Published in the December 2010
issue of the Canadian
Nuclear Society Bulletin, Vol.30, No.4.
The Curious Case of CANDU
by Jeremy Whitlock
Ladies and gentlemen, our next subject is a curious case indeed. At the time of CANDU's birth in 1962, it was readily appreciated that it was an odd creature with certain needs - but nobody suspected that it was, it seems, inverted in time. That is to say, it was born advanced, with features of a mature and progressive industry, and as it aged the rest of the global industry caught up with it.
When first encountered, CANDU had not yet been named. It was simply "Nuclear Power Demonstration", and it lived in a deep hole at the top of a cliff near a hydro dam. It was small (about 20 megawatts electrical), much like designs only now being proposed for distributed or small-grid applications. It had passive safety features - slow kinetics, low energy density, low excess reactivity, distributed pressure boundary, large heat sink - and as you've just heard: earthen (in fact, bedrock) containment. These are all features you will note are finding their way into design proposals today.
NPD, as it was called, was designed with replacement of individual fuel channels (if required) in mind. Aging management, in fact, has been a natural part of CANDU operation from the beginning. With its pressure boundary in the high neutron flux of the core, it was required at an early age to know as much about the longevity of the boundary material as possible. Much later, such concerns came to light in the rest of the global industry as it naturally aged.
As CANDU grew it retained many of its passive safety features, but it moved above ground like the rest of the world's designs. It always used natural uranium however, and therefore required none of the enrichment technology that decades later would cause considerable consternation in the hands of questionable state nuclear programs. Its natural uranium design also allowed it to make the most efficient use of the earth's fissionable ores, in particular uranium or thorium. Resource efficiency and thorium usage have emerged as topical items only recently, as plans for expanding the global reactor fleet lead to dour projections of uranium supply.
Similarly, CANDU's lack of a pressure vessel has always decoupled it from the planet's large forging capabilities, the importance of which became clear as the first orders for new LWRs were placed at the outset of the nuclear renaissance.
By the time CANDU had grown to its Pickering size it was being controlled by digital computers, long before this became commonplace in the rest of the global nuclear industry. Digital control is of particular benefit to CANDU with its more distributed control system, but it's a good idea with any large reactor as everyone else soon realized.
Other things were realized as the world's nuclear industry aged. Intermediate storage of spent nuclear fuel delays the need for a repository while freeing up space in the storage pools. A particularly useful and long-practiced activity in CANDU due to its high fuel throughput (and quite easily implemented), dry storage transfer will now see increasing interest as the rest of the world runs out of storage space with few long-term options ready to go.
Another area is safeguards, where the world must yet come to grips with how it will protect the variety of advanced technologies and fuel cycles - some on-load refuelled - against proliferation efforts. CANDU has much to offer in this respect, having dealt with similar issues for decades and emerging as the most comprehensively safeguarded commercial reactor on the planet.
Ladies and gentlemen, at this stage in its development CANDU finds itself still slightly ahead of its cousins, but the gap is closing. Meanwhile there are signs of crossing, as the mainstream nuclear industry is just now waking up to the value of a neutron-efficient machine with on-load refuelling capability, plus the non-proliferation benefit of natural uranium fuel cycles, while CANDU is taking evolutionary steps away from these fundamentals.
A curious case, to be sure.