According to the description in the rulebook to use a Power in fettered mode you have to roll half your dice pool rounded up.

What are the rules for a Primaris Psyker with a Psi Rating of 1? The dice pool is 1 therefore half the dice pool is 0.5 rounded up to 1. Does this mean that the Psyker can consider all Power rolls to be fettered?

Technically, since you always round up, yes. In practice, I'd suggest you just say a Psy 1 psyker simply doesn't have sufficient control to manifest powers at a Fettered level. I'd be pretty surprised if a Primaris Psyker started Ascension with a Psy Rating of 1 anyway. What on earth have they spent all their xp on?

Well it is possible to have a Psy Rating of 1 if the character was a Nascent Psyker before Ascension.

Technically, Imperial Psyker is the only route to Primaris Psyker, but there could indeed be allowances made by a GM in an established campaign that would allow a Nascent Psyker to go from their former career into Primaris.

I'd still stand by Psy 1 being too low in skill and experience to use powers at the Fettered level. It's going to be assumed that Primaris Psykers are Rank 6 or 7 (as they should be, coming from the Imperial Psyker career) and the system then grants them an added safety measure resulting from intensive training and honing of their talents. To me, it doesn't make sense for a Psy 1 (or maybe even up to Psy 4) psyker to have that sort of fine control.

The Ascension rulebook states that Nascent Psykers are eligible to select the Mind Opens (or something like that I cannot remember the exact name right now) transistion package which can lead to the Primaris Psyker, Sage, Interogator and Inquisitor career paths.

The description of this package states that if the character does not have a psy rating (Nascent Psykers do not have a Psy Rating) then they must start with a rating of 1.

It seems to me that a Nascent Psyker could become a Primaris Psyker (a very weak one though). It could be said that such a character is the equivalent of a Dual Class character in DnD.

Ah, yes, I thought there might be something like that.

I still really think that a Psy 1 psyker should not be allowed to do powers Fettered. To me, that's an Ascension-level bonus that best fits a well-experienced psyker. Doing powers with no risk whatsoever from the very start feels inappropriate from both a fluff and game balance perspective to me, and part of the fun of playing a psyker from the beginning is struggling with Perils of the Warp - it tempers you, how often you do powers and so on. It teaches the player restraint as well as the character! (Or it doesn't, and it's a very short-lived character because either the Warp eats them, or the other PCs kill them.)

Ultimately though, since it's not covered in the rules explicitly, and you always round up, half Psy Level for a Psy 1 psyker would indeed be 1.

Well, though it's not really DH RAW, Only War is now available and allow Fettered/Push mechanics for regular Imperially Sanctioned Psychers in the Guard. It uses a different system (the one used from RT and on, if I understand things correctly), and the chance of a phenomena is constant at 10% (rolling doubles on the WP test to focus the power)