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#151 2020-06-28 17:58:36

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 23,078

Re: Nukemobiles on Mars

post 30 starts for about 5 talk of pebble reactor on first page also a single post on page 3 by joshnh

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#152 2020-08-09 15:01:22

SpaceNut
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Posts: 23,078

Re: Nukemobiles on Mars

kbd512 wrote:

GW,

I've taken a second look at using Strontium-90 for in-space and surface power applications, mostly using direct thermal power supplied to advanced supercritical CO2 turbines.  Sr90 may be less desirable compared to Pu238, but it's very plentiful compared to Pu238 and much cheaper.  We also have many thousands of tons of the stuff since it's a daughter product of fission.  We'd need 5,435kg of Sr90 to produce 2.5MWt at BOL and 1.5MWt after 20 years of service.  I estimate the radiator mass at 500kg or less, though even if that was doubled it wouldn't be a major problem.  I was also thinking that at least part of the radiator mass could be supported by the sides of the gondola to keep the CG as low as possible.  The SNAP-50 reactor design of the 1960's was to dissipate a 1,448kWt thermal load at 1156F using 1,100 pounds of 316 SS tubing and Copper fin radiator and that radiator design was 700ft^2 (from the boiling Potassium reactor design Calliban pointed out).  I believe that was single-sided, as it would be in this application, although I could be wrong about that.

Some of the radiator mass would be integrated with the vehicle mass.

Sr90 Characteristics:
Density: 2.375g/cm^3 near the melting point
Half Life: 28.1 years
BOL Thermal Power Output: 460W/kg
BOL Surface Temperature: 700C to 800C

Pu238 Characteristics:
Density: 19.329g/cm^3
Half Life: 87 years
BOL Thermal Power Output: 540W/kg
BOL Surface Temperature: 1050C

Anyway, the idea is that the RTG and radiator mass is split between two tracked prime movers, so each tracked prime mover nominally produces 1MWe at BOL or 750kWe at EOL, one at the front and one at the end of an overland version of a railway boxcar.  The primary construction material for the prime movers would be the 5083 Aluminum alloy used by the M113 APC, along with forged steel track links and Thoraeus metallic rubber clad road wheels to better withstand low temperatures.  The vehicle design will be similar to the Swedish Stridsvagn Strv-103, meaning it will use hydraulic suspension to increase ground clearance for travel and to "kneel" when not in operation to better facilitate loading / unloading operations.  If you google, "E-10 Jagdpanzer", visualize this vehicle without a gun and a giant articulating hitch on top to support one end of an Aluminum railway gondola car and that's pretty much what I had in mind.  I figure we'll use the T157 track shoe for ease of disassembly with a speed wrench, despite it's weight per foot (71.4 pounds per foot).  It's reasonably stout, 21 inches wide, and used on the M2 Bradley IFV- a vehicle in the same tonnage range as the load we're trying to support on Mars.

We're going to stipulate that each prime mover weighs 12t, even with the mass of the RTG and gas turbine, since each prime mover will be a single seat vehicle with limited interior volume.  The SCO2 turbine will provide direct mechanical power to an automatic transmission.  For reliability, a miniature thermoelectric RTG will supply electrical power to onboard vehicle electronics and life support equipment.  A hydraulic pump will be attached to the gas turbine's accessory case to supply hydraulic power for the articulating suspension and for mounting / dismounting the Aluminum gondola at the mine and ore refining facility.

A BNSF aluminum rotary gondola railcar weighs 41,900 lbs / 19,045kg empty and its load limit on a railway is 244,100 lbs / 110,955kg.  This type of railcar is used for transport of metal ores.  We're going to stipulate that the weight of the steel trucks required for railway use will be replaced with structural reinforcement of the car for overland operations and a pair of articulating "trailer hitch" type structures to connect the gondola to the pair of prime movers.  The total mass is 154,000kg on Earth and 58,520kg on Mars.

The complete vehicle with max payload weighs 128,978lbs on Mars, for 20.8hp/ton, when loaded like a train railcar.  That's slightly better than the British Challenger 2 MBT, supplied with 19.2hp/ton via its 1,200hp Perkins diesel.  Maximum off-road speed for Challenger 2 is 40km/h, so I expect that over a graded trail, this vehicle could manage the same speed.

That's a heck of a lot more mass and power to move this vehicle acceptably well over an off-road environment than a standard railcar requires by rail, but it should still work.  Since there probably won't be any paved roads or trains on Mars for quite some time, this solution still permits transport of 110t railcar load using roughly the same transport technology we use here on Earth in mining operations.

Seems like it can scale down as a kilowatt unit seems possible.

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#153 2021-01-19 03:36:46

Calliban
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From: Northern England, UK
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Re: Nukemobiles on Mars

An idea worth revisiting.  Nuclear reactors have better power-weight ratio if scaled up to large sizes.  This is largely due to shielding requirements.  In addition, larger reactors suffer less from neutron leakage and can therefore make use of lower enriched uranium.  Important because LEU is commercially much easier to source than HEU, which is almost impossible to procure unless you are a military operation.

A nuclear powered ground vehicle would work well on Mars if it were scaled up to the point where the nuclear reactor could achieve good power-weight.  That means that nuke mobiles will be large compared to most vehicles on Earth.  It fits well with the idea of a travelling base that gradually roams the Martian surface exploring the environment.  You want drive systems that are high torque but low speed.  A base travelling at 1km/hour could circumnavigate the planet in about 2 years.  Shorter range ground vehicles could explore the terrain around it and catch up with it later.  The base itself could weigh thousands of tonnes.

Sr-90 would be a useful power source for much smaller vehicles, as it does not require heavy shielding.  The problem is that it is a fission product and therefore requires isotopic separation from spent nuclear fuel.  That is a complex and expensive industrial operation.  It may be some time before there is enough nuclear power capacity on Mars to allow such a programme to get started.

Last edited by Calliban (2021-01-19 03:42:43)


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#154 2021-01-19 09:02:18

tahanson43206
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Re: Nukemobiles on Mars

For Calliban ... This is a topic worth developing ...

Would you be willing to provide leadership (to the extent you have time from your many other obligations) for young people who might be interested in developing equipment along the lines you've described.

We are operating in By Invitation mode, and SpaceNut has been advertising for new members in the main Mars Society web site.

It seems possible SpaceNut might be willing to post an invitation from you for the kind of person you'd like to work with.

The large moving base concept is on SpaceNut has posted about before.

(th)

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#155 2021-01-19 18:29:50

SpaceNut
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Re: Nukemobiles on Mars

Short of RTG for nuclear the mass of a reactor shielding is only part of the issue for using one in proximity to manned use on a vehicle.
What we have not explored is specific trade offs for which radioactive element is chosen for the reactor as to how much mass shifts for shield to power level drop from that change.
So far I have not seen the calculations or mass for shielding the KRUSTY reactor for a mobile use as it would be plenty for continous use...

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#156 2021-01-19 22:02:55

kbd512
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Re: Nukemobiles on Mars

SpaceNut,

Increasing the shielding doesn't decrease the power output of a reactor, but it does decrease the power-to-weight ratio.  If the reactor doesn't have to move, then it can be planted in the ground wherever it's required.  Absent nuclear reactors, I don't think battery powered vehicles will be very practical, since you can't use them when people are normally awake without lots of extra battery packs sitting on the charger.

Antius and I already discussed the halving thicknesses of prototypical nuclear shielding materials like Tungsten and Lithium Hydride.  For a heavy tracked off-road vehicle, I don't see a show-stopper problem with operating a properly shielded reactor in close proximity to people inside the vehicle, but the solution won't be light.

Although the following link describes minimum shielding mass requirements for a large 2.5GWth reactor core, it's useful for understanding how neutron and gamma fluxes drive shielding material placement (why they put lighter low-Z material closer to the edge of the reflector and why heavier high-Z material is placed further away; although still quite heavy, it actually economizes on shielding mass), thus the mass of the final solution:

Weight Optimization of Reactor Shielding using Transmission Matrix Methods

This link details neutron radiation shielding more appropriate for the small modular reactors we'd most likely see in use on Mars:

Neutron Shielding for Small Modular Reactors

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#157 2021-02-08 17:32:40

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 23,078

Re: Nukemobiles on Mars

kbd512 wrote:

SpaceNut,

SCCO2 is a necessary technology for heat engines, the only type of 24/7/365 power plant in existence, for use here on Earth and on Mars, whether solar or nuclear.  Both photovoltaics and batteries degrade over 25 years (photovoltaics) or 5 years (Lithium-ion), we don't have good methods for recycling them, and we can never make enough with available raw materials to satisfy demand.  A glass encased mylar film or polished stainless steel reflector does not degrade appreciably within a human lifetime (we even have a few select examples of well-cared-for non-stainless steel from the Middle Ages in good physical condition).  A molten salt thermal power storage liquid and CO2 power transfer fluid do not degrade at all under the conditions we would use them, and both materials are abundant / easy to obtain / low-cost, both here and on Mars.  A high-grade steel SCCO2 turbine does not appreciably degrade over 25 years.

We have had diesels running continuously in oil fields for longer than photovoltaics have existed.  We have had solar thermal and nuclear thermal since the beginning of time.  Until such time as the new hotness in battery and solar technology are recyclable, the simple heat engine is the way forward.  We need to recycle the CO2 by pulling it out of the atmosphere.  The only energy equation that matters is whether or not we have more power output than power input.  We can get that from solar thermal or nuclear thermal, as well as liquid hydrocarbons by way of H2O and CO2 recycling.  Without those things, technologically advanced human civilization ends.  One day, many years from now, we may master photovoltaics and batteries, but we're nowhere close to that technology goal despite chasing it for the better part of half a century now without achieving the result we're after.  All power technologies have their uses and I'm onboard with developing all of them, but heat engines are the bedrock of modern human civilization.

Calliban wrote:

This belongs in the nuke mobile topic, but it occurred to me after reading kbd's post.  If we are going to use a nuclear reactor to melt phase change material, then it would make sense using that material as a gamma shield.

We would mount a sodium cooled fast reactor on a trailer along with its SCO2 power generation machinery.  The phase change tank would sit in the middle of the trailer.  The reactor would be at the back.  The SCO2 machinery and generator would be mounted forward of the phase change tank, partially shielding it from high gamma fluxes during reactor operation.  The reactor would be surrounded by a tank of water, which is just thick enough to reduce fast neutron fluxes during operation and thereby prevent activation of materials on the trailer.

At the end of each day, the rover would be detached from the trailer and would drive several hundred metres away from it on battery power.  The rover would be positioned so that the phase change tank obscures view of the reactor.  The nuclear reactor would then be run and the heat would melt the phase change material whilst the crew sleeps.  A few hours before sunrise, the reactor would shut down, allowing a few hours for short lived fission products to decay.  After a few hours of shut down, the rover reverses back and hooks the trailer back up.  The vehicle then drives on stored heat, which runs the SCO2 power generation loop.  The phase change tank would serve a useful secondary function of absorbing decay heat from the shutdown reactor.

This set up appears to allow a minimally shielded nuclear reactor to power a rover.  By relying upon distance instead shielding as the dose reduction measure, a nuclear reactor can be made quite compact.  There needs to be enough shielding to prevent dangerous gamma doses from fission products to people walking around the trailer during the day.  But this is a few inches of steel, around a core that is perhaps 30cm in diameter.  Such an arrangement could therefore be very compact.  It also has the advantage of providing a detachable powerplant, that can be removed from the rover at a destination and used to power equipment or provide process heat.  If we needed to work around a reactor during operation, a shield bunker could be dug out.

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#158 2021-02-11 19:14:10

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 23,078

Re: Nukemobiles on Mars

SpaceNut wrote:

Which brings me to size matters in why KRUSTY make sense  for mars use not to much mass, steady 10kw electrical, plus a plausible 30kw of IR heat make use of and its already a molten salt unit.
Use an up draft chimney to heat compress Mar's Co2 and create power from the updraft.

kbd512 wrote:

SpaceNut,

This is really "nuke mobile" content, but in response...

We could design a "bespoke" KiloPower reactor that only generates thermal power output to split CO2 into CO and O2, or to heat a tank of molten salt.  The 10kWe KiloPower reactor is actually generating 40kWt, all of which can be used to supply process heat.  At the temperatures involved, a molten salt could store roughly twice as much power per unit mass as a Lithium-ion battery.  If the crewed vehicles in a convoy were using tanks of molten salt to provide thermal input power to 50% efficient SCCO2 gas turbine, then you have a 100% reusable solution that doesn't require re-generation of chemicals, has roughly the same gravimetric energy density as Lithium-ion, but without the battery degradation problems.  The "battery" still weighs 8t, but it will last until the ceramic containment unit cracks.

If the process heat is used to re-heat the molten salt, then it would take a bit over 100 hours, or 4 days, to recharge a single battery.  The insulated tanks lose less than 1% of the stored heat per day, so it's practical to recharge several vehicles with a single reactor, over a few weeks.  If a single tracked vehicle carried 4 reactor cores, then all of the vehicles in the convoy can be recharged in less than a week.  A foot of steel surrounding each coffee can should be sufficient to shield the reactors.  Maybe a stainless steel foam impregnated with Boron can shield against both neutrons and gamma for less weight.  It'll be really heavy, but it should work.

Well I could not agree more so we would want a shield around the reactor facing outward and one on the inside facing any crew but the tank of molten salt would act as the over all shadow shield for lowering exposure when in use.

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#159 2021-02-12 10:12:46

Calliban
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From: Northern England, UK
Registered: 2019-08-18
Posts: 1,154

Re: Nukemobiles on Mars

The most mass efficient shielding solution for the reactor, assuming you intend to run it when the vehicles in your convoy have stopped, would be to dig a pit beneath the vehicle and then lower the core into it.  You can do that by lowering the entire trailer bed.  This dramatically reduces the minimum safe distance from the reactor.  You only need enough permanent shielding to protect the power conversion machinery on the trailer during operation and to shield the gamma from residual fission product decay when the reactor is not in operation.

Last edited by Calliban (2021-02-12 10:18:58)


Interested in space science, engineering and technology.

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#160 2021-02-12 10:28:02

SpaceNut
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Re: Nukemobiles on Mars

Can we turn on and off the reactor that fast and then what is the decay rate. Since we would be moving by day light most likely the reactor is on during the night and then each day we would turn it off...
Then the batteries can only make use of some of the thermal salt energy before we will have issues with the reactor restarts taking longer to fire up.

Gave the main KRUSTY topic a bump and here is a link to its design
https://ntrs.nasa.gov/api/citations/201 … 012354.pdf

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#161 2021-02-12 14:20:35

kbd512
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Re: Nukemobiles on Mars

I still think the most practical solution is to properly shield the reactors and to affix them in-place in the un-crewed vehicle(s) in the convoy.  That way, the reactors can operate at constant power output and constantly re-heat onboard tanks of molten salt, for transfer or mixing with lower temperature molten salt in the crewed vehicles being supplied, by using a simple transfer pipe and pump to pump the molten liquid between storage tanks in each of the vehicles.

A Titanium tank could hold the molten salt and a ceramic thermal barrier / corrosion resistant coating and a light aerogel insulator blanket wrapped around the tank, would inhibit heat transfer from the tank to the environment.  A simple thin-wall vacuum thermos design is all that's required to hold the molten salt, since it's at near-zero pressure.  The closed-loop SCCO2 system would have the heater pipe or pipes run through the tank to transfer thermal power from the molten salt to the SCCO2, and a Copper radiator would be the thermal sink.

Since all parts of the vehicle chassis design except the road wheels are high grade steel with ceramic thermal barrier coatings, accidentally spilling some molten salt on top of the vehicle won't do anything at all to the vehicle, even if it's not cleaned off afterwards.  I expect that there will be some drips and minor spills during transfer operations.

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#162 2021-02-12 15:03:39

Calliban
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From: Northern England, UK
Registered: 2019-08-18
Posts: 1,154

Re: Nukemobiles on Mars

If the reactor trailer is carrying molten salt anyway, then it would make sense using the salt as a reactor shield.  Fast neutrons streaming out of the core will generate gamma rays if they interact with high Z materials.  That's why water gets used as a neutron shield.  But if the molten salt layer is thick enough it will absorb the excess gamma rays.  The half thickness of materials varies with gamma ray energy.  It would be interesting to work out what thickness of salt would be needed to reduce gamma ray dose to sufficiently low levels.

Last edited by Calliban (2021-02-12 15:15:26)


Interested in space science, engineering and technology.

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#163 2021-02-12 18:02:05

SpaceNut
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Re: Nukemobiles on Mars

Not wanting to really have an extra trailer car or un-crewed following rover to the mix as its one more risk factor that can go wrong.

Kustry's output is constant due to the reactor pouring in thermal energy but a cycled reactor and seperate pools of salt would not sustain unless there is some size to it to make sure we can go the time frame without any issues from the power unit.

I would rather make the reactor smaller so as to not let it cycle and to always ensure we do not have any issues.

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#164 2021-02-13 20:49:11

kbd512
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Re: Nukemobiles on Mars

Calliban,

Give how much salt is required to supply 4MWh, it might be practical to properly shield the reactor core.  Putting high-Z Tungsten right up to the edge of the reflector would drastically increase the secondary gamma production associated with neutron activation, meaning it would dominate the radiation escaping from the core, which is undesirable from a mass-optimized shielding perspective, but an Inconel can containing molten Lithium Hydride should work.  LiH melts at around 690C, but it boils at 900C to 1,000C, which means reactor operating temperatures are well within limits for using low-Z LiH to inhibit neutron activation.

The basic concept is that the molten salt surrounds the heat pipes from the reactor's core for thermal power transfer to the surrounding salt tank, but the core itself is already substantially shielded such that radiation activation of the salt should be minimal.  Rather than the enormous Aluminum or Copper radiators and inefficient conversion of heat to electricity, 100% of the core's thermal power is being absorbed by a tank of molten salt.  FLiBe (Fluorine Lithium Beryllium) seems to be the salt of choice for containing fuel, because it doesn't readily eat neutrons and melts at a reasonably low temperature.  However, that's undesirable for this application.  Actually, it depends upon whether or not the reactor is fast or thermal spectrum, with respect to the neutron absorb behavior of FLiBe.

Anyway, I've given this some more thought and this is what I've come up with:

* Use Uranium Dioxide powder as the fissile core material, or liquid Uranium metal.
* Use molten Boron instead of molten FLiBe for thermal power storage.  At 2680C, molten Boron stores 2.68MWh/m^3.
* Run the core hotter than hades to melt the Boron, which UO2 should be able to handle.
* Use Tungsten piping in conjunction with liquid Titanium metal in the primary coolant loop.
* Store the molten Boron in a thin wall Tungsten tank with a ceramic coating to act as a thermal barrier between the reactor / salt tank assembly and the rest of the vehicle.
* The rest of the heat engine assembly will use SCCO2 in a secondary loop, also run through the molten Boron tank.

People who think the liquid Sodium fast reactors were a little extreme should examine this thing.  I like it.  It solves the radiation absorption problems so that no extra vehicle is required.  The power pack will be quite heavy, but no worse than a battery powered vehicle.  With further development work, a TPV array could convert the heat from the molten Boron directly into electricity, so that would exclude the requirement for gas turbines and associated plumbing to convert the heat into useful power output.  There would be no moving components except for the molten Titanium in the primary loop.

All that said, a 250kWe output reactor could be properly shielded with the amount of weight involved in the energy storage scheme, at which point it makes a whole lot more sense to drive the vehicle from a generator in the core's primary coolant loop, foregoing storage of thermal power.  The above nonsense was directly related to the idea of using a very low power output reactor, which makes very little sense in the grand scheme of things.

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#165 2021-09-15 07:21:52

Mars_B4_Moon
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Registered: 2006-03-23
Posts: 884

Re: Nukemobiles on Mars

The 1958 Ford Nucleon
https://www.ans.org/news/article-3058/t … -its-time/

a vehicle was to be powered by a small nuclear reactor in the rear of the vehicle.

dangers of the car would be the radiation, nuclear waste and a possibility of a small nuclear meltdown.

The car was never built and never went into production, but it remains an icon of the Atomic Age of the 1950s.

It still features in pop culture,  the post-apocalyptic computer video game Fallout 3 contains wrecked cars resembling the Nucleon.

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#166 2021-09-15 07:58:51

kbd512
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Re: Nukemobiles on Mars

We were talking about giving a nuclear powered rover to a highly trained and skilled crew of astronauts for use on another planet 50% further from the Sun than Earth is so they could truly explore without undue concern over being dead within hours if their batteries or solar panels failed, given that those electronic-dependent devices frequently fail in the far more benign environment of Earth.  Mars is also subject to periodic months-long near black-out dust storms that render solar panels paperweights.  This was certainly not a proposal to give John Q. Public a nuclear powered motor vehicle to use to tool around on the local freeway or do burnouts in the Wal-Mart parking lot.  We routinely launch RTGs aboard space probes that contain a far greater radioactive inventory than any nuclear reactor (prior to turning it on) when they're launched, so the incredible range and power that a nuclear reactor provides, in comparison to all alternatives, seemed like a reasonably good idea.  If we were 50% closer to the Sun than Earth is and operating on a planet without dust storms that last for months, then we could and certainly would opt for solar panels and batteries.

Incidentally, the radiation released from coal-fired power plants to produce 1950s era gas powered vehicles greatly exceeded that of all nuclear reactors and all nuclear power accidents, but the media hysteria over nuclear reactors melting through the center of the Earth and ending up somewhere in China nixed the plans to provide CO2-free power, back before the current crop of religious zealots masquerading as environmentalists thought that was what everyone should do.

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#167 2021-09-15 14:21:14

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,571
Website

Re: Nukemobiles on Mars

The radioisotope U-238 stuff is a rather dilute but long-lasting form of power.  The two 1-ton rovers are driven by it,  but it only makes a few hundred watts.  It has less radiation to shield stuff from that more conventional reactors,  but it still poses some risks.  I rather doubt a few hundred watts driving around at a slow walk's speed is suitable for a manned rover.  There's nothing left over for any life support.

A sort of updated conventional reactor is the "kilopower" sort of thing,  where the core is maintained hot by fission,  and its heat drawn out with heat pipes to a heat exchanger.  That heat exchanger plus the waste heat rejection device (a radiator) are the hot and cold for a closed cycle heat engine.  It has an energy conversion efficiency of just about 25%,  which means if it had a 10 KW thermal rating,  it produces actually 2.5 KW electric (the other 7.5 KW is REQUIRED to be radiated away as waste heat by the radiator or other heat rejection device.  Similarly,  40 KW thermal is 10 KW electric  with 30 KW heat to be rejected.  Etc.  If you don't get the heat rejected,  you don't get the electricity,  and your hardware likely tries to melt down.  This is serious thermodynamics here.

So if 10 KW thermal,  that's 2.5 KW = 2500 watts of electricity,  compared to a few hundred watts for the radioisotope thing.  Lots more electricity is much more in line for the needs of a manned rover with life support as well as instruments and propulsion,  and is also suitable for much higher travel speeds,  more like the rovers we had on the moon during Apollo.

But,  the radiation hazard is quite severely lethal,  because this is really a functioning fission reactor,  and the basic unit design provides about nothing in the way of shielding.  The intended use was as a stationary power plant,  buried down in a well,  or enclosed by a tall,  thick berm.  So,  in reality,  to use kilopower for a rover,  you must have a battery vehicle that you charge up at night off the kilopower electricity.  Basically,  the same sort of electric rover Apollo used on the moon.  Except this one gets recharged and used again,  repeatedly. 

For long trips beyond battery range,  you have to install the infrastructure of kilopower charging stations along the route,  as a necessary prerequisite.  Eventually,  as the route becomes well-traveled,  you could string wires and power vehicles with them like the old electric trolleys and some trains.   With multiple distributed nuclear generators,  you get both redundancy and lower line losses.

There's other ways to do this,  of course.  But the nuclear-sourced battery vehicles,  later supplanted with overhead-wire electric propulsion on real paved roads or tracks,  is something we already know would work. 

Just food for thought.

GW

Last edited by GW Johnson (2021-09-15 14:29:36)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#168 2021-09-15 17:07:52

kbd512
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Posts: 4,612

Re: Nukemobiles on Mars

GW,

They deliberately tried to melt the reactor down by removing the heat pipes, but that didn't happen, because it can't happen, because it's a function of the core's surface area to volume ratio.  You can hack off the entire heat pipe system while the reactor is running at full rated output and it won't melt down.  Doing that would render the unit inoperable, but it wouldn't pose a health hazard.  Dave Poston thought that "feature" was important for a space-rated reactor to have, so that was specifically tested, six ways to Sunday.  There's a brief thermal power transient and then core thermal power output swiftly drops like a rock, because it's a feedback design that produces more thermal power output as more power is demanded / taken off by the power generating equipment, up till the point where you start over-heating the core, which produces a negative feedback that lowers power output.  This design is also not like a normal naval reactor where you can quickly throttle up and down, meaning it takes some minutes to ramp up to full output, but only seconds to kill the reaction using the single control rod.  Net net, you need a battery or super capacitor or hydraulic reservoir for bursts of accelerating power, because the reactor is always trying to establish an equilibrium thermal power output.  It can do load following to a limited extent once it's fully warmed up, but not like a battery and certainly not like a super cap.  It's a lot like a large shipboard diesel engine, in that regard.

The 25% thermal-to-electrical conversion efficiency rating for KiloPower has to do with the fact that it's a direct / simple cycle power takeoff, whereby the thermal power is transferred using NaK through a heat pipe system and the heat exchanger (large blocks of Copper or Aluminum alloy), which comprises the radiator assembly.  The reactor operates at a temperature where a cycle more akin to what NREL uses with their Supercritical CO2 gas turbine could easily achieve a 40% conversion efficiency with dual-pass / re-heat of the working fluid.  The reactor operating temperature precludes achieving higher efficiencies, but allows for the use of low-cost / commonly available steel alloys.  There was nothing fru-fru or "botique" about the reactor's design, all well proven materials and technologies in common use at NASA and elsewhere.

President Trump said no weapons-grade material will be allowed in space, so the reactor is a little bigger using High-Assay LEU / HALEU instead of the HEU that NASA originally intended to use to get rid of existing stockpiles of the material.  Even so, it's not that much, and it's not a deal breaker for what is essentially a Mars variant of the M113 armored personnel carrier equipped with a properly shielded nuclear reactor and some batteries and electric drive motors.  It's definitely more demanding in terms of mass allocated to shielding and core inventory, but still perfectly doable, and opens up the transfer of said reactor technology to qualified civilians, who will never be allowed to possess HEU or any other weapons grade materials.

U238 decay heat is approximately 550W/kg when the material is fresh from being irradiated in a reactor core, so you'd need 45kg and some change to produce 10kWe with a 40% conversion efficiency.  This would be much lighter than a KiloPower reactor up to a point, given a bespoke design that uses a Stirling engine rather than a TEG device, but the cost of U238 and limited availability means it's not practical.  The U238 alone would cost more than the entire KiloPower reactor by a wide margin.  U238 currently costs NASA $8.8M/kg to have custom fabricated for them, whereas the total cost of materials in KiloPower is under $100K, excepting the turbo-generators ($14K a pop, IIRC, and there's 4 to 8 of them), with nearly all other costs associated with transport, fabrication, and labor, which amount to a unit with a price tag of $1M to $2M, delivered to the launch vehicle.  Total program costs are beyond this to maintain all the infrastructure, but the marginal cost is very low, even when compared to the solar panels that NASA uses on Mars, which run $1M/kW.  Launch costs greatly exceed the cost of the reactor and all other associated costs.

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