Pebble Bed Reactor - Criticisms of The Reactor Design

Criticisms of The Reactor Design

The most common criticism of pebble bed reactors is that encasing the fuel in combustible graphite poses a hazard. When the graphite burns, fuel material could potentially be carried away in smoke from the fire. Since burning graphite requires oxygen, the fuel kernels are coated with a layer of silicon carbide, and the reaction vessel is purged of oxygen. While silicon carbide is strong in abrasion and compression applications, it does not have the same strength against expansion and shear forces. Some fission products such as xenon-133 have a limited absorbance in carbon, and some fuel kernels could accumulate enough gas to rupture the silicon carbide layer.]) from May 2008">citation needed]]] Even a cracked pebble will not burn without oxygen, but the fuel pebble may not be rotated out and inspected for months, leaving a window of vulnerability.

Some designs for pebble bed reactors lack a containment building, potentially making such reactors more vulnerable to outside attack and allowing radioactive material to spread in the case of an explosion. However, the current emphasis on reactor safety means that any new design will likely have a strong reinforced concrete containment structure. Also, any explosion would most likely be caused by an external factor, as the design does not suffer from the steam explosion-vulnerability of some water-cooled reactors.

Since the fuel is contained in graphite pebbles, the volume of radioactive waste is much greater, but contains about the same radioactivity when measured in becquerels per kilowatt-hour. The waste tends to be less hazardous and simpler to handle. Current US legislation requires all waste to be safely contained, therefore pebble bed reactors would increase existing storage problems. Defects in the production of pebbles may also cause problems. The radioactive waste must either be safely stored for many human generations, typically in a deep geological repository, reprocessed, transmuted in a different type of reactor, or disposed of by some other alternative method yet to be devised. The graphite pebbles are more difficult to reprocess due to their construction, which is not true of the fuel from other types of reactors. Proponents point out that this is a plus, as it is difficult to re-use pebble bed reactor waste for nuclear weapons.

Critics also often point out an accident in Germany in 1986, which involved a jammed pebble damaged by the reactor operators when they were attempting to dislodge it from a feeder tube (see THTR-300 section). This accident released radiation into the surrounding area, and probably was one reason for the shutdown of the research program by the West German government.

In 2008, a report 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. Main points of discussion are

• No possibility to place standard measurement equipment in the pebble bed core, i.e. pebble bed = black box
• 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.
• improper temperatures in the core (more than 200 °C above calculated values)
• necessity of a pressure retaining containment
• 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.

There is significantly less experience with production scale Pebble Bed Reactors than Light Water Reactors. As such, claims made by both proponents and detractors are more theory-based than based on practical experience.