Sentence:  Upon the expiration of a period deemed adequate for demonstration purposes for each plant , but <WSD>not</WSD> to exceed such twelve year period , the Secretary shall proceed as promptly as practicable to dispose of any plants so constructed by sale to the highest bidder , or as may otherwise be directed by Act of Congress . 
Meaning of the word marked with <WSD> tags:  negation of a word or group of words