Sentence:  Upon the <WSD>expiration</WSD> of a period deemed adequate for demonstration purposes for each plant , but not 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:  a coming to an end of a contract period