A DR reader writes (4/5/95):

Boron is a common component in control rods for fission reactors. 
During the Manhattan Project the folks at Hanford and Los Alamos 
were forbidden to wash their clothes in boron bleach due to fears 
it would interfere with the development of the a-bomb.

Grolliers Encyclopedia Entry...

boron
{bohr'-ahn}

Boron is a metalloid chemical element with properties intermediate 
between those of carbon and aluminum.  Its chemical symbol is B, 
atomic number 5, and atomic weight 10.811.  Boron is relatively 
rare, constituting only 3 ppm of the Earth's crust.  It is most 
commonly found in the BORATE MINERALS borax, and kernite.  The 
element was first isolated by Sir Humphry DAVY in 1807 and by 
Joseph GAY-LUSSAC and Louis THENARD in 1808.

Boron exists in an amorphous form as an extremely hard, blackish 
brown powder, and in three crystalline forms that look like 
metals.  Crystalline boron is second only to diamond in hardness 
but is too brittle for use in metals.

Research suggests that boron may be nutritionally important. 
Apparently it helps to maintain appropriate body levels of 
minerals and hormones needed for bone health.

Uses

The low electrical conductivity of boron increases greatly as its 
temperature is raised.  At certain temperatures, therefore, boron 
behaves as a SEMICONDUCTOR, and it is often added to germanium and 
silicon to increase their electrical conductivity.  The use of 
cubic boron nitride as a high-temperature semiconductor is also 
being explored.

Small additions of boron to steel appreciably increase the 
hardness of the alloy.  Boron is also used in the production of 
pure, strong metals to remove the oxygen and nitrogen dissolved in 
the metal or chemically bound to it, and it is used to absorb fast 
neutrons in nuclear reactors.

The most important boron compound is borax, which has been used in 
pottery glazes since the Middle Ages.  Borax deposits were first 
found in Tibet, and borax was brought to Europe by the Arabs.  It 
is still important in the ceramic industry.  Borax combines with 
chromium, copper, manganese, cobalt, and nickel to form 
beautifully colored compounds.  Borax beads were once used as a 
reagent in the detection of these elements.  Borax is also 
important in the production of borosilicate glass, which has a 
high refractive index and is suitable for the manufacture of 
lenses.  Other applications of borax include the impregnation of 
textiles and wood to make them fire resistant; softening water for 
laundry;  and as a flux in brazing (dissolution of oxides).  A 
weak base, borax is also used in buffer solutions and photographic 
developers.

Since boron is important in the calcium cycle of plants, borax or 
boric acid is often added to boron-poor soils as a fertilizer.  
Boric acid is obtained by the action of strong acids on borax and 
is used as a mild disinfectant.  Although its toxicity is low, it 
is not completely harmless.  Its use as a food preservative is 
prohibited in many countries.

Boron Chemistry

Boron has three valence electrons and forms covalent compounds 
almost exclusively.  It generally forms planar, three-bonded 
compounds with 120 degree bond angles.  These compounds have only 
six bonding electrons.  The boron atom can attain a rare gas 
configuration, with eight bonding electrons, in reactions of 
molecules with free electron pairs.  Boron forms compounds with 
nitrogen in a similar way.

Bibliography:  Alekseev, N.  V., et al., Structural Chemistry of 
Boron and Silicon (1986);  Greenwood, N.  N., The Chemistry of 
Boron (1975);  Wells, A.  F., Structural Inorganic Chemistry, 5th 
ed.  (1983).