19 USC 2137 - Reservation of articles for national security or other reasons

(a) National security considerations 
No proclamation shall be made pursuant to the provisions of this chapter reducing or eliminating the duty or other import restriction on any article if the President determines that such reduction or elimination would threaten to impair the national security.
(b) Action taken under other laws 
While there is in effect with respect to any article any action taken under section 2253 of this title, or section 1862 or 1981 of this title, the President shall reserve such article from negotiations under this subchapter (and from any action under section 2132 (c) of this title) contemplating reduction or elimination of
(A) any duty on such article,
(B) any import restriction imposed under such section, or

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(C) any other import restriction, the removal of which will be likely to undermine the effect of the import restrictions referred to in subparagraph (B).

In addition, the President shall also so reserve any other article which he determines to be appropriate, taking into consideration information and advice available pursuant to and with respect to the matters covered by sections 2151, 2152, and 2153 of this title, where applicable.