Under The Hood With Duncan Williams – Areva NP Improves Its Fuel Matrix

Areva NP Improves Its Fuel Matrix

 - By Duncan Williams -

One of the most important aspects of light water nuclear reactors is removing heat from the reactor core.  The reactor core is the portion of the reactor that contains the reactor fuel assemblies and is where the nuclear reactions occur.  Creating massive amounts of heat, these fuel assembles are made up of many individual fuel components (e.g., fuel pellets or plates) that are all properly aligned and grouped together.  If heat is not removed from these fuel assemblies, catastrophic results could result similar to the Three Mile Island incident.

Many companies, like Areva NP Inc, are focusing their research on improving the heat removal capabilities of each individual fuel component mentioned above.  For example, a patent recently issued to Areva NP (U.S. Patent No. 7,587,018) claims that adding silicon carbide to these individual fuel components improves the reactor core’s ability to cool them.

The type of fuel component described in Areva’s patent has three well-known parts: (1) a fuel matrix, (2) a zirconium alloy cladding, and (3) reactor coolant.  The fuel matrix is made up of uranium dioxide (UO2) particles that are spaced apart and held in place by various composite materials.  During the nuclear fission process, the UO2 particles emit energy resulting in extremely high temperatures in the fuel matrix.  The UO2 particles also release neutrons, some having enough energy to rip through matter like a bullet.  However, zirconium alloy is capable of withstanding the bombardment of these high-energy neutrons without being distorted from its original shape.

For this reason, a layer of zirconium alloy, known as the zirconium cladding, surrounds the fuel matrix.  In addition to helping the fuel component maintain its structural integrity, it also serves to protect the fuel matrix from the corrosive effects of the reactor coolant.  The purpose of the coolant flowing around the zirconium coated fuel matrix is to transfer heat away from the reactor core and ultimately cooling the fuel assemblies.  In a nutshell, heat created in the center of the fuel matrix must first travel through the remainder of the fuel matrix, then through the zirconium cladding, before finally reaching the reactor coolant.

But the heat travels through the zirconium cladding at a faster rate than it travels through the fuel matrix.  The zirconium cladding has a relatively high thermal conductivity, which means that heat quickly passes through the cladding and is subsequently transferred to the reactor coolant.  In contrast, the UO2 fuel matrix has a relatively low thermal conductivity, which means that heat tends to build up in the fuel matrix before it can be passed on to the zirconium cladding.

Due to the poor heat transfer characteristics of the fuel matrix, reactors must limit their overall power level in order to minimize the peak temperatures in the fuel matrix.  By increasing the heat transfer capabilities of the fuel matrix, however, higher power levels may be achieved with little to no increase in peak temperatures.  Thus, many companies are focusing on the development of a UO2 fuel matrix which has a relatively high thermal coefficient allowing it to operate at higher power levels with minimal temperature increases.

A patent recently issued to Areva NP claims that impregnating a porous UO2 fuel matrix with silicon carbide increases its thermal coefficient.  The patent describes a process that begins by submersing the porous UO2 fuel matrix in a precursor liquid, typically allylhydridopolycarbosilane.  The submersed fuel matrix is then pressurized in order to force the precursor liquid deep into the pores of the fuel matrix.  A curing process is then performed, typically using heat, which turns the liquid into a solid polymer.  The fuel matrix is then subjected to another heat curing process which turns the solid polymer into discrete silicon carbide particles which are homogenously embedded in the fuel matrix.  The result is a fuel matrix that contains both UO2 particles and silicon carbide particles evenly distributed throughout.  The patent claims that the characteristics of such a fuel matrix allows for greater heat transfer allowing the reactor to operate at a higher sustained power.

The patent explains several reasons why this method of impregnating the fuel matrix is superior to other methods, for example chemical vapor infiltration.  Chemical vapor infiltration results in an uneven mixture of silicon carbide throughout the fuel matrix.  In other words, some portions of the fuel matrix have a higher density of silicon carbide whereas other portions may have no silicon carbide.  This leads to uneven burning of the UO2 particles, resulting in a shorter overall reactor core lifetime.  Chemical vapor infiltration is also more expensive due to the costly machinery involved.  Also, the methyltrichlorosilane used in chemical vapor infiltration results in the release of harmful hydrogen chloride gas.  The patent also points out that a fuel matrix containing silicon carbide is compatible with most of the currently existing light water reactors, and hence no other modification to the reactor core would be necessary.

Of interest:

CEZUS Takes Stake In Zirco To Better Serve Growing Worldwide Nuclear Demand

Zirco Products is a jointly owned company by Sumitomo Metal Industries and Kobe Special Tube  - By Linton Levy - Zirco Products, Japan's largest manufacturer of Zirconium cladding tubes for nuclear power plants, and CEZUS (Compagnie Européenne du Zirconium, France), a subsidiary of AREVA, have agreed on the latter's 33.30 % ...

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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