Containment Protocols:
It is currently undetermined as to how effectively RPC-733 can be contained.
Condense to: "The efficacy of RPC-733's containment is currently undetermined."
All RPC-733-Δ instances are uncontainable, but appear to be contained within RPC-733 for the time being.
Reword slightly: "While uncontainable, all RPC-733-Δ instances remained confined within RPC-733."
Description:
From available footage it has been concluded that the dimension contains an alternate timeline that ensued during the late 1940s, in which a cosmic threat annihilated all life on earth, the remains of which are patrolled by RPC-733-Δ.
Hmm, feels a bit iffy, let's rewrite this to: "It has been determined that RPC-733 produces an alternative timeline occurring during the 1940's, where an extra-planetary hazard eliminated all life on Earth, the remains of which are patrolled by RPC-733-Δ."
annihilated all life on earth
Capitalize 'Earth.' (but since I rewrote the involved sentence prior ignore this)
They are speculated to be 250 - 300 m tall.
Substitute 'speculated' -> 'estimated.' Close the spacing between the hyphen and the two measurements as well.
Noticeable features include
Replace 'noticeable' -> 'notable.'
… as all available footage of active RPC-733-Δ instances is shot on black and white film…
You can replace 'black and white' -> 'monochromatic.'
… of a strong energy that is not currently understood.
Reword slightly to: "… of intense energy that is currently undetermined."
procedure; If it has spent too much time
Substitute your semicolon for a period, begin a new sentence here.
If it has spent too much time in an alternate dimension (exact time has yet to be determined), the RPC-733-Δ instance will create a rift in space-time, returning itself to its place of origin.
Condense to: "If remaining in an outside dimension for a period of time, the aforementioned instance materializes a spatial rift, thereby returning itself to its original location."
RPC-733-Δ is considered the primary force utilised to initiate the aforementioned threat, and is assumed to be a mere foot soldier.
I'm rather confused on what you specify here. It would be helpful to clarify in this section of the paragraph (or just let me know in a response).
When in Authority custody, the memory banks of the RPC-733-Δ instance harbored various reports by RPC-733-Ω about the progress of their creation.
Substitute 'when in' -> 'while in.' Also, you should transplant the paragraph of which the discovery takes place into a supplementary section instead of the main description, therefore you can focus on one topic at a time, which is describing it.
RPC-733-Δ are the result of work by a powerful entity
Show, don't tell. Remove 'powerful.'
and is actively attempting to reach them, presenting a very real threat to this dimension.
Again, same thing, remove the part of the sentence proceeding the comma. End it after 'them.' Let the reader come to their own premonitions.
Addendum:
Nothing notable in the audio logs or subsequent addendum.
Overview: So overall, this piece is 'eh.' I was somewhat content with the anomalous properties of the RPC, and I did derive a 'Terminator'-esque feeling from it, specifically with how it describes the skeleton-like entities that are composed of an extremely durable material (though I'm not sure whether that was your first source of inspiration of this piece, if at all). However, I felt as though nothing particularly outstanding came from them apart from "they destroy things." You should make an effort to derive from their anomalous properties in a method that distinguishes it from said trope, as you could lose your reader's attention otherwise.
Second, the recording logs feel like your typical "good intentions gone bad" sequence. You need to establish something that allows the reader to connect with Dr. Ivanov. For instance, you could say that he was previously penalized by the Soviets, and that this project is a potential outlet to commute whatever sentence he was put up against initially provided it serves as a significant contribution in the war effort. From there on, you can document his faithful attitude toward the project, before understanding the limitless capabilities of the said minerals that he previously discovered.
For now, I don't feel it's as ready as it should be. I recommend, in addition to my grammatical suggestions, to work primarily on both the later paragraphs of the description, as well as the audio logs in the manner described above.