Offshore Platform and Depleted Reservoir Conversion in the Energy Transition Perspective

New hypotheses for reusing platforms reaching their end-of-life have been investigated in several works, discussing the potential conversions of these infrastructures from recreational tourism to fish farming. In this perspective paper, we discuss the conversion options that could be of interest in the context of the current energy transition, with reference to the off-shore Italian scenario. The study was developed in support of the development of a national strategy aimed at favoring a circular economy and the reuse of existing infrastructure for the implementation of the energy transition. Thus, the investigated options include the onboard production of renewable energy, hydrogen production from seawater through electrolyzers, CO2 capture and valorization, and platform reuse for underground fluid storage in depleted reservoirs once produced through platforms. Case histories are developed with reference to a typical, fictitious platform in the Adriatic Sea, Italy, to provide an engineering-based approach to these different conversion options. The coupling of the platform with the underground storage to set the optimal operational conditions is managed through the forecast of the reservoir performance, with advanced numerical models able to simulate the complexity of the phenomena occurring in the presence of coupled hydrodynamic, geomechanical, geochemical, thermal, and biological processes. The results of our study are very encouraging, because they reveal that no technical, environmental, or safety issues prevent the conversion of offshore platforms into valuable infrastructure, contributing to achieving the energy transition targets, as long as the selection of the conversion option to deploy is designed taking into account the system specificity and including the depleted reservoir to which it is connected when relevant. Socio-economic issues were not investigated, as they were out of the scope of the project

Identification of accident sequences for the DEMO plant

Safety studies are performed in the frame of the conceptual design studies for the European Demonstration Fusion Power Plant (DEMO) to assess the safety and environmental impact of design options. An exhaustive set of reference accident sequences are defined in order to evaluate plant response in the most challenging events and compliance with safety requirements.The Functional Failure Mode and Effect Analysis (FFMEA) has been chosen as analytical tool, as it is a suitable methodology to define possible accident initiators when insufficient design detail is available to allow for more specific evaluation at component level. The main process, safety and protection functions related to the DEMO plant are defined through a functional breakdown structure (FBS). Then, an exhaustive set of high level accident initiators is defined referring to loss of functions, rather than to specific failures of systems and components, overcoming the lack of detailed design information. Nonetheless reference to systems or main components is always highlighted, as much as possible, in order to point out causes and safety consequences. Through the FFMEA a complete list of postulated initiating events (PIEs) is selected as the most representative events in terms of challenging conditions for the plant safety. All the four blanket concepts of the European DEMO reactor have been analysed

Safety assessment: perspectives for next generation nuclear plants

Safety assessment and risk analysis are recognized as a priority in the development of next gen-eration nuclear systems (Generation-IV reactors and full-scale fusion reactor –DEMO-) and demand a recon-sideration of the safety philosophy currently applied to the existing nuclear stations. Since their innovative physics and technology and the preliminary design phase of some of the concepts, their safety assessment has to rely on the basis of nuclear safety and technological neutral methodology. In order to satisfy this necessity, a bibliographic survey on nuclear and non-nuclear international standards and best practices is performed. By comparing them, this work tries to reach a new and more systematic approach, based on functional safety, suitable for dealing with the unique challenges of the innovative nuclear facilities, in order to guarantee that safety achievement is intended to be “built-in” rather than “added-on” by influencing the concept evolution from its earliest stages

CFD modelling of an accidental pressurised gas release

Risk assessment usually requires to simulate hundreds of different accidental scenarios in order to identify the most potentially critical events. The objective of this work is to improve and optimize the use of a Two Steps CFD model in order to minimise the number of simulations needed this is achieved thanks to a sensitivity analysis on the main parameters characterising a release event in a typical congested industrial environment

Models and tools for the simulation of exhaust dispersion in oil and gas offshore platforms

In the framework of worker safety on offshore oil&gas platforms, the respect of Threshold Limit Values is particularly critical, due to the number of pollutant sources and to the peculiarly congested layout of the installation. For example, relevant sources of emissions are diesel engines and gas turbines that emit CO and NOx affecting workers health and causing temperature gradients, harmful for helideck operations. The present study is focused on the wind-obstacles interaction and its consequences on the pollutant dispersion in a platform deck, in order to improve the knowledge about tools and models that may be adopted to better predict the pollutant concentration and stagnation areas with different conditions of wind directions and intensities. This paper presents the results of the adoption different turbulence models, two of which have not been tested yet in offshore CFD simulations: LES and DES

Hydrogen leakages in a congested aircraft environment: a CFD simulation method

The option of using hydrogen as a fuel for propulsion of aircraft has been investigated in the recent past especially in combination with long endurance unmanned mission targets (Helinet, Helios). This application has proved to be challenging mostly due to the low volumetric density of hydrogen, which needs to be compressed at very high pressures to be confined in the narrow volumes allowed by aircrafts structures. Aerial, as well as automotive, applications of hydrogen pose also the issue of weight for the total storage solution adopted: high pressures may mean thick layers for vessels and consequently high weights for unit of mass of hydrogen stored. Composite materials have helped in reducing the weight but remain tough to be adopted for vessels large enough to store the hydrogen mass necessary for long trips. Liquid hydrogen has only been adopted so far for aerospace applications and just for boosting rather than for endurance. Instead, hydrogen can be efficiently used for fuelling auxiliary systems on board and for ground services, helping to reduce the environmental impact, also regarding the idling phase. Fuel cells that are supplied with hydrogen can provide the electricity needed by all the auxiliary equipment, from air conditioning systems, to controls and avionics, to lighting and security services. Although smaller quantities of hydrogen are needed on board to supply only the auxiliaries rather than for propulsion, still there is a need for pressurising the gas and so to have a pressurised feed line that runs into the congested environment of an aircraft where ventilation is anyway usually present. In view of the experience gained in the oil and gas offshore sector, where flammable and pressurised gases may be released due to failures in the feed lines, we propose an innovative approach to investigate the possible hazards deriving from the use of pressurised hydrogen in aeronautics. Hydrogen releases may happen due to failures all along the lines but, statistically, ruptures are more frequent in lower pressures sections that are potentially less protected. A hydrogen supply line can cover pressures that range from 350 bars of the storage vessel to the nearly ambient pressure when it supplies the fuel cells. This induces to take greater care of possible mid-pressure (10-15 bars) releases and of their consequences. Ruptures are seldom catastrophic, while more often they are represented by small diameter cracks. The release through these ruptures is supersonic and it soon slows down also due to the scattering with obstacles in the aircraft environment. Modelling of the entire phenomenon is a challenging task for Computational-Fluid-Dynamic analysts as some variables such as pressure, and therefore velocity, have too strong variations throughout the domain. Yet, CFD remains the best tool to predict the dispersion and possibly the dangerous (i.e. above the - very low- flammability limit of hydrogen) accumulation of gas. Our proposal is to split the phenomenon in two phases and to study them separately with a coupling based on the parameters that are more relevant to describe the evolution: velocity and concentration. First, the supersonic release of hydrogen from the rupture, and the consequent compressible effects, are modelled in a domain that is large enough to contain the deceleration of the gas up to dispersion-like rates: this domain is, however, smaller than the full domain where we wish to study the entire phenomenon. Second, data related to speed and concentration calculated on the surface of this domain are given as boundary conditions for the simulation of the dispersion phase. Preliminary applications of this method to hydrogen releases from 10 mm hole ruptures at a pressure of 10 bar have provided interesting results especially with the supersonic release phase, that is the most challenging for the CFD simulation due to the intrinsic characteristics of hydrogen as a very light gas. In particular, addressing the supersonic release phase may allow estimating the effect of impact of the jet release onto the first obstacle (in the form of thermal stresses). The final coupling of the two phases can provide a dispersion pattern within a congested environment that can be validated in field tests, in the same way as it is being done with this method applied to natural gas releases in offshore platforms

A novel approach to high-pressure gas releases simulations

Risk-relevant plants like Oil & Gas (O&G) or nuclear ones are subjected to strict safety regulations. Risk Assessment is mandatory for these plants, and the damage quantification is a crucial step which has to be carefully addressed. Nowadays the state of practice for consequences estimation entails the use of semi-empirical methods which permit a fast evaluation of the large number of accidental scenarios needed for a Quantitative Risk Assessment (QRA). However, in case of large and congested industrial environments like offshore platforms or equipment inside nuclear primary containment buildings, the aforementioned methods usually lead to an overestimation of the damage areas, for example because they neglect the space congestion which highly affects the accident evolution. A more accurate analysis can be performed using the Computational Fluid Dynamics (CFD), nonetheless its high computational cost represents an important drawback. In this work a CFD approach, called Source Box Accident Model (SBAM), is presented. It models high-pressure gas releases (> 10 bar) in congested environments guaranteeing a good computational cost-accuracy compromise. The aim of SBAM is to permit a fast consequences estimation (evaluation of flammable/toxic areas, etc.) via CFD, in order to have a simulations time compatible with the plants design schedule. The long-term objective is to integrate the CFD contribution in a safety driven design process. SBAM is based on the splitting of the multi-physics and multiscale phenomena characterizing the accident: the initial supersonic compressible release and the successive low speed dispersion. The first one is simulated in a small domain called Source Box (SB) and the second one in the case study domain. The two simulations are coupled in a suitable way, through proper parameters which are extensively discussed. This work presents a detailed description of SBAM and two different analyses: a sensitivity study on the coupling parameters and a numerical benchmark which uses a standard CFD simulation as reference. The sensitivity analysis shows that the coupling is a crucial step of the method and the coupling parameters must be treated in the most accurate way. The numerical benchmark shows that SBAM is not introducing significant errors with respect to a standard CFD simulation and in addition, permits a relevant simplification in the simulation setup and computational cost reduction

Models and tools for the simulation of exhaust dispersion in oil and gas offshore platforms

In the framework of worker safety on offshore oil&gas platforms, the respect of Threshold Limit Values is particularly critical, due to the number of pollutant sources and to the peculiarly congested layout of the installation. For example, relevant sources of emissions are diesel engines and gas turbines that emit CO and NOx affecting workers health and causing temperature gradients, harmful for helideck operations. The present study is focused on the wind-obstacles interaction and its consequences on the pollutant dispersion in a platform deck, in order to improve the knowledge about tools and models that may be adopted to better predict the pollutant concentration and stagnation areas with different conditions of wind directions and intensities. This paper presents the results of the adoption different turbulence models, two of which have not been tested yet in offshore CFD simulations: LES and DES