The Foundations of Chemistry

slows neutrons but does not absorb them all. The next most efficient is “heavy water”

(deuterium oxide,^21 H 2 O or^21 D 2 O). This is so expensive that it has been used chiefly in

research reactors. A Canadian-designed power reactor that uses heavy water is more

neutron-efficient than light water reactors. This design is the basis of the many reactors

in Canada.

Control Rods

Cadmium and boron are good neutron absorbers.

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The rate of a fission reaction is controlled by the use of movable control rods, usually

made of cadmium or boron steel. They are automatically inserted in or removed from

spaces between the fuel rods. The more neutrons absorbed by the control rods, the fewer

fissions occur and the less heat is produced. Hence, the heat output is governed by the

control system that operates the rods.

Cooling System

Two cooling systems are needed. First, the moderator itself serves as a coolant for the

reactor. It transfers fission-generated heat to a steam generator. This converts water to

steam. The steam then goes to turbines that drive generators to produce electricity.

Another coolant (river water, sea water, or recirculated water) condenses the steam from

the turbine, and the condensate is then recycled into the steam generator.

The danger of meltdown arises if a reactor is shut down quickly. The disintegration of

radioactive fission products still goes on at a furious rate, fast enough to overheat the fuel

elements and to melt them. So it is not enough to shut down the fission reaction. Effi-

cient cooling must be continued until the short-lived isotopes are gone and the heat from

their disintegration is dissipated. Only then can the circulation of cooling water be stopped.

The 1979 accident at Three Mile Island, near Harrisburg, Pennsylvania, was due to

stopping the water pumps too soon andthe inoperability of the emergency pumps. A

combination of mechanical malfunctions, errors, and carelessness produced the over-

heating that damaged the fuel assembly. It did not and could not explode,although melting

of the core material did occur. The 1986 accident at Chernobyl, in the USSR, involved

a reactor of a very different design and was far more serious. The effects of that disaster

will continue for decades.

Shielding

It is essential that people and the surrounding countryside be adequately shielded from

possible exposure to radioactive nuclides. The entire reactor is enclosed in a steel contain-

ment vessel. This is housed in a thick-walled concrete building. The operating personnel

are further protected by a so-called biological shield, a thick layer of organic material

made of compressed wood fibers. This absorbs the neutrons and beta and gamma rays

that would otherwise be absorbed in the human body.

Breeder Reactors

The possibility of shortages in the known supply of^235092 U has led to the development of

breeder reactors,which can manufacture more fuel than they use. A breeder reactor is

1028 CHAPTER 26: Nuclear Chemistry

The neutrons are the worst problem
of radiation. The human body contains
a high percentage of H 2 O, which
absorbs neutrons very efficiently. A
new weapon, the neutron bomb,
produces massive amounts of neutrons
and so is effective against people, but
it does not produce the long-lasting
radiation of the fission atomic bomb.