The total mass of reactants in the fusion chamber is below milligram, some of which is bound in stable isotopes. Even if all of it escaped, it would be far from catastrophic.
The reaction itself cannot run away on its own because it requires a delicate balance in temperature and density, which will be immediately disturbed if there was a breach in containment.
The walls will be activated by neutrons, but short of blowing the reactor up, there’s not much chance of materials escaping in a significant amount to pose a danger.