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GE Hitachi - PRISM Reactor

 - By Duncan Williams -

One of the most vexing issues facing the nuclear power industry today is what to do with the spent nuclear fuel after it has been used in a nuclear reactor.  Currently, used fuel is safely stored in pools of water or in dry casks at the nuclear plant site.  But as for a long-term solution, there is no consensus as to disposing of, or permanently storing, the spent nuclear fuel. 

To this end, GE Hitachi Nuclear Energy, a joint venture between General Electric and Hitachi, is researching a reactor design which would use recycled spent nuclear fuel instead of creating new fuel.  This design is not new, as it was originally developed in Idaho at the Argonne National Laboratory back in the 1980s and 1990s.  GE Hitachi calls the design the Power Reactor Innovative Small Module (PRISM), which is a key component to a new fuel cycle in which spent nuclear fuel is reused instead of being stored.

The PRISM reactor would use recycled nuclear fuel from a reprocessing facility known as the Advanced Recycling Center (ARC).  The ARC would be located at the PRISM reactor site, and would likely use a patented electrometallurgical process in order to separate and isolate the uranium from the spent nuclear fuel.

This process is described in U.S. Patent No. 7,638,026, issued on December 29, 2009, and assigned to the Department of Energy.

The patent generally describes an electroplating process using molten lithium chloride (LiCl) as the transport medium.  The spent fuel is placed in a porous basket (12) near the anode (18), and a small voltage (1.34) is applied.  Various chemical and electrolytic reactions cause the uranium to leach out of the porous basket (12) and migrate through the molten lithium chloride towards the cathode (22).  The end result is the production of carbon dioxide gas and a layer of uranium coating the cathode (22).  The uranium can then be used to form the nuclear fuel in the PRISM reactor.

This process highlights the most recent development in a long history of research related to PRISM technology.  Last year, the American Nuclear Society awarded Charles Boardman, who retired from GE in 2001, with the prestigious Walker Lee Cisler Medal for his decade-long work relating to the development of this technology.

Since his retirement, Boardman has continued to work with government entities and private corporations in order to bring this technology to fruition.  Mr. Boardman now has the attention Congress, who is eager to find a solution to the ever-growing inventory of spent nuclear fuel in storage.

The PRISM reactor operates in a fundamentally different way than the reactors widely in use today.  All nuclear reactors emit two kinds of neutrons based on the energy level of the neutron.  Low energy neutrons are known as thermal neutrons, while the higher energy neutrons are called fast neutrons.  Currently, most nuclear reactors in America rely on thermal neutrons to sustain a fission chain reaction.  These thermal neutron reactors typically use uranium-235 as the main component in the nuclear fuel.  In order to increase the number of thermal neutrons, thermal neutron reactor designers choose materials which slow down the fast neutrons in order to turn them into useful thermal neutrons.

The size of the hydrogen and oxygen particles in water happen to be of the same magnitude as neutrons.  When neutron particles collide with hydrogen and oxygen particles, some of the kinetic energy from the neutron is transferred to the water molecule, much like a cue ball hitting another billiard ball of the same size.  This reduces the energy level of fast neutrons and turns them into thermal neutrons.  The thermal neutrons can then be absorbed by uranium-235, which undergoes further reactions to produce even more neutrons in order to sustain a fission chain reaction.  This is why normal water is used as a coolant in most thermal neutron reactors.

Fast reactors, in contrast to thermal reactors, utilize fast neutrons for a sustained fission chain reaction.  Therefore, the materials used in fast reactors tend to maximize the number of fast neutrons.  For this reason, water is unsuitable as a coolant because it tends to turn fast neutrons into thermal neutrons.  Instead of water, liquid sodium is typically used as a coolant because the sodium atoms are so much larger and heavier than neutrons that when they collide the neutrons simply ricochet off the sodium atom - much a like a small bullet ricocheting off a thick plate metal.  Thus, liquid sodium is the coolant of choice in fast reactors because it can effectively transfer heat away from the nuclear fuel, while at the same time maximizing the number of fast neutrons.

According to the PRISM Preapplication Safety Evaluation report submitted to the NRC in February of 1994, the PRISM reactor will be cooled by liquid sodium.  Sodium becomes a liquid at 208 F, and boils at 1621 F.  Since the PRISM reactor is designed to operate between 640 F – 905 F, the properties of sodium make it an excellent coolant.  These documents also indicate that the fuel used in the PRISM reactor will contain a combination of recycled uranium, plutonium and zirconium.  Whereas most nuclear fuels in modern reactors are an oxide of uranium, the PRISM fuel will be metallic and will not contain metal oxides.

These documents also detail the operational and safety features of the PRISM reactor.  In order to shutdown the reactor quickly, the PRISM reactor includes control rods which can be inserted into the reactor core in order to absorb neutrons.

As shown in the diagram, the control rods include an absorber bundle which is designed to shutdown the reactor in one-fifth of a second.  While the concept of control rods is common to most nuclear reactors, the PRISM reactor also includes a notably unique shutdown feature.  The PRISM reactor design includes an Ultimate Shutdown System (USS), which can be manually actuated in the event that the reactor control rods cannot be inserted into the core.

Upon actuation, balls made of boron carbide (B4C) are dropped into an otherwise empty cavity in the reactor core.  The boron absorbs the excess neutrons in the reactor and disrupts the fission chain reaction - effectively shutting it down completely.

As can be shown in the diagram, the primary liquid sodium system is located within the containment vessel.  The liquid sodium level is maintained about 4 feet from the top of the reactor vessel.  Four electromagnetic (EM) pumps are suspended from the top of the reactor vessel and have no moving parts – the liquid metal sodium is propelled by the use of magnets in the pump.

The EM pumps circulate the liquid sodium downward to the core inlet plenum, where the liquid sodium is at its coolest.  As the liquid sodium flows up through the core containing the nuclear fuel, heat is transferred from the fuel to the sodium.

The upper internals structure (UIS) houses instrumentation which measures critical properties such as flow velocity, temperature, neutron flux levels, etc.  Once the liquid sodium reaches the top of the reactor vessel, it is at its hottest.  The sodium then flows downward through one of the two intermediate heat exchangers (IHX) which is attached to, and suspended from, the top of the reactor vessel.

As can be seen in the diagram, the primary liquid sodium enters through the side of the IHX and then flows downward before exiting the IHX.  The heat from the primary liquid sodium is transferred through the metal structure of the IHX to the liquid sodium flowing through intermediate side.  Although the flow path of the intermediate liquid sodium is not clear from the diagram, it enters the IHX through the top of the IHX and flows to the bottom, where it then U-turns and flows upwards where it finally flows out of the IHX just below the point where it entered.

The intermediate liquid sodium is then circulated through another heat exchanger called a steam generator.  As can be seen in the diagram from the Idaho National Laboratory’s website, the intermediate liquid sodium transfers heat through the metal structure of the steam generator to water which is circulating on the secondary side.  The heat turns the water to pressurized steam, which is then used to turn generator-turbines in order to produce electricity.

To satisfy containment requirements specified by the NRC, the reactor vessel will be located below ground level inside a reactor silo. 

At least part of the reason for this is that liquid sodium would ignite spontaneously if it comes into contact with air.  Although liquid sodium becomes extremely radioactive due to neutron activation during normal reactor operation, most (all but two) radioactive sodium isotopes have a half-life of less than a minute, and therefore pose minimal long-term risks. 

The purpose of having the intermediate liquid sodium loop is a precautionary measure to allow reactor operators time to shutdown and isolate the primary liquid sodium system in the event that the radioactive liquid somehow leaks into the intermediate loop.

By turning off the pumps in the intermediate loop and halting flow through the steam generator, any radioactivity can be isolated to the primary or intermediate system.  In addition, the PRISM design includes a containment dome made of 1.5 inch thick carbon steel which shields workers and the environment from radiation.  The diagram also shows seismic shock-absorbers that protects the reactor vessel in the event of an earthquake.

Conclusion
In a letter to Nuclear Regulatory Commission (NRC), dated March 19, 2009, GE Hitachi indicates that it intends to submit a design application to the NRC regarding the PRISM reactor in mid-2011.  Although the science behind the PRISM seems sound, the initial costs of building the ARC, as well as the costs of building the PRISM reactor itself, is the biggest hurdle GE Hitachi must overcome.  Once GE Hitachi finds an initial customer with deep pockets willing to pay these costs, the PRISM project is poised to move forward with the NRC approval process. 

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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