In designing repositories for HLW and spent nuclear fuel, an important consideration is heat output. During the operational period, temperatures can be controlled by ventilation but, after backfilling and closure, this is no longer possible and temperatures will rise. To avoid damage to the engineered barriers, it is usual to specify a limiting peak temperature on the surface of the waste canister (often 100 °C). This is implemented by increasing the spacing between adjacent waste packages so that the volume of rock that is available to absorb and conduct away the heat is sufficiently large to keep temperatures at acceptable levels. Increasing the size of the repository in this way is expensive and may also rule out some otherwise acceptable sites. Both are reasons why heat-producing waste may need to be cooled for decades before it can be disposed.

Long-lived ILW, which also requires deep disposal, has a much lower heat output so that, even when the waste packages are stacked so as to completely fill the emplacement tunnels, the temperature increases are no more than a few tens of degrees. This reduces the cost of deep disposal for this type of waste.