Based on your assumptions, the fusion reactor was probably a gray project, contracted through companies that were legally bound to keep the research and development highly classified, similar to classified military tech like the B2 Stealth Bomber:
During its design and development, the Northrop B-2 program was a gray project before its revelation to the public. Unlike the Lockheed F-117 program, which was a black project, the type of military project of which very few people knew even existed while it was being designed and developed, more people within the United States federal government knew about the B-2 and more information about the project was available. Both during development and in service, there has been considerable importance placed to the security of the B-2 and its technologies. Staff working on the B-2 in most, if not all, capacities have to achieve a level of special-access clearance, and undergo extensive background checks carried out by a special branch of the Air Force.
For the manufacturing, a former Ford automobile assembly plant in Pico Rivera, California, was acquired and heavily rebuilt; the plant's employees were sworn to complete secrecy regarding their work. To avoid the possibility of suspicion, components were typically purchased through front companies, military officials would visit out of uniform, and staff members were routinely subjected to polygraph examinations. The secrecy extended so far that access to nearly all information on the program by both Government Accountability Office (GAO) and virtually all members of Congress itself was severely limited until the mid-1980s. Northrop (now Northrop Grumman) was the B-2's prime contractor; major subcontractors included Boeing, Hughes Aircraft (now Raytheon), GE, and Vought Aircraft.
In 1984, a Northrop employee, Thomas Cavanaugh was arrested for attempting to sell classified information to the Soviet Union; the information was taken from Northrop's Pico Rivera, California factory. Cavanaugh was eventually sentenced to life in prison and released on parole in 2001.
The B-2 was first publicly displayed on 22 November 1988 at United States Air Force Plant 42 in Palmdale, California, where it was assembled. This viewing was heavily restricted, and guests were not allowed to see the rear of the B-2. However, Aviation Week editors found that there were no airspace restrictions above the presentation area and took photographs of the aircraft's then-secret rear section with suppressed engine exhausts from the air, to the USAF's disappointment. The B-2's (s/n 82-1066 / AV-1) first public flight was on 17 July 1989 from Palmdale to Edwards AFB.
In October 2005, Noshir Gowadia, a design engineer who worked on the B-2's propulsion system, was arrested for selling B-2 related classified information to foreign countries. Gowadia was convicted and sentenced to 32 years in prison for his actions.
The B-2 bomber is still classified technology, even though it is now deployed in service. People can see it flying around, but there are still many secrets to its design:
The bomber does not always fly stealthily; when nearing air defenses pilots "stealth up" the B-2, a maneuver whose details are secret.
So we know that a government can, through the methods used on the B-2, create a technology that no one else knows much of anything about. Why they would do that for fusion is up to your story.
I can't tell you why people in your story make choices, but if you are looking for inspiration, there are a couple possibilities:
Dominance through simple energy superiority and independence:
Countries succeed by leveraging resources. Sources of energy have become one of the most important resources. Fossil fuels are a big deal. Countries that have it or can get it prosper. Those who can't fall behind. If your country has fusion but others do not, you race ahead of them, making yourself a dominant power. You can make fusion-powered war machines and they can't. Your electricity is so cheap that manufacturers start moving operations to your country, boosting your economy. They wouldn't make that move if fusion will soon be coming to their current location. Your street lights and electric trains run for almost free. Your citizens can charge their electric cars for almost free. Your GDP would explode. If you share this tech, then the world advances together. If you withhold it, then you pull far ahead of the others.
Keep control of the reactors, but sell the electricity for money and political or strategic deals.
The superpower that has the fusion tech under government control could make deals to sell the power output, but not the tech itself, to other countries. If another country wants to be able to buy the cheap power, they have to give the superpower some land (of the superpower's choosing). The superpower will build a reactor and also a military base on that land. The reactor will be on super-power sovereign territory, and protected by the base, but the small nation's power grid can connect some power lines to the output. This is a sweet deal for the superpower. They get to build new military outposts and also sell electricity. Since the small nation is now dependent on the electricity, they are close to being a vassal-state of the superpower, who can threaten to cut the power if they misbehave.