Nuclear Reactor Safety

Over the last few weeks, I have received numerous questions about nuclear safety.  I believe a short review of nuclear hazards would be of benefit before discussing safety systems.

A little over a decade after the success of the first sustained critical reaction experiment in 1942, the first commercial nuclear power plant attained criticality, giving birth to a new industry.  The perception of the nuclear power industry has changed dramatically over its short lifetime.  From the dreams of nuclear powered “everything” - ships, planes, trains, buses, and utilities (“power too cheap to meter”), public perception has changed from acceptance to fear and distrust.  With the association with nuclear weapons, zealous critics, and two major accidents, the rise in safety concerns should be no surprise.  It is interesting though, how many deaths and injuries have occurred in non-nuclear industries, and yet the public regards them as safe.  It is also interesting to note that had not the operators disabled or overrode the installed safety system, neither of the above nuclear accidents would have occurred.

A long time ago, I had a chance discussion with a nuclear plant-engineering manager.  During the course of the conversation, I was quite surprised at the blasé safety attitude with respect to nuclear power plants.  “After all, what’s the big deal?  A reactor is simply a different heat source.  Everything else is the same.”  Unfortunately, Three Mile Island and Chernobyl exemplified the “big deal.”

Well, a nuclear reactor is another heat source, but not “just” another heat source.  There are some significant differences.  The most important, and most widely known, is the consequences of failure to contain the products of the fission process (the nuclear ash), to allow it to escape into the environment.  Nuclear fuel (uranium) is relatively harmless when fabricated, handled and loaded into the reactor.  During the fission process, the uranium is split into two new atoms, called fission fragments, releasing a relatively large amount of energy (on an atomic scale), and an average of 2.5 neutrons.  The same two atoms are not produced with each fission event, but rather a spectrum of atoms are released over the many events.  And there are billions and billions of fission events occurring each second in a nuclear reactor.  Unfortunately, these fission fragments are unstable isotopes of atoms.  They decay, becoming different isotopes, giving off energy in the form of radiation during the process.  This radioactive energy is very harmful to every living thing, plants, animals and humans, in varying degrees depending on the isotope, and how exposed, externally or internally.  The effect of allowing this material into the environment is the same as the fallout from several nuclear weapons.  After all, it is the same material, just produced more slowly in a nuclear reactor.

Nuclear reactors are designed, first and foremost, to contain the products of fission.  In western reactors, there are three containment barriers.  First there is the fuel container within the reactor, generally thousands of sealed metal tubes, rods or plates, fabricated to the highest quality standards.  Second there is a closed loop of very pure water, which transports the fission energy away from the reactor.  Water is converted to steam by the fission energy.  The steam passes through a turbine-generator, is condensed and the eventually returned to the reactor to be reheated.  Note that while this loop serves to generate steam, it also serves to cool the reactor, keeping it in thermal equilibrium.  Should one or more fuel elements fail, mechanically or due to overheating, the fission fragments would be contained in the closed cooling water loop (as occurred at TMI).  Most reactors (except the Chernobyl type) have a third barrier, called a containment building.  It is a large steel lined, concrete structure completely enclosing the reactor and it’s cooling loops.  It is designed to completely contain all of the coolant should a major failure leak occur in the cooling loops, and all of the water flashed to steam.  So even if fission fragments were released into the cooling loop and the loop leaked, fission fragments would be held in the containment building.

The second most important difference of a nuclear reactor is the fission fragment decay process.  In addition to releasing harmful radioactivity, the process also releases heat, called decay heat.  The amount of heat can be substantial, as much as 7% immediately after reactor shutdown, to 1% one hour after shutdown and continually decreasing.  This is a major difference compared to a fossil (coal, oil or gas) fueled boiler.  In a fossil boiler, should an unsafe situation arise, automatic safety systems isolate the fuel supply and the reaction and heat generation stops.  In a nuclear reactor, should an unsafe situation arise, automatic safety systems trigger the insertion of neutron absorbing rods, which stops the fission process.  It does not completely stop the generation of heat however!  Remember, substantial heat is being generated from the decay of fission fragments, and nothing can be done to stop it!  If this heat is not removed, the fuel elements will continue to increase in temperature until they melt and pool at the bottom of the vessel.  If cooling is not reestablished, the decay heat process will continue until the vessel melts releasing the molten fuel onto the containment-building floor.  Because of the decay heat phenomena, nuclear reactor systems must be designed to never, never lose the ability to remove heat from the core.  Hence, numerous, redundant control and emergency systems are installed and maintained to insure that there will always be a method of removing decay heat under all creditable failure scenarios.

The challenge to equipment manufactures, designers, operators and regulators, is to insure that systems will always be available to first shutdown the reactor and to continuously remove the resulting decay heat. 

 Updated by Brian Smith on June 2, 2002.