23391
U has a much higher fission efficiency by low-energy ( thermal ) neutrons , compared e.g. with U. Most uranium chemistry studies were carried out on uranium-238 owing to its long half-life of 4.4 years .
 U
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