Fuel Rods In A CANDU Reactor

March 10, 2019

Table Of Contents

1. What Are Fuel Rods?
2. Background
   1. History Of Nuclear Reactors
   2. Production Of Uranium Rods
3. Components of Uranium Rods in a CANDU reactor
   1. Pellet
   2. Zirconium Alloy Rod/Tube
   3. Zirconium Alloy Plug
   4. Zirconium Alloy Endplate
   5. Pressure Tube
4. How Uranium Rods Work
5. Conclusion
6. Reference
7. Glossary

 

 

 

What Are Fuel Rods?

Fuel rods are long metal tubes that are usually made out of zirconium alloy metal, which contain small pellets made out of fissionable material. These fuel rods are key to generating high amounts of energy in a reactor. They are able to generate high amounts of energy by going through a process called fission. The rods are later placed into assemblies, and are loaded into the reactor core. Small pellets are made out of about 0.7% to 5% of uranium-235 and the rest is made out of uranium-238.

These fuel or uranium rods are located inside of a CANDU reactor. CANDU stands for Canada Deuterium Uranium, which is a pressurized heavy water reactor. Unlike the fuel rods in other nuclear reactors, such as pressurized water reactor, boiling water reactor and advanced gas-cooled reactor, the fuel rods in a CANDU reactor are positioned horizontally and in circular bundles. Figure 1 shows a diagram of a CANDU reactor core, where the rods can be seen positioned horizontally in a Calandria. Figure 2 shows a diagram of a boiling water reactor core, where the rods are positioned horizontally. These circular bundles also do not contain control rods because the control rods are located outside of these fuel rod bundles. The main difference between CANDU reactor fuel rods and other fuel rods is that CANDU fuel rods do not need to be enriched. This means that CANDU fuel rods are only made up of about 0.7% of uranium-235 and 99.3% of uranium-238. Not having to enrich the fuel, is a big advantage because it saves a lot of time and money. CANDU reactors also use about 25% to 30% less mined uranium than the light water reactors [1].

Figure 1: Candu 6 Reactor Core

                                                                         Figure 2: Boiling Water Reactor Core                                                                               

Background

1. History of Nuclear Reactors.

Uranium was first discovered by a German chemist, Martin Klaproth, in 1789, in a material called pitchblende which is known as uraninite. He also discovered the element zirconium which is now used in nuclear reactors. In 1896, Henri Becquerel discovered radioactivity of Uranium. Gilbert LaBine discovered Canada’s first uranium deposit in 1930 and Harold Clayton Urey discovered deuterium in 1931. In 1939, Niels Bohr and John Wheeler published a theory of nuclear fission. In 1942, there was a proposal that the deuterium production should be moved to Canada [2]. During the year, the first nuclear reactor was built by Enrico Fermi, and was named CP-1 or Chicago Pile 1. Chicago Pile 3 was constructed on May 15, 1944, and was the first reactor built using heavy water and unenriched uranium [3].

1. Production of Uranium Rods.

First the uranium ore is mined and processed through a mill where it is crushed into smaller pieces. Then it is mixed with water and leached with sulphuric acid. This process dissolves the uranium oxides and leaves the other minerals undissolved. Another method that can be used to extract uranium is called in situ leaching. This method requires groundwater with a lot of oxygen to be injected into the uranium ore. The ore gets dissolved and then pumped out of the ground leaving most of the ground intact. Then, the uranium is filtered and separated by ion exchange. This produces a uranium oxide concentrate, which is also known as “yellowcake”. The uranium oxide concentrate is usually sent for enrichment for other nuclear reactors but for CANDU reactors, the uranium oxide concentrate goes straight into a fuel fabrication plant where it is converted to uranium dioxide powder. This powder is then compressed into a form of a small pellet and heated to create a hard ceramic material. These pellets are then inserted into fuel rods which are usually made from zirconium alloys [4].

Components of Uranium Rods in a CANDU reactor

The uranium rods in a CANDU reactor can be broken up into smaller parts which are shown in Figure 3. Calandria is the nuclear reactor core that holds these uranium rods and is shown in Figure 4. Zirconium alloy metal is used because of its low neutron absorption, and its high resistance to heat and corrosion.

1. Pellet

Small ceramic pellets which are made of uranium dioxide. These uranium pellets are about 10 to 13 millimeters in length and 8 to 13.5 millimeters in diameter. The uranium pellets are stored in rods that are made of zirconium alloy.

1. Zirconium Alloy Rod/Tube

Zirconium alloy or zircaloy-2 is used in CANDU reactors and is composed of tin 1.2-1.7%, iron 0.07-0.2%, chromium 0.05-0.15%, and nickel 0.03-0.08% [5].

1. Zirconium Alloy Plug

Small circular zircaloy plug used to close the zircaloy rod to keep the uranium pellets inside.

1. Zirconium Alloy Endplate

Circular zircaloy grid used to hold the uranium rods together creating a fuel bundle.

1. Pressure Tube

Zircaloy tube that holds many fuel bundles together. It is around 6 meters in length and 0.104 meter in diameter.

 

 

Figure 3: Diagram showing dimensions and parts of the fuel bundle

Figure 4: Assembly of the fuel pellet into the Calandria.

How Uranium Rods Work

The uranium rods go through a process called fission. Nuclear fission is the process of splitting apart large nuclei, which in this case is uranium-235. This can be achieved by launching a thermal neutron into an unstable nucleus. The energy that is released from fission comes from the repulsion force between the protons. Each proton repels another proton with about 20 Newtons of force. This is a lot of force because a proton has a really small mass of  1.6726 x 10^-27 kilograms, and the distance between the protons is also extremely small. During this process some of the mass is converted into energy [6]. When the radioactive nuclei of uranium-235 splits, it can split into two different elements while releasing more neutrons. Uranium-235 can split into krypton-92 and barium-141 releasing 3 neutrons. It can also split into neptunium-93 and caesium-140 releasing 2 neutrons. The released neutrons hit other uranium-235 nuclei creating another fission reaction which releases more neutrons, thus creating a chain reaction.

Figure 5: Two possible fission reactions.

Unlike uranium-235, uranium-238 is not a fissile isotope. This means that it is unable to undergo a fission reaction by absorbing a thermal neutron. Therefore, uranium-238 is turned into plutonium-239 which is a fissile isotope. The way this works is a fast neutron with energy higher than 1 megaelectronvolt is launched at uranium-238 isotope, and this creates uranium-239 isotope. The half-life of the new uranium-239 isotope is about 23.5 minutes, and it will decay into neptunium-239. Neptunium-239 has a half-life of about 2.4 days, which will decay into plutonium-239 [7].

Figure 6: Process of nuclear fission for uranium-238.

Conclusion

Fuel rods in a CANDU reactor are designed to use natural uranium which makes it easier since the fuel can now be locally manufactured, and requires less money since the uranium does not have to be enriched. Uranium rods are made to undergo fission reactions and to generate a lot of electricity throughout the process. These rods are helping to replace other sorts of combustion energy and reduce air pollution. Uranium rods have a really long half-life and are dangerous to the environment. The fission caused in the uranium rods is also very dangerous because it could go out of control and cause a nuclear meltdown as seen in the past.

Despite the dangers of radiation and accidents in the past, nuclear reactors will help to reduce the emissions of carbon dioxide into the atmosphere. New nuclear reactors are being created which contain more safety designs. Thorium is also being experimented with which could potentially be more efficient but also more dangerous.

 

 

 

 

 

 

 

 

 

References

Title Page Picture

World-nuclear-news.org. (2019). World Nuclear Association – World Nuclear News. [online] Available at: http://www.world-nuclear-news.org/ENF-Chinese_reactor_trials_Candu_fuel_reuse-2403101.html [Accessed 11 Mar. 2019].

Figure 1

Nuclearfaq.ca. (2019). The Canadian Nuclear FAQ – Section A: CANDU Technology. [online] Available at: http://www.nuclearfaq.ca/cnf_sectionA.htm [Accessed 11 Mar. 2019].

Figure 2

Encyclopedia Britannica. (2019). Boiling-water reactor | physics. [online] Available at: https://www.britannica.com/technology/boiling-water-reactor [Accessed 11 Mar. 2019].

Figure 3

Shoesmith, D. (2019). Used Fuel and Uranium Dioxide Dissolution Studies – A Review. [online] researchgate.net. Available at: https://www.researchgate.net/figure/Typical-CANDU-fuel-bundle_fig3_238622817 [Accessed 11 Mar. 2019].

Figure 4

Nuclearsafety.gc.ca. (2019). Nuclear Power Plant Safety Systems – Canadian Nuclear Safety Commission. [online] Available at: http://nuclearsafety.gc.ca/eng/reactors/power-plants/nuclear-power-plant-safety-systems/index.cfm?pedisable=true [Accessed 11 Mar. 2019].

Figure 5

World-nuclear.org. (2019). How does a nuclear reactor make electricity? – World Nuclear Association. [online] Available at: http://www.world-nuclear.org/nuclear-basics/how-does-a-nuclear-reactor-make-electricity.aspx [Accessed 11 Mar. 2019].

Figure 6

Machine Design. (2019). What’s the Difference Between Thorium and Uranium Nuclear Reactors?. [online] Available at: https://www.machinedesign.com/whats-difference-between/whats-difference-between-thorium-and-uranium-nuclear-reactors [Accessed 11 Mar. 2019].

[1] J.M.K.C. Donev et al. (2016). Energy Education – CANDU reactor [Online]. Available: https://energyeducation.ca/encyclopedia/CANDU_reactor. [Accessed: March 11, 2019].

[2] Brown, M. (2009). Canada’s Nuclear History. [online] Cns-snc.ca. Available at: https://www.cns-snc.ca/media/history/canadian_nuclear_history.html [Accessed 11 Mar. 2019].

[3] Atomic Heritage Foundation. (2019). Heavy Water Reactors. [online] Available at: https://www.atomicheritage.org/history/heavy-water-reactors [Accessed 11 Mar. 2019].

[4] How uranium ore is made into nuclear fuel – World Nuclear Association”, World-nuclear.org, 2019. [Online]. Available: http://www.world-nuclear.org/nuclear-basics/how-is-uranium-ore-made-into-nuclear-fuel.aspx. [Accessed: 25- Mar- 2019].

[5] Sciencedirect.com. (2019). Zirconium Alloys – an overview | ScienceDirect Topics. [online] Available at: https://www.sciencedirect.com/topics/materials-science/zirconium-alloys [Accessed 11 Mar. 2019].

[6] J.M.K.C. Donev et al. (2018). Energy Education – Nuclear fission [Online]. Available: https://energyeducation.ca/encyclopedia/Nuclear_fission. [Accessed: March 25, 2019].

[7] Nuclear Power. (2019). Uranium 238. [online] Available at: https://www.nuclear-power.net/nuclear-power-plant/nuclear-fuel/uranium/uranium-238/ [Accessed 11 Mar. 2019].

[8] J.M.K.C. Donev et al. (2017). Energy Education – Uranium enrichment [Online]. Available: https://energyeducation.ca/encyclopedia/Uranium_enrichment. [Accessed: March 11, 2019].

Glossary

Calandria – a circular drum which contains the moderator, the fuel rods and the control rods. It is also known as the reactor core.

Control rods – metal rods usually made out of boron, silver or cadmium. These rods are used to control the fission reaction.

Deuterium – known as heavy water which contains heavy hydrogen (normal hydrogen has no neutrons but hydrogens in heavy water have 1 neutron).

Enrichment  – process that is used to increase the concentration of uranium-235 from about 0.72% to up to 5%. This process converts uranium into a gaseous state. Then, the gas is placed into a gas centrifuge (cylindrical tube containing an electric motor) [8]. The gas centrifuge spins and since uranium-235 is lighter than uranium-238, most of the uranium-235 stays in the middle of the gas centrifuge. The separated uranium gas is collected and transformed back into its solid state.

Fissile – the ability of going through a fission reaction when capturing a slow or a thermal neutron (uranium-235 is fissile while uranium-238 is not).

Fission – process of when a bigger nucleus splits into smaller and lighter nuclei.

Fissionable material – material that is able to undergo a fission reaction only if the neutron they absorb contains enough kinetic energy.

Ion exchange – exchange of ions or a charge between an insoluble solid and a solution.

Isotope – forms of an element that contain the same number of protons but a different number of neutrons.

Oxide – a compound of oxygen combined with another element.

Pressurized Heavy Water Reactor –  water reactor that uses deuterium oxide as it’s neutron moderator (In CANDU, the heavy water which is deuterium oxide, is used to slow down high velocity neutrons).

Thermal neutron –  neutron that has the same energy and temperature as its surroundings.

Zirconium alloy – combination of zirconium metal and other metals like tin, iron, chromium and nickel.