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In the unlikely event of a nuclear power plant long duration station black-out, as in the Fukushima Daiichi (1F) severe accident (SA), it was recognized that the suppression chamber (S/C) functions of heat sink and fission product (FP) scrubbing will degrade, resulting in the S/C pressure increase, reduction of the scrubbing efficiency and subsequent necessity of venting operations. Consequently, a relatively large amount of FPs, in particular highly volatile elements (e.g. CH3I), is likely to be dispersed into the environment. As a method to evaluate the degradation of the pool characteristics under discharge of steam and non-condensable gases through vent pipes and steam through different quencher geometries of make-up systems, an experimental campaign was recently started at the SIET research laboratory in Italy. Two different quencher geometries, representing vent pipes and the reactor core isolation cooling (RCIC) exhaust pipes in 1F2 and 1F3, were adopted. Several combinations of steam and air mass flow rates were tested to scale down the main conditions occurred during the 1F SA. Measurements of pool water temperature in different locations and visualization with high-speed camera represent the main outcome of the experimental activity. The preliminary results have demonstrated that a relatively small concentration of air in the steam flow is able to suppress the occurrence of chugging of the steam, with reduced mixing in the pool. Both RCIC quenchers adopted induced large chugging at the bottom of the pool which are effective to avoid temperature stratification, thanks to the large water recirculation and vertical mixing within the pool. At decreased subcooling, mixing in the pool ceases and the quenchers with holes disposed in the vertical direction, as in the RCIC exhaust pipe of the 1F unit 3, introduce intense stratification that drastically reduces the condensation efficiency of the S/C pool. Quencher of 1F2 RCIC does not present stratification possibly dependent on the distance of the pipe outlet to the pool floor. Given the reduced size of the pool compared to the plant scale, the observed phenomena should not be extrapolated for the whole S/C. The objective of the ongoing experimental activity is to construct a database based on the high-speed filming, measurements of major quantities such as water temperature, steam pressure and FPs concentration to foster the development of physical models for both lumped parameter SA codes and detailed computational fluid dynamics software, in an effort to enhance the understanding of the complex phenomena following the 1F accident.

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.

AbstractThis 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.

A preliminary feasibility study and scope analysis for a demonstrator (demo) of the SUstainable Proliferation-resistance Enhanced Refined Secure Transportable Autonomous Reactor (SUPERSTAR) has been performed. Preliminary core design studies have been carried out focused on maximizing the power level compatibly with natural circulation cooling and transportability requirements, while meeting the foremost goals of (i) providing energy security and proliferation resistance thanks to a long life core design, (ii) minimizing the reactivity swing over the fuel lifetime, and (iii) flattening the radial power profiles, as demanded by the choice of wrapper-less fuel assemblies and by the stringent technological constraints imposed by the short-time-to-deployment feature. Once established appropriate geometrical pin and fuel assembly specifications, a suitable active height allowing the system to be cooled by free-flowing lead has finally been set through parametric T/H analyses. Fuel cycle calculations have been then performed to optimize both the fresh fuel composition and the radial enrichment zoning. Moreover, the use of several absorbing materials has been investigated in order to guarantee enhanced safety by incorporating control elements having a net density greater than that of the surrounding lead coolant. A complete static neutronic characterization of the resulting core has been finally accomplished.

•A Gen IV SMR is studied for cogeneration application.•The cogeneration system consists of advanced lead-cooled fast reactor integration into PCM storage modules for district heating.•The PCM storage modules are designed to be compatible with the NPP operating conditions.•Techno-economic analysis of the cogeneration plant has been performed.•The flexibility of the cogeneration plant in serving heat demand has been increased.

The aim of this paper is to present a fractional scaling analysis (FSA) application for a system with interacting components where multiple figures of merits need to be respected during complex transient accident scenario with several consecutive time sequences. This paper presents FSA application to the International Reactor Innovative and Secure (IRIS) reactor and Simulatore Pressurizzato per Esperienze di Sicurezza 3 (SPES3) integral effects test (IET) facility. The FSA was applied for the small break loss of coolant accident (SBLOCA) on the direct vessel injection (DVI) line as the most challenging transient scenario. The FSA methodologies were applied for two figures of merits: (1) reactor and containment vessels pressure responses, and (2) reactor vessel water collapsed level response. The space decomposition was performed first. The reactor vessel and containment vessel were divided in components so that important phenomena and their consequences can be evaluated in each of them. After that, the time decomposition in consecutive time sequences was performed for the considered transient (DVI SBLOCA) based on the starts, or ends, of the defining events. The configuration of the system in each time sequence might be different and dependent on the control system actions connecting, or disconnecting, various components of the system due to the valves openings, or closings. This way, the important phenomena and their consequences can be evaluated for each component and time sequence. Also, this paper presents and discusses options for deriving nondimensional groups and calculation of distortions between prototype and model responses for complex transients containing multiple consecutive time sequences. The input data for scaling analysis are based on the results of RELAP/GOTHIC analysis performed for IRIS and RELAP analysis performed for SPES3. The scaling analysis was applied iteratively several times for different IRIS and SPES3 configurations. Based on the intermediate results, some components in the IRIS and SPES3 were redesigned so that the distortions between IRIS and SPES3 responses are decreased.