having googled the cat business, you have to propos that the cat will be dead in the time it takes the air to run out in the vacuum filled steel chamber
this will be a lot sooner than the radioactive substance (which could also be porposed woudl kill the cat) runs out and breaks the cat killing capsule with the cat killing acid in it

to presume that the cat will be alive indeffinitly is a falicy because you have sealed the cat in it's doom when you have closed the chamber door
if you were not to open the door within a matter of minutes the cat would certainly perish

(all this is assuming that there is a small bax the cat is going to be placed into, not a big steel room the size of an oil tanker which would give the cat a lot longer to live because the air would be more sustainable and it also doesn't prusme that there would be food and water left inside for said cat to nourish itself on for the duration of int experimental incarcaration)