5 ms·

It's not quite like that. > An atomic bomb is a prompt fast supercritical system. Yes, it is, but more precisely it is a fast hypercritical system. The diffe

It's not quite like that. > An atomic bomb is a prompt fast supercritical system. Yes, it is, but more precisely it is a fast hypercritical system. The difference is this: in a supercritical system the ratio of neutrons in a new generation vs the old generation (generally denoted by k) is higher than one. For example 1.001 qualifies as supercritical. In a hypercritical configuration, k is higher than 2. In nature things generally evolve in a continuous way. To get to 2 you need to first get to 1 and 1.001, for example. For a bomb, it is very important to get to 2 extremely quickly, so that the assembly does not start the chain reaction while k is 1.001, or 1.1. To do that, you use either a gun or high explosives, to quickly reassemble a subcritical configuration into a hypercritical one, before a stray neutron has a chance to start a barely supercritical reaction (also known as a "fizzle"). With a reactor, there's no way to get an atomic bomb effect, because you go through the point of k=1.001. When you get there, there are plenty of neutrons around that this results in a "fizzle". Heat is produced and the core dilates and the ratio becomes low again. This results in "prompt excursions", each one lasting less than one second, each one some sort of a "bomb fizzle". These excursions are annoying, but they are not Hiroshima. > I will not be surprised if their approach ends up being unlicensable in the US because safety cannot be assured by analysis under accident conditions. What you are saying is a Catch-22. NRC can't approve a new design because it doesn't know how it performs under accident conditions, but then you can't know how something performs under accident conditions if you don't build it, and you don't build it if NRC does not approve it. The fact is, Russia has been operating the BN-800 sodium-cooled fast reactor for 10 years [1], and the smaller BN-600 for 45 years. So sodium-cooled fast reactors can work, and Russia likes them enough that it plans to build a bigger version, BN-1200. They don't explode. You could say "not yet", and sure thing, this means that the NRC needs to work hard to cover all the bases. But this is not impossible. [1] https://en.wikipedia.org/wiki/BN-800_reactor https://en.wikipedia.org/wiki/BN-800_reactor

deleted 2y ago
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pfdietz 2y ago
You are assuming that as the core starts to expand, it doesn't expand in a way that increases k (for example, suppose liquid sodium is violently expelled from collapsing coolant channels; this could increase k). Can this be assured? Because if not, the yield could become very large. Granted, this doesn't seem likely, but can you assure it's impossible, in any possible accident, and any possible geometry of fuel and voids? > What you are saying is a Catch-22. NRC can't approve a new design because it doesn't know how it performs under accident conditions, but then you can't know how something performs under accident conditions if you don't build it, and you don't build it if NRC does not approve it. Even if you build it, and even if there is an accident, you don't know how it behaves under accident conditions. You just know how it behaved in that particular accident. It might be easier for developers if we could build thousands a fast reactors and let some of them explode to get some statistics on what works, but that's not how the NRC is arranged, and even that would not ensure all the catastrophic risk had been retired.
- credit_guy 2y ago
  Here's some of the software Terrapower wrote in order to model the neutron economy of the Natrium reactor [1]. Such software has been written ever since the early days of the atomic era. Nuclear engineers know how k behaves, they don't need to make assumptions. Here's a 101 on reactivity feedback effects by the NRC [2]. The US has built all sorts of nuclear reactors, sodium-cooled included. Even one that was sodium cooled and used molten (hot) plutonium as fuel [3]. The experience with prompt criticality excursions is (for the better or worse) extensive [4]. The NRC absolutely has the ability (and duty) to analyze all possible accident conditions for this reactor. This is the page it maintains for Natrium [5]. They have already received and reviewed thousands of documents for it. We would not be at the point that they allow the building to start if they think there's a slight chance the reactor would be "unlicenseable". [1] https://terrapower.github.io/armi/ https://terrapower.github.io/armi/ [2] https://www.nrc.gov/docs/ml1214/ml12142a130.pdf https://www.nrc.gov/docs/ml1214/ml12142a130.pdf [3] https://www.osti.gov/biblio/4206527 https://www.osti.gov/biblio/4206527 [4] https://en.wikipedia.org/wiki/Prompt_criticality#List_of_accidental_prompt_critical_excursions https://en.wikipedia.org/wiki/Prompt_criticality#List_of_acc... [5] https://www.nrc.gov/reactors/new-reactors/advanced/who-were-working-with/licensing-activities/pre-application-activities/natrium.html https://www.nrc.gov/reactors/new-reactors/advanced/who-were-...

deleted 2y ago

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pfdietz 2y ago

You are assuming that as the core starts to expand, it doesn't expand in a way that increases k (for example, suppose liquid sodium is violently expelled from collapsing coolant channels; this could increase k). Can this be assured? Because if not, the yield could become very large. Granted, this doesn't seem likely, but can you assure it's impossible, in any possible accident, and any possible geometry of fuel and voids? > What you are saying is a Catch-22. NRC can't approve a new design because it doesn't know how it performs under accident conditions, but then you can't know how something performs under accident conditions if you don't build it, and you don't build it if NRC does not approve it. Even if you build it, and even if there is an accident, you don't know how it behaves under accident conditions. You just know how it behaved in that particular accident. It might be easier for developers if we could build thousands a fast reactors and let some of them explode to get some statistics on what works, but that's not how the NRC is arranged, and even that would not ensure all the catastrophic risk had been retired.

credit_guy 2y ago
Here's some of the software Terrapower wrote in order to model the neutron economy of the Natrium reactor [1]. Such software has been written ever since the early days of the atomic era. Nuclear engineers know how k behaves, they don't need to make assumptions. Here's a 101 on reactivity feedback effects by the NRC [2]. The US has built all sorts of nuclear reactors, sodium-cooled included. Even one that was sodium cooled and used molten (hot) plutonium as fuel [3]. The experience with prompt criticality excursions is (for the better or worse) extensive [4]. The NRC absolutely has the ability (and duty) to analyze all possible accident conditions for this reactor. This is the page it maintains for Natrium [5]. They have already received and reviewed thousands of documents for it. We would not be at the point that they allow the building to start if they think there's a slight chance the reactor would be "unlicenseable". [1] https://terrapower.github.io/armi/ https://terrapower.github.io/armi/ [2] https://www.nrc.gov/docs/ml1214/ml12142a130.pdf https://www.nrc.gov/docs/ml1214/ml12142a130.pdf [3] https://www.osti.gov/biblio/4206527 https://www.osti.gov/biblio/4206527 [4] https://en.wikipedia.org/wiki/Prompt_criticality#List_of_accidental_prompt_critical_excursions https://en.wikipedia.org/wiki/Prompt_criticality#List_of_acc... [5] https://www.nrc.gov/reactors/new-reactors/advanced/who-were-working-with/licensing-activities/pre-application-activities/natrium.html https://www.nrc.gov/reactors/new-reactors/advanced/who-were-...