New low for US workers' radiation dose
American nuclear safety regulators said that 2007 saw the lowest ever collective radiation dose to the country's nuclear workers. Regulators said they wanted to keep up the downward trend.
American nuclear safety regulators said that 2007 saw the lowest ever collective radiation dose to the country's nuclear workers.
The Nuclear Regulatory Commission said all US citizens receive average of 3.6 mSv per year from a range of sources such as cosmic rays, medical treatment and radon. In recent years, workers at the USA's 104 nuclear power reactors received an average of an additional 1.6 mSv per year - which compares to the annual occupational limit of 50 mSv per year.
Managers of nuclear plants record the radiation doses that individual workers receive and compile average statistics to monitor overall trends in radiation safety. These averages are reported to the NRC which then calculates an 'average of averages' called the collective dose. In 2007, the NRC said this collective dose was 0.97 man-Sv - the lowest figure ever.
NRC chair Dale Klein said the commission would continue to encourage plants with collective doses above the average in order to keep up the downward trend in dose rates.
The NRC explained that the collective dose across the country's 69 pressurized water reactors (PWRs) was 0.69 man-Sv, while at the 35 boiling water reactors (BWRs) the figure was 1.54 man-Sv.
Worldwide the situation is similar - even a little better. The collective radiation doses for workers at nuclear power plants which reported to the World Association of Nuclear Operators (Wano) were 0.55 man-Sv for PWRs and 1.12 man-Sv for BWRs in 2007. Wano seeks to help nuclear operators across the world share experience in strict confidence, with the aim of increasing levels of safety and plant performance simultaneously.
Different strokes
The reason for the differing collective doses at pressurized water reactors (PWRs) and boiling water reactors (BWRs) comes down to a basic design difference between the two reactor types.
BWRs employ a single loop of coolant water that drives the plant's steam turbines after passing through the reactor, whereas PWRs have two loops.
The primary loop of a PWR cools the reactor, while a secondary one drives the steam turbines using heat transferred from the primary loop by large steam generators. Because the two bodies of water are kept separate, routine radioactive products from the reactor core remain in the primary loop and never leave the reactor containment building. This means that more of a PWR's sytems can be maintained without the strictest radiation controls.
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