 Hello, welcome to the lectures on the actinide chemistry. In this lecture series, we will be discussing about the actinides, their relevance and why one should be studying the chemistry of actinides. This first lecture, I will be discussing the introduction, discovery and synthesis of actinide elements. But the first question comes to our mind is why actinide elements? Because some of the early actinide elements like uranium and plutonium, they are having application in the nuclear fuel cycle as the fuels, the fissile isotope of uranium and plutonium that is 235 uranium and 239 plutonium can be used as a fuel material in the nuclear reactors. And that is why it is very, very important to study the chemistry of actinides. So, first I will give some example why this nuclear fuel cycle? Here, I have given a schematic of the nuclear fuel cycle where you start with the uranium ore. So, that is how it is the mining of the uranium ore and this also involves the mining of the thorium ore because thorium can also give the uranium 233 which will be seen in the course of this lecture series that uranium 233 also is used in the Ease-WR reactors. So, there it can be used as the fissile element. And 233 is used as the fissile element in the Aase-WR reactors. So, therefore, thorium dioxide and uranium dioxide they are obtained from the mining and the milling operations, then these elements and their compounds like the oxides. The oxides of thorium and uranium, in this case the uranium oxide is given for 12 fabrication. Some cases if you need enriched uranium, then enrichment is also one of the steps before the fuel fabrication. Then after fuel fabrication it goes to the nuclear reactor and after the nuclear reactor is operated for a certain period of time, then the spent fuel where the fissile content has depleted significantly is removed from the nuclear reactor core and it is kept in a storage called the spent fuel storage where the radioactivity is decreased the force of the time and afterwards the spent fuel reprocessing is done where the value looks like uranium and plutonium will be recovered. In case of the Aase-WR fuel then we have to recover thorium and also the uranium 233 and uranium and plutonium they can be used as the mixed oxide fuel or the mox fuel and again it goes back for fuel fabrication and to the reactor. So, this is the nuclear fuel cycle or a closed nuclear fuel cycle which will be discussing subsequently in this lecture. Now the chemistry of actinides it is very very important because of its application in the nuclear reactors also after the nuclear reactor operation is over the fuel is removed from the reactor core and then as I mentioned there is something called the spent fuel reprocessing. There also the chemistry of actinide is important where the uranium and plutonium is recovered using a process called PX process which will be discussing subsequently in one of the lectures in greater detail then the raffinate which is coming out of the spent fuel reprocessing is taken out for the nuclear waste management program and there this actinides and the commercial products they are vitrified and kept in the deep geological repositories. They are also the chemistry of actinide is important mostly the minor actinides like abyssm, curium and lepturium their chemistry one needs to know such that this nuclear waste management can be carried out very very efficiently. Finally there is also this effect of some of the hazardous actinides which are having large amount of radio toxicity their effect on the environment is also very important to know that is why one needs to study the chemistry of actinides in such cases the species and migration of actinides mainly that of plutonium one needs to understand. Now apart from this application the nuclear fuel cycle and also this environmental chemistry of actinides it is also a challenging study where this actinide chemistry one needs to study because of their very interesting and challenging chemistry why it is a challenging because of the high radioactivity of the actinides their short applies but also one of the very important features of the actinide elements is their variable oxidation states and also very high radio toxicity in view of this the manipulations in the laboratory is very very challenging with the actinides. Now what are the actinides? Now we know them as the actinide elements and it appears that they are similar to the lanthanides but about 80 years back when this man made actinides were not discovered so that time it was not known which is the actinide series and now we know that this actinides or actinoids they are called their egg block elements where the electrons are filled in the five-way permittence. The actinide series starts from actinium which has this electronic configuration of 5F0, 6D1 and 7S2 and it is with Lorentzium with the electronic configuration of 5F, 14, 6D1 and 7S2 are considered to have similar physical and chemical properties. This is mostly the later part of the actinides they have the similar chemical properties and physical properties though actinium does not have any F electrons it is considered part of the series similar to lanthanum in the lanthanide series. General electronic configuration of the actinides given here the outer core of the radon is there and you have this electron filling of these electrons in the SP, D and F cells as given here. The electronic configuration will be discussed separately in another lecture. Actinides give rise to decay series such as the 4M with the thorium 232, 4M plus 1 that is the actinium 237 series and 4M plus 2, 238 uranium and 4M plus 3 that is 235 uranium series which I am sure you have already studied in the schools. Now, coming to whether these actinides are available in the nature we know that these actinides like uranium, thorium they are available in the nature in a very large extent and also you know this actinium and protactinium also available in the nature. But what about the other actinides like plutonium they were not available in the nature and they were synthesized around 1940 by Seabourg and in research group however interestingly this plutonium was detected in the nature and trace quantity of plutonium was actually seen in the natural uranium 238 deposited such as one in the Oklo. This is called a natural fission reactor was suggested to have existed in Oklo, Gabon, Africa. It was discovered by Francis Perrin in 1972 and this is the only location where this phenomena of self-perceiving nuclear fission reaction is known to have occurred. This is thought to have taken place approximately 1.7 billion years ago and probably continued for a few thousand years. Now, why this was considered like this? This is because of the high amount of 235 uranium in that particular mine which is maybe more than whatever we are seeing maybe reason maybe around 3.1 percent of 235 was probably existing at that time. Now it is much less than that as you will be seeing in the subsequent slides. So one thing is that relatively large fraction of 235 uranium was existing in the ores and secondly there are also signatures of this fission because the neodymium isotope monitoring was done for burn-up measurements and from that isotropic ratio it was found out that definitely there is a change in this whatever expected from the normal and that fission fission this is a neodymium fission product this signature was seen. Then ruthenium-99 fraction was mowed and after the reactor operated for several thousand years and stopped thereafter due to the lower 235 content and increased neutron poisons. So this suggested that plutonium was existing in this Oclo mines. Now coming to the periodic table, these actinides in the early part of the 20th century their actinides were not placed in the periodic table the way we see them now. So they were considered as similar to the transition elements so that we will be discussing subsequently and as you can see that only actinium, thorium, protactinium and uranium were discovered by that time. When 1913 Fershand and Gehring identified a short-lived isotope of protactinium in the half-life of 1.17 minutes while studying the uranium-238 decay. So this is how these actinides were discovered first we will see the discovery of actinides that is the uranium is the earliest known actinide known as early as 79 AD by the Romans and its oxide extracted from pitch-blend. It was also used as a coloring agent in glass in the Czech Republic during the medieval era but the discovery of the element is credited to the German scientist Martin Heinrich's clock quotes in 1789. The uranium metal was prepared by Eugen Pelegot in 1841 and the atomic mass of uranium was then calculated as 120. Mendeleev arranged the then known elements in the periodic table and corrected the atomic mass of uranium as 240 based on its periodic laws. Coming to thorium it was discovered by Morton S. Mark in 1828 and Virgilius named it in 1829. Uranium and thorium are primordial elements and 232 thorium starts the 4 NDK series as mentioned with a final stable product being 208 lead. Actinium was discovered in 1899 by Adred Debian and F. Giesel identified and isolated the element actinium in pitch-blend. In 1934 500 milligram of protactinium was isolated from pitch-blend but it can be easily formed now by irradiating thorium 230 by N gamma reaction giving 231 protactinium or thorium 232 by N gamma reaction giving 233 protactinium. Protactinium 233 is an important isotope as it is decaying 233 uranium which is a fissile element using the ASWR reactors. Now the interesting thing is the discovery of neptunium. So Edwin Macmillan is the first to have discovered neptunium in the Berkeley Radiation Laboratory. In 1934 however Enrico Fermi he has published a paper suggesting the discovery of element number 93 after irradiating uranium with neutrons. So now Fermi believed very strongly that he has discovered a different element because it was giving lot of radioactivity. Now this was disproved by Van Grosje who suggested the possible production of protactinium which was disproved later on and Nodak who suggested that uranium might have fragmented to 2 to 3 pieces of other radionuclides. This is of course before the nuclear fission was discovered but Fermi stuck to his claim and he said that he has discovered element 93 at that point though which was to be similar to the group 7 elements including manganese and rhenium. So as uranium chemistry is similar to that of molybdenum and tungsten similar way the element number 93 was believed to be similar to that of manganese and rhenium which are again transition elements. But the chemistry of the so-called element number 93 as claimed by Fermi didn't match to that of rhenium and that is how this claim was proven run. Subsequently the Japanese physicist Nishina and Kimura they omitted uranium with vast neutrons around 1940 or so and they discovered uranium 237 which is a half-life of 6.75 days. So this discovery was correct but the amount of uranium 237 which was isolated by them was too less because subsequently it was understood that uranium 237 decays to neptronium 237 but the neptronium 237 has a very long half-life and also this uranium 237 decaying to neptronium 237 with a 6.75 day half-life. So the neptronium 237 which would have formed with a very very less quantities that is how Nishina and Kimura they could not detect neptronium 237 otherwise they would have been the discoverer of this element. However after the discovery of fission Edwin McMullen he wanted to carry out some experiments to measure the fission use of the uranium target in the cyclotron at the Berkeley radiation laboratory in 1939. So he carried out some experiments where he irradiated the uranium target by the neutrons coming from the Berillian target bombarded by the 8 Nubian neutrons in the 37 inch cyclotron. So he has used basically the thermal neutrons and he has seen that this uranium target which he has used basically a paper he has taken on which he has spread the uranium metal compound and there he was irradiating with neutrons and he found that this activity actually what he did is he got he measured the profiles actually he has taken several aluminum foils which are on 0.5 milligram per centimeter square this thick and he measured the activity as a function of the range actually the aluminum foils he has subsequently dismantled and then he has measured as a function of this length or the centimeter in air and he found this profile like this he has seen and this is how he got so this is actually decaying so this is attributed to the fission products and he found that there was a large amount of activity was still there this is because of the aluminum foils. So subsequently what he carried out some experiments he did where where he taken a paper actually instead of the aluminum he has taken paper and then he has packed those papers and then he found that very interesting thing he has absorbed so he has plotted the activity as a function of hours and what he has seen is that there is a fission product capture these activities following this train this is a fission product capture and there was a 23 minutes half life pattern was seen this is the 23 minutes and of course there was another one 2.3 days half life so this was basically he has seen the signature of 23 minutes half life one radionic light and 2.3 days half life another radionic light so this was as I was introducing so he thought that there is definitely some new element has formed but he wanted to prove it how to prove it so he carried out subsequently another experiment with the chemistry and this will be the next slide so this experiment the chemistry experiment was carried out by Emilio Segre who is the discoverer of titanium and Segre was an expert in the chemistry of rhenium so when Macmillan approached Segre so he carried out the chemistry of rhenium for this hydroelectric radioactivity what he did is he reacted with hydrofluoric acid in oxidizing conditions and it behaved like a rare earth concluding that it is a fission product and not a new element so that is how the Macmillan he thought that probably whatever he has discovered it may be a fission product but not a new element but after sometime he collaborated with Phil Abelson the same experiment he has carried out instead of oxidizing conditions in reducing conditions and what he observed that there was a precipitation of this new element was possible like thorium so that is how he suggested that it is not a rare earth element and it is probably a different element so then what he did he repeated the experiment in oxidizing conditions and precipitated by sodium acetate so then he concluded that it may be similar to that of uranium so finally Macmillan he concluded that the 23 minutes radianiclide is nothing but rhenium 239 which is actually neutron activated product of rhenium 238 which is present in the natural uranium and at 2.3 days half life radioactivity is nothing but a new element which is similar to that of uranium and he named it as neptunium so this is a reaction which is giving neptunium 239 that is 238 uranium captures the neutron giving 239 uranium which undergoes beta decay giving 239 neptunium neptunium subsequently this 237 isotope of 237 neptunium was discovered but in that case the reaction can be twofold it can be carried out by 235 uranium which captures a slow neutron giving 236 uranium and which again captures another neutron giving 237 uranium which undergoes beta decay to give 237 neptunium another reaction is 238 uranium can react with fast neutrons giving 237 uranium plus 1 neutron then this 237 uranium again undergoes beta decay giving 237 neptunium so this is how neptunium was discovered now beyond neptunium subsequently many other actinates were discovered 240 to 38 plutonium was discovered by siever, wall, kennedy and macmillan in 1940 and this was called as element 94 where the uranium was irradiated by neutrons from this cyclogram facility as the Lawrence radiation laboratory and this was the first plutonium isotope which was discovered subsequently of course plutonium 239 was discovered by siever later again by irradiating uranium with slow neutron and this is the reaction uranium 238 which captures the neutron giving 239 neptunium as already we have seen and it undergoes beta decay giving 239 plutonium so this 239 plutonium is very very important it is a fissile isotope of plutonium it is used in our reactor and subsequently it was used in the atom bomb now the production of heavier actinates between 1944 and 1974 12 trans plutonium elements were added to the periodic table and element 95 and 96 were discovered that is the amyxium and curium as we know today they were discovered from the 239 plutonium so the reactions are given here plutonium 239 when it is undergoing a nuclear reaction with helium 4 gives 242 curium plus neutron plutonium 239 can also undergo a neutron capture reaction giving 240 neutron 240 plutonium which again captures another neutron to give 241 plutonium which undergoes beta decay to give 241 amyxium this 241 amyxium can also capture 1 neutron as given here to produce 242 amyxium and this 242 amyxium can undergo beta decay to give 242 curium so this is how this curium and amyxium were discovered by Simon and his colleagues and subsequently in 1949 the bombardment of amyxium and curium by helium ions accelerated by the 16 cyclotron it produced element number 97 which is known as bercelium today and element number 98 which is known as californium the reactions are given here where amyxium 241 reacts with helium 4 to give 244 bercelium and curium 242 reacts with helium 4 to give 245 californium subsequently 1952 the thermonuclear explosion was carried out and it has indicated the formation of 253 californium you see from here that means a large number of neutrons are captured by uranium 238 to give uranium 253 which decays to californium 253 and this californium 253 subsequently decays to iron scallium 253. 255 formium also was detected in this thermonuclear explosion and subsequently in ORNL this high flux isoprop reactor that is HF IR was built with a neutron flux around 10 to the power 15 neutrons per centimeter square per second and lot of this heavier actinides were synthesized using this high flux isoprop reactor and which has a transpermium elements however was not possible and they were subsequently carried out by some nuclear reactions with some sort of fusion reactions were carried out which will be discussing subsequently. Now these are the isotopes of the actinide elements shown here from atomic number 89 to 103 as we see here 89 is actinium which was of course we are known before this synthetic actinide elements were produced in 1940. So, thorium also was known, protactinium was known and uranium was known and from neptunium onwards as I have mentioned these were man-made actinides and this number of isotopes written here see there are nearly 33 isotopes of actinium starting from 204 actinium to 236 actinium and mostly these are transient they are not very very stable isotopes. Thorium there are 32 isotopes starting from 207 thorium to 238 thorium and the naturally occurring isotope of course is 232 thorium with a 100 percent observance. Protactinium 30 isotopes are there starting from 211 to 240 protactinium again many of these are transient uranium there are 27 isotopes starting from 214 uranium to 219 uranium then 221 to 240 uranium and 242 uranium after these naturally occurring isotopes are given here that is 234 uranium, 235 uranium and 238 uranium with the abundance of 0.0055 percent for 234, 0.72 percent for 235 and 99.27 percent for 238 uranium now all these heavier actinides starting from neptunium they are man-made similarly for the man-made actinide elements the number of isotopes are listed in this table.