The NILUS reactor for the co-production of electricity and potable water

The NILUS-1000 reactor, an innovative PWR with integrated layout and full natural circulation, is proposed for the adoption in those countries where the production of potable water is a major concern for the population, and where the nuclear culture could not be so developed and firmly established as the complexity of the nuclear industry would require. The NILUS concept is briefly depicted, together with the reference desalination process and plant size selected for the coupling with the 1000MWth nuclear reactor. The safety systems and the “hybrid” containment, a compact steel pressure containment with a pressure suppression pool, are described. The results of the preliminary safety analysis are reported for an ATWS accident, showing the validity of the concept design and the large grace period achieved in this beyond DBA transient

Multiple nuclear power plants investment scenarios: Economy of Multiples and Economy of Scale impact on different plant sizes

“Deliberately small” nuclear reactors are making their way on the market, not as a mere shift backwards to the small scale of first commercial reactors, but as concepts designed to foster modularization, simplification and serial production. They are proposed by manufacturers worldwide (SMART, 4S, SSTAR, mPower, Nuscale, etc.) and are also intended to address developed electricity markets. The idea of an economic attractiveness of Small and Medium sized Reactors (SMR) is counterintuitive, due to the loss of Economy of Scale on a capital intensive investment. Nevertheless a broader understanding of capital costs drivers has shaped a new concept of Economy of Multiples, that applies on multiple NPP deployment. It relies on learning accumulation to mitigate construction costs of later NPP units; design modularization to exploit the benefits of “serial” production; co-siting economies to decrease the incidence of fixed and site-related costs. We assume that smaller NPP size fosters design modularization and simplifications, with related cost savings. While the effect of modularization on construction costs has been modeled, the estimation of design-based savings may be the upmost arbitrary and controversial, but the underlying assumption is that the lower the plant size, the higher may be the “Design cost-saving factor”. The dynamic and benefits of the Economy of Multiples of SMR have already been investigated on a case study of a stand-alone Large Reactor (LR) against four SMR deployed on a single site. The two alternative investment projects have been evaluated on their economic performance and profitability. But Economy of Multiples is not a privilege of SMR. This work aims to analyze at what extent and conditions the Economy of Multiples holds against the Economy of Scale, when NPP of different sizes are deployed in multiple units, considering that the Economy of Multiples smoothes its benefits with the increase in number of units installed and that the maximum size of the sites is a limit to its application on LR. The limit case-study of “Very Small Reactors” (VSR) is investigated, representing a massive NPP deployment and a huge loss of Economy of Scale. Our analysis is performed by mean of INCAS (INtegrated model for the Competitiveness Analysis of Small-medium modular reactors) Polimi’s proprietary simulation code. Scenario simulations are run managing the Design cost-saving factor of each SMR fleet size as a parameter; its value is calculated in order to achieve the same level of economic performance of LR investment scenario. In other words we have determined the required design simplification effort needed by each NPP size, in order to attain the economic performance of the equivalent LR deployment scenario. Our results show that the Economy of Multiples holds as a competitive edge for Medium and Small Reactors even when nuclear site may host multiple LR: 8-9% design cost saving is able to grant the same economic performance of a fleet of LR, even with higher construction cost estimates. On the contrary, VSR need to achieve more stretching degree of design simplification and related cost savings (up to 15%) in order to be competitive with LR

An Evaluation of SMR Economic Attractiveness

The nuclear “renaissance” that is taking place worldwide concerns the new build of GW size reactor plants, but smaller GenIII+ NPP (Small ModularReactors, SMR) are on the verge to be commercially available and are raising increasing public interest. These reactor concepts rely on the pressurized water technology, capitalizing on thousands of reactor-years operations and enhancing the passive safety features, thanks to the smaller plant and equipment size. On the other hand, smaller plant size pays a loss of economy of scale, which might have a relevant impact on the generation costs of electricity, given the capital-intensive nature of nuclear power technology. The paper explores the economic advantages/disadvantages ofmultiple SMR compared to alternative large plants of the same technology and equivalent total power installed. The metrics used in the evaluation is twofold, as appropriate for liberalized markets of capital and electricity: investment profitability and investment risk are assessed, from the point of view of the plant owner. Results show that multiple SMR deployed on the same site may prove competitivewith investment returns of larger plants, while offering, in addition, unique features that mitigate the investment risk

Monte Carlo Approach to Dynamic PSA: Neural Solution of Equations Describing Core Transients

The PSA analysis of a real plant represent a formidable com-putational task usually afforded either with an analytical approach based on the theory of the Markov chains or with a Monte Carlo simulation. In our opinion this latter methodology, thanks to its unique flexibility features, represents the only viable approach to the problem when time dependencies have impacts on the analy-sis: examples are time dependent transition rates (ageing), timing of the protection, control and safety systems, operator actions etc. Moreover the PSA analysis of a real plant demands taking into account the process variable dynamics when the evolution of the underlying physical process interacts with the system hardware configuration, e.g. when the process variables influence the failure rates or activate the protection systems. The inclusion of these dy-namic aspects dramatically burdens the analysis: a solution could be presently attempted only through short cuts to the solution of the deterministic equations governing the evolution of the process variables. In the present paper we consider the application of a multi-layered neural network for the solution of the mathematical mod-els related to the core behaviour of a PWR under varying thermal-hydraulic conditions. Since the neural network works very rapid-ly, this approach seems to be a good candidate for being included in a Monte Carlo dynamic PSA code which requires solving thou-sands of times the model equations relating to the different hard-ware plant configurations. Possible approximations thereby intro-duced could be tolerated if comparable with those following from the uncertainties of the stochastic parameters. The time reduction advantage is expected to increase when the future parallel com-puters become widely available

Analysis of Different Containment Models for IRIS Small Break LOCA, using GOTHIC and RELAP5 Codes

Advanced nuclear water reactors rely on containment behaviour in realization of some of their passive safety functions. Steam condensation on containment walls, where non-condensable gas effects are significant, is an important feature of the new passive containment concepts, like the AP600/1000 ones. In this work IRIS reactor was taken as reference, and the relevant condensation phenomena involved within its containment were investigated with different computational tools. In particular, IRIS containment response to a Small Break LOCA (SBLOCA) was calculated with GOTHIC and RELAP5 codes. IRIS containment drywell was modelled with RELAP according to a sliced approach, based on the two-pipe-with-junction concept, while it was simulated with GOTHIC testing several modelling options, regarding both heat transfer correlations and volume and thermal structure nodalization. The influence on containment behaviour prediction was investigated in terms of drywell temperature and pressure response, Heat Transfer Coefficient (HTC) and steam volume fraction distribution, and internal recirculating mass flowrate. The objective of the paper is to compare the capability of the two codes in modelling of the same postulated accident, thus to check the results obtained with RELAP5, when applied in a situation not covered by its validation matrix. The option to include or not droplets in fluid mass flow discharged to the containment was the most influencing parameter for GOTHIC simulations. Despite some drawbacks, due e.g. to a marked overestimation of internal natural recirculation, RELAP confirmed its capability to satisfactorily model the IRIS containment

Load following with Small Modular Reactors (SMR): A real options analysis

Load following is the potential for a power plant to adjust its power output as demand and price for electricity fluctuates throughout the day. In nuclear power plants, this is done by inserting control rods into the reactor pressure vessel. This operation is very inefficient as nuclear power generation is composed almost entirely of fixed and sunk costs; therefore, lowering the power output doesn't significantly reduce generating costs and the plant is thermo-mechanical stressed. A more efficient solution is to maintain the primary circuit at full power and to use the excess power for cogeneration. This paper assesses the technical-economic feasibility of this approach when applied to Small Modular Reactors (SMR) with two cogeneration technologies: algae-biofuel and desalinisation. Multiple SMR are of particular interest due to the fractional nature of their power output. The result shows that the power required by an algae-biofuel plant is not sufficient to justify the load following approach, whereas it is in the case of desalination. The successive economic analysis, based on the real options approach, demonstrates the economic viability of the desalination in several scenarios. In conclusion, the coupling of SMR with a desalination plant is a realistic solution to perform efficient load following

Plutonia-Thoria Fuel Cycle as Starting Solution for a wider Thorium Use

This paper is focused on a description of thoria fuel option. Our opinion is that this option, beyond being a valuable way to exploit the energy content of plutonium without further breeding it, may be a starting point for introducing an Uranium-Thorium fuel cycle, based on a different strategic context with respect to past proposals. The option is based on the adoption of current or advanced PWRs, the latter characterised by a reduced fuel power density, always adopting conventional fuel rods and assemblies. A three-batches full core loading scheme is assumed. The thoria-plutonia composition is determined by the constraints to obtain at Beginning Of Life (BOL) a non positive void coefficient, and to reach a burnup as high as possible. Different fuel compositions and pellet radius are considered. The plutonium content is in the range of 4.5÷15%, mainly depending on the plutonium quality, namely Weapon Grade (WG) or Reactor Grade (RG). The results are in terms of dynamic coefficients, life duration, plutonium consumption and final isotopic compositions. These fuels show the capability to destroy about 40÷60% of total plutonium for RG, while this figure rises to 65÷70% for WG. These values are well above those obtained by MOX option. A variant to eliminate any proliferation concern foresees the addition of small quantities of 238U at the expenses of a reduction of the fuel burnup and its capacity in burning RG plutonium, while the opposite occurs for WG. The low boron worth is not different from the MOX one, being related mainly to the plutonium content, and much less to the chosen fertile isotope. Therefore, modifications of control devices for a full core strategy is not expected to be different in the two cases. The results confirm the viability of this proposal, apt to future variants, including those connected to accelerator actinide burning solution. An irradiation experiment, expe-cted to take place at Halden HWBR in the context of the ENEA participation to the Halden Project, is the main part of the Inert Matrix – Thoria fuel R&D activity presently underway as a Polytechnic of Milan–ENEA co-operation

Natural Circulation and Integrated Layout Pressurized Water Reactor

A natural circulation and integrated layout PWR is here proposed by a preliminary feasibility study. Its main features are: low unit power and core power density, the same temperatures and pressures of current PWRs, no fuel melting scenario, passive protection systems, pressure suppression containment, long fuel cycle duration and no boron in the primary water. The power station can be designed for electrical power production (700 MWth - 225 MWe), or for combined electricity and heat production (200 MWth); this last solution seems viable even for urban siting. Here only the 225 MWe plant is described. The reactor startup is obtained by heating with a constant and low neutronic power, and a possible procedure is indicated and analyzed. The ATWS of a steam line break accident in hot standby conditions is studied by a suitable calculation program and the results are positive

Experimental Characterization of Two-Phase Flow Instability Thresholds in Helically Coiled Parallel Channels

Among the various types of instabilities affecting vapor generation in boiling systems, Density Wave Oscillation (DWO) occurrence within parallel channels is depicted. Parallel channel instability may represent a critical concern for the operation and safety of the once-through steam generators adopted in GenIII+ and GenIV nuclear reactor concepts. Extensive attention is required to determine the safe operating regime of a two-phase heat exchanger, by evaluating the instability threshold values of system parameters such as thermal power, flow rate, pressure, inlet temperature and exit quality. While the amount of published experimental work in the field of DWOs investigation in parallel straight tubes is overwhelming since the ’60, scarce attempt has been dedicated to the helical-coiled tube geometry. Conversely, coiled pipes are foreseen for applications to steam generators of the next generation NPPs, due to compactness and higher efficiency in heat transfer. The paper deals with the results of an experimental campaign on flow instability occurrence in two electrically heated helically coiled parallel tubes. In the framework of the IRIS project, a full-scale open-loop experimental facility simulating the thermal-hydraulic behavior of a helically coiled steam generator has been built and operated at SIET labs in Piacenza (Italy). The facility comprises two helical tubes (1 m coil diameter, 32 m length, 8 m height), connected via lower and upper headers. In order to excite flow unstable conditions starting from stable operating conditions, supplied electrical power was gradually increased up to the appearance of permanent and regular flow oscillations. Several flow instability threshold conditions were identified, in a test matrix of pressures (80 bar, 40 bar, 20 bar), mass fluxes (600 kg/m2s, 400 kg/m2s, 200 kg/m2s), and inlet subcooling (from -30% up to ~0). The long test section feature and the helical-coiled tube geometry render the present facility a quite unique test case in the outline of two-phase flow instability experimental studies. Parametric effects of the operating pressure, flow rate and inlet subcooling on the threshold power are discussed. The period of oscillations is also discussed