Hybrid Gate Selection and SCRAM Baselines for Reactor-Scale Fault Tree Subtrees

This paper presents a standalone, audit-ready pipeline that selects a small set of quantum-feasible reactor-scale fault tree subtrees and establishes classical baselines for those candidates using SCRAM. The starting point is a reactor-scale subtree working set containing 30,885 subtrees extracted from a Phase 2B operational model. Gate 1 applies structural criteria, qubit upper bound criteria, and circuit depth proxy criteria consistent with near term quantum resource constraints, selecting 776 subtrees (2.513% of the working set). Gate 2 converts each Gate 1 candidate into an OpenPSA Model Exchange Format (MEF) representation, validates the MEF with SCRAM 0.16.2, and computes prime implicants for the top event. In the frozen Gate 2 run, all 776 candidates validate successfully and all 776 runs complete successfully. Across the 776 candidates, SCRAM returns between 3 and 5 prime implicants per subtree (median 3 for n = 5 and n = 6 basic events; median 4 for n = 8). Order-2 implicants constitute 85.83% of all 2,859 prime implicants identified. The order distribution exhibits a clear structural regularity: n = 6 subtrees yield exclusively order-2 implicants, while n = 5 and n = 8 subtrees exhibit mixed-order patterns attributable to their distinct gate topologies. SCRAM execution times are tightly clustered, with a run time median of 143.1 ms and a 95th-percentile of 176.1 ms across all 776 candidates. A depth proxy cross-validation against offline FakeTorino transpilation confirms that the Gate 1 depth criterion operates conservatively: the proxy value of 22 uniformly underestimates realized depth (39 excluding measurement gates, a 17-gate underestimate; 40 including measurement gates, an 18-gate underestimate), but all realized depths remain well within the 50-gate threshold. The resulting artifact bundle provides a defensible, fully reproducible classical baseline for subsequent hybrid and quantum evaluation.