Wednesday, April 6, 2011

Nuclear Reactors, Chernobyl, and Japan

Part 1: Nuclear Reactors:

What is a Nuclear Reactor?

A nuclear reactor is a device that triggers a nuclear chain reaction and controls and sustains them at a steady rate. Fission, the process used in nuclear reactors, takes place when bombarding a nucleus with another particle. It starts off with one neutron, which then produces more neutrons once it is bombarded with another particle, and so on as a chain reaction (1). Controlled fission is used in order to produce useful energy to use as electricity. Fission is a self-sustaining reaction, meaning it can keep itself going, and therefore is very good energy source that releases large amounts of energy in the form of heat. Although this is a very helpful aspect of fission, it can also be dangerous because it can get out of control very quickly and safety precautions need to be taken (3). Before being able to understand the disasters at Chernobyl and Japan and what went wrong to cause the disasters, it is first necessary to understand how a nuclear reactor works and the parts within it.

Parts of a Nuclear Reactor:
  • The core is where the radioactive material is stored. The most common type of fuel used in nuclear reactors is Uranium-235. (4)
  • The moderator is a material in the core that slows down neutrons without absorbing or reacting with them so that can continue the chain reaction and produce more fission. In most moderators water is used but in the moderator at Chernobyl grid graphite was used. (5)
  • Control rods are used to absorb neutrons. They can be inserted or withdrawn from the reactor core in order to control the rate of reaction. When the control rods are inserted into the core, they absorb a large amount of neutrons, therefore slowing the fission process down. When the rods are pulled out of the reactor core, fewer neutrons are absorbed, therefore speeding up the fission process. These rods are made of steel and contain a large amount of material that can absorb neutrons and are usually composed of boron. Fuel rods are narrow cylinders that contain small pellets of uranium fuel. (5)

  • The coolant is a material that passes through the core that is used to transfer heat from the fuel to a turbine. In more simple terms, the nuclear reaction produces heat, and the coolant carries the heat away. This also serves the function of keeping the reactor cool enough to prevent a meltdown. (1)
  • The steam generator is part of the cooling system. It is used to boil water and produce steam for the turbine. The turbine then transfers the heat from the coolant to produce electricity. (4)
  • Containment is the steel and concrete structure around the reactor core that separates the reactor from the environment to prevent radiation from escaping. (1)
  • Cooling towers release the excess heat that cannot be converted into energy. (1)

All of these components work together to produce useful energy in the form of electricity. Nuclear reactions in fuel rods heat water, and then the steam drives a generator to produce electricity. Cold water from the sea or rivers heat and converts steam back to water. The water cycles back to the core to be reheated and repeat the process. 

Part 2: Disaster at Chernobyl

What Happened at Chernobyl?

The disaster that took place at Chernobyl is considered to be the worst nuclear reactor disaster ever. Chernobyl is located in Ukraine near the capital city of Kiev. The nuclear reactor is a RMBK design, meaning that it has graphite as opposed to water in the moderator. On April 25, 1986, reactor 4 at Chernobyl was scheduled for routine maintenance and was shut down. It was decided that an additional test would also be performed in order to see how long turbines could supply power to keep the cooling system going in case of a power outage (2).
At 1:00 am on April 25, the operators at Chernobyl began to reduce power for the test, but the test was delayed for 9 hours due to the need of power in the nearby town, Kiev. This resulted in difficulties with the work shifts, leaving the unprepared night shift to run the test. On April 26, the operators carried on the power reduction of the reactor, but due to a mistake by an operator, the power drastically dropped and was too low to run the test. To try to increase the power to keep the test running, operators raised and removed all but six control rods from the core at 1:00 am. Yet again, this was a terrible mistake by the operator (2). The minimum safe operating number of control rods is 30, yet there were only 6 control rods in the reactor core at that time, making it very unstable (3). The operators did not think it was not ideal to shut down the reactor, so they disabled the automatic shutdown system to continue the test. The purpose of the test was to see how the reactor worked under low power, so it was necessary to shut down the automatic system. Once the reactor was considered stable enough, the engineers decided to continue the test (2).
At the time the test was continued, both turbines were shut down and there was a reduction in the water flow, causing power to rise. This caused the reactor to boil and overheat, and the water coolant started turning to steam. The power drastically increased. (2) At that moment the manual shutdown button was pushed and the control rods that were originally taken out to increase power, were inserted back into the core. Due to the design of the control rods and the way they were inserted, it relocated the coolant and drove all activity to the lower core. (3) This dramatically increased the power to 120 times its full power. At 1:23 am, two explosions occurred. (2)

Explosions and Evacuation:

The explosions were caused because the pressure from the excess steam, which was supposed to go to the turbines that had been shut down, broke pressure tubes. Fuel pellets in the core began to explode, causing the explosions. The roof of the reactor was blown off and radioactive contents and burning graphite exploded outwards. A large fire was started when air was sucked into the reactor and ignited flammable carbon monoxide. Firefighters were rushed to the scene of the large fire and were successful in putting out the large fire nine days later (2). The firefighters poured water on the fires and tried to stop them with sand, but it proved to be ineffective. They then resorted to stopping the fires with lead and nitrogen (6). Along with the large fire at the core, 30 additional small fires were started near the accident. The Soviet Union did not notify the people about this disaster until the day after it happened. The radiation level decreased for a short period of time and the Soviet Union tried to cover up the fact that this disaster ever happened. The radiation levels soon rose and evacuation was ordered two days after the explosion. Those that stayed in towns near the reactor were ordered to remain indoors so they would not be exposed to the radiation. There was a 30 km evacuation zone around the reactor and those near it had to be relocated. (2) 

How have they tried to fix the situation at Chernobyl?

One of the main problems at Chernobyl was that it didn’t have a containment, a steel or concrete structure around it to protect the environment from radioactive material. Once the explosions occurred, a large amount of radioactive debris escaped from the building and led to many long-lasting health effects. The Soviets decided in order to try to prevent radiation from escaping the reactor they would build a large shell around it to contain the radiation. The shell was made out of concrete and steel and would permanently cover the entire reactor. This is known as the Sarcophagus (2).


Health and Environment Effects:

The workers who helped in the clean up process were not equipped with proper protective gear and were exposed to high amounts of radiation. This led to sickness and many deaths among these workers who were some of the first to treat the reactor immediately after the accident. 31 people died immediately after the explosion, and it is predicted that more than 100,000 people have already died and as people continue to be exposed to radiation, that number will continue to increase (6). Chernobyl released as much as 100 times the radioactive contamination of the Hiroshima bomb (3). Over 300,000 people were resettled due to the disaster, yet millions still continue to live in the contaminated area. Rainfall and wind can also transport radiation to other countries. Radioactivity is still found in large amounts in Belarus, Ukraine, and Russia (3).
            Radiation puts people’s health into jeopardy by causing cancer and birth defects. Thyroid cancer is especially common among young children ages 0-14, and other cancers such as leukemia affect people exposed to radiation. Cesium-137 is a harmful substance from radiation that has a half-life of 30 years. Due to this long half-life, it is still found today in soils and foods in Europe. The contaminated soil cannot grow crops because it will contain radiation in foods and will harm people (7). Pripyat, a town very close to Chernobyl that housed plant workers is now a ghost town because everyone was ordered to evacuate from it (6). 


Part 3: Japan

What happened in Japan?

-       Explosions in reactors 1, 2, and 3.
-       Water was boiled down and exposed the rods. This caused the rods to overheat because there was no water to cool them. The extreme heat caused the explosion (9).
-       Now Japan is pumping cool seawater into the reactor to try to regulate the temperature and cool it down to prevent more explosions from occurring (9).
-       People are taking iodine pills to try to prevent the possibility of getting illnesses or cancer from the radiation. This helps protect the thyroid gland so it can’t absorb harmful iodine (8).  

Similarities between Fukushima and Chernobyl:

-       Evacuation was carried out in areas that were heavily impacted by radiation
-       Radiation entered the environment 
-    People reacted in fear 

Differences between Fukushima and Chernobyl:

            There are more differences than similarities:
-       Perhaps the main difference between the two is the reason each explosion occurred. The disaster as Chernobyl was caused when a new test was taking place and was due to careless mistakes by operators, whereas Fukushima was caused by a natural disaster – tsunami (9).
-       Another key difference between the two is the containment. Chernobyl released a much larger amount of radiation into the environment than Fukushima has. Fukushima has containment around it – heavy steel and concrete – to protect the environment from the radiation inside the core. Chernobyl on the other hand did not have one, so when it exploded the radiation was immediately released into the environment. The containment at Fukushima is keeping in a large amount of radiation (9).
-       The designs of the reactors were also different. Chernobyl used graphite in its moderator as opposed to water used in Fukushima. Graphite catches on fire easily, therefore causing a large explosion (8).
-       The Chernobyl plant was used to process plutonium weapons as well as supply electricity – Fukushima only to supply electricity (8).

Should we be worried?

-       No, the radiation is not as bad as Chernobyl and it will not spread to the U.S.
-       Fukushima has containment, so the radiation released into the environment is not as much.
-       Measures are being taken to control and watch the radiation.