One thing about THAAD, the “kill” is accomplished by impact, not explosion. When something impacts at the velocity we are talking about the material is disintegrated into atomic “sand”. As others have indicated, a nuclear explosion must occur precisely as designed, any error will result in a non-nuclear break up but as already understood, the impact with the THAAD kill vehicle will result in a cloud of nuclear particles and no nuclear explosion. The cloud will spread in the upper atmosphere where it will add to the Van Allan radiation belts around the planet. They will be in the space atmosphere and will add to this radiation but this is considered preferable to a ground detonation. Right?
I didn’t think a THAAD kill was high enough to add to the Van Allen belts. The addition would minuscule cat best.