Dottorato in ENERGIA E AMBIENTE

Titolo della tesi: Investigation of Design Extended Conditions and related mitigation strategies for integral Pressurized Water Reactors

This thesis contributes to the Euratom SASPAM-SA project, which investigates whether and how knowledge and operating experience from large light-water reactors (LWRs) can be transferred to integral pressurized water reactors (iPWRs) to support European licensing analyses for severe accidents and Emergency Planning Zones. Advanced iPWR concepts are designed with inherent and passive features that reinforce the first three levels of Defence-in-Depth, yielding reductions in Core Damage Frequency; nonetheless, robust provisions for prevention and mitigation at Levels 4 and 5 remain indispensable, and deterministic studies of postulated sequences are required. Against this background, the project adopts two generic iPWR concepts selected for their proximity to near-term deployment in Europe and for the distinct evolutionary features they embody relative to operating large reactors: one with submerged containment and an electric power of about 60 MWe, and one with a dry containment, extensive passive safety functions, and an electric power of about 300 MWe. The intent is not to assess proprietary designs but to derive general statements about the applicability of internationally recognized severe-accident tools to classes of iPWRs that share common features under postulated severe-accident conditions, with scenarios characterized by severity rather than by probability due to the generic nature of the concepts. Within this framework, the thesis pursues a twofold objective. First, it evaluates the capability of the integral severe-accident code MELCOR to simulate transients in the 300 MWe generic configuration, hereafter Design 2, with emphasis on the interconnection of multiple passive systems during both design-basis and design-extended scenarios. Second, it explores In-Vessel Melt Retention (IVMR) feasibility for iPWR using W-STORM, a zero-dimensional, fast-running Python tool developed by Sapienza University of Rome in collaboration with the French Authority for Nuclear Safety and Radiation, aimed at quantifying the lower-head thermal loads that may arise after core relocation. The combined approach provides complementary views: MELCOR offers system-level insight into passive mitigation and containment–primary coupling over long transients, whereas W-STORM supplies rapid, preliminary quantification of the parameters that govern lower-head integrity in the post-relocation steady configuration, guiding where integral simulations and higher-fidelity models should concentrate. For the MELCOR activity, a complete input deck was developed for the generic Design 2 plant. Because no proprietary data were used, the nodalization and plant parameters were derived from engineering scaling of the SPES-3 integral facility, supplemented by public information for IRIS-type small modular reactors. The integral reactor pressure vessel encloses the core, helical-coil once-through steam generators, reactor coolant pumps, and pressurizer, so large-break loss-of-coolant accidents are precluded by design and attention focuses on small-break LOCA initiators. Passive functions modeled include decay-heat removal through steam-generator-side heat exchangers submerged in an elevated pool, automatic depressurization in two stages to enable passive make-up, high- and low-pressure borated injections, pressure suppression within the drywell to condense released steam, natural circulation establishment in the integral vessel after pump coast-down aided by riser-to-downcomer connections, and reactor cavity flooding intended to submerge the lower head for potential external cooling during severe accident conditions. After establishing steady state, a design-basis sequence was postulated as a double-guillotine rupture of one Direct Vessel Injection line, producing a small-break LOCA. A seventy-two-hour transient was simulated to capture the timing and interaction of passive systems. Subsequently, three design-extension condition B sequences, sharing the same initiating event, were examined to probe the consequences of unavailability in selected passive functions: one with unavailability of the automatic depressurization system and emergency heat removal, one with unavailability of all modeled safety functions, and one with unavailability confined to the emergency heat removal while depressurization remains available. To aid interpretation, results were organized in phenomenological windows that track the evolving state of the core and containment from blowdown and boil-off through uncovery, relocation, reflooding, and stabilization. The design-basis case exhibited the expected progression of passive mitigation. Depressurization and secondary-side heat removal, combined with steam condensation in the drywell and the drainage of condensate to the reactor cavity, sustained long-term gravity-driven make-up and removed decay heat without core damage over the seventy-two-hour window. The magnitudes and trends of the principal variables and actuation timings were consistent with prior expectations based on SPES-3/IRIS-derived information, offering qualitative confidence that MELCOR can represent iPWR passive-system phenomenology in design-basis conditions. In the design-extension analyses, the early blowdown signature—a sharp drywell pressure rise beyond its design value—was similar across cases, but subsequent behavior diverged as passive functions became unavailable or delayed. When both early depressurization and secondary-side removal were unavailable, core heat-up advanced, with distinct drywell pressure spikes aligned with degradation windows and a substantially higher cumulative hydrogen production than in the other sequences. When all passive systems were unavailable, the evolution remained broadly similar in shape, albeit with timing differences, and extensive relocation proceeded without effective mitigating pathways until late interactions. By contrast, in the last scenario, the second automatic depressurization stage directly coupled the vessel and containment, enabling long-term injection and cavity flooding that restored inventory and avoided complete core damage, eventually achieving full reflood. The drywell pressure in this case exhibited typical oscillations driven by filtered venting, with higher frequency consistent with the stronger coupling induced by depressurization venting. Across cases, the phenomenological-window framework proved useful in locating the intervals where MELCOR’s models—natural circulation in the integral geometry, steam condensation and pressure suppression capacity, coupling between vessel and containment under depressurization—dominate outcomes and in pinpointing data needs for future assessment. The W-STORM activity addresses the In Vessel Melt Retention stage in integral Pressurized Water Reactors. The tool represents a stratified corium pool settled in the lower plenum, composed of metallic and oxidic layers that exchange heat internally and with the vessel wall under a specified containment pressure. A simplified thermochemical equilibrium model determines layer compositions and effective properties; a geometrical module determines layer ordering and dimensions; and a heat-transfer module partitions the internal heat fluxes and computes the inner-wall temperature. The residual lower-head thickness is obtained through an iterative ablation procedure that updates the thickness based on the difference between the computed inner-wall temperature and the local melting threshold until convergence within a tight tolerance of 0.1 K. Although not intended to capture full dynamics, the tool rapidly quantifies which parameters exert first-order influence on focusing effects and peak wall loads in iPWR geometries and thereby indicates whether external cooling strategies envisioned for the generic configuration are likely to afford adequate margins. The tool is able to identify specificities in iPWRs with respect to LWR in IVMR, such as thick oxide crusts, higher fractions of heat to the metal layer, high radiative heat transfer and limited focusing effect. A sensitivity analysis is carried out with such tool, strengthening the need for models for Heavy Metal stratification and liquid metal freezing in SA codes, as well as highlighting correlations between selected parameters.