34 C.F.R. § 650.31
How does an individual accept a fellowship?
(a) An individual notified by the Secretary of selection as a fellow shall inform the Secretary of the individual's acceptance in the manner and time prescribed by the Secretary in the notification.
(b) If an individual fails to comply with the provisions of paragraph (a) of this section, the Secretary treats the individual's failure to comply as a refusal of the fellowship.