Power for nothing and your hyperbole for free.
The massive temblor and ensuing tsunami have seriously changed the lives of most Japanese and have tragically ended many lives as well. This tragedy is on par for other tragedies such as Hurricane Katrina and the Tsunami at Banda Aceh. The triple whammy is coming in the form of a nuclear disaster that, if you focus on Western Media, is putting this event on par with Chernobyl.
I am not a nuclear physicist nor am I a power plant engineer, but I have been studying previous nuclear accidents and the construction of safer nuclear facilities as a result of those accidents.
The two accidents that I studied were both intriguing and informative in light of the current situation at the nuclear power plant in Fukushima. I, of course, read many articles and viewed many documentaries on Chernobyl, which is the most severe nuclear accident in the history of nuclear power: International Nuclear Event Scale (INES) Level 7. The other event was a more obscure, INES Level 4, event that occurred in the Idaho desert in 1961. The event was at the country’s nuclear power research laboratories at the National Reactor Testing Station, now known as Idaho National Laboratory. The reactor in this event was known as SL-1 or Stationary Low-power Reactor Number One. It was designed to be small and portable so that it could be used at remote Army outposts for generating 200 kW of power.
The story of the SL-1 shows the aspects of a nuclear reactor on a scale that most laymen can understand and the event that killed three maintainers (the only fatalities from nuclear reactors in the U.S.) helped spur many safety precautions that are now standard in the design and functioning of nuclear reactors.
There are two types of reactor designs used in the majority of power plants and they are Boiling Water Reactors (BWR) and Pressurized Water Reactors (PWR). The main difference between the two is that in a PWR there is a third water circuit that allows heated water to vaporize to run the steam turbines. In the BWR that circuit is combined in the containment vessel surrounding the fuel rods where the water is allowed to boil in the low pressure and high heat generated by the fission of uranium atoms. The water in a PWR is not allowed to boil when under pressure in the containment vessel. The water has three main functions in both types of reactors. Firstly it is there to cool the fuel rods during fission. Next, it acts as a moderator that slows the speed of neutrons released in the fission, or chain reaction, to moderate the reaction and the efficiency of the reaction. Finally it is converted to steam and used as thermal power to run the turbines and generators.
The SL-1 was a BWR that had been shut down during the Christmas break in 1960. A three-man crew came in on January 3, 1961 to prepare the reactor for start up. In a reactor there is a control rod made of carbon and cadmium or some other material such as indium or silver to capture all the neutrons. To restart the reactor the control rod is slowly removed from the core to initiate fission. The neutrons are what split the atoms in the fission and when captured, by the control rod reinserted into the core, they no longer create fission. The SL-1 had 5 of these cadmium control rods and all were inserted in between the fuel rods when the reactor was shut down, or SCRAMed, before the Christmas break. These control rods were also detached from their drive motors for special maintenance and needed to be reattached, hence the 4 inches they had to be removed manually.
The term “SCRAM” comes from the earliest safety precaution used on the first reactors. A rope was tied to the control rods and used to remove the rods to start the fission. A man with an axe was positioned near the rope in case of emergencies to chop the rope to let gravity reinsert the rods for shut down. This man was known as the Safety Control Rod Axe Man or S.C.R.A.M. The control rod, in this case, has been referred to as: The Sword of Damocles.
At about 9:01 P.M. on January 3, 1961 an alarm was sounded at a fire station 10 miles from the SL-1 site. The coded 2-2-1 alarm indicated a temperature spike at the SL-1 facility, but indicates the facility as a whole including the administrative offices there. This was the third alarm that day for that facility. The two previous alarms were related to a faulty detector in the building’s boiler facility. The fire crew arrived at the facility about 9 minutes after the alarm was sounded. They first checked the alarm that they had check twice already that day. It was not the culprit. Further inspection soon revealed a radiation alarm in the control room. The fire fighters went back out and retrieved meters to test the level of radiation and these instruments proved too small in scale to accurately measure the Roentgens/hour that were present, and the needles went off the gauges. The SL-1 had exploded.
The outer walls of the reactor room contained the explosion of the SL-1, even though the facility wasn’t designed to do so. The cylindrical building that housed the reactor now contained the radiation released in the explosion. The heroic efforts of the fire fighters over the next 5 hours ended with the extraction of two of the casualties from the highly reactive facility. The first removed was Navy Electricians Mate McKinley who was still breathing but died on the way to a nearby hospital. The second casualty removed was the body of Army specialist Byrnes who had died instantly in the blast. The last victim of the reactor explosion was Army specialist Richard Legg who had been pinned to the ceiling of the reactor room by one of the control rods. It took several days to devise a plan to extract the body of Richard Legg, by using a large makeshift stretcher attached to the end of a crane boom that was inserted through a freight door and positioned under the body to catch it when dislodged from the ceiling.
The incident report concluded that the central control rod had been pulled far beyond its 4-inch maximum limit of travel. This resulted in the instant restart of the reactor and exceeded the prompt excursion of the reaction. All of the water surrounding the core was instantly and explosively vaporized. It is still not known; do to lack of survivors, as to why the rod was pulled so far out, so quickly.
The things learned from this accident were invaluable to the future of reactor design. One possible reason for the over-extension of the rod was due to Boron chips, deteriorating in the slide channel for the control rod led to a sticking situation. Today’s control rods are spring-loaded to the inserted position to overcome sticking. Also, the rods are never manually removed from the core in today’s reactor maintenance. Another innovation came in the form of containment. Today’s reactors have 3 levels of containment: the Zircaloy tubes for the uranium pellets, the reactor vessel and an outer steel/concrete containment shell. The biggest thing learned from the SL-1 incident was that once the water moderator is removed from the fuel rods, the fission also ceases. This is very important when looking at how reactor cores are moderated, especially when looking at Chernobyl, which used graphite blocks to moderate neutrons.
The incident at Chernobyl could have been far worse than it was, and it was very bad to begin with. The design of the reactor at Chernobyl was quite different than the BWR and the PWR. The fuel rods at Chernobyl were inserted into a grid of graphite blocks to form the core. The core was cooled by water and control rods were used to start and stop the fission. However, the graphite allowed for positive pressure voids, which allowed the fission to continue for a longer period of time after the control rods were insert to shut down the core. This can be viewed as a sort of “dieseling” for reactors. The other risk is that even when the water is vaporized the graphite moderator continues the chain reaction and meltdown is inevitable in a situation where the coolant level drops or is vaporized. The reactor at Chernobyl did have an outer containment, but it was filled with sand rather than concrete. This cheap solution to containment actually was the saving grace at Chernobyl. When the reactor went supercritical, all the coolant was vaporized and the containment vessel around the core exploded. This explosion also breached the outer containment and released the sand, which in turn encased the molten core in glass that oozed through the lower levels of the facility as it cooled and solidified. This actually contained much of the radiation and the fissile material and stopped the chain reaction. If it had not, this core could have gone from fission to fusion according to scientists that inspected the facility for years after the incident. Fusion is what makes an atomic warhead a weapon of Mass Destruction. Fission is used to bump the materials into a situation known as fusion and the energy released can be as great as 50 megatons, which the Soviets achieved with their Tsar Bomba warheads. Scientists and physicist claim that there was enough material at Chernobyl to create at least a 50-megaton explosion and if the sand hadn’t encased this material, it was highly probable that this would have occurred. The largest atomic bomb ever detonated by the U.S. was “Castle Bravo” which yielded 15 megatons, double the expectations of the physicists that created it. The top of the mushroom cloud reached 70,000 feet into the stratosphere with a width of 9 miles. This detonation eliminated an island at the Bikini Atoll.
Looking at the past and going over the incidents that lead to safety innovations in nuclear reactors, we can see that the dangers are immense. The U.S. learned quite a lot from its research out in the deserts of Idaho and our innovations in reactor construction make them the safest in the world. For example: the 3 levels of containment kept Three-mile Island from releasing deadly radiation.
General Electric leads the world in reactor construction and it is their 40-year old Mark I PWR that sits in Fukushima’s Daiichi facility and is the center of current nuclear concerns. In addition to the 3 levels of containment there is an outer structure that houses the whole reactor and its containments. This building is what exploded several hours into the incident after the earthquake. The earthquake would have triggered the spring-loaded control rods to SCRAM the reactors immediately. The reactors still have residual heat produced by the slight amount of free neutrons that do not immediately get absorbed by the control rods (about 3% of the heat when running at critical fission). The coolant is needed to completely cool the core after shutdown, but this was lost as a result of the earthquake and loss of power. The redundancy generators also failed due to a tsunami that took out the generators. The heat is enough to cause a partial meltdown of the core and this molten uranium and zircaloy can act as a moderator and restart the fission. The resulting build-up of pressure from vaporized coolant that the plant workers are desperately introducing to the core, has to be released to maintain the integrity of the reactor vessel. This release of, essentially, radioactive hydrogen was being contained in the outer building, but it built up too much causing the explosion of the outer building. The radioactive hydrogen will lose its dangerous levels of radiation within a few minutes; hence the reason the engineers must of felt it necessary to contain it within the building; to allow it to decay before release to the atmosphere. Reports are that the 3 levels of containment are still intact. In the worse case scenario, the core could go into complete meltdown and melt through the bottom of the containment vessel. In the containment shell below the vessel is a bed of graphite (the moderator at Chernobyl) that will encase the molten fissile material and contain the further release of neutrons. This may be a lesson from Chernobyl but I couldn’t find anything on the decision to use graphite in the bottom of the containment shell.
This reactor is one of the best designs available and is the result of many years of incidents and reactor failures at the testing facilities in Idaho. There is little to worry about outside of the immediate area where Japanese officials are already evacuating nearly 140,000 people from a 12-mile radius around the reactor in question. Potassium Iodide is also being distributed to those in the area to combat the deadly affects of radioactive Iodine 131, which has been detected in small amounts. When a reactor does experience troubles, the whole world sits on the edge of their seats because of the many tragedies in the past, but this incident should prove that even when hit with earthquakes and tsunamis a modern reactor can be “fought” by the technicians to prevent disastrous release of deadly radiation and explosively reactive materials (other than the hydrogen).
Though this is still a highly volatile situation and will, unfortunately, result in some injuries and deaths due to radiation, the affects of it will never reach the west coast of the U.S. as many fear it will. The only comparison between this event and Chernobyl will be that we’ve come a long, long way in safety since the days of SL-1 and Chernobyl. If the reactor at Chernobyl were the one struck by an 8.8 earthquake and further damaged by a 30-foot tsunami, it would have killed millions by now.
In the past this powerful source of energy, fission, was the Sword over our heads. In modern reactors, the roles of Damocles and the Sword have switched to their proper positions. We are the Sword, or control, over the head of some awesome power; the power of fission.
