Byproduct-Material Accountancy

Every gram of regulated isotope and every activated component is tracked as byproduct material from creation to disposition, closing to the operational mass balance.

Accountancy as continuous conservation

Byproduct-material accountancy answers a regulator's core question: is every quantity of regulated radioactive material accounted for at all times? L7 answers it continuously by identifying accountancy with the operational isotope mass balance. Tritium and helium-3 inventories, and the activation inventory of components, are the same numbers the plant uses to run itself, so accountancy is never a reconstruction after the fact.

Material balance areas

The plant is divided into material balance areas (breeding blanket and tritium plant, isotope storage and shipping, neutron-service positions, activated-component handling). Each area maintains its own balance, and inter-area transfers are logged as paired debit and credit entries so material moving between areas is never lost or double-counted. The plant balance is the sum of area balances plus logged inflows and outflows.

# an inter-area transfer is a paired, conserving event
def transfer(material, qty, src_area, dst_area, t):
    assert qty <= balance(src_area, material, t)
    debit(src_area, material, qty, t)      # leaves source
    credit(dst_area, material, qty, t)     # enters destination
    log_custody(material, qty, src_area, dst_area, t)
    # plant total conserved by construction

Decay is an accountancy term, not an error. Tritium decays to helium-3; the accountancy debits tritium and credits helium-3 by the decay law over each period, so a falling tritium inventory and rising helium-3 inventory are explained by physics, not flagged as loss. Only residuals beyond measurement uncertainty are treated as anomalies to investigate.

Activated components are byproduct material too. When a first-wall module or electrode is removed, its activation inventory (estimated by the twin's neutronics from its exposure history) enters the accountancy as it moves to waste handling or reprocessing, so the material does not disappear from the ledger when it leaves the vessel.

Accountancy runs today on twin-modeled creation, decay, and activation at design stage. The same balance areas and transfer logic carry real measured material from FOAK ~2030 onward, so the regulatory accounting method is validated before it is relied upon.