Nuclear is an adjective.
Most English adjectives do not have separate singular and plural forms.
Perhaps you were thinking of the noun nucleus (singular) or nuclei (plural).
The singular form of the demonstrative pronoun 'these' is this.
The singular form of "that" is "it."
The singular form is cuff.
it is already in its singular form.
"Stratum" is the singular form of stratum. The plural form is "strata".
The form hypothesis is the singular form. The plural form is hypotheses.
A singular form of dice is a die
The singular form of feet is foot.
The singular form of Oases is, Oasis.
Phenomenon is the singular form of phenomena.
The singular form of chromosomes is a chromatid.
"Wash" is the singular form of "wash."