Isotope Accountancy Interface

Accountancy is the slowest, most auditable physical interface: a reconciled mass balance for tritium, helium-3, and deuterium across both machines.

Why accountancy is an interface

Every gram of tritium and helium-3 that moves through the plant is regulated material. The accountancy interface is where physical sensing (stream assays, inventories, containment status) meets a reconciled ledger. It is not a control loop — it is the audit surface that regulators, offtake, and the fleet view all read from.

What it reconciles

• Tritium: ~4 kg/yr class product plus in-plant inventory and decay.
• Helium-3: ~1.97 kg/yr byproduct class, tracked against the 400x demand gate.
• Deuterium: throughput and purity from the supply interface.
• Neutron budget: 14 MeV source used to close the breeding estimate.

# closing the isotope ledger (L7, auditable)
for iso in ['T','He3','D']:
    balance[iso] = opening[iso] + produced[iso] \
                   - consumed[iso] - decay[iso] - transfers[iso]
    residual[iso] = measured_closing[iso] - balance[iso]
    assert abs(residual[iso]) <= mba_limit[iso]   # material-balance limit

Owner and traceability

Owner: L7 for the ledger; L4 supplies campaign-level flows; L2 guarantees that every input reading is timestamped and traceable. The material-balance limit is the tolerance within which the books must close; a residual outside it is an investigation trigger, not a control action.

Calibration is the strictest of any interface: each assay traces to a reference standard with documented uncertainty. All figures are design-and-simulation class; the ledger structure is built now so it is auditable from FOAK onward. No economics appear here — this is material accountancy only.