Radioactive isotopes

28.44 - Radioactive chemical elements and radioactive isotopes (including the fissile or fertile chemical elements and isotopes) and their compounds; mixtures and residues containing these products. 2844.10 - Natural uranium and its compounds; alloys, dispersions (including cermets), ceramic products and mixtures contaming natural uranium or natural uranium compounds 2844.20 - Uranium enriched in U 235 and its compounds; plutonium and its compounds; alloys, dispersions (includin cermets), ceramic products and mixtures contain~nguranium enriched in 235, plutonium or compounds of these products 2844.30 - Uranium depleted in U 235 and its alloys, dispersions (including containing uranium depleted products ounds; thorium and its compounds; , ceramic products and mixtures , thorium or compounds of these 2844.40 - Radioactive elements and isotopes and com ounds other, than those of subheading 2844.10, 2844.20 or 2844.30; a loys, dispersions (including cermets), ceramic roducts and mixtures containing these elements, isotopes or compounds; ra ioactive residues f 2844.50 - Spent (irradiated) fuel elements (cartridges) of nuclear reactors (I) ISOTOPES The nuclei of an element, defined by its atomic number, always contain the same number of protons, but they may have different numbers of neutrons and, consequently, will be of different mass (different mass number). Nuclides which differ only in the mass number and not in the atomic number, are called isotopes of the element. For example, there are several nuclides with the same atomic number 92 which are all called uranium, but their mass number ranges fiom 227 to 240; they are designated, for example, as uranium 233, uranium 235, uranium 238, etc. Analogously, hydrogen 1, h drogen 2 or deuterium (classified in heading 28.45) and hydrogen 3 or tritium are isotopes o hydrogen. B The important factor in the chemical behaviour of an element is linked to the amount of the positive electric char e on the nucleus (number of protons); this determines the number of orbital electrons whic actually affect the chemical properties. F Because of this, different isotopes of an element whose nuclei have the same electrical charge but different masses, will have the same chemical properties but their physical properties wll vary fiom one isotope to another. Chemical elements are composed either of a single nuclide (monoisotopic elements) or of a For example, natural chlorine, mixture of two or more isotopes in known of 75.4 % chlorine 35 and in both the free and combined states, 24.6 % chlorine 37 (which gives it its When an element is composed of a mixture of isoto es, its constituent parts can be separated for example by difbsion throu porous tubes, by e ectro-magnetic separation or by fractional electrol sis. Isotopes can a so be made by bombarding natural elements with neutrons or charge particles of high kinetic energy. dY P P For the purposes of Note 6 to this Chapter and of headings 28.44 and 28.45, the term isotopes covers not only isotopes in their pure state but also chemical elements whose natural isotopic composition has been artificially modified by enrichin the elements in some of their isoto es (wh~chis the same as depleting them in some other$, or by converting, thmugh a nuc ear reaction, some isotopes into other, artificial isotopes. For exam le, chlonne of atomic wei 35.30 obtained by enriching this element to contam 85 % of ch orine 35 (and consequently y depleting it to contain 15 % of chlorine 37) is considered as an isotope. f f P It should be noted that elements existing in nature in the monoisotopic state, e.g., beryllium 9, fluorine 19, aluminium 27, hosphorus 31, manganese 55, etc., are not to be considered as isotopes, but are to be classi led, ~neither the free or the combined state, according to the case, in the more specific headings relating to chemical elements or to their compounds. ? Radioactive isotopes of these same elements obtained artificially (e.g. Be 10, F 18, A1 29, P 32, Mn 54) are, however, to be considered as isotopes. d Since artificial chemical elements generally with an atomic number greater than 92, or transmanic elements) do not have a ixed isotopic composition but one whch varies according to the method of obtaining the element, it is impossible m these cases to distinguish between the chemical element and its lsotopes for the purposes of Note 6. This heading covers only those isotopes which ossess the phenomenon of radioactivity (described below); stable isotopes, on the other han ,are classified in heading 28.45. d' Certain nuclides, whose nuclei are unstable, whether in the pure state or in the form of compounds, emit complex radiations producing physical or chemcal effects such as : (1) Ionisation of gases. (2) Fluorescence. (3) Fogging of photographic plates. These effects make it possible to detect these radiations and to measure their intensity by using, for example, Gei er-Muller counters, proportional counters, ionisation chambers, Wilson chambers, bubble ow counters, scintillation counters, and sensitised films or plates. This is the phenomenon of radioactivity; chemical elements, isotopes, compounds and, in general, substances that display it are called radioactive. (110 RADIOACTIVE CHEMICAL ELEMENTS, RADIOACTIVE ISOTOPES AND THEIR COMPOUNDS; MIXTURES AND RESIDUES CONTAINING THESE PRODUCTS (A) Radioactive elements. Within this heading fall the radioactive chemical elements referred to in Note 6 (a) to this Cha ter, namely : technetium, romethium, polonium and all elements of greater atomic num er, such as astatine, ra on, francium, radium, actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium and lawrencium. ! f These are elements generally composed of several isotopes which are all radioactive. On the other hand, there are elements composed of mixtures of stable and radioactive isotopes, such as potassium, rubidium, samarium and lutetium (heading 28.05), which, because the radioactive isotopes have a low level of radioactivity and constitute a relatively low percentage of the mixture, can be considered as practically stable and thus do not fall in this heading. On the other hand, the same elements ( otassium, rubidium, samarium, lutetium), if enriched in their radioactive isotopes (K 4 , Rb 87, Sm 147, Lu 176, respectively), are to be considered as radioactive isotopes of this heading. B (B) Radioactive isotopes. To the natural radioactive isotopes otassium 40, rubidium 87, samarium 147, and lutetium 176 already mentioned, may e added uranium 235 and uranium 238, which are discussed in more detail in Section (IV)below, and certain isotopes of thallium, lead, bismuth, olonium, radium, actinium or thorium, which are often known by a name different om that of the cones onding element. This name refers rather to the element fiom which they were derived y radioactive conversion. Thus, bismuth 210 is called radium E, polonium 212 is called thorium C' and actinium 228 is called mesothorium II. g g g Chemical elements which are normally stable may nonetheless become radioactive either after bombardment with particles havlng a very high kinetic energy (protons, deuterons) issuing fiorn a article accelerator (cyclotron, synchrotron, etc.) or after absorbing neutrons in a nuc ear reactor. P The elements thus transformed are called artificial radioactive isotopes. Of these, a b u t 500 are known at present, of which close to 200 are already being used in applications. Apart fi-om uranium 233 and the plutonium isotopes, which are r scussed tlCa1 later, some of the most important are : hydrogen 3 (tritium), carbon 14, sodium 24, hosphorus 32, sulphur 35, potassium 42, calcium 45, chromium 51, iron 59, cobalt 60, L Tton 85, strontium 90, yttrium 90, palladium 109, iodine 131 and 132, xenon 133, caesium 137, thulium 170, iridium 192, gold 198, and polonium 210. Radioactive chemical elements and radioactive isotopes transform themselves naturally into more stable elements or isotopes. The time required for the quantity of a given radioactive isotope to decrease to one-half that initially present is known as the half-life or transformation rate of that i s o t o ~ It ~. varies fiom a fiaction of a second for c e q j n highly radioactive isotopes 0.3 x 1 - for thorium C') to billions of years (1.5 x 10 years for samarium 147) an constitutes a convenient yardstick of the statistical instability of the nucleus concerned. d Radioactive chemical elements and isotopes fall in this heading, even when mixed together or with radioactive compounds, or w ~ t hnon-radioactive materials (e.g., ?processed irradiated targets and radioactive sources), rovided that the specific radioactivity of the product is greater than 74 Bq/g (0.002 ~AC!I/& (C) Radioactive compounds; mixtures and residues containing radioactive substances. The radioactive chemical elements and isotopes of the present heading are oRen used in the form of compounds or products which are " labelled (i-e., contain molecules with one or more radioactive atoms). Such compounds remain classified in this heading, even when dissolved or dispersed in, or mixed natural1 or artificially with, other radioactive or non-radioactive materials. These elements an isotopes are also classified in this heading when in the form of alloys, dispersions or cermets. B Inorganic or organic compounds, chemical1 or otherwise constituted of radioactive chemical elements or radioactive isotopes, an solutions thereof, still fall in this headin , even if the specific radioactivity of these compounds or solutions is below 74 Bq g (0.002 pCi1g); on the other hand, alloys, dispersions (including cermets), ceramic products and mixtures containing radioactive substances (elements, isotopes or corn ounds thereof) fall in this heading if their specific radioactivity 1s greater than 74 Bqlg (0. 02 @i/g). The radioactive elements and isotopes, ve rarely used in their free form, are commercially available in chemical compounds or a loys. Apart from compounds of fissile and fertile chemical elements and isotopes, which are mentioned in Section (W)below on account of their characteristics and importance, the most important radioactive compounds are : ~7 f (1) Radium salts (chloride, bromide, sulphate, etc.) used as radiation sources for treating cancer or for certain experiments m physics. (2) Compounds of radioactive isotopes referred to under (m[) (B) above. Artificial radioactive isotopes and their compounds are used : (a) In industry, e.g., for metal radio aph for measuring the thickness of sheets, plates, in an inaccessible container; for facilitating etc.; for measuring the level of vulcanisation; to tngger off polymerisation or grafting of several organic compounds; for the manufacture of luminous paint (mixed, for example, with zinc sulphde); for clock and watch dials, instruments, etc. fqui& (b) In medicine, e. ., for diagnosing or treating certain diseases (cobalt 60, iodine 131, gold 198, phosp oms 32, etc.). f (c) In agriculture, e.g., for sterilising a cultural produce; to prevent germination; for studies of fertiliser application or of ertiliser absorption by plants; to induce genetic mutations thus improving strains, etc. (cobalt 60, caesium 137, phosphorus 32, etc.). F' (d) In biology, e.g., for studying the functioning or development of certain animal or vegetable organs (tritium, carbon 14, s o & m 24, hosphorus 32, sulphur 35, potassium 42, calcium 45, iron 59, strontium 90, iodine 1 1, etc.). (e) In physical or chemical research. Radioactive isotopes and their compounds are normally put up in the form of powders, solutions, needles, thread or sheets. They are generally contained in glass ampoules, in hollow platinum needles, in stainless steel tubes, etc., which are packed in anh-radiation metal outer containers (general1 of lead), the choice of thickness of which depends on the degree of radioactivi of t e isotopes. In accordance with certain international agreements, a special la el must then be affixed to the container, giving particulars of the isotope contained therein and its degree of radioactivity. '% K Mixtures may include certain neutron sources formed by associating (in a mixture, alloy, combinations, etc.) a radioactive element or isotope (radium, radon, antimony 124, americium 241, etc.) with another element (be Ilium, fluorine, etc.) in such a way as to produce a (y,n) or (a,n) reaction (introduction o a y-photon or an a-particle, respectively, and emission of a neutron). However, a11 assembled neutron sources, ready to be introduced into nuclear reactors to initiate a fission chain reaction, are to be considered as reactor components and consequently are to be classified in heading 84.01. Microspheres of nuclear fuel coated with layers of carbon or silicon carbide intended for introduction into spherical or prismatic fuel elements fall in this heading. Also included in this heading are the roducts used as luminophores, which have small uantities of radioactive substances ad?ded to make them self-luminescent, provided that %e resulting specific radioactivity is greater than 74 Bqig (0.002 pCilg). Of the radioactive residues, the most important from the point of view of re-use are : a water : after a residence time of varying length in a nuclear reactor, some of the euterium in the heavy water is converted, by absorption of neutrons, into tritium and thus the heavy water becomes radioactive. (1) Irradiated or tritiated hea (2) Spent (irradiated) fuel elements (cartrid es), generally very high1 radioactive, mainly used for the urpose of recovering the issile and fertile materia s contained in them (see Section below). B i (IV)PISSILE AND FERTILE CHEMICAL ELEMENTS AND ISOTOPES AND COMPOUNDS THEREOF; MIXTURES AND RESIDUES CONTAINING THOSE SUBSTANCES (A) Fissile and fertile chemical elements and isotopes. Certain of the radioactive chemical elements and isoto es mentioned in Section @I) have a high atomic mass, for example thorium, uranium, p utonium and americium, of which the nucleus of the atom has a particularly complex structure. These nuclei, when subjected to the action of subatomic particles (neutrons, protons, deuterons, tritons, a particles, etc.) may absorb these particles, thereby increasing their instability to a degree sufficient to cause them to split into two nuclei of medium weight with neighbouring masses (or more rarely into three or four fra ents). This disiate ation liberates a considerable amount of energy and is accompanieEY the formation o secondary neutrons. It a known as the process of fission or nuclear bipartition. P F Fission only seldom occurs spontaneously or under the action of photons. The secondary neutrons released at the time of fission may cause a second fission to take place thus creating secondary neutrons and so on. The repetition of this process produces a chain reaction. The probability of fission is generally very high for certain nuclides (U 233, U 235, Pu 239) if slow neutrons are used, i.e., neutrons of an average speed of a proximate1 2,200 dsec. (or an ener of 1/40 of an electron volt (eV)). As this spee corresponJ approximately to that of e molecules of a fluid (thermal motion) slow neutrons are also sometimes called thermal neutrons. a C At present, fission caused by thermal neutrons is that most used in nuclear reactors. For this reason, the term fissile is commonly used to describe isotopes which under o fission b thermal neutrons, particularly uranium 233, uranium 235, plutonium 239 and t e chemica elements that contain them, particularly uranium and plutonium. f r Other nuclides, such as uranium 238 and thorium 232 only undergo direct fission by fast neutrons and are commonly considered, not as fissile, but as fertile. The " fertility " comes fiom the fact that these nuclides can absorb slow neutrons, giving rise to the formation of plutonium 239 and uranium 233, respectively, which are fissile. In thermal nuclear reactors (with slowed-down neutrons), since the energy of secondary neutrons released b fission is much hiper (approximately 2 million e these neutrons have to be slowed own if a chain reaction is to take place. This can be ac ieved by means of moderators, i.e., products with a low atomic mass (such as water, heavy water, certain hydrocarbons, graph~te,beryllium, etc.) which, although they absorb art of the ener the neutrons by a succession of shocks, do not absorb the neutrons emselves or a sorb Of only a negligible proportion of them. "1: d tE P In order to start and maintain a chain reaction, the average number of secondary neutrons produced by fission must more than compensate the neutrons lost by the phenomena of capture and escape not leading to fission. The fissile and fertile chemical elements are listed below : (I) Natural uranium. Uranium in the natural state is composed of three isotopes : uranium 238, which forms 99.28 % of the total mass, uranium 235 which represents 0.71 %, and a negligible uantity (about 0.006 %) of uranium 234. Consequently, it can be considered as both a Zssile element (because of its U 235 content) and a fertile element (because of its U 238 content). Uranium is mainly extracted from pitchblende, uraninite, autunite, b m e r i t e , carnotite or torbernite. It is also obtained from other secondary sources, such as residues from the manufacture of supe hosphates or gold-mine waste. The normal. process is reduction of the tetrafluori e by means of calcium or magnesium, or by electrolysis. 2' element, very heavy (specific gravity 19) and hard. It but tarnishes on contact with the oxygen in the air, oxidises and ignites rapidly when in contact with air. Uranium is normally marketed in the form of ingots ready for polishing, filing, rolling, etc. (to produce bars and rods, tubes, sheets, wire, etc.). (2) Thorium. Since thorite and orangite, though very rich in thorium, are rare, thorium is mainly obtained fiom monazite which is also the source of rare-earth metals. The im we metal takes the form of an extremely pyro horic gre powder. It is obtaine by electrolysis of the fluorides or by reduction o the fluori es, chlorides or oxides. The resulting metal is purified and sintered in an inert atmosphere and transformed into heavy steel-grey ingots (s ecific gravity 11.5); they are hard (although softer than uranium) and oxidtse rap1fly on contact with air. P B B These in ots are rolled, extruded or drawn to produce sheets, rods, tubes, wire, etc. Natural t orium consists essentially of the isotope thorium 232. f Thorium and certain thorium alloys are mainly used as fertile materials in nuclear reactors. Thorium-ma esium and thorium-tungsten alloys, however, are used in the aircraft industry or in t e manufacture of thermionic devices. r Articles or parts of articles, made of thorium of Sections XVI to XI.are excluded from this heading. (3) Plutonium. Industrial plutonium is obtained by irradiating uranium 238 in a nuclear reactor. It is very heavy (specific avity 19.8), radioactive and highly toxic. It is similar to uranium in appearance, an m its oxidising propensities. F It is put up in the same commercial forms as enriched uranium and requires the greatest care m handling. The fissile isotopes include : (1) Uranium 233 : this is obtained in nuclear reactors from thorium 232, which is transformed successively into thorium 233, protactinium 233 and uranium 233. (2) Uranium 235 : this is the only fissile uranium isotope which occurs in nature, being present in the proportion of 0.7 1 % in natural uranium. To obtain uranium enriched in U 235 and uranium depleted in U 235 (enriched in U238), uranium hexafluoride is submitted to isotopic separation by the electro-magnetic, centrifugal or gas-diffusion processes. (3) Plutonium 239 : this is obtained in nuclear reactors from uranium 238, which is successively transformed into uranium 239, neptunium 239 and plutonium 239. Also to be mentioned are certain isotopes of transplutonium elements such as californium 252, americium 241, curium 242 and curium 244, which can give rise to fission (whether spontaneous or not) and which can be used as intense neutron sources. Of the fertile isotopes, apart fiom thorium 232, depleted uranium (is. depleted in U 235 and consequently enriched in U 238 should be mentioned. This metal is a by-product of the production of uranium enriche in U 235. Because of its much lower cost and the large quantities available, it replaces natural uranium, especially as a fertile material, as a protective screen against radiations, as a hea metal for the manufacture of fly-wheels or m the preparation of absorbent compositionsxetters) used for purifying certain gases. d Articles or parts of articles, made of uranium depleted in U 235, of Sections XVI to XIX are excluded from this heading. @) Compounds of fissile and fertile chemical elements or isotopes. The following compounds, in particular, fall in this heading : (I) of uranium : (a) the oxides U02, U308,and U03 (b) the fluorides UF4 and m6(the latter sublimes at 56 OC) (c) the carbides UC and UCz (d) the uranates Na2U207and (NH4)U207 (e) uranyl nitrate U02(N03)2.6 H20 (f) uranyl sulphate U02S04.3 H20 (2) of plutonium : (a) the tetrafluoride PuF4 (b) the dioxide Pu02 (c) the nitrate P u O ~ ( N O ~ ) ~ (d) the carbides PuC and Pu2C3 (e) the nitride PUN. The uranium or plutonium compounds are mainly used in the nuclear industry, either as intermediates or as finished products. The uranium hexafluoride is usually presented in sealed containers; it is rather toxic and should therefore be handled with care. (3) of thorium : r (a) oxide and h droxide. Thorium oxide (Thoz) (thoria) is a whitish-yellow powder, inso uble in water. The hydroxide (TII(OH)~)is hydrated thoria. Both are obtained from monazite. They are used m the manufacture of as-mantles, as refractory products or as catalysts (acetone synthesis). The oxi e is used as fertile material in nuclear reactors; C @) inorganic salts. These salts are usually white. The most important are : (i) thorium nitrate, appearing in the more or less hydrated state as crystals, or as powder (calcined nitrate). It is used to prepare luminescent paints. Mixed with cerium nitrate it is used to impregnate gas-mantles; (ii) thorium sul hate, a crystalline owder, soluble in cold water; thorium hydrogen su phate and alkali dou le sulphates; i' P (iii) thorium chloride (ThC14), anhydrous or hydrated, and oxychloride; (iv) thorium nitride and thorium carbide. Used as refractory products, as abrasives or as fertile materials in nuclear reactors; (c) organic compounds. The best known organic compounds are thorium forrnate, acetate, tartrate and benzoate, all used in medicine. Alloys, dispersions (including cermets), ceramic products, mixtures and residues containing fissile or fertile elements or isotopes or inorganic or organic compounds thereof. The principal products in this group are : (1) Alloys of uranium or plutonium with aluminium, chromium, zirconium, molybdenum, titanium, niobium or vanadium. Also uranium-plutonium and ferro-uranium alloys. (2) Dispersions of uranium dioxide (UOz) or of uranium carbide (UC) (whether or not mixed with thorium dioxide or thorium carbide) in graphite or polyethylene. (3) Cermets consisting of various metals (e.g. stainless steel) together with uranium dioxide (UOz) plutonium dioxide (PuOz uranium carbide UC) or plutonium carbide (PuC) (or these compounds mixed with orium oxide or car ide). b These products in the form of bars, plates, spheres, threads, powder, etc., are used either for the manufacture of fuel elements or, sometimes, directly in the reactors. Bars, plates and spheres, contained in a sheath and fitted with special attachments for handling purposes, fa11 in heading 84.01. (4) Spent or irradiated fuel elements (cartridges), that is, those which, after more or less extensive use, must be replaced (e.g., because the accumulation of fission products is hampering the chain reaction or because the sheath has deteriorated). After sufficiently long storage in ve deep water to cool them and to allow their radioactiviz to decrease, these fue elements are trans orted in lead containers, to speci ised installations equip ed for the recovery o the residual fissile material, of the fissile the transformation or fertile elements (which are usually material derived contained in fuel elements) and of fission products. gem ?