Why CANDU is the way it is by Alan Rose A capacity crowd gathered in the Bennett Room of the J.L. Gray centre on May 26 to hear Gordon Brooks, AECL Vice-President and Chief Engineer from 1983 until his retirement in 1991, give his views on the reasons why CANDU is the way it is. Brooks has been active in nuclear engineering since 1959 and has participated in the full evolution of the CANDU power reactor system. Brooks informed the audience that a "CANDU origins and evolution working group" has been formed of retirees in the CANDU design field who are tapping their own memory banks and doing documented research and interviews with old timers from the early days to produce monographs on the critical design decisions that characterised the development of the CANDU power reactor. He said that these papers would be available on the AECL intra-net for the use of future design engineers and possibly would be collected into a single volume for publication in book form. Brooks chose to identify a 1954 study team called the Nuclear Power Group, under Harold Smith of Ontario Hydro, as the starting point of CANDU development even though the reactor name was not adopted until later. The group comprised nine engineers from Canadian utilities, manufacturers and engineering companies with Chalk river providing assistance in physics, metallurgy and chemistry. AECL was at this time heavily involved in the design and construction of NRU. The team started with a vertical reactor concept since ZEEP, NRX and NRU were all vertical reactors. The concept was of a pressure vessel with heavy water moderator and coolant to preserve the use of natural uranium as fuel. Because Canadian industry was not capable of manufacturing pressure vessels of the size required the concept of pressure tubes to hold the fuel in a low pressure calandria was adopted. This in turn led to a horizontal reactor to accommodate on power refuelling and the NPD reactor was designed on this principle and became the prototype of the CANDU. Brooks related anecdotes on how design decisions were made on the many components of a CANDU, such as choosing the fuel bundle length. Scientists had asked for 50 centimetres but the engineers were not metric at the time and rounded it down to 19.5 inches. Auxiliary equipment such as boilers and steam generators became groups of small units rather than single large units based on the availability of these items on the market. He noted that the on-power, bi-directional fuelling prompted the pioneering of digital computer control in Douglas Point. Brooks went on to describe the sequence of construction of Douglas Point, Pickering and Bruce and the CANDU 6 export model, each phase being designed and built before the previous one started up, thereby losing any advantage from operating experience of previous designs. He noted that safety considerations prompted first the addition of containment to the design problem and later the requirement for two separate shutdown systems, necessitating back fitting in some cases, which complicated the designers task. A lengthy question period followed the presentation, equally distributed between young engineers and a number of old timers that were also present.