A safer nuclear reactor for the future

Kelvin Teo

A nuclear power plant

A nuclear power plant

Nuclear power has somehow become an option of choice for policy-makers seeking to meet energy needs of their countries and at the same time reduce carbon dioxide emissions. When the road to nuclear power has been chosen, it is then the matter of choosing from another set of paths to take. This set of paths refer to the type of nuclear reactors to establish, of which there exist a variety of choices. There are a number of proposed theoretical nuclear reactor design concepts, which are currently under research. The term for these theoretical designs is Generation IV reactors and they are expected to come into service by 2030 at the very least.

Two types of Generation IV nuclear reactors will be explored in greater detail here – the Pebble Bed Modular Reactor and the Thorium Molten Salt reactor. Originally invented and tested in Germany during the 1970s and 1980s, the Pebble Bed Modular Reactor is making a comeback of its own at the Massachusetts Institute of Technology, China and South Africa. Recent WikiLeaks documents has also revealed the Singapore government’s interest in the Pebble Bed Reactor; the republic was reported to be playing a waiting game, holding out to see if the U.S. Nuclear Regulatory Committee approves the Pebble Bed Reactor technology.

The Thorium Molten Salt Reactor, on the other hand, was experimented on from 1964 to 1969 at the Oak Ridge National Laboratories, USA. Like the Pebble Bed Modular Reactor, the Thorium Molten Salt Reactor has undergone a renaissance of some sorts. An international consortium from U.S., Japan and Russia are developing a reactor dubbed the Fuji Molten Salt Reactor based on the same concept as the one experimented earlier at the Oak Ridge National Laboratories. China has also joined the race, announcing earlier this year that it plans to develop a thorium-fuelled molten salt reactor.

What are the essential features of the core of both reactors? The Pebble Bed Modular Reactor core comprises 360,000 uranium-fuelled pebbles each of which is the size of a tennis ball. Each ball contains 9 grams of uranium. Uranium is packed in smaller sized microspheres and is encased by a material known as silicon carbide. There are 10,000 – 15,000 of such microspheres within one ball and they are embedded within a graphite matrix. As for the Thorium Molten Salt Reactor, it can be inferred from its label that the fuel, thorium, is dissolved in a molten salt such as lithium fluoride.

The following provides a summary of the advantages of the Pebble Bed Modular and Thorium Molten Salt Reactors respectively.

Pebble Bed Modular Reactor

Click to enlarge

  • Modular design, i.e. allows the parts of the plant to be assembled “Lego” style.
  • Meltdown-proof, due to passive cooling mechanisms. Even if the coolant system is knocked off, passive cooling mechanisms via convection mainly and additionally conduction plus radiation will dissipate the heat.
  • Uses an inert coolant, helium.
  • Allows for online refuelling, i.e. the plant does not have to be shutdown during the refuelling process. Used fuel pebbles may be removed and new fuel pebbles can be added even during operation

Thorium Molten Salt Reactor

  • Uses thorium, which is in relatively abundant supply.
  • Meltdown-proof, even if the molten salt containing the fuel leaks, it will solidify upon exposure to room temperature.
  • Operates at low pressure, which reduces the risk of a pressure-mediated blast.
  • Clean form of energy, produces wastes which are mainly fission products.
  • Allows for refuelling online, without the need for the plant to shutdown

It appears that both Pebble Bed Modular and Thorium Molten Salt Reactors are feasible candidates for our next generation of reactors. However, a 2004 research publication by Ralph Moir and Edward Teller from the Lawrence Livermore National Laboratory, California, that suggests safer designs for the Thorium Molten Salt Reactor may have tipped the scales in its favour of being a safer nuclear reactor in comparison with the Pebble Bed Modular Reactor.

They suggested an underground Thorium Molten Salt Reactor in which the burning of the nuclear fuel, thorium within the molten salt, takes place 10 metres underground. The essential safety feature of this molten salt reactor is that the nuclear fission reactions, i.e. the radioactive part, are meant to take place underground, while the part of the plant that is located above ground comprises electric generators fed by non-radioactive liquid. Hence, what is located above the ground is essentially non-radioactive. If there is an unfortunate spill of radioactive materials, it will essentially remain underground. Any residual heat is carried by the heat exchangers to be dissipated within the surroundings above the ground.

This underground design for the Thorium Molten Salt Reactor addresses one potential Achilles heel of the Pebble Bed Modular Reactor – the fact that the latter does not have a containment building, unlike conventional nuclear reactors. Since the reactor relies on convection as a cooling process, a containment building is absent. Thus, the only thing that protects against a radiation breach is the integrity of pebbles – they must be defect-free.

However, for every 10,000 microspheres within a pebble, one will harbour a defect, which results in radioactive materials entering the coolant. However, if we assume one out of every 10,000 microspheres has a defect, the amount of radioactive materials released is still considered too small to warrant an emergency response. Ultimately, however, safety of reactor is dependent on manufacturing quality of the pebbles, and the fact that there are many of such pebbles within the reactor carries with it certain risks.

Hence, the safety features of an underground Thorium Molten Salt Reactor cannot be ignored. In addition, the fact that it is underground offers protection against potential terrorist attacks, and its overall design makes it hard for those intending to harness the fuel for weapon production purposes. Thus, this reactor has an advantage over the Pebble Bed Modular Reactor and pips the latter in the safety category. It is a more viable choice where public safety concerns take high priority.

Photo courtesy of Martin Prochnik, Flickr Commons. Pebble Bed Reactor diagram courtesy of University of California at Berkeley. Thorium Molten Salt Reactor diagram courtesy of Wikimedia Commons.