Pebble-bed reactor
These TRISO particles consist of a fissile material (such as 235U) surrounded by a ceramic coating of silicon carbide for structural integrity and fission product containment.
The core is cooled by a gas that does not react chemically with the fuel elements, such as helium, nitrogen or carbon dioxide.
Such high temperatures allow higher thermal efficiencies than possible in traditional nuclear power plants (up to 50%) while the gases do not dissolve contaminants or absorb neutrons as water does, so the core has less in the way of radioactive fluids.
The concept was first suggested by Farrington Daniels in the 1940s, inspired by the innovative design of the Benghazi burner by British desert troops in WWII.
The HTR-10 prototype was developed into China's HTR-PM demonstration plant, which connects two reactors to a single turbine producing 210 MWe, operating commercially since 2023.
[6] The uranium, thorium or plutonium nuclear fuels are in the form of a ceramic (usually oxides or carbides) contained within spherical pebbles a little smaller than the size of a tennis ball and made of pyrolytic graphite, which acts as the primary neutron moderator.
The pebble design is relatively simple, with each sphere consisting of the nuclear fuel, fission product barrier, and moderator (which in a traditional water reactor would all be different parts).
Much of the cost of a conventional, water-cooled nuclear power plant is due to cooling system complexity, which is not a factor in PBRs.
Such reactors do not need to operate well at the varying neutron profiles caused by partially withdrawn control rods.
Pebbles travel from the bottom to the top about ten times over a period of years, and are tested after each pass.
Doppler broadening therefore creates a negative feedback: as fuel temperature increases, reactor power decreases.
[citation needed] Even in the event that all supporting machinery fails, the reactor will not crack, melt, explode or spew hazardous wastes.
In a safety test using the German AVR reactor, all the control rods were removed, and coolant flow was halted.
However, the effect was not anticipated, and since the reactor was cooled by ambient air in an open cycle, the process could not be reliably controlled, and led to a fire.
It slows neutrons effectively, is strong, inexpensive, and has a long history of use in reactors and other high temperature applications.
For example, pyrolytic graphite is also used, unreinforced, to construct missile reentry nose-cones and large solid rocket nozzles.
Some fission products such as 133Xe have limited absorbance in carbon, so some fuel kernels could accumulate enough gas to rupture the silicon carbide.
[citation needed] Some designs do not include a containment building, leaving reactors more vulnerable to attack.
[citation needed] In 2008, a report[13][14] about safety aspects of Germany's AVR reactor and general PBR features drew attention.
The crucial insight was to combine fuel, structure, containment, and neutron moderator in a small, strong sphere.
[citation needed] The leak in the steam generator leading to this accident was probably caused by high core temperatures (see criticism section).
Even though the power generation used primary coolant, it was reported that the AVR exposed its personnel to less than 1/5 as much radiation as a typical light water reactor.
During this examination it was revealed that the AVR was the world's most heavily beta-contaminated (strontium-90) nuclear installation and that this contamination was present as dust (the worst form).
Trying to restart the pebbles' movement by increasing gas flow stirred up dust, always present in PBRs, which was then released, unfiltered, into the environment due to an erroneously open valve.
[citation needed] In spite of the limited amount of radioactivity released (0.1 GBq 60Co, 137Cs, 233Pa), a commission of inquiry was appointed.
The handling of this minor accident severely damaged the credibility of the German pebble-bed community, which lost support in Germany.
Pebble debris and graphite dust blocked some of the coolant channels in the bottom reflector, as was discovered during fuel removal after final shut-down.
[citation needed] In 2004 China licensed the AVR technology and developed a reactor for power generation.
[22] In June 2004, it was announced that a new PBMR would be built at Koeberg, South Africa by Eskom, the government-owned electrical utility to operate at 940 °C (1,720 °F).
[24] The reactor was never completed and the testing facility was decommissioned and placed in a "care and maintenance mode" to protect the IP and the assets.
