Special Containment Procedures: SCP-XXXX along with its movable part is to be contained in a locked, medium security vault, under direct surveillance of the Security staff, to prevent its use outside of authorized testing.
Could be simplified. Vaults are implicitly locked, I don't understand yet what the "movable part" is, and if the SCP might do something even when left alone then the containment procedures should clarify this, and what to do in such an event.
Due to instances of SCP-XXXX-C being highly unpredictable, tests may be conducted under overseeing of the Chief of Security in the adjacent cell.
Bit awkward. I'd go with "Due to the highly unpredictable nature of SCP-XXXX-C instances, tests may be conducted only under the Chief of Security's supervision."
The room is to be properly equipped with automated decontamination procedures, and incinerators are to be activated if deemed necessary by Security.
A procedure is just a list of instructions. Did you mean to equip the room with something more specific? If so, what is that specific thing? And maybe clarify that the incinerators are manually activated, I guess.
X-Ray and other methods of scanning revealed less of its interiors than expected, however it can be inferred that it operates as a cold fusion and/or fission reactor. No radiation was ever detected during testing.
I'd go with "provided little conclusive information." I'm not quite sold on this thing operating via cold fusion. That second part could probably just be dropped entirely.
I also don't quite understand how the incident log ties in to the rest of the SCP. And that [DATA EXPUNGED] is unnecessary, given the description of the thing that was supposedly expunged. Go for it with both barrels; either give us details to work with or leave it to the imagination.
Lastly, I'd personally classify this as Safe, since the SCP does nothing if not activated. But others may disagree.
Overall, the concept is interesting, but the execution could use some work. Some fat could be trimmed (e.g. word choice, shorter sentences, unnecessary explanations, etc.).
Crit of the other SCP is coming up soon. Keep it up, this one is interesting!