Set pasta water to boil.
Cook the shrimp as per instructions on package. Do not overcook.
Remove from heat, drain and allow to cool.
Prepare radishes and scallions.
Meanwhile, cook the pasta.
Drain when al dente.
Rinse with cold water to stop cooking process and prevent from sticking.
Drain again.
Place the pasta in a large serving bowl.
Mix in the mayonnaise, the lemon juice, salt and pepper.
Add the rest of the ingredients and toss, mixing well.
Sprinkle with a dash of cayenne pepper, or if you prefer Tabasco sauce.
Place in refrigerator for an hour or more.
Serve cold.