Isotope Inventory and Mass-Balance Accounting

A continuous mass balance tracks every gram of tritium and helium-3 from creation through burn, decay, retention, and shipment, with no unexplained gaps.

Conservation as the accounting principle

Isotope accounting rests on conservation: what is created must equal what is burned, retained, decayed, lost, and shipped. L7 maintains a continuous mass balance for tritium and helium-3 per unit and per fleet, reconciling measured inventory against modeled flows every accounting period. A discrepancy beyond measurement uncertainty is an anomaly, surfaced immediately rather than absorbed.

The balance equation

# per-period tritium balance (kg); must close within uncertainty
#  bred      : from lithium blanket (twin neutronics)
#  burned    : consumed sustaining D-T plasma
#  decayed   : lambda * inventory * dt
#  shipped   : offtake out
#  loss      : process/permeation loss
d_inventory = bred - burned - decayed - shipped - loss
residual = measured_inventory_change - d_inventory
assert abs(residual) <= k * meas_uncertainty   # else flag anomaly

Tritium and helium-3 differ in the balance: tritium decays (into helium-3, which the balance credits back), while helium-3 is stable and its scarcity makes shipment-versus-retention the dominant term. The accounting captures the tritium-to-helium-3 decay coupling so a helium-3 inventory rise from decaying tritium is not mistaken for unexplained gain.

This mass balance is the same data the compliance layer needs for byproduct-material accountancy, so there is one authoritative ledger, not a separate operational count and regulatory count. Every entry carries provenance (measurement, method, uncertainty, timestamp) so the ledger is auditable end to end.

The residual term is a diagnostic in its own right. A persistent nonzero residual can indicate permeation loss, a diagnostic drift, or an unmodeled breeding effect; feeding it back to the twin's neutronics improves the production model. Mass balance thus serves both accounting and physics validation.

The ledger runs today on twin-modeled flows and reports design-stage quantities. At FOAK ~2030, measured inventory replaces modeled inventory in the same equation, and the residual becomes a real reconciliation between physics prediction and hardware.