Plutonium-238: Use, Origin and Properties
We hear a lot about the radioisotope plutonium-238 these days, due to ongoing use of Pu-238 by NASA in satellites and space probes and controversy over that use. The present publicity is bringing to light past uses of plutonium-238 in American and Soviet satellites (which are also controversial) and other uses.
When I searched, I found only hints to the origin and properties of Pu-238.
Powering Satellites and Space Probes
 Electrical and electronic equipment needs a steady supply of electrical power over its lifetime. Satellites and space probes consist entirely of electronic equipment (imagers, sensors, computers and radio transmitters and receivers) and systems to contain, protect, navigate, and power that equipment. On earth, virtually all electronic equipment receives its electrical power from convenient power lines connected to power plants. There is no chance of delivering power electrically into space. The required power cable, if possible at all, would be hopelessly impractical.
 Power could be delivered from earth through focused light beams or radio waves. It is conceivable and it may even be practical for earth satellites.
 All the problems of powering portable electronic equipment in remote places on earth apply to space equipment. Batteries don't last very long before needing recharge or replacement. Generators don't last very long before they need refueling. Unfortunately, satellites and space probes cannot come home periodically for a recharge.
 Lots of electronic equipment derives electrical power from its local environment. Solar cells, solar engines and windmills are in use every day, around the world. Solar cells have been used in space for many years, but they have limitations. Solar cells need large surface areas, so they are mounted on elaborate unfolding umbrellas. These umbrellas are expensive, fragile and vulnerable. Solar cells in orbiting satellites can't make electricity in the dark shadow of the earth, so they need storage batteries and more electronics. Storage batteries wear out. Further from the sun, the brightness is less, so bigger solar batteries are needed to compensate. Solar cells can be damaged by meteorites and radiation. Excess capacity is needed to compensate for anticipated damage.
 Lots of electronic systems carry with them all the energy they need. By attaching a large enough battery or fuel tank, the system can operate for a long time. The longer the intended life of the equipment, the more stored energy it needs to carry.
 Batteries carry a small amount of energy relative to their weight. Attempts to make practical battery-powered motor vehicles remind us that chemical fuels carry far more energy per weight, even if we include the weight of the engine. Also, batteries tend to self-discharge, limiting their lifespan even if no energy is drawn from them.
 A fueled engine and generator offers substantial energy per weight. An improved generator has been developed that has no moving parts other than the ions within it. It has been used successfully in many manned space programs. The "fuel cell" consumes hydrogen and oxygen and delivers electrical power and heat (and water as the reaction product). The fuel cell is a new kind of power source. Note that the terminology becomes confusing or amibiguous as these new non-classical power sources emerge. The fuel-cell is like a cell or battery in that it produces electricity directly from ionic reactions between gases in its electrodes. The fuel-cell is like a generator in that it consumes fuel and can be recharged by replenishing that fuel. It uses fuel and releases a combustion product, but no actual flame takes place. Yet it burns the fuel in the sense that it uses it up and releases energy.
Stored Nuclear Energy for Satellites
 The highest known energy content per weight is found in the release of nuclear energy. Scientists understood this long before they ever split the atom. Radium is rare naturally occurring element. It is also radioactive. The principal isotope is Radium-226. It emits alpha, beta and gamma radiation. Much of the radiation is converted to heat. (4.871MeV energy released per disintegration.) It has a half-life of 1600 years. Even with such a long half-life, it has a measurable heat output. Scientists measured the heat output and the half-life of the decay. Multiplying these two values together tells (approximately) the amount of energy released by the decay of radium. (The decay product of Radium, Radon-222, is itself radioactive, so the total energy stored in radium is actually much larger.) The result was astounding: the radioactive decay of radium releases millions of times more energy per mass than any known chemical reaction. A gram of radium stores more energy than a ton of coal. Scientists dreamed of radium powering steamships, if only they could speed up its energy release (and find a larger supply of radium). (No way was found to speed up the release of useful energy from radium. They had to discover artificial radioactivity and then the nuclear chain reaction, which today powers submarines and aircraft carriers.) There probably wouldn't be any detectable amounts of radium on earth today except that it forms (ultimately) from the decay of U-238, a much longer-lived isotope with a half-life of 4,468,000,000 years. Radium's rate of heat output per weight is low; a very heavy piece would be required to power a space-probe. A better candidate would have a shorter half-life than 1600 years.
 By definition, all radioisotopes release energy. Much of the energy readily converts to heat, which can be harnessed. The heat can power a motor which can turn a generator and make electricity. The heat can be applied to a thermocouple, which generates electricity directly from that heat, with no moving parts. Such a power source is variously known as: RTG, Radioisotope Thermal Generator, advanced radioisotope power system, plutonium-fueled power source, nuclear battery, ... . The plutonium is like a fuel, because it is used up and converted into a less-valuable residue. But it doesn't burn in the conventional sense; no air or oxidizer is consumed. It is a single reactant that releases its energy by a decay reaction, whose rate cannot be controlled or stopped. It is like a generator because it converts heat from the isotope into electricity. The RTG is like a battery because it appears as a sealed unit that consumes elements within itself, storing the spent material within itself and delivering power to the outside. It is not a battery because it must constantly be relieved of waste heat or it will overheat or even melt.
 Each radioisotope has a different half-life and mode of decay. Each radioisotope can be evaluated for its potential usefulness as a heat source. Radium is not the only naturally-occurring radioisotope. For example, polonium-210 is an alpha-emitter, so it releases approximately the same energy per weight as radium. It has a half-life of 138.376 days, so it releases that energy 4223 times faster than radium. Unfortunately the same short half-life also makes much rarer than radium. There would not be any detectable polonium on earth except that it is formed from the decay of radium, a much longer-lived isotope. Polonium has the opposite problem as radium: it decays too quickly. A small amount will yield plenty of power, but after 3 years the power output will be less than 1% of the original. Though polonium might actually have been used for space power [ www.webelements.com/webelements/elements/text/Po/uses.html ], a better candidate will have a longer half-life than 139 days.
 Before the discovery of neutron sources, most radioisotopes were available only as trace elements, or inseparably blended with their more abundant non-radioactive stable isotopes. Certain unusual isotopes were available by chemically purifying one element, letting it set, and then chemically extracting the desired decay product. Particle accelerators and neutron sources made many more isotopes available, in measurable quantities. The same process of purification followed by extraction applies, except that bombardment occurs in between, not just simple aging. Nuclear reactors produce huge quantities of neutrons because they multiply the level of bombardment by millions. This makes possible the production of quantities and types of radioisotopes previously unimaginable. Nuclear reactors also produce huge quantities of radioisotopes as by-products: both fission fragments and products of incidental bombardment.
 Searching the huge variety of radioisotopes now available, by their mode of decay, rate of decay, and cost to produce, researchers hit upon plutonium-238 as a nearly ideal heat source. It has a half-life that is short enough to give it a high rate of heat production, but long enough to ensure useful operation of an RTG for decades. It decays (primarily) by alpha-emission, which yields a lot of heat and little or no penetrating radiation. Pu-238 decays into U-234, which is long-lived (half-life 245500 years).
 The calculations proved successful. Pu-238 has been made into light-weight, long-lasting, super-reliable electrical power sources. Pu-238 is now established as a power source for space probes and satellites.
So where does Pu-238 come from?
 Plutonium is not found in nature, it must be produced artificially. Seemingly, everyone has heard of U-238 and Pu-239, but Pu-238 just doesn't seem to add up.
 From a Commerce & Business Daily posting I found out that Pu-238 is produced by irradiating targets of pure neptunium-237 (Np-237). [available on WWW as: http://groups.google.com/groups?selm=76tkp6%24571%241%40pula.financenet.gov&output=gplain or (if this URL doesn't work), go to Google Groups [ http://www.google.com/grphp?hl=en&tab=wg&q= ] and search on terms like these:
plutonium-238 "irradiation services" )
 This makes sense. Let the Np soak up some neutrons, and a "simple" two-element chemical separation completes the job, with little else to worry about. (The target rods will contain little or no fission product or other by-products.)
 Where does Np-237 come from? Information on this is sparse. In only one place I see mention that it is a by-product of plutonium production.
 Based on published nuclide tables [ possibly similar to www.nndc.bnl.gov/wallet/nwccurrent.html ], I speculated that the following sequence is the probable origin of the available quantities of Np-237:
 U-235 absorbs a neutron and the compound nucleus (some call it metastable U-236) has a half-life of 121ns.
But some fraction of U-235 becomes ground-state U-236, which is long-lived (Half-life 23My) and can't be separated from the other uranium.
If the reactor runs for a long time, or especially if the fuel is repeatedly reprocessed, then the U-236 concentration in the U fuel increases toward some equilibrium concentration.
U-236 probably mostly absorbs thermal neutrons and becomes U-237, half-life 6.75d. (So very little of it has a chance to absorb another neutron to become U-238.) U-237 beta-decays to NP-237, which is again long-lived (Half-life 2.144My).
 ... or does this account for most of the Np-237?: occasionally a neutron hits U-238 and instead of bouncing off, or sticking and making U-239, or knocking off a different neutron leaving U-238, it knocks out 2 neutrons, making making U-237, half-life 6.75d. U-237 beta-decays to Np-237, which is long-lived (Half-life 2.144My).
 What happens to Np-237? Whether it remains in the fuel, or is separated out and bombarded as a pure target rod, the Np-237 eventually eats one neutron, becoming Np-238. Np-238 is short-lived (2.117d), so very little will absorb another neutron and become (useful) Np-239. Most of the Np-238 beta-decays to Pu-238, which is long-lived (87.7y).
 If the Pu-238 sits in the reactor long enough, it will absorb a neutron and become Pu-239 fuel.
Now the questions begin to multiply.
If this was fuel in a commercial fuel cycle with reprocessing (which is no longer done in the U.S.), then they'd probably want to remove the Np-237.
Np-237 started as U-235 which absorbed 2 neutrons. However, it would take 2 more neutrons to make Np-237 fissionable by neutron #3, so it is probably a net loss to leave it in the fuel.
[It doesn't even pay as a "blanket" in a breeder arrangement. Even if breeding Np-237 into Pu-239 pays off, cheap abundant Th-232 and U-238 are both more economical because each needs only 1 neutron to become fissionable. Np-237 still needs 2 neutrons, and trying to extract fuel from it involves handling nasty Pu-238.]
But breeding isn't being done and commercial power fuel isn't even reprocessed any more in the U.S..
For weapons production, I think they'd sometimes use pure U-238 target rods. (Iraq was caught trying to have a U-238 fuel-rod shaped assembly manufactured in Canada.)
The extraction of those targets would be very simple (only 2 or 3 elements).
The recipe is: run a load of crappy mixed-up fuel to high burnup in the middle to make neutrons.
Run pure U-238 target rods around the outside to short exposures to make the purest possible product with the simplest possible extraction.
But something tells me they usually run short cycles in the middle and reprocess the mixed U-235 236 237 238 239 + Np-237 238 239 + Pu-238 239 + fission-products mess, regardless of whether or not they had pure U-238 targets on the outside.
Calling Np-237 a by-product means they deliberately removed it from the cycle.
If they ran their fuel to high-burnup in the middle, they could have reprocessed that fuel or used new fuel, whichever was economical at the time. (If they ran low-burnup, they had no choice but to reprocess.)
If they reprocessed, it was probably not much harder to extract the Np to get it out of the way. Uranium, neptunium and plutonium are standard outputs of the extraction process used in weapons facilities.
Now, there's a weird thing about the Np.
Right after a short cycle, "all" of the U-239 (23.45m) has decayed to Np-239, but much less of the U-237 (6.75d) has decayed to Np-237, so very little Np-238 will have had a chance to be formed.
If they extract the Np right away, it will be mostly Np-239 (2.3565d) ready to decay into Pu-239, with traces of inseparable Np-237 (long-lived) and inseparable Np-238 (2.117d) ready to decay into inseparable, very-undesirable Pu-238. I believe they would let this Np sit and then extract it again after it decays. The resulting Pu will be product (mostly Pu-239 with traces of Pu-238). The remaining Np is nearly pure Np-237, which they would remove (because it is worthless) and store somewhere, forever (because it is hazardous).
Whether they've extracted the uranium or left it in the residue, they might benefit from letting this sit a while as well. Any trace of U-237 (6.75d) decays to Np-237, giving another opportunity to extract Np (would be nearly-pure Np-237) and get it out of the cycle by similarly removing it and storing it somewhere.
They have to run short cycles and remove the Np-237 to minimize the formation of Pu-238 in reprocessed fuel, just like they first discovered the need to run short-cycles to minimize the formation of Pu-240 even in new fuel. Just as traces of Pu-240 (6564y, alpha) contamination raise the alpha background of Pu-239 (24100y, alpha) enough to make it useless for weapons, traces of Pu-238 (87.7y, alpha) contamination should ruin it even more quickly. [But Pu-238 does have a lower spontaneous fission rate than Pu-240.]
So all this NP-237 has been set aside and is stockpiled in the US and former USSR. I'd be curious just how much is around. Many tons of pure Pu-239 have been produced for the arsenals; multiply that by 0.001 or 0.01 or 0.1(?) to guess how much Np-237 is piled up. There must be at least a ton of it in the world, probably separate and pure because of the second plutonium extraction.
Np-237 is one thing that's even more worthless than "depleted uranium", U-238. It's not fuel and it's useless for breeding fuel or weapons. But it is sensitive because it can be bred into the extremely-poisonous (minimum 274x as carcinogenic as Pu-239) but useful heat source Pu-238.
Causes for concern:
Potential security risks:
If they set the Np aside in a hurry way-back-when, while it still contained traces of undecayed Np-239, then the aged Np will contain traces of pure Pu-239. If outside operators handling these rods get a chance to sneak in a quick extraction cycle, they will obtain some pure Pu-239. DOE could prevent this, not by extracting Pu-239 from the rods (it is illegal), but by making sure that the rods are precontaminated with Pu-238. Also, if someone substitutes U-238 for the NP-237, they will have diverted the full process into Pu-239 production.
Isn't this reprocessing?
Another problem with this whole operation is politics and hypocrisy. During the debates on whether the U.S. should burn our old weapons-grade plutonium for fuel or waste it by contaminating it and burying it, some high official came forward and said that if we burn this plutonium we are contradicting the example that we are trying to set for the rest of the world: that reprocessing should not be done because it is uneconomical and risky (for mishaps as well as diversion). [[What did they decide on this, anyway?]] Where are those people and their objections now? They declare that burning plutonium extracted 55 to 13 years ago under a different dispensation is a wrong thing to do today and a wrong example to set today. Shouldn't they be objecting just the same to this proposed and ongoing use of Np-237, which originated only from the very same fuel reprocessing done during those very same weapons production operations? The proposed process will place this reprocessed neptunium into commercial power reactors to irradiate it and convert it into an isotope of plutonium, and then chemically extract it (reprocess it) for final use as a power source. Of course there are some differences: the extraction is simpler, nowhere near as dirty as reprocessing reactor fuel elements. None of the material involved could ever be used for weapons purposes (except as noted above). None of the material could ever be used as economical reactor fuel; in fact the production makes a subtraction from the efficiency of whatever reactor is used.
Part of what makes reprocessing so bad is the hazards of handling the radioactive materials involved. The processing involved in Np-237 Pu-238 production is vastly different. The hazard from fission-product is essentially nil, because there is no fission product involved. But Pu-238 is 247 times more toxic (by weight) than Pu-239. Both Pu-238 and Pu-239 are routinely handled with rubber gloves because the radiation is so non-penetrating. But a single dust speck of either one will cause certain lung cancer if inhaled. If Pu-239 dust gets loose in a room, then that room becomes uninhabitable, but if Pu-238 dust gets loose in a room, then the whole building becomes uninhabitable. That is the magnitude of the handling risk of this process. (Note, polonium is even more active, 231x more active than Pu-238. It is so energetic it has a reputation for not staying in the containers where it is put.)
Oops, killed earth's human population, just for a space battery:
The planned uses border on insanity when sanely contemplated. If a space-probe such as Cassini burns up in earth's atmosphere, it could kill millions. I've heard that the Environmental Impact Statement for one of these probes says that if it malfunctions in the worst way, then it could adversely affect one billion people. I've seen published statements minimizing the dangers of the probes. One of these "worst-case" analyses was calculated mistakenly using the data for Pu-239. It neglected to factor in the increased activity of Pu-238 relative to Pu-239. Because Pu-238 is 247 times more radioactive than Pu-239, a "lethal particle" of Pu-238 is only 1/247 the mass of a "lethal particle" of Pu-239. Correcting the worst-case analysis from Pu-239 to Pu-238: the same mass, optimally divided, would yield 247 times as many lethal particles and therefore 247 times as many fatailities, using the same statistics as the original. But an additional adjustment is needed: a particle 1/247 the mass has 1/247 the volume and therefore 1/6.27 the diameter. The smaller particle would fall about 6.27 times slower. That would multiply the fatal inhaled doses again by 6.27, using the original statistics. The two corrections mean a combined multiplication of 1549 times, , when translating from Pu-239 to an equal mass of Pu-238.
The 72.3 pounds of Pu-238 on the Cassini, as a biological hazard, is about the equivalent biological hazard of 17,858 pounds of Pu-239. However, a guesstimated 11,000 pounds of Pu-239 have already been dispersed in the atmosphere by nuclear weapons tests. Whatever horrors resulted from atmospheric weapons tests, a worst-case Cassini burn-up would merely double it. The conclusions remain controversial.
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posted 1999.03.01 revised 2007.07.17