The Seven Last Infra-Uranium Elements to Be Discovered

The term “infra-uranium,” meaning before uranium, is one that I have proposed by contrast to the better-known term transuranium elements that are discussed in the following chapter. The present chapter concerns the last seven elements that formed the missing gaps in the old periodic table that ended with the element uranium. After Moseley developed his X-ray method, it became clear that there were just seven elements yet to be isolated among the 92 naturally occurring elements or hydrogen (#1) to uranium (#92). This apparent simplicity is somewhat spoiled by the fact that, as it turned out, some of these seven elements were first isolated from natural sources following their being artificially created, but this raises more issues that are best left to the next chapter of this book. The fact remains that five of these seven elements are radioactive, the two exceptions being hafnium and rhenium, the second and third of them to be isolated. The first of the seven final infra-uranium elements to be discovered was protactinium, and it was one of the lesser-known predictions made by Mendeleev. In his famous 1896 paper, Mendeleev indicated incorrect values for both thorium (118) and uranium (116). (See figure 1.6.) A couple of years later, he corrected both of these values and showed a missing element between thorium and uranium (figure 4.4). In doing so, Mendeleev added the following paragraph, in which he made some specific predictions. . . . Between thorium and uranium in this series we can further expect an element with an atomic weight of about 235. This element should form a highest oxide R2O5, like Nb and Ta to which it should be analogous. Perhaps in the minerals which contain these elements a certain amount of weak acid formed from this metal will also be found.. . . The modern atomic weight for eka-tantalum or protactinium is 229.2. The apparent inaccuracy in Mendeleev’s prediction is not too surprising, however, since he never knew that protactinium is a member of only four “pair reversals” in the entire periodic table.

Synthetic Elements

The periodic table consists of about 90 elements that occur naturally ending with element 92 uranium. This lack of precision is deliberate since one or two elements such as technetium were first created artificially and only later found to occur naturally on earth. This kind of occurrence provides a foreshadowing of things to come when we begin to discuss the transuranium elements, meaning those beyond uranium that have been artificially synthesized. Chemists and physicists have succeeded in synthesizing some of the elements that were missing between hydrogen (1) and uranium (92). In addition, they have synthesized a further 25, or so, new elements beyond uranium, although, again, one or two of these elements, like neptunium and plutonium, were later found to exist naturally in exceedingly small amounts. The existence of superheavy elements raises a number of interesting questions that pertain to the field of philosophy of science and also sociology of science. In fact, the very question of whether these superheavy elements actually exist needs to be dissected further, as it will be in the course of this chapter. The synthetic elements are extremely unstable, and only the lightest ones among them have been created in amounts large enough to be observed. Roughly speaking, the heavier the atom, the shorter its lifetime is. For example, the heaviest element for which there is now conclusive evidence is element 118, a few atoms of which have been created in just one single isotope form and with a half-life of less than a millisecond. Laypersons and specialists alike have asked themselves in what sense these elements can really be said to exist. The superheavy elements also have philosophical implications for the study of the periodic system as a whole and the question of whether there is a natural end to chemical periodicity. A related question, which has now become quite pressing, is the possible extension of the periodic table to include a new g-block which in formal terms should begin at element 121.

Homogenous recycling of transuranium elements from irradiated fast reactor fuel by the EURO-GANEX solvent extraction process

The EURO-GANEX process was developed for co-separating transuranium elements from irradiated nuclear fuels. A hot flow-sheet trial was performed in a counter-current centrifugal contactor setup, using a genuine high active feed solution. Irradiated mixed (carbide, nitride) U80Pu20 fast reactor fuel containing 20 % Pu was thermally treated to oxidise it to the oxide form which was then dissolved in HNO3. From this solution uranium was separated to >99.9 % in a primary solvent extraction cycle using 1.0 mol/L DEHiBA (N,N-di(2-ethylhexyl)isobutyramide in TPH (hydrogenated tetrapropene) as the organic phase. The raffinate solution from this process, containing 10 g/L Pu, was further processed in a second cycle of solvent extraction. In this EURO-GANEX flow-sheet, TRU and fission product lanthanides were firstly co-extracted into a solvent composed of 0.2 mol/L TODGA (N,N,N′,N′-tetra-n-octyl diglycolamide) and 0.5 mol/L DMDOHEMA (N,N′-dimethyl-N,N′-dioctyl-2-(2-hexyloxy-ethyl) malonamide) dissolved in Exxsol D80, separating them from most other fission and corrosion products. Subsequently, the TRU were selectively stripped from the collected loaded solvent using a solution containing 0.055 mol/L SO3-Ph-BTP (2,6-bis(5,6-di(3-sulphophenyl)-1,2,4-triazin-3-yl)pyridine tetrasodium salt) and 1 mol/L AHA (acetohydroxamic acid) in 0.5 mol/L HNO3; lanthanides were finally stripped using 0.01 mol/L HNO3. Approximately 99.9 % of the TRU and less than 0.1 % of the lanthanides were found in the product solution, which also contained the major fractions of Zr and Mo.