Boil water in a 3 quart sauce pan.  Add the shrimp and boil for 3 minutes. Remove them from the water and set aside. Reserve 16 shrimp as garnish, 2 for each bowl of soup.

In a 3-4 quart saucepan (a small Dutch Oven would be perfect for this) over medium heat, melt the butter. Add the onions, season with salt and pepper, and cook until soft, about 4-5 minutes. Add the tomato paste and cook for another 3-4 minutes, stirring continually to prevent the paste from burning. 

Add the clam juice and rice. Bring to a boil. Reduce the heat to a simmer and cover. Cook for about 15-20 minutes until the rice is tender.

Remove the lid and add the shrimp. Heat through for about 1 minute.

Working in batches, add the soup to a blender or food processor and puree until smooth.  Return the soup to the saucepan and whisk in the half and half and cayenne pepper. Taste and make any needed salt and pepper adjustments at this time..