Advanced experimental fuel rods loaded into a Russian nuclear reactor
The first Russian-made nuclear reactor fuel bundles with experimental ATF (Advanced Technology Fuel) rods have been loaded into Unit 2 of the Rostov nuclear power plant in southwest Russia. The reactor resumed operations earlier this month after a scheduled shutdown for maintenance and reloading.
The Rostov site is home to four VVER-1000 reactors (Figure 1), which entered commercial operation in 2001, 2010, 2014 and 2018, respectively. The core of the VVER-1000 reactor design contains 163 fuel assemblies, each with 312 fuel rods. Typically, a third of the fuel assemblies are replaced during an outage for recharging. Three of the fuel assemblies loaded during the last refueling of Rostov Unit 2 contain experimental fuel rods (12 rods in each assembly) that will demonstrate the innovative fuel in a commercial reactor.
ATF improves reactor safety
ATF is resistant to severe beyond design basis accidents in nuclear power plants that experience a loss of coolant in the reactor. Even in the event of a heat removal failure in the core, the ATF is designed to maintain its integrity long enough to prevent a zirconium-vapor reaction from inducing the release of hydrogen. ATF is of crucial importance for improving the integral safety and reliability of nuclear power.
The fuel assemblies loaded at Rostov include two types of experimental fuel rods (Figure 2). Both use uranium dioxide, but six of the bars are coated with a zirconium alloy containing a chromium coating, while the other six are coated with 42XHM, which is a chromium-nickel alloy.
The experimental fuel bundles are expected to last three fuel cycles, each lasting approximately one and a half years. After the pilot operation, the rods will be allowed to cool for a period in the unit’s spent fuel pool, after which the bundles will undergo post-irradiation examinations in hot cells at the Atomic Reactor Research Institute. from Dimitrovgrad, in the Ulyanovsk region.
“The irradiation of fuel in a commercial reactor is an important step for the further commercialization of this product,” said Natalia Nikipelova, president of TVEL Fuel Co., Rosatom’s fuel division, in a statement to POWER. “The development of ATF is the main global trend in the nuclear power industry, which aims to bring the safety of nuclear power plants to a new level, virtually eliminating the likelihood of accidents having a negative impact on the environment. Some ATF options with an alternative fuel matrix could also make operations more economically efficient without increasing the level of uranium enrichment. According to international analysts, this product will dominate the global market in the relatively short term, ”she said.
Further tests in progress
Rosatom said POWER that two fuel assemblies with samples of VVER size fuel rods and pressurized water reactor (PWR) were manufactured at the end of 2018. They were loaded into the water loops of the MIR research reactor of the Institut de recherche sur atomic reactors. MIR is a loop-type research reactor, designed primarily to test fuel elements, fuel assemblies and other core components for different types of operational and under development nuclear reactors.
The assemblies loaded into the MIR are somewhat different from the actual fuel used in power reactors. Instead, they’re little packages made for research. And while the assemblies loaded into the Rostov and MIR reactors have two variants of cladding, the MIR assemblies also contain different types of fuel. In addition to uranium dioxide fuel, the MIR project is testing uranium-molybdenum alloy fuel pellets with increased density and thermal conductivity.
Based on the research results, Rosatom plans to choose the optimal combination of structural and fuel materials, taking into account technical and economic performance. After two irradiation cycles, Rosatom reported that all fuel rods loaded in the MIR water loops remain hermetically sealed, which is a positive sign, indicating that the tests are going well.
Other advances and commercialization
This year, Rosatom expanded its ATF program with another fuel matrix material option: uranium disilicide. A spokesperson for the company said POWER Uranium disilicide has a number of advantages. First, it has a high density and high uranium content, which can allow the introduction of longer fuel cycles without increasing the level of enrichment. Second, it has high thermal conductivity and low heat capacity, which means less heat accumulated in the reactor core and lower cladding temperatures. Third, the lower operating temperature can improve the performance characteristics of the fuel in general.
There is a well-defined process in the nuclear industry for testing and marketing new types of fuel. The Rosatom project team chose ATF fuel rods with a standard uranium dioxide fuel matrix for the Rostov project as a conservative measure. This is consistent with the practice of fuel development programs around the world, which generally evolve at an incremental rate as new fissile and structural materials are introduced. The research, design and testing of accident-tolerant fuel in TVEL Fuel Co. is provided and coordinated by the Bochvar Institute for High-Tech Scientific Research on Inorganic Materials.
Many analyzes must be performed before a reactor is fully loaded with new fuel bundles (Figure 3). Rosatom explained that decisions about cladding materials are usually a big part of the first step, but the next step can involve changes in the fuel matrix. Uranium-molybdenum alloy and uranium disilicide fuels not only have better thermal conductivity properties, but also higher density. This could help advance fuel cycle economics, which will be important in the future. “To put it simply, a pellet will contain more enriched uranium and produce more energy,” the spokesperson said.
Rosatom has expertise in PWR fuel fabrication and has developed its own PWR fuel assembly design called TVS-K. “If our partners among the PWR operators are interested in supplies of TVS-K in ATF version, and the market is in fact surely interested, we will provide such an option,” said the spokesperson. POWER.
–Aaron Larson is the editor-in-chief of POWER (@AaronL_Power, @POWERmagazine).