The attack merely infected a system that was not connected to any critical controls in the nuclear facility. Dtrack malware was developed by Lazarus Group, a hacking group believed to be working on behalf of North Korea’s state spy agency. The attack was limited only to an Internet-connected computer used for administrative purposes, which is isolated from any mission-critical system at the facility. If a hacker who wants to sabotage a nuclear facility, the first step would be collecting much information about the targeted organization, including type of devices and possible ways to jump through gaps.
Source: https://thehackernews.com/2019/10/nuclear-power-plant-cyberattack.html