Page 133 - Dynamics and Control of Nuclear Reactors

CHAPTER

11
Nuclear reactor safety

11.1 Introduction
Reactor safety is an important issue, deserving of a complete treatment. However, a
full treatment is beyond the scope and purpose of this book. But all nuclear engineers
need to know what can happen when things go bad. There have been accidents in
several research reactors and in three nuclear power reactors. Reactor dynamic char-
acteristics played a major part in determining the way one of the nuclear power plant
accidents unfolded.

11.2 Reactor safety principles
Two safety functions must occur following an abnormal safety-related event in a
power reactor. The fission reaction must be quenched (usually by immediate inser-
tion of safety rods, a reactor “scram”). This stops most of the fission power produc-
tion, but some fissions continue and the inventory of radioactive elements continues
to decay and produce heat. The decay heat power level depends on the operating his-
tory of the reactor, but initially is typically around 6% of previous reactor power. The
second essential function is continued cooling of the reactor core. Cooling is required
to prevent a temperature rise due to the continued power production and to prevent
fuel melting. Backup power systems (emergency Diesel generators) are available in
Generation II reactors to provide electricity to keep the coolant pumps running in the
event of the loss of normal electrical power. Also, plants use auxiliary pumps, whose
function is to come online if normal feedwater pumps are inoperable or have been
valved out, which is a serious violation of procedure.
It should be noted that newer reactor designs include emergency cooling systems
that do not require electrical power. Cooling water enters the reactor by gravity or
from pressurized vessels.

11.3 Early accidents with fuel damage
Accidents occurred in the early days of reactor deployment. The worst were those in
which fuel melting occurred. Accidents with fuel melting include those described
briefly in the following section.
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