As part of the Office of Nuclear Energy's Next Generation Nuclear Plant (NGNP) Program, the Advanced Gas Reactor (AGR) Fuel Development Program has achieved a new international record for irradiation testing of next-generation particle fuel for use in high temperature gas reactors (HTGRs).
The AGR Fuel Development Program was initiated by the Department of Energy in 2002 to develop the advanced fabrication and characterization technologies, and provide irradiation and safety performance data required to license TRISO particle fuel for the NGNP and future HTGRs. The AGR Fuel team used the Idaho National Laboratory’s (INL) unique Advanced Test Reactor (ATR) in a nearly three-year pivotal irradiation test to subject more than 300,000 nuclear fuel particles to an intense neutron field and temperatures around 1,250 degrees Celsius.
INL researchers say the particle fuel experiment set the world record for particle fuel performance by consuming a maximum of 19 percent burn-up of the initial low-enriched uranium content, with an average burn-up of 16 percent for all of the fuel tested. The maximum 19 percent burn-up achieved is more than double the previous record set by similar particle fuel experiments run by German scientists in the 1980s, and more than three times that achieved by current light water reactor fuel. Additionally, none of the fuel particles experienced failure since entering the extreme neutron irradiation test environment of the ATR in December 2006.
"This level of performance is a major accomplishment," said Dr. David Petti, Director of the Very High Temperature Reactor Technology Development Office at the U.S. Department of Energy's INL. The purpose of the fuel program is to develop this particle fuel, produce experimental data that demonstrates to the Nuclear Regulatory Commission (NRC) that the fuel is robust and safe, and re-establish a U.S. fuel manufacturing capability for high temperature gas reactors. INL has been working with Babcock and Wilcox Inc., General Atomics and Oak Ridge National Laboratory (ORNL) to establish standards and procedures for the manufacture of commercial-scale HTGR fuel. The overarching goal of the AGR Fuel Program is to qualify coated nuclear fuel particles for use in HTGRs such as the NGNP. Developing particle fuel capable of achieving very high burn-up levels will also reduce the amount of used fuel that is generated by HTGRs.
"An important part of our mission is the development and exploration of advanced nuclear science and technology,” said Dr. Warren F. "Pete" Miller, assistant secretary for Nuclear Energy. “This achievement is an important step as we work to enable the next generation of reactors, decrease fossil fuel use in industrial applications, make fuel cycles more sustainable, and reduce proliferation risks.
"AGR-1" is the first of eight similar experiments which aim to confirm designs and fabrication processes and performance characteristics for HTGR fuel. The AGR-1 test specimens contain TRISO (TRistructural ISOtropic) particle fuel with uranium oxicarbide (UCO) fuel kernels each surrounded by three coating layers that serve as mini-containment vessels to hold in radioactive fission products. The ORNL applied the layers to UCO kernels to produce the TRISO coated fuel particles and then embedded them into a graphitic matrix material to form cylindrical “compacts” that were shipped to the INL where they were placed into graphite holders inserted into the six capsules inside the AGR-1 test train.
The AGR-1 experiment was inserted into the ATR and allowed for each of six capsules containing the particle fuel specimens to be monitored and controlled separately. The 18-foot-long test rig included a long, curved tube containing its “umbilical cord” with instrumentation wiring and gas flow tubes that were attached to the side of the ATR pressure vessel and a straight section that held the six independent capsules with particle fuel specimens inside the reactor core region.
Inside the ATR, the fuel specimens were subjected to neutron irradiation roughly between 2 to 3 times higher than what they would experience inside an actual HTGR. This allows INL researchers to gain irradiation performance data for nuclear fuel and materials in a shorter time. The AGR-1 test was completed on November 6, 2009, at the end of the ATR cycle, and achieved over 600 effective full-power days of irradiation time. None of the fuel particles released an amount of fission product gases that would indicated failure of the three coating layers that surround each particle during the entire irradiation campaign. The INL technical team monitored the AGR fuel for several of parameters including individual capsule time-dependent peak and average temperatures, individual fission product isotopic gas release rates, and the particle fuel "burn-up," which is a measure of the percent of uranium fuel which has undergone fission reactions.
Although the experiment has been removed from the ATR, researchers still have more work to do before the AGR-1 test campaign will be finished. Post irradiation examination (PIE) will begin at INL and ORNL hot cell facilities and allow scientists to examine the fuel closely so that the fuel and its coating layers can be evaluated for degradation patterns and other characteristics. In addition, controlled higher temperature testing in special furnaces is planned to determine the safety performance of the fuel under postulated accident conditions. These PIE activities will last another two years.
The Next Generation Nuclear Plant Program aims to use a high temperature gas reactor to produce high temperature process heat and hydrogen used by many industrial facilities in daily operations, and to support the broader goal of developing the next generation of nuclear power systems that provide abundant carbon-free electricity on a 24/7 basis.
Current operating commercial light water reactors using steam thermodynamic cycles can produce electricity with only 30 to 33 percent efficiency and their fuel assemblies are limited to approximately 5 percent uranium burn-up by NRC regulations. HTGRs with helium-only gas turbo-generators can operate at much higher efficiency levels between 44 and 48 percent. By using TRISO particle fuel that could safely achieve higher burn up levels, for example, 12 to 16 percent, far less uranium resources would be needed to produce a given amount of electricity. Essentially, the NGNP TRISO fuel particles will be able to produce significantly more usable energy (electricity, process heat and hydrogen) with the same amount of uranium fuel.
Excellent TRISO particle fuel irradiation performance must be demonstrated before high temperature gas reactors can be licensed and co-located with these complementary industrial facilities. Reaching this world record peak burn-up of 19 percent without any particle failure demonstrates the robustness of this particle fuel design. Future AGR fuel tests, conducted over the next decade will include TRISO particle fuel made on a prototypical industrial-scale basis to further prove its irradiation performance for the specific fuel design that will be used for the DOE NGNP demonstration reactor.