There's only a few grams of hydrogen in the reactor's plasma, it's reaction with oxygen wouldn't be much more exciting than just losing containment. There are engineering challenges that have to be addressed but no worse than the 6 MW research reactor I used to walk by every day to my college classes in the middle of a dense city.

The proliferation risk of someone using the neutron flux to produce an atomic or dirty bomb are real but that exists no matter where it is.