The tritium required for ITER will be supplied from the CANDU production in Ontario, but while Ontario may be able to supply 8 kg for a DEMO fusion reactor in the mid-2050s, it will not be able to provide 10 kg at any realistic starting time. The tritium required to start DEMO is uncertain within a wide margin; stocks would likely have to be shared if more than one fusion reactor is built. It is in theory possible to start up a fusion reactor with little or no tritium, but at an estimated cost of $2 billion per kilogram of tritium saved, it is not economically sensible. The US has started producing tritium for defence purposes by irradiating tritium-producing burnable absorber rods containing lithium in a commercial (government-owned) light water power reactor. Argentina, China, India, the Republic of Korea, and Romania also have CANDU-type reactors, in which tritium is generated in the heavy water, however not all these countries extract tritium from their reactors. The Republic of Korea actively extracts tritium from their heavy water reactors, and Romania has definite plans to extract tritium from theirs. Some heavy water reactor tritium production scenarios with varying degrees of optimism are presented, with the assumption that only Canada, the Republic of Korea, and Romania make tritium available to the fusion community. CANDU and similar heavy water reactors could in theory generate additional tritium in a number of ways: (a) adjuster rods containing lithium could be used, giving 0.13 kg per year per reactor; (b) a fuel bundle with a burnable absorber has been designed for CANDU reactors, which might be adapted for tritium production; (c) tritium production could be increased by 0.05 kg per year per reactor by doping the moderator with lithium-6. If a fusion reactor is started up around 2055, governments in Canada, Argentina, China, India, South Korea and Romania will have the opportunity in the years leading up to that to take appropriate steps: (a) build, refurbish or upgrade tritium extraction facilities; (b) extend the lives of heavy water reactors, or build new ones; (c) reduce tritium sales; (d) boost tritium production in the remaining heavy water reactors. All of the alternative production methods considered have serious economic and regulatory drawbacks, and the risk of diversion of tritium or lithium-6 would also be a major concern.