The dome at Dounreay under construction in the early 1950s
The moment nuclear power began to be generated inside the dome at the Dounreay nuclear power complex 50 years ago is being recalled.
The experimental fast breeder at Caithness achieved criticality on 14 November 1959.
Journalist Chapman Pincher described the radioactive energy within the steel sphere at the time as a "caged tiger".
A book and release of archive images, including a visit by the Queen Mother, are helping to mark the anniversary.
Criticality is a self-sustaining chain reaction producing energy and radioactivity.
Staff at Dounreay had worked for four years to reach that point.
Former staff have reportedly said in the past that some employees took leave on the day of criticality because it had not been done using fast breeder reactor technology before and there remained some unanswered questions about what might happen.
In the 1950s, fast breeder reactors were seen as a more efficient way to harness the energy contained in uranium.
The fuel in the reactor core could be burned more intensely and some of the energy released could be captured to create or "breed" a new fuel - plutonium.
But the reaction was so intense that pressurised water or gas normally used for cooling the reactor were found to be inadequate.
Scientists used liquid metal instead.
Dounreay staff at the Dome in more modern times
Almost 170,000 litres of a sodium and potassium alloy, known as NaK, were poured into the experimental fast reactor at Dounreay.
It was deemed effective at controlling the intense heat, but the toxic alloy posed a serious risk to health and would ignite easily if mishandled.
Dounreay Site Restoration Limited (DSRL), the company leading the decommissioning of the site, said what remains of the system is regarded as one of the biggest hazards in the UK.
Last week, previously unpublished images of the late Queen Mother visiting Dounreay were released on the internet.
The photographs are from visits she made to the site in 1957 and 1961 while she was staying 20 miles away at Castle of Mey.
DSRL has made the pictures available on its website.
Last week also saw DSRL taken over by Babcock International Group.
The move ends the UK Atomic Energy Authority's presence at Dounreay after 55 years and completes the privatisation of its entire workforce at the site.
The total cost of dismantling and cleaning up the site is expected to run to £2.7bn.
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