USES OF RADIONUCLIDES●The uranium is stored inside the reactor core in alloy tubes called fuel rods. A
minimum mass of^235 U is needed in the reactor in order to ensure that there are
enough^235 U nuclei to absorb the neutrons emitted during the induced fission, so
that further fission may continue. This critical massof^235 U depends upon the
shape of the uranium fuel. It is usually several kilograms.●Many of the fission products (such as^14256 Ba and^9236 Kr) are intensely radioactive in
their own right. This makes the intensity of nuclear radiation in the nuclear core
thousands of millions of times more intense than that generated by the natural
radioactivity of the uranium radioisotopes alone. (A typical fission reactor may
contain about 200 tonnes of uranium. Once fission has started, the radiation in-
tensity of the reactor core increases by a factor of about 100 000 000 000.) Elabor-
ate shielding (including thick concrete walls) is therefore needed to protect
workers and members of the public.●A chain reaction cannot be allowed to proceed unchecked in a nuclear reactor
because, although there is no danger of a nuclear explosion, the tremendous
heat generated by uncontrolled fission might cause structural damage to the re-
actor itself. This would make it very likely that some of the intensely radioactive
material in the core of the reactor would escape into the surrounding environ-
ment – this happened at Chernobyl. For this reason, control rodsmade of
neutron-absorbing material (such as boron or cadmium) are lowered into the
nuclear reactor. To speed up fission, some of the control rods are withdrawn.
When demand for electricity is low (as at night), extra control rods are lowered
into the reactor.●The nuclei and neutrons produced as fission products possess considerable kinetic
energy and, due to collisions with the reactor coolant, this energy is utimately
407Fig. 21.5The main parts of a nuclear reactor. Heat is transferred outside the nuclear reactor
using a liquid or gas as coolant, such as water or carbon dioxide. The coolant then transfers the
heat to a heat exchanger which converts water to steam to drive turbines which produce
electricity.