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#126 2016-01-29 03:18:28

kbd512
Member
Registered: 2015-01-02
Posts: 1,187

Re: Nukemobiles on Mars

JoshNH4H wrote:

kbd512-

For the purposes of this discussion, I went and looked up the neutron absorption and scattering coefficients for the four isotopes we've been discussing.  I found them here.  Neutron cross sections as follows (measured in Barns, or 1e-28 square meters):

___Isotope____Scat. XS____Abs. XS___

U-233_________12.9_______575_____
U-235_________14.0_______681_____
Pu-239_________7.7_______1020____
Am-242m_______*__________*_____

*As it turns out, the nuclear cross-sections for Am-242m are not available on this page, which is normally quite exhaustive.  Could you point me to your source for the claim that "Am242m has one the highest cross sections of any common isotope"?  As your entire argument for Americium rests on this claim, and I can't find any evidence for it, our disagreement is moot if it's untrue or unverifiable.

Source 1

Source 2

Source 3

Source 4

Source 5

Pseudo Source 1 - cross sections from 4500b - 6900b have been listed in various sources, with 6000b - 6600b being the most common

I wish there were more varied sources available that were free, but that's what I could easily dig up.

JoshNH4H wrote:

As to the significance of critical mass, I've argued that it doesn't matter at all for the size of the reactor and the total shielding mass.  You argued at first that it would make the core smaller, but are now arguing that a smaller critical mass reduces the required amount of moderator/reflector which would result in mass savings.  I have three responses to that claim.

Most of mass of the core is everything else that is not fissile material.  SAFE-400 uses a heavy Be neutron reflector, for example.  My argument about reducing critical mass was directed at everything required to sustain fission.  If a reactor requires a neutron reflector that doubles the diameter of the core, then the major constraint on core dimensions is the reflector.  I'm pretty sure I mentioned the reflector dimensions in just about every response.

JoshNH4H wrote:

The first is that it probably doesn't matter all that much.  You've given as an example a 400 kg reduction in reactor mass.  If you'll accept my ballpark of $50,000/kg landed on Mars, that's a savings of $20,000,000 over a non-americium reactor.  Especially because you've suggested that the vehicle would be reused, that amount of savings will likely be more than eaten up by the cost of developing Americium reactor technologies and actually producing any significant amount of Americium.

I'll take a 400 kg reduction in reactor mass, but I think the shielding mass savings will amount to more than 400 kg.  Even if it's only 400 kg, that's more food and water or spare parts.

With respect to cost, if my proposal to develop a nuclear reactor is expensive, then your proposal to use internal combustion engines on Mars is surely just as expensive.  Nobody has ever built an Am242m reactor and nobody has ever built a methalox plant and refueling system for use on Mars.

With a nuclear powered tracked rover all you'll likely send to Mars for subsequent missions is replacement tracks, road wheels, and electronics.  If we replaced all of those components on every subsequent mission, you're looking at about a 1.5t payload of replacement parts for the band tracks / sprockets / idlers / road wheels, maybe 2t with packaging, and perhaps 8 hours of the astronauts' time.  For four vehicles, that's 8t of replacement parts.

The M113 and MTVL variants are constructed from 5083 aluminum plate that varies in thickness from 20MM to 44MM.  The vehicle has a 44MM glacis, 38MM sides / rear / roof, 28MM belly).  That's probably a little overboard for a place where no one is likely to shoot at you.  I think our vehicle could get away with using 20MM or even 15MM of plate.  I'm going to go way out on a limb and say there's probably not much need for spall liners, turret rings, top hatches, fuel tanks, or smoke grenade launchers.

JoshNH4H wrote:

My second response is that critical mass (Or critical size) is still probably a better figure of merit than cross-section.  The reason for this is that fission is a result of both free neutrons released per fission (n) and their likelihood of causing another fission event, (p).  For a barely-critical mass:

n•p=1

p is a function of the neutron cross-section, the density and positioning of fissile material, and the probability than an absorbed neutron will cause a fission event.  The critical mass takes all of these into account, while the neutron cross-section doesn't, and that's why i think that the critical mass is a better way at looking at the compactness of a reactor core.

The figures of merit for this application are the volumes and masses of all the components required to construct a properly shielded reactor operated in very close proximity to humans.  The fissile material is a very small percentage of the reactor's mass, so its ability to sustain fission without a voluminous and heavy neutron reflector is far more important than negligible critical mass differences between U235, Pu239, and Am242m.  Reducing the core's volume, to the extent practical, is what's required for a properly shielded reactor mounted in a manned vehicle.

JoshNH4H wrote:

My third response is that, for whatever reason, shielding is typically made from neutron reflectors and not from neutron absorbers.  I can think of a couple possible reasons for it but it's just speculation.  The first thing is that neutron absorbers will tend to become radioactive once they absorb more than one neutron, and by doing this add to the problem of tertiary radiation.  Second is that when a nucleus absorbs a neutron, it normally emits a pretty high energy gamma ray, which is actually harder to shield against than a neutron.  In any case, Hydrogen is a good neutron reflector but not a particularly good neutron absorber.  I don't actually know how big of a problem neutrons vs x-rays and gammas are for shielding (Do you, and can you cite it?), but I do know that you were talking about using lighter elements and that seems to be the only logical reason why.

Read the MSR Final Report for more information about the neutron and gamma radiation you could expect from SAFE-400.

Gamma and neutron radiation are both major problems for this application.  The astronauts are going to spend most of their time in the cargo compartment of the MTVL to put 5M between them and the reactor.  We're going to use ration storage bags for additional shielding.  I would prefer to keep the water tank in the belly of the cargo compartment as both a counter mass to the far forward reactor mounting (over the drive sprockets) and as a way to keep the computers and ECLSS away from water by mounting them high in the cargo hold (ideally all electronics and electrical connections would be completely sealed but if that's not feasible then separation is the next best thing), but it could be moved if the radiation from the reactor requires a water wall between the reactor and crew.

I believe Antius suggested hafnium hydride for neutron shielding, tungsten for gamma shielding, and an active cooling system that sucks in CO2, liquefies it, and uses it as coolant.

JoshNH4H wrote:

In principle, reducing the diameter of the fuel rods will increase the rate of heat transfer, but in practice I'm sure any design already takes this into account and makes the diameter as small as possible.  My guess would be that the constraint on fuel rod diameter is a mechanical one, insofar as fuel rods that are too long and skinny are more likely to break and cause a containment breach.  Strong fuel rods are particularly important in an application (Let's say a land vehicle) that will be subjecting the core to mechanical stresses while it's operating.

There was to be a higher power output variant of SAFE-400 that would use smaller heat pipes and fuel pins.

JoshNH4H wrote:

Neither the production of fissionable amounts of Americium nor the development of an Americium-fueled reactor is impossible.  But it is something new!  It's something that's never been done before in the history of our species.  And that's where the cost comes in.  Good design is expensive, and we need good design.  Think about building a house.  Any able-bodied person could do it, in theory.  But in practice, making a building takes skill, tools, materials, and knowledge of how to use them.  If I left you in a field with all the materials you needed to build a house but no construction experience, it would take time to figure out what you needed to do and how to do it.  I don't know who you are or what your occupation is, but imagine how hard it would be to do it if you had to figure it out from scratch.  That's where the cost comes in.  And when it comes to nuclear technology, progress is typically impeded by a strong desire not to give anyone cancer or worse. Impossible? No. Expensive? Probably.

The same argument can be made for setting up an ISPP plant on Mars or using internal combustion engine vehicles on another planet.

JoshNH4H wrote:

A design study on the matter would probably be pretty cheap and definitely be worth it.  My point wasn't that the idea isn't worth studying, it's that the study hasn't been done and therefore the claims you are making are mostly baseless.

The same could be said of trucking LH2 to another planet tens of millions of miles away so we can use internal combustion engines.  Save the output of the ISPP plant to fuel that massive MDV/MAV that everyone is salivating over.  If possible, fuel or top off the chemical kick stage for TEI.

JoshNH4H wrote:

Wikipedia calls the M113 (after which you say this is based) an "armored personnel carrier".  I suppose that's not a tank because it lacks a turret, but it sure looks like one.

I want to remove approximately half the armor.  It won't look much different when we're done, but it'll weigh significantly less than a stock MTVL and still be far more durable than the wheeled soda can rovers that NASA wants to use.  A little extra mass devoted to extreme durability won't hurt.

JoshNH4H wrote:

Oh, and by the way:

Yesterday, 1/27, you wrote:

Even if the core dimensions of the Pu239 fueled reactor is only 9" x 12", and the core dimensions of the Am242m is 6" x 12", the Pu239 fueled reactor has a core volume that has increased by more than 100%.  The shielding has to envelop the reactor.  It's mass and volume will also increase.  It's easy to see why core dimensions matter so greatly for mobile applications of nuclear power.

But two days before, on 1/25, you wrote:

If I thought using Pu239 as fuel would be politically feasible at all, I'd use readily available Pu239 and call it a day.

Yesterday, 1/27, you wrote:

Why the discrepancy?

The original exercise was to simply determine that mounting a reactor in a vehicle was feasible.  All that's absolutely required for this reactor to function as intended (provide 100kWe, don't overheat, and don't kill or injure the crew) is a shadow shield to protect the crew and CO2 coolant loop to protect the reactor.  Plutonium would likely require a smaller reflector, the core dimensions could be decreased by refining SAFE-400's basic design (use fissile material that doesn't require a reflector as thick as the original UN fuel and transfer more heat).  Shielding requirements weren't likely to vary dramatically if output was the same, and I didn't think the effort and expense of Am242m fuel was worth the problems.  However, when people hear "Plutonium", they generally think "nuclear warhead".

After thinking about it a bit more, I thought it would be really nice if the crew could walk in front of the vehicle without getting BBQ'd by the unshielded core.  Sooner or later, someone would have to decommission the reactor.  The ability to be in close proximity to a decommissioned core for brief periods of time seemed like a worthwhile design criteria so that the astronauts could bury the core.

Last edited by kbd512 (2016-01-29 11:38:18)

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#127 2016-01-29 10:41:30

Antius
Member
From: Cumbria, UK
Registered: 2007-05-22
Posts: 974

Re: Nukemobiles on Mars

By making reasonable extrapolations from the MCNP study that Spacenut kindly provided us, we have effectively shown that a nuclear powered rover based upon the SAFE-400 reactor is feasible, that about 10tonnes of shielding are required to keep operator dose beneath 2rem/year for a 2000hour/year driving exposure at a crew separation distance of 5m.  If total vehicle mass is 20te and the CO2 power conversion system is 50% efficient, the vehicle can achieve a similar power to weight ratio as a Challenger tank, which suggests a practical top speed of 25mph over rough ground and 40mph on flat solid surfaces.

That was based upon the assumption of a LiH/tungsten shadow shield protecting about 30% of core circumference in the direction of the crew and a much thinner shield designed to prevent back-scatter on the remainder of the reactor.

The effects of shielding with other provisions and internal bulkheads were not included within the estimate.  It was noted later on that HfH2 has almost three times the hydrogen density of LiH, about 14 times the mass density and has a very high thermal-neutron cross-section.  We can expect that a HfH2 neutron shield would require only about 1/3rd the thickness of an LiH shield for the same contact dose rate and the tungsten shield surrounding it would be much thinner too.  So it may even be possible to reduce shielding mass from around 10 tonnes to as little as 2 tonnes.

The above estimation is based upon an enriched uranium fuelled SAFE-400.  I would point out that critical radius for the fissile material is not the only consideration in reducing reactor size.  Assuming that power output is intended to remain the same, there may be thermal hydraulic difficulties in cooling a smaller core.  But if reactor diameter can be reduced from 30cm to 20cm, the required shielding mass is reduced by around 20%.  So for a 400KWth reactor using Pu/Am fuel and HfH2 + tungsten shielding, it is possible that total reactor mass could come in beneath 2 tonnes, including shielding.

Bit of a problem with the idea of using liquid CO2 as a working fluid.  Although Martian night temperatures are beneath CO2 triple point, the latent heat of evaporation of CO2 is 590KJ/kg.  This means that quite a lot of heat would need to be dumped at low temperatures in order to liquefy the CO2 and a big radiator would be needed.  Assuming 5000kg of CO2 are gathered over 8 hours of night and radiator temperature is 220K, then 770m2 of radiator would be needed.  The good news is that this could probably be a lightweight roll-up polymer.  Ammonia would make an excellent working fluid as it would be liquid over the required temperature range.

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#128 2016-01-29 20:11:18

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 10,650

Re: Nukemobiles on Mars

Is there a way to estimate the shielding that Mars soils would give if the shielding space was a container for regolith to be put into. It would allow for the distance from the reactor to be increased to accomidate the amount of regolith soil to make the enhanced shield work. This would give a mass savings as well for landing the vehicle and could in a pinch allow for additional cargo to be loaded into the vehicle.

I do agree that ammonia would be the favored working fluid for the radiators to use.

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#129 2016-01-30 16:37:18

kbd512
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Registered: 2015-01-02
Posts: 1,187

Re: Nukemobiles on Mars

SpaceNut,

The samples taken by our Mars rovers puts the average bulk density of the regolith between 1.4g/cm3 and 1.6g/cm3.  A number of samples have been taken over several years, so I presume those numbers are pretty accurate.  You're going to need at least a couple meters of regolith to permit humans to approach within 10m of the reactor.  The gamma attenuation of the regolith will be poor.

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#130 2016-01-31 21:58:40

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 10,650

Re: Nukemobiles on Mars

Thanks for the numbers...

I was thinking of placing the life support recirculating racks next to the wal closest to the reactor and leaving a bit of room to allow access to all the devices with in it. Then next would be the consumable food stock which will be with a quantity of water to place in between the crew and the reactor as well. These added steps will improve the isolation distance and lower thr exposure that the crew would and could receive....

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#131 2016-01-31 22:09:24

JoshNH4H
Mod and Martian
From: New York, NY, USA, Earth, Sol
Registered: 2007-07-15
Posts: 2,193
Website

Re: Nukemobiles on Mars

kbd512-

Re: Sourcing of Americium, thanks for the links.  Objection withdrawn.

Most of mass of the core is everything else that is not fissile material.  SAFE-400 uses a heavy Be neutron reflector, for example.  My argument about reducing critical mass was directed at everything required to sustain fission.  If a reactor requires a neutron reflector that doubles the diameter of the core, then the major constraint on core dimensions is the reflector.  I'm pretty sure I mentioned the reflector dimensions in just about every response.

Sure, neutron reflector mass matters.  My response to your claims are that:

1) The reflector mass is going to be pretty small, and won't matter that much.  Your estimate was 400 kg, which combined with my estimate of $50,000/kg on the surface is $20,000,000.  You proposed reuse of the vehicle, so these savings are non-recurring.

2) Savings on reflector mass will be partially canceled out by an increase in the shielding mass that is necessary.  This is less important if you're using regolith shielding instead of landed-mass shielding.  Your claim is that as the dimensions of the core increase, the size of the shadow shield and therefore its mass also increases.

I thought about this, and I realized that neutron cross-section is still not the figure of merit.  Yes, the most important figures-of-merit are mass and size.  But in terms of predicting how much fissile material you need for any geometry with a certain amount of shielding the best number to look at is the critical mass of the isotope.  If extra neutron reflectors are a big problem, just increase the portion of the fuel rod that is made from fissile material.  You might suffer from a small reduction in operating temperature but it's probably worth it compared to the mass penalty you're claiming the car would suffer.

With respect to cost, if my proposal to develop a nuclear reactor is expensive, then your proposal to use internal combustion engines on Mars is surely just as expensive.  Nobody has ever built an Am242m reactor and nobody has ever built a methalox plant and refueling system for use on Mars.

Here's what you're proposing:

-New infrastructure to create and separate many kilograms of Am-242m using breeder reactors and presumably centrifuges
-A reactor for use in space (The SAFE-400 wasn't a reactor)
-A nuclear electric car

These are big-ticket development items, each very challenging in its own way and requiring know-how that we'll have to develop.

Here's what I'm (somewhat rhetorically) countering with:

-Solar arrays and power storage for Mars
-Sabatier reactors and electrolysis units (Possibly but not necessarily a chemical reactor for producing methanol)
-A car utilizing an internal combustion engine that can run on methane or methanol, possibly with some CO2 looped back from the exhaust and probably with radiators to condense the water out of the CO2 exhaust*

What I hope you notice is that the nuclear electric car requires a few big-ticket new technologies that don't exist now, while the solar/chemical car requires new systems using existing technologies.  A Martian solar farm takes some development, but the panels already exist and batteries do too.  (Or flywheels!)  I'm sure industrial sabatier reactors are much bigger than the one we would use, but there's nothing impossible about building one.  Zubrin notes in The Case for Mars that he built one himself.  The car would take some design, of course, but we have quite a large automotive industry in this country and could adapt existing vehicles, or at least existing components.

I'm sure you agree that not all new technology development projects cost the same amount.  The mission will probably use laser communications, but that won't break the bank.  Nor will a new spacesuit better suited for heavy usage on a planetary surface.  We spent billions of dollars developing the SP-100; How much does it cost for Ford to implement some small changes to their engines?

The question of Americium vs. Plutonium, however, is mostly irrelevant.  Really the most important thing about this to me is that a nuclear powered car is absolutely crazy.

*There are plenty of reasons for this.  The water can be recycled a drinking water, used to reconstitute food, and used as shielding.  It's probably bad to add too much water vapor to the intake as it could condense into ice when it's mixed with the oxygen.  It's also likely that the water would freeze out as soon as it was vented, and this could block up the vent or whatever it's near.


-Josh

If you try to talk to me about cold fusion or propellantless drives I will ignore you.
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#132 2016-02-01 01:03:52

kbd512
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Registered: 2015-01-02
Posts: 1,187

Re: Nukemobiles on Mars

JoshNH4H wrote:

Sure, neutron reflector mass matters.  My response to your claims are that:

SAFE-400 component masses:

Core contains 146 kg worth of UN fuel pellets and fuel rod cladding, so 366kg must be the mass of the reflector, control drums, and drum operating controls.

Each Brayton cycle generator weighs 72 kg, so 144 kg for both generators to produce 100kWe.

I put radiator mass at 270 kg using the suggested carbon-carbon panels, which includes the heat pipes, but no mass figure was provided since the radiator solution was not developed.  This could be decreased by using a more efficient material, but Antius and I think an active cooling solution is required.  Perhaps the primary loop will still use a liquid metal due to the thermal hydraulic issues Antius noted with using smaller heat pipes, but I would like to use a secondary open loop system that uses Martian CO2.

JoshNH4H wrote:

1) The reflector mass is going to be pretty small, and won't matter that much.  Your estimate was 400 kg, which combined with my estimate of $50,000/kg on the surface is $20,000,000.  You proposed reuse of the vehicle, so these savings are non-recurring.

Any significant increase in core diameter substantially increases the mass of the shielding required.  It's not the mass of the reflector I'm primarily interested in minimizing, although for this application I'd like to minimize that too, it's the mass of the shielding.

Am241 can be obtained for $1500 per gram.  If Am242m can be obtained for a similar price, that's 13.3 kg of fissile material for $20M.  Many tons of Am241 are put into casks every single year.  Why not separate it from the spent fuel and irradiate it using the breeder reactor in France to produce Am242m?

JoshNH4H wrote:

2) Savings on reflector mass will be partially canceled out by an increase in the shielding mass that is necessary.  This is less important if you're using regolith shielding instead of landed-mass shielding.  Your claim is that as the dimensions of the core increase, the size of the shadow shield and therefore its mass also increases.

Beryllium primarily scatters neutrons.  All materials absorb neutrons to one degree or another, but Beryllium's mass is at least an order of magnitude higher and it does not efficiently absorb neutrons as a result.  The reflector is not a neutron mirror.  The neutrons are flung in all directions, including outward, which is something we don't want.  The Be reflector doesn't substantially reduce neutron shielding requirements.  It reduces neutron energy by roughly 10% per incident collision or thereabouts and has a negligible effect on gamma shielding.  Beryllium metal is neither a high-Z nor low-Z material.  It's used primarily for its neutron scattering properties.

A hydrogen rich material works best for neutron absorption, masses between hydrogen atoms and neutrons being roughly equal.

JoshNH4H wrote:

I thought about this, and I realized that neutron cross-section is still not the figure of merit.  Yes, the most important figures-of-merit are mass and size.  But in terms of predicting how much fissile material you need for any geometry with a certain amount of shielding the best number to look at is the critical mass of the isotope.  If extra neutron reflectors are a big problem, just increase the portion of the fuel rod that is made from fissile material.  You might suffer from a small reduction in operating temperature but it's probably worth it compared to the mass penalty you're claiming the car would suffer.

Regarding your figure-of-merit, critical mass is only of paramount importance if you're trying to make a bomb.  Think about what you're asking for.  SAFE-400 already uses 97% enriched UN fuel.  Do you want to remove the fuel rod cladding, use pure HEU, or something else (optimize the fuel rod geometry, optimize the reflector geometry, and/or optimize the reflector material)?

For this specific application, where both mass and volume have a dramatic effect on the practicality of the solution, there's a limit to what will work without substantially modifying the vehicle or changing operations in a way that adversely affects the mass of the complete solution.

That would be the reason why using another vehicle or trailer, which still requires a properly shielded reactor for all practical purposes since humans will live and work in close proximity for many months at a time, is not a solution that reduces the overall mass of the complete portable power solution.

If we're going to make this nuclear reactor so large and heavy that another vehicle is required, I'd rather just find a way to deal with the operational problems that solar power and batteries represent.

I posted a link to a new lithium ion battery technology that may make energy storage problems with existing lithium ion batteries a moot point:

New Lithium Ion Battery Technology

If we can do the same thing with solar panels, I'd happily take the advanced battery and solar panel technology.  I'm not fixated on using a specific technology.  I'm fixated on never running out of power and using power technology that's practical for this extreme application.

JoshNH4H wrote:

Here's what you're proposing:

-New infrastructure to create and separate many kilograms of Am-242m using breeder reactors and presumably centrifuges
-A reactor for use in space (The SAFE-400 wasn't a reactor)
-A nuclear electric car

Yep.  That's the plan.

JoshNH4H wrote:

These are big-ticket development items, each very challenging in its own way and requiring know-how that we'll have to develop.

There seems to be some sort of recurring notion that nuclear reactors must necessarily be insanely expensive.  The fissile materials are indeed expensive, but micro reactors aren't exactly the pinnacle of reactor design except in volume/mass efficiency.

JoshNH4H wrote:

Here's what I'm (somewhat rhetorically) countering with:

-Solar arrays and power storage for Mars
-Sabatier reactors and electrolysis units (Possibly but not necessarily a chemical reactor for producing methanol)
-A car utilizing an internal combustion engine that can run on methane or methanol, possibly with some CO2 looped back from the exhaust and probably with radiators to condense the water out of the CO2 exhaust*

Let's just move Mars 50% closer to the Sun than it is now.  That oughta fix that distance problem.

How much would this proposed Sabatier reactor and electrolysis unit weigh?

How long would it take to manufacture enough fuel to operate the rovers?

Where will the power to run the Sabatier reactor come from?

How are you going to fuel the rover?  The oxidizer and fuel are both cryogens that require some form of active cooling, even on Mars.

Why not save the rocket fuel for the MAV's?  The more fuel we have for the MAV's, the elaborate the MAV's can become.

JoshNH4H wrote:

What I hope you notice is that the nuclear electric car requires a few big-ticket new technologies that don't exist now, while the solar/chemical car requires new systems using existing technologies.  A Martian solar farm takes some development, but the panels already exist and batteries do too.  (Or flywheels!)  I'm sure industrial sabatier reactors are much bigger than the one we would use, but there's nothing impossible about building one.  Zubrin notes in The Case for Mars that he built one himself.  The car would take some design, of course, but we have quite a large automotive industry in this country and could adapt existing vehicles, or at least existing components.

The existing solar and battery technology isn't good enough, but I hope that will change in the near future.  Some things won't change, though.  The energy density of chemical reactions pale in comparison to nuclear reactions.

JoshNH4H wrote:

I'm sure you agree that not all new technology development projects cost the same amount.  The mission will probably use laser communications, but that won't break the bank.  Nor will a new spacesuit better suited for heavy usage on a planetary surface.  We spent billions of dollars developing the SP-100; How much does it cost for Ford to implement some small changes to their engines?

The first statement is true.  The second is an open question, but has good evidence of feasibility backing it.  The third is a requirement for all practical purposes.  The fourth is an attempt to compare apples and oranges.  SP-100 and SAFE-400 are not comparable.  The fifth should take proper account of the fact that no Ford engine burns LOX and LCH4 made from the atmosphere of another planet using LH2 sent from Earth.

JoshNH4H wrote:

The question of Americium vs. Plutonium, however, is mostly irrelevant.  Really the most important thing about this to me is that a nuclear powered car is absolutely crazy.

Am242m vs Pu239 vs U235 is entirely relevant to development of a practical portable nuclear fission reactor.  Portable nuclear power is not crazy.  People losing their minds over nuclear power is crazy.  We're using it on another planet and you're coming up with every possible excuse not to use it.  Apart from the unfounded viability excuses, these same people will spend any amount of money on alternative technologies to avoid a very simple truth of physics and chemistry.  Nuclear reactions produce orders of magnitude more energy per reaction than any chemical reaction ever could.

JoshNH4H wrote:

*There are plenty of reasons for this.  The water can be recycled a drinking water, used to reconstitute food, and used as shielding.  It's probably bad to add too much water vapor to the intake as it could condense into ice when it's mixed with the oxygen.  It's also likely that the water would freeze out as soon as it was vented, and this could block up the vent or whatever it's near.

A nuclear reactor can bake so much water out of the Martian regolith using waste heat that we'll likely dump grey water over the side of the rover rather than try to process it.  I'm sure someone from the EPA will be along shortly to give us a speech on Martian climate change and the environmental impact of grey water discharge to Martian dirt.

Last edited by kbd512 (2016-02-01 02:05:33)

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#133 2016-02-01 12:27:40

Antius
Member
From: Cumbria, UK
Registered: 2007-05-22
Posts: 974

Re: Nukemobiles on Mars

Am241 is predominantly an alpha emmitter and has been proposed as a long term RTG isotope.  It is a neutronic poison.  The problem with Am242m is that would need to be separated by centrifuge from Am241.  Both are highly radioactive, 241 being 50 times more radiotoxic than plutonium.  Am242m is a powerful gamma emmitter which would complicate isotope separation and fuel fabrication.  Consider as well that with a half-life of 141 years, after 7 years some 3% of your fuel has decayed into curium 242, a neutronic poison.

Unless americium provides a very significant engineering advantage over plutonium, there would appear to be no logic in going down that route.  There are tonnes of reactor grade plutonium seperated in french and UK reprocessing plants.  NASA could buy it without much difficulty.  The US has its own reserves of weapons grade material which would easily be available in the quanities needed.

We already know that SAFE-400 can do the job using enriched U.  A Pu or Am core would be a design improvement but not a game changer.

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#134 2016-02-03 22:05:58

kbd512
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#135 2016-02-18 04:35:51

kbd512
Member
Registered: 2015-01-02
Posts: 1,187

Re: Nukemobiles on Mars

Any thoughts on the nuclear resonant battery idea?  Supposedly the Nucell Strontium-90 batteries could produce 7.5kWe using a gram of the material.  I have no idea what the volume or mass of the device was.  Sounds hokey, but stranger things have worked.

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#136 2016-02-18 07:17:45

Terraformer
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From: Lancashire
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Posts: 2,508
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Re: Nukemobiles on Mars

I suggested using strontium earlier. Though you're four orders of magnitude off, since the decay of Sr-90 only gives 0.536 W/g. Which is still a very good figure for a power source, since it means you might be able to get 100 We/kg overall, or 100 kWe from a 1 tonne device.

The problem is, it requires launching radioactive material into orbit. If you make sure it won't break up if it accidentally re-enters, and launch it over the ocean so it will sink to the bottom in the event of a failure, you should be able to reduce the risk to a negligible amount. But good luck persuading the vocal minority of that.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#137 2016-02-18 19:27:05

SpaceNut
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Re: Nukemobiles on Mars

Never heard of it kbd512....
A bit of google and here it is Basically, the device is an LCR tank circuit oscillating at its self-resonant frequencywith L standing for inductance, C is the capacitance and R is resistance.....
RESONANT NUCLEAR BATTERY MAY AID IN MITIGATING THE GREENHOUSE EFFECT BY PAUL M. BROWN a device captures the kinetic energy imparted to alpha and beta particles when a particle-emitting isotope decays rather than thermal heat as in the typical use as an RTG. This sounds simular to the ECat that is using isotopes as well for generating power using nickle and water in its technology.

Another simular technology is Nuclear battery breakthrough Research by a team led by Jae W Kwon at the University of Missouri in the USA has opened the door for the development of a new generation of water-based batteries powered by beta radiation.

Not all that much appears to have been done since some initial work.....

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#138 2016-02-19 00:56:46

kbd512
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Re: Nukemobiles on Mars

Terraformer wrote:

I suggested using strontium earlier. Though you're four orders of magnitude off, since the decay of Sr-90 only gives 0.536 W/g. Which is still a very good figure for a power source, since it means you might be able to get 100 We/kg overall, or 100 kWe from a 1 tonne device.

Is that 100kWe out of a 1t RTG that uses Sr-90 or are you saying we need 1t of Sr-90 to obtain 100kWe?

Terraformer wrote:

The problem is, it requires launching radioactive material into orbit. If you make sure it won't break up if it accidentally re-enters, and launch it over the ocean so it will sink to the bottom in the event of a failure, you should be able to reduce the risk to a negligible amount. But good luck persuading the vocal minority of that.

That's a problem with any nuclear power source.

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#139 2016-02-19 13:14:49

Terraformer
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Re: Nukemobiles on Mars

I mean you might/should be able to get 100 kWe out of a 1 tonne nuclear battery. I don't know what the Russians have actually gotten though.

Most space reactors don't involve launching them with substantially radioactive cores though. The idea is usually to turn them on once they're safely in space.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#140 2016-02-19 20:42:24

SpaceNut
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Re: Nukemobiles on Mars

I got thinking about a nuclear batteries construction as wanting to be simular to a chemical battery but would use particles as indicated in the resonant article but I would also make use of the oposite particle decay as well.

The arrangement would be a plate next to positive particle emmision and then an insulator to the next pllate that has a negative particle emission and continued until the output power is achieve much as a battery does to build up the voltage and currents. So long live particle emission is a requirement or we would be replacing them quickly. The size and thickness of the plates as well as distance between them makes for the output parameters to change in the way a battery does. Some plates might be solid while others may be more like a screen to cause charge potential to change such that a current will flow. In many ways its sort of like the old vacumn tubes with plates and grids.....

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#141 2016-02-20 20:03:37

kbd512
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Re: Nukemobiles on Mars

SpaceNut,

If resonant nuclear batteries actually function the way the patent holder states, I would bypass fission reactor development entirely for space applications in favor of this technology.  Any power source that requires no cooling system and directly converts radiation into electricity is pure genius, assuming it actually works.

There's probably an upper limit to how well this technology scales, but the inventor thought a 100kWe battery the size of a 55 gallon drum was entirely possible.  Personally, I think development of smaller 10kWe units makes the technology more adaptable to the potential applications.

For example, GPS and communications satellites - one unit, a large surface habitat module - two or three units, unpressurized rover - one unit, large pressurized rovers - up to ten units, and a practical ISRU - up to twenty units.

For in-space propulsion, one or two larger units that provides up to 250kWe may work best.  We'd have to determine how much the device would weigh and whether or not any benefit over solar panels could be achieved at the distances from the Sun that the unit would operate at.  My guess is that the required mass of solar panels like UltraFlex would be lighter at 1AU distances or closer.

I've given some more thought to heating the tracks and road wheels of the rover, too.  Attaching RHU's directly to the road wheel hubs, drive sprocket, and idler wheel makes the overall solution far less complicated than embedding heating elements, even if it's heavier.  There's no electrical heating elements to engineer into the body of the running gear of the vehicle.  Since there's no heavy reactor shielding or cooling solution required, whatever small increase in weight required to mount RHU's on the running gear is not a major problem.

Last edited by kbd512 (2016-02-21 04:10:59)

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#142 2016-02-29 19:25:22

SpaceNut
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Re: Nukemobiles on Mars

kbd512 wrote:

Rover Crew Sustainment and Power Requirements:

MOXIE is supposed to be a 1% scale demonstrator of the device that NASA wants to use to make propellant on Mars.  The demonstrator requires roughly ~168W of power and produces 22g/hr of oxygen.  If MOXIE is scaled up 60%, it can make the maximum amount of oxygen that NASA says an astronaut would consume in a day, 35g/hr.  Alternatively, 2 units similar to the demonstrator would produce 44g/hr.  The rover would likely require six such units to provide backup oxygen generation capability in the event of failure of any two units, with four units active at any given time.  That's 672W just to produce oxygen.  The dimensions of MOXIE is 30cm x 23.5cm x 23.5cm, but I can't seem to find the mass.  Maybe we can get away with using four units or fewer with a highly efficient CL-ECLSS.

I have no idea how much power is required to bake the water out of the Martian soil, but we need to know this in order to provide for the astronauts water requirements.  I expect that the every-day EVA's will result in a loss of water as well as oxygen.

What are the power, mass, and volume requirements for two 95% efficient CL-ECLSS capable of sustaining two crew members in a 2M by 5M inflatable enclosure with a 1M x 2M inflatable airlock that the astronauts open and close at least four times per day?

For a rover with a mass of 1t carrying a payload with a mass of 1t to 1.5t, how much power is required to move at 50kph?  The rover only weighs .95t on Mars with a 1.5t payload.  So for a rover that weighs roughly three times what Curiosity weighs on Mars, how much electrical power is required to move it over relatively smooth terrain between 25kph and 50kph?

To keep things simple, let's say the rover is a 4 wheel design like a normal light vehicle and each wheel supports 237.5kg of the rover's weight.  Each wheel would have its own electric motor.  Let's say each hub motor can provide power from .5kW to 2.5kW, and is 85% efficient.  If the rover is driving over sand, how much power would the vehicle consume at a roughly constant speed of 25kph?

If no nuclear battery technology is forthcoming and we're limited to rollup solar panels and batteries, what's the mass of the battery pack required to permit 8 to 12 hours of off-road travel over soil with a consistency similar to sand?  If we had a 20kWh battery pack is this doable?  If there are 5 hour mid-day stop for recharging the batteries, is that sufficient time to recharge a 20kWh battery pack?

2 Person Rover Concept of Operations:

* 1 to 3 days of travel to site of interest

7AM to 10AM travel
10AM to 3PM stop for battery recharge
4PM to 9PM travel

* 1 to 2 days of exploration of site of interest

Astronauts deploy solar panels at 7AM during the first EVA and then repack them at 5PM when they come in from the second EVA
7AM to 11AM EVA-1
1PM to 5PM EVA-2

* 1 day of preparation for travel to next site of interest (rover maintenance, samples cataloging and cursory examination, route planning)

Astronauts deploy solar panels at 7AM during the first EVA and then repack them at 5PM when they come in from the second EVA (EVA's for solar panel deploy/stow and rover inspection and maintenance only)

Does a 20kWh battery pack provide enough power for movement and replenishment of lost oxygen in that time frame?  I was thinking that while the rover isn't moving, it would generate oxygen and water for the crew.  While on site in the exploration area, the crew would simply lay the solar panels out during the morning EVA and then pack them up after the afternoon EVA.  I have no idea how much power the ECLSS would consume or what a realistic battery discharge/depletion rate is for the type of off-road travel encountered.

I know that the flexible roll-up arrays are less efficient than the crystalline arrays, but their flexibility should mean that 1.5M square array panels can simply be laid out on the ground near the rover versus some sort of deployment mechanism that requires mechanical devices to function properly.  The astronauts would use flexible nylon pouches for solar panel storage and brushes to wipe off dust.  Each astronaut would carry his or her panels in the rolled up pouch through the airlock, deploy the panel squares, and connect detachable power cables from the panels to the rovers.

Anyway, just some thoughts about how this could work and a lot of questions.

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#143 2016-03-19 22:38:09

SpaceNut
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#144 2016-03-22 02:24:21

kbd512
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Re: Nukemobiles on Mars

SpaceNut,

If there is a case to be made for use of nuclear power for Mars rovers, then resonant nuclear batteries are surely at the forefront of the argument.  The Strontium-90 in the batteries is readily available nuclear waste, so it costs very little to obtain, is only beta active as are all decay products, and no fissionable isotopes are required for power production.  Resonant tank circuits are not exactly cutting edge technology, either.  We may only be using a few grams of the material per mission, but at least we're using it for something productive instead of burying it.

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#145 2016-03-22 19:21:41

SpaceNut
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Re: Nukemobiles on Mars

Lets see if with a bit more research links and documents that its is going to be possible....found on this page that there is another type http://www.meridian-int-res.com/Energy/Betavoltaics.htm which seems to act like a solar cell panel in that it uses a semiconductor to collect the beta particles to create current....found it encouraging to see that

A small prototype the size of a soup can produced a continuous 75W

would be possible for the resonant type.

The weight of the strontium-90 used to generate 75 watts of power in the Nucell prototype is approximately the same as the weight of 2 millimeters of wire cut off the end of a small paper clip.

http://www.scribd.com/doc/8929973/Nucle … ery#scribd
http://www.world-nuclear-news.org/EE-Nu … 09148.html this one talks about materials...

Kwon's team developed a battery using a strontium-90 beta source, with a platinum coated titanium dioxide electrode to collect and convert energy into electrons. The cell contains a water-based semiconducting material, which provides shielding from the radioactive source and absorbs the kinetic energy of the beta particles. When the liquid absorbs radiation energy, radiolysis takes place and free radicals - highly reactive but short-lived chemical species - are produced. These can also be converted into electricity, boosting the cell's power output still further.

A Modular Design for Nuclear Battery Technology


https://nuclearhistory.wordpress.com/20 … ct-nucell/ indicates the

Brown’s first prototype power cell produced 100,000 times as much energy per gram of strontium-90 (the energy source) than the most powerful thermal nuclear battery yet in existence. The Nucell battery yielded 7500 watts per gram of strontium-90. Compare this to an advanced device recently developed by the US Dept. of Energy Byproducts Utilization Program. Their state-of-the-art thermal nuclear battery produced 0.063 watts per gram of strontium-90.

Long-lasting, water-based nuclear battery developed

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#146 2016-03-23 05:40:44

Terraformer
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Re: Nukemobiles on Mars

Huh. That doesn't make sense. I'm sure the decay energy is far less than that...


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#147 2016-03-23 11:55:39

kbd512
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Re: Nukemobiles on Mars

Terraformer wrote:

Huh. That doesn't make sense. I'm sure the decay energy is far less than that...

The resonant nuclear battery (RNB) doesn't attempt to convert heat from decay or fission into electricity.  Those processes are extremely inefficient.  It's a direct conversion process.  The RNB uses an alpha or beta active radiation source, a thin foil with a mass of approximately 1g that's housed in a stainless steel cylinder that has been evacuated, to induce an electrical motive force in the windings of a LCR tank circuit in close proximity to the radiation source (Strontium-90 foil because it and all decay chain products are beta emitters, so no gamma shielding is required; the stainless steel container for the circuit and beta source are gross overkill for the application).  The "resonant" part of the RNB has nothing to do with the nuclear material and everything to do with the circuit.  The circuit is resonant at a specific frequency and is "tuned" with a capacitor.  The last challenge to overcome was frequency stability.  However, in actual tests the electrical power produced from 1g of Strontium-90 using a device roughly the size of a large soup can is approximately 7.5kW.

The phenomenon doesn't require an alpha or beta emitter to demonstrate.  There's a very simple principle at work here.  The device itself is very simple and has no moving parts, as far as I can tell.  There's probably more money to be made from enriching and storing Pu-238 than there is from turning a radioactive waste product, like Strontium-90, into a power source.

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#148 2016-03-23 13:01:54

Terraformer
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Re: Nukemobiles on Mars

That still doesn't deal with the fact that Sr-90 doesn't put out 7.5 MW/kg, by far. Are you sure you're not mistaken by 3 orders of magnitude?

Still, 20 kg of Sr-90 is not much for a 150 kW powerplant. I think we can afford to launch that. Even if it's 5x lower, because of the need to include a system that will ensure it will rapidly sink to the bottom of the ocean in the event of a launch failure, that's still a high enough power density to make nukemobiles feasible.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#149 2016-03-23 15:09:30

kbd512
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Re: Nukemobiles on Mars

Terraformer wrote:

That still doesn't deal with the fact that Sr-90 doesn't put out 7.5 MW/kg, by far. Are you sure you're not mistaken by 3 orders of magnitude?

From the Scribd document posted by SpaceNut: "The magnetic energy givenoff by alpha and beta particles is several orders of magnitude greater than either the kinetic energy or the direct electric energy produced by these same particles."

Terraformer wrote:

Still, 20 kg of Sr-90 is not much for a 150 kW powerplant. I think we can afford to launch that. Even if it's 5x lower, because of the need to include a system that will ensure it will rapidly sink to the bottom of the ocean in the event of a launch failure, that's still a high enough power density to make nukemobiles feasible.

It's not the quantity of material that's a problem.  It's the volume, mass, and heat rejection requirements of the entire solution.

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