Are there serious environment risks with fusion reactors?

My understanding was that very little radioactive waste was created from a fusion reactor and what little there is will decay pretty quickly (decades).

A fusion reactor creates a much more intense flux of neutrons than any fission reactor, which will transmute into radioactive isotopes any substance from which a shield will be made.

So the quantity of radioactive waste will certainly not be little, but more likely much greater than in a fission reactor.

Nevertheless, because there is more freedom in the design of the neutron shield than in a fission reactor, it is likely that it is possible to find such compositions where most of the radioactive waste will decay quickly enough, so that there will remain only a small quantity of long-lived radioactive waste.

However, until someone demonstrates this in reality, it is still uncertain how much radioactive waste will be generated, because this depends on many constructive details.

A lot of components of a fusion reactor, e.g. pipes for cooling fluid and the like, will become damaged by the neutrons and they will have to be replaced periodically, after becoming radioactive. The amount of such waste will depend a lot on the lifetimes of such components. For now it is very uncertain how much time such components will resist before requiring maintenance.

The big risk is tritium leakage.

To show the scale of the problem: if the world were powered by Helion's reactors (for all primary energy), and the tritium produced were just released into the environment and mixed completely with all water on the planet (including oceans, lakes, rivers, ground water, and ice), then it would lift all that water above the US regulatory limit for tritium in drinking water. All the water, including everything in every ocean.

You are correct. Radioactive materials from fusion reactors are not a significant environmental threat. The bigger problem with fusion reactors is that nobody has yet built a controlled terrestrial fusion reactor that produces net power.

Well we don't have working fusion reactor topology yet but the current "reactor" components are low level waste so safe within 40-100 years. Which is still a hell of a long time. Also they still will require biological shielding and associated materials are quite difficult to deal with (concrete etc).

I expect that the longevity of their attention is considerably less than this, particularly if the LLM boom crashes. ROI will not pay for the disposal later down the line.