Full text of Dr. Miller's paper also available (PDF, 340 kb).
Low-Cost Heavy Water Crucial to Future of CANDU, Speaker Saysby Jeremy Whitlock
"Heavy water: Manufacturers' Guide for the Hydrogen Century" was the theme of a well-attended public seminar by Alistair Miller of AECL, hosted by the Canadian Nuclear Society on Thursday evening, January 25 in the J.L.Gray Centre.
Dr. Miller has been in the heavy water business for 35 years, joining Chalk River in 1966 as a newly minted chemical engineer from the University of London. A resident of Deep River, Dr. Miller is an expert on the physical chemistry of heavy water, and was President of the Canadian Society of Chemical Engineering from 1997 to 1998.
It is crucial to the future of CANDU technology, says Dr. Miller, to learn how to generate large volumes of heavy water at the lowest possible cost. A typical CANDU reactor takes 450 tonnes of heavy water, adding enormously to the capital cost of these machines. The heavy water is needed to enable a fission chain reaction using natural uranium - a relatively inexpensive process once the initial load of heavy water is procured.
Finding heavy water is as simple as finding water, for it occurs as roughly one part in 7000 within nature. Interestingly, there is a 30% variation in this figure across the country, with the leanest heavy water found in Alberta snowfall, and the highest concentrations in the Arctic ocean. The difference is mainly due to rainfall patterns, since atmospheric water vapour tends to include less heavy water.
The tricky step, however, is separating the heavy water from the lighter variety. Dr. Miller summarized a number of ways to do this, each with problems and merits. Simple water distillation (the process favoured by nature) involves huge volumes of feedwater. This is the method used only in the final stage of commercial production, and in upgrading facilities located at each CANDU station.
Electrolysis was the method first used to separate heavy water during World War Two, but it requires enormous energy input. More recently tuned lasers have been used, achieving unsurpassed separation efficiency, but with great complexity and expense.
The lion's share of the heavy-water separation used for Canada's domestic and export reactors was achieved in massive towers that mixed hydrogen-sulphide gas and water at two different temperatures. These plants are all shut down, and would be too costly to use with today's lower heavy-water demand and stricter environmental regulations.
This has prompted AECL to develop newer, simpler technologies based on the reaction between hydrogen gas and water. The reaction is efficient, but won't happen without a catalyst of some kind, and as it turns out the catalyst chosen must be protected against contact with water in order to work. Hence the "wet-proofed catalyst" technology perfected at Chalk River Laboratories over the decades.
Two technologies based on this concept have been developed, one using trustworthy electrolysis to generate the hydrogen (better for upgrading), and the other using reformed hydrogen from methane (better for bulk production). In both cases implementation relies on a third party producing the hydrogen; for example, AECL has installed a prototype unit on the sidestream of a plant in Hamilton, Ont., where the Air Liquide company commercially generates reformed hydrogen from methane.
Dr. Miller concluded his talk with an optimistic look at the "hydrogen century" that he believes we are now entering. Hydrogen, particularly in fuel cells, will be the fuel of choice he says, and the current popular method for separating hydrogen will have to be replaced by nuclear-powered electrolysis if the net result is to be cleaner air. Dr. Miller suggests that CANDU technology is a natural partner in this scenario, driving the electrolysis cleanly, and deriving heavy water on the side.