There are a host of particular features that I envision for a RKKV which are not otherwise well-written about, so I want to quality my answer by saying that I have not seen these details described in other places.
- Nano-particles make the most sense as the projectile. This comes from a balance between minimizing penetration depth, avoiding unwanted disruptive interactions along the way, manageability, and charge-to-mass ratio
- The particle would be moved along in a particle accelerator
- The energy source for the acceleration would be a capacitor bank onboard the ship, heat energy is deposited directly into them and the magnetic coils (not much because of superconductors, most energy goes into the projectile)
- To the question, considering the safety aspects, engineers would either strive very hard to get it up-to-speed in a linear accelerator, or they would not put critical equipment in the free-path trajectory in a circular accelerator
The best analog to the safety situation on Earth would be the LHC. There are genuine safety concerns for the case where the confining coils lose power, and then the beam of particles plows into the wall and penetrating into the surrounding soil.
To be perfectly blunt about a battle situation, the biggest problem might be the need for such a large power plant and sufficient capacitive storage to generate and hold enough energy to power this weapon. Whatever ship contained the weapon would not be able to move very fast due to this extremely heavy loading. Yield will scale up in direct proportion to its mass. So in a "battle" (if such a thing can ever happen in space in the traditional form), the ship would be a sitting duck for enemy fire because it would have very poor ability to maneuver itself.
Capacitors are very good at fast energy release, so I would favor the case of the linear accelerator. You impart and electrical charge to the particle, and then dump all the juice you have into it within a lightning-fast process. If someone went wrong technically, it would still exit along its intended trajectory, but not at full energy.
Since the particle is small, you would be looking at a highly relativistic speed like 0.999c. As such, it is almost perfectly invulnerable to premature detection. Those killed in the initial blast it causes will die with literally no warning.
This would be safer than a missile, but anytime you cary an enormous amount of energy onboard, you risk premature release of that energy, and having those capacitors accidentally discharge could effectively blow its own crew up. Since no energy storage is perfect, I don't think you can ever fully eliminate such possibilities, even if it is well-engineered and the chances of it happening are remote.