 What episode number in the series was 'igo nuclear'?
CREATE TABLE table_2576 (
    "No. in series" text,
    "No. in season" text,
    "Title" text,
    "Directed by" text,
    "Written by" text,
    "Original air date" text,
    "Production code" text,
    "U.S. viewers (millions)" text
)
SELECT "No. in season" FROM table_2576 WHERE "Title" = 'iGo Nuclear'