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The ratio of the rate of formation of fissile nuclei to the rate at which the specified fissile inventory is consumed, under stated reaction and accounting conventions. A conversion ratio below one indicates a replacement shortfall in that inventory. It is distinct from neutron multiplication: a reactor's instantaneous neutron balance and its long-term fuel-inventory balance are different questions.

The conversion ratio usually tends to increase as moderation is reduced. A harder spectrum gives greater importance to capture in fertile fuel, while less water can reduce some parasitic neutron absorption. These tendencies can increase replacement of fissile material during irradiation. Competing leakage, fast-fission and spectral effects mean the exact change is not fixed by the hydrogen-to-heavy-metal ratio alone.

Higher conversion can support greater discharge burnup and a longer nuclear fuel cycle length, but neither follows automatically. The improvement tends to slow the loss of useful fissile inventory during irradiation. The same change of lattice, however, lowers beginning-of-cycle reactor reactivity under the usual under-moderated assumptions. With enrichment unchanged, the available initial reactivity margin may become inadequate before any later benefit from improved conversion can be realized.

Under the same-rod-number and same-height assumptions, the initial heavy-metal mass of nuclear fuel $M_{\mathrm{HM}}$ and thermal power $P$ are unchanged. Thus the rate of accumulating energy-per-mass burnup is unchanged:For a constant-power irradiation interval $t$, $B=Pt/M_{\mathrm{HM}}$. A longer achievable interval would consequently increase the burnup accumulated in that interval. This relation alone says nothing about whether that interval remains possible.

If sufficient initial reactor reactivity, shutdown capability and thermal margins are retained, slower depletion can allow increased discharge burnup and a longer cycle. If initial reactivity is too low, or the reduced boiling margin or fuel limits become controlling, either can instead decrease. Discharge burnup also depends on batch residence and refuelling strategy, so it is not generally identical to burnup accumulated in one operating cycle.

The appropriate qualitative answer is therefore conversion tends upward; achievable discharge burnup and cycle length are conditional on the full reactivity history and operating limits. An increased conversion ratio by itself does not determine a longer cycle.

The production rate is ${\sigma }_{c\mathrm{Th}}\varphi N_{\mathrm{Th}}$, while depletion includes both capture and fission of the fissile population, ${\sigma }_{a\mathrm{U}}\varphi N_{\mathrm{U}}$. Thus the conversion ratio in this stipulated model isAt zero instantaneous flux this expression describes the limiting ratio of the two reaction rates. The total absorption coefficients are $a=3.08+0.02=3.10\mathrm{b}$ and $b=7.10+57.10=64.20\mathrm{b}$, soIt is below one: fissile nuclei are initially removed slightly faster than this simplified chain replaces them. Consequently this model is not initially self-replacing in fissile inventory. Inventory replacement is distinct from the instantaneous neutron balance that determines criticality.