Reason Magazine 2014 Quartal 2 Seite 14

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Massive Power Failure A dangerous transformer fire shuts down a nuclear power plant I n December 2013 according to a report in the Los Angeles Times a transformer fire at Arkansas Nuclear One plant near Russellville forced a shutdown of the unit and ultimately necessitated installation of a replacement transformer Although the fire which took place in a switchyard outside the plant caused neither injuries nor any radiation releases it was a serious disruption for a multibillion dollar facility And as is often the case with unex pected transformer failures the cause was not immediately obvious Indeed in scores of incidents sparked by weather events or just wear and tear transformers burn or even explode causing disruption and injury and incurring substantial costs It s an ongoing challenge for utilities facility operators and the public sector According to details of the Arkansas incident contained in Nuclear Regulatory Commission documents At approximately 0748 CST on Dec 9 2013 an electrical fault occurred resulting in a fire and explosion on the ANO Arkansas Nuclear One Unit 2 Unit Auxiliary Trans former This caused a unit trip and a loss of power to Startup 3 Transformer which is one of the two off site power feeds to ANO Unit 2 ANO Unit 2 is currently in a stable shut down condition With Startup 3 and Unit Aux Transformer unavailable power was lost to the Reactor Coolant Pumps and Circulating Water Pumps RCS Reactor Coolant Sys tem natural circulation is in progress remov ing core decay heat Emergency Feedwater actuated due to low steam generator levels and is supplying both steam generators The unit is steaming to the atmosphere 2A 1 and 2A 3 are powered from SU Startup 2 Trans former 2A 4 is powered from 2K 4B Emer gency Diesel Generator As a result of the failure Arkansas Nuclear One Unit 2 had to be cooled down via natural circulation with heat being removed from the reactor via emergency feedwater and downstream dump valves to the atmosphere According to the NRC doc uments the accident also released a minor unknown amount of mineral oil into nearby Lake Dardanelle requiring further reports of the incident to emergency and environmen tal officials Just what is it that causes transformers to fail so spectacularly and suddenly To answer that question it helps to understand what a transformer does A transformer con sists of two very long strands of wire that are wound around a common core typically made from iron providing an electromag netic coupling between the windings Oper ating under alternating current a trans former cannot operate with direct current the device can perform functions such as converting one voltage to another stepping up or stepping down They can also provide a degree of isolation between different parts of an electrical system In operation transformers also produce heat sometimes dissipated through oils and cooling fins While transformers on the whole are reliable external interference and internal cumulative hazards and wear and tear can and often do lead to failure Transformer problems can come from a variety of sources such as improper installation or electrical loading including issues with unanticipated harmonics lack of maintenance and testing as well as from lightning strike grid distur bance and interference from tree branches or even small animals A chronic challenge is maintaining proper insulation For instance the coiled wires within a transformer are designed to function only when those wires are prop erly insulated from each other Over time The charred side of an auxiliary building at Arkansas Nuclear One in Russellville Arkansas USA is pictured on Dec 9 2013 following a report of a transformer fire that was extin guished forcing utility officials to take a unit at the facility offline AP Photo The Courier Joshua Mashon 14 Reason ISSUE 2 2014 theLATEST