Part 4 of A Historic Review of the Canadian Nuclear Industry: Medical Isotopes and the CANDU Reactor

The below is part four of a five-part series detailing the history of Canada’s nuclear industry written by Michael Alexander Sinclair. It was originally submitted as an essay on November 15, 2017 for Ryerson University’s HST 701: Scientific Technology & Modern Society, and has since been modified for publication onto the Mackenzie Institute website.

There are several critical radioactive isotopes important to medicine. They are used as diagnostic tools in cancer treatments. When Canadians built the first research reactor, it allowed for the first time the commercial manufacturing of rare isotopes. By inserting specific target elements into areas of the reactors with high neutron flux, it was possible to artificially create desired isotopes.^[1] One isotope in particular, cobalt-60, became, and remains, the main source for chemotherapy around the world and is used in approximately 70 per cent of all cancer radiation treatments globally.  This Canadian scientific technology has helped approximately 35 million cancer patients across the world since it was first used in London, Ontario’s Victoria Hospital on October 27, 1951. Cobalt-60 allowed doctors to treat deep tissue tumours without harming the skin. This discovery was unparalleled in importance for cancer treatment since the discovery of X-rays in 1895.[2] For example, following the development of medical applications for radioisotopes, the cure rate for cervical cancer rose to 75 per cent from 25 per cent.

Another major isotope produced by the Canadian nuclear reactors is molybdenum-99. This isotope naturally decays to become technetium-99, which is used in up to 80 per cent of all nuclear diagnostic procedures. This important isotope is used to create images of the brain, thyroid, parath yroid, lungs, liver, kidney, spleen and bone marrow as well as being used in heart disease diagnosis.^[4] Used domestically, this isotope is used in approximately 24,000 of the 30,000 diagnostic image procedures performed every week.^[5] As much as 40 per cent of the world’s production of molybdenum-99 and cobalt-60 was being produced by Canada when the Chalk River reactors were still being used for isotope production. [6] ^[7] The Canadian nuclear industry created entire fields of medicine: nuclear imaging and chemotherapy. The profound effect of Canadian produced isotopes on the world medical community is one of the greatest successes of the Canadian nuclear industry. While research reactors were producing isotopes, Canadian scientists and engineers were not idle. The local expertise developed on Canadian soil also enabled Canada to join the exclusive list of nuclear-powered countries.

Since the 1940s, Canada has designed, built and has even exported its own type of nuclear power reactor: a heavy water moderated reactor designed to use natural uranium as fuel named CANada Deuterium Uranium Reactor (CANDU).^[8] The first full scale CANDU reactor ever built was called the Nuclear Power Demonstration (NPD) . Built in 1962, it provided the proof of concept of nuclear power generation within Canada. Heavy water moderated reactors in Canada were driven by social and economic conditions at the time. Canadians wanted to develop a technology within their own borders as much as possible. The country The reactor core  features pressure tubes, essentially hollow cylinders, which are much easier to manufacture.The CANDU reactor uses natural uranium fuel, made possible through the use of heavy water moderation and zirconium alloys, meaning that fuel enrichment is not necessary. By designing a reactor to use natural uranium, CANDU fuel is and avoids waste created by enrichment processes.^[9]

Nuclear reactors operate on the fundamental generation of heat energy from self-sustained, controlled fission reactions that is then converted to electrical power. Given the necessary hazardous, radioactive materials and extreme conditions for these reactions, reactors require an extreme level of control to operate safely. Loss of control can lead to a failure to sufficiently cool the nuclear fuel within the core causing core melt-downs, radioactive containment breaches and disastrous environmental consequences.[10] Through the use of natural uranium fuel and the ability to constantly re-fuel while online, CANDU reactors have relatively lower reactivity feedback and Power Coefficient of Reactivity (PCR), an incredibly important safety factor that describes how rates of reactivity change as a function of fuel temperature, potential losses of coolant, and coolant temperature. Heavy water kinetics, which are several orders of magnitude lower than light water, also reduce both instantaneous and delayed kinetic effects that could cause instability in the reactor. Thus, the CANDU core reaction is easier to shut down in the case of upset conditions  that could result in loss of control of the nuclear reaction.[12] Due to the fact that the CANDU reactor output is more stable, shutdown procedures have much greater time to react. The core also contains a huge volume of heavy water surrounding the reaction, kept at a relatively low temperature. This mass of heavy water acts as a gigantic heat sink, especially due to water’s large heat capacity, which means that the ability to overheat and thus any chance of a melt-down is greatly reduced. Additionally, CANDU reactors operates near atmospheric pressure, so the systems within the reactor are operating in mild conditions, again allowing for if necessary. CANDU reactors are also built with systems capable of shutting down the nuclear reaction within two seconds to prevent accidents. Neutron absorbing rods, common in most reactor types, that are suspended above the core by electromagnets and liquid neutron poison, less common, stored under compressed helium are both designed to activate in case of electrical power loss.[13] Further, CANDU reactors feature divided thermal loops that contain further divided pressure tubes, which causes any defects or losses of coolant to be very localized and to have a very small effect on the core overall due to this passive isolation. The above characteristics make CANDU reactors inherently safer to operate than alternative designs, something incredibly important due to the potentially devastating consequences of nuclear power reactor disasters. The history of CANDU reactor safe operation is yet another international success for the Canadian nuclear industry.

One very important aspect of CANDU reactors, made possible through low reactivity of their fuel cycle, is their ability to refuel while online, avoiding expensive shutdowns.[15] This is achieved through two identical fueling machines on either side of the fuel channels within the core, one that loads new bundles into one side which pushes old bundles out the other end which are then collected by the other.[16] There are a lot of problems that accompany shutting down nuclear reactors, especially the massive Light water reactors require complete shut downs every 18 to 24 months for large scale re-fueling operations, which typically last 4 to 6 weeks. Direct costs of shutting down light water reactors for refueling can be greater than $800,000 per day, and account for up to 25 per cent of annual operating budgets. This does not account for lost profits from not producing power or replacement power costs, which is can be as great as $500,000 per day.[17] CANDU fuel is manufactured in short bundles, and the core pressure tubes allow spent fuel to easily be exchanged for new fuel. This also allows for any defective bundles to easily be removed from the reactor before they can become a bigger problem, avoiding expensive repairs. This gives CANDU reactors a higher capacity factor, also called capability factor, a metric for how much power is produced versus the theoretical maximum operational power able to be produced. In 2004, for example, the 10 CANDU-6 type reactors in the world achieved an 87.44 per cent annual capacity factor, an exceptionally high figure for nuclear power reactors in general but not uncommon for CANDUs.[19] The flexibility provided by moveable fuel bundles while remaining online also allows CANDU reactors to achieve higher fuel burn-up, meaning that more of the fissile uranium is used up from the fuel and less is wasted. This flexibility also allows operators the ability to even out power distribution across the reactor, which reduces localized hot areas creating a much more symmetrical fuel load throughout the core and a more stable power output.[20]

Stay tuned for the final installment of the series coming soon.