68004
A few particularly fissile and readily obtainable isotopes ( notably U , U and Pu ) are called nuclear fuels because they can sustain a chain reaction and can be obtained in large enough quantities to be useful .
 U, Pu
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 U, Pu