New Mars Forums

Official discussion forum of The Mars Society and MarsNews.com

You are not logged in.

Announcement

Announcement: We've recently made changes to our user database and have removed inactive and spam users. If you can not login, please re-register.

#26 2016-01-11 20:56:02

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

Re: Nukemobiles on Mars

The coolant system for Mars will be simular to that of the ISS as there is nothing to wick the heat away from without expending energy to use the soil laying on a radiator directly and changing it often....

So that is why CO2 would not be loaded into the cooling loop until it lands as the mass puts it out of capability to drop to the surface.

http://www.space.com/21059-space-statio … aphic.html
iss-ammonia-leak-150114b-02.jpg?1421258251

The International Space Station’s active thermal control systems (ATCS) pump fluids through closed-loop pipes. A liquid-ammonia coolant loop along the station’s main truss keeps the station’s electricity-generating solar panels cool.

Evaporational cooling not something we can do on mars....
http://www.nei.org/News-Media/News/News … ower-plant

We have seen what can happen when cooling does not work and it needs to be even better than what we have here as there is no safe shutdown or alternative cooling for a crew using a nuclear reactor.

Offline

#27 2016-01-12 06:56:53

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

Re: Nukemobiles on Mars

Concerning the choice of shielding material: As the reactor vessel and shield are separated by a vacuum, the heat flux recieved by the shield will be whatever is radiated across the gap.  If the reactor is operating at 1000K, then the radiant heat flux will be 56.7kW/m2.  Added to that is the hard radiation energy flux (about 5% of operating power = 0.05x400kw=20kW).  Trying to dump 77kW of heat into ambient martian conditions at 100C would require a big radiator.  Many reactors employ thermal insulation to limit the assault on the shielding, but this would add weight and volume.  The problem is reduced somewhat if the incoming propellant feed (i.e. liquid CO2 is used to cool the shield).  But I would agree water isn't ideal in terms of the temperature limits it imposes.  It is worth noting that most reactors use composite shielding.  The first shield in most PWRs is a stainless steel baffle plate, which limits irradiation of the pressure vessel.  A hydride (zirconium?) might make a more temperature resistant shield, but even these have thermal dissociation temperature limits.

The other option is distance.  Does the reactor have to be physically built into the vehicle?  Or could you tow it a few metres behind the vehicle?

Offline

#28 2016-01-12 11:08:00

Terraformer
Member
From: Lancashire
Registered: 2007-08-27
Posts: 2,508
Website

Re: Nukemobiles on Mars

Or even keep it on orbit, and beam the power down every pass. Use fuel cells to even out the power supply and demand.

You could even use solar panels instead with such a system...


"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

Offline

#29 2016-01-12 16:45:56

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

Re: Nukemobiles on Mars

Antius wrote:

Concerning the choice of shielding material: As the reactor vessel and shield are separated by a vacuum, the heat flux recieved by the shield will be whatever is radiated across the gap.  If the reactor is operating at 1000K, then the radiant heat flux will be 56.7kW/m2.  Added to that is the hard radiation energy flux (about 5% of operating power = 0.05x400kw=20kW).  Trying to dump 77kW of heat into ambient martian conditions at 100C would require a big radiator.  Many reactors employ thermal insulation to limit the assault on the shielding, but this would add weight and volume.  The problem is reduced somewhat if the incoming propellant feed (i.e. liquid CO2 is used to cool the shield).  But I would agree water isn't ideal in terms of the temperature limits it imposes.  It is worth noting that most reactors use composite shielding.  The first shield in most PWRs is a stainless steel baffle plate, which limits irradiation of the pressure vessel.  A hydride (zirconium?) might make a more temperature resistant shield, but even these have thermal dissociation temperature limits.

Radiator dimensions would be between 125m2 and 150m2.  That's too bulky and heavy for this application.  The only option left is an active cooling system.  I was thinking about a high pressure CO2 coolant loop that vents some of the coolant to cool the heat pipe outlet.  I don't know how much CO2 the pumps could compress versus how much would be given up to cool the heat pipes, so I don't know whether or not this solution is viable or has inordinate power and space requirements.  The idea was to constantly replenish the CO2 tank from the Martian atmosphere while venting some of the CO2 using an open loop system.

The reactor mass is what it is.  The only way to significantly reduce the core diameter is to use an exotic fuel like Am-242m, but then you're talking about designing an entirely new reactor and I'm not interested in that.  The shielding mass is substantial, but I was thinking that uranium carbide for gamma shielding and boron carbide for capture of fast neutrons would keep the shielding volume within acceptable limits.

Antius wrote:

The other option is distance.  Does the reactor have to be physically built into the vehicle?  Or could you tow it a few metres behind the vehicle?

The reactor has to be built into the vehicle for practical reasons.  The M113 is more than capable of supporting the weight of the reactor and shielding, especially on Mars.  The goal is to not end up with a M113 that weighs more than its Earth-bound counterpart, or power-to-weight will suffer.  Using two 40kW electric motors that can produce 100hp, with a vehicle weight of up to 18t should have very similar performance characteristics to its Earth-bound counterpart.

Towing the reactor behind the vehicle or hoisting the reactor on an "A" frame above the vehicle only introduces new mass, complexity, and operational problems, even if it looks attractive from a shielding mass reduction standpoint, and the reactor still requires shielding.  Maybe the solution is battery powered rovers and a separate vehicle carrying the reactor.  There again, more mass, complexity, and operational problems.  The simplest solution, but least attractive in terms of mass, is to properly shield the reactor.

Even if the reactor was towed or "A" frame mounted above the vehicle, the reactor still has to have a shadow shield.  The astronauts still have to exit the rover.  Hoisting the reactor atop the vehicle means the convoy has to concern itself with the relative elevation of vehicles in the immediate vicinity, cooling requirements don't go away, and now you have to have a power cable transfer the power back to the vehicle.

Everything works better from a space and weight efficiency standpoint if the reactor can be placed in the hull between the drive sprockets at the front of the vehicle.  If the reactor, generator, cooling pumps, CO2 tank, and electric motors are all packaged into a removable power pack, then a robot can remove it for servicing and disposal.

Offline

#31 2016-01-13 01:12:37

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

Re: Nukemobiles on Mars

SpaceNut,

A pebble bed reactor might work for a stationary plant, assuming you can make lightweight high-temperature resistant radiators that can run for years without requiring significant maintenance.  Irrespective of what reactor design is selected, the problems of shielding and waste heat management remain.  I don't think reactors that have greater waste heat rejection problems to contend with work in your favor on Mars.

Offline

#32 2016-01-13 22:23:03

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

Re: Nukemobiles on Mars

Working on locating documents as they relate to shielding...

PEBBLE BED MODULAR REACTOR (PBMR) - A POWER GENERATION LEAP INTO THE FUTURE

Modelling long-range radiation heat transfer in a pebble bed reactor

Shielding Design to Obtain Compact Marine Reactor

Application of MCBEND to Pebble Bed Modular Reactor (PBMR) Shielding Analysis

[url=http://www.janleenkloosterman.nl/reports/thesis_buijs_2012.pdf]Calculation of the Danco Factor in Pebble Bed
Reactors using Empirical Chord Length Distributions[/url]

Shielding calculations in the CUD area of the PBMR Design

Happy reading....

Offline

#33 2016-01-14 03:02:53

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

Re: Nukemobiles on Mars

SpaceNut,

After reading through the documents, there was nothing there that I saw that related to research into making the core diameter of the reactor smaller.  The pebble bed reactors are designed to produce more heat, but that's not the problem with this application.

Incidentally, I determined what the required shielding materials would weigh for humans in such close proximity to the reactor and even the shadow shield is not feasible unless the core diameter is significantly reduced.  It looks like an Americium fueled reactor would be required.  The diameter would be between four and eight inches.  If you can make the core diameter that small, then the required shielding is within the realm of feasibility.  An active cooling solution is still required, unless someone has a good solution for mounting a 40' x 40' radiator on a vehicle.

A nuclear power solution for mobile applications is a tough nut to crack.  I don't think it's possible to build an adequately shielded reactor weighing less than 2t.  Even then, standing in front of the vehicle is not recommended.

Offline

#34 2016-01-14 05:52:59

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

Re: Nukemobiles on Mars

kbd512 wrote:

SpaceNut,

After reading through the documents, there was nothing there that I saw that related to research into making the core diameter of the reactor smaller.  The pebble bed reactors are designed to produce more heat, but that's not the problem with this application.

Incidentally, I determined what the required shielding materials would weigh for humans in such close proximity to the reactor and even the shadow shield is not feasible unless the core diameter is significantly reduced.  It looks like an Americium fueled reactor would be required.  The diameter would be between four and eight inches.  If you can make the core diameter that small, then the required shielding is within the realm of feasibility.  An active cooling solution is still required, unless someone has a good solution for mounting a 40' x 40' radiator on a vehicle.

A nuclear power solution for mobile applications is a tough nut to crack.  I don't think it's possible to build an adequately shielded reactor weighing less than 2t.  Even then, standing in front of the vehicle is not recommended.

Remember, if you are planning to use atmospheric CO2 as a working fluid, then that fluid can be sacificial.  In other words, you don't need to cool it and recondense it after it has passed through the expander cycle, you just vent it into the atmosphere along with any residual heat that it contains.  So dumping wasteheat during operation need not be an issue.  You can suck in replacement CO2 at very little energy cost during the Martian night.  The amount of enthalpy change of CO2 heated from 220K to 1000K dwarfs the energy needed to collect more.

The radiator only needs to be sized for decay heat dumping.  That's about 6% of reactor shutdown power and declines to about 1% within a minute of shutdown.  A flat plate panel will radiate about 50KW of heat per square metre at 1000K.  So for every MW of reactor power you need about 1m2 of radiator panel.  I think your plan is therefore doable.  Care must be taken to ensure that the reactor trips when the CO2 store runs dry.  For extra shielding, the geometry of the vehicle can be arranged such that provisions are used to provide additional shielding.

Last edited by Antius (2016-01-14 05:57:36)

Offline

#35 2016-01-14 12:06:15

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

Re: Nukemobiles on Mars

Antius wrote:

Remember, if you are planning to use atmospheric CO2 as a working fluid, then that fluid can be sacificial.  In other words, you don't need to cool it and recondense it after it has passed through the expander cycle, you just vent it into the atmosphere along with any residual heat that it contains.  So dumping wasteheat during operation need not be an issue.  You can suck in replacement CO2 at very little energy cost during the Martian night.  The amount of enthalpy change of CO2 heated from 220K to 1000K dwarfs the energy needed to collect more.

Antius,

Increasing the core diameter, even if you're using a heat source that provides greater efficiency for electric power generation, doesn't help you decrease shielding mass required to adequately protect the crew.  The crew is less than a meter from the reactor.  The problem I'm trying to resolve is adequate shielding for humans in such close proximity to the reactor.

The cooling system is an operational issue.  A 12 meter by 12 meter radiator could be erected atop the vehicle, but it's a decidedly unappealing solution, even if it only weighs 270 kg with all the fixtures, piping, and insulation required to protect the 5083 alloy from the heat.  Using a passive cooling solution requiring that much working space around the vehicle creates a situation that could lead to damage to the cooling solution and therefore the reactor.  I consider that to be enough of a problem to rule out using a passive-only cooling solution.

Antius wrote:

The radiator only needs to be sized for decay heat dumping.  That's about 6% of reactor shutdown power and declines to about 1% within a minute of shutdown.  A flat plate panel will radiate about 50KW of heat per square metre at 1000K.  So for every MW of reactor power you need about 1m2 of radiator panel.  I think your plan is therefore doable.  Care must be taken to ensure that the reactor trips when the CO2 store runs dry.  For extra shielding, the geometry of the vehicle can be arranged such that provisions are used to provide additional shielding.

A radiator designed to handle decay heat could be mounted to the top of the vehicle and remain within its surface area.

Regarding the internal design of the rover, ideally the end caps of the reactor core would be axially aligned with the drive sprockets, which are at the front of the vehicle on a M113 / MTVL.  The reactor core and its shielding, coolant pumping system, coolant reservoir, and electric generators would be packaged together for reactor servicing and disposal.  The electric motors would be separated from the rest of the power pack to permit separate servicing of the electric motors.  A bulkhead would separate the driver's compartment pressure vessel from the power pack compartment and another bulkhead would separate the driver's compartment pressure vessel from the cargo compartment pressure vessel.

Ideally, the crew would only enter the driver's compartment for critical vehicle maneuvering or EVA so both astronauts don't have to suit up.  The crew would spend most of their time in the cargo compartment, driving the vehicle using an iPad.  That keeps the crew about 2 meters from the reactor for most of their time and puts more shielding and distance between them and the reactor.

Offline

#36 2016-01-14 12:25:16

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

Re: Nukemobiles on Mars

I made a few crude calculations using half-thickness data from this website.
http://large.stanford.edu/courses/2012/ … heodosis1/
The half-thickness for lead is given as 1cm.  This corresponds to an attenuation coefficient of 0.8, which looks about right for 1MeV photons based upon data provided in this document.
http://www.eichrom.com/PDF/gamma-ray-at … -rev-4.pdf
Neutrons tend to scatter more easily than gamma-rays.  So I am going to assume that what shields out gamma will also shield out neutrons.
A fission reaction yields about 3.5% of energy as gamma rays, so a 400kW reactor will put out 14kW in gamma rays.  A dose of 1 gray is 1J/kg of tissue.  An average human being has a mass of 50kg, a frontal area of 1m2 and a tissue depth of ~0.2m.  So a flux of 1W/m2 would deliver a dose rate of 0.01gray/second.  The unshielded flux from the core at a distance of 1m is 14,000/4π = 1114.1W/m2.  So that’s 40KGray/hour.  A dose of 0.01mGray per hour would result in accumulated dose of 20mGray per year (20mSV or 2rem per 2000hours).  So dose must be reduced by a factor 4E9 over unshielded value.  That’s 32 half thicknesses.  To provide that much shielding around the entire reactor would require almost exactly 1m3 of lead shielding.  That’s 11.34 tonnes.  With the reactor and power conversion system is included, let’s say a round 12tonnes.
If power output is 100kWe, then P/W ratio is 8.3W/kg.  That’s not good.  It’s about the same P/W ratio as a rickshaw.  It’s even worse when you factor in the weight of the rest of the vehicle.  Still, electric motors provide lots of torque at low speed, Mars has lower gravity and no air resistance and going faster than 25mph won’t be desirable over most Martian terrain anyway.  At 20km/h, a nuclear rover could circumnavigate the entire planet in 41 days of continuous driving.
If you are using stored liquid CO2 as a propellant and venting it back out into the atmosphere, the power conversion system may be up to 50% efficient.  In that case, specific power for a 20t vehicle would be 10W/kg.  That’s about the same as a fuel-cell powered car.
Weight might be reduced a little further if stored provisions can be used to shield the crew whilst the reactor is in operation.  Let's say we put the reactor in the middle of the vehicle for weight distribution purposes and use drinking water, waste water, food and stored CO2 for additional shielding between the core and crew.  When shut down, dose should not be a problem.  Doubling the distance between the reactor and crew to 2m reduces dose rate by a factor of 4 and stored provisions should be equivelent to another half-thickness.

Last edited by Antius (2016-01-14 12:34:23)

Offline

#37 2016-01-14 14:30:02

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

Re: Nukemobiles on Mars

Antius,

Using the same rough calculations, the required halving thickness of depleted uranium would be 6.4cm, but I'm going to use 7.62cm.  If my hypothetical Am242 fueled reactor has a 6" core diameter and is 12" long (supposedly using Am242 will reduce the required core diameter by 70% to achieve the same thermal output as a reactor fueled with U235, but I'm using a 50% core diameter reduction and 60% reduction in core length reduction to account for the volume of the heat pipes and any other aspects not considered), then if the reactor core was completely encased in DU (obviously not possible since heat pipe outlets are required), then the gamma shielding would weigh ~1000kg.  That takes care of gamma.

Now we have to account for the fast neutrons.  Are thermal neutrons also a problem?  Boron carbide with a DU backing?  The idea is to minimize the diameter of the DU

Could we use Iodine as a working fluid for the turbine?  That should improve specific power over the working fluid selected for SAFE-400.

Offline

#38 2016-01-14 15:24:34

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

Re: Nukemobiles on Mars

Antius wrote:

If power output is 100kWe, then P/W ratio is 8.3W/kg.  That’s not good.  It’s about the same P/W ratio as a rickshaw.  It’s even worse when you factor in the weight of the rest of the vehicle.  Still, electric motors provide lots of torque at low speed, Mars has lower gravity and no air resistance and going faster than 25mph won’t be desirable over most Martian terrain anyway.  At 20km/h, a nuclear rover could circumnavigate the entire planet in 41 days of continuous driving.

The rover should be more than capable of 40km/h.  The entire draw to this concept, for me, is a vehicle that doesn't run out of power, can store provisions for 250 days of a nominal 500 day surface stay, and doesn't have a logistics tail.  Here on Earth, the M113's and MTVL's exert low ground pressure have much better traction than comparable wheeled vehicles of equivalent weight.  Physics still applies on Mars.

The four vehicle convoy concept was designed for a crew of four astronauts.  The convoy provides habitation redundancy, provisions for the other 250 days of the 500 day surface stay, spare parts for the rovers, and scientific equipment for field geology.  Only two of the four rovers will be manned at any given time.  The other two are tele-robotically operated mobile spares that travel ahead of the manned vehicles and send video to the astronauts so they can decide whether or not something ahead looks interesting.

The idea was to do away with a heavy inflatable habitat module that anchors the astronauts to a single landing site in favor of providing habitation redundancy and mobility in the form of completely functional mobile habitats.  That's RV'ing Martian style.  The entire point to going there is field science.  You can't do that with a battery operated soda can on casters.  You need heavy duty, purpose-built off-road vehicles that are durable.

Antius wrote:

If you are using stored liquid CO2 as a propellant and venting it back out into the atmosphere, the power conversion system may be up to 50% efficient.  In that case, specific power for a 20t vehicle would be 10W/kg.  That’s about the same as a fuel-cell powered car.
Weight might be reduced a little further if stored provisions can be used to shield the crew whilst the reactor is in operation.  Let's say we put the reactor in the middle of the vehicle for weight distribution purposes and use drinking water, waste water, food and stored CO2 for additional shielding between the core and crew.  When shut down, dose should not be a problem.  Doubling the distance between the reactor and crew to 2m reduces dose rate by a factor of 4 and stored provisions should be equivelent to another half-thickness.

Putting the reactor in the middle of the vehicle is not desirable from a space efficiency or vehicle redesign standpoint.  The MTVL is about 6m long.  If you sit in the back of the habitat, you're slightly more than 5m from the reactor.  We're taking a standard MTVL hull made from 5083 aluminum, centering the reactor over the drive sprockets, and using a heavy PE water tank in the rear of the vehicle that provides a counter-weight to balance the vehicle.

Offline

#39 2016-01-14 18:58:34

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

Re: Nukemobiles on Mars

I can see that a battery storage would mean that the reactor would be cycling as it does not need to be on all the time as the power useage is much lower when not driving such as at night. This would be especially true for a reactor that has an over abundance of power coming out. Use lead acid batteries as part of the shield closest to the crew men. Use the drinking water infront of the batteries between them and the crew giving additional shielding. A waste collection tank could be between the batteries and the reactor for some more.... Each of these would change the weight and thickness of the shield for the reactor....

Offline

#41 2016-01-14 21:10:27

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

Re: Nukemobiles on Mars

SpaceNut wrote:

I can see that a battery storage would mean that the reactor would be cycling as it does not need to be on all the time as the power useage is much lower when not driving such as at night. This would be especially true for a reactor that has an over abundance of power coming out. Use lead acid batteries as part of the shield closest to the crew men. Use the drinking water infront of the batteries between them and the crew giving additional shielding. A waste collection tank could be between the batteries and the reactor for some more.... Each of these would change the weight and thickness of the shield for the reactor....

I think any significant energy storage capacity provided by lead acid batteries would add too much weight to a vehicle that already has to be very thoughtfully designed to keep the delivered payload weight to 20t.  I also think cycling a high heat reactor every day is probably bad for long term reliability.  I will say that that option should certainly be tested to determine its viability as a method to reduce crew radiation exposure.  I don't think that using lead as a shielding material is dense enough for a mobile application, but every bit of shielding helps.

Offline

#42 2016-01-15 09:42:52

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

Re: Nukemobiles on Mars

This is a fascinating paper.  Many thanks for posting it Spacenut.  I believe it allows us to refine our estimates somewhat.

The material most effective for gamma shielding at reasonable cost is tungsten.  Although osmium is slightly denser it is extremely costly.  For neutron shielding, Lithium Hydride provides the best attenuation.  The study is explicit that it does not consider the thermal stability of LiH under intense radiation fields.  One interesting conclusion of the study is that the optimum shielding configuration is a sandwich of LiH (inside) and tungsten on the outside.  This is because fast neutrons collide inelastically with tungsten and produce high energy gamma rays.  So a weight optimum shield must first screen the fast neutron flux (LiH shield), with additional gamma shielding after the neutron shielding (Tungsten).

The study concludes that for a 100kWth nuclear heat source, some 85cm of LiH and 10cm of tungsten are needed to reduce dose rates outside the shield to 11.4rad/hr (0.11gray/hour).  For a reactor of 400kWth, the doserate at the shield surface is presumably 4 times greater (0.46gray/hour).  The shield must extend around the bottom of the reactor to prevent backscatter into the vehicle, but can be thinner in this area (10-15cm LiH and 2cm tungsten).

For the 400kWth reactor considered in our study, a shadow shield covering 1/3rd of the reactor radius in the direction of the crew compartment, would weigh 7515kg, for the sandwich shield featured in the study.  The bottom (backscatter) shield would weigh 36kg for the 0.3m diameter core.  If we assume that a shield equivalent to the backscatter shield surrounds the rest of the reactor as well (i.e. the part facing away from the crew) then its mass would be 2385kg.  Total shield mass is therefore 9936kg.  But this is the shield thickness required to reduce surface dose rate down to 0.46gray/hour.  We noted at the beginning of our study that the target dose rate for humans was 0.01mGray per hour.  So dose must be reduced by a factor of 46,000.  How to achieve that?

One option is distance.  If the reactor is located at the front of the vehicle and the crew sit at the back, or vice versa.  That would put 5-10m between the crew and reactor – enough to reduce dose by a factor of 25-100 (let’s say 50).  How do we reduce dose by another factor of 920?  Human tissue has only 40% of the density of silicon (assumed as target material in the study) so in reality flux must be reduced by a factor of 370.  That’s 8.5 half-thicknesses.  How to provide that extra shielding?  If tungsten is used, then another 4.25cm must be added to the shadow shield, which would increase its mass by another 2850kg.

A better option would be to arrange the geometry of the vehicle such that the stored CO2 propellant, batteries, equipment, water, food, waste, etc. were between the reactor and the crew driving cab.

Summary: Using a composite LiH/W shield, total shielding mass would be ~10 metric tonnes.  This assumes a crew dose rate during operation of 0.01mGray/hour (1mRem/hour), a separation distance from the reactor core of 7m and additional shielding of ~3 tonnes provided by various objects and provisions between the core and driving cab.  This fits in with the original estimate of 20 tonnes total weight and P/W ratio of 10W/Kg.

Offline

#43 2016-01-15 10:04:37

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

Re: Nukemobiles on Mars

One way of reducing the shielding mass by amount 70% would be to carry the reactor on a trailer.  At the end of each drive, detach it, drive it 100m from the main vehicle and then use it to charge lead acid batteries attached to the forward end of the trailer.  The batteries provide both energy storage and residual shielding.  It would work even better if there were a convenient rock to put it behind.

Offline

#44 2016-01-15 10:46:53

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 2,712
Website

Re: Nukemobiles on Mars

A nuclear power reactor is likely always too big,  heavy,  and dangerous to put right in the vehicle with you.  But the idea of a nuclear power trailer is a good idea.  Solve cg problems when you split it into two vehicles like that.  You'll still need some shielding,  but parking 100 m away at night is a good way to reduce exposure.  Nothing at all wrong with a 100 m extension cord,  either. 

Tank tread (track-laying) design is really good for rough ground,  but I really don't think rubber is a good idea,  even if the glass transition temperature problem is solvable.  Rubber is just not tough enough to resist being cut up by the sharp-edged rocks that show up over a lot of Mars.  That's the same mistake they made with Curiosity and its aluminum wheels.  The rocks win,  they're tougher.  That job takes steel,  and it needs to be good very cold,  which carbon steel is not.

May I suggest a 300-series stainless?  Good to cryogenic temperatures,  tough but ductile,  easily workable,  easily weldable.  Not a rare expensive exotic,  either. 

GW


GW Johnson
McGregor,  Texas

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

Offline

#45 2016-01-15 12:56:46

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

Re: Nukemobiles on Mars

Antius wrote:

The material most effective for gamma shielding at reasonable cost is tungsten.  Although osmium is slightly denser it is extremely costly.  For neutron shielding, Lithium Hydride provides the best attenuation.  The study is explicit that it does not consider the thermal stability of LiH under intense radiation fields.  One interesting conclusion of the study is that the optimum shielding configuration is a sandwich of LiH (inside) and tungsten on the outside.  This is because fast neutrons collide inelastically with tungsten and produce high energy gamma rays.  So a weight optimum shield must first screen the fast neutron flux (LiH shield), with additional gamma shielding after the neutron shielding (Tungsten).

I think the most effective gamma shielding is not tungsten, it's depleted uranium.  Halving thickness for DU is 2MM - 2.8MM, depending on the exact material used.  The halving thickness for W is 3.3MM.  You can't use materials that aren't optimal for the job and get decent results.  The US produces thousands of tons of DU every year and has to invent uses for it.  I think the optimum shielding configuration is D38 - B4C - D38.

11.34g/cm3 - Pb

19.05g/cm3 - D38

19.25g/cm3 - W

Antius wrote:

The study concludes that for a 100kWth nuclear heat source, some 85cm of LiH and 10cm of tungsten are needed to reduce dose rates outside the shield to 11.4rad/hr (0.11gray/hour).  For a reactor of 400kWth, the doserate at the shield surface is presumably 4 times greater (0.46gray/hour).  The shield must extend around the bottom of the reactor to prevent backscatter into the vehicle, but can be thinner in this area (10-15cm LiH and 2cm tungsten).

I think the study avoids using the most effective materials and shielding configuration for gamma attenuation.

Neutron activation of DU has been studied extensively.  Here on Earth, we put the DU on the inside and the PE or stainless steel on the outside.

Antius wrote:

For the 400kWth reactor considered in our study, a shadow shield covering 1/3rd of the reactor radius in the direction of the crew compartment, would weigh 7515kg, for the sandwich shield featured in the study.  The bottom (backscatter) shield would weigh 36kg for the 0.3m diameter core.  If we assume that a shield equivalent to the backscatter shield surrounds the rest of the reactor as well (i.e. the part facing away from the crew) then its mass would be 2385kg.  Total shield mass is therefore 9936kg.  But this is the shield thickness required to reduce surface dose rate down to 0.46gray/hour.  We noted at the beginning of our study that the target dose rate for humans was 0.01mGray per hour.  So dose must be reduced by a factor of 46,000.  How to achieve that?

Use more effective shielding materials in a configuration that's more effective.  How much of a problem is neutron activation of DU?

We need a copy of this:

Atlas of Gamma-ray Spectra from the Inelastic Scattering of Reactor Fast Neutrons

And a complete copy of this:

New Data and Directions for Neutron Activation Analysis

This research is fairly instructive about why the high-Z material is closest to the reactor:

Shielding Analysis of Depleted Uranium Silicate Filler Concept for Spent Fuel Canister Designs

This lecture seems to suggest that secondary gamma emission same thing:

Lecture 6 - Rad. Interaction with Matter 6 - Neutron Shielding

Antius wrote:

One option is distance.  If the reactor is located at the front of the vehicle and the crew sit at the back, or vice versa.  That would put 5-10m between the crew and reactor – enough to reduce dose by a factor of 25-100 (let’s say 50).  How do we reduce dose by another factor of 920?  Human tissue has only 40% of the density of silicon (assumed as target material in the study) so in reality flux must be reduced by a factor of 370.  That’s 8.5 half-thicknesses.  How to provide that extra shielding?  If tungsten is used, then another 4.25cm must be added to the shadow shield, which would increase its mass by another 2850kg.

Unfortunately, distance is not much of an option for a mobile application.  There are simply too many complications associated with placing the reactor outside the vehicle and it still requires shielding.

Antius wrote:

A better option would be to arrange the geometry of the vehicle such that the stored CO2 propellant, batteries, equipment, water, food, waste, etc. were between the reactor and the crew driving cab.

If the core diameter can be sufficiently reduced with better nuclear fuels and the shielding configuration is optimal for the application, do we really need to redesign the interior?

Antius wrote:

Summary: Using a composite LiH/W shield, total shielding mass would be ~10 metric tonnes.  This assumes a crew dose rate during operation of 0.01mGray/hour (1mRem/hour), a separation distance from the reactor core of 7m and additional shielding of ~3 tonnes provided by various objects and provisions between the core and driving cab.  This fits in with the original estimate of 20 tonnes total weight and P/W ratio of 10W/Kg.

Using sub-optimal materials in a sub-optimal configuration for the shielding will ensure that the shielding is heavier than it needs to be.  The high-Z material should be closest to the reactor.  The secondary gamma emission from inelastic scattering is not as much of a problem as the extreme emissions coming from fissioning Am242.

I think an optimal configuration would use B4C reactor reflector to contend with thermal neutrons and use D38 - B4C -D38 shielding.

Edit: I realized this post was not clear, so I wanted to clarify some things:

1. The primary neutron source from the core is fast neutrons.  If the reactor is the conventional SAFE-400, versus an AM242 fueled design, the the thermal neutrons are attenuated by the reflector and pin cladding and don't significantly penetrate beyond the walls of the reflector.  If the reactor is fueled with AM242, then some thermal neutrons will penetrate past the very thin reflector.  The thermal neutrons from the AM242 fueled reactor can be attenuated with a neutron absorber like B4C.

2. If using a radioactive material for shielding is unattractive, for whatever other reasons that don't correlate to its effectiveness for the intended purpose, then even tungsten is a very effective gamma attenuator.  However, either material will be too heavy if it's wrapped around a low-Z material.  Secondary gamma emission is not nearly as much of a problem as gamma emissions from the core.

3. The only neutrons we're getting from the core are fast neutrons, so secondary gamma emission from neutrons with lower energies (the primary source for secondary gamma) will be even less of an issue.  I think a thick gamma attenuator in direct contact with the core, followed by a thick neutron attenuator wrapped around the gamma attenuator, and finally a thin gamma attenuator wrapped around the neutron attenuator is the way to lick this problem while keeping mass within the limits of what a light tracked vehicle can carry.

Last edited by kbd512 (2016-01-15 14:26:29)

Offline

#46 2016-01-15 15:06:11

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

Re: Nukemobiles on Mars

A fast fission in DU would yield 7MeV of gamma rays and 5MeV of fast neutrons, with an RBE of 20.  So using DU right up to the core might not be dose optimal.  But I am entering the realm of hunches and guesses.  I don't have the time to design a dose optimal nuclear rover.

The important conclusion as far as I am concerned is that a nuclear rover is a workable concept.  If what you say about the use of DU is correct, then a better design might shave a few tonnes off of the 10tonnes shielding estimate.  I do think a trailer mounted core is a better idea.  Even a long power cable will not weigh as much as all that shielding.  We could even shovel a load of Mars dirt in front of the core before we switch it on.

The original lunar mission concept for SP-100 involved digging a pit for the reactor and using the bulk surface material as a shield.  If the core is only 60cm high and 30cm wide, then digging a pit at the end of each day should be a 5 minute job.  If we dig a pit 1m deep and drop the core into it and park the vehicle 100m away, we might feasibly reduce shielding mass to almost zero.  There would still need to be enough to protect power cycle equipment and electrics, but a lot less than 10 tonnes.

Last edited by Antius (2016-01-15 15:25:12)

Offline

#47 2016-01-15 15:49:02

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

Re: Nukemobiles on Mars

Antius wrote:

A fast fission in DU would yield 7MeV of gamma rays and 5MeV of fast neutrons, with an RBE of 20.  So using DU right up to the core might not be dose optimal.  But I am entering the realm of hunches and guesses.  I don't have the time to design a dose optimal nuclear rover.

I think that's exactly what you have to do for a mobile application of nuclear power.

Antius wrote:

The important conclusion as far as I am concerned is that a nuclear rover is a workable concept.  If what you say about the use of DU is correct, then a better design might shave a few tonnes off of the 10tonnes shielding estimate.  I do think a trailer mounted core is a better idea.  Even a long power cable will not weigh as much as all that shielding.  We could even shovel a load of Mars dirt in front of the core before we switch it on.

Digging a pit each day might be a 5 minute job, but that means performing an EVA once a day or using earth moving equipment.  Then you'd need a crane, winch, or other mechanical advantage to lower the reactor into the pit.  It's hard to say which solution would have the lowest mass without knowing what's required to implement it.  It's certainly doable, but earth moving adds more complexity from an operational standpoint.

Antius wrote:

The original lunar mission concept for SP-100 involved digging a pit for the reactor and using the bulk surface material as a shield.  If the core is only 60cm high and 30cm wide, then digging a pit at the end of each day should be a 5 minute job.  If we dig a pit 1m deep and drop the core into it and park the vehicle 100m away, we might feasibly reduce shielding mass to almost zero.  There would still need to be enough to protect power cycle equipment and electrics, but a lot less than 10 tonnes.

I like that idea because it moves the reactor further away from the astronauts when in operation, but how can you guarantee that the soil in the area you're traveling through won't be so densely packed that machinery is required to dig the pit or so soft that you have to dig a much larger pit?  If large rocks or other natural obstructions are available, I'd try to use them to minimize effort expended to obtain natural shielding.

Offline

#48 2016-01-15 18:10:45

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

Re: Nukemobiles on Mars

A pit reactor might require spending the last hour or two of the day looking for the right place to stop.  Not ideal I guess.  Reminds me of some rubbish camping trips I went on in the 80's.

So far as dose is concerned, all evidence suggests that it is linearly proportional to risk.  That means that a higher dose one day isn't so much of problem if dose is lower other days.  It's the year total that counts, unless you are literally burning the whole dose budget in one day.

In terms of mass of shielding, Spacenut's paper suggests that bulk surface material would weigh an order of magnitude more than the LiH/W shield.  This shield was 1m thick and had an areal mass density of 2.7 tonne per square metre.  Coincidentally, that's about the same as bulk Martian regolith.  So burying the reactor in a 1m deep pit is more than adequate if separation distance is 100m.  If the reactor were placed behind a solid rock a couple of metres wide that would also do an adequate job.  Using distance alone without any shielding, would require about 60km of distance, which clearly is no good.

Offline

#49 2016-01-15 22:41:32

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

Re: Nukemobiles on Mars

http://energy.gov/ne/nuclear-reactor-te … r-reactors

Small Modular Reactors (SMRs) are nuclear power plants that are smaller in size (300 MWe or less) than current generation base load plants (1,000 MWe or higher). These smaller, compact designs are factory-fabricated reactors that can be transported by truck or rail to a nuclear power site.

http://www.world-nuclear.org/info/Nucle … -Reactors/

http://www.greentechmedia.com/articles/ … evelopment

http://www.westinghousenuclear.com/New- … ar-Reactor

So maybe small is not small enough lets go with micro as that seems to be sub 1 Mwe size which would be more than plenty for a RV rover...

Toshiba Builds 100x Smaller Micro Nuclear Reactor

Toshiba has developed a new class of micro size Nuclear Reactors that is designed to power individual apartment buildings or city blocks. The new reactor, which is only 20 feet by 6 feet, could change everything for small remote communities, small businesses or even a group of neighbors who are fed up with the power companies and want more control over their energy needs.
The 200 kilowatt Toshiba designed reactor is engineered to be fail-safe and totally automatic and will not overheat. Unlike traditional nuclear reactors the new micro reactor uses no control rods to initiate the reaction. The new revolutionary technology uses reservoirs of liquid lithium-6, an isotope that is effective at absorbing neutrons. The Lithium-6 reservoirs are connected to a vertical tube that fits into the reactor core. The whole whole process is self sustaining and can last for up to 40 years, producing electricity for only 5 cents per kilowatt hour, about half the cost of grid energy.

Toshiba expects to install the first reactor in Japan in 2008 and to begin marketing the new system in Europe and America in 2009.

Offline

Board footer

Powered by FluxBB