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PostPosted: Nov 20, 2014 10:01 am 
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High end space solar panels system cost around $400/Watt-peak last time I checked. So over $1000/Watt average power delivered.

Pu238 puts out 0.5 Watt/gram, if you get 10% efficiency you'd need 20 grams per Watt average power. So looks to me you can afford to pay quite a bit for the Pu238, tens of $ per gram and it would compete. The canister and shielding should not cost much but a tiny high end free piston stirling system might hurt a bit.

Pu238 is expensive because of the tiny amounts currently demanded. Maintaining a production infrastructure for a kg of material is always going to cost a lot per gram.

Still it is true that Sr-90 is a good alternative especially for terrestrial applications.

There are many remote terrestrial and marine applications where either sunlight isn't reliably available (like a south pole research station in winter) or the environment is too nasty for solar panels (like heavy marine environment).


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PostPosted: Nov 20, 2014 1:08 pm 
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Pu238 is expensive because of the tiny amounts currently demanded. Maintaining a production infrastructure for a kg of material is always going to cost a lot per gram.


If thorium becomes commonly used in power reactors, we will have a lot more of Pu238. But the demand will stay very low.

Solar panels for spatial probes are fine if we stay in space and not beyond the orbit of Jupiter. So there is a lot of places where we can advantageously use RTGs.

The last big robot Mars Science Laboratory uses RTGs and can work day and night, summer and winter contrary to Opportunity if I don't make mistake. The sunshine is low on the surface of Venus due to the clouds and the night lasts 243 terrestrial days (to be fair it would be difficult to use anything on the surface on Venus). There is the surfaces of Mercury and of asteroids where we can use solar pannels, but even in these places we can question the use of solar panels. If, by misfortune, the probe lands in a place where there is not enough sunshine, like Philae did, we can loose the probe. Spatial missions are very difficult so if we can decrease the difficulties by using RTGs, it may worth the thinking and the price. There are also some places on Mercury and the Moon where there may be ice but these interesting places are always in the shadows.


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PostPosted: Nov 21, 2014 7:19 pm 
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fab wrote:
If thorium becomes commonly used in power reactors, we will have a lot more of Pu238. But the demand will stay very low.


I disagree. To me that sounds like claiming 640Kbytes of RAM is enough for any computer.

I remember a few years ago that titanium got cheap enough that people weren't using it in just airplanes any more. People were thinking of using it as a building material. I don't recall if titanium girders were actually used to make office buildings since the price jumped back up again rather quickly. Point is that if something gets cheap enough then people will find new uses for it.

If Pu238 gets cheap enough then it might replace solar panels in even communication satellites. If it gets cheap enough then I can see it being used in all kinds of places. One example I can think of is fishing boats. They are always in cold water so heat is always needed. Put a thermocouple on it and it can provide backup power for communication and navigation. It's going to be heavy so put it in the keel to help keep the boat upright.

I realize that putting radioactive material in a fishing boat right now would be a legal and political nightmare but the same used to also be true for depleted uranium.

The reason we use Pu238 RTGs in spacecraft is because for the given size and mass its the only thing we have. If Pu238 gets cheaper then it can compete with other technologies where things like solar panels are now being used. If it gets much much cheaper then I'll be heating my house with a Pu238 powered furnace.

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PostPosted: Nov 21, 2014 9:33 pm 
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We already had problems with RTGs in the past. In USSR there have been Sr90 RTGs used to power lights for airports and such. After the collapse of USSR these RTGs have been taken by mens who didn't know what it was, they were badly irradiated. Having Pu238 spilled in populated areas will be very bad, at least for public relation. We must also protect these sources against terrorism, if they are everywhere that seems impossible.

In this forum we like to speak about safety of new reactors and I have seen here scenarios way beyond Fukushima and Chernobyl. After Fukushima the designers have to imagine reactors capable of withstanding incredible scenarios to convince the public that Nuclear Power is safe. It seems strange to me to try to make the reactors resist to crazy scenarios whereas there are RTGs everywhere.

As Cyril said we can surely increase the use of RTGs in some domains where the sources are protected or difficultly accessible ( big ships, army stations, scientific stations, etc...). But putting RTGs everywhere is not possible I think. It will not be economical also, heating the houses with cogeneration heat or with electricity produced by nuclear reactors built in series will be cheaper I think.


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PostPosted: Nov 22, 2014 12:18 am 
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fab wrote:
We must also protect these sources against terrorism, if they are everywhere that seems impossible.


How is Pu238 a terrorism concern? The element and a good number of its decay products are alpha emitters with relatively long half-lives, which makes it worthless for a dirty bomb. Pu238 is not fissile and converting it to something fissile is just as easy as taking uranium or thorium out of the dirt and turn that into something fissile, so its worthless to make a nuclear weapon. Its about as biologically hazardous as any of a number of more common heavy metals, so not much value as a biological weapon. It's quite possible I'm missing a very important detail but I'm not seeing a terrorism aspect to be concerned about.

fab wrote:
But putting RTGs everywhere is not possible I think. It will not be economical also, heating the houses with cogeneration heat or with electricity produced by nuclear reactors built in series will be cheaper I think.


I agree that putting RTGs everywhere is not economically feasible. I'm just speculating that if we were able to produce Pu238 cheaply enough there are all kinds of places where it would be useful as an energy source. I'm just thinking that we could have all kinds of ways we could make Pu238 work for us. I'd have a car where a dead battery never keeps it from starting, and I'd never have to scrape ice off the windshield. I could have a portable camping lantern that could provide heat an light for years and never have to change the batteries.

I'm just saying that the cheaper something gets the more uses we tend to find for it. It's basic economics. If RTGs can be made cheaper than lead-acid batteries then people are going to put RTGs in their cars instead. It gets more complex than that real quick though because a lead acid battery lasts about five years before it won't hold a charge, and it needs constant recharging from an alternator. A RTG would not need an alternator, it can provide electric power for decades, and the waste heat could heat the vehicle cabin. Wikipedia tells me that the radiation shielding needed to keep a Pu238 RTG safe is 2.5mm of lead, sounds easy enough to do.

Seems to me that a Pu238 RTG is no more dangerous than a lead acid battery and we put those in cars, so if we can make them cheap enough then why would I not use one everywhere I could?

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PostPosted: Nov 22, 2014 8:45 am 
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AIUI 238Pu is actually fissile.


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PostPosted: Nov 22, 2014 10:52 am 
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E Ireland wrote:
AIUI 238Pu is actually fissile.

Yes, well fissionable. Only with fast neutrons. Still cant make a bomb because of heat generation.


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PostPosted: Nov 22, 2014 1:57 pm 
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There have been a lot of accidents in the past with radioactive sources loosed or mis-handled and lot of people irradiated, even in controled environments with profesionnals like nuclear medecine. I am strongly against the spread of radioactive material everywhere but maybe I am wrong.

1 kg of Pu238 has an activity of 6.33 * 10^14 Bq and a thermal power of 567 W

A single washing machine of 3000 W need a RTG with around 26 kg of Pu238 (if we take a generous efficiency of 20 % for the RTG).

If these 26 kg are spilled in the environment that is around 165 *10^14 Bq released in the envronment (16.5 PBq). That is greater than the amount of radioactivity due to Cesium 137 released in the atmosphere by Fukushima (around 15 PBq) which has lead to the permanent evacuation of the area within 20km from the power plant (we can discuss about the pertinence of these evacuations but the psychological effects for the population are here).

Pu238 has an half-life 3 times greater than Cs137, and contrary to Cesium which has a biological half-life of less than one year, it seems that Plutonium stays in your body for your entire life (if I am not mistaken also). The alpha particule of the Pu238 is 11 times more energetic than the beta of the Cs137.

Of course Plutonium seems less mobile than Cesium and we can wonder how to largely spread it into the environment but I don't want to take the risk to have terrorists releasing large amounts of Pu238 in a big city like London or New-York.

It is the same story for Sr90.

But maybe I am mistaken, I see in the literature that dirty bombs or other radiological terrorist attempts are not big deal, so I surely don't understand something.


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PostPosted: Nov 23, 2014 12:33 am 
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fab wrote:
There have been a lot of accidents in the past with radioactive sources loosed or mis-handled and lot of people irradiated, even in controled environments with profesionnals like nuclear medecine. I am strongly against the spread of radioactive material everywhere but maybe I am wrong.


Not all radioactive material is equal. I can recall instances of Cobalt-60 not being properly handled and resulted in people dying or getting ill. U-238 is considered safe enough to use as counterweights in aircraft and boats. The use in aircraft is not as prevalent as it was since it was found that under the right conditions it can burn. The health hazard Pu-238 poses is somewhere in between the two, leaning more towards the U-238 than the Co-60.

fab wrote:
A single washing machine of 3000 W need a RTG with around 26 kg of Pu238 (if we take a generous efficiency of 20 % for the RTG).

If these 26 kg are spilled in the environment that is around 165 *10^14 Bq released in the envronment (16.5 PBq). That is greater than the amount of radioactivity due to Cesium 137 released in the atmosphere by Fukushima (around 15 PBq) which has lead to the permanent evacuation of the area within 20km from the power plant (we can discuss about the pertinence of these evacuations but the psychological effects for the population are here).


I was not serious about using a Pu-238 furnace to heat my house. For giggles I did some math to figure out just how much Pu-238 I'd need to keep my house warm and if I did my math right it'd take 35kg. (If someone wants to check my math I'll note that my current furnace is rated at 60,000 BTU/hr.) That's a lot of plutonium. I do not believe we'd ever get to a point where the price of Pu-238 would ever get low enough to make that practical.

Where I was not joking is the use of Pu-238 in something like emergency power sources for ships at sea. That's a place where longevity, reliability, and portability get to where such a thing might be practical. A power source with one kg of Pu-238 would produce about 500 watts of heat and 50 watts of electricity. For a person out at sea with a busted engine on their boat that would be a life saver to get on a radio to call for help and keep from freezing until help arrives.

fab wrote:
It is the same story for Sr90.


It is not the same story as for Sr-90. Sr-90 is a beta emitter, is readily dissolved in water and taken up by plant life, and with it's shorter half-life produces more radiation per mass. Pu-238 is an alpha emitter, tends to stay where it lands, and is much less radioactive than many other fission products.

fab wrote:
But maybe I am mistaken, I see in the literature that dirty bombs or other radiological terrorist attempts are not big deal, so I surely don't understand something.


If we are going to play the terrorist card on Pu-238 then we'd have to ban all kinds of stuff. What of mercury? How much of a biological hazard would there be if the same mass of mercury were spread over a populated area by en explosive? I could think of all kinds of dangerous materials that are readily purchased that could be used in creating panic and mayhem.

I'll steal a line from one of Mr. Sorensen's presentations, when you fill up your car with gasoline do you think of how much napalm you could make from that to burn down houses? No, you think about how many miles that can carry you. We put mercury in our light bulbs. Our batteries contain lead, lithium, cadmium, and all kinds of caustic, acidic, and heavy metal containing chemicals. Gasoline can be used to make napalm. Diesel fuel and fertilizer is what was used to make a crater in Oklahoma City. Despite the many ways these current power sources could be misused they are easy obtained. I believe the current laws on radioactive materials is way out of proportion to the relative harm they can do.

If we can make our own Pu-238 in the USA in sufficient quantity then I can see all kinds of applications for it beyond powering probes into deep space. Materials from Flibe Energy tell me that a 1000 MW LFTR can produce 15kg of Pu-238 per year. The internet tells me that the world consumes 15 terawatts of power. If that power was produced by LFTRs then we'd have an annual production of 225000 kg of Pu-238, which is really close to the world annual lithium production.

I'm thinking that if we're going to bring up the terrorism aspects of Pu-238 then I could come up with a lot of more dangerous stuff that are already on the market. Taken too far and this fear over terrorism would mean that little Ralphie couldn't even get his Red Ryder BB gun for Christmas.

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PostPosted: Nov 23, 2014 6:02 am 
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Slightly off topic:

Quote:
If we are going to play the terrorist card on Pu-238 then we'd have to ban all kinds of stuff. What of mercury? How much of a biological hazard would there be if the same mass of mercury were spread over a populated area by en explosive? I could think of all kinds of dangerous materials that are readily purchased that could be used in creating panic and mayhem.


Is there an easy way to detect these other terrorist possibilities? It seems to me that a radiological dirty bomb might be easier to clean up after because you can use a geiger counter or similar device to find the stuff.


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PostPosted: Nov 23, 2014 10:26 am 
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leaning more towards the U-238 than the Co-60


Pu238 has an activity of 6.3*10^14 Bq/kg whereas U238 is at 1.2*10^7 Bq/kg and Co60 at 4,4*10^16 Bq/kg.

So there is a factor of around 70 between Pu238 and Co60 and a factor of around 50 000 000 between Pu238 and U238.
I think that we can't compare Pu238 and U238 even if they are alpha emitters. Moreover if we loose the cooling of a piece of Pu238 the cladding may be damaged if the piece of PuO2 is sufficiently big, this requires special attention compare to U238.

Quote:
It is not the same story as for Sr-90.


What I wanted to say is that the quantities of radioactivity involved are very high whatever we use Pu238 or Sr90. The lost of only one big RTG is in the same order of magnitude than the long term radioactivity involved in major nuclear disasters.

Quote:
I'm thinking that if we're going to bring up the terrorism aspects of Pu-238 then I could come up with a lot of more dangerous stuff that are already on the market.


But the perception of the public about nuclear is irrationnal, we must remember it. If the public perception and the safety autorities were rational, the entire western world would have been already powered by simplified water reactors at a cost of 1/10 of the cost of actual new reactors, and still be the safest form of power by far. Terrorism plays with fear and the public is afraid of radiation, the public is not afraid of mercury or lead (otherwise they should have closed all the coal plants) not afraid by dust particules, not too afraid of conventionnal bombs in stadiums, not afraid by the misuse of chemical products, not afraid of climate change, not afraid of chemical polution, not afraid of car accidents, not afraid to smoke, but they are afraid of radiations and the terrorists know it.

An other radiological disaster could be the end of the nuclear industry.
I know that Pu238 is an alpha emitter with a low mobility but personaly I don't want to see it widely spread unless the perception of the public about nuclear changes. Radioactive materials have been lost in the past a lot of times so I think that Pu238 requires special attention. Maybe I overestimated the risks, maybe I fall myself in irrational fear, for now I think the world is not ready for common use of Pu238.


Last edited by fab on Nov 23, 2014 11:06 am, edited 2 times in total.

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PostPosted: Nov 23, 2014 10:56 am 
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It seems to me that a radiological dirty bomb might be easier to clean up after because you can use a geiger counter or similar device to find the stuff.


It depends of the area of contamination. At Chernobyl for example the area is simply to large, it is too costly to remove all the Cesium (in fact it seems that the area is habitable so it is useless to remove the Cesium but it is an other subject). But we can ask ourselves how the terrorists can spread radioactives materials in such large areas. I am not smarter than terrorists but I prefer to not speak about this subject on this forum.


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PostPosted: Nov 23, 2014 12:19 pm 
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Use of strontium 90
For solubility problem, use insoluble salts like fluoride.
For land use, bury it in a suitable concrete.
For sea use, use a floating power unit towed by a cable.
It can be widely used for remote towers or relay stations too.


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PostPosted: Nov 23, 2014 1:46 pm 
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fab wrote:
Pu238 has an activity of 6.3*10^14 Bq/kg whereas U238 is at 1.2*10^7 Bq/kg and Co60 at 4,4*10^16 Bq/kg.


You are comparing alpha, beta, and gamma radiation as if they are equivalent, they are not. An alpha particle can be stopped by a few inches of air, or the dead layer of skin on our bodies. Beta particles can be stopped with a thin sheet of metal. Gammas require very heavy shielding to prevent harm. A loss of shielding on a sample of Co-60 could, and has, result in many people becoming ill and possibly die. Loss of shielding on a sample of Pu-238 or U-238 means it's safe from a few feet, perhaps inches, and you certainly don't want to eat it.

Pu-238 should be handled with gloves, goggles, and mask, to prevent it entering the body. It would be much like how any other heavy metal, like lead, mercury, cadmium, or lithium should be handled. Even if someone were to take some Pu-238 and spread it around would anyone even notice? The alpha particles it emits as it decays are so easily shielded against that to pick it up on a geiger counter one one have to be within inches to detect it. Spread far and wide the radiation would be difficult to pick out from the background gammas.

To me it seems that Pu-238 is an almost magical substance. It can produce a lifetime of heat and electricity in a very small package. It's alpha emissions are so weak that just the casing needed to harness the heat is enough to protect people from the radiation. To poison someone with it would require busting it up into small pieces and practically spooning it into their mouth, which makes it no different than other materials we use to power the machines we use.

fab wrote:
Moreover if we loose the cooling of a piece of Pu238 the cladding may be damaged if the piece of PuO2 is sufficiently big, this requires special attention compare to U238.


Now I regret even joking about heating my house with a Pu-238 furnace. Yes, a sufficient quantity of any dangerous substance requires special handling. Small amounts of lithium, cadmium, and mercury are of such little concern that people discard of batteries containing these substances in their household waste. If large quantities are such a concern then treat them carefully.

I'm thinking that if we could make Pu-238 cheap enough, and I believe we can, that we could put it in places where we might normally use other power sources. In my previous example of a car battery that would never run dead I do not propose the Pu-238 unit that replaced the lead-acid batter should be treated with complete disregard. I believe we can treat them with the same care and not pose a hazard to public health. The Pu-238 from the battery, just like the lead we use now, should be taken out and recycled or disposed of properly. Handle it with gloves, goggles, mask, and great care.

It would require production of Pu-238 of a much greater scale than we have now before we can consider using it to keep our cars starting in the winter, I'm just having fun here. But I don't think it'd be too much to think we could use it to power devices in some extreme environments on Earth, as well as in space, in the not too distant future.

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PostPosted: Nov 23, 2014 10:09 pm 
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fab wrote:
Quote:
It seems to me that a radiological dirty bomb might be easier to clean up after because you can use a geiger counter or similar device to find the stuff.


It depends of the area of contamination. At Chernobyl for example the area is simply to large, it is too costly to remove all the Cesium (in fact it seems that the area is habitable so it is useless to remove the Cesium but it is an other subject). But we can ask ourselves how the terrorists can spread radioactives materials in such large areas. I am not smarter than terrorists but I prefer to not speak about this subject on this forum.
Removing cesium is quite simple. Plant industrial hemp or mustard. Both do a good job of bioaccumulating cesium. Harvest and anaerobically digest to reduce the contaminated volume/mass substantially. If they had started that around Chernobyl, the places treated would be effectively cesium free by now (actually 10 to 15 years ago).

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