Cook the chicken breasts until tender; drain and chill.
In the meantime, in a large pot cook the pasta until al dente; drain and rinse under cold water.Mix the mustard, yogurt, salt and pepper into the pasta.
Cut the chicken into bite-sized pieces and add it to this
mixture.Fold in the pasta,grapes, peas and celery.
Toss well, cover and refrigerate for about an hour; serve.