thorium cycle, by conversion of thorium-232 (Th-232) to U-233), part gets fissioned while still in the reactor—contributing to energy output and reducing the quantity of fissile material that otherwise would need to be provided in fresh fuel—and part is discharged from the reactor in the spent fuel.

The only way to achieve conversion ratios near zero, as would be essential if the aim were to burnup surplus WPu without generating new fissile plutonium or U-233 in the process, would be to use fuel containing no U-238 or Th-232. Such "nonfertile" fuels are possible and have served as the basis for some high-temperature gas-cooled reactor (HTGR) designs. But, as we discuss later, their development for use in existing light-water, heavy-water, or fast reactors would require considerable effort and a corresponding investment of time and money. While nonfertile fuels for HTGRs are closer to availability, more development and higher costs would be involved in using these reactors than in using existing reactor types with fertile fuels. Even with nonfertile fuels in hand, it would not be easy to burn an initial stock of WPu down to zero, for reasons to which we now turn.

An important part of the difficulty of "burning up" fissile material completely is that, for fertile and nonfertile fuels alike, the fuel tends to lose either its structural integrity or its capacity to sustain a chain reaction long before its fissile content is exhausted. It tends to lose its structural integrity because of the combined effects of cyclic thermal stresses, corrosion, and the structural damage caused by fission neutrons and the fission-product tracks, as well as the problems posed by the pressure from gaseous fission products; when these take too high a toll, the result is excessive leakage of fission products into the reactor coolant, generating problems in maintenance and compliance with environmental standards. ¹⁰ Or, before the fuel starts to lose structural integrity, its reactivity may fall below the level required, because of the combination of diminishing density of fuel nuclei as these are burned up and growing density of fission products, some of which are strong neutron poisons (absorbers). The amount of fission energy derivable from fuel before this happens can be increased-within the limits of fuel structural integrity—by increasing the initial concentration of fissile nuclei. This measure may require the addition to the fuel of "burnable poisons" to offset the high reactivity that would otherwise be associated with the high initial fuel density.¹¹ It may be attractive for economic reasons (to reduce the amount of fuel that must be fabricated for a given energy output)-or to