I'm home sick today and while I can still sit up I'm trying to go over some interesting stuff about historical nuclear reactors that were designed to exploit plutonium as a fuel, a matter which is of more critical importance to humanity than it has ever been.
I most recently reviewed current thinking about plutonium in a series of diaries on the Indian commercial nuclear program, the last member of the series being The Light of Day: India's Fast Breeder Nuclear Reactor: Some Technical Comments. (Pt. 7)
Besides its enormous value as an energy source, plutonium is one of the most interesting elements in the periodic table. The paper from the primary scientific literature that I will discuss today, comes from two American scientists, Hecker and Stan, out of respectively, Stanford and Los Alamos National Laboratory, and is entitled "Properties of plutonium and its alloys for use as fast reactor fuels." The reference is Journal of Nuclear Materials 383 (2008) 112–118.
It was an American chemist - one of the greatest chemists (and human beings) of all time - Glenn Seaborg, who developed the concept of "Actinides" as a separate group in the periodic table that is roughly analogous to the lanthanides, a series of elements that begin with lanthanum (element 57) and end with lutetium, (element 71). The remarkable thing about the lanthanides is that they all behave, more or less, in a similar way and are mostly characterized by having an oxidation state of +3, meaning that a common oxide is would have a formula of Ln2O3 where Ln is any lanthanide. A notable (but by no means only) exception is the oxide of cerium, which has an oxide with a +4 oxidation state, CeO2 that is widely used in self cleaning ovens and similar systems.
It was Seaborg's great insight that the elements after actinium, element 89, should correspond not to the elements in the above block in the table, but roughly to the lanthanide series which had only recently become understood.
The groups on the bottom of the periodic table, if you are looking at it, should actually be another step down, but the table is not written that way very much, since it would become too wide to put on a sheet of paper.
The shape of the periodic table is derived from quantum mechanics and types of suborbitals that are designated by s, which can contain two electrons, and which are being filled in groups 1 and 2, corresponding to lithium and beryllium and elements below them, p, which can contain 6 electons corresponding to groups 13 through 18, headed by boron through neon, and d, headed by scandium through zinc which can contain 10 electrons.
The lanthanides and actinides correspond to a f suborbital which can contain 14 electrons.
The lanthanides all behave roughly analogously because the electron shells being filled as they add protons are all 4f electrons, meaning they have a "f" shape and are in the 4th period. In general 4f electrons do not participate in chemical bonding, and therefore do not cause much chemical change to the elements.
5f electrons however, behave somewhat differently.
It was not intuitively obvious that this case - the f suborbital case - should correspond to the actinides. Indeed the known members of the series of actinides when Seaborg began his work, actinium, thorium, protactinium, and uranium behaved more like d elements than f elements.
Thorium, for instance, forms almost exclusively compounds with the +4 oxidation state, and is closer in chemistry to the elements zirconium and hafnium than it is to say, uranium. It has an oxide whose formula is ThO2, like the cerium in self-cleaning ovens but unlike cerium, it does not form an oxide of the form Th2O3. Protactinium has most commonly and oxidation state of +5, rather like Vanadium and Tantalum, although lower oxidation state compounds are known. Uranium is more like chromium, with multiple oxidation states than it is like neodymium. The existance of high oxidation states of uranium has always been essential to uranium chemistry, most notably in the case of the high oxidation state +6 fluoride, UF6, which is easily sublimed into a gas.
Nevertheless we now understand that actinium, thorium, protactinium, and uranium are properly considered actinides and not d metals.
The element that stands on the cusp between true f orbital behavior and d orbital behavior is plutonium, a fascinating element whose chemistry brings together many aspects of inorganic chemistry.
The paper cited mentions a few of these interesting properties.
They put what I have discussed above this way:
the 5f electrons of the light actinides behave more like the 5d electrons of the transition metals than the 4f electrons of the rare earths. At the very beginning of the actinide series, there is little f electron influence and, hence, one finds typical metallic crystal structures, few allotropes, and high melting points. This behavior is best illustrated in the connected phase diagram across the actinides in Fig. 2 [1]. As more f electrons are present (up to plutonium), they participate in bonding (that is, they are itinerant, much like the d electrons in transition metals) and the crystal structures become less symmetric, the number of allotropes increases, and the melting points decrease. At americium and beyond, crystal structures typical of metals return, the number of allotropes decreases, and the melting points rise – all indications of the f electrons becoming localized or chemically inert, much like the 4f electrons in the rare earths. Since plutonium sits right at the transition point from itinerant to localized 5f electrons, it exhibits many unusual properties [2].
What are these unusual properties?
Plutonium first of all has many phases or including many metal phases having different volumes. (A common metal that has different phases is tin. When tin is cooled sufficiently a noise is heard, the "tin cry" which is connected to a phase change) Cold tin is brittle whereas warmer tin is quite malleable. (Different phases of substances was the issue evoked by Kurt Vonnegut in "Cat's Cradle," his science fiction novel about a state of water called "Ice-9." As it happens at extreme pressures different phases of solid water are known, but none have the fanciful thermodynamics described in Vonnegut's novel.)
Some phases of plutonium metal have an interesting property - evoked in the paper - of being lighter than molten plutonium, and thus, like water, these phases float in the molten metal. (Other materials having this property, solid phases that are actually less dense than the liquid phase include silicon and gallium.
The multiple phases of plutonium have caused engineering problems for pure metal fuels in nuclear reactors (and, actually in nuclear weapons) since the metal expands and contracts as it changes phase. For this reason plutonium is stabilized by inserting an element which - like a lanthanide - has a +3 oxidation state. In nuclear weapons, the element often chosen for this purpose is gallium.
It is also known that certain metals having a +4 oxidation state can result in metastable plutonium metal, and this is the basis for proposals for Plutonium Zirconium alloys in nuclear fuels.
(There are even better options than this in my opinion, but they have not been explored but should be when and if advanced plutonium having higher isotopes become available - as they inevitably will be in the case that humanity survives climate change. I will not discuss these here.)
Plutonium also forms several eutectics, which are solutions of two solid substances that when combined yield a lower melting point than either of the pure substances. The most famous example of a eutectic is the mixture of salt and water, which melts at -21C (O F, which is the basis of the Fahrenheit temperature scale) much lower than the melting point of pure water (O C, 32F) or salt. Most solders used in plumbing are eutectics.
Plutonium forms eutectics with metals like iron, meaning that the pure metal is extremely corrosive towards iron and steel. The iron/plutonium eutectic contains about 6 atoms of plutonium for each atom of iron and melts at the low temperature of only 410C. The melting point of pure iron is 1811C. The melting point of pure plutonium is about 640C.
Thus it is relatively easy to dissolve iron in liquid plutonium, a problem for a steel reactor.
The IFR program that was foolishly canceled in 1994 as "unnecessary" got around this problem by using an alloy that was a mixture of plutonium, zirconium and uranium. This ternary alloy had excellent properties for using metal fuels.
The IFR cancellation was a damn shame in my opinion, and a great technical loss for the United States. Obviously more than a decade of development has been irretrievably lost because of this dubious decision.
The advantage of metal fuels as opposed to oxide fuels is their high thermal conductivity compared to oxides. Interestingly the thermal conductivity of pure plutonium metal - as might be expected because of its very complex phase behavior - shows wide swings.
In modern times, computational methods for the examination of complex mixtures and their phases have become available owing to the increased computational power that did not frankly exist during the last burst of nuclear power development. These methods, known as CALPHAD have allowed for great advances in materials science, and may simplify the development of plutonium based options.
Right now the world inventory of plutonium is on the order of a few thousands of tons, including several hundreds of tons of weapons grade material that must be denatured in order to achieve nuclear weapons disarmament. My personal feeling is that we should raise our plutonium inventories by a few thousand metric tons if we are to save humanity, but that all plutonium should be subject to long term neutron fluxes in commercial power reactors and controlled and audited by the IAEA.
I am very fond of the plutonium inventory management schemes and planning put forth by the CEA, the French nuclear agency. Before anyone starts telling me how wonderful thorium is - something I concede - I note that thorium will be more or less useless without access to plutonium. We need plutonium and there is no rational reason to fear the element inappropriately.
For this reason it is advisable to continue to work on the physics and chemistry of this most exciting and interesting of metals.