according to scientists who were deeply involved in nuclear development in the pak chong-hui era , north korean scientists who worked for the nuclear development program , pyongyang 's big science program , can be divided into the first , second , and third generations -- those who defected to the north around the time of the korean war forming the first , their children who defected to the north with them and who later studied in the soviet union making up the second , and those who were educated in north korea , then studied in the soviet union and who are currently working in the nuclear development program representing the third generation .