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Like they wouldn't have boron near a nuclear reactor. I'm a PWR man myself, but I am 99% sure that GE BWR designs have a couple of big tanks of borated water s

Like they wouldn't have boron near a nuclear reactor. I'm a PWR man myself, but I am 99% sure that GE BWR designs have a couple of big tanks of borated water specifically for a LOCA. I highly doubt they're having reactivity problems, it's probably just managing the decay heat, and now the radiation leaks, that's the problem. "Just" is a relative word, here, of course. Boiling water reactors are simpler, cheaper, but generally aren’t made anymore because they are perceived as being less safe. That’s because the exotic coolant in the pressurized water reactor can contain boric acid which absorbs neutrons and can help (or totally) control the nuclear reaction. You can’t use boric acid or any other soluble boron-laced neutron absorbers in a boiling water reactor because doing so would contaminate both the cooling system and the environment. He's completely wrong about industry adoption of BWRs. There are two BWR's planned to be built in the US (along with 3 or 4 PWRs), and I believe that China has contracted with GE for a few as well (along with 4 Westinghouse PWRs and maybe a few Areva ones too). PWRs are preferred largely because of their higher power densities (a BWR core that produces the same power must be larger) and simpler nuclear calculations and control strategies (two-phase flow makes calculations much more difficult, and it's harder to calculate correct positions for control blades (whose effects are highly localized) than it is to calculate the correct boron concentration (whose effects are smeared over the whole core)). However, now that computers are faster and us nuclear engineers no longer have the excuse of slow computers to hide behind, PWRs are looking to move away from relying on Boron concentration as the main form of control (the Westinghouse AP1000, specifically, relies much more on rod movement than the AP600), because of the cost of performing regular boron dilutions. He's right that BWRs are simpler and cheaper - about half the moving parts.

rdl 16y ago
Is one of the advantages of the BWR that you don't need a large pressure vessel? I remember news articles about JSW in Osaka being the only source for the huge forgings required for PWR pressure vessels, and only having the capacity for ~4 reactors per year.
- neutronicus 16y ago
  Not really. BWRs need larger pressure vessels, PWRs need thicker ones. I believe that JSW is the only place (also I think there's one in Germany, too) where you can forge a reactor pressure vessel in one piece, but that if you're willing to bolt two pieces together there are a few more options. It may be that this is more feasible for a BWR than a PWR, but I actually think that BWR pressure vessels are usually more expensive. Where BWRs really save all the money is that you don't need to buy, maintain and replace pressurizers, steam generators, and a ton of piping and pumps that need to be rated for 2500 PSI, as well as a whole bunch of instrumentation for measuring and controlling boron dilution levels. You'll be paying more for engineering services, since all the calculations will take longer, but I can only imagine it's worth it, since, high-margin as engineering services are, they're chump change compared to how much the reactor costs.
patrickgzill 16y ago
I thought the new designs were classified as ALWRs - advanced light water reactors (light water meaning they use regular water).
- neutronicus 16y ago
  However they're "classified", some of them (ABWR, ESBWR) work by boiling water inside the core at atmospheric pressure and using the steam to turn a turbine, and some of them (AP1000, EPR) work by heating pressurized (borated) water inside the core, which then transfers heat to other (pure) water at a steam generator, which (pure) steam is then used to turn a turbine.
masklinn 16y ago
> Like they wouldn't have boron near a nuclear reactor. They probably do, the question is whether they can carry it to the site faster than the US navy, and whether the boron tanks have not been compromised.
- neutronicus 16y ago
  The question is also, and I didn't make it clear enough in my original post: "Wait, they need boron?" Boron is a neutron absorber, and is therefore used to control the fission rate. You add boron when you're afraid of the fission rate getting out of control (e.g. Chernobyl). This is a legitimate concern here for two reasons: 1) A BWR is significantly more reactive (conducive to a high fission rate) at cold zero power than at hot full power, because cold water is a better neutron moderator than steam. 2) Xenon-135 and Samarium-149 are by-products of nuclear fission that have an effect on reactivity similar to boron. Their half-lives are on the order of hours, so when you crank down the fission rate, a couple hours later you also crank down the concentrations of Xe-135 and Sm-149, which, if you're not careful, can cause the reactor to go supercritical (and possibly prompt supercritical - a form of criticality in which everything happens approximately a thousand times faster - pretty much the worst-case reactivity excursion scenario) again a few hours after shutdown. So, basically, you need enough negative reactivity from somewhere, either control rods or borated water, to counteract these two reactivity insertions. The Japanese reactor is almost certainly designed so that inserting all of the control rods into the core will kill any and all reactivity increases after shutdown. What is most likely occurring is that there is very little nuclear fission inside the reactor right now. All the power inside the reactor is coming from decaying fission products, and it's probably on the order of kilowatts, it's just that the when the flow rate through the core drops from gallons per second to essentially nothing, a few kilowatts per cubic foot will get you pretty damn hot pretty damn quick. At this point some of the fuel rods have probably failed as well (if not melted), so the water in the reactor may be nastier than usual. Unless the control rods have failed or are in the process of failing, I doubt that boron is even necessary for reactivity control, except as insurance. My educated guess is that their problems are entirely thermal- and containment-related, and that there is no danger of a reactivity accident, since a xenon transient or a condensation transient would have run its course by now, so the control rods probably have enough reactivity worth to keep the reactor subcritical indefinitely.
closu11 16y ago
yeah that's kind of gay
willgodfrey 16y ago
Just an FYI, the additional reactors going in in South Texas are ABWR's, not BWR's.

rdl 16y ago

Is one of the advantages of the BWR that you don't need a large pressure vessel? I remember news articles about JSW in Osaka being the only source for the huge forgings required for PWR pressure vessels, and only having the capacity for ~4 reactors per year.

neutronicus 16y ago
Not really. BWRs need larger pressure vessels, PWRs need thicker ones. I believe that JSW is the only place (also I think there's one in Germany, too) where you can forge a reactor pressure vessel in one piece, but that if you're willing to bolt two pieces together there are a few more options. It may be that this is more feasible for a BWR than a PWR, but I actually think that BWR pressure vessels are usually more expensive. Where BWRs really save all the money is that you don't need to buy, maintain and replace pressurizers, steam generators, and a ton of piping and pumps that need to be rated for 2500 PSI, as well as a whole bunch of instrumentation for measuring and controlling boron dilution levels. You'll be paying more for engineering services, since all the calculations will take longer, but I can only imagine it's worth it, since, high-margin as engineering services are, they're chump change compared to how much the reactor costs.

patrickgzill 16y ago

I thought the new designs were classified as ALWRs - advanced light water reactors (light water meaning they use regular water).

neutronicus 16y ago
However they're "classified", some of them (ABWR, ESBWR) work by boiling water inside the core at atmospheric pressure and using the steam to turn a turbine, and some of them (AP1000, EPR) work by heating pressurized (borated) water inside the core, which then transfers heat to other (pure) water at a steam generator, which (pure) steam is then used to turn a turbine.

masklinn 16y ago

> Like they wouldn't have boron near a nuclear reactor. They probably do, the question is whether they can carry it to the site faster than the US navy, and whether the boron tanks have not been compromised.

neutronicus 16y ago
The question is also, and I didn't make it clear enough in my original post: "Wait, they need boron?" Boron is a neutron absorber, and is therefore used to control the fission rate. You add boron when you're afraid of the fission rate getting out of control (e.g. Chernobyl). This is a legitimate concern here for two reasons: 1) A BWR is significantly more reactive (conducive to a high fission rate) at cold zero power than at hot full power, because cold water is a better neutron moderator than steam. 2) Xenon-135 and Samarium-149 are by-products of nuclear fission that have an effect on reactivity similar to boron. Their half-lives are on the order of hours, so when you crank down the fission rate, a couple hours later you also crank down the concentrations of Xe-135 and Sm-149, which, if you're not careful, can cause the reactor to go supercritical (and possibly prompt supercritical - a form of criticality in which everything happens approximately a thousand times faster - pretty much the worst-case reactivity excursion scenario) again a few hours after shutdown. So, basically, you need enough negative reactivity from somewhere, either control rods or borated water, to counteract these two reactivity insertions. The Japanese reactor is almost certainly designed so that inserting all of the control rods into the core will kill any and all reactivity increases after shutdown. What is most likely occurring is that there is very little nuclear fission inside the reactor right now. All the power inside the reactor is coming from decaying fission products, and it's probably on the order of kilowatts, it's just that the when the flow rate through the core drops from gallons per second to essentially nothing, a few kilowatts per cubic foot will get you pretty damn hot pretty damn quick. At this point some of the fuel rods have probably failed as well (if not melted), so the water in the reactor may be nastier than usual. Unless the control rods have failed or are in the process of failing, I doubt that boron is even necessary for reactivity control, except as insurance. My educated guess is that their problems are entirely thermal- and containment-related, and that there is no danger of a reactivity accident, since a xenon transient or a condensation transient would have run its course by now, so the control rods probably have enough reactivity worth to keep the reactor subcritical indefinitely.

closu11 16y ago

yeah that's kind of gay

willgodfrey 16y ago

Just an FYI, the additional reactors going in in South Texas are ABWR's, not BWR's.