Harwell is the name of a British government centre for atomic (nuclear) research, next to the village of Harwell in Oxfordshire, in southern England. After the Second World War, Britain put a large science laboratory there to study the atom and nuclear energy — both weapons research in the early years and, later, a lot of peaceful science. People still say Harwell to mean that research campus, not only the little village. It is a proper name, so it takes a capital H. You will meet it in news or books about British science or nuclear history, not in everyday chat unless you are talking about that place. There is another, much smaller Harwell in Nottinghamshire; that is a different place. The famous research centre belongs to the Oxfordshire village.