Burnup
Encyclopedia
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 both as the fraction of fuel atoms that underwent fission in %FIMA (fissions per initial metal atom) and as the actual energy released per mass of initial fuel in gigawatt-days/metric ton of heavy metal (GWd/MTHM), or similar units.
(MTU) and operates at full power for 1 year, the average burnup of the fuel is (3000MW*365)/24 metric tonnes = 45.63 GWd/MT, or 45,625 MWd/MTHM (where HM stands for heavy metal, meaning actinides like Uranium, Plutonium, etc.).
Converting between percent and energy/mass requires knowledge of κ, the energy released per fission event. A typical value is 200 MeV/fission. With this value, the maximum burnup of 100%, which includes fissioning not just fissile
content but also the other fissionable nuclides, is equivalent to about 938 GWd/MT. Nuclear engineers often use this to roughly approximate 10% burnup as just less than 100 GWd/MT.
The actual fuel may be any actinide
that can support a chain reaction, including uranium, plutonium
, and more exotic transuranic fuels. This fuel content is often referred to as the heavy metal to distinguish it from other metals present in the fuel, such as those used for cladding. The heavy metal is typically present as either metal or oxide, but other compounds such as carbides or other salts are possible.
s were typically designed to achieve about 40 GWd/MTU. With newer fuel technology, and particularly the use of nuclear poison
s, these same reactors are now capable of achieving up to 60 GWd/MTU. After so many fissions have occurred, the build-up of fission product
s poisons the chain reaction and the reactor must be shut down and refueled.
Some more-advanced light-water reactor designs are expected to achieve over 90 GWd/MT of higher-enriched fuel.
Fast reactors are more immune to fission-product poisoning and can inherently reach higher burnups in one cycle. In 1985, the EBR-II reactor in Idaho took metallic fuel up to 19.9% burnup, or just under 200 GWd/MT.
The Deep Burn Modular Helium Reactor (DB-MHR) might reach 500 GWd/MT of transuranic elements.
In a power station, high fuel burnup is desirable for:
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.
s 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.
limiting factor, is predominantly from medium-lived fission products, particularly 137Cs and 90Sr. 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
.
, 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
.
Nuclear power
Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...
technology, burnup (also known as fuel utilization) is a measure of how much energy is extracted from a primary nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
source. It is measured both as the fraction of fuel atoms that underwent fission in %FIMA (fissions per initial metal atom) and as the actual energy released per mass of initial fuel in gigawatt-days/metric ton of heavy metal (GWd/MTHM), or similar units.
Measures of Burnup
Expressed as a percentage, burnup is simple: if 5% of the initial heavy metal atoms have undergone fission, the burnup is 5%. 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) plant uses 24 metric tonnes of enriched uraniumEnriched uranium
Enriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight...
(MTU) and operates at full power for 1 year, the average burnup of the fuel is (3000MW*365)/24 metric tonnes = 45.63 GWd/MT, or 45,625 MWd/MTHM (where HM stands for heavy metal, meaning actinides like Uranium, Plutonium, etc.).
Converting between percent and energy/mass requires knowledge of κ, the energy released per fission event. A typical value is 200 MeV/fission. With this value, the maximum burnup of 100%, which includes fissioning not just fissile
Fissile
In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...
content but also the other fissionable nuclides, is equivalent to about 938 GWd/MT. Nuclear engineers often use this to roughly approximate 10% burnup as just less than 100 GWd/MT.
The actual fuel may be any actinide
Actinide
The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...
that can support a chain reaction, including uranium, plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
, and more exotic transuranic fuels. This fuel content is often referred to as the heavy metal to distinguish it from other metals present in the fuel, such as those used for cladding. The heavy metal is typically present as either metal or oxide, but other compounds such as carbides or other salts are possible.
History
Generation II reactorGeneration II reactor
A generation II reactor is a design classification for a nuclear reactor, and refers to the class of commercial reactors built up to the end of the 1990s...
s were typically designed to achieve about 40 GWd/MTU. With newer fuel technology, and particularly the use of nuclear poison
Nuclear poison
A neutron poison is a substance with a large neutron absorption cross-section in applications, such as nuclear reactors. In such applications, absorbing neutrons is normally an undesirable effect...
s, these same reactors are now capable of achieving up to 60 GWd/MTU. After so many fissions have occurred, the build-up of fission product
Fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...
s poisons the chain reaction and the reactor must be shut down and refueled.
Some more-advanced light-water reactor designs are expected to achieve over 90 GWd/MT of higher-enriched fuel.
Fast reactors are more immune to fission-product poisoning and can inherently reach higher burnups in one cycle. In 1985, the EBR-II reactor in Idaho took metallic fuel up to 19.9% burnup, or just under 200 GWd/MT.
The Deep Burn Modular Helium Reactor (DB-MHR) might reach 500 GWd/MT of transuranic elements.
In a power station, high fuel burnup is desirable for:
- Reducing downtime for refueling
- Reducing the number of fresh nuclear fuel elements required and spent nuclear fuelSpent nuclear fuelSpent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor...
elements generated while producing a given amount of energy - Reducing the potential for diversion of plutoniumPlutoniumPlutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
from spent fuel for use in nuclear weaponNuclear weaponA nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
s
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
Online refuelling
In nuclear power technology, online refuelling is a technique for changing the fuel of a nuclear reactor while the pile is critical.Online refuelling has been provided for three main reasons:...
, 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.
Fuel requirements
In once-through nuclear fuel cycleNuclear fuel cycle
The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in...
s 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
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
and of the plutonium bred from the 238U
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
to be utilised, reducing the uranium requirements of the fuel cycle.
Waste
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 repositoryDeep geological repository
A deep geological repository is a nuclear waste repository excavated deep within a stable geologic environment...
limiting factor, is predominantly from medium-lived fission products, particularly 137Cs and 90Sr. 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
Nuclear reprocessing
Nuclear reprocessing technology was developed to chemically separate and recover fissionable plutonium from irradiated nuclear fuel. Reprocessing serves multiple purposes, whose relative importance has changed over time. Originally reprocessing was used solely to extract plutonium for producing...
, 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
Specific activity
In nuclear sciences and technologies, "activity" is the SI quantity related to the phenomenon of natural and artificial radioactivity. The SI unit of "activity" is becquerel, Bq, while that of "specific activity" is Bq/kg. The old unit of "activity" was curie, Ci, while that of "specific activity"...
.
Proliferation
Burnup is one of the key factors determining the isotopic composition of spent nuclear fuelSpent nuclear fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor...
, 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
Plutonium-240
Plutonium-240 is an isotope of the metal plutonium formed when plutonium-239 captures a neutron. About 62% to 73% of the time when Pu-239 captures a neutron it undergoes fission; the rest of the time it forms Pu-240. The longer a nuclear fuel element remains in a nuclear reactor the greater the...
and 242Pu
Plutonium-242
Pu-242 is one of the isotopes of plutonium, the second longest-lived, with a half-life of 373,300 years.242Pu's halflife is about 15 times as long as Pu-239's halflife; therefore it is 1/15 as radioactive and not one of the larger contributors to nuclear waste radioactivity.242Pu's gamma ray...
.
Cost
One 2003 MIT graduate student thesis concludes that "the fuel cycle cost associated with a burnup level of 100 GWd/MTHM is higher than for a burnup of 50 GWd/MTHM. 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."External links
- High Burnup LWR Fuel Modeling and Optimization at the MIT website.
- Improved Nuclear Fuel Pellet Design and Process to Eliminate the RIM Effect (abstract) discusses the effect of high burnup at fuel pellet extremities.
- Basic Requirements of High Burn-up fuels in LWRs