Under The Hood With Duncan Williams - The ACR-1000: The first light-water, low-enriched uranium CANDU reactor

The ACR-1000: The first light-water, low-enriched uranium CANDU reactor

 - By Duncan Williams -

Earlier this year, India and the Atomic Energy of Canada, Limited (AECL), announced an agreement to develop a competitive pricing model for the ACR-1000 nuclear power plant.  This potential deal comes on the heels of a recent change in Canada’s foreign policy, announced on November 28, 2009, which now explicitly allows Canadian firms to import and export controlled nuclear materials, equipment and technology to and from India.  Canada had previously imposed a ban on nuclear trade with India after controversial nuclear tests were conducted by India in 1974.  The ACR-1000 reactor plant could become the first Canadian technology exported to India in over three decades.

click for full sizeThe ACR-1000 is one of the latest advances in the mostly Canadian-developed reactor technology widely known as the CANDU reactor.  The CANDU has been exported to various countries including South Korea, China, India, Argentina, Romania and Pakistan.  The term CANDU stands for CANada Deuterium Uranium reactor.  The Atomic Energy of Canada Limited (AECL), the equivalent to the U.S. Department of Energy, was issued a trademark in 1984 for the term “CANDU.”

Unlike conventional reactors, which use enriched uranium as its fuel, CANDU reactors are capable of sustaining the fission process using naturally occurring uranium – eliminating the need for any enrichment facilities.  Naturally occurring uranium contains about 99.2% uranium-238, and .72% uranium-235.  Uranium-238 tends to absorb neutrons having high energy levels, and typically does not emit other neutrons as a result of neutron absorption. 

Due to its properties, a fission chain reaction cannot be established using only uranium-238 as nuclear fuel.  In contrast, uranium-235 absorbs low energy neutrons, resulting in the release of even more neutrons allowing the fission chain reaction to be sustained.  Due to uranium-235’s unique ability to absorb slow energy neutrons, and its tendency to emit neutrons as a result, most conventional reactors use enriched uranium as nuclear fuel.  Although expensive, the enrichment process required for manufacturing this type of nuclear fuel increases the percentage of uranium-235 from .72% to about 4%.

In order to help promote a fission chain reaction using natural uranium as a fuel source, the CANDU reactor surrounds the nuclear fuel with a substance known as “heavy water.”  Heavy water is water that has a higher concentration of deuterium oxide than ordinary water.  When a high energy neutron collides with a deuterium oxide molecule, as opposed to a water molecule, it is more likely to slow down and be redirected towards a uranium-235 particle.  In this respect, the deuterium oxide acts as a neutron moderator, since it reduces the energy of high-energy neutrons.  In contrast, ordinary water particles absorb too many neutrons to allow a fission chain reaction using naturally occurring uranium as fuel.  Thus, CANDU reactors use heavy water as a moderator in order to provide enough low energy neutrons to sustain a fission chain reaction within the natural uranium nuclear fuel.

Canada’s lack of uranium enrichment capacity in the late 1940s and early 1950s, when CANDU was first being developed, led researchers to choose natural uranium as a fuel source.  Accepting a license to use enriched uranium from countries like the United States would require the licensee to allow intrusive inspections so that the proper usage of the fuel could be verified.  Canada chose instead to develop nuclear reactors that were fueled by naturally occurring uranium.  After all, Canada has over 8% of the world’s total uranium – even more than the United States.

click for full sizeThe naturally occurring uranium fuel is manufactured into pellets, which are then stacked into fuel rods inside a giant cylindrical vessel known as the calandria.   As shown in the diagram, the calandria is filled with heavy water to slow down the fast neutrons resulting from the fission process.  This massive tank of liquid also serves as an emergency source of water in case of a major loss of coolant casualty, ensuring that water will be available to cool the reactor in an emergency.

Inside the fuel rods, the pellets are stacked against one another.  For example, U.S. Pat. No. 4,036,691, issued on July 19, 1977, to the AECL, describes a fuel rod of an early CANDU reactor.  As can be seen in the diagram of the fuel rod from the patent, fuel pellets (6) are stacked against one another inside the cladding (1) of the fuel rod.  

Once the pellets are securely in place, a heat resistant disc (14) made of graphite is placed on one end.  A spacer (16) holds the disc (14) against the fuel pellets (6), allowing for a chamber (8) that allows for fission product gasses to collect as the pellets undergo fission.

click for full sizeThe entire fuel rod is then sealed up on both ends and arranged into a fuel bundle, as seen in the picture.  This is a picture of a fairly recent 37-rod fuel bundle design for a CANDU reactor.

 

click for full sizePreviously patented versions of this design can be seen in U.S. Pat. No. 3,941,654, issued on March 2, 1976, to Canadian General Electric.  The 18-rod fuel bundle design shown in this diagram from the patent shows the fuel rods (12) attached to the end plate (14) to form a fuel assembly.  [fuel bundle patent design]  These assemblies are strung together to form a long assembly and are then inserted into a fuel channel (53) between the end walls (51, 52) of the calandria.   

click for full sizeThe volume of the calandria (50) is filled with heavy water to serve as a moderator to slow down fast neutrons.  In order to remove heat, heavy water coolant enters at (48), flows over the fuel bundles, and exits through (49).

The ACR-1000: A Break From a Canadian Tradition
Instead of exporting nuclear technology relating to heavy water coolant and natural uranium fuel, as in previous CANDU designs, Canada intends to sell a light water coolant reactor containing fuel that has been enriched to between 2-4% uranium-235.  Capable of producing 1165 MWe, the ACR-1000 is the first CANDU reactor to use low-enriched uranium as fuel and light water to cool the fuel. 

Design documents obtained from AECL show that the fuel channel is similar to the earlier CANDU fuel channel designs except that light water coolant, instead of heavy water, flows directly over the fuel bundles.  Heavy water is still used as a moderator and is circulated within the calandria.  By using light water coolant, the heavy water volume of the ACR-1000 is reduced by over 60% compared to previous versions.  This reduces costs associated with initially filling the reactor as well as the inevitable periodic refills during normal use.

click for full sizeInstead of forming pellets from naturally occurring uranium, fuel pellets for the ACR-1000 are formed from low-enriched uranium in the form of uranium dioxide.  The pellets contain varying densities of the uranium dioxide particles, depending on its position in the core, in order to optimize the power distribution throughout the core.  Additionally, the fuel bundles in the ACR-1000 contain 43-fuel rods, which is the most rods of any previous CANDU fuel bundle.

 

click for full sizeThe low-enriched uranium fuel has a longer lifespan than the fuel made from naturally occurring uranium, leading to fewer refueling operations as well as reducing the overall volume of spent nuclear fuel.  The ACR-1000 reactor plant has a design lifespan of 60 years.

The ACR-1000 is designed to be refueled while it is operating – an advantage over any pressurized light water reactor design.  Whenever the nuclear fuel inside the fuel rod is spent, the fuel rod can easily be withdrawn from the calandria and a new one inserted in its place with little to no disruption in the operation of the reactor.  The actual withdrawal and insertion of the fuel rods is performed by remotely-operated refueling mechanisms.

click for full sizeThus, it seems that Canada is moving away from its previous CANDU design that uses naturally occurring uranium as a fuel source.  As such, the ACR-1000 represents a significant shift in reactor design from previous CANDU reactors.  The results of Canada’s pending deal with India will be a telling sign as to whether or not this design will be competitive with other light water reactors in the global nuclear industry.
 

 

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About Duncan Williams
Duncan Williams graduated from the University of Florida in 1994 with a B.S. in Physics, and a minor in mathematics.  Upon graduation, he was commissioned  in the U.S. Navy where he completed training in the Navy’s Nuclear Propulsion program.  He then served onboard an aircraft carrier, the USS Theodore Roosevelt, as a reactor control division officer.  Onboard, he was responsible for the operation and maintenance of the electrical and mechanical components that make up the reactor control systems.  This includes the control rod drive mechanisms, the reactor safety and emergency systems, the reactor coolant pump systems, and the ion exchangers.  He also developed and implemented ship-wide reactor safety drills in order to educate sailors in reactor safety.

Duncan then transferred to the U.S. Naval Academy, where he served as a senior instructor teaching Thermodynamics to senior cadets.  While serving as an instructor at the Naval Academy, Duncan attended night law school at the George Washington University Law School.  After receiving his J.D. in 2004, he resigned his commission and began working as an intellectual property associate with Kenyon & Kenyon LLP.  While at Kenyon & Kenyon, he drafted numerous patents relating to medical devices, electronic devices, telecommunications, as well as other technologies.  He also has experience in all stages of patent litigation, and has represented numerous Fortune 500 companies in protecting their intellectual property rights.  Duncan is currently an intellectual property associate at Blank Rome LLP.

If you have questions, comments, or know of a patent that you think Duncan should review E-mail Duncan Williams>> duncan@nuclearstreet.com 

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