Small Modular Reactors in Italy’s Energy Transition: Integrating Long-Term Planning and Hourly Weather Stress Testing

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Abstract
Energy is increasingly regarded as a strategic national asset. Long-term planning must therefore evaluate environmental sustainability, security of supply, system reliability and the affordability of system costs together. This energy trilemma becomes harder to assess when systems rely more heavily on variable renewable energy. As a result, energy planning requires more robust modelling tools, able to connect long-term system evolution with operational performance under realistic conditions. Energy-system models used for long-term planning often simplify chronology through representative or averaged time slices. This abstraction makes large multi-year optimisation problems tractable, but it may understate the operational consequences of variable renewable energy, prolonged low-resource events and hourly adequacy requirements. This paper develops and applies a soft-linked modelling framework for Italy in which a dynamic multi-carrier energy-system optimisation model, Hypatia, defines long-term pathways across electricity, heat and hydrogen, while an hourly model, Calliope, tests selected target-year configurations under 8760-hour chronology. The coupling transfers Hypatia capacities for 2030, 2040 and 2050 to Calliope, where all non-battery technologies are fixed at their inherited levels, while additional battery capacity can be selected endogenously to meet hourly flexibility requirements.. The hourly stage is run under a Typical Meteorological Year and three critical historical weather years selected from a 45-year solar and wind dataset, including the lowest combined capacity-factor year and one-week and two-week Dunkelflaute events. Four policy-relevant cases are analysed: a conservative reference pathway, a VRES-led net-zero pathway with doubled renewable deployment limits, and two nuclear-plus-CCS sensitivities with high and low nuclear capital costs. Results show that temporal aggregation materially affects scenario interpretation. In the VRES-led net-zero case, Calliope increases 2050 battery requirements from 473 GWh in Hypatia to 26107 GWh, while the corrected system cost rises from 699 to 1866 billion € under the two-week Dunkelflaute case. By contrast, firm low-carbon capacity reduces the adequacy correction: in the low-costs nuclear case, 2050 battery needs remain within 161–183 GWh and the cost correction is within 10 billion €. The results show that operational and weather-year stress testing can fundamentally change the assessment of long-term decarbonisation pathways, highlighting the role of nuclear capacity in achieving carbon neutrality while maintaining system adequacy and limiting the cost and storage requirements associated with prolonged renewable scarcity.

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