700 research outputs found
Is the concentration of dark matter halos at virialization universal?
No full textSeveral recent studies suggest a correlation between dark matter halo mass and the shape of the density profile. We reanalyze simulations from Ricotti in which such a correlation was first proposed. We use a standard analysis of the halo density profiles and compare the old simulations to new ones performed with Gadget2, including higher resolution runs. We confirm Ricotti's result that, at virialization, the central log slopes a, at 5%-10% of the virial radius, are correlated with the halo mass and that the halo concentration is a universal constant. Our results do not contradict the majority of published papers: when using a split power law to fit the density profiles, due to the α - c degeneracy, the fits are consistent with halos having a universal shape with a = 1 or 1.5 and concentrations that depend on the mass, in agreement with results published elsewhere. Recently, several groups have found no evidence for convergence of the inner halo profile to a constant power law. The choice of the split power-law parameterization used in this Letter is motivated by the need to compare our results to previous ones and is formally valid because we are not able to resolve regions where the slope of the fitting function reaches its asymptotic constant value. Using a nonparameterized technique, we also show that the density profiles of dwarf galaxies at z ∼ 10 have a log slope shallower than 0.5 within 5% of the virial radius. © 2007. The American Astronomical Society. All rights reserved
IRIS-LIKE REACTOR CONFIGURATION AND MAIN PASSIVE SAFETY STRATEGY FOR A SUBMERGED SMR DEPLOYMENT
No full textКонцепция модульного реактора малой мощности (МРММ) интегральной компоновки, который эксплуатируется внутри капсулы/защитной оболочки под водой, т.е. в море или в искусственном озере, обладает очень интересными возможностями для повышения ядерной безопасности и заслуживает тщательного изучения. В последние годы эта тема исследуется в Миланском политехническом институте, начиная с концепции Flexblue, с последующим развитием анализа безопасности, направленного на определение базовой конфигурации МРММ и стратегии обеспечения его безопасности на основе пассивных систем. В докладе приведен анализ этих работ. На первом этапе мощность МРММ была определена с помощью кодов CFD путем исследования естественной конвекции морской воды вокруг нагретой капсулы с целью оценки мощности остаточного тепловыделения, которая может быть отведена пассивным способом через капсулу/защитную оболочку. Затем была определена предварительная конфигурация МРММ, исходя из первоначальной мощности IRIS – 1000 МВт (тепл.) и компоновки (общая высота 22 м) и их пересмотра; оборудование и системы первого контура были выбраны из соображений соответствия ограничениям подводной капсулы/защитной оболочки, т.е. по мощности остаточного тепловыделения и высоте капсулы 13 м. Принятая стратегия обеспечения безопасности является полностью пассивной и рассчитана на использование морской или озерной воды в качестве постоянного теплоприемника. В соответствии с уроками, извлеченными из фукусимской аварии, были рассмотрены два сценария референтных аварий – полное обесточивание (SBO) и длительное охлаждение после аварии с потерей теплоносителя (LOCA). Для этой цели применялся системный код Relap5. Результаты продемонстрировали успешные характеристики систем безопасности. Кроме того, были спроектированы две экспериментальных установки, предназначенные для валидации численных моделей. Заключительные замечания относятся к основным проблемам, с которыми придется столкнуться при развертывании подводных МРММ.The concept of an integral Small Modular Reactor (SMR) operating inside a hullcontainment located in a submerged environment, i.e. the sea or an artificial lake, owns very interesting potentialities for the improvement of nuclear safety and it is worth investigating it accurately. In recent years, Politecnico di Milano has addressed the topic, moving from a study on the Flexblue concept, then developing safety analyses aimed at defining a basic SMR configuration and its main safety strategy, based on passive systems. This paper reviews those activities. As a first step, the power output of the SMR has been identified with a CFD study on the natural convection of the sea water around the heated hull, aimed at estimating the decay power that could be passively rejected through the hull-containment. Then, a preliminary SMR configuration has been defined, moving from the original IRIS size (1000 MWth) and layout (e.g. 22 m total height) and revisiting it, as well as the primary components and systems, in order to fit the submerged hull-containment constraints, e.g. the decay power and the hull height (13 m). The safety strategy adopted is fully passive and relies on the sea or lake waters as the permanent heat sink. According to Fukushima lessons learned, two reference accident scenarios have been studied, i.e. the Station Black-Out (SBO) and the longterm cooling after a Loss Of Coolant Accident (LOCA). The Relap5 system code has been adopted. The results show the successful performances of the safety systems. Moreover, two experimental facilities aimed at validating the numerical models have been designed. Final comments refer to the main challenges to be faced by the submerged SMR deployment
Nuclear energy: basics, present, future
No full textThe contribution is conceived for non-nuclear experts, intended as a synthetic and simplified overview of the technology related to energy by nuclear fission. At the end of the paper, the Reader will find a minimal set of references, several of them on internet, useful to start deepening the knowledge on this challenging, complex, debated albeit engaging energy source
Transuranic Waste Burning Potential of Thorium Fuel in a Fast Reactor
No full textWestinghouse Electric Company (referred to as “Westinghouse” in the rest of this paper) is
proposing a "back-to-front" approach to overcome the stalemate on nuclear waste management
in the US. In this approach, requirements to further the societal acceptance of nuclear waste are
such that the ultimate health hazard resulting from the waste package is “as low as reasonably
achievable”. Societal acceptability of nuclear waste can be enhanced by reducing the long-term
radiotoxicity of the waste, which is currently driven primarily by the protracted radiotoxicity of the
transuranic (TRU) isotopes. Therefore, a transition to a more benign radioactive waste can be
accomplished by a fuel cycle capable of consuming the stockpile of TRU “legacy” waste
contained in the LWR Used Nuclear Fuel (UNF) while generating waste which is significantly
less radiotoxic than that produced by the current open U-based fuel cycle (once through and
variations thereof).
Investigation of a fast reactor (FR) operating on a thorium-based fuel cycle, as opposed to the
traditional uranium-based is performed. Due to a combination between its neutronic properties
and its low position in the actinide chain, thorium not only burns the legacy TRU waste, but it
does so with a minimal production of “new” TRUs. The effectiveness of a thorium-based fast
reactor to burn legacy TRU and its flexibility to incorporate various fuels and recycle schemes
according to the evolving needs of the transmutation scenario have been investigated.
Specifically, the potential for a high TRU burning rate, high U-233 generation rate if so desired
and low concurrent production of TRU have been used as metrics for the examined cycles.
Core physics simulations of a fast reactor core running on thorium-based fuels and burning an
external TRU feed supply have been carried out over multiple cycles of irradiation, separation
and reprocessing. The TRU burning capability as well as the core isotopic content have been
characterized. Results will be presented showing the potential for thorium to reach a high TRU
transmutation rate over a wide variety of fuel types (oxide, metal, nitride and carbide) and
transmutation schemes (recycle or partition of in-bred U-233). In addition, a sustainable scheme
has been devised to burn the TRU accumulated in the core inventory once the legacy TRU
supply has been exhausted, thereby achieving long-term virtually TRU-free
Real time active vision for the mobile robot PARIDEProceedings of IECON'94 - 20th Annual Conference of IEEE Industrial Electronics
No full textLONG-TERM SUMP NATURAL CIRCULATION IN A SUBMERGED SMALL MODULAR REACTOR
No full textThis work investigates the performances of the passive safety systems of a submerged and transportable Small Modular Reactor (SMR) after a Loss Of Coolant Accident (LOCA). The focus of the activity concerns the long-term period, addressing the feasibility of the “depressurized and flooded” safe state, i.e. a targeted situation where the reactor containment is flooded by the injection of water from a large safety tank. Decay heat is removed by sump natural circulation and rejected through the metal containment to the surrounding water, which acts as an infinite heat sink. Following the accidental event and the operation of the safety injection systems, the safe state is expected to provide a continuous and efficient cooling of the fuel rods, ensuring a potentially unlimited grace period with no electrical input or human intervention required. The study employs a numerical approach and simulations are performed using the 1D system code RELAP5. The nodalization process identifies three macro-components, i.e., the reactor pressure vessel, the reactor containment and the safety tank, connected by the recirculation lines. Conservative boundary and initial conditions are set to simulate the transient starting at 7h30’ after the scram of the reactor. Results assess the effectiveness of the sump natural circulation under the reference conditions. The main outcomes also show the good potentialities of the heat exchange systems, highlighting the safe operation of the passive safety systems for at least 21 days after the reactor scram. The sensitivity analysis identifies the nodalization of the reactor containment as a modeling and numerical issue, deserving further investigation.
KEYWORD
The application of nodal method for dynamic analysis of TRIGA Mark II
No full textIn this paper, Nodal dynamic method is introduced to model the TRIGA Mark II reactor of the University of Pavia as a dynamic system for the total operative power range (i.e. 0–250 kW) using a zero dimensional thermal hydraulic method. Neutronic and thermal hydraulic models are coupled in order to demonstrate reactor dynamic behavior. The reactor is divided into bi-dimensional zones and simulated in Serpent 2 to obtain two group cross sections for each specified zone as well as kinetic parameters. And they are applied in nodal method for the analysis of reactor dynamic behavior. Point kinetic and nodal dynamic methods are utilized for the second core configuration as the neutronic model. Different reactivities (i.e. 190, 104, 78, 40 pcm) are inserted by control rods in different power levels (i.e. 1, 50, 100, 150 kW) for the reactor dynamic analysis. A program is written in MATLAB to couple neutronic and thermal hydraulic models. A system of ordinary differential equations are produced and solved in space state model. The calculated and experimental power excursion results are in good agreement with less than 1% difference. The results between nodal method and point kinetic method are very consistent with less than 0.05% of difference
Core Physics Studies and TRU Burning Potential of a Thorium-based Fuel Fast Reactor
No full textTo overcome the stalemate on nuclear waste management in the US, Westinghouse is proposing a “waste-centric” approach. It consists of setting specifics for the nuclear waste aimed at furthering its acceptability from a societal viewpoint and then pursuing the nuclear system with the best potential to satisfy such requirements [1].
Public acceptability of nuclear waste is viewed to be tied to reducing the required isolation time to a viable minimum. In the U-based cycle, the isolation time is dictated primarily by the protracted radiotoxicity of the transuranic (TRU) isotopes that are generated. Switching to a thorium-based cycle is an appealing option for the potential of reducing the production of TRU [2] while efficiently burning the legacy supply [3].
The focus of this study is to assess the impact of various fuel types (i.e. oxide, nitride and metal) on the transmutation performance of a fast reactor (FR) employing thorium fuel
Flow boiling heat transfer in a helically coiled steam generator for nuclear power applications
No full textForced convection boiling of water was experimentally investigated in a 24 mlong full-scale helically coiled steam generator tube, prototypical of the steam generators with in-tube boiling used in small modular nuclear reactor systems. Overall, 1575 axially local and peripherally averaged heat transfer coefficient measurements were taken, covering operating pressures in the range of 2–6 MPa, mass fluxes from 200 to 800 kg m-2 s-1 and heat fluxes from 40 to 230 kW m-2. The heat transfer coefficient was found to depend on the mass flux and on the heat flux, indicating that both nucleate boiling and convection are contributing
to the heat transfer process. Seven widely quoted flow boiling correlations for straight tubes fitted the present helical coil databank with a mean absolute percentage error within 15–20%, which was comparable with the experimental uncertainty of the measured heat transfer coefficient values, thus indicating that curvature effects on flow boiling are small and negligible in practical applications
- …
