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Mason Brooks
Mason Brooks

Burnup


In nuclear power technology, burnup (also known as fuel utilization) is a measure of how much energy is extracted from a primary nuclear fuel source. It is measured as the fraction of fuel atoms that underwent fission in %FIMA (fissions per initial metal atom)[1] or %FIFA (fissions per initial fissile atom)[2] as well as, preferably, the actual energy released per mass of initial fuel in gigawatt-days/metric ton of heavy metal (GWd/tHM), or similar units.




burnup



Expressed as a percentage: if 5% of the initial heavy metal atoms have undergone fission, the burnup is 5%FIMA. If these 5% were the total of 235U that were in the fuel at the beginning, the burnup is 100%FIFA (as 235U is fissile and the other 95% heavy metals like 238U are not). In reactor operations, this percentage is difficult to measure, so the alternative definition is preferred. This can be computed by multiplying the thermal power of the plant by the time of operation and dividing by the mass of the initial fuel loading. For example, if a 3000 MW thermal (equivalent to 1000 MW electric at 30% efficiency, which is typical of US LWRs) plant uses 24 tonnes of enriched uranium (tU) and operates at full power for 1 year, the average burnup of the fuel is (3000 MW365 d)/24 metric tonnes = 45.63 GWd/t, or 45,625 MWd/tHM (where HM stands for heavy metal, meaning actinides like thorium, uranium, plutonium, etc.).


Fast reactors are more immune to fission-product poisoning and can inherently reach higher burnups in one cycle. In 1985, the EBR-II reactor at Argonne National Laboratory took metallic fuel up to 19.9% burnup, or just under 200 GWd/t.[4]


It is also desirable that burnup should be as uniform as possible both within individual fuel elements and from one element to another within a fuel charge. In reactors with online refuelling, fuel elements can be repositioned during operation to help achieve this. In reactors without this facility, fine positioning of control rods to balance reactivity within the core, and repositioning of remaining fuel during shutdowns in which only part of the fuel charge is replaced may be used.


On the other hand, there are signs that increasing burnup above 50 or 60 GWd/tU leads to significant engineering challenges[6] and that it does not necessarily lead to economic benefits. Higher-burnup fuels require higher initial enrichment to sustain reactivity. Since the amount of separative work units (SWUs) is not a linear function of enrichment, it is more expensive to achieve higher enrichments. There are also operational aspects of high burnup fuels[7] that are associated especially with reliability of such fuel. The main concerns associated with high burnup fuels are:


In once-through nuclear fuel cycles such as are currently in use in much of the world, used fuel elements are disposed of whole as high level nuclear waste, and the remaining uranium and plutonium content is lost. Higher burnup allows more of the fissile 235U and of the plutonium bred from the 238U to be utilised, reducing the uranium requirements of the fuel cycle.


In once-through nuclear fuel cycles, higher burnup reduces the number of elements that need to be buried. However, short-term heat emission, one deep geological repository limiting factor, is predominantly from medium-lived fission products, particularly 137Cs (30.08 year half life) and 90Sr (28.9 year half life). As there are proportionately more of these in high-burnup fuel, the heat generated by the spent fuel is roughly constant for a given amount of energy generated.


Similarly, in fuel cycles with nuclear reprocessing, the amount of high-level waste for a given amount of energy generated is not closely related to burnup. High-burnup fuel generates a smaller volume of fuel for reprocessing, but with a higher specific activity.


Burnup is one of the key factors determining the isotopic composition of spent nuclear fuel, the others being its initial composition and the neutron spectrum of the reactor. Very low fuel burnup is essential for the production of weapons-grade plutonium for nuclear weapons, in order to produce plutonium that is predominantly 239Pu with the smallest possible proportion of 240Pu and 242Pu.


One 2003 MIT graduate student thesis concludes that "the fuel cycle cost associated with a burnup level of 100 GWd/tHM is higher than for a burnup of 50 GWd/tHM. In addition, expenses will be required for the development of fuels capable of sustaining such high levels of irradiation. Under current conditions, the benefits of high burnup (lower spent fuel and plutonium discharge rates, degraded plutonium isotopics) are not rewarded. Hence there is no incentive for nuclear power plant operators to invest in high burnup fuels."[9]


To understand "burnup," it helps to know more about the uranium that fuels a reactor. Before it is made into fuel, uranium is processed to increase the concentration of atoms that can split in a controlled chain reaction in the reactor. The atoms release energy as they split. This energy produces the heat that is turned into electricity. In general, the higher the concentration of those atoms, the longer the fuel can sustain a chain reaction. And the longer the fuel remains in the reactor, the higher the burnup.


In other words, burnup is a way to measure how much uranium is burned in the reactor. It is the amount of energy produced by the uranium. Burnup is expressed in gigawatt-days per metric ton of uranium (GWd/MTU). Average burnup, around 35 GWd/MTU two decades ago, is over 45 GWd/MTU today. Utilities now are able to get more power out of their fuel before replacing it. This means they can operate longer between refueling outages. It also means they use less fuel.


The burnup level affects the fuel's temperature, radioactivity and physical makeup. It is important to the NRC's review of spent fuel cask designs because each system has limits on temperature and radioactivity. How hot and how radioactive spent fuel is depends on burnup, as well as the fuel's initial makeup and conditions in the core. All these factors must be taken into account in designing and approving dry storage and transport systems for spent fuel.


Nuclear fuel is encased in metal cladding. In the reactor, this cladding reacts with cooling water. The reaction forms oxide on the outside (similar to rust) and releases hydrogen. These processes begin slowly, then start to accelerate as the fuel reaches burnup of 45 GWd/MTU. Anything higher has historically been considered high burnup. But in reality there is no sharp line between low and high burnup. It is a continuum. That means the difference between fuel burned to 45 GWd/MTU and 46 or 47 GWd/MTU can be very small.


When spent fuel is placed in a dry storage system and the water is removed, the temperature of the fuel temporarily increases and the makeup of the cladding can change. This change can be more pronounced in high burnup spent fuel, which prompted the NRC to evaluate whether the cladding can become less "ductile," or less pliable, as it cools. To address the technical concern, the NRC and the U.S. Department of Energy sponsored research programs to evaluate the performance of high burnup spent fuel. This research showed that the cladding remains ductile and the dry storage systems and transportation packages can safely hold the fuel.


Based on these reviews, the NRC has certified systems for dry storage and transportation of high burnup spent fuel. Because low burnup spent fuel has been around longer and there is more of it, there are more systems for low than for high burnup spent fuel. However, as more data has become available on the performance of high burnup spent fuel, the NRC has certified dry storage and transportation systems for higher burnups for an initial term up to 20 years.


Operating experience since dry storage began in 1986 and short-term tests show that both low and high burnup spent fuel can be stored and transported safely. The NRC has sponsored testing at Oak Ridge National Laboratory on high burnup fuel under stresses greater than the loads expected during normal storage and transport. These tests have shown that high burnup fuel is very strong. However, the NRC wants to continue to obtain valuable information on high burnup spent fuel stored in the U.S., which is why users of dry storage systems will evaluate results from a demonstration program of an actual loaded dry storage system. These results will provide added confirmation of the safety of dry storage and transport of aged high burnup spent fuel.


The primary focus of research today is to get more data to support the continued safety of dry storage systems for high burnup spent fuel beyond the initial 20-year storage term. The research is designed to ensure that the existing data is accurate as the fuel gets older. Results are expected to confirm that the fuel will remain safe for transport even after extended storage.


The Department of Energy is sponsoring two research programs on high burnup spent fuel. The Research Cask Program, run jointly with the nuclear industry and with regulatory oversight by the NRC, is currently underway. In this study, high burnup spent fuel was loaded into a dry storage system fitted with instruments to provide temperature readings and allow sampling of the gas inside. These readings, combined with tests on the fuel assemblies and inspection of the cask's interior after years of dry storage, will provide more data. The results will enhance our understanding of what happens to high burnup spent fuel in a storage system as it cools over time.


The second study focuses on the characteristics, material properties, and performance of high burnup fuel rods with similar irradiation histories as those loaded in the research cask. These "sister rods" will be tested against the con ditions as measured in the research cask as well as against conditions modeled for other dry storage systems that have different thermal profiles and histories. All this work will help system designers, users and regulators better understand how to ensure high burnup spent fuel will remain safe in dry storage over the long term and during eventual transportation to a centralized storage or disposal facility. 041b061a72


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