Speaking as a person who has built a number of neutron collimators: you can not collimate the neutrons inside of a reactor.
Your question seems to be about comparing the motion of neutrons in a reactor to the motion of photons in a laser. Let's talk briefly about how a laser works:
You have some "gain medium" which you can undergo a "population inversion," where more of the medium is in an excited state than in the ground state.
Some thermodynamically-unimportant fraction of this gain medium relaxes to the ground state, emitting a photon.
This photon stimulates the coherent emission of other photons from other parts of the gain medium.
The coherent light builds up in intensity because the gain medium is inside of a resonant optical cavity.
The optical cavity has such a large quality factor ("$Q$-factor") that the trapped coherent light becomes extremely intense, and some of it is allowed to leak out.
While a long, linear optical cavity is the most common design for macroscopic lasers, like the historically important ruby laser or the various kinds of gas or dye lasers, the optical cavity can actually have any shape. In a diode laser, where the gain medium is a piece of semiconductor, the volume of the optical cavity is defined by the size of the diode, which is extremely small. The laser light emitted by a diode has a huge divergence, and to make a "ray" requires some aligning optics. That's very different from the intrinsically long optical cavity in something like a helium-neon laser, where the light which leaks out is already fairly well collimated, because nonparallel light has a low gain in the cavity.
If you like, you can think of the fissile material in a reactor as a "gain medium" which turns neutrons into more neutrons. But crucially, the new neutrons are incoherent. In laser emission, each stimulated photon has the same energy as the photon which stimulated it. But in a reactor, you need a population of thermal or colder neutrons to induce fission, while the fission neutrons are released in a high-energy state. A crucial part of keeping a reactor going is removing the heat from the new fission neutrons, so that they can thermalize and induce the next fission.
If you are at a reactor and you'd like a linear beam of neutrons, what you do is to cut a hole in the side of the reactor, so that some of the neutrons exit the hole instead of contributing to the reaction. You can keep more of the neutrons if you put a tube between the hole in your reactor wall and the place you'd like the neutrons to go. There is an entire sophisticated technology ecosystem about removing neutrons from a reactor core or other neutron source and bringing the neutrons to a distant experiment.
In such a neutron beam, on the outside of a neutron reactor, a "collimator" is a piece of neutron absorber with a hole in it, to make the beam smaller. For example, you might have a gap in your neutron guide for some apparatus, such as a polarizer or a velocity selector. In the gap, the beam diverges a little bit, and the collimating absorber eats up all of the neutrons who would miss the neutron inlet of your next piece of equipment.