Denaturation (fissile materials) Wikipedia
Denaturation of fissile materials suitable for nuclear weapons is the process of transforming them into a form that is not suitable for weapons use and can not easily be reversely transformed. For Uranium 235 this is straightforward, by mixing it with Uranium 238, but for plutonium it is more difficult and/or less effective, because other plutonium isotopes are either also suitable for weapons or not available and not practical to produce, while mixing with another element allows chemical separation.
The situation with Uranium-233 is more drastic. Decay of the attached Uranium-232 produces Thorium-228 with a radioactive half-life of 1.9 years and several short-lived daughter nuclides; these daughters include some very hard gamma-ray emitters like Thallium-208 and Lead-212. After approximately one single year the alpha activity of these decay products is several hundred curies per kilogram of U-233, and the gamma penetration radiation is a thousand times larger to some than from the plutonium. Aged U-233 is self-protected radiologically from diversion.