TimelineJun 4, 2026actionSmall reactorsPrivately developed Mark-Zero reactor goes critical at Idaho National Laboratory, DOE says[1][2][3]1KBOIJun 4, 20262The National News DeskJun 4, 20263World Nuclear NewsJun 5, 2026Key score 0.66iNovelty 0.47i4 sources, 4 outletsAll 4 sources