Much of its yield came from its thermonuclear reaction, making it a true , and the United Kingdom became the third nation to possess one.
The painting is in the collection of the.
The third, Grapple 3, also had a low yield.
A further test, Grapple X, exceeded expectations.
Grapple Y, in April 1958, yielded about three and remains the most powerful British nuclear weapon ever tested.