In this patent corpus, a passive safety system refers to reactor safety mechanisms that remove decay heat or shut down a reaction without relying on active pumps, external power, or operator action. The most-cited record in this dataset, US4753771A, describes exactly this: a subsystem that circulates coolant solely by natural convection to remove decay heat. Filers use the term to distinguish these designs from active safety systems that depend on powered pumps and valves, which matters for both regulatory classification and claim scope.

The assignee ranking in this dataset covers 38 companies, with one leader holding 35 records against a fifth-place holder at 9 and a tenth-place holder at 5. The top 5 filers combined account for 113 records, or 61.4% of all 184 records in scope, and the top 10 combined reach 79.3%. That leaves a long tail of single- or few-filing entrants below the leading group, so freedom-to-operate risk is concentrated rather than evenly spread.

Filings peaked at 24 in 2018 and the most recent complete three-year span, 2021 to 2024, moved from 6 to 5 filings, a -17% change. Because patent publication typically lags filing by around 18 months, the years closest to the 2026-08-31 data cut-off are still incomplete and should not be read as evidence of a slowdown. The safest reading is that the field passed its filing peak in 2018 and has since settled into a lower, steadier volume rather than continuing to accelerate.

US4753771A, assigned to Westinghouse Electric Corporation, claims a passive safety subsystem that circulates reactor coolant solely by natural convection between two branches of the coolant circuit, bypassing the steam generator, using a heat exchanger and a valve in a specific arrangement. With 170 citations, it is the most-cited record in this dataset, meaning later filers have repeatedly had to route around this specific branch-and-bypass-plus-heat-exchanger combination rather than the general idea of natural-convection cooling. Designs using forced circulation, different bypass topology, or a different heat-rejection path can sit outside its core claims.

The under-claimed branches sit outside the dominant G21C and G21D reactor and plant classes, in areas such as valves and taps (F16K, 2.2% of 184 records), solid-state and semiconductor tie-ins (H10N, H01L, H02N, each under 5%), and separation or turbine integration (B01D and F01D, 1.1% each). These low record counts suggest passive-safety concepts have not been densely claimed at the component level, even though they are implicated in the core cooling-loop claims. A first claim tying a specific passive-safety trigger to one of these component classes, rather than to the cooling loop itself, is more likely to clear the dense prior art around the core mechanism.