The ranked assignee list covers 44 companies, with the leader alone holding 21 of the 123 records in scope — roughly a fifth of the field. The top 5 combined account for 70 records, or 56.9% of the total, and the top 10 together hold 74.8%. Below that tier, filing fragments quickly into single- and few-filing entrants, so the practical picture is a small group of dense holders sitting above a long, active tail.

Filings rose from 3 in 2021 to 11 in 2024, a +267% increase over that three-year span, and 2024 is the most recent year the data can treat as complete. 2025 shows a higher raw count of 16, but publication typically lags filing by around 18 months, so that figure and 2026's are still filling in rather than representing a slowdown or a peak that has passed. Treat the 2021-2024 window as the reliable growth signal.

US9384863B2 claims an apparatus for retaining molten core material outside a Generation IV reactor after a severe accident, built around an inner/outer wall arrangement, a vapor rising channel formed against a pressure vessel, and a deflector creating a coolant falling channel. It is a specific structural arrangement for core-melt retention, not a claim over molten-material retention generally, so designs using different channel geometries or different coolant flow paths for the same accident-mitigation function are not automatically blocked. Anyone working on severe-accident containment hardware should still map their design against this claim's specific wall and channel configuration before finalising geometry.

Inside this dataset's scope, fusion-adjacent coolant interfaces (G21B, 5.7% of records), radiation shielding for modular units (G21F, 5.7%) and neutron flux instrumentation (G01T, 7.3%) all carry far fewer filings than core reactor hardware under G21C, which touches 67.5% of records. That gap does not mean these areas are technically easier — it means fewer claims have been staked there yet, which is where a well-drafted first claim has more room to establish position rather than immediately colliding with dense prior art.

The two most-cited records in this set — both concerning multi-layered ceramic tube fuel containment barriers — carry 175 and 97 citations respectively, far ahead of anything filed more recently. This is a structural feature of citation counting inside any searched corpus: older filings have simply had more years to accumulate citations from subsequent work. High citation counts signal influence on the field's prior art base, not that the underlying technology is still the most commercially relevant approach today.