Page 134 - Dynamics and Control of Nuclear Reactors

130 CHAPTER 11 Nuclear reactor safety

11.3.1 Accidents
NRX: An accident occurred on December 12, 1952 at the NRX reactor in Canada [1].
NRX is a heavy-water moderated, light-water cooled reactor. During shutdown,
while working on the pneumatically operated control rod drives, the operator
wrongly opened valves, causing control rods to be withdrawn. While acting to cor-
rect the problems, there was a miscommunication between operators, and control
rods were further withdrawn. This caused a rapid increase in reactor power, with
resulting fuel melting.
SRE: An accident occurred in July 1959 at the Sodium Reactor Experiment
(SRE) in California. SRE was a sodium cooled, graphite moderated reactor [2]. Tet-
ralin (an organic liquid used as a sealant in the SRE pumps) leaked into the sodium
coolant. Solid carbon resulted from degradation of the tetralin. The carbon restricted
coolant flow through the core. Some of the fuel overheated and melted.
SL-1: An accident occurred on December 21, 1960 at the SL-1 reactor in Idaho
[3]. During preparations to restart the shutdown reactor, a control rod was withdrawn
manually to attach to its drive mechanism. It was withdrawn too far and prompt crit-
icality occurred. Power increased rapidly, causing an explosion in the reactor.
Windscale: An accident occurred on October 10, 1957 at the Windscale reactor
in Britain [4]. Windscale was an air cooled, graphite moderated reactor. A graphite
fire occurred because of release of Wigner energy in the graphite. Wigner energy is
energy stored in a material because of dislocation in the atomic structure of atoms
caused by neutron absorption. This stored energy is released as heat when the atoms
spontaneously relax from the dislocated condition. Wigner energy caused a fire in the
graphite that burned for several days, destroying the reactor.
Fermi-I: An accident occurred on October 5, 1966 at the Fermi-I reactor in Mich-
igan [5]. Fermi-I was a sodium cooled fast reactor. A metal plate dislodged from its
position beneath the reactor core. It flowed to the bottom of the core, blocking flow to
several fuel elements. Some fuel overheated and melted.
Lucens: An accident occurred on January 29, 1969 at the Lucens reactor in Swit-
zerland [6]. Lucens was a carbon dioxide cooled, heavy water moderated reactor.
During shutdown moisture condensed on fuel elements, causing corrosion. Corro-
sion products accumulated in the flow path, restricting coolant flow. The fuel over-
heated and melted.
Russian Submarines: Russian nuclear submarines experienced an astonishing
number of accidents (too many to describe here) [7].

11.3.2 Assessment
One of the lessons learned from this limited experience is the importance of avoiding
a loss-of-coolant event. Generation-II and -III reactors feature back-up cooling sys-
tems. But even they have failed in subsequent reactor accidents.
Some of the accidents described above resulted in the death of plant workers, and
direct exposure to radiation or radioactive contamination outside of the plant