How to select radicand

Another way

Cases[t1, Sqrt[x_] :> x, Infinity]
(* {2, 3, 5, 6, 7, 31, 10, 11, 17} *)

# & @@@ Cases[t1, _Power, All]

 {2, 3, 5, 6, 7, 31, 10, 11, 17}

Assuming all expressions are either product of a square root or square root on its own:

First /@ Cases[t1, Sqrt[_], Infinity]
(* result: {2, 3, 5, 6, 7, 31, 10, 11, 17} *)

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