In 2008, a report[12][13] about safety aspects of the AVR reactor in Germany and some general features of pebble-bed reactors have drawn attention. The claims are under contention.[14] Main points of discussion are
1. No possibility to place standard measurement equipment in the pebble-bed core, i.e. pebble bed = black box
2. Contamination of the cooling circuit with metallic fission products (Sr-90, Cs-137) due to the insufficient retention capabilities of fuel pebbles for metallic fission products. Even modern fuel elements do not sufficiently retain strontium and cesium.
3. improper temperatures in the core (more than 200 °C above calculated values)
4. necessity of a pressure retaining containment
5. unresolved problems with dust formation by pebble friction (dust acts as a mobile fission product carrier)
Rainer Moormann, author of the report, requests for safety reasons a limitation of average hot Helium temperatures to 800 °C minus the uncertainty of the core temperatures (which is at present at about 200 °C).
The pebble-bed reactor has an advantage over traditional reactors in that the gases do not dissolve contaminants or absorb neutrons as water does, so the core has less in the way of radioactive fluids. However, the pebbles generate graphite particulates that can blow through the coolant loop and will absorb fission products if fission products escape the TRISO particles.
