Abstract

Spain's long-term energy strategy combines rapid renewable expansion with the progressive phase-out of nuclear power by 2035. Since the timing of reactor retirements can significantly affect the long-term electricity balance, this study compares four nuclear phase-out scenarios: the current official retirement schedule (SCN_40) and three lifetime-extension pathways allowing reactor operation up to 50, 55, and 60 years (SCN_50, SCN_55, and SCN_60, respectively). All scenarios use the same demand and renewable deployment trajectories. A target-driven daily simulation developed with LEAF-EB combines historical generation and demand patterns with projections derived from the PNIEC and Spain's long-term decarbonization strategy. For each scenario, 1000 Monte Carlo simulations estimate clean electricity surpluses and the raw clean generation gap. Surpluses are allocated to BESS charging, exports, and synthetic methane production, while the gap is covered by BESS discharge, cross-border imports, methane reconversion, and residual backup. Over 2028–2050, extending reactor operation from the official schedule to 60 years reduces the modeled cumulative clean generation gap by 39.1% and electricity imports by 38.5%, while exports increase by 72.4%. BESS discharge covers 7.2–11.4% of the gap, while imports account for approximately 21% across all scenarios. Electricity recovered from stored synthetic methane increases from 77.2 to 153.2 TWh, raising its contribution from 8.1% in SCN_40 to 26.5% in SCN_60. The residual backup requirement decreases by 61.0%, from 599.3 to 233.5 TWh, but still covers 40.4% of the gap in SCN_60 and is required on 14.0% of simulated days. Extending nuclear operation therefore reduces the magnitude and frequency of balancing requirements but does not eliminate the need for firm backup generation under the modeled assumptions.