AUSTRALIAN ATOMIC ENERGY COMMISSION RESEARCH ESTABLISHMENT LUCAS HEIGHTS A BRIEF SURVEY OF PROCESSES FOR HEAVY WATER PRODUCTION R. K. RYAN ABSTRACT A brief review is given of methods for the production of nuclear grade heavy water, including water electrolysis, distillation, and chemical exchange processes. Present world production comes mainly from the U. S. A., however Canadian plants will shortly produce much of the world supply. These plants use the HgO/HgS process, the only one developed to industrial scale. Sufficient develop- ment of the NH,/Hp process has proceeded for its use industrially. Heavy water production as a by-product of established industries appears attractive provided a process matched to the feed supply is available. There are many possible processes that have not been developed. CONTENTS Page No; 1. INTRODUCTION 1 2. SOURCES t 1 3. SEPARATION METHODS 2 3.1 Water Electrolysis . 2 —3.2 Distillation 2 3.2.1 Hydrogen distillation 3 3.2.2 Methane distillation 3 3.2.3 Ammonia distillation 3 3.3 Chemical Exchange Processes 3 3.3.1 HgS/HpO exchange (GS process) 4 3.3.2 Hp/NH, exchange 4 r c o 3.3.3 H20/H2 exchange 5 4. PRESENT WORLD PRODUCTION 5 5. INDUSTRIES CAPABLE OF PRODUCING BY-PRODUCT HEAVY WATER 6 6. ADDITIONAL POSSIBLE PROCESSES FOR HEAVY WATER PRODUCTION 6 7. SUMMARY . 6 8. ACKNOWLEDGEMENTS 7 9. REFERENCES 7 Table 1 Feed Volume Requirements to Produce 100 short tons of D?0 per Year Table 2 Separation Factors of D^O Processes Table 3 Heavy Water Plants in Operation and Under Construction Table 4 Heavy Water Plants Proposed Table 5 Variables Affecting Production and Costs - GS Process Table 6 Industries Capable of Producing By-Product Heavy Water Table 7 Potential Processes Examined (Barr and Drews 1960) Appendix 1 Definition of Separation Factor Figure 1 Heavy Water Inventory v. Unit Rating in Megawatts (Electrical) (After Cochran 1966) Figure 2 Chemical Exchange Processes 1. INTRODUCTION The heavy water inventories for various reactors differ as shown by Cochran (1966) in a chart of heavy water inventory per MWe versus unit ratings (repro- duced in Figure IK Cochran, discussing the chart, suggested: "Curve 1 represents heavy water inventory resulting from a recent Canadian heavy water reactor design optimization for a range of unit ratings. This curve appears compatible with the majority of known heavy water reactor designs. However, there are exceptions at considerably lower inventory levels. # Bohunice has an inventory of 0.5 short tons per MWe because of its low burnup, about 3,000 MWd/tonne. The boiling light water reactor with 1.42 ** per cent. U02 enrichment, according to paper AECL-2010, would have an in- ventory of about 0.34 short tons per MWe. Accordingly, curve 2 represents a possible minimum inventory level that may be encountered in the future". Estimates of heavy water requirements for any country thus depend on the size and number of reactors to be built. Assuming initial reactor installations would be in the range 200 to 500 MWe, the heavy water inventory would be 0.8 to 1.0 short tons/MWe, if natural uranium is assumed to be the fuel. 2. SOURCES Theoretically any hydrogen-containing compound is a potential source of deuterium and, hence, heavy water. However, many compounds have insufficient hydrogen content to be used economically as a source of deuterium. One of the important requirements for a source of nuclear grade heavy water is a pure feed material. Very large quantities of feed material are necessary to recover rather small quantjties of heavy water since the deuterium content of natural hydrogen sources, for example HgO and CH^, is only about 0.015 mole per cent. (Rae 1965> Bebbington and Thayer 1959^. The quanti ty of feed material is further increased by the inefficiency of recovery in some processes. For example, in the GS (Girdler-Spevak or Girdler-Sulphide) process only 20 per cent.of the deuterium present is recovered (Becker 1962K The only feed materials available in suf- ficient quantity are hydrogen, ammonia, methane and water and typical examples of the quantities required to separate 100 short tons of DgO per year are given in Table 1. * A 150 MWe heavy water power reactor in Czechoslovakia for completion in 1968. ** Pon et al. (1964). 3. 2. 3.2.1 Hydrogen distillation 3. SEPARATION METHODS Hydrogen distillation has a high separation factor, but its disadvantage The several methods available for the separation of deuterium from the feed, is the extremely low boiling point of liquid hydrogen, about 20°K. include electrolysis, distillation, arid chemical exchange. Flynn (I960) described a pilot plant for the distillation of hydrogen The"'separation factor (a) is defined as the ratio of the heads abundance to produce deuterium, which was built in 1958 at the National Standards Labor- ratio to the tails abundance ratio (Appendix 1). Separation factors for several atories cryogenic engineering laboratory at Boulder. The preliminary concen- possible processes are compared in Table 2. With other factors equal, the process tration stage, which was the only one piloted, consisted of a 6 inch diameter with the higher separation factor is the belter process. Although the separation tower with an equivalent D00 production rate of 45 lb/8,000 hour year. Stouls & * factor is of great importance since it determines plant sixe, flow rates, etc., (1965), in a patent described an apparatus for the production of deuterium by there is no correlation between Ct and the economic prospects of a process. The hydrogen distillation. At Nangal in India (Table 3) a hydrogen distillation energy consumption and plant cost have to be carefully weighed in each case. plant for the production of deuterium is operated in association with a ferti- lizer plant (Gupta 1965). Becker (1962) in discussing the recovery of deuterium found water electrolysis too costly for pre-enrichment but suitable for final concentration because the 3.2.2 Methane distillation process is simple and easily controlled. Water distillation also is useful only British American Oil Company is developing methane distillation as a source for final concentration, but low temperature hydrogen distillation, which has a of heavy water. Methane distillation is useless at atmospheric pressure (sep- high separation factor, is attractive provided the difficulties associated with aration factor - 1.0002). However, the factor increases at elevated pressure, the low temperatures are overcome. He found that chemical exchange methods are being largest at about 600 p.s.i.a. (Pogorski 1965). Further improvements are most economic, particularly the GS process, which has been run industrially at obtainable by application of ma*gnetic and electric fields and also by use of Savannah River. The H20/H2 and H2/NH3 exchange processes are preferable because additives to give ternary systems. There is no problem from vei^y low temper- of their higher theoretical separatiots factor, but. are not so highly developed. atures as required in the hydrogen distillation process, and the process may 3.1 Wat er Electrolysis actually make use of high pressure liquefied methane if available, for example, Water electrolysis has a high separation factor (Table 2.}. However, the in U. S. A. where helium is recovered from natural gas (C.N.T. 1966a). Shamsul Huq (1966) discussed this process for use in conjunction with fertilizer pro- energy requirements are very high. Some of the energy may be recoverable by duction from natural gas. fuel cells, but generally the process can only be used where power costs are j extremely low and even then only for final concentration (Bebbington et al.1964). 3.2.3 Ammon ia d i st illati on s Alvarez (Alvarez et al.1964) and Otero (Otero and Gispert, x964, Otero et al •, Barr and Drews (1960) discussed ammonia distillation as a possible process I960) are developing electrolytic cascades and burners and are piloting this for deuterium recovery. The vapour pressure and latent heat of ammonia compared process. with water would be advantageous in this distillation if the separation factor 3.2 Distillation were large enough. However^ it is too low for economic deuterium recovery. (Table 2). Four different distillation processes are of interest. Water distillation *» is only of interest for final concentration where simplicity and operational 3.3 Chemical Exchange Processes safety rather than economics are of paramount importance. Proctor and Thayer Chemical exchange depends upon minor differences in the chemical (1962) described an improved vacuum distillation method. The other three pro- properties of isotopes causing a shift in equilibrium of chemical reactions. cesses are (a) hydrogen distillation, (b) methane distillation, and (c) ammonia Examples are: distillation. '.4 5. 4. improved performance using cesium and rubidium amide catalysts. This process (1) may be either monothermal or bithermal. (Figure 2). The small plant at HDO H2S(g) ;» U) 8 Mazingarbe in France (Table 3) is of monothermal type (Rae 1966) and has been H (2) HD(g) 2(g) discussed by Lafrancois (1964) . He suggested that the bithermal exchange may (3) become economically preferable if the low temperature tower is held at -70°C HDD KD(g) or lower. Exchange processes are of two types, monothermal and bithermal (Figure 2). Rae (private communication) has indicated that assessments by A.E.C.L. Monothermal processes allow the recovery of most of the deuterium in the of heavy water processes have shown that the NHj/Hg process does not appear feed as the gas is produced by chemical decomposition of the liquid and then as attractive as at first thought. Pilot plant studies will be undertaken contacted with the incoming liquid, the temperature of the reaction being when it is clear which process offers the most promise of worthwhile cost chosen to give a sufficiently large separation factor.