That is possible. But the writer has chosen to write it another way. They have dropped the subject 'particles' - i.e. in full it would be 'These particles block or destroy...'. This is possible since the subject is clear from the previous sentence.

It should be block tiny radioactive particles is just blocking the blood supply not destroying them.

Thank you, Matteo. I get it now.

You're making this unnecessarily complicated. These and they are both pronouns. Simple enough? But these, the plural form of this, falls into a category called demonstrative pronouns. The antecedent for these in the sentence you've mentioned is particles. Since particles is plural, these is used. They can be used correctly, but the author might want to make a point that the specific particles aforementioned are the ones that are in fact doing the destroying or blocking. This is more of an issue of style rather than syntax or semantics.