The web search results confirm that China has been testing and developing its own nuclear technology (e.g., CF3 fuel testing at Qinshan). However, none of the provided evidence explicitly states that China 'recently completed a key test at its next-generation, domestically developed nuclear reactor' as a singular, definitive event. The evidence is suggestive but lacks a single, definitive source confirming the completion of a 'key test' at a specific time.

The claim provides specific historical data (from three reactors to 60 operating reactors) and future development plans. While the general topic of China's expanding nuclear capacity is present in the web search results, the specific numerical progression (3 to 60) and the framing of becoming the 'world's leading atomic powerhouse' are not directly confirmed by multiple independent sources. The evidence is general regarding China's nuclear growth but does not corroborate the specific statistics provided.