Cut green tops from leeks, and discard. Chop white portions.
Melt butter in a Dutch oven; add leeks and celery, and sauté until tender. Add flour, and cook, stirring constantly, 3 minutes. Stir in broth, 3 cups water, and salt; bring to a boil. Reduce heat, and simmer, stirring occasionally, 15 minutes.
Microwave cream cheese and yogurt in a 2-quart microwave-safe bowl at HIGH for 15 to 20 seconds or until warm; whisk until smooth. Whisk in egg yolks; gradually whisk in 2 cups leek mixture. Stir cream cheese mixture and pepper into leek mixture. Cook over low heat, stirring occasionally, until thoroughly heated. (Do not boil.) Sprinkle with chives, if desired.