The first nuclear weapons to be delivered to Scampton arrived during 1958 and comprised twenty kiloton (20kt) atomic bombs given the Rainbow Code, Blue Danube. They were replaced by the smaller Yellow Sun Stage 1 (Mk. 1s) which were the first of the UK's operational thermonuclear weapons. The development of the stand-off nuclear missile Blue Steel required the construction of new specialist buildings: the Missile Servicing and Storage Building (MSSB) which was erected between the main hangars and the airfield, and the highly volatile High Test Peroxide (HTP) and kerosene fuel storage buildings which were located at some distance from the MSSB.