They are inside the containment dome and the radiation can’t get outside it.
Get enough leaking tubes on the reactor side of the heat exchanger and voila, you get radioactive water in the clean circuit.
I love nuclear power and I blame this on A) Mitsubishi and B) whoever accepted the design if it was modified.
We need to get Mitsubishi to fix the issue, pay for the downtime and let them get on with making clean electricity.
Oh yeah, if the design was modified, who signed off on the initial design as well as the final product? If Doctors are willing to sacrifice their license to hand out pain killers by the handful, I can see an inspector being more interested in the green stuff in the paperwork, than the results of the actual tests or design.