subcritical nuclear experiment

nuclear bomb test

specialist; expert

criticality, the critical (state, point, pressure, etc.)

underground nuclear test

atomic nucleus / (algebra) kernel / (astronomy) core, nucleus / cell nucleus

experiment

to know, be aware of / to learn, acquire, / to be acquainted with, to know (someone) / (colloquial, in the negative) to care in the least about, to be concerned with,

speciality/specialty; subject of study; subject of expertise; area of interest; profession

he / distal demonstrative, something far off removed from both speaker and listener: that / boyfriend