5 ms·

What applies for fission products should also apply for control rods in fast spectrum. If control rods do not absorb fast neutrons, how a fast reactor is contro

What applies for fission products should also apply for control rods in fast spectrum. If control rods do not absorb fast neutrons, how a fast reactor is controlled? The cross section of fuel in fast spectrum also decreases. Fast reactors have larger fissile holdup. For every atom of fission fragment there are more fuel atoms in fast reactor than in thermal reactor. Fast reactors can run longer because fission products are dilute in the fuel. But, reprocessing is needed even for fast reactors to close the fuel cycle. The advantages like U238 or Pu239 fission or fast fission factor can be also obtained in heterogeneous thermal spectrum reactors. (Example: A CANDU is heterogeneous reactor. It can burn more plutonium and it can use LWR SNF-fuel without reprocessing.) For a fluid-fuel reactor the fuel acts only as fuel and heat exchange takes place outside reactor core. Fuel region in a fluid-fuel reactor can be as thick as it is needed. So, reactor can be designed for maximum multiplication factor. (Solid fuel rods can't be very thick, because they are also heat exchangers.) Slide 6 in this presentation. http://www.thoriumenergyworld.com/uploads/6/9/8/7/69878937/seaborg_technologies_thec18_slides.pdf http://www.thoriumenergyworld.com/uploads/6/9/8/7/69878937/s... Edit: This is a reply for: "I prodded him gently on it: "Oh, you mean the fast neutrons don't get readily absorbed in fission products, not the slow ones, right?" He doubled down. Some poor schmuck is funding this guy."

acidburnNSA 8y ago
Similar to fission products, neutron control material (like Boron) indeed does not absorb fast neutrons as much. Fortunately it still does absorb fast neutrons enough to control fast reactors (with which we have 430 reactor-years of experience or so). Indeed fast reactors need higher fissile concentration to be critical. Some kinds of fast reactors (like the Bill Gates Traveling Wave Reactor idea, which, disclaimer: I have professional connections to) can breed up plutonium in spent fuel and then burn it down without reprocessing (to be fair, the spent fuel would have to be hot refabricated into metallic fuel first, but that's still not separations/reprocessing). This kind of reactor needs enrichment once (to start up) and then can just be fed natural, DU, or SNF and it will run on a stream of it happily "forever" (until the vessel life is reached, at which point you transfer the core to a new machine and keep on shuffling). Only fast neutron systems can do such a thing. The Fast Mixed-Spectrum Reactor idea of BNL in 1980 was similar. Many fast-neutron MSRs are the same. Fast fission is not all that's at play in a fast reactor. It's all about Eta (neutrons released vs. neutrons absorbed in fuel). In a fast reactor, it skyrockets around 0.5 - 1 MeV because of three different physical facts: neutrons released per fission goes up with fast incident neutrons, fission cross section stays flat for all actinides around this range, and capture cross sections drop off towards zero around this range. This is discussed on: https://whatisnuclear.com/fast-reactor.html#havingmore https://whatisnuclear.com/fast-reactor.html#havingmore Having extra neutrons around means you can afford to invest more in breeding fissile material, and only fast neutrons can get you to the point where reactivity is flat or increasing instead of decreasing as fission products are produced. The only exception is Thorium fuel cycle, where U-233 releases a lot of neutrons even with thermal neutron absorption. In that case you have to be removing the fission products as they are created with separations, and this basically is only practical with fluid fuel. This was the idea of the Molten Salt Breeder Reactor project of Oak Ridge in the late 1960s. With reprocessing, you can definitely re-concentrate the remaining fissile material and get a thermal reactor critical again to burn it. But you cannot meaningfully use the other actinides as fuel, which is the whole idea of "running on spent fuel". Regarding that slide deck, I love the idea of molten fuel in tubes. I investigated it heavily once upon a time, being super excited about it. I ran into the problem that while salt fuel mass densities are low, separating them out even more physically makes the fissile density annoyingly low, to the point that I couldn't get the reactor performance I wanted for my then target market. I still think this concept is very interesting so I hope these folks do well. They won't be extracting any non-fissile energy from SNF though until they replace that moderating fluid with something like lead, gas, or sodium and change their fuel salt to a chloride. CANDUs can't use straight-up SNF out of a LWR without first stripping out the fission products. They're an example of "burning down the fissile material even better" but not "extracting the 100x more energy from the fertile U238". BTW the safety example in that slide deck is a bit disingenuous by suggesting that traditional reactors are not physically stable. If you know those guys you might want to tell them. Overmoderation and Doppler and the NRC ensure that they are inherently stable at power. These things aren't like the F-22 or whatever, requiring active systems to stay in the air. The engineered safety systems are simply for removing decay heat, which can be done passively in Gen III+ plants and indefinitely in any Gen-IV plant.
- achalhp 8y ago
  Fission products have larger cross section than enriched Boron-10. It is all about concentration; that is the number of B-10 atoms per number of atoms of fuel. Here is the cross sections plot I took yesterday: https://imgur.com/a/uWb0SAa https://imgur.com/a/uWb0SAa Robert Steinhaus' Question for fast reactor folks: "While the theoretical case for fast reactors being used to burn nuclear waste down to fission products with short half lives has been made for decades, there has in all that time not been a single demonstration of a fast reactor actually experimentally burning any significant quantity (kilograms) of separated Minor Actinides and Transuranics down to fission products to a batch completeness exceeding 90%. The proposal of using fast reactors to treat nuclear waste has been vigorously put forward for decades. Why do fast reactor proponents not demonstrate with one existing fast reactor the burn up of some kilograms of separated long half life Minor Actinide waste to prove the technical feasibility of fast reactors for the application of waste burning and waste treatment?" Yes, more neutrons are released for Pu239 fast fission. In CANDU, U238 fast fission is more because 99% of fuel is U238. If we increase Pu concentration Pu fast fission happens as well. U238 fast fission is ~3% of CANDU's power. This is because all neutrons are born fast. A FFR (MSR) can adjust heterogeneity as required, because heat transfer takes place outside the core. A MSR can have thicker fuel regions and higher lattice pitch than a CANDU and exploit fast fission further. CANDU can use straight-up SNF. Check this or search it: http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/29/013/29013478.pdf http://www.iaea.org/inis/collection/NCLCollectionStore/_Publ... "These things aren't like the F-22 or whatever, requiring active systems to stay in the air." That is exactly how pressuriser in a PWR works. A control system loop with Temp & pressure sensors with heater and water injection. Almost all reactors are directly synced with grid, reactors will participate in tiny load adjustments and there is also cooling water temperature variations in a day. MSRs can be isolated from the power conversion using a thermal salt reserve. That is called inherent safety. Water temperature variations or frequency control wont hit the reactor. Even if MSR is connected to grid directly, there is no DNB control, the boiling point margin is high. If Xenon is removed with >90% efficiency, -ve reactivity can take care of leftover xenon as well, no control rod circus for MSR till a xenon equilibrium is attained.
  - acidburnNSA 8y ago

acidburnNSA 8y ago

Similar to fission products, neutron control material (like Boron) indeed does not absorb fast neutrons as much. Fortunately it still does absorb fast neutrons enough to control fast reactors (with which we have 430 reactor-years of experience or so). Indeed fast reactors need higher fissile concentration to be critical. Some kinds of fast reactors (like the Bill Gates Traveling Wave Reactor idea, which, disclaimer: I have professional connections to) can breed up plutonium in spent fuel and then burn it down without reprocessing (to be fair, the spent fuel would have to be hot refabricated into metallic fuel first, but that's still not separations/reprocessing). This kind of reactor needs enrichment once (to start up) and then can just be fed natural, DU, or SNF and it will run on a stream of it happily "forever" (until the vessel life is reached, at which point you transfer the core to a new machine and keep on shuffling). Only fast neutron systems can do such a thing. The Fast Mixed-Spectrum Reactor idea of BNL in 1980 was similar. Many fast-neutron MSRs are the same. Fast fission is not all that's at play in a fast reactor. It's all about Eta (neutrons released vs. neutrons absorbed in fuel). In a fast reactor, it skyrockets around 0.5 - 1 MeV because of three different physical facts: neutrons released per fission goes up with fast incident neutrons, fission cross section stays flat for all actinides around this range, and capture cross sections drop off towards zero around this range. This is discussed on: https://whatisnuclear.com/fast-reactor.html#havingmore https://whatisnuclear.com/fast-reactor.html#havingmore Having extra neutrons around means you can afford to invest more in breeding fissile material, and only fast neutrons can get you to the point where reactivity is flat or increasing instead of decreasing as fission products are produced. The only exception is Thorium fuel cycle, where U-233 releases a lot of neutrons even with thermal neutron absorption. In that case you have to be removing the fission products as they are created with separations, and this basically is only practical with fluid fuel. This was the idea of the Molten Salt Breeder Reactor project of Oak Ridge in the late 1960s. With reprocessing, you can definitely re-concentrate the remaining fissile material and get a thermal reactor critical again to burn it. But you cannot meaningfully use the other actinides as fuel, which is the whole idea of "running on spent fuel". Regarding that slide deck, I love the idea of molten fuel in tubes. I investigated it heavily once upon a time, being super excited about it. I ran into the problem that while salt fuel mass densities are low, separating them out even more physically makes the fissile density annoyingly low, to the point that I couldn't get the reactor performance I wanted for my then target market. I still think this concept is very interesting so I hope these folks do well. They won't be extracting any non-fissile energy from SNF though until they replace that moderating fluid with something like lead, gas, or sodium and change their fuel salt to a chloride. CANDUs can't use straight-up SNF out of a LWR without first stripping out the fission products. They're an example of "burning down the fissile material even better" but not "extracting the 100x more energy from the fertile U238". BTW the safety example in that slide deck is a bit disingenuous by suggesting that traditional reactors are not physically stable. If you know those guys you might want to tell them. Overmoderation and Doppler and the NRC ensure that they are inherently stable at power. These things aren't like the F-22 or whatever, requiring active systems to stay in the air. The engineered safety systems are simply for removing decay heat, which can be done passively in Gen III+ plants and indefinitely in any Gen-IV plant.

achalhp 8y ago
Fission products have larger cross section than enriched Boron-10. It is all about concentration; that is the number of B-10 atoms per number of atoms of fuel. Here is the cross sections plot I took yesterday: https://imgur.com/a/uWb0SAa https://imgur.com/a/uWb0SAa Robert Steinhaus' Question for fast reactor folks: "While the theoretical case for fast reactors being used to burn nuclear waste down to fission products with short half lives has been made for decades, there has in all that time not been a single demonstration of a fast reactor actually experimentally burning any significant quantity (kilograms) of separated Minor Actinides and Transuranics down to fission products to a batch completeness exceeding 90%. The proposal of using fast reactors to treat nuclear waste has been vigorously put forward for decades. Why do fast reactor proponents not demonstrate with one existing fast reactor the burn up of some kilograms of separated long half life Minor Actinide waste to prove the technical feasibility of fast reactors for the application of waste burning and waste treatment?" Yes, more neutrons are released for Pu239 fast fission. In CANDU, U238 fast fission is more because 99% of fuel is U238. If we increase Pu concentration Pu fast fission happens as well. U238 fast fission is ~3% of CANDU's power. This is because all neutrons are born fast. A FFR (MSR) can adjust heterogeneity as required, because heat transfer takes place outside the core. A MSR can have thicker fuel regions and higher lattice pitch than a CANDU and exploit fast fission further. CANDU can use straight-up SNF. Check this or search it: http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/29/013/29013478.pdf http://www.iaea.org/inis/collection/NCLCollectionStore/_Publ... "These things aren't like the F-22 or whatever, requiring active systems to stay in the air." That is exactly how pressuriser in a PWR works. A control system loop with Temp & pressure sensors with heater and water injection. Almost all reactors are directly synced with grid, reactors will participate in tiny load adjustments and there is also cooling water temperature variations in a day. MSRs can be isolated from the power conversion using a thermal salt reserve. That is called inherent safety. Water temperature variations or frequency control wont hit the reactor. Even if MSR is connected to grid directly, there is no DNB control, the boiling point margin is high. If Xenon is removed with >90% efficiency, -ve reactivity can take care of leftover xenon as well, no control rod circus for MSR till a xenon equilibrium is attained.
- acidburnNSA 8y ago