PFR safety and licensing

The achievement of this policy was quantified in a probabilistic risk assessment (PRA) for the plant, judged against the frequency everity criterion recommended for a prototype reactor at a remote location. PRA was used throughout PFR s history to improve the quality of the safety arguments. The 1974 risk assessment was revised in 1984, and again in 1990 as part of the preparations for licensing of the AEA by the Nuclear Installations Inspectorate (Nil). 

There were multiple barriers between fuel and the environment. The fuel in each fuel pin was hermetically sealed within a strong stainless steel clad the fuel pins were immersed in a sodium pool able to retain chemically a number of important fission products, should a fuel pin fail the coolant was contained within the primary contaiiunent (the reactor vessel, the biological shield roof and, surrounding the reactor vessel, the leak jacket) over the biological shield roof was the secondary containment building which incorporated a post-incident cleanup plant. The latter ensured that any radioactive release to the environment, even following a major incident would be kept to a minimum. 

There must be a highly reliable means of removing decay heat from the primary circuit. To insure against non-availability of the secondary circuits and steam plant, PFR was provided with the three independent decay heat rgection loops. 

Inherent safety properties. PFR possessed inherent safety features which would allow it to survive a range of very improbable incidents, even in the highly unlikely circumstances of failure of the engineering safeguards. The major inherent safety features were natural circulation (within the NaK filled decay heat rejection loops and of the primary sodium coolant) and the reactor s negative temperature and power coefficients. 

Loss of electrical supplies to the PFR would result in a trip of the reactor and the run down of the main coolant pumps. Because of flywheels the primary pump speed halving time was 10 seconds, and the pumps would stop after approximately 200 seconds but for the clutching-in of the continuously running pony motors which maintained a pump speed of 10%. Should all three pony motors or their clutches fail to operate, reactor experiments demonstrated that natural circulation of the hot sodium in the core would transfer the core decay heat to the 900 tones of primary coolant. The decay heat could then be transferred to the atmosphere via the naturally circulating decay heat rgection system without the sub-assembly outlet temperatures exceeding normal operating levels. 

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