Is Pu-238 a Viable Alternative to Pu-239 for Nuclear Energy?

[truhaht]

I know what fission is and what fusion is. Who needs em? I figure that most of the energy content of particles emitting from naturally radioactive materials is lost to distant locales. Am I right? If you took a sizable chunk of radioactive mineral, like uranium, whether refined or not, whether enriched or not, if that chunk were encased in a layer of lead then wouldn't it get warm after a bit? And wouldn't it stay warm for nearly forever? Please: could I use an arrangement like that to heat my cabin?

 

[cesiumfrog]

Sure, and you can get electricity too (like deep space probes).

But what's wrong with making even more efficient use of these minerals, as a reactor does?

 

[mgb_phys]

The energy output form uranium from natural alpha radioactivity is tiny ( a couple of miCi/kg) Even for Plutonium you only get 10W/kg from spontaneous fission.

 

[truhaht]

Even for Plutonium you only get 10W/kg from spontaneous fission.

10W/kg is humble alright, but it sounds like a start -- IF it can be contained/harnessed as described. You're referring to refined plutonium, yes? (not the ore). I can't figure out what MiCi stands for. You said "alpha" and I think you mean electrons, right? Is there more for the lead to absorb than just that? Aren't there also photon emissions? How about beta radioactivity? Just the 10W?? hmmm..

 

[mgb_phys]

Uranium on it's own emits Alpha particles - the good news is that they aren't very dangerous, they can be stopped by a sheet of paper, the bad news is that they don't have a lot of energy.
Natural uranium ore emits about 12million alpha particles/kg/s (the milliCurie is an old unit) each particle carries around 4Mev or 6x10^-13 Joules of energy
So a total of 7.6x10^-6 Joules/kg/s or 7.5Watts from 1000tons!
Thats the reason for Rutherford's famous statement that getting energy from radiation was moonshine.

The energy in reactors come from the much more energetic fission (splititng) of atoms, For natural uranium the rate of splitting is very low, only U235 splits an it makes up only 0.2% of natural uranium. Thats why 'weapons grade' uranium needs to be enriched to contain much more U235, or you can also use plutonium it has a higher rate of splitting - but that's trickier to get hold of.
You can increase the rate of splitting by putting a bigger lump together (the critical mass) or adding another material (a moderator) to improve the efficency - but then you have built a reactor!

There is a kind of reactor called a pebble bed which is almost what you suggest. Small beads of reactor fuel in a big pile (inside a container!) that simply get hot enough to boil water but not hot enough to meltdown

 

[vanesch]

Moved to engineering (it is about a practical application, not a scientific question of nuclear physics).

Just some notes: as to plutonium, there's no ore of it, it is man-made in nuclear reactors.

The problem with heating something with nuclear reactions is that most of the time, by the time it gets hot, the radiation is already lethal for a long time. So you need shielding, radiation protection and all that. It was the joke of the cold fusion of Pons and Fleishmann: if they really had seen some heat in their non-shielded apparatus, they wouldn't have lived to tell the world! (after that, they "invented" the radiationless fusion, but ok...) In a nuclear power plant inside the reactor, the radiation levels are so high that you get a lethal dose in less than a millisecond.

While it is true that alpha radiation itself is harmless outside of the body (very short range of the particles), most alpha emitting also goes with gamma emissions.

 

[Vanadium 50]

Just some notes: as to plutonium, there's no ore of it, it is man-made in nuclear reactors.

True, but it's also true that there is a trace amount in uranium ore. It's produced by the same processes as in reactors, just on a much, much slower timetable.

 

[truhaht]

Thank you for satisfying my curiosity

 

[signerror]

There are things like what you describe, harnessing energy directly from radioisotopes; these are used in exotic places where refueling is not an option. However, these are rare and very expensive: isotopes radioactive enough to have meaningful power output must have very short half-lives, so none of these occur naturally on earth. (Most isotopes are created in astrophysical nuclear reactions, either in stars or (for the heavier ones) in supernovae. The nuclei on Earth come from these primordial sources and are billions of years old; so the only significant radioisotopes here are those with billion-year half lives - Th-232, U-235, U-238, and K-40, and their daughter products.) Thus only synthetic radioisotopes are usable, and these can only feasibly be created in nuclear reactors (or perhaps particle accelerator neutron sources). So radioisotope power is really just a way of storing energy created in nuclear reactors.

One way is just like what you describe - collecting the decay heat of radiation, perhaps using it to generate electricity in a heat engine. Radioisotope thermoelectric generators (RTGs) are used in some exotic applications where refueling is not an option - pacemakers (long-lived surgical implants), remote naviagation beacons (common in the former USSR), and space probes to the outer solar system (too far from the sun for solar panels).

Wikipedia: Radioisotope thermoelectric generators

Plutonium-238 glowing incandescently from its own decay heat:
http://img340.imageshack.us/img340/7905/773pxradioisotopethermoxl8.jpg

RTG-powered pacemaker with plutonium:

2.5 Ci * http://www.nndc.bnl.gov/chart/reCenter.jsp?z=94&n=144 = 680 milliwatts

Oak Ridge: Plutonium Powered Pacemaker (1974)

Another method is to use beta emitters (nuclei which emit high-energy electrons when they decay) to excite phosphors (chemicals which emit light when excited by energetic electrons or photons). This a light that never goes out. Tritium (hydrogen-3) is common today in battery-less emergency signs.

Wikipedia: Self-powered lighting

http://epa.gov/radtown/exit-signs.html

Tritium in a phosphor-lined vial on a keychain: $29.00
http://www.unitednuclear.com/trasergreen300.jpg

 

[Morbius]

I know what fission is and what fusion is. Who needs em? I figure that most of the energy content of particles emitting from naturally radioactive materials is lost to distant locales. Am I right?

truhaht,

I'm afraid you are wrong. Most radioactive decay reactions only give you a few MeV [ million electron
volts ] of energy.

The fission reaction gives you about 200 MeV of energy from a nucleus that is about 200 amu in
mass - so you get about 1 MeV / amu of energy per unit mass of fuel.

D-T fusion gives you 17.6 MeV of energy from a reactants that have a mass of 5 amu - so you
get about 3 MeV / amu of energy per unit mass of fuel for D-T fusion.

You get a LOT more energy from fission and fusion.

Dr. Gregory Greenman
Physicist

 

[Naty1]

And fission and fusion converts only a tiny proportion of material to energy...1 or 2% in fission. So a lot of "debris" is left...
I like the concept of harnessing the cosmological constant inherent in every piece of space...it's called dark energy or negative pressure...antigravity...but that's not likely in our life times.

 

[mheslep]

And fission and fusion converts only a tiny proportion of material to energy...1 or 2% in fission. So a lot of "debris" is left...

Careful there. No 'material' nuclear particles are converted to energy in fission or fusion reactions. The total binding energy of the nucleus/nuclei changes in those nuclear reactions; that binding energy has an equivalent relativistic mass. There's probably a PF sticky FAQ on this somewhere that states the case more completely than I am able.

 

[vanesch]

Actually, I wonder, for thermo-electric generators powered by Pu-238, does one do an isotopic separation on the plutonium to get pure Pu-238 ?

 

[signerror]

Good question. If Pu-238 is produced from Np-237(n,gamma) (Np-238 beta decays), and it seems that Pu-238 has a much higher (n,gamma) cross section than Np-237 in the thermal spectrum (looking it up on NNDC), then it would make sense that "Pu-238" created in light-water thermal reactors would actually be mostly Pu-239. But I've never heard of hot-cell isotopic separation - uranium enrichment is of course very low-radiation stuff. IIRC Kirk Sorensen claimed that it's next to impossible, in the context of proliferation and the thorium fuel cycle, where short-lived U-232 contaminates fissile U-233.

 

[vanesch]

Good question. If Pu-238 is produced from Np-237(n,gamma) (Np-238 beta decays), and it seems that Pu-238 has a much higher (n,gamma) cross section than Np-237 in the thermal spectrum (looking it up on NNDC), then it would make sense that "Pu-238" created in light-water thermal reactors would actually be mostly Pu-239.

Ah, that's it. First, Np-237 is extracted from nuclear spend fuel, and then separately re-irradiated in a neutron flux.
You can work with low production fractions, and then the production rate of Pu-238 (proportional to the density of Np-237) is larger than the production rate of Pu-239 (proportional to the density of Pu-238), like they do in production reactors for Pu-239 (with U-238 this time). If you only convert, say, 1% of the Np-237 into Pu-238 before separation, you get pretty pure Pu-238.

 

[truhaht]

Ah, that's it. First, Np-237 is extracted from nuclear spend fuel, and then separately re-irradiated in a neutron flux.
...you get pretty pure Pu-238.

And now our enemies have the recipe they've longed for? :uhh:

 

[signerror]

Ah, that's it. First, Np-237 is extracted from nuclear spend fuel, and then separately re-irradiated in a neutron flux.
You can work with low production fractions, and then the production rate of Pu-238 (proportional to the density of Np-237) is larger than the production rate of Pu-239 (proportional to the density of Pu-238), like they do in production reactors for Pu-239 (with U-238 this time). If you only convert, say, 1% of the Np-237 into Pu-238 before separation, you get pretty pure Pu-238.

Oh, so obvious!

And now our enemies have the recipe they've longed for?

Relax. Everyone knows how make plutonium - it's no use if you don't already have a working nuclear reactor (no other source of neutrons is powerful enough). Also, a subtle point: Pu-239 is the fissile isotope usable in weapons, whereas Pu-238 is a non-fissile isotope used in RTGs as a heat source.

 

[Astronuc]

Actually, I wonder, for thermo-electric generators powered by Pu-238, does one do an isotopic separation on the plutonium to get pure Pu-238 ?

http://consolidationeis.doe.gov/background.html

The nuclear infrastructure required to produce RPS is comprised of three major components: (1) The production of Pu-238; (2) the purification and encapsulation of Pu-238 into a fuel form; and (3) the assembly, testing, and delivery of the RPS to the Federal users. The three major components of the existing infrastructure, including their current status, are briefly described below:

Production of Pu-238: The Pu-238 production process consists of the fabrication of neptunium-237 (Np-237) targets, irradiation of the targets in a suitable irradiation facility, and the recovery of Pu-238 from the irradiated targets through chemical processing. In the past, Pu-238 was produced at DOE's Savannah River Site (SRS), using reactors that are no longer operating. After SRS stopped producing Pu-238, DOE satisfied its Pu-238 requirement by using DOE's available inventory in storage at LANL. This inventory was augmented by Pu-238 purchased from Russia for use in space missions. DOE analyzed the need for reestablishment of Pu-238 production capability in the Programmatic Environmental Impact Statement for Accomplishing Expanded Civilian Nuclear Energy> Research and Development and Isotope Production Missions in the United States, Including the Role of the Fast Flux Test Facility (NI PEIS) (DOE/EIS-0310), issued in December 2000. On the basis of the analysis in the NI PEIS, DOE issued a Record of Decision (ROD) (66 FR 7877, January 26, 2001) to reestablish Pu-238 production capability at ORNL using the Radiochemical Engineering Development Center (REDC) for the fabrication of targets and extraction of Pu-238 from the irradiated targets. The Advanced Test Reactor (ATR) located at the Idaho National Engineering and Environmental Laboratory (also referred to as the Idaho Site), supplemented by the High Flux Isotope Reactor (HFIR) located at ORNL, would be used in the irradiation of targets, and the irradiated targets would be returned to REDC/ORNL for extraction of Pu-238. This decision, however, has not yet been implemented and the DOE has expended no resources to establish the Pu-238 production at the Oak Ridge Site.

Np-237, the feed material for fabrication of targets for Pu-238 production, had been stored at the SRS where Pu-238 was historically produced. In the NI PEIS ROD, DOE decided to transfer this material to ORNL since the Pu-238 capability was planned to be reestablished there. However, Np-237 is a special nuclear material and, after the events of September 11, 2001, it required a higher level of security than could be reasonably provided at REDC/ORNL. Therefore, DOE amended the ROD for the NI PEIS to change the storage location for Np-237 from ORNL to the Idaho Site (69 FR 50180, August 13, 2004). Np-237, in the form of an oxide, will be shipped from SRS to the Idaho Site beginning in FY 2005 (and ending in FY 2006) for storage until needed for Pu-238 production.

Purification and Encapsulation of Pu-238: Pu-238 is purified and encapsulated in a metal capsule and welded closed. These fuel capsules are used as a heat source in the RPS. The purification and encapsulation work is currently conducted within the Technical Area-55 (TA-55) complex at LANL. The finished Pu-238 fuel capsules are shipped from LANL for assembly of the RPS at the Idaho Site.

. . . .

 

[Morbius]

And now our enemies have the recipe they've longed for? :uhh:

truhaht,

It's a pretty OBVIOUS thing to do. Additionally, Pu-238 production doesn't give one the recipe for
a bomb. One needs fissile Pu-239 for a nuclear weapon - not radioactive Pu-238.

In fact, Pu-238 is something you want to LEAVE OUT OF a nuclear weapon - the Pu-238 doesn't
help in the fission reaction - and its radioactivity and heat just complicate matters.

Dr. Gregory Greenman
Physicist

 

[vanesch]

In fact, Pu-238 is something you want to LEAVE OUT OF a nuclear weapon - the Pu-238 doesn't
help in the fission reaction - and its radioactivity and heat just complicate matters.

I understand why one wants to leave out Pu-238, but nevertheless, the k_inf is more than 2.5 in its own fission spectrum. It is not fissile in a thermal spectrum, but it is in a fast spectrum.

 

[Morbius]

It is not fissile in a thermal spectrum, but it is in a fast spectrum.

vanesch,

Actually, Pu-238 IS fissile. At room temperature 0.025 eV;
Pu-238 has a (n,fission) microscopic cross section of about 15 barns.

That is in comparison to about 700 - 800 barns for Pu-239.

So Pu-238 is fissile - but it isn't going to help much - and the
negatives are overwhelming.

A nuclide can't be not "fissile" in a thermal spectrum but fissile in
a fast spectrum- by definition.

The word "fissile" means that the nuclide will fission with thermal neutrons.

If a nuclide like U-238 that will only fission with neutrons of energy above
a certain threshold - like with fast neutrons - those nuclides are termed "fissionable".

U-238 is NOT "fissile" - but it is "fissionable".

Dr. Gregory Greenman
Physicist

 

[vanesch]

A nuclide can't be not "fissile" in a thermal spectrum but fissile in
a fast spectrum- by definition.

The word "fissile" means that the nuclide will fission with thermal neutrons.

If a nuclide like U-238 that will only fission with neutrons of energy above
a certain threshold - like with fast neutrons - those nuclides are termed "fissionable".

U-238 is NOT "fissile" - but it is "fissionable".

Dr. Gregory Greenman
Physicist

You are right concerning terminology. I should have said fissionable, not fissile.

I just wanted to point out that a big enough lump of Pu-238 will undergo just as well a fast power excursion (if you see what I mean) as a big enough lump of Pu-239, as its k_inf value is far above 1. (which is not the case for U-238 btw).

 

[Morbius]

You are right concerning terminology. I should have said fissionable, not fissile.

I just wanted to point out that a big enough lump of Pu-238 will undergo just as well a fast power excursion (if you see what I mean) as a big enough lump of Pu-239, as its k_inf value is far above 1. (which is not the case for U-238 btw).

vanesch,

For high energy neutrons, the fission cross-sections for Pu-238 and Pu-239 are quite comparable.

The problem is that you are not going to be able to get the Pu-238 to the same densities that one
can with Pu-239.

The problem with looking at just k-infinity is that infinite sized devices are really tough to transport
to the target.

Dr. Gregory Greenman
Physicist