Science News - April 9, 2011

electricity from the atom boiling-water reactors like the ones at the fukushima Daiichi plant use heat from nuclear fission to make steam, which powers a turbine to generate electricity. the steam is then condensed back into liquid water for another trip through the reactor. the design uses water both as a coolant and to absorb neutrons, keeping the nuclear reaction under control.

Reactor
vessel
Steam
Fuel rods
Control rods
Turbine
Condenser
Generator
Pump
Pump
Containment structure
Electricity
gen gas, formed in reactions of water with
the hot zirconium rods, also built up and
exploded at two of the reactor buildings,
releasing radioactive by-products of fission
like iodine-131 and cesium-137.
the reactors weren’t the only potential
source of radiation emitted by the crippled
plant. spent fuel—depleted in uranium
but containing highly radioactive fission by-
products — is stored in pools of water in
the same buildings that house the fuku-
shima Daiichi reactors. the flow of water
through those pools was vulnerable to the
same power-supply problems that plagued
the reactors, and the tanks began heating
up. in at least one case, the spent fuel
appears to have caught fire.
as plant workers and emergency
responders fought to bring the plant
under control, radiation spread. at first,
the Japanese government reassured
the public that no radiation had spread
beyond the plant itself, and cautioned
people living nearby to stay indoors. but
by the evening of march 12 the govern-
ment had evacuated a 12-mile radius
around the site, and within a week of the
earthquake monitoring had revealed con-
tamination of local food and water.

though the accident’s full health impact won’t be known for decades, the plant workers and emergency responders who struggled to bring the plant under control are at greatest risk. radiation levels around unit 3 reached 400 millisieverts per hour on march 15. health effects depend on how long someone is exposed to such levels, says kelly Classic, a medical health physicist at the mayo Clinic in rochester, minn. for comparison, 400 millisieverts is about the dose people would receive from 40 Ct scans of the abdomen. natural background rates of radiation average about 3 millisieverts per year. in the long term, exposures above even 10 millisieverts might somewhat elevate an individual’s risk of developing cancer. — Janet Raloff and Alexandra Witze

the magnitude 9. 2 earthquake that hit alaska on March 28,
1964 (March 27 local time) devastated downtown anchorage.

at magnitude 9.0, the march 11 earthquake in Japan was the fourth largest since 1900. though there is no theoretical limit to an earthquake’s size, it is extremely unlikely that motion along a known tectonic fault could produce an event of magnitude 10.0 or larger.