Marco E. Ricotti

Evaluation of erosion-corrosion in multiphase flow via CFD and experimental analysis

Abstract A numerical simulation is proposed of erosion–corrosion phenomena in four-phase flows comprising two immiscible liquids, gas and particulate solid. The simulation geometry is a pipe bend and the evaluated quantity is the wall erosion–corrosion brought about by the flow of a fluid mixture of two liquid phases, one of which is corrosive, plus a gas phase flow and a solid phase. A computational fluid dynamic tool has been adopted for the simulation of the flow field inside the piping and for the simulation of the particle trajectories and impact rates. As far as corrosion is concerned, a passivating and an actively corroding metallic material have been considered. Erosion model parameters have been derived from experiments correlating particle impact angle and erosion rate. Corrosion model parameters have been obtained from electrochemical measurements. The effects of the key operating parameters (fluid flow velocity, particulate content and gas volume fraction) have been evaluated by a two-level design of experiments approach. The single most important effects on synergistic damaging and on the ratio of corrosive to overall damaging have been identified. Erosion-enhanced and erosion-limited effects of flow conditions have been highlighted for the passivating and for the actively corroding alloys, respectively.

Neural network approach to sensitivity and uncertainty analysis

Abstract Computer simulation of the dynamic evolution of complex systems has become a fundamental tool for many modern engineering activities. In particular, risk-informed design projects and safety analyses require that the system behavior be analyzed under several diverse conditions in the presence of substantial model and parameter uncertainty which must be accounted for. In this paper we investigate the capabilities of artificial neural networks of providing both a first-order sensitivity measure of the importance of the various parameters of a model and a fast, efficient tool for dynamic simulation, to be used in uncertainty analyses. The dynamic simulation of a steam generator is considered as a test-bed to show the potentialities of these tools and to point out the difficulties and crucial issues which typically arise when attempting to establish an efficient neural network structure for sensitivity and uncertainty analyses.

FINANCIAL CASE STUDIES ON SMALL- AND MEDIUM-SIZE MODULAR REACTORS

Nowadays interest in small- to medium-size modular reactors (SMRs) is growing in several countries, including those economically and infrastructurally developed. Such reactors are also called “deliberately small reactors” since the reduced size is exploited from the design phase to reach valuable benefits in safety, operational flexibility, and economics. A rough evaluation based only on the economies of scale could label these reactors as economically unattractive, but that approach is incomplete and misleading. An economic model (INCAS - INtegrated model for the Competitiveness Assessment of SMRs) is currently being developed by Politecnico di Milano university within an international effort on SMR competitiveness fostered by the International Atomic Energy Agency, suitable to compare the economic performance of SMRs with respect to large reactors (LRs). INCAS performs an investment project simulation and assessment of SMR and LR deployment scenarios, providing monetary indicators (e.g., internal rate of return, levelized cost of electricity, total equity employed) and nonmonetary indicators (e.g., design robustness, required spinning reserve). This paper presents the general features and purpose of the INCAS model, detailing the input required, and points out the main differences with other simulation codes. INCAS is applied to evaluate the financial attractiveness of an investment in four SMRs with respect to a single LR with the same power generation capacity installed, in different deployment scenarios. Then, a sensitivity analysis highlights the degree of elasticity of the key output parameters for the investors, with respect to the most sensitive input parameters. Given the uncertainties of the main input parameters, INCAS results are affected by uncertainties as well. However, the financial output parameters provide a general understanding on the investment economics: INCAS shows that the economy of scale is not the only cost driver, because the economies of multiples may compensate for most of the gap in the economic performance of the SMRs. The uncertainties that affect the input data and the model do not allow declaration of a straightforward and neat economic performance superiority of SMRs versus LRs, or vice versa. Nevertheless, some trends have been highlighted. In particular, in “supported” market scenarios, where overnight construction costs have the highest incidence and the market conditions are less volatile, the most suitable strategy is to pursue the economies of scale. In contrast, SMRs behave better in “merchant” scenarios, where the cost of financing is higher and financial risk is sensitive. A “modular” investing strategy with a step-by-step power block deployment process allows lower financial exposure and less capital at risk and may mitigate the impact of scenario uncertainties on a project’s profitability.

STEAM GENERATOR OF THE INTERNATIONAL REACTOR INNOVATIVE AND SECURE

IRIS (International Reactor Innovative and Secure) is a light water cooled, 335 MWe power reactor which is being designed by an international consortium as part of the US DOE NERI Program. IRIS features an integral reactor vessel that contains all the main reactor coolant system components including the reactor core, the coolant pumps, the steam generators and the pressurizer. This integral design approach eliminates the large coolant loop piping, and thus eliminates large loss-of-coolant accidents (LOCAs) as well as the individual component pressure vessels and supports. In addition, IRIS is being designed with a long-life core and enhanced safety to address the requirements defined by the US DOE for Generation IV reactors. The design of the steam generators, which are internally contained within the reactor vessel, is a major design effort in the development of the integral IRIS concept. The ongoing design activity about the steam generator is the subject of this paper.

Neutronic analysis of U-free inert matrix and thoria fuels for plutonium disposition in pressurised water reactors

Abstract Inserting reactor-grade (RG) or weapons-grade (WG) plutonium in uranium-free matrices and burning it in light water reactors (LWRs) is an option gaining a wider consensus in the nuclear community. The main results of our neutronic studies performed in the last few years on this subject are reported. Our attention was mainly concentrated on two kinds of matrices: inert matrix in the form of calcia-stabilised zirconia, and thoria. Both materials are likely to exhibit excellent behaviour under irradiation (already demonstrated for thoria fuels) and high chemical stability. Direct disposal of spent fuel should be made feasible and attractive. A preliminary neutronic analysis was performed on these U-free fuels, imposing the constraint of maintaining the same assembly design and cycle length of a standard enriched-uranium fuel. In particular inert matrix fuel (IMF) showed a high plutonium burning capability, but associated with unacceptable feedback coefficients. Therefore, a whole IMF core results unfeasible, and only a partial core loading is possible. The solution then studied consists in replacing ≈21% of the pins of a standard enriched-U subassembly with IMF pins. Detailed assembly and core calculations were performed. A crucial aspect is the choice of a suitable burnable poison, which has to dampen the power peaks in the different fuel pin types without life penalisation. Among the considered poisons, a thin boron coating on the IMF pellets resulted the only effective one. Preliminary IMF pin cell calculations and the detailed ones gave similar results in terms of burnt plutonium fractions: 90% of fissile and 73% of total plutonium is burnt when RG plutonium is used. The main drawbacks of this fuel are the limited core loading capability and the lack of in-pile technological validation. In the case of Pu–Th fuels, pin cell calculations showed that increasing the plutonia content, decreasing the thoria content, and decreasing the pellet diameter are all possible ways to reach a longer fuel cycle and a higher percentage of burnt plutonium. Attained values for RG–Pu are ⩾80% and >60% for the fissile and total plutonium, respectively. The use of IMF is an effective solution to proliferation concerns, while some concerns remain for thoria fuels because of the production of 233 U. This, however, can be eliminated by a small addition of 238 U. Long time radiotoxicity is scarcely affected by these fuels with respect to conventional MOX.

Dynamic simulation of a steam generator by neural networks

Abstract Numerical simulation by computers of the dynamic evolution of complex systems and components is a fundamental phase of any modern engineering design activity. This is of particular importance for risk-based design projects which require that the system behavior be analyzed under several and often extreme conditions. The traditional methods of simulation typically entail long, iterative, processes which lead to large simulation times, often exceeding the transients real time. Artificial neural networks (ANNs) may be exploited in this context, their advantages residing mainly in the speed of computation, in the capability of generalizing from few examples, in the robustness to noisy and partially incomplete data and in the capability of performing empirical input–output mapping without complete knowledge of the underlying physics. In this paper we present a novel approach to dynamic simulation by ANNs based on a superposition scheme in which a set of networks are individually trained, each one to respond to a different input forcing function. The dynamic simulation of a steam generator is considered as an example to show the potentialities of this tool and to point out the difficulties and crucial issues which typically arise when attempting to establish an efficient neural network simulator. The structure of the networks system is such to feedback, at each time step, a portion of the past evolution of the transient and this allows a good reproduction of also non-linear dynamic behaviors. A nice characteristic of the approach is that the modularization of the training reduces substantially its burden and gives this neural simulation tool a nice feature of transportability.

Pioneering role of IRIS in the resurgence of Small Modular Reactors

Abstract This paper presents an overview of the first 10 years of the IRIS project, summarizing its main technical achievements and evaluating its impact on the resurgence of small modular reactors (SMRs). SMRs have been recurrently studied in the past, from early days of nuclear power, but have never gained sufficient traction to reach commercialization. This situation persisted also in the 1990s; the focus was on large reactors based on the presumed common wisdom of this being the only way to make the nuclear power plants competitive. IRIS is one of several small reactor concepts that originated in the late 1990s. However, the specific role and significance of IRIS is that it systematically pursued resolving technology gaps, addressing safety, licensing, and deployment issues and performing credible economics analyses, which ultimately made it possible—together with other SMR projects—to cross the “skepticism threshold” and led the making of a convincing case—domestically and internationally—for the role and viability of smaller reactors. Technologically, IRIS is associated with a number of novel design features that it either introduced or pursued more systematically than its predecessors and ultimately brought them to a new technical level. Some of these are discussed in this paper, such as the IRIS Safety-by-Design, security by design, the innovative thermodynamic coupling of its vessel and containment, systematic probabilistic risk assessment-guided design, approach to seismic design, approach to reduce the emergency planning zone to the site boundary, active involvement of academia, and so on. Many individuals and organizations contributed to that work, too many to list individually, and this paper attempts to pay tribute at least to their collective work.

The SPES3 Experimental Facility Design for the IRIS Reactor Simulation

IRIS is an advanced integral pressurized water reactor, developed by an international consortium led by Westinghouse. The licensing process requires the execution of integral and separate effect tests on a properly scaled reactor simulator for reactor concept, safety system verification, and code assessment. Within the framework of an Italian R&D program on Nuclear Fission, managed by ENEA and supported by the Ministry of Economic Development, the SPES3 facility is under design and will be built and operated at SIET laboratories. SPES3 simulates the primary, secondary, and containment systems of IRIS with 1 : 100 volume scale, full elevation, and prototypical thermal-hydraulic conditions. The simulation of the facility with the RELAP5 code and the execution of the tests will provide a reliable tool for data extrapolation and safety analyses of the final IRIS design. This paper summarises the main design steps of the SPES3 integral test facility, underlying choices and phases that lead to the final design.

COMPETITIVENESS OF SMALL-MEDIUM, NEW GENERATION REACTORS: A COMPARATIVE STUDY ON CAPITAL AND O&M COSTS

Smaller size reactors are going to be an important component of the worldwide nuclear renaissance. An inappropriate application of the economy of scale would label the small-medium size reactors as not economically competitive with larger plants because of capital costs (

Preliminary Safety Analysis of the IRIS Reactor

/kWe) and O&M costs (