Will SpaceX ever go nuclear propulsion?

[JasonAW3]

Jason: Actually, you're wrong. Thin film PV, in combo with either regen fuel cells or (more relevant nowadays with recent advances) state of the art Lithium Ion or Lithium sulfur (both of which are better than older regen fuel cells) beats nuclear power pound for pound and volume stowage wise for surface power on Mars. See this paper: http://systemarchitect.mit.edu/docs/cooper10.pdf

And in-space, PV trounces nuclear (ie how much power for a given mass) until you get past the asteroid belt. It's not even fair, solar is like 5-10x more powerful (if you compare existing or historical in-space nuclear to existing solar, OR credible new developments for nuclear compared with credible new developments for solar). That's why no one has nuclear powered satellites anymore.

The problem with PV on Mars is that your PV panels build up an electric charge from both use as well as dust storms, after a while, simply brushing off the panels doensn't work so well because the dust is now electrostatically stuck on the PV cell faces.  Plus, the fine dust would start to cloud the surface of the PV cells just from simple abrasion.  This is a small part of why the Mars rovers using PV cells are slowly but surely becoming unable to generate power.

    MIND YOU, this has not happened NEARLY as fast as anyone at NASA expected, thus the decade plus mission on a rover that was supposed to only last 90 days.  But there has, over the years, been a noticable and steady drop off of power that the cells can generate.

    PV in space, becomes, essentually, a much larger target than a Nuclear Power plant.  We've recently seen evidence of this from impacts on the ISS PV arrays, which has been hit before.  PV cell circuit interconnectivity has been a major plus here, allowing the system to continue to function with only minute power loss.

     Out at Mars, the amount of light received at noon is about 40% of what it is on Earth, which means a significant drop of of power generation.  Plus, there is the Asteroid Belt to contend with.  While not actually in the Belt, Mars is FAR closer to it and as such is far more prone to meteor impacts than Earth, ESPECIALLY with an atmospheric pressure that is about 1% of that of Earth's.  Should you be able to extend the PV array's photoreceptivity deeper into the UV side as well as visible light, you could generate more power, but still not as much as on Earth.

As an augment to nuclear power, PV arrays are a great idea, but they too, have their technical issues to work out for the most efficent use on Mars.

[guckyfan]

PV cells do lose capacity over time. True. But the rovers are still in excellent shape, the drop was small. And not due to sticky dust.

Micrometeorites are not an issue on Mars unlike space. The atmosphere will absorb them. Bigger meteorites will probably be more frequent than on earth but still rare. They may take out a single panel out of many thousands every few years. Early science fiction where every ship in space was hit by meteorites have fortunately proven wrong. Not few probes have even passed through the "thick" of the asteroid belt and not been hit by anything that caused problems. They did get the occasional micrometeorite hit which would not happen on Mars with its atmosphere.

True about the 40% at noon. But even thick dust storms are nowhere near in density compared to clouds on earth. They scatter light more than they absorb. Much of it is still available for PV cells. Few places on earth receive much more light than Mars  near equator on average over a year. None that are inhabited. Even a thin transparent cloud cover will absorb so much that the available light will drop below Mars levels. If you have ever used a good camera you know that.

But true that power production will peak at noon and be much lower in the morning and in the evening. Good efficient batteries will be needed. Fortunately batteries get better fast.

In a later stage of development large nuclear facilities may be useful. But they have the problem of cooling. Nuclear plants on earth use huge amounts of water for cooling and sometimes need to scale back during summer when water becomes scarce. Cooling will be possible but drive up cost a lot.

[Robotbeat]

Jason: Actually, you're wrong. Thin film PV, in combo with either regen fuel cells or (more relevant nowadays with recent advances) state of the art Lithium Ion or Lithium sulfur (both of which are better than older regen fuel cells) beats nuclear power pound for pound and volume stowage wise for surface power on Mars. See this paper: http://systemarchitect.mit.edu/docs/cooper10.pdf

And in-space, PV trounces nuclear (ie how much power for a given mass) until you get past the asteroid belt. It's not even fair, solar is like 5-10x more powerful (if you compare existing or historical in-space nuclear to existing solar, OR credible new developments for nuclear compared with credible new developments for solar). That's why no one has nuclear powered satellites anymore.

The problem with PV on Mars is that your PV panels build up an electric charge from both use as well as dust storms, after a while, simply brushing off the panels doensn't work so well because the dust is now electrostatically stuck on the PV cell faces.  Plus, the fine dust would start to cloud the surface of the PV cells just from simple abrasion.  This is a small part of why the Mars rovers using PV cells are slowly but surely becoming unable to generate power.

    MIND YOU, this has not happened NEARLY as fast as anyone at NASA expected, thus the decade plus mission on a rover that was supposed to only last 90 days.  But there has, over the years, been a noticable and steady drop off of power that the cells can generate.

    PV in space, becomes, essentually, a much larger target than a Nuclear Power plant.  We've recently seen evidence of this from impacts on the ISS PV arrays, which has been hit before.  PV cell circuit interconnectivity has been a major plus here, allowing the system to continue to function with only minute power loss.

     Out at Mars, the amount of light received at noon is about 40% of what it is on Earth, which means a significant drop of of power generation.  Plus, there is the Asteroid Belt to contend with.  While not actually in the Belt, Mars is FAR closer to it and as such is far more prone to meteor impacts than Earth, ESPECIALLY with an atmospheric pressure that is about 1% of that of Earth's.  Should you be able to extend the PV array's photoreceptivity deeper into the UV side as well as visible light, you could generate more power, but still not as much as on Earth.

As an augment to nuclear power, PV arrays are a great idea, but they too, have their technical issues to work out for the most efficent use on Mars.

That is all taken into account, and PV still comes out ahead by a large margin.

[Vultur]

Jason: Actually, you're wrong. Thin film PV, in combo with either regen fuel cells or (more relevant nowadays with recent advances) state of the art Lithium Ion or Lithium sulfur (both of which are better than older regen fuel cells) beats nuclear power pound for pound and volume stowage wise for surface power on Mars. See this paper: http://systemarchitect.mit.edu/docs/cooper10.pdf

And in-space, PV trounces nuclear (ie how much power for a given mass) until you get past the asteroid belt. It's not even fair, solar is like 5-10x more powerful (if you compare existing or historical in-space nuclear to existing solar, OR credible new developments for nuclear compared with credible new developments for solar). That's why no one has nuclear powered satellites anymore.

The problem with PV on Mars is that your PV panels build up an electric charge from both use as well as dust storms, after a while, simply brushing off the panels doensn't work so well because the dust is now electrostatically stuck on the PV cell faces.  Plus, the fine dust would start to cloud the surface of the PV cells just from simple abrasion.  This is a small part of why the Mars rovers using PV cells are slowly but surely becoming unable to generate power.

    MIND YOU, this has not happened NEARLY as fast as anyone at NASA expected, thus the decade plus mission on a rover that was supposed to only last 90 days.  But there has, over the years, been a noticable and steady drop off of power that the cells can generate.

Maybe to some degree, but IIRC a cleaning event earlier this year got Opportunity up to 94% of its original capacity. So most of the loss does seem to be easily removable dust.

(And Opportunity apparently tends to get cleaning events on hillsides, so static panels placed on hills would probably do better than Opportunity has, since they would be on the hill all the time.)

[macpacheco]

There is NOTHING great about water cooled reactors except for the fact they are operational and commercially available. The reason we have them is cause the big old US Navy paid for the development of the first one, for the simple reason that the Navy was used with dealing with high pressure water and high pressure steam handling. It was always seen as a kludge to get the world started on nuclear power. Even in the 50s the concept of fast sodium reactors and molten salt reactors were already worked on in research labs.
Even fast sodium reactors are much safer than light water reactors as far as risk of a serious accident, due to the same fundamental reason molten salt reactors are even safer: Low pressure operation (near ambient pressure).
Water cooled reactors are enormous ultra high pressure cookers (75-150 atmospheres), while salt/metal cooled reactors operate at an average pressure of one atmosphere.
This wasn't the first time that the military conservativeness did us a sizeable diservice picking inferior technology.

[Jim]

This wasn't the first time that the military conservativeness did us a sizeable diservice picking inferior technology.

Unsubstaniated

[llanitedave]

There is NOTHING great about water cooled reactors except for the fact they are operational and commercially available. The reason we have them is cause the big old US Navy paid for the development of the first one, for the simple reason that the Navy was used with dealing with high pressure water and high pressure steam handling. It was always seen as a kludge to get the world started on nuclear power. Even in the 50s the concept of fast sodium reactors and molten salt reactors were already worked on in research labs.
Even fast sodium reactors are much safer than light water reactors as far as risk of a serious accident, due to the same fundamental reason molten salt reactors are even safer: Low pressure operation (near ambient pressure).
Water cooled reactors are enormous ultra high pressure cookers (75-150 atmospheres), while salt/metal cooled reactors operate at an average pressure of one atmosphere.
This wasn't the first time that the military conservativeness did us a sizeable diservice picking inferior technology.

The U.S. Navy also developed a molten salt reactor for the U.S.S. Seawolf, SSN 575.  It was soon replaced with a standard light water reactor.  Why?

"Sodium also has a small fission capture cross section which formed ³H as a free gas in the primary system. This complicated system operation since ³H is radioactive, would mix with the H₂ from Na-H₂O reaction and had to be contained."

Also, according to Hyman Rickover, liquid sodium reactors were "expensive to build, complex to operate, susceptible to prolong shutdown as a result of even minor malfunctions, and difficult and time-consuming to repair."

Now, maybe the Navy didn't allow enough time to perfect the design, but it shows that they were never the panacea you seem to think of them as.

http://en.wikipedia.org/wiki/USS_Seawolf_%28SSN-575%29

[Robotbeat]

Don't have to use sodium in a molten salt reactor.

[Nilof]

Liquid metal reactors =/= molten salt reactors. Both are nuclear reactors and operate at near ambient pressure but that's as far as the similarity goes. Pure sodium as a coolant in a submarine is indeed a PITA. At these high temperatures it will spontaneously ignite in contact with air and explode in contact with water.

Either way, for space only high temperature reactors are really viable. In-space nuclear reactors are to a large extent limited by radiator mass. The specific power of the reactor system scales with the fourth power of the operating temperature, which is why nobody has ever made a lightwater reactor for use in space.

[macpacheco]

Seawolf was a fast sodium reactor.
The Russians instead did a lead cooled fast reactor which gave the sub tremendous power with a very light reactor. It can outrun its contemporary Los Angeles Class Sub (at least in unclassified reviews).
Sodium reacts with water and oxygen. With water it produces H2 very quickly and exothermally, leading to spontaneous combustion (or explosion). With Oxygen it oxidizes very quickly (releasing lots of heat, but not quite a fire).
Lead doesn't react with water or oxygen in such extreme ways, far more manageable.
But a Lead cooled reactor essentially must never be shutdown, if the coolant fully solidifies it can destroy the reactor.
A molten salt reactor have none of those issues.
The classical molten salt coolant (F Li Be) does solidify bellow 350C but it doesn't expand like lead. And salts are at rock bottom of chemical stability, almost as stable as a noble gas, but far denser.
Optimum in every way:
 Higher density = more compact
 Higher thermal conductivity = more compact
 The demonstration MSR experiment at ORNL was shutdown almost every weekend. The coolant+fuel mixture solidified in the drain tank, which was re-heated every monday morning and pumped back into the main loop without issue.
 Must be heated to extreme temps to become a gas (1400C+) = high temperature margins
 Most sodium reactors use solid fuels, most MSR use molten fuel mixed with the coolant = impossible to have loss of coolant accident, as the fuel and the coolant is kept as a single mixture, without the presence of the moderator and specifically designed geometries, criticality is prompting terminated.
 Typically runs in the thermal spectrum, but fast MSR reactors have been proposed for its ability to burn spent nuclear fuel with just a simple pyro reprocessing (remove fission products, keep all fissile and fertile nuclear material).

[ArbitraryConstant]

most MSR use molten fuel mixed with the coolant = impossible to have loss of coolant accident

Again, seems to me this doesn't make loss of coolant impossible, it means escaped coolant is murderously radioactive and self-heats. And, again, safety systems can hope to prevent this, but boasting of certainty simply tells us is that you either aren't being candid about the real risk or have no idea what it is.

[sheltonjr]

Why would the leak material self heat?  Earth and Mars reactors would drain the leaked material to a tank designed to passively remove decay heat.

A in space reactor would have an outer shell that would contain and transfer the decay heat to space

A LWR is designed to hold heat while operating which is a problem after it is shut down. The solid fuel cannot be moved to a less insulating tank.  This another safety advantage of the MSR

[ArbitraryConstant]

Why would the leak material self heat?

Because it contains isotopes with half lives short enough that the heat is significant.

Earth and Mars reactors would drain the leaked material to a tank designed to passively remove decay heat.

I agree that's plan 'A', but the claim I'm responding to is that it's impossible that this will fail.

[Robotbeat]

Liquid sodium, no, it's molten salt. VERY different. Nobody on this thread proposed using molten sodium.

[macpacheco]

most MSR use molten fuel mixed with the coolant = impossible to have loss of coolant accident

Again, seems to me this doesn't make loss of coolant impossible, it means escaped coolant is murderously radioactive and self-heats. And, again, safety systems can hope to prevent this, but boasting of certainty simply tells us is that you either aren't being candid about the real risk or have no idea what it is.

Actually, F Li Be salts even with the fuel with maximum fission product concentration self plug small leaks. Decay heat is insufficient to maintain the core material molten without the moderator and proper fuel geometry to maintain criticality.
Plus MSR traditional design has a catch pan that drains into the drain tank, so that any core leaks go into the drain tank. Since the core fluid is at atmospheric pressure there is no pressure trying to squirt that stuff sideways.
BTW the reactor isn't designed to actually leak, its strictly a safety feature.
There is also the freeze plug that melts in case of reactor overheat and can be manually molten by stopping the blower, so in case the reactor looses electrical power it shuts itself down, no computers or human intervention required.
Any terrestrial and marine reactor must have a secondary containment that surrounds all of that, so that murderously radioactive material is still inside containment even in all of those scenarios, and since there is nothing inside the secondary containment to produce anything flammable or high pressure, the secondary containment can be much smaller (hence much cheaper) and can be thinner while increasing safety margins, plus MSR reactors are so compact they can be easily installed below ground level, with lateral and bottom containment calculated per needed margins, but top containment as thick as water cooled reactors to protect from 9/11 and other similar scenarios. Water cooled reactors tend to require very tall secondary containment to fit all steam leak scenarios making it unattractive for underground installation.
May I suggest:

[Robotbeat]

...okay, now imagine getting all that kind of safety in any kind of flight-weight system. This is why SpaceX will never go with nuclear propulsion. That weight is not a big deal on Earth, very big deal in space.

[ArbitraryConstant]

May I suggest:

Seen it, irrelevant to my point. All the reasons you give are reasons to suppose risk will be lower with MSR than LWR, which is another way of saying that none of your reasons even attempt to show that leak is impossible. Hence the boast of impossible is easily identified as exaggeration.

And you're starting from an advantaged position with me since I have a basic familiarity with nuclear technology, a nuanced discussion with me is at least possible. Mostly people just dismiss such boasts out of hand.

[Nilof]

Because it contains isotopes with half lives short enough that the heat is significant.

If you mean the fission products, some LFTR reactor designs have a filtering mechanism where these are removed from the reactor. This is one MAJOR advantage of liquid fuel, since this is impossible in solid-fueled reactors as the products are trapped inside the reactor and can cause neutron poisoning. This allows a much higher burnup rate for the fuel. If you mean neutron activation, that is negligible for FLiBe as long as you have purged your lithium of Li-6.

It also means that in the case of an actual accident/containment vessel breach, there is no built-up radioactive material that can escape. That risk has been transferred from the working reactor to the reprocessing facillity.

This is somewhat off topic though since for an in-space reactor you're more or less equally screwed either way in case of an accident, regardless of what design you choose. The reactor being critical to the mission is usually more important than whatever failure mode it may have.

[ArbitraryConstant]

If you mean the fission products, some LFTR reactor designs have a filtering mechanism where these are removed from the reactor. This is one MAJOR advantage of liquid fuel, since this is impossible in solid-fueled reactors as the products are trapped inside the reactor and can cause neutron poisoning.

Nope, this incorrectly suggests fission products generally are filtered out. Xenon-135 is removed as a neutron poison, and I suppose there's other gasses that go with it, but most of them stay in the salt and are quite capable of causing significant decay heat after shutdown.

It also means that in the case of an actual accident/containment vessel breach, there is no built-up radioactive material that can escape.

Unequivocally false.

[Darkseraph]

The costs will outweigh the benefits for a company like spacex. If one of those reentered the atmosphere, even if there were no direct casualties, they'd be finished as a business. I think segregating cargo from crew delivery, and using different forms of electrical propulsion for just the cargo would have the most benefit for both Mars, Asteroids and the Moon.