A nuclear reactor is a powerful source of radiation, since fission reactions produce neutrons and gamma rays, both of which are highly penetrating radiations. A reactor must have aptly designed shielding around it to absorb and reflect this radiation in order to protect technicians, other reactor personnel and the general public from exposure.
In many recent reactors, the shield consists of a thick concrete structure around the reactor system referred to as the biological shield. This shield may contain heavy metals, such as lead or steel, which are more effective absorbents of gamma rays, and heavy aggregates may be used in the concrete itself to achieve the same purpose.
These shields are expensive and contribute immensely to the huge cost of nuclear energy .
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d.) gravity pulled particles of dust and gas together to form planets.
distance, luminosity, brightness, radius, chemical composition, and temperature