Basically, shielding is just that. We shield it, which we do all the time in X-rays and treating cancers; we shield the people we don't want exposed. The steel acts as a shield. If you breach it, of course, radioactivity will get out.
These are very safe as they are. You could go with what-ifs for many different things that might happen, but as the intact boiler is being shipped, there's very low-level waste.