4 ms·

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.

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
Whoops: the dominant control reaction in B-10 is (n,alpha), not (n,gamma)! That's a common mistake. As you'll see, it's larger than most fission products across the board See https://imgur.com/6ODUcRx https://imgur.com/6ODUcRx. There are a few Hafnium and Europium nuclides that can beat it but they're pretty expensive so most people stick to Boron control. Regarding that paper on CANDUs, I'd like to see some lattice physics calcs supporting that. Has anyone run CASMO on it? I would be absolutely shocked if you took a core of 55 MWd/kg (avg) LWR spent fuel, put it straight in a CANDU with no separations (all FP inventory accounted for) and saw it push through an additional normal amount of burnup. Today, most CANDU's use ~2% enriched feed, I believe. PWRs are not unstable. They are in their most critical configuration during operation. If they heat up, the water density goes down. Neutrons fail to be moderated and the source of thermal neutrons back into the fuel goes down, and the chain reaction shuts down. This is a regulatory requirement (GDC 11 of the NRC). Unstable cores run away in power excursions when poked (see Chernobyl). Modern cores don't do that. They have negative MTC and Doppler. I'm well aware of what inherent safety is. EBR-II, a sodium-cooled fast reactor, was the first to actually demonstrate passive shutdown and passive decay heat following unprotected loss of heat sink and unprotected loss of flow. BTW I don't advocate for fast reactors to burn SNF. It's much more economical to just dispose of it and mine new uranium. I just was pointing out that some people messed up their attempts to burn SNF in thermal reactors.
- achalhp 8y ago
  Yeah, I missed it the first time. You commented when I started editing it. I took (N,TOT) this time. Fission product cross sections is still higher. https://imgur.com/a/pjoNv1p https://imgur.com/a/pjoNv1p CANDU: DUPIC fuel cycle is not demonstrated. Canadians don't run PWRs. India, China and South Korea run PWR and CANDU, not much advanced R&D in India. Chinese CANDU in Qinshan may be testing this. US is most advanced in nuclear R&D. Why not build a CANDU in the US and demonstrate this? I was talking about pressuriser and DNB. If there is no pressuriser (PWR) and constant adjustment of recirculation flow (BWR), clad may get damaged and fuel may melt because of DNB or denucleate boiling. Not inherently safe, engineered systems are needed. (In BWR fuel itself is engineered top-to-bottom with different enrichment levels and different burnable poison concentrations.) Control rod adjustments are needed till a xenon equilibrium is attained. Also flux flattening circus, boron dilution circus etc. Operators are needed to babysit a LWR or CANDU. This is not like diesel generator or PV panel. Passive shutdown does not mean passive operation like a diesel. I don't think first generation of MSRs can be like diesel generators. Operators are needed. But, there is potential for nuclear reactors to be like diesel generators with fluid-fuel reactors. (Edit: Reply to "Today, most CANDU's use ~2% enriched feed, I believe." No. ALL heavy water reactors run with natural uranium in most tubes, depleted uranium or thorium in few pressure tubes for flux flattening circus. No enriched uranium in any tubes.)

acidburnNSA 8y ago

Whoops: the dominant control reaction in B-10 is (n,alpha), not (n,gamma)! That's a common mistake. As you'll see, it's larger than most fission products across the board See https://imgur.com/6ODUcRx https://imgur.com/6ODUcRx. There are a few Hafnium and Europium nuclides that can beat it but they're pretty expensive so most people stick to Boron control. Regarding that paper on CANDUs, I'd like to see some lattice physics calcs supporting that. Has anyone run CASMO on it? I would be absolutely shocked if you took a core of 55 MWd/kg (avg) LWR spent fuel, put it straight in a CANDU with no separations (all FP inventory accounted for) and saw it push through an additional normal amount of burnup. Today, most CANDU's use ~2% enriched feed, I believe. PWRs are not unstable. They are in their most critical configuration during operation. If they heat up, the water density goes down. Neutrons fail to be moderated and the source of thermal neutrons back into the fuel goes down, and the chain reaction shuts down. This is a regulatory requirement (GDC 11 of the NRC). Unstable cores run away in power excursions when poked (see Chernobyl). Modern cores don't do that. They have negative MTC and Doppler. I'm well aware of what inherent safety is. EBR-II, a sodium-cooled fast reactor, was the first to actually demonstrate passive shutdown and passive decay heat following unprotected loss of heat sink and unprotected loss of flow. BTW I don't advocate for fast reactors to burn SNF. It's much more economical to just dispose of it and mine new uranium. I just was pointing out that some people messed up their attempts to burn SNF in thermal reactors.

achalhp 8y ago
Yeah, I missed it the first time. You commented when I started editing it. I took (N,TOT) this time. Fission product cross sections is still higher. https://imgur.com/a/pjoNv1p https://imgur.com/a/pjoNv1p CANDU: DUPIC fuel cycle is not demonstrated. Canadians don't run PWRs. India, China and South Korea run PWR and CANDU, not much advanced R&D in India. Chinese CANDU in Qinshan may be testing this. US is most advanced in nuclear R&D. Why not build a CANDU in the US and demonstrate this? I was talking about pressuriser and DNB. If there is no pressuriser (PWR) and constant adjustment of recirculation flow (BWR), clad may get damaged and fuel may melt because of DNB or denucleate boiling. Not inherently safe, engineered systems are needed. (In BWR fuel itself is engineered top-to-bottom with different enrichment levels and different burnable poison concentrations.) Control rod adjustments are needed till a xenon equilibrium is attained. Also flux flattening circus, boron dilution circus etc. Operators are needed to babysit a LWR or CANDU. This is not like diesel generator or PV panel. Passive shutdown does not mean passive operation like a diesel. I don't think first generation of MSRs can be like diesel generators. Operators are needed. But, there is potential for nuclear reactors to be like diesel generators with fluid-fuel reactors. (Edit: Reply to "Today, most CANDU's use ~2% enriched feed, I believe." No. ALL heavy water reactors run with natural uranium in most tubes, depleted uranium or thorium in few pressure tubes for flux flattening circus. No enriched uranium in any tubes.)