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The Chernobyl nuclear power plant had 4 operating thermal neutron RBMK reactors moderated by a graphite stack. The core was cooled by water circulating through zirconium-niobium pressure tubes (the water also acted as a neutron absorber). The power level in the core was controlled by boron carbide absorber rods with graphite tips. At the time of the accident, a test was being conducted on an off-line reactor to determine whether the power generated during spin down of the turbogenerator by its own inertia would be sufficient to power the reactor coolant pumps in the event of a loss of external electrical power, thereby providing more times for the backup diesel generators to be run up and brought on-line. A ‘prompt critically’ temperature runaway developed, and high-pressure steam leaked into the reactor, blowing off the top cover. The reaction of water and steam with the zirconium fuel rod cladding and graphite moderator core generated a mixture of hydrogen (H2) and carbon monoxide (CO) which caused an explosion large enough to blow the concrete roof off the reactor building and disperse radioactive particles across much of western Europe.
Ask yourself and your crew:
- How can something like this happen here (e.g. on our site)?
- What safety measures (i.e. procedures, controls/barriers) do we have in place to mitigate the risk?
- How do we know the risk controls/barriers are working?
- What improvements or changes should we make to the procedures, controls/barriers or the way we work?
Original content courtesy of IChemE Safety Centre
