Chernobyl &
Three Mile Island
One of the distortions that has been used to try and convince Nevadans of the dangers of nuclear waste is to compare Yucca Mountain to the Three Mile Island reactor accident in 1979 or to the Chernobyl reactor disaster that occurred in Russia in 1988. For example, NWPO consultant Dr. Shrader-Frechette in a quoted speech to students from the UNLV environmentral studies program stated:
"Chernobyl was not a worst case disaster." [Shrader-Frechette, Kristin; speech to UNLV Environmental Studies sponsored by NWPO, 3/15/93]
This was meant to imply Yucca Mountain could even be more dangerous. In fact, Chernobyl was exceedingly close to being a worst case nuclear reactor disaster. Three Mile Island (TMI) was not a disaster at all and is classed as an accident because there were no casualties. Yucca Mountain in contrast is not a nuclear reactor, but a nuclear waste repository, an exceedingly different technology from nuclear reactors.
A geologic repository is designed to minimize the energy levels of the nuclear materials it encloses by keeping away moderators (like water) that enhance nuclear reactions and by using neutron poisons (like Boron) to stop chain reactions. Chernobyl, TMI and other nuclear reactors are on the other hand designed to produce tremendous amounts of energy by promoting nuclear reactions. Power creating engines like reactors are by nature subject to catastrophic failure, while a repository is by nature designed to minimize energetic processes.
Commercial nuclear waste in the U.S. is mostly in the form of inert reactor pellets and some glassified material. Chemically, nuclear spent fuel is a non-explosive ceramic, related to an inert ashtray. It takes either a runaway nuclear reaction, a very hot furnace, or an intense explosion to create the conditions necessary to disperse these ceramic materials on the wind.
Chernobyl created exactly those conditions, becoming an incredibly hot, explosive nuclear furnace. This chaain of events is extremely unlikely to occurr in nuclear reactors as implemented in the United States, evidenced by the lack of casualties and impact at Three Mile Island where there was no massive dispersal of radioactive material. Instead, the Chernobyl disaster has everything to do with the criminally sloppy design of Russian power plants. In contrast to both Chernobyl and TMI, nuclear waste to be stored at Yucca Mountain will lack the reaction mechanisms necessary to turn it into a massively destructive release of nuclear material. Reviewing the Chernobyl disaster will highlight the huge differences in failure modes separating a nuclear wste repository and a nuclear reactor.
DESIGN FACTORS LEADING TO CHERNOBYL DISASTER
The first nuclear reactor that was built was done under the stadium at the University of Chicago in 1942 The reactor was composed of a "pile" of blocks of graphite (carbon) on the order of six inches square, and from this we derive the term "reactor pile". Selected blocks of the graphite contained a slug of uranium fit into a cylindrical cavity within the block so that it was possible to position uranium precisely within the graphite pile. In other words, the nuclear reactor was a pile of carbon bricks in which uranium was dispersed.
The reason for this design is that the graphite blocks slow radioactively emitted neutrons down to the right speed needed to keep a nuclear chain reaction going. In the game of pool, a billiard ball that goes too fast will jump off the table and not hit anything; the same holds true for neutrons within a reactor which must travel at the right speed to split uranium atoms and cause energy to be emitted. The graphite blocks not only slow neutrons down, but they also keep the uranium in the pile in a fixed geometry and provide a heat conduction medium to draw away the energy that is generated. This is the same general design as used at the Chernobyl plant forty years later.
Most U. S. nuclear reactors (including Three Mile Island) evolved away from the graphite pile design to water enclosed systems (Pressurized Water Reactors (PWR) or Boiling Water Reactors BWR)) for a number of reasons both historical and involving safety. Our power reactors were originally modeled after the reactors used in the Navy's nuclear submarine program, developed during Admiral Hyman Rickover's long and memorable career. A high-performance submarine requires compactness, high power density and water cooling and for better or worse our land based reactors owe much of their design emphasis to Admiral Rickover's nuclear submarine fleet.
A number of safety factors inherent in water moderated reactors decrease the risk of nuclear accidents by American nuclear power plants in comparison to graphite pile reactors of the type Chernobyl represents. When a pressurized water reactor, or PWR for short, looses its coolant the reactor automatically shuts down. This is because the water itself is used as a moderator in place of graphite to slow neutrons to the proper speed to continue chain reactions in fissionable uranium atoms. Added measures of safety are also added to American reactors in the form of various safety systems, the most obvious being the reinforced concrete containment building which houses the reactor core. These safety features all played a role in diminishing the impact at TMI.
The reason some graphite core piles still exist in the U.S. and the reason the Russians used this design in predominance is that the nuclear reactions these reactors generate produce as a byproduct large quantities of weapons grade plutonium. It's also cheaper to build such a pile and the energy efficiency is greater, though we need to stress that efficiency comes at the expense of safety.
The Russian reactors are continuously "milked" of their plutonium on a regular basis by removing spent fuel, while U.S. commercial reactors requires a yearly shutdown; another difference between American and Russian reactor designs. The Russians omitted reinforced concrete containment shell in their reactor designs because the use of overhead cranes to pull fuel assemblies out of the core made a bulky containment structure impractical. In other words, the Russian reactors walk a safety tightrope without the failsafe net a containment structure provides.
Consequently, you cannot design a much more efficient way to disperse radioactive contamination over the countryside than Chernobyl. Graphite only burns at high flame temperatures, around 1800 degrees Fahrenheit. Once Chernobyl reached a meltdown state and its huge stack of graphite ignited, blowing the top off the reactor building, there was no stopping such an inferno, or even slowing it down. Yucca Mountain has zero in common with such a nuclear incinerator.
Many accounts of the Chernobyl disaster blame the problems on operator errors as they conducted a test on the energy producing capacity of the turbines after a power sutdown. While there is no doubt operator errors initiated the meltdown chain of events, according to V.M. Chernousenko, a Director for the Task Force for the Rectification of the Consequences of the Accident at Chernobyl, the power plant was designed to fail. In Chernousenko's book Chernobyl: Insight From The Inside, he writes:
It was the very emergency system which is supposed to shut the reactor down reliably and quickly, whatever its condition, that caused it to run away. The explosion occurred 5 seconds after the emergency button was pressed. Three seconds after the button was pressed, when the power level was at 520 MW, the emergency alarms were activated by the power rise from the preceding level of 200 MW and by the sharpness of the rise.
Thus the accident prevention system failed to trip the reactor, not just in the normal situation, but even in this extreme situation. . . .
The 211 control rods could be inserted into the core at a rate of about 40 cm per second. The height of the core is 7 meters. Thus the time required for full insertion of the rods into the core averages 18-20 seconds. . .
Thus, Professor B.G. Dubovskii states:
"An emergency system which takes 18-20 seconds to operate is not a protection system at all - it is a parody of such a system. Normal emergency systems, as used in reactors all over the world, come into operation in just a few seconds (up to 5 seconds at most). At least, that is true of the rapidly acting subsystems. [V.M. Chernousenko; Chernobyl: Insight from the Inside, Springer Verlag, 1991, p. 76]
Chernobyl was from the beginning an failure prone reactor in which most normal safety measures were compromised or ignored. At Three Mile Island when a failure of primary cooling pumps occurred, backup systems responded quickly in time to prevent a disaster. The unsafe elements of the Russian design are completely absent from the technology being envisioned for Yucca Mountain.
THE ACCIDENT AT THREE MILE ISLAND
In contrast to the Chernobyl disaster, the accident at Three Mile Island led to no fatalities because automatic shutdown features and backup systems prevented the runaway situation in the Russian design. Because of the massive containment structure at Three Mile Island, the amounts of radioactive material the local population was exposed to was trivial, mostly in the form of gaseous bleed off of radioactive iodine. According to Bernard Cohen, the differences between the TMI accident and Chernobyl are substantial:
. . . containment provides a broad range of protection to (a) reactor against external forces, such as a tornado hurling an automobile, a tree, or a house against it, an airplane flying into it, or a large charge of chemical explosive detonated against it. In a meltdown accident, however, the function of the containment is to hold the radioactive material inside. Actually, it need only do this for several hours, because there are systems inside the containment for removing radioactivity from the atmosphere. One type blows the air through filters in an operation similar in principle to that of household vacuum cleaners. In another, water sprinklers remove the dust from the air. Thus, if the containment holds even for several hours, the health consequences of a meltdown would be greatly mitigated. In the Three Mile accident, there was no threat to the containment. The investigations have therefore concluded that even if there had been a complete meltdown and the molten fuel had escaped from the reactor, the containment would very probably have prevented the escape of any large amount of radioactivity. In other words, even if the Three Mile Island accident was a "near miss" to a complete meltdown ( a highly debatable point), it was definitely not a near miss to a health disaster.
The Chernobyl reactor did not have a containment anything like those used in U.S. reactors. Analyses have shown, that if it had used one, virtually no radioactivity would have escaped, there would have been no threat to human health, and the world would probably would have never heard about it. [Cohen, Bernard; The Nuclear Energy Option,Plenum, 1990, p77]
THE YUCCA MOUNTAIN REPOSITORY
In contrast to both the Chernobyl and Three Mile Island reactors is Yucca Mountain, a repository specifically designed to:
1) Operate in a non-critical state (excluding groundwater that would increase the decay rate of radioactive materials).
2) Have no combustible materials (no graphite) which could disperse radioactive materials.
3) Be under a 800 feet of rock containment (versus in essence zero containment at Chernobyl).
4) "Poison" the radioactive material with neutron absorbing materials like boron (to further diminish nuclear reactions).
5) Make radioactive material escape pathways as long as possible, through hundreds of feet of rock vertically and miles horizontally.
6) Slow radioactive migration through natural absorbants like zeolite mineral.
Combined with other safety features, the Yucca Mountain nuclear waste repository is substantially safer than nuclear power reactors. Of all the potential disaster mechanism remotely possible, local intrusions of water into the repository seems to be the greatest cause of concern.
Unfortunately, because of somewhat artificial language in the Nuclear Waste Policy Act and other regulations, for licensing purposes safety mechanisms are now limited to "non-engineered" barriers. Of course, engineered solutions might well enhance safety beyond the natural barriers to radiation transport already present at Yucca Mountain.. One such mechanism would be a drainage system for the repository. Another would be continuous monitoring for ten thousand years. Many other engineered solutions could also enhance long term safety.
Even so, Yucca Mountain has an inherent degree of safety because it is by design the opposite technology from Chernobyl, Three Mile Island and other reactor systems. The repository lacks power generating capability through absence of moderators that would create a criticality situation. It also lacks the incinerator-like characteristics (graphite blocks) that made Chernobyl a verifiable disaster. Consequently, comparisons to reactor disasters of limited use in evaluating the safety of the mountain.
ÿ