Proton-Lead Power
Posted: Tue 07 Jan 2014, 00:15:50
Just to be clear nobody has built one for power generation at this time in history, however the basic systems had been used since the 1930's and the technology is well understood. The issue is mostly the same as the issue for Fusion reactors, we know this works but building one that generates more power than it consumes has not been done yet and sadly nobody is working on one today either.
Testing done by physicists in the 1920's and 1930's demonstrated that if you hit an atom hard enough you can smash it into bits, which is where the concept of high energy physics being centered around "atom smashers" comes from. They also learned even way back then that atoms centered around Nickle and Iron are the most naturally tightly bound nuclei, Iron 56 and Nickle 62 specifically have the highest binding energy per atom. That means any fusion process that leads to the Iron or Nickle atoms produces energy and also any atom bigger than Iron or Nickle that gets smashed where one of the pieces is close to this mass also releases energy. When unstable atoms like the Actinides gets hit with a proton or neutron with enough energy it will fission, that is how a nuclear reactor works. As a ball park rule the isotopes we call fissionable meaning Uranium 233, U-235, Plutonium 239, Pu-241 are all so delicately balanced that it doesn't take any outside energy from the neutron to cause them to fission. If you use an atom smasher firing protons they have to have enough velocity expressed as electron Volts to overcome the positive charge of the target , but that is all the energy they need. As soon as they come into contact most of the atoms of those four isotopes will fission.
When a atom fissions the two biggest pieces will form smaller atoms, one of them will mass about 40% of the original mass and the other about 60%. Usually one or more neutrons are also released and frequently an alpha particle, a Helium nucleus also gets released in the break up. Because the Actinides are very heavy nuclei the resulting pieces are much closer to Iron and Nickle on the periodic table so energy that was contained in the original atom is released in the break up as radiation and as the violent motion of the fragmentary pieces and the neutron or neutrons shot out. Because Actinides like Uranium are so massive both pieces that result are usually much more massive than Iron or Nickle. However because some isotopes are so easily fissionable it is relatively easy to design a power plant that can maintain a chain reaction to generate heat.
A Proton-Lead system however can not carry on a chain reaction. The way the Proton-Lead system works is fundamentally simple, you use a particle accelerator to shoot protons at a lead core. You have to fire them with around 17 MeV just to get them past the repulsion effect of the Lead nuclei, but at energies that low the Proton is just absorbed and the nuclei is converted from Lead into Bismuth which doesn't accomplish very much for the person who built the atom smasher. When you turn the energy of the smasher up to about 50 MeV things start to happen that are much more interesting. First I need to point out that 98.5% of Lead is from three isotopes, Pb-206, Pb-207 and Pb-208. Pb-208 comes almost entirely from the decay pathway of Uranium and makes up just over 51% of all Lead on Earth. Unfortunately Pb-208 is at one of the 'Islands of Stability' which is to say that because of the way the Neutrons and Protons are arranged it is one of the most stable substances known to man making it harder to smash. Pb-206 is about 24% of all Lead and Pb-207 is about 22%, and there are traces of about 1.5% of Pb-204 as well in natural Lead found here on Earth. These three together make up about 49% of all earth Lead and because they do not sit on the island of stability they are somewhat easier to smash than Pb-208. When you smash Lead with a Proton moving with 50 MeV or more of energy it fissions just like Uranium or Plutonium do, that is to say it makes two main fragments of about 40% and 60% of the original mass plus ejecting one or more neutrons and radiation. However because Lead is not fissionable with slow Neutrons there is no chain reaction, regular fission neutrons have about 3 MeV to 10 MeV of energy which is not high enough to smash a Pb-204/206/207/208 nucleus and most of them just bounce around in elastic scattering collisions for a second or so losing a tiny fraction of their energy with each collision until they come to rest. A free Neutron has a half life of under 15 minutes, three hours after the atom smasher is shut off all of the free neutrons will have decayed into Protons.
If you turn up the power output on the atom smasher and fire the Protons at say 999 MeV instead of 50 then they will really smash the Lead nuclei they hit, only because they are hitting so hard the Lead doesn't just fission into two main pieces, it shatters into many small pieces and knocks several neutrons out of the debris at very high energy, around 100 MeV. Those kind of neutrons do have enough energy to fission other Lead nuclei, however the chain stops there because the neutrons released by the second set of atoms smashed are back in the 3 MeV to 10 MeV range. What is worse is ramping up the energy that high takes a lot more energy input for the accelerator so to get a positive energy return becomes much more difficult. Think of it this way, you can fire 19 Protons at 55 MeV and have several of them smash a Lead atom into pieces for the same energy input that will fire one Proton at 990 MeV. The very high speed Proton is almost guaranteed to smash the first Lead nuclei it hits straight on exploding it and releasing half a dozen 100 MeV neutrons and a dozen more in the typical 3 MeV to 10 MeV range. If all six of those high energy neutrons manage to fission another Lead nuclei then your total yield is seven nuclei smashed instead of about ten for the moderate speed Protons. From an energy in vs energy out point of view moderate speeds are better.
Each smashed nuclei from the 204/206/207 isotopes will yield around 150 MeV of energy, plus the input energy from the 55 MeV Proton minus losses of around 50% to elastic collisions will give a net yield around 170 MeV compared to an average yield from a Uranium atom of around 220 MeV. This means the energy out is 15-20% lower for a smashed Lead atom instead of smashing Uranium, but the safety factor is unparalleled. Shut the accelerator off and within seconds the smasher is no longer smashing lead. Within three hours the Neutrons have all decayed into Protons and only the fission fragments remain giving off residual heat. Turn the power on it gets hot, turn the power off it cools back down. Any Protons that are captured by the Lead without smashing it convert the nuclei into Bismuth, but Bismuth is also a useful fuel for an Proton accelerator power plant so that is not a major issue.
If they are so great why doesn't anyone build them? Well honestly, it is just much easier to build a regular Uranium Fission power plant and if you design it correctly it will maintain a chain reaction for years or even over a decade without needing more fuel. Civilian power reactors typically recycle ever 12 to 24 months exchanging a third or the core when they do so but military power reactors like the USN uses will run for 15 or more years between refueling cycles. The logical design for these Proton accelerator systems would be a molten pool of Lead with a very good vacuum in the chamber. Many of the fission fragments are gasses or would become gasses at molten lead temperatures so filters and cooling system on the vacuum system would be a must. Because you are not worried about maintaining a chain reaction with the neutrons released by the process you don't care about neutron poisons like Xenon or Gadolinium that forms some of the fission fragments in the 60% of Lead mass range. Because Lead isotopes are in the 204-208 range of masses the fragments with form around isotopes 80 and 125 nucleons in mass. This puts the two peak fragment sizes down a few elements from the Uranium/Plutonium peaks, but not very far. Most of the fragment sizes will overlap between any of the targets of the atom smasher.
Then there is the other factor, if you are smashing Lead, or Bismuth there is no way to make a real weapon of mass destruction out of it. That means most countries are just not interested in doing the research needed to prove if a proton accelerator Lead atom smasher is economically competitive. As much as people despise the facts Nuclear power is a side growth from nuclear weapon production. Fusion research got started because it is an outgrowth of nuclear weapon studies and research to build the H-bomb, if there had been no H-bomb program the basic research needed to understand how Fusion even works might still be waiting for funding. Accelerator systems for scientific study of collisions has been advanced, but no money has gone down the side channel to develop a power supply based on the knowledge gained. Lead is relatively speaking, a cheap material and abundant enough to be worth exploiting. Lead is about 5.5 times more abundant in the crust of the Earth than Uranium is, and 49% of its isotopes have a high energy yield in a Proton accelerator compared to 0.7% of Uranium in a light water reactor.
Testing done by physicists in the 1920's and 1930's demonstrated that if you hit an atom hard enough you can smash it into bits, which is where the concept of high energy physics being centered around "atom smashers" comes from. They also learned even way back then that atoms centered around Nickle and Iron are the most naturally tightly bound nuclei, Iron 56 and Nickle 62 specifically have the highest binding energy per atom. That means any fusion process that leads to the Iron or Nickle atoms produces energy and also any atom bigger than Iron or Nickle that gets smashed where one of the pieces is close to this mass also releases energy. When unstable atoms like the Actinides gets hit with a proton or neutron with enough energy it will fission, that is how a nuclear reactor works. As a ball park rule the isotopes we call fissionable meaning Uranium 233, U-235, Plutonium 239, Pu-241 are all so delicately balanced that it doesn't take any outside energy from the neutron to cause them to fission. If you use an atom smasher firing protons they have to have enough velocity expressed as electron Volts to overcome the positive charge of the target , but that is all the energy they need. As soon as they come into contact most of the atoms of those four isotopes will fission.
When a atom fissions the two biggest pieces will form smaller atoms, one of them will mass about 40% of the original mass and the other about 60%. Usually one or more neutrons are also released and frequently an alpha particle, a Helium nucleus also gets released in the break up. Because the Actinides are very heavy nuclei the resulting pieces are much closer to Iron and Nickle on the periodic table so energy that was contained in the original atom is released in the break up as radiation and as the violent motion of the fragmentary pieces and the neutron or neutrons shot out. Because Actinides like Uranium are so massive both pieces that result are usually much more massive than Iron or Nickle. However because some isotopes are so easily fissionable it is relatively easy to design a power plant that can maintain a chain reaction to generate heat.
A Proton-Lead system however can not carry on a chain reaction. The way the Proton-Lead system works is fundamentally simple, you use a particle accelerator to shoot protons at a lead core. You have to fire them with around 17 MeV just to get them past the repulsion effect of the Lead nuclei, but at energies that low the Proton is just absorbed and the nuclei is converted from Lead into Bismuth which doesn't accomplish very much for the person who built the atom smasher. When you turn the energy of the smasher up to about 50 MeV things start to happen that are much more interesting. First I need to point out that 98.5% of Lead is from three isotopes, Pb-206, Pb-207 and Pb-208. Pb-208 comes almost entirely from the decay pathway of Uranium and makes up just over 51% of all Lead on Earth. Unfortunately Pb-208 is at one of the 'Islands of Stability' which is to say that because of the way the Neutrons and Protons are arranged it is one of the most stable substances known to man making it harder to smash. Pb-206 is about 24% of all Lead and Pb-207 is about 22%, and there are traces of about 1.5% of Pb-204 as well in natural Lead found here on Earth. These three together make up about 49% of all earth Lead and because they do not sit on the island of stability they are somewhat easier to smash than Pb-208. When you smash Lead with a Proton moving with 50 MeV or more of energy it fissions just like Uranium or Plutonium do, that is to say it makes two main fragments of about 40% and 60% of the original mass plus ejecting one or more neutrons and radiation. However because Lead is not fissionable with slow Neutrons there is no chain reaction, regular fission neutrons have about 3 MeV to 10 MeV of energy which is not high enough to smash a Pb-204/206/207/208 nucleus and most of them just bounce around in elastic scattering collisions for a second or so losing a tiny fraction of their energy with each collision until they come to rest. A free Neutron has a half life of under 15 minutes, three hours after the atom smasher is shut off all of the free neutrons will have decayed into Protons.
If you turn up the power output on the atom smasher and fire the Protons at say 999 MeV instead of 50 then they will really smash the Lead nuclei they hit, only because they are hitting so hard the Lead doesn't just fission into two main pieces, it shatters into many small pieces and knocks several neutrons out of the debris at very high energy, around 100 MeV. Those kind of neutrons do have enough energy to fission other Lead nuclei, however the chain stops there because the neutrons released by the second set of atoms smashed are back in the 3 MeV to 10 MeV range. What is worse is ramping up the energy that high takes a lot more energy input for the accelerator so to get a positive energy return becomes much more difficult. Think of it this way, you can fire 19 Protons at 55 MeV and have several of them smash a Lead atom into pieces for the same energy input that will fire one Proton at 990 MeV. The very high speed Proton is almost guaranteed to smash the first Lead nuclei it hits straight on exploding it and releasing half a dozen 100 MeV neutrons and a dozen more in the typical 3 MeV to 10 MeV range. If all six of those high energy neutrons manage to fission another Lead nuclei then your total yield is seven nuclei smashed instead of about ten for the moderate speed Protons. From an energy in vs energy out point of view moderate speeds are better.
Each smashed nuclei from the 204/206/207 isotopes will yield around 150 MeV of energy, plus the input energy from the 55 MeV Proton minus losses of around 50% to elastic collisions will give a net yield around 170 MeV compared to an average yield from a Uranium atom of around 220 MeV. This means the energy out is 15-20% lower for a smashed Lead atom instead of smashing Uranium, but the safety factor is unparalleled. Shut the accelerator off and within seconds the smasher is no longer smashing lead. Within three hours the Neutrons have all decayed into Protons and only the fission fragments remain giving off residual heat. Turn the power on it gets hot, turn the power off it cools back down. Any Protons that are captured by the Lead without smashing it convert the nuclei into Bismuth, but Bismuth is also a useful fuel for an Proton accelerator power plant so that is not a major issue.
If they are so great why doesn't anyone build them? Well honestly, it is just much easier to build a regular Uranium Fission power plant and if you design it correctly it will maintain a chain reaction for years or even over a decade without needing more fuel. Civilian power reactors typically recycle ever 12 to 24 months exchanging a third or the core when they do so but military power reactors like the USN uses will run for 15 or more years between refueling cycles. The logical design for these Proton accelerator systems would be a molten pool of Lead with a very good vacuum in the chamber. Many of the fission fragments are gasses or would become gasses at molten lead temperatures so filters and cooling system on the vacuum system would be a must. Because you are not worried about maintaining a chain reaction with the neutrons released by the process you don't care about neutron poisons like Xenon or Gadolinium that forms some of the fission fragments in the 60% of Lead mass range. Because Lead isotopes are in the 204-208 range of masses the fragments with form around isotopes 80 and 125 nucleons in mass. This puts the two peak fragment sizes down a few elements from the Uranium/Plutonium peaks, but not very far. Most of the fragment sizes will overlap between any of the targets of the atom smasher.
Then there is the other factor, if you are smashing Lead, or Bismuth there is no way to make a real weapon of mass destruction out of it. That means most countries are just not interested in doing the research needed to prove if a proton accelerator Lead atom smasher is economically competitive. As much as people despise the facts Nuclear power is a side growth from nuclear weapon production. Fusion research got started because it is an outgrowth of nuclear weapon studies and research to build the H-bomb, if there had been no H-bomb program the basic research needed to understand how Fusion even works might still be waiting for funding. Accelerator systems for scientific study of collisions has been advanced, but no money has gone down the side channel to develop a power supply based on the knowledge gained. Lead is relatively speaking, a cheap material and abundant enough to be worth exploiting. Lead is about 5.5 times more abundant in the crust of the Earth than Uranium is, and 49% of its isotopes have a high energy yield in a Proton accelerator compared to 0.7% of Uranium in a light water reactor.