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About Valar Atomics At Valar Atomics, we're redefining what's possible in energy. Our mission is to make clean, high-temperature nuclear power scalable—unlocking abundant energy for industry, hydrogen, and next-generation manufacturing. We are a team of builders, engineers, and operators who believe nuclear energy should be fast to deploy, factory-made, and built for the real world. Our first pilot plant in Orangeville, Utah will demonstrate how advanced nuclear systems and fuel fabrication can power the future. Joining Valar Atomics means becoming part of a company where autonomy, ownership, and impact exist at every level. The Team   As part of the  Engineering  organization, Models & Simulations develops advanced analytical models that support engineering design, performance prediction, and system optimization. The team enables informed decision-making through computational analysis and simulation.   The Role   The Senior Thermal-Hydraulics Systems Engineer leads system-level (1-D / network) thermal-hydraulics modelling of the reactor’s primary and secondary loops, balance of plant and power-conversion cycles using Flownex SE. Within the ModSim Engineering Group, the role builds and validates integrated plant models that capture steady-state performance and transient behavior – start-up and shutdown, load changes, loss-of-flow and trip scenarios – and that inform component sizing, control philosophy and cycle optimization (Brayton and supercritical-CO₂). The engineer defines the systems-modelling approach, reconciles Flownex results with CFD-derived coefficients and supplier data, and translates system analyses into engineering decisions and inputs for the wider design and safety effort. Key Responsibilities   Build, run and maintain integrated Flownex SE network models of primary/secondary loops, balance of plant and power-conversion cycles for steady-state and transient analysis. Characterize and size components (pumps, compressors, heat exchangers, valves, turbomachinery) and evaluate cycle performance and optimization across Brayton and sCO₂ architectures as required. Analyze plant transients and control response (start-up/shutdown, load-follow, loss-of-flow, trips) and provide system boundary conditions to the CFD and FEA streams while consuming their loss/heat-transfer coefficients. Produce calculation packages and system design-basis documentation under NQA-1; provide inputs to safety analysis and interface with detailed CFD models and neutronics power distributions. Lead verification and validation of system models, set systems-modelling standards, support regulator/customer reviews, and mentor junior systems analysts. Mentor engineers and train junior team members Standardize Flownex modelling methods, component libraries and templates, and support the Stream Lead in scoping, estimating and scheduling systems work. Basic Qualifications     Bachelor's degree or higher in Mechanical, Nuclear, Chemical or Aerospace Engineering with emphasis on thermodynamics, heat transfer or thermal-hydraulics systems. 5+ years of professional thermal-hydraulics / plant-systems analysis experience, including lead responsibility on integrated system or cycle models. Preferred Skills and Experience   Master’s degree (MSc/MEng) or higher in Mechanical, Nuclear, Chemical or Aerospace Engineering with emphasis on thermodynamics, heat transfer or thermal-hydraulics systems. Flownex SE expertise strongly preferred; otherwise demonstrable depth in a comparable systems/transient code (RELAP5, TRACE, GOTHIC, MELCOR, Modelica/Dymola, Aspen HYSYS Dynamics, Thermoflow / GateCycle) with willingness to adopt Flownex. Strong applied thermodynamics, heat transfer, and fluid mechanics; closed-loop power-cycle design (Brayton, supercritical-CO₂). Heat-exchanger thermal/hydraulic design and rating; turbomachi