Page "Pebble bed reactor" Paragraph 8

Much of the cost of a conventional, water-cooled nuclear power plant is due to cooling system complexity.

These are part of the safety of the overall design, and thus require extensive safety systems and redundant backups.

A water-cooled reactor is generally dwarfed by the cooling systems attached to it.

Additional issues are that the core irradiates the water with neutrons causing the water and impurities dissolved in it to become radioactive and that the high pressure piping in the primary side becomes embrittled and requires continual inspection and eventual replacement.