Saturday, June 11, 2011

Nuclear Technology Basics: Part 5 Light Water Reactors


Part 1

Part 2

Part 3

Part 4

Light water reactors are the most common type of nuclear reactor that has been built in the world so far. These reactors use regular water as a neutron moderator in addition to a coolant. Although there are different sub-types of light water reactors, they are all similar in their design and operation.

1. The Pressurized Water Reactor (PWR)

This design was covered in detail in my last post regarding nuclear energy. It uses a heat exchanger consisting of heated water in the primary coolant loop flowing through a pipe that is surrounded by cool water in the secondary coolant loop. This takes place in the steam generator where the heated water in the primary coolant pipe produces steam from the feedwater that is pumped in from the secondary coolant loop. The steam is then used to drive the turbine and produce electricity.

Moderator Type: Normal water

Technology: Generation II

Existing examples: Ubiquitous around the world.


-It is a very familiar Generation II design that has been around since the mid-20th century.

-The majority of reactors used for electricity generation are of this design.

-It is also very safe as the fuel rods are held upwards using electromagnets. If the electricity is disabled, the fuel rods will automatically drop into the reactor vessel and cause the reactor to shut down.


-It must use fuel rods that have had their uranium-235 content enriched up to four percent, necessitating increased costs for fuel production.

-Ninety percent of the available uranium-235 within the spent fuel pellets remains un-fissioned, which means that the spent fuel pellets must either be reprocessed into usable fuel or disposed of.

-The reactor must be shut down to engage in refueling.

-Some of the actinides that remain in the spent fuel such as cesium-137 and strontium-90 are a cause for concern and have half-lives that are measured in decades. Although many of the remaining radioisotopes in spent fuel from light water reactors are only weakly radioactive, they have half-lives measuring thousands of years.

-Although there are no hurdles to reprocessing spent fuel from a technical and industrial standpoint, sociopolitical hysteria has effectively blocked any and all discussion of such a practice and has also prevented the construction of a centralized location where the spent fuel can be moved for long-term storage.

-A combination of politics and perceived investment risks has effectively prevented the construction of any new nuclear electricity generation facilities for decades in many countries.

Variants: European Pressurized Reactor (EPR), Advanced Passive 1000 (AP1000)

2. The Boiling Water Reactor (BWR)

The BWR is similar to the PWR in that it uses ordinary water as a coolant and neutron moderator. However, unlike the PWR it lacks a pressurizer and steam generator as water is heated into steam within the reactor vessel itself. This is the second most utilized reactor design in the world.

Moderator type: Normal water

Technology: Generation II

Existing Examples: Ubiquitous, as with the PWR.


-The design is one that nuclear engineering projects are very familiar with.

-It is of a simpler design than the PWR.

-Because of the nature of the BWR there are options for passive and active safety systems that are not available with the PWR.

-Like the PWR it is a very safe design.


-Because of the nature of coolant circulation within the upper half of the reactor chamber, the reactor still requires active coolant flow to remove residual heat even after the reactor has shut down.

-The control rods are inserted in the bottom of the reactor chamber rather than the top which requires an active hydraulic system rather than a passive fail safe using electromagnets like in the LWR.

-The reactor must be shut down to be refueled.

-A large portion of the uranium-235 in the fuel rods remains un-fissioned like with the PWR.

-The nature and quantity of the spent fuel produced by BWRs is the same as with PWRs and the same sociopolitical hurdles stand in the way of closing the fuel cycle or building a centralized location for spent fuel in some countries.

Variants: Advanced Boiling Water Reactor (ABWR), Simplified Boiling Water Reactor (SBWR), Economic Simplified Boiling Water Reactor (ESBWR)

3. Supercritical Water Reactor (SCWR)

This is a theoretical design that would utilize water as a coolant and neutron moderator. The water is heated and pressurized past the critical point in which liquid water and gaseous steam become virtually indistinguishable. Because of the higher operating temperature, it would be able to reach a higher degree of thermal efficiency than either the BWR or the PWR. Supercritical water is a less efficient moderator than water in its liquid state but it is also a better conductor of heat. Because of this, less moderating fluid might be needed. This might lead to the operation of a SCWR as a fast neutron reactor. The control rods would likely be inserted into the top of the reactor chamber like in the PWR. The design is being researched by several dozen countries across the world.

Moderator Type: Normal water in a supercritical state.

Technology: Generation IV

Existing Examples: None, as the reactor is still theoretical


-The greater thermal efficiency would allow the reactor to utilize the Brayton cycle

-The higher efficiency of the reactor would allow for a smaller reactor and smaller components

-The design of the SCWR is simpler making it cheaper to build and maintain.

-Fast breeder SCWR designs would be able to "burn up" more of the actinides produced during fission, leading to less actinides in the spent fuel and the spent material produced would have a half-life of an average of three hundred years.

-Breeder variant SCWRs would be quite flexible in the fuel they would be able to use as feedstock ranging from spent fuel from other reactors to thorium, to decommissioned nuclear warheads.


-Supercritical water presents unique challenges from a metallurgical and chemical standpoint when designing the ductwork for the circulating coolant and moderator.

-Activating a SCWR requires more complex operations than a BWR or PWR.

-Designing the reactor core to avoid a positive void coefficient would require a fairly high degree of complexity which would raise the cost of construction.

Variants: Fast Spectrum Supercritical Water Reactor (FSSCWR), Supercritical Heavy Water Reactor (SCHWR)


Light water reactors are the most common reactor types in the world today. This is because they have proven to be both reliable and relatively simple to operate, and have become to be regarded as "traditional" within the nuclear industry. However, they are by no means the final word in nuclear fission technology. The SCWR does show some promise, but there are other reactor designs that would be even more impressive in their application for electricity generation and for breeding more fuel.

That concludes my post on the light water reactor family. Next time, we will look at heavy water reactors. If you have any questions or comments, feel free to ask me as I welcome your interest.

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