Russia's war in Ukraine has strained oil and gas supplies.  A study published by Columbia University's Center on Global Energy Policy said the nuclear sector would also be affected, since Russia is also the dominant player in the global supply chain for nuclear reactor technology.  The report was co-authored by Paul Dabbar, a former DEPUTY secretary of the U.S. Department of Energy, and Matthew Bowen, a research scholar at Columbia's Center for Global Energy Policy.  
Of the 439 nuclear reactors in operation in 2021, 38 are in Russia, and 42 other nuclear power plants in other countries are built with Russian reactor technology.  By the end of 2021, there are 15 more plants under construction using Russian technology.  
Reducing or eliminating dependence on Russia's nuclear supply chain will vary by country and demand.  If a country has not yet built a nuclear reactor, they can decide from the outset not to sign a contract with Russia, with the US, France, South Korea and China as "viable" supplier options.  
Second, if a country already has a Russian nuclear reactor, it needs Russia to seek repair parts and services.  In such cases, countries can also get repair assistance from Pennsylvania-based Westinghouse.  
Then there's fuel.  Nuclear fission reactors are fueled by enriched uranium.  Russia accounts for about 6% of the world's annual raw uranium production.  Russian supplies could be replaced if other countries that mine uranium increase their mining.  
However, uranium does not go directly from the mine to the reactor.  It must be converted and enriched before it can be used as fuel for nuclear reactors.  Russia is a dominant player in this regard.  In 2020, Russia had 40% of the world's total uranium conversion infrastructure;  46% of the world's total uranium enrichment capacity.  
In addition to Russia, these uranium conversion and enrichment capabilities exist in Canada, France, Germany, the Netherlands, the United Kingdom and the United States.  These are "sufficient to replace at least part" of the conversion and enrichment required for western nuclear reactors, but it is unclear whether they will fully replace Russia's capabilities on a global scale. The United States also needs to be ready for fuel to go into advanced reactors currently under development that require uranium enrichment to 15 to 19.75 percent, compared with the 3 to 5 percent enrichment used in conventional light-water reactors currently operating in the United States.  This highly enriched uranium (HALEU) fuel is currently available only from Russia on a commercial scale.  
"More investment in mining, conversion and enrichment facilities is needed to completely free the Western nuclear fuel chain from Russian involvement," Dabbar and Bowen wrote in their report.  However, adding sufficient new conversion capacity and enrichment capacity will take years to complete."  But to persuade private companies to devote money and resources to uranium infrastructure, they need a commitment from the government not to resume Russian supplies.  "The concern is that in a year or two, maybe less, if Russian uranium products are to be allowed back into the national market, it will lead to investment by private companies."  In the US, there is only one uranium conversion facility (in Illinois), which has been idle since November 2017, reopening "pending market improvement and customer support".