The Neutron Economy

Every fusion neutron is a resource: it can breed tritium, multiply into more neutrons, or be lost, and balancing this budget sets the breeding ratio.

Neutrons as a budget

In a deuterium-tritium plant, the 14 MeV neutrons carry the energy and drive breeding. Each neutron can be captured in lithium to make tritium, multiplied into additional neutrons by certain materials, absorbed uselessly by structure, or leaked out. The tritium breeding ratio is the outcome of how this neutron budget is spent.

Levers on the budget

Designers raise the breeding ratio by adding neutron multipliers, enriching lithium-6 for reliable capture, retaining lithium-7 for its extra-neutron reaction, improving blanket coverage to reduce leakage, and choosing low-absorption structural materials. Each lever trades against engineering complexity and against the machine's other requirements.

• Multipliers add neutrons to the budget
• Lithium enrichment tunes capture reliability
• Full blanket coverage reduces leakage
• Low-absorption structures preserve neutrons for breeding

Coupled to breeding and waste

The neutron economy sets the achievable breeding ratio and also drives material activation, linking the fuel cycle to the waste case. See the breeding-ratio lever and lithium-6 versus lithium-7.

Spending a scarce budget wisely

Each fusion neutron is spent only once, so the breeding ratio is ultimately an accounting of how carefully that budget is allocated. Multipliers add to the budget, enrichment directs it toward reliable capture, full coverage prevents leakage, and low-absorption structures avoid wasting it. Every lever trades against engineering complexity, which is why the neutron economy is one of the defining design problems of a breeding machine.