Fission FAQ v 1.5
Posted: Wed 25 Jan 2006, 21:46:32
I am attempting to put together a Fission FAQ file so we do not have to keep regurgitating the same argument over and over. Please let me know of any mistakes I make and I will edit in corrections. It would also help if this were a Sticky topic and stays where people can find it right off the bat.
First the math behind Fission. Fission is a process that takes place on a nucleus level where fundamental particles interact to rearrange the building blocks of chemical isotopes into new chemical isotopes. Because these processes take place inside the nucleus of chemical isotopes they are dubbed 'nuclear', however both Fission, where the Nucleus splits and Fusion where the Nucleus grows are Nuclear, they rearrange the building blocks to change a chemical from one isotope into another isotope.
When a nucleus of any element heavier than Iron is fissioned and the fragments are larger than Iron the result is a release of energy. This is a result of the fact that Iron is at the peak of the binding curve of energy, the particles that make up the nuclease of Iron are packed as tightly together as physical laws allow nucleons to be in normal matter. While the difference in the binding energy between Iron and Hydrogen is the greatest difference on the curve, the difference between the Actinide elements and Iron is great enough to release copious amounts of energy. An atom of Uranium 235 has a binding energy of about 218.89 MeV, Iron Fe-58 has a binding energy of 505+-2 MeV.
When a nucleus of U-233, U-235, Pu-239 or Pu-241 fissions the binding energy is expressed as kinetic energy as the fission fragments speed away from each other, because the fission can result in a large range of fragment isotopes and a variable number of neutrons the energy release is averaged over a spectrum of possible results. U-233 averages 197.9 MeV, U-235 averages 202.5 and Pu-239 averages 207.1 Plugging these averages into formulas give an energy yield as follows;
For U-233 one gram fissioned yields 24.22654024 MWh(t)
For U-235 one gram fissioned yields 24.78966346 MWh(t)
For Pu-239 one gram fissioned yields 25.35278668 MWh(t)
To convert Thermal MWh into Electric MWh the heat released has to go through some sort of conversion process, typically this involves boiling water and using the steam to drive a turbine which in turn spins a generator to produce electricity. Not all Fission plants operate by simply heating water and using steam, some of the more efficient reactor designs heat a gaseous fluid like Helium or Carbon Dioxide which is used to directly drive a gas turbine, then used to heat water into steam and drive a steam turbine. This double use of the heat yields a considerable improvement in energy efficiency. An average fission plant converting heat directly into steam is about 33% efficient, a gas cooled reactor is closer to 50% efficient.
Assuming the average reactor world wide produces 1000 MWe and is 33% efficient at converting heat to electricity it will produce 3030 MWt by Fissioning 122 grams of U-235 or 119 grams of Pu-239 per hour of operation. When a fission reactor fueled with Uranium starts up for the first time all of the fission takes place in the U-235 in the fresh fuel, however as the reactor continues to run a small percentage of the Uranium 238 which makes up the bulk of the fresh fuel is converted into Pu-239. Each 12 to 18 months after the first start of the reactor the system is shut down for heavy maintenance and refueling. During the refueling cycle about one third of the used or 'spent' fuel is removed and placed in a large cooling pool for temporary storage. The remaining two thirds of the fuel is examined for damaged elements and rearranged in the core of the reactor where it is joined by a one third core of fresh fuel. When the reactor maintenance and refueling is completed and the reactor is started back up only the fresh fuel is pure Uranium, the remaining two thirds has a mix of Uranium, fission products, and Plutonium. After another 12 to 18 months the reactor again shuts down for heavy maintenance and refueling and again one third of the fuel is replaced with fresh fuel. From that point on until the reactor is decommissioned the core will be a mixture of one third fresh(first cycle), one third second cycle fuel and one third, third cycle fuel. The fresh fuel used in most reactors is all Uranium Oxide, the third cycle fuel has more Plutonium than U-235 undergoing fission in it.
Many European reactors now use a Mixed Oxide fuel loading where 30% of the fuel elements contain recycled Plutonium instead of enriched Uranium. For these reactors when one third of the fuel is replaced 10% of the total fuel is replaced with MOX elements. Any standard light water reactor can use this much MOX fuel without any modification to the reactor itself, however because of the differences in the way that Plutonium fissions compared to Uranium if the MOX exceeds 50% of the total fuel loading the reactor must either be designed for MOX or modified to operate correctly with it. The MOX fuel currently being used in Europe is made by recycling ‘spent’ fuel which has been used in a reactor for three to five years to recover the Uranium and Plutonium in it. On average ‘spent’ fuel has 1% mixed Plutonium isotopes in it that can be chemically separated from the Uranium and fission waste and recycled as MOX fuel. Most of this recovered Plutonium is currently being mixed with depleted Uranium in a concentration of about 7% vs. 93% Uranium but several other mixtures are being explored as options for future fueling systems. In the USA and FSU each 34 tons of weapons grade Plutonium is being mixed in 5% vs. 95% Uranium MOX elements and is to be used as fuel for civilian fission power reactors. Two other mixtures under study are 10% vs. 90% for Plutonium recovered from ‘spent’ MOX fuel elements, and a mix of 1% Plutonium, 5% U-235, 94% U-238, both of which are intended to allow Plutonium to be recycled repeatedly in standard reactors until it is all consumed. A final fuel under study in South Korea is TMOX-RG which has a mixture of reactor grade Plutonium, depleted Uranium and Thorium. TMOX-RG has the benefit of consuming Plutonium while producing Uranium-233 and is designed to be used as a replacement fuel for standard reactors.
EDIT ONE
More fuel information for those who remain interested. When a standard Generation II PWR of 1000 MWe is studied some assumptions are made. These include not only the MWt and conversion efficiency for heat to electricity but also the fuel enrichment ratio and 'burn time' which is calculated in terms of MW days per ton of Actinide metal. For a standard 1000 MWe Generation II reactor fresh fuel is assumed to be enriched to 3.5% and fuel is assumed to be 'burned' for 35,000 MWd/t. Recently these figures have been changed as reactors have been 'uprated' to operate more efficiently, use a more highly enriched fuel, and 'burn' the fuel longer between refueling cycles.
With the old standard of 3.5% U-235 enrichment the fuel would be 'burned' in the reactor for 3 years during which time the U-235 enrichment would fall from 3.5% to .84%. That would imply to the inexperienced that 2.66% U-235 was fissioned, however this figure is not correct. Nearly 20% of the time when a U-235 nucleus captures a neutron it does not fission, instead it is transmuted into U-236. U-236 is not a good nuclear fuel in thermal reactors, instead it has the tendency that when a neutron is captured it almost always transmutes into U-237 instead. U-237 is highly radioactive with a short half life, it almost always decays into Neptunium-237, the most stable isotope of Neptunium, before it interacts with another Neutron. Np-237 like U-236 does occasionally fission when it captures a neutron but predominantly it becomes Np-238 which is like U-237 in that it decays rapidly into another chemical, in this case Plutonium-238. Pu-238 has only a 6% chance of fissioning when it captures a neutron, the remaining 94% of the time it simply transmutes into Pu-239, which is a good fission fuel but not as good as U-235. In the case of U-235 about 81% of the time it fissions, for Pu-239 the figure is close to 62% of the time fission occurs.
Because standard reactor fuel starts out 3.5% U-235 and 96.5% U-238 most of the Pu-239 and heavier isotopes come from the U-238+n=U-239~~Np-239~~Pu-239 chain of reactions, but if you add up the U-236, Np-237 and Pu-238 in spent fuel you get .52%, or about 20% of the 2.66% U-235 which has been consumed during the fuel cycle. 2.66%*.20=.532%
So that explains where 2.66%-.52%=2.14% of the fission waste comes from, it was the U-235 fissioned during the fuel cycle. But when 'Spent' fuel is removed from the reactor it typically has 4% fission waste and 1% Plutonium and higher actinide isotopes in it. The remaining 1.86% fission waste comes from the Plutonium bred in the reactor which has undergone fission during the operation of the reactor. The operation of the reactor breeds not only the 1.86% of the fuel mass Plutonium fissioned, it breeds enough that about .9% plutonium remains in the fuel at the end of its life in the reactor. 1.86%+.9%=2.76% Uranium converted into Plutonium. 1.86% Plutonium fissioned + 2.14% U-235 fissioned=4. 2.76/4=.69, which is the conversion ratio of fuel fissioned to fuel bred for this spent fuel assay. Because this PWR reactor replaces 69% of its own fuel consumption while operating it is able to run for a much longer time. If you replaced the 96.5% U-238 in the fresh fuel with an inert matrix like SiC (Silicon Carbide) the reactor would not be able to use the fuel for 3 to 5 years per cycle, instead the fuel U-235 would have to supply all of the fission fuel and would run out in about 14 months. Only the fact that Fertile fuels in the core undergo conversion into fissile fuels make it economical to operate NPP's.
First the math behind Fission. Fission is a process that takes place on a nucleus level where fundamental particles interact to rearrange the building blocks of chemical isotopes into new chemical isotopes. Because these processes take place inside the nucleus of chemical isotopes they are dubbed 'nuclear', however both Fission, where the Nucleus splits and Fusion where the Nucleus grows are Nuclear, they rearrange the building blocks to change a chemical from one isotope into another isotope.
When a nucleus of any element heavier than Iron is fissioned and the fragments are larger than Iron the result is a release of energy. This is a result of the fact that Iron is at the peak of the binding curve of energy, the particles that make up the nuclease of Iron are packed as tightly together as physical laws allow nucleons to be in normal matter. While the difference in the binding energy between Iron and Hydrogen is the greatest difference on the curve, the difference between the Actinide elements and Iron is great enough to release copious amounts of energy. An atom of Uranium 235 has a binding energy of about 218.89 MeV, Iron Fe-58 has a binding energy of 505+-2 MeV.
When a nucleus of U-233, U-235, Pu-239 or Pu-241 fissions the binding energy is expressed as kinetic energy as the fission fragments speed away from each other, because the fission can result in a large range of fragment isotopes and a variable number of neutrons the energy release is averaged over a spectrum of possible results. U-233 averages 197.9 MeV, U-235 averages 202.5 and Pu-239 averages 207.1 Plugging these averages into formulas give an energy yield as follows;
For U-233 one gram fissioned yields 24.22654024 MWh(t)
For U-235 one gram fissioned yields 24.78966346 MWh(t)
For Pu-239 one gram fissioned yields 25.35278668 MWh(t)
To convert Thermal MWh into Electric MWh the heat released has to go through some sort of conversion process, typically this involves boiling water and using the steam to drive a turbine which in turn spins a generator to produce electricity. Not all Fission plants operate by simply heating water and using steam, some of the more efficient reactor designs heat a gaseous fluid like Helium or Carbon Dioxide which is used to directly drive a gas turbine, then used to heat water into steam and drive a steam turbine. This double use of the heat yields a considerable improvement in energy efficiency. An average fission plant converting heat directly into steam is about 33% efficient, a gas cooled reactor is closer to 50% efficient.
Assuming the average reactor world wide produces 1000 MWe and is 33% efficient at converting heat to electricity it will produce 3030 MWt by Fissioning 122 grams of U-235 or 119 grams of Pu-239 per hour of operation. When a fission reactor fueled with Uranium starts up for the first time all of the fission takes place in the U-235 in the fresh fuel, however as the reactor continues to run a small percentage of the Uranium 238 which makes up the bulk of the fresh fuel is converted into Pu-239. Each 12 to 18 months after the first start of the reactor the system is shut down for heavy maintenance and refueling. During the refueling cycle about one third of the used or 'spent' fuel is removed and placed in a large cooling pool for temporary storage. The remaining two thirds of the fuel is examined for damaged elements and rearranged in the core of the reactor where it is joined by a one third core of fresh fuel. When the reactor maintenance and refueling is completed and the reactor is started back up only the fresh fuel is pure Uranium, the remaining two thirds has a mix of Uranium, fission products, and Plutonium. After another 12 to 18 months the reactor again shuts down for heavy maintenance and refueling and again one third of the fuel is replaced with fresh fuel. From that point on until the reactor is decommissioned the core will be a mixture of one third fresh(first cycle), one third second cycle fuel and one third, third cycle fuel. The fresh fuel used in most reactors is all Uranium Oxide, the third cycle fuel has more Plutonium than U-235 undergoing fission in it.
Many European reactors now use a Mixed Oxide fuel loading where 30% of the fuel elements contain recycled Plutonium instead of enriched Uranium. For these reactors when one third of the fuel is replaced 10% of the total fuel is replaced with MOX elements. Any standard light water reactor can use this much MOX fuel without any modification to the reactor itself, however because of the differences in the way that Plutonium fissions compared to Uranium if the MOX exceeds 50% of the total fuel loading the reactor must either be designed for MOX or modified to operate correctly with it. The MOX fuel currently being used in Europe is made by recycling ‘spent’ fuel which has been used in a reactor for three to five years to recover the Uranium and Plutonium in it. On average ‘spent’ fuel has 1% mixed Plutonium isotopes in it that can be chemically separated from the Uranium and fission waste and recycled as MOX fuel. Most of this recovered Plutonium is currently being mixed with depleted Uranium in a concentration of about 7% vs. 93% Uranium but several other mixtures are being explored as options for future fueling systems. In the USA and FSU each 34 tons of weapons grade Plutonium is being mixed in 5% vs. 95% Uranium MOX elements and is to be used as fuel for civilian fission power reactors. Two other mixtures under study are 10% vs. 90% for Plutonium recovered from ‘spent’ MOX fuel elements, and a mix of 1% Plutonium, 5% U-235, 94% U-238, both of which are intended to allow Plutonium to be recycled repeatedly in standard reactors until it is all consumed. A final fuel under study in South Korea is TMOX-RG which has a mixture of reactor grade Plutonium, depleted Uranium and Thorium. TMOX-RG has the benefit of consuming Plutonium while producing Uranium-233 and is designed to be used as a replacement fuel for standard reactors.
EDIT ONE
More fuel information for those who remain interested. When a standard Generation II PWR of 1000 MWe is studied some assumptions are made. These include not only the MWt and conversion efficiency for heat to electricity but also the fuel enrichment ratio and 'burn time' which is calculated in terms of MW days per ton of Actinide metal. For a standard 1000 MWe Generation II reactor fresh fuel is assumed to be enriched to 3.5% and fuel is assumed to be 'burned' for 35,000 MWd/t. Recently these figures have been changed as reactors have been 'uprated' to operate more efficiently, use a more highly enriched fuel, and 'burn' the fuel longer between refueling cycles.
With the old standard of 3.5% U-235 enrichment the fuel would be 'burned' in the reactor for 3 years during which time the U-235 enrichment would fall from 3.5% to .84%. That would imply to the inexperienced that 2.66% U-235 was fissioned, however this figure is not correct. Nearly 20% of the time when a U-235 nucleus captures a neutron it does not fission, instead it is transmuted into U-236. U-236 is not a good nuclear fuel in thermal reactors, instead it has the tendency that when a neutron is captured it almost always transmutes into U-237 instead. U-237 is highly radioactive with a short half life, it almost always decays into Neptunium-237, the most stable isotope of Neptunium, before it interacts with another Neutron. Np-237 like U-236 does occasionally fission when it captures a neutron but predominantly it becomes Np-238 which is like U-237 in that it decays rapidly into another chemical, in this case Plutonium-238. Pu-238 has only a 6% chance of fissioning when it captures a neutron, the remaining 94% of the time it simply transmutes into Pu-239, which is a good fission fuel but not as good as U-235. In the case of U-235 about 81% of the time it fissions, for Pu-239 the figure is close to 62% of the time fission occurs.
Because standard reactor fuel starts out 3.5% U-235 and 96.5% U-238 most of the Pu-239 and heavier isotopes come from the U-238+n=U-239~~Np-239~~Pu-239 chain of reactions, but if you add up the U-236, Np-237 and Pu-238 in spent fuel you get .52%, or about 20% of the 2.66% U-235 which has been consumed during the fuel cycle. 2.66%*.20=.532%
So that explains where 2.66%-.52%=2.14% of the fission waste comes from, it was the U-235 fissioned during the fuel cycle. But when 'Spent' fuel is removed from the reactor it typically has 4% fission waste and 1% Plutonium and higher actinide isotopes in it. The remaining 1.86% fission waste comes from the Plutonium bred in the reactor which has undergone fission during the operation of the reactor. The operation of the reactor breeds not only the 1.86% of the fuel mass Plutonium fissioned, it breeds enough that about .9% plutonium remains in the fuel at the end of its life in the reactor. 1.86%+.9%=2.76% Uranium converted into Plutonium. 1.86% Plutonium fissioned + 2.14% U-235 fissioned=4. 2.76/4=.69, which is the conversion ratio of fuel fissioned to fuel bred for this spent fuel assay. Because this PWR reactor replaces 69% of its own fuel consumption while operating it is able to run for a much longer time. If you replaced the 96.5% U-238 in the fresh fuel with an inert matrix like SiC (Silicon Carbide) the reactor would not be able to use the fuel for 3 to 5 years per cycle, instead the fuel U-235 would have to supply all of the fission fuel and would run out in about 14 months. Only the fact that Fertile fuels in the core undergo conversion into fissile fuels make it economical to operate NPP's.