Soon after the Second World War, it became evident to the UK that the US had no intention of sharing their newly acquired nuclear weapons technology despite the UK's assistance in the Manhattan Project. As such the British government set about building its own atomic arsenal; leading to the worst nuclear meltdown in UK history.
Despite being a partner in the Manhattan Project, along with Canada and the US, and being assured access to the technology, the British government saw nothing for its efforts at the end of WWII when the US congress passed legislation prohibiting the dissemination of its nuclear advances. Not to be pushed aside as a superpower by the US and Russia, Britain rushed to develop its own megaton-class indigenous nuclear weapon systems. It wasn't just the fear of losing its position in the international hierarchy that spurred the weapon's development; the British government was also worried that the atmospheric testing of nukes could be outlawed in the coming years (which it was), preventing them from publicly demonstrating the these weapons' capabilities.
Thus Project Grapple began. The result: Orange Herald, 117kg of U-235 surrounded by lithium deuteride, and the first British nuclear device to use an external neutron source. On May 31, 1957, a 30-inch (0.75m) Herald exploded in the skies over Malden Island in the Pacific, yielding a 720kt blast and making Orange Herald the largest fission device ever tested.
While the Orange Herald test was a success, the annual UK production of weapons-grade U-235 was barely 120kg a year, just enough to make a single bomb. The 1958 US-UK Mutual Defence Agreement solved that problem. By foregoing further independent weapons development, the UK would be allowed utilise a derivative of the Americans' technology, providing significant cost savings as well as the ability to buy enriched uranium from the US, use that in its civil nuclear reactors, and sell the resulting plutonium back to the US.
Despite its success, Orange Herald was not particularly popular with the scientists assigned to the project. "I thought that Orange Herald was a stupid device." Dr. Bryan Taylor told the documentary team behind, Windscale: Britain's Biggest Nuclear Disaster, "It wasn't elegant, it was a dead-end design and couldn't be taken any further." What's more, the large amount of fissionable material demanded by the device has been blamed for the meltdown and fire at the Windscale nuclear reactor complex in Cumberland.
On October 10, 1957, the core of the Unit 1 reactor (which was being used for defence research) melted down, then caught fire and burned for nearly three days before being put out. During that time, the fire spewed massive amounts of radiation—specifically iodine-131, which has been linked to thyroid cancer—into the atmosphere which then spread across the UK and Western Europe. Surprisingly, despite being ranked a level 5 on the 7-point International Nuclear Event Scale, authorities did not evacuate the surrounding areas and a 2010 study found no lingering health issues or rampant cancers among workers involved in the accident and subsequent clean-up. The site is expected to be completely detoxified by the end of next year. [Wiki 1, 2, 3 - Nuclear Weapon Archive - Damn Interesting - EO Earth - Britannica]