Sunday, July 3, 2011

Nuclear Technology Basics: Part 7 Graphite Moderated Reactors

Introduction

Part 1

Part 2

Part 3

Part 4


Part 5

Part 6

Many reactor designs use graphite as a moderator. Graphite is not as effective of a moderator as heavy water, but it is cheaper and it also has a low degree of neutron capture like heavy water. This makes it possible to use un-enriched or natural uranium as fuel. Graphite is somewhat susceptible to corrosion and annealing because the moderator blocks are often located in the hottest part of the reactor. Graphite also has a tendency to expand with prolonged neutron exposure. However, modern reactor designs have mitigated these issues so graphite remains a viable choice for a neutron moderator. The earliest energy reactor designs were graphite moderated, but some generation IV designs also use graphite as a neutron moderator.

1. Gas-Cooled Graphite Moderated Reactors (GCRs)



These reactors use graphite as a neutron moderator, but use pressurized carbon dioxide gas to cool the reactor. The primary example of this reactor type is the now obsolete MAGNOX design, so named for the "magnox" alloy of magnesium and aluminum which was used in the cladding of the reactor's core. The MAGNOX reactor grew out the nuclear weapons production program in Britain, but it also served a dual role for energy production.

The reactor itself consists of a pressure vessel similar to the BWR, with the fuel rods inserted in the top of the pressure vessel like in the LWR. The fuel rods of the reactor are surrounded by blocks of graphite, serving as the neutron moderator. These are situated inside the pressure vessel where carbon dioxide gas is heated by the fuel rods and flows upward and out of the hot gas duct connected near the top of the pressure vessel. The gas is pushed through a heat exchanger where water circulating within a closed loop is heated by the hot carbon dioxide and the resulting steam is used to turn a turbine. As the gas cools, it exits via the cool gas duct at the bottom of the heat exchanger and is sent back to the pressure vessel where it is heated again to complete the cycle.

A more advanced type of GCR was developed from the MAGNOX reactor, which is known as the advanced gas-cooled reactor or AGR. The AGR is similar to the design of the MAGNOX except that the heat exchanger of the AGR is contained within the reactor vessel itself instead of being outside of it like in the MAGNOX reactor. The AGR requires its fuel to be enriched unlike the MAGNOX design because the cladding of the AGR is made out of steel which has a tendency to capture neutrons. The original design of the AGR used beryllium cladding but this proved to be too costly, as beryllium is a very difficult material to process because of its high melting point and its affinity for oxygen at very high temperatures.

The AGR was built to overcome some of the MAGNOX reactor's shortcomings as the AGR was specifically built for energy production rather than the dual production of military grade plutonium. It was seen as a potential challenger to LWR designs, as the AGR was designed to allow refueling while the reactor was still in operation like in the MAGNOX reactor, but the fuel rod removal equipment was shown to be very prone to failure. In addition the design of the AGR was overly complex which added to the costs of construction and operation. In short, GCR reactors have a history of being fraught with technical difficulties and it is unlikely that any more of these types of reactors will be built.

Moderator Type: Graphite

Technology: Generation I-II

Existing Examples: Eighteen GCRs continue to operate in the UK, both of the MAGNOX and AGR designs.

Advantages

-Using a gas as a coolant allowed for higher operating temperatures and thermal efficiency.

-Some designs can use naturally occurring uranium without requiring further enrichment.

-It is more efficient in its fuel utilization than light-water moderated designs.

-It can be refueled without having to shut down the reactor.

-The resulting spent fuel can be stored in a more compact manner because it generates less heat when coming out of the reactor as it is less reactive.

Disadvantages:

-The fuel rod removal systems of these reactors was prone to technical problems.

-The design of the reactors was overly complex it often led to malfunctions.

-These reactors were more costly to build and operate than some reactor types.

-The first reactors of this type were not optimized for power production.

Variants: MAGNOX, AGR

2. High Power Channel-Type Reactors
(RBMKs)




This was a Russian design, and it was the reactor responsible for the infamous Chernobyl incident. This was both because of inherent flaws in this reactor's design as well as the fact that the staff on duty during the Chernobyl incident had attempted to run an unauthorized experiment with the reactor during its operation. With that being said, this reactor type is now considered to be obsolete.

The Reaktor Bolshoy Moshchnosti Kanalniy (RBMK) means "high power channel-type reactor" in Russian. It was similar to the MAGNOX design except it was water-cooled instead of gas-cooled and it heated water directly into steam within a pair of steam separators located inside the reactor pit which was used to turn dual turbines for energy. Because of the graphite moderator, the RBMK can run on unenriched uranium and the fuel rods can also be changed while the reactor is still in operation.

Part of the problem with this reactor design is the fact that it had a very high void coefficient. As the reactor is water-cooled, increases in temperature and pressure can cause the coolant to boil away and turn into steam, and the intensity of nuclear fission rises as the heat increases as the graphite moderator enables fission to continue. The control rods in the Chernobyl reactor were also controlled manually rather than automatically during an emergency. The Chernobyl accident resulted in a loss of coolant flow, which caused massive amounts of heat to build up in the core of the reactor triggering a positive feedback loop. The overheated core immediately vaporized the coolant within the reactor causing a huge steam explosion, similar to that of an overheated boiler within a steam engine.

The resulting steam explosion scattered radioactive particles from the core for hundreds of miles. The graphite surrounding the control rods was ignited by the heat of the reactor core and the roof of the reactor contained bitumen which also started burning. The official death toll released by the Soviet government was a total of thirty one deaths, and most of these were reactor workers and rescue personnel but many more people were thought to be sickened by the release of radioactive particles. To this day, an exclusion zone surrounding the reactor has been declared off-limits to humans but wildlife appears to be thriving there and the degree of ambient radioactivity has dropped considerably since the incident. If the Chernobyl reactor had a containment dome over it like all modern reactor designs do today, the effects of the meltdown on the workers and nearby populace would have been negligible. However, while the Chernobyl disaster and the resulting casualties was indeed a tragedy that could have been prevented with the proper engineering precautions, it was by no means the most severe industrial accident in the modern world. When compared to the Bhopal incident in India or the Banqio dam collapse in China or the yearly death toll resulting from a fossil fuel-based infrastructure dwarfs that of Chernobyl several times over.

Ever since the Chernobyl incident, the few remaining RBMK reactors still in operation have had their safety systems updated to prevent something like this from ever happening again. As it is, RBMKs only exist in Russia. Because this design of reactor is considered to be obsolete, it is unlikely that any new RBMKs will be built in the future.


Moderator Type: Graphite

Technology: Generation I-II

Existing Examples: Eleven in Russia, one in Lithuania.

Advantages

-It can use naturally occurring uranium without requiring further enrichment.

-The reactor generated a lot of electricity with its dual-turbine design

-It can be refueled without having to shut down the reactor.

Disadvantages:

-This design has a very high void coefficient.

-The control rods were under manual instead of automatic control

-The safety features of this reactor design are obsolete

-The Chernobyl disaster has effectively ended interest in the RBMK design.

Variants: None, other than RBMKs with modified safety features.

3. High Temperature Gas-Cooled Reactors
(HTGRs)



Improvements on GCRs has led to interest in developing gas-cooled reactors with a higher operating temperature that would allow for a greater degree of thermal efficiency and a higher degree of fuel utilization than the previous GCR designs. In addition, these reactors are also of a simpler design and use gas turbine systems for power generation leading to a more compact turbine assembly. Unlike previous GCR reactors, HTGRs use helium rather than carbon dioxide as a coolant because of the fact that it is inert even at higher temperatures leading to less corrosion on the piping systems of the circulating coolant.

Early experimental HTGR designs such as the Fort Saint Vrain Generating Station in the US, and the THTR-300 in Germany experienced technical problems or financial difficulties. However, the pebble bed reactor (PBR) shows great promise. The PBR is revolutionary in that its fuel is contained within spherical pellets of graphite that are piled within a chamber inside the reactor vessel. The pyrolytic graphite shells surrounding the fuel cores serves as a neutron moderator while the helium gas that circulates through the spaces between the pebbles serves as a coolant, avoiding the complex piping systems needed for designs that utilize fuel rods. These fuel pellets are circulated through the reactor every thirty seconds or so to be inspected for damage, allowing worn pellets to be removed and new ones to be put into circulation without shutting down the reactor for refueling. In addition, the reactor can hypothetically use graphite shelled pellets of thorium or fuel made from decommissioned nuclear warheads. Finally, the PBR pellets lack the fuel density to allow a meltdown to happen making a meltdown of the reactor physically impossible.

Of course, the PBR carries its own disadvantages, mainly that the PBR design utilizes a once-through fuel cycle and the graphite-sealed fuel pellets are very difficult to recycle in addition to increasing the volume of spent fuel to be disposed of by up to fifty percent. While there is certainly no shortage of fissile material that can be used in a PBR, once-through fuel cycles that make it difficult to engage in nuclear reprocessing are inherently wasteful.

Even though the PBR was originally a German design, political pressure within Germany's government has effectively stalled any and all research for nuclear power generation for the foreseeable future. There was interest in the PBR in South Africa, but political hurdles surrounding the construction of the Koeburg reactor has scared away investors and the reactor project was mothballed in 2010 by the South African government. The only active PBR project that remains is in China where the HTR-10 prototype at Tsinghua University is scheduled to be commissioned in 2013.

Moderator Type: Graphite

Technology: Generation III

Existing Examples: None currently in operation.

Advantages

-The fuel pellet design makes meltdowns physically impossible.

-The reactor core is cooled by helium gas which is chemically inert at high temperatures.

-The coolant system of the PBR is cooled passively and its simple design eliminates the extensive piping of active cooling systems.

-It can be refueled without having to shut down the reactor.

-Alternative fissile fuels can hypothetically be made into pellets to be used by this reactor design.

Disadvantages:

-The PBR uses a once-through fuel cycle.

-The pellet design of the reactor makes the reprocessing of spent pellets very difficult.

-The graphite shells of the spent pellets increases the total volume of material to be disposed of by up to fifty percent.

Variants: Dragon, TTR-300, Peach Bottom Unit 1, Fort St. Vrain 1, PBR, PBMR

4. The Very High Temperature Reactor (VHTR)

The VHTR (Very High Temperature Reactor) is a generation IV design that could be said to be an extension of HGTR reactor designs. It is also cooled with helium gas and uses graphite as a moderator. The core of pebble bed VHTRs would be similar to the PBR while prismatic block VHTRs would have its fuel shaped into fuel rods that are inserted in holes drilled into the hexagonal graphite blocks surrounded by control rods. The helium gas would be heated by circulating around the control rods to be carried to a heat exchanger to heat up water to spin a turbine.

True to its name, the VHTR concept operates at temperatures reaching 1000°C. This would make it very useful for providing process heat to carry out many industrial applications such as cheap hydrogen production and hydrocarbon reactions. The reactor would be extremely safe in addition to having a very high degree of thermal efficiency because of its high operating temperature. While this reactor design would have a once-through fuel cycle, the VHTR has a much higher fuel utilization ratio and the resulting spent fuel would have a much shorter half life than fuel from traditional LWRs. Prototype VHTRs are under development in China and the US has expressed interest in the VHTR for its Generation IV program.


Moderator Type: Graphite

Technology: Generation IV

Existing Examples: None currently in operation.

Advantages

-The fuel pellet design of the pebble bed variant makes meltdowns physically impossible.

-The reactor is an extremely safe design.

-The reactor core is cooled by helium gas which is chemically inert at high temperatures.

-The coolant system of this reactor is very simple.

-It can be refueled without having to shut down the reactor.

-Alternative fissile fuels can hypothetically be made into pellets to be used by this reactor design.

-The very high temperatures lend themselves to higher thermal efficiencies.

-The reactor would serve a dual purpose as a source of process heat for thermochemical production.

Disadvantages:

-It uses a once-through fuel cycle.

-The pellet design of the PBR variant VHTR makes the reprocessing of spent pellets very difficult.

-The graphite shells of the spent pellets of the PBR variant of the VHTR increases the total volume of material to be disposed of by up to fifty percent.

Variations: PBR VHTR, Prismatic Block VHTR

4 comments:

Frnk Sclt said...

Would there be any increase in the ability of the graphite moderator performance by adding diamond dust to the graphite. Diamond dust, like graphite is a form of carbon. It is used in abrasives and also as a polishing medium.

Runescape 2007 Gold said...

Would likely there always be just about any surge in light beer the actual graphite moderator functionality by having precious stone dirt to the graphite. Precious stone dust, such as graphite is really a way of as well as. It really is employed in abrasives and also as a new sprucing medium.
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Irvin said...

This is gorgeous!

Parshva Jhaveri said...

YES.But your blog fails to answer this question:Why is graphite used as a moderator? (the chemistry reason)

And if you are passionate about nuclear physics.. why are you an art student?