Page "Pebble bed reactor" Paragraph 9

In contrast, a pebble bed reactor is gas-cooled, sometimes at low pressures.

The spaces between the pebbles form the " piping " in the core.

Since there is no piping in the core and the coolant contains no hydrogen, embrittlement is not a failure concern.

The preferred gas, helium, does not easily absorb neutrons or impurities.

Therefore, compared to water, it is both more efficient and less likely to become radioactive.