Heavy water natural abundance

The only large-scale use of deuterium in industry is as a moderator, in the form of D2O, for nuclear reactors. Because of its favorable slowing-down properties and its small capture cross section for neutrons, deuterium moderation permits the use of uranium containing the natural abundance of uranium-235, thus avoiding an isotope enrichment step in the preparation of reactor fuel. Heavy water-moderated thermal neutron reactors fueled with uranium-233 and surrounded with a natural thorium blanket offer the prospect of successful fuel breeding, ie, production of greater amounts of (by neutron capture in thorium) than are consumed by nuclear fission in the operation of the reactor. The advantages of heavy water-moderated reactors are difficult to assess. 

The very low D/H natural abundance ratio (0.015% = 150 ppm) is responsible for the high cost of heavy water. Materials balance requires a minimum of 7x 103 mol feed per mol of product, and that increases even more for reasonable values of tails analysis (in some plants the feed/product ratio has reached nearly 4 x 104). At peak Canadian production, 800t year-1, this amounted to feeds of 3 x 107t year-1. Clearly that figure demands a cheap and easily accessible feed (i.e. water), or alternatively, requires deuterium production to be parasitic on some large industrial process, for example the production of NH3 for fertilizer, or petrochemical processing. 

Deuterium or "Heavy Hydrogen". D2 gas, mw 4.03 fr p 13-95°K, bp 20.57°K, d of liq 169 g/liter was first isolated in concns sufficient for positive identification by Urey et al at Columbia Univ in 1931. Deuterium is a stable isotope and occurs in natural hydrogen, water and other H-bearing compds in an av abundance of 0.015 mole %. It is of interest to research workers as a tracer in biological processes in chem reactions. There is now commercial production of Deuterium St Heavy Water (Ref 2). It... 

Deuterium is abundant, naturally occurring and in wide use now as D20 in heavy-water-mo derated reactors. Tritium is a radioactive isotope with a 12.3-year half-life and does not occur in natnre. Tritium emits an electron and decays to stable helium-3. 

Grosse etal measured highly concentrated heavy water samples produced from surface waters that contained concentrations of natural tritium approximately one million times higher than the original water. Observations have shown that concentrated samples are indeed radioactive to a level corresponding to a natural abundance of tritium of about 10 by atomic ratio. 

Heavy water, D2O, is of interest as a nuclear reactor moderator. A typical power reactor requires some 200 tons of heavy water. Since the natural abundance of deuterium is 1/7000, an annual production of 200 tons of D2O requires the processing of a hydrogenous feed at a rate between 2 million ton/yr., for 100% extraction, to 20 million ton/yr., for 10% extraction. Only water, sulfuric acid, and petrochemicals are processed in this amount and can supply such quantity of feed material. 

Use of heavy water both as a neutron moderator and as a coolant in fission reactors has resulted in its worldwide demand and many heavy water plants (HWPs) have been constructed. This interest in heavy water, both scientific and technological, has been responsible for the generation of immense information regarding its various aspects. In this article, the nuclear and other properties of heavy water, its production from natural abundance, upgrading and detritiation of downgraded and irradiated water in the reactor are reviewed and an attempt is made to present an overview of the major aspects of heavy water management in nuclear power generation. 

In 1959, Sulzer Brothers designed and built a plant to distill electrolytic hydrogen enriched to six times the natural abundance of deuterium, which was available at the ammonia plant of Emswerke AG, at Ems, Switzerland [HI]. At this plant, about 2 MT/year of heavy water were... 

The natural abundance of deuterium in water is, however, only 150 p.p.m. and hence much electrolysis is necessary to product a significant percentage of heavy water. In practice, it is best achieved by using a cascade of cells through which the water passes as it becomes enriched. This would now only be an economic procedure if cheap electricity was available and the hydrogen gas could be used fruitfully. 

Evidently, throughout all these processes the isotopic composition of uranium remains in its natural abundance form, that is, containing 0.72% of U. Uranium of this composition is suitable for use as nuclear fuel in reactors that operate with heavy water (DjO) such as CANDU reactors or graphite (such as the old Magnox reactors) as the moderator for slowing neutrons. In this case, the UO2 is ground and sintered to form pellets that will be placed in fuel elements (see Chapter 2 for analytical procedures to characterize these pellets). As an example. Frame 1.4 describes the process used in India for production of UO2 powder, pellets, and fuel elements as an example of the processes in the UCF. 

Small cells are utilized to electrolyse deuterium or tritium containing water There are two applications for these cells (t) they may be operated in a similar fashion to conventional water clectrolysers but producing deuterium or tritium gas (in place of pure hydrogen) frorn DjO. DHO or HTO (2) (and more commonly) the cells may be used to concentrate the amount of deuterium or tritium in the electrolyte (DTO DHO HTO or DjO), This is made possible by kinetic factors which determine that hydrogen is evolved more rapidly than deuterium or tritium, e.g. hydrogen is evolved from 2 to 10 times faster than deuterium. The natural abundance of deuterium in water is very low (c. 150 mg dm" Hence, extensive electrolysis is required to produce a stgnih-cant level of heavy (dcutcriatcd) water. 

In fact, Canada did use heavy water as the moderator in nuclear power plants, which used natural-abundance uranium. These days heavy water is obtained using the Girdler process, which employs an exchange reaction for which the equilibrium constant is very favorable (A - 1.01) ... 

Monodeuterated water HDO has an abundance in natural water of approximately 600 ppm. One of the methods used in early attempts to separate the two isotopes was fractional distillation, for which the separation factor at 1 atm was found to be 1.026. Although this procedure was ultimately superseded by the more efficient chemical-exchange processes for larger-scale production of heavy water, distillation remains the separation method of choice for upgrading small amounts of heavy water that have been contaminated by atmospheric water vapor. Distillation is in this case carried out at reduced pressure to take advantage of the higher separation factor. Suppose we wish to carry out the distillation of H2O-HDO at ambient temperatures, i.e., at subatmospheric... 

Previous experimental work on plate efficiency and other design parameters such as allowable vapor velocity, plate spacing, weir height, and downcomer area permitted the design of a pilot plant. The pilot plant is capable of increasing the concentration of HD in from its natural abundance, 0.03 per cent, to 3 per cent, and has a capacity equivalent to the production of 45 pounds of heavy water in an 8000 hour year. 

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