I asked a consultant urological radiologist why a prostate doesn't work after RT, if the X-rays only kill the cancer cells.
The answer was that the non-cancerous prostate glandular cells would still work. However, the repeated exposure of the prostate muscle to daily X-rays results in the generation of lots of fibrous material in the muscle as it heals, and it can't contract to expel the fluid. Also, many of the small ducts may be blocked by fibrosis. A prostate is typically around 5% effective after RT, but this can vary from nothing in some people, to very significant recovery for others.
Someone I know has recovered over half his ejaculate having finished RT and HT. It's worth noting that over half of the semen comes from the seminal vesicles, and the prostate contributes a smaller proportion. If someone had their prostate but not seminal vesicles irradiated, they may still ejaculate something looking like semen, but missing the prostate contribution (which is a clear fluid, so not obviously missing). If that applies to you, I think you could try an experiment to confirm if the prostate contribution is missing. A substantial part of the contribution from the prostate is PSA. PSA's job is to liquify the semen so the sperm can swim out, and this normally happens within a few minutes of ejaculation - semen will go runny and clearer. If yours doesn't, you are probably missing the prostate's contribution to it.
If you had spread in/near the seminal vesicles or were a high risk patient (PSA => 20, or Gleason >= 8, or T3 or N1), you will normally have your seminal vesicles included in the RT target area too, in which case they are likely to be very ineffective afterwards too. Even if they are working, they might not still have functioning ejaculatory ducts through the irradiated prostate, but if you get white coloured semen, then your seminal vesicles are working to some extent, and managing to get their contribution through the prostate.