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It is going to be highly radioactive at some point (when it's in use), so it does need shielding for that point. It needs to be able to survive any kind of cat

It is going to be highly radioactive at some point (when it's in use), so it does need shielding for that point. It needs to be able to survive any kind of catastrophic detonation/break-up/crash landing you can think of, because the alternative is spreading a large amount of radioactive material directly into the atmosphere/onto the Earth's surface. For example, the Challenger orbiter itself was fine, but when the SRB blew up it took up the orbiter with it. So if there'd been any nuclear materials in the orbiter, even if they weren't used at launch, they still would've needed serious shielding. And it doesn't matter if the consequences of said radioactive material release aren't actually that bad in the grand scheme of things (like Fukushima looks not to have been) -- what matters is that the public reaction to such an event would preclude the possibility of ever launching it again.

arethuza 6y ago
I suspect the public should be worried more about the thousands of other rockets sitting around with nuclear materials on top! Edit: Regarding the Challenger accident - the crew compartment survived the explosion and the crew were still alive for a while after :-( https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disaster#Cause_and_time_of_death https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disas...
- CydeWeys 6y ago
  It didn't survive the crash though, which is what you'd need to prevent a radioactive materials release.
  - catalogia 6y ago
    If they're launching it over the ocean (they would be, if they were launching from Kennedy or Vandenberg) they could just say "YOLO" and let the reactor fall into the ocean. It certainly wouldn't be the first time a nuclear reactor was dumped into the ocean (either by accident or deliberately.) It's obviously not a popular thing to do, but ocean water does provide a lot of shielding.. https://en.wikipedia.org/wiki/USS_Thresher_(SSN-593) https://en.wikipedia.org/wiki/USS_Thresher_(SSN-593) https://en.wikipedia.org/wiki/Lenin_(1957_icebreaker)#Nuclear_accidents https://en.wikipedia.org/wiki/Lenin_(1957_icebreaker)#Nuclea...
    - CydeWeys 6y ago
      Doesn't solve the problem of it blowing up on the pad, or near the pad while still over land, or crashing into land. Rockets have gone the wrong way before and been intentionally self-exploded mid-air.
      - catalogia 6y ago
        While true, a reactor blown up over land would also be simpler to clean up; while a reactor scattered over the ocean bed would be easier to leave in place.
        CydeWeys 6y ago
        "Simpler" here is relative. It would probably still cost billions of dollars and would create enough of a PR disaster to permanently forestall an NTR from ever launching again.
      - deleted 6y ago
        [deleted]
    - dwheeler 6y ago
      Water is incredibly good at radiation shielding. An awesome description of this is here: https://what-if.xkcd.com/29/ https://what-if.xkcd.com/29/
      - CydeWeys 6y ago
        Water is not remotely as good as lead is by mass as a radiation shield. If you aren't already hauling many cubic meters of water for other reasons (and you wouldn't be), it absolutely wouldn't make sense to bring it along solely for that purpose. Water also has the severe problem of being liquid at habitable temperatures, so you either have to continuously spend energy to freeze it or deal with absolutely ruinous slosh. The linked article is talking about the radiation shielding properties of what essentially amounts to a large swimming pool's worth of water. You know how much that would weigh?? Water is not a good radiation shield, it's just cheap here on Earth, so we use large quantities of it in applications where weight doesn't matter.
rbanffy 6y ago
> It is going to be highly radioactive at some point (when it's in use), so it does need shielding for that point. We can use two things as shielding - propellant and distance. For the first, we would want to make sure we don't use all the propellant with the reactor running hot, but decrease power output as we run out of propellant. As for distance, we may want to add a foldable structure between the propulsion section and the habitable section that would be extended to its full length prior to starting the nuclear reactor.
- CydeWeys 6y ago
  The long foldable structure would add considerable weight* and complexity (and thus risk), and would also be unsuitable for use as a first or second stage. So, this would maaaaybe be acceptable to handle the Hohmann transfers on a crewed mission to Mars, but not for much else. * It needs to be strong enough to handle the full acceleration of the engines.
  - rbanffy 6y ago
    > It needs to be strong enough to handle the full acceleration of the engines. I don't think we can expect to have high accelerations with NTRs anyway, so I don't think it'd need to be particularly robust. Also, propellant tank walls can be structural elements here too. If a foldable structure isn't practical, we could just assemble it from rigid elements lofted on other launches.
Sharlin 6y ago
The nuclear fuel doesn't really need to be shielded any better during launch than the existing RTGs used on space missions. Scaremongering notwithstanding, those are pretty much impossible for a mere launch accident to "blow to bits" in a way that exposes the PuO, and so would the fuel rods of a proper reactor. At least unless the reactor itself suffers a catastrophic excursion and blows itself to bits à la Chernobyl No. 4, but we've become pretty good at building reactors that don't do that.
- DennisP 6y ago
  And the RTGs use an isotope that provides significant energy just from radioactive decay. By contrast, uranium in a reactor is barely radioactive at all, before the reactor has been turned on.

arethuza 6y ago

I suspect the public should be worried more about the thousands of other rockets sitting around with nuclear materials on top! Edit: Regarding the Challenger accident - the crew compartment survived the explosion and the crew were still alive for a while after :-( https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disaster#Cause_and_time_of_death https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disas...

CydeWeys 6y ago
It didn't survive the crash though, which is what you'd need to prevent a radioactive materials release.
- catalogia 6y ago
  If they're launching it over the ocean (they would be, if they were launching from Kennedy or Vandenberg) they could just say "YOLO" and let the reactor fall into the ocean. It certainly wouldn't be the first time a nuclear reactor was dumped into the ocean (either by accident or deliberately.) It's obviously not a popular thing to do, but ocean water does provide a lot of shielding.. https://en.wikipedia.org/wiki/USS_Thresher_(SSN-593) https://en.wikipedia.org/wiki/USS_Thresher_(SSN-593) https://en.wikipedia.org/wiki/Lenin_(1957_icebreaker)#Nuclear_accidents https://en.wikipedia.org/wiki/Lenin_(1957_icebreaker)#Nuclea...
  - CydeWeys 6y ago
    Doesn't solve the problem of it blowing up on the pad, or near the pad while still over land, or crashing into land. Rockets have gone the wrong way before and been intentionally self-exploded mid-air.
    - catalogia 6y ago
      While true, a reactor blown up over land would also be simpler to clean up; while a reactor scattered over the ocean bed would be easier to leave in place.
      - CydeWeys 6y ago
        "Simpler" here is relative. It would probably still cost billions of dollars and would create enough of a PR disaster to permanently forestall an NTR from ever launching again.
    - deleted 6y ago
      [deleted]
  - dwheeler 6y ago
    Water is incredibly good at radiation shielding. An awesome description of this is here: https://what-if.xkcd.com/29/ https://what-if.xkcd.com/29/
    - CydeWeys 6y ago
      Water is not remotely as good as lead is by mass as a radiation shield. If you aren't already hauling many cubic meters of water for other reasons (and you wouldn't be), it absolutely wouldn't make sense to bring it along solely for that purpose. Water also has the severe problem of being liquid at habitable temperatures, so you either have to continuously spend energy to freeze it or deal with absolutely ruinous slosh. The linked article is talking about the radiation shielding properties of what essentially amounts to a large swimming pool's worth of water. You know how much that would weigh?? Water is not a good radiation shield, it's just cheap here on Earth, so we use large quantities of it in applications where weight doesn't matter.

rbanffy 6y ago

> It is going to be highly radioactive at some point (when it's in use), so it does need shielding for that point. We can use two things as shielding - propellant and distance. For the first, we would want to make sure we don't use all the propellant with the reactor running hot, but decrease power output as we run out of propellant. As for distance, we may want to add a foldable structure between the propulsion section and the habitable section that would be extended to its full length prior to starting the nuclear reactor.

CydeWeys 6y ago
The long foldable structure would add considerable weight* and complexity (and thus risk), and would also be unsuitable for use as a first or second stage. So, this would maaaaybe be acceptable to handle the Hohmann transfers on a crewed mission to Mars, but not for much else. * It needs to be strong enough to handle the full acceleration of the engines.
- rbanffy 6y ago
  > It needs to be strong enough to handle the full acceleration of the engines. I don't think we can expect to have high accelerations with NTRs anyway, so I don't think it'd need to be particularly robust. Also, propellant tank walls can be structural elements here too. If a foldable structure isn't practical, we could just assemble it from rigid elements lofted on other launches.

Sharlin 6y ago

The nuclear fuel doesn't really need to be shielded any better during launch than the existing RTGs used on space missions. Scaremongering notwithstanding, those are pretty much impossible for a mere launch accident to "blow to bits" in a way that exposes the PuO, and so would the fuel rods of a proper reactor. At least unless the reactor itself suffers a catastrophic excursion and blows itself to bits à la Chernobyl No. 4, but we've become pretty good at building reactors that don't do that.

DennisP 6y ago
And the RTGs use an isotope that provides significant energy just from radioactive decay. By contrast, uranium in a reactor is barely radioactive at all, before the reactor has been turned on.