1. In a large pot of boiling salted water, cook pasta according to package instructions. Drain and return to pot.
2. In a small bowl, finely mash 1 egg yolk with a fork (reserve remaining yolk for another use). Add yogurt, mayonnaise, and lemon juice; stir until smooth. Add to pasta, then stir in egg whites, radishes, celery and pickle. Season with salt and pepper.
3. Refrigerate in an airtight container for up to 3 days. Sprinkle with chives just before serving.