Idle speculation from someone who is not a nuclear or a chemical engineer:

I'm suddenly wondering if the pressure relief valves for the primary coolant system could be designed with ignition circuits. The idea would be to burn off any hydrogen in a controlled fashion as it exits the coolant system, rather than giving it a chance to accumulate and later detonate. It would be similar to the way oil rigs burn off unwanted natural gas.

No doubt there's some very good reason for not doing this that I'm just not aware of. :-P