Michael,
although the worst kept secret in nuclear engineering is that there is a huge anomaly in how many "free" neutrons one gets from a deuterium moderator.
Would you care to elaborate?
My former neighbor was a Canadian plant engineer at Pickering during the early days of Candu. These reactors have performed exceptionally well over the years and required fewer refuelings than initially thought. Many operate at over 100% of design. OK you may say they were conservative with the original specs - but that appears not to have been the case. Have you ever known a modest Canadian? - they are almost the equal of Texans (in terms of bluster). I was also told that there is some level of official dis-information in the published material. Try getting that verified.
The higher than expected neutron flux, and better performance, is coming from the moderator/coolant. My impression was that there had been a concerted effort in Canada NOT to let this become common knowledge. This reactor is a big expert item for them. I have seen nothing published which documents this information on accelerated heavy-water degradation, as it was relayed to me. In fairness, if you have ever known any Canadian engineers, then you may realize they have a high capacity to consume beers and tell tall-tales; however, nothing I have heard from others in the US leads me to believe otherwise. On the contrary, many reactor designers here thought the Candu was going to be a huge blunder and are amazed at its success ... therefore, other than the fact that this is not in "print," it is very believable... and needless to say, many important things, in any field or endeavor, are not in print.
As for Robin's question - the only thing I seem to remember is that they will get something like a tenth of a percent per year of decrease in purity (there are many tons of heavy water involved however) and that the moderator is continually being upgraded because of this. If the purity gets too low, then the moderator starts absorbing part of the flux and will shut the thing down. Initially, they never figured on any of this dynamic situation ,which is heavily to their benefit as it turns out. If you make it in tonnage, heavy water is cheap. BTW I don't remember the exact numbers, but that tenth of a percent yearly is probably one heck of a lot of cheap neutrons.
The markup on heavy water, and Candu reactors, puts $tarbucks and Micro$oft to shame.
Jones
