In the News: Pebble Bed Modular Reactor
The Pebble Bed Modular Reactor (PBMR) is a new type of high temperature helium gas-cooled nuclear reactor, which builds and advances on world-wide nuclear operators' experience of older reactor designs. The most remarkable feature of these reactors is that they use attributes inherent in and natural to the processes of nuclear energy generation to enhance safety features.
PBMR's are designed to produce 110MW each which means that 30,000 average homes could be sustained by one such reactor. More than one PBMR can be located in a facility thus creating energy parks. It is possible for a PBMR energy park to be made up of a maximum of 10 modules which share a common control center. This system allows sequential construction of modules to match users' growth requirements; as the area grows, so more modules can be added to meet the industrial and domestic needs for electricity in an area.
A single PBMR reactor would consist typically of a single main building, covering an area of 1,300 square meters (50 x 26 m). This area is far less than the area covered by a rugby field or even a soccer field. The height of the building would be 42m, some of it below ground level, depending on the bed rock formations as the building would sit on bed-rock. The part of the building that would be visible above ground is equivalent to a six story building. There would be a unit control room, a high voltage switch yard, and a cooling tower for inland facilities and a sea pump-house for coastal facilities.
Helium gas is passed into the reactor and flows over the fuel pebbles in which a chain reaction is taking place. The helium is heated to a temperature of 900º and pressure increases to 69 bars inside the reactor. The heated helium gas flows through to the turbine which in turn drives a generator. The helium gas then goes through to a very effective recuperator which gives up much of its heat to the helium which is just about to re-enter the reactor - pre or re-heating the helium. The lower-energy helium gas is then passed through the pre-intercooler and inter-cooler and low pressure compressor and high pressure compressor before returning to the reactor core at 540º. Water is used only on the cooling systems.
Two turbo compressors pump helium around the PBMR circuit. Each turbo-compressor sits on its own shaft.
The fuel used in a PBMR consists of "spheres" which are designed in such a way that they contain their radioactivity. The PBMR fuel is based on proven high quality fuel used in Germany.
Each sphere is about the size of a tennis ball and consists of an outer graphite matrix (covering) and an inner fuel zone. The fuel zone of a single sphere can contain up to 15,000 "particles". Each particle is coated with a special barrier coating, which ensures that radioactivity is kept locked inside the particle. One of the barriers, the silicon carbide barrier, is so dense that no gaseous or metallic radioactive products can escape (it retains its density up to temperatures of over 1,700ºC). The reactor is loaded with over 440,000 spheres - three quarters of which are fuel spheres and one quarter graphite spheres - at any one time. Fuel spheres are continually being added to the core from the top and removed from the bottom. The removed spheres are measured to see if all the uranium has been used. If it has, the sphere is sent to the spent fuel storage system, and if not, it is reloaded in the core. An average fuel sphere will pass through the core about 10 times before being discharged. The graphite spheres are always re-used. The graphite spheres are used as a moderator. They absorb and reduce the energy of the neutrons so that these can reach the right energy level needed to sustain the chain reaction.
The design of the of PBMR fuel makes it easy to store the spent fuel, because the silicon carbide coating on the fuel spheres will keep the radioactive decay particles isolated for approximately a million years, which is longer that the activity even of plutonium.
Because the PBMR fuel can be stored on site for at least 80 years, special casks for transporting the spent fuel and storing it at a remote location such as the nuclear waste disposal site. There is no intention to reprocess the spent fuel as this is more difficult than with Koeberg-type fuel. The PBMR fuel also has a greater "burn-up" which makes it less valuable to recycle. More of the useful uranium present in the fuel is used while in the reactor
The spent coated particle fuel can be disposed of in a deep under-ground repository. (Coated particle fuel will maintain its integrity for up to ~ 1 million years in a repository, ensuring that spent fuel radionuclides are contained for extremely long periods of time. The plutonium will have decayed away completely in 250,000 years)
The PBMR is walk-away safe. Its safety is a result of the design, the materials used and the physics processes rather than engineered safety systems as in a Koeberg type reactor.
The peak temperature that can be reached in the reactor core (16,000ºC under the most severe conditions) is far below any sustained temperature (2,000ºC) that will damage the fuel. The reason for this is that the ceramic materials in the fuel such as graphite and silicon carbide - are tougher than diamonds.
Even if a reaction in the core cannot be stopped by small absorbent graphite spheres cooled by the helium, the reactor will cool down naturally on its own in a very short time. This is because the increase in temperature makes the chain reaction less efficient and it therefore ceases to generate power. The size of the core is such that it has a high surface area to volume ratio. This means that the heat it loses through its is more than the heat generated by the decay fission products in the core. Hence the reactor can never (due to its thermal inertia) reach the temperature at which a meltdown would occur. The plant can never be hot enough for long enough to cause damage to the fuel.
The helium itself, which is used to cool the reaction, is chemically and radiologically inert: it cannot combine with other chemicals; it is non-combustible, and non-radioactive.
Because oxygen cannot penetrate the helium, oxygen in the air cannot get into the high temperature core to corrode the graphite used in the reaction or to start a fire. If, through some accident, the helium gas duct (inlet and outlet lines) is ruptured, it would take some nine hours for natural air to circulate through the core. Even if this could happen, it would only lead to less than 10-6 (one millionth) of the radioactivity in the core being released per day. That means that the amount of activity released in 24 hours under this very severe (and recoverable) situation would be some 10,000 times less than that requiring any off-site emergency actions. To avoid such a total failure of the main gas ducting it is designed to leak before it breaks, so that the depressurization will be gradual and cannot lead to such a rupture.
The helium pressure inside the closed cycle gas turbine is higher than the air pressure outside it, so nothing can get inside the nuclear circuit to contaminate it.