3 ms·

> The paper mentions that they used the software MCNP to estimate this factor, but it does not mention how they treated the change of phase. When the projectil

by jjk166 3y ago

> The paper mentions that they used the software MCNP to estimate this factor, but it does not mention how they treated the change of phase. When the projectile reaches the temperature they describe (11600 K), the mean velocity of the hydrogen ions is about 16 times higher than the mean velocity of the uranium atoms, so basically all the hydrogen is gone. The amount of moderator just went from a certain number to zero. The hydrogen having a higher mean velocity doesn't mean it goes anywhere. When the uranium and moderator are initially vaporized the iron shell is still intact (there isn't enough time for the heat to conduct to it nor for the protons from the ionized hydrogen to escape) so everything is still contained within the projectile at that point. There are control drums that act both to moderate neutrons while the projectile is in motion and to keep the barrel subcritical when the projectile is clear of the section. In the paper they report that K_eff peaks right before the uranium is vaporized and then drops off rapidly. > there's no discussion of the neutron economy inside the barrel. The paper models the projectile and barrel as a single system. Power generation in the barrel is approximately 1/25th that in the projectile. > why was this paper published in Acta Astronautica, and not in a nuclear physics of nuclear engineering journal? The application is indeed to rockets, but the meat of the paper is the nuclear fission. The meat of the paper is generating and containing a high speed plasma projectile that just happens to be generated by fission. It's not trying to do anything innovative on the nuclear side beyond throwing a weird (and for non-space applications irrelevant) configuration into standard simulation software.

credit_guy 3y ago
Just to be very clear, are you saying this paper looks good to you? You are not seeing any red flags? Like, for example, the fact that the 7th entry of Table 2 contains 0% Uranium, but still has a K_eff of 1.095. What does that even mean? Or like the following sentence on page 15: > The MCNP6 modeling and calculation of the moderating ability of the fuel projectile that evaluated the uses of Lithium-6, Lithium-7, Boron-10, Boron-11, Carbon-12, and H2O Why would you even consider Boron-10, which is one of the strongest neutron poisons? Maybe it's a mistake, right? Well, 3 of the rows in Table 1 contain Boron-10, and one (the ninth row) is the "champion" at the amount of energy production. A funny champion, considering that again it does not include any Uranium. As for the iron not vaporizing, I suspect you are basing your conclusion on this statement: > Because the projectile heats up so fast, there is no time for conductive heating of the iron. Since all the naturally occurring isotopes of iron have a thermal neutron absorption cross section less than 2.5 barns, direct neutron heating will not dominate. Therefore, it is highly probable that the iron sheath around the bullet will remain a solid far deeper into the barrel. That "highly probably" is a bit vague. What exactly happens when the iron sheath disintegrates? Will it change phase instantaneously in its entirety, or from the inside to the outside. And if it's the second, will shrapnel hit the barrel? If It does, it's kind of a problem, isn't it? > Power generation in the barrel is approximately 1/25th that in the projectile. Does this mean that after firing 25 projectiles (i.e. after 25 seconds) the power generation in the barrel is equal to the power that is sufficient to vaporize the projectile? You could say it's not a biggie, because the barrel is much more massive, but it's finite. What happens after 250 seconds, or 2500 seconds? And more importantly, what happens to the neutron economy in the whole system after a good amount of the uranium in the barrel undergoes fission under this neutron bombardment? Are we going to have Xenon poisoning at some point? I could go on, but you get the idea.

by jjk166 3y ago

credit_guy 3y ago

Just to be very clear, are you saying this paper looks good to you? You are not seeing any red flags? Like, for example, the fact that the 7th entry of Table 2 contains 0% Uranium, but still has a K_eff of 1.095. What does that even mean? Or like the following sentence on page 15: > The MCNP6 modeling and calculation of the moderating ability of the fuel projectile that evaluated the uses of Lithium-6, Lithium-7, Boron-10, Boron-11, Carbon-12, and H2O Why would you even consider Boron-10, which is one of the strongest neutron poisons? Maybe it's a mistake, right? Well, 3 of the rows in Table 1 contain Boron-10, and one (the ninth row) is the "champion" at the amount of energy production. A funny champion, considering that again it does not include any Uranium. As for the iron not vaporizing, I suspect you are basing your conclusion on this statement: > Because the projectile heats up so fast, there is no time for conductive heating of the iron. Since all the naturally occurring isotopes of iron have a thermal neutron absorption cross section less than 2.5 barns, direct neutron heating will not dominate. Therefore, it is highly probable that the iron sheath around the bullet will remain a solid far deeper into the barrel. That "highly probably" is a bit vague. What exactly happens when the iron sheath disintegrates? Will it change phase instantaneously in its entirety, or from the inside to the outside. And if it's the second, will shrapnel hit the barrel? If It does, it's kind of a problem, isn't it? > Power generation in the barrel is approximately 1/25th that in the projectile. Does this mean that after firing 25 projectiles (i.e. after 25 seconds) the power generation in the barrel is equal to the power that is sufficient to vaporize the projectile? You could say it's not a biggie, because the barrel is much more massive, but it's finite. What happens after 250 seconds, or 2500 seconds? And more importantly, what happens to the neutron economy in the whole system after a good amount of the uranium in the barrel undergoes fission under this neutron bombardment? Are we going to have Xenon poisoning at some point? I could go on, but you get the idea.