On March 11, 2011 an earthquake and tsunami severely damaged four nuclear reactors at the Fukushima Number 1 power plant on the northeast coast of the Japanese island of Honshu.

            The Unit One reactor is a boiling water design fueled with about eighty tons of uranium dioxide in zirconium alloy fuel rods. The primary concrete containment vessel surrounds the core of the reactor and the secondary concrete containment vessel included upper levels which contained pools for storing fuel rods and irradiated equipment.

            Around 3 PM Unit One was shut down in response to the earthquake which shook the reactor and broke pipes. Around 5:30 PM all the electrical power generated by the reactor stopped. Emergency batteries were supposed to take over to provide power for monitoring and control systems but Unit 1’s backup batteries were damaged when the tsunami struck and could not provide emergency power. Fifteen minutes later, TEPCO, the company that managed Fukushima Number 1, declared a Nuclear Emergency Situation because they could not confirm that emergency cooling systems were injecting coolant into the core of Unit 1. Radioactive steam was released into the secondary containment vessel to reduce pressure in the primary vessel.

            At first, after the quake, TEPCO used the isolation condenser system to cool Unit 1 but after ten minutes, they shut down the isolation condenser and turned on the emergency cooling injection system which sprayed coolant into the reactor core. After a half hour, the loss of electrical power to the reactor disabled the spray cooling system. The operators were unable to restart the isolation condensers for a half hour and they functioned intermittently after that. The condensers should have been able to cool the core for eight hours but they failed to perform as expected. It was revealed later that TEPCO had changed the original arrangement of pipes feeding the isolation condensers without notifying government regulator. This may have contributed to the problems at Unit 1.

            By midnight, the core coolant levels were dropping and TEPCO announced that there might be a release of radioactivity from Unit 1. Early on March 12th radiation levels were rising in the Unit 1 turbine building. TEPCO said that they might need to relieve the pressure by venting the rising pressure which would release radioactivity into the environment. This was excused on the premise that there would be very little radioactivity and it would be blown out to sea by prevailing winds. During the night, the isolation cooling system failed and around noon on March 12th, the pressure was relieved by venting and water was injected into the system.

            The heat continued to rise in the core as the cooling water was boiled off and released as steam. The lost of electrical power interfered with the operation of coolant pumps and fans. Increasing radioactivity was detected outside the reactor complex including cesium-137 and idodine-131 which indicated that the coolant levels in the core had dropped so low that the fuel rods were exposed and were melting. Building pressure had to be relieved by manually opening the valves because of the loss of electrical power.

            At 3 PM on March 12th, there was an explosion in the Unit 1 reactor building which blew out the walls on the upper levels and collapsed the roof. The exposure of the zirconium alloy fuel rods to live steam resulted in a reaction that generated hydrogen which resulted in the explosion. The primary containment was not breached but significant radioactivity was released. There was concern that some of the fuel rods may have dropped through the bottom of the core.

            At 10 PM, the Japanese government order that seawater be pumped into the Unit 1 reactor to cool the core although this would ruin the reactor. Boric acid was added to the seawater as it was pumped in by fire trucks. After taking about ten hours to fill the core, the seawater took about ten days to cool down the core. Over the next few days, repairs continued and electrical power was restore to Unit 1. Pressure continued to build requiring reduction of water flow and steam venting. Salt water corroded the zirconium alloy shell of the fuel rods. By March 24 it was determined that the fuel rods in the upper level cooling power had been exposed.

            Work continued for the next several months to bring the core temperate down and restore the various monitoring and control. Temperature of the core was reduced to under 100 degrees Celsius by August, bringing the reactor closer to “Cold Shutdown Status.” By October, a concrete cover for the Unit 1 reactor had been completed.

            The Hanford nuclear facility contains fifty three million gallons of high-level radioactive and chemical waste. These wastes were generated when corrosive chemicals were used to dissolve spent fuel rods to retrieve plutonium. The result was hot radioactive corrosive liquid was. The waste is stored near the Columbia River in huge underground tanks in what is called the Hanford Tank Farm. There are one hundred and seventy seven tanks and most of them are well past their design lifetime. One third of the tanks are known to have leaked some of their contents.  Estimates of the amount leak vary from a million gallons to six million gallons. In addition to being highly radioactive and toxic, the material in the tanks also generates gases that must be vented in order to prevent explosions.

            One hundred and fourty nine single-shell tanks were built between 1943 and 1964. During World War II, there was a shortage of stainless steel so they used cheaper and weaker carbon steel on tanks build during the war. Even after the war when the supply of stainless steel returned to normal levels, they continued to use carbon steel to build the tanks. The twenty double-shell tanks were built between 1977 and 1986 and they were also constructed with carbon steel. The tanks were designed to last 20 years and were never intended as a permanent storage system. The waste is acidic and highly corrosive. It has been eating away at the lining of the tanks since they were filled. One third of the tanks have been eaten thought completely in places and they are leaking the waste into the environment.

            The operators at the site monitor the conditions in the tanks and move waste from older single-shell tanks to twenty eight newer double-shelled tanks when required but space in the new tanks is limited. Robotic systems have been developed to handle the extremely dangerous and toxic waste but they are not always employed and workers are endangered when required to deal with the waste. The single-shelled tanks are considered “closed” when ninety nine percent of the waste has been removed. To date, only seven of the one hundred and fourty nine old tanks have been closed.

            The only way to permanently stabilize the waste is to create glass logs to contain the waste in a solid form. This process is called vitrification. A vitrification plant is being built that will be able to process some of the waste but not all of it. 

            It is not known exactly what is happening at the Hanford Tank Farm because the government has denied the severity of the problem for years and has never adequately funded a complete analysis of the site. A number of clean-up projects have been proposed, funded and started over the years but the site is still the most serious nuclear waste problem in the United States. The main priority of the cleanups is to minimized further leakage from the aging tanks and to stabilize the waste to prevent catastrophic explosions. Three hundred million dollars a year are being spent to operate the Tank Farm.

 

            In 1943 the US government established a nuclear research site in south-central Washington State at the town of Hanford on the Columbia River. That area had been used by Native American tribes for thousands of years and was the location for a reservation. The Yakima Indians were moved to another reservation west of the old reservation in 1943 to make way for the construction of the Hanford nuclear site. They still retain the right to monitor the health of the natural environment at the Hanford nuclear site.

            The Hanford site currently covers about six hundred square miles of desert with the Columbia River forming the north and east boundaries of the site. The area gets less than ten inches of rainfall annually. The site was considered idea because of its isolation and abundant water supply. Some of the people living nearby were moved to other locations. There are three separate areas for reactors, chemical separation facilities and waste storage. Some of the land that was originally part of the Hanford site has been returned to civilian use.

            The first plutonium production reactor in the world was built at the Hanford site as part of the Manhattan Project to develop nuclear weapons during World War II. Plutonium was produced there for the first test nuclear bomb and for the bomb dropped on Nagasaki, Japan.

            During the period known as the Cold War following World War II, the site was expanded to include nine nuclear reactors and five plutonium processing installations. The plutonium installations produced plutonium for almost sixty thousand bombs in the U.S. nuclear weapons program. Scientists at Hanford developed many techniques and technologies used in the evolving nuclear industry. The site has been mostly decommissioned but still hosts the Columbia Generating Station’s nuclear reactor which generates power for the grid as well as research facilities including the Pacific Northwest National Laboratory.

            Most of the reactors were shut down by 1971 with the last reactor shut down in 1987. They were entombed in concrete. Unfortunately, radioactive isotopes were released into the air and into ground water that reached the Columbia River because the early safety and waste disposal techniques were still evolving and were not adequate. The health of the environment and people living in the area were impacted by the radioactive releases.

            Weapons production at Hanford ended in 1991 with the collapse of the Soviet Union and the end of the Cold War. Over fifty million gallons of high level liquid nuclear waste was left by the years of weapons production, most of it stored in metal tanks. Twenty five million cubic feet of solid nuclear waste was buried which contaminated over two hundred square miles of groundwater beneath the site.

            Hanford is the site of the biggest environmental cleanup project in the United States. Two thirds of the high-level nuclear waste in the United States is located at Hanford.

 

 

 

 

 

            On March 11, 2011 a tsunami was triggered by a powerful earthquake off the northeast coast of the Japanese island of Honshu. The huge wave crashed into the coastal Fukushima Number 1 nuclear power plant in Okuma, Fukushima Prefecture and caused a major nuclear disaster.

            The Daiichi plant contains six GE boiling water reactors operated by the Tokyo Electric Power Company. The plant was started operation 1971 and ultimately generated almost five gigawatts of electrical power making it one of the fifteen biggest commercial nuclear power plants in the world.

            On March 11, 2011 reactors 5 and 6 were in what is called cold shutdown where the pressure of the coolant is at regular sea level atmospheric pressure and the temperature is under two hundred degrees Fahrenheit. Reactor 4 had been defueled (had its fuel removed.)

            Following the earthquake, reactors 1, 2 and 3 automatically shut down and emergency diesel generators switched on to provide electricity for reactors controls and coolant circulation pumps. The tsunami caused by the earthquake hit the site and flooded the rooms containing the emergency generators. It also severed the connection of the plant to the electrical power grid. Access to the site was inhibited by the flooding. Newer generators had been built above the flood zone but their connection to the reactors was not protected and was flooded, preventing their use. Attempts to bring in mobile generators failed because they could not be connected to the site power grid.

            The reactors were designed for the coolant to continue to circulate for four to eight hours without the circulation pumps operating. After this time elapsed, the operating reactors began to overheat and eventually went into meltdown where the cores of the reactors become so hot that they melted. Hydrogen was generated from overheating of the zinc alloy sheaths of the fuel rods and this resulted in several explosions also occurred. The government ordered seawater to be pumped into the reactors to cool them which completely destroyed them. Water levels also dropped in the pools where fuel rods were kept when they were not in the reactor cores. This raises the prospect of fires and release of more radioactivity. People were evacuated from a circle around the site twelve miles in diameter. People working on the disaster suffered radiation poisoning. Electrical power was eventually restored to some of the reactors and allowed cooling systems to begin operation again. The melting fuel and the exposed rods in the cooling pools remain a grave concern more than a year after the

            Radioactive isotopes were released into the atmosphere, the soil and the ground water. Dangerous levels of cesium have been detected over 30 miles from the site. Sale of food grown near the site and use of tap water near the site have been restricted. The Japanese government and TEPCO have been criticized for incompetence in reacting to the disaster and poor communication with the public. Increased radioactivity from the Fukushima disaster has been measured in the Pacific ocean water, and in the air, water and soil of North America. The Fukushima disaster was ultimately rated as Level 7 on the International Nuclear Event Scale.

 

 

 

 

 

 

            In April of 1986 there was a major nuclear accident at the Chernobyl nuclear power plant in Ukraine about two miles Prypiat, a city of about fifty thousand people on the Dnieper river near border with Belarus   Ukraine was part of the Soviet Union at that time and the power plant was under the direct control of authorities in Moscow. As with Kyshtym, there was not enough attention paid to safety systems and procedures.

            The Chernobyl power plant contained 4 reactors built between 1970 and 1983 based on a unique Soviet boiling water reactor design. Ordinary water was used as a coolant with graphite acting as a neutron moderator. Control rods were raised or lowered to control the power output of the reactors.

            As the reactor heats up, bubbles or voids of steam reduce the density of the water in the core which causes a drop in neutron absorption and an increase in the reactivity of the core. In this particular reactor design, the amount of bubbles or the void coefficient has a very strong influence over reactor temperature and output.

            On April 25 the operators were preparing the reactor number 4 for a test of the turbines ability to continue to spin and drive the circulation pumps after main electrical power was shut off. Early on April 26 automatic shut down mechanisms were disabled before the test.

            With the reactor in a very unstable condition, the control rods were inserted into the core and caused a severe power surge. The very hot fuel and cold water reacted to produce fuel fragmentation, increase in steam production and an increase in pressure. The pressure opened the one thousand ton reactor cap which ruptured the fuel channels and jammed the control rods which had only been inserted halfway. Water dumped into the core by the ruptured cooling system fed increased steam production throughout the core. A huge steam explosion released fission products into the atmosphere. A few seconds later a second explosion likely caused by hydrogen from zirconium alloy interacting with steam blew out fragments of the fuel and graphite from the core. About three hundred tons of graphite was thrown out of the core resulting in the fuel becoming incandescent and staring fires. These fires were the main cause of the release of around fourteen times ten to the eighteenth power Becquerels of radioactivity into the environment.

            For twelve hours, around three hundred tons of water per hour were dumped into the half of the reactor that hadn’t been destroyed but was ultimately stopped to prevent flooding of reactors 1 and 2. Over the next eight days, five thousand tones of boron, dolomite, sand, clay and lead were poured into the burning core from helicopters in an attempt to put out the fires and stop the release of radioactivity. Over half a million works were ultimately involved in trying to contain the disaster. The eighteen billion ruble cost of fighting the fires and cleaning up the site dealt a severe blow to the economy of the Soviet Union. Five years later the Soviet Union disintegrated.

            The Chernobyl accident was the worst release of radioactivity in history from a civilian nuclear power plant. Serious disruption of the social and economic life of large populations in Ukraine, Belarus, Russia and countries in Europe resulted. The radiation release spread over much of Europe Iodine-131 and cesium-137 were a major threat to public health. Thirty operators were killed in the accident and a few more died later. Over two hundred people received radiation injuries and around thirty later died from the effects of the radiation. Many children developed thyroid cancer as a result of exposure to iodine-131. The accident was rated as a level 7 on the International Nuclear Event Scale.

 

 

 

 

 

 

            In 1957, there was a serious nuclear accident in Ozyorsk, Russia at the Mayak nuclear fuel reprocessing plant. Ozyosk was one of the “closed cities” that the Soviet Union built to carry out for highly classified research and industry. Ozyorsk was not on any public maps so the disaster has been referred to under the name of the nearest town that was on public maps, Kyshtym.          

            In a rush to catch up with the United States in nuclear weapons development, the Soviets built the Mayak reprocessing plant between 1945 and 1948 without great concern for safety or the environment. The six reactors on the site discharged irradiated cooling water directly into Lake Kyzyltash and high-level radioactive waste was dumped into a nearby river.

            In 1953, they built storage for liquid nuclear waste underground. Steel tanks were mounted on concrete bases and a cooling system was created to deal with heat generated by continuing radioactive decay in the waste. The monitoring systems which were created were not sufficient to deal problems arising from the cooling systems and the contents of the tanks.

            In September of 1957, the cooling system failed in one of the tanks and was never repaired. The tank contained about eighty tons of liquid waste. The temperature in the tank continued to rise and the liquid waste was dried out through evaporation. The ammonium nitrate and acetates in the dried waste eventually exploded with a force equivalent to one hundred tons of TNT, blowing the one hundred and sixty ton concrete lid off the tank. Up to fifty microcuries of radiation were released into the atmosphere.

            Over the next twelve hours, the radioactive plume spread to two hundred miles northeast over the east Ural region of western Russia. Three hundred square miles of the landscape were contaminated with cesium-137 and strontium-90. This contaminated area has been given the name of the East-Ural Radioactive Trace. Over the next two years ten thousand people in twenty two villages were evacuated. Early evacuations took place without explanations to the people being moved.

            Because of the secrecy surround the Soviet nuclear program, only vague reports of a the release of radioactivity over Russia from a terrible accident appeared in April of 1958. In 1968, the Soviet Union created the East-Ural Nature Reserve and forbid any unauthorized access to the Reserve in order to conceal the effects of the accident. Eventually, it was revealed that supposed laboratory experiments on the effects of radiation on plants and animals published in Soviet scientific journals were actually reports on the impact of the accident on the environment. Finally, in 1976, Zhores Medvedev revealed what had really happened at Ozyosk. IN 1979, a Freedom of Information Act request filed with the CIA revealed that the CIA had learned about the accident in 1957 but had withheld the information in order to protect the new US nuclear industry. The Soviets finally declassified their records on the disaster in 1990.

            Ultimately, it is estimated that over eight thousand people died because of the accident at Ozyosk. The disaster is rated as a six on the International Nuclear Event Scale.

 

 

 

 

 

 

 

 

            In 1957, there was a serious nuclear accident in Ozyorsk, Russia at the Mayak nuclear fuel reprocessing plant. Ozyosk was one of the “closed cities” that the Soviet Union built to carry out for highly classified research and industry. Ozyorsk was not on any public maps so the disaster has been referred to under the name of the nearest town that was on public maps, Kyshtym.          

            In a rush to catch up with the United States in nuclear weapons development, the Soviets built the Mayak reprocessing plant between 1945 and 1948 without great concern for safety or the environment. The six reactors on the site discharged irradiated cooling water directly into Lake Kyzyltash and high-level radioactive waste was dumped into a nearby river.

            In 1953, they built storage for liquid nuclear waste underground. Steel tanks were mounted on concrete bases and a cooling system was created to deal with heat generated by continuing radioactive decay in the waste. The monitoring systems which were created were not sufficient to deal problems arising from the cooling systems and the contents of the tanks.

            In September of 1957, the cooling system failed in one of the tanks and was never repaired. The tank contained about eighty tons of liquid waste. The temperature in the tank continued to rise and the liquid waste was dried out through evaporation. The ammonium nitrate and acetates in the dried waste eventually exploded with a force equivalent to one hundred tons of TNT, blowing the one hundred and sixty ton concrete lid off the tank. Up to fifty microcuries of radiation were released into the atmosphere.

            Over the next twelve hours, the radioactive plume spread to two hundred miles northeast over the east Ural region of western Russia. Three hundred square miles of the landscape were contaminated with cesium-137 and strontium-90. This contaminated area has been given the name of the East-Ural Radioactive Trace. Over the next two years ten thousand people in twenty two villages were evacuated. Early evacuations took place without explanations to the people being moved.

            Because of the secrecy surround the Soviet nuclear program, only vague reports of a the release of radioactivity over Russia from a terrible accident appeared in April of 1958. In 1968, the Soviet Union created the East-Ural Nature Reserve and forbid any unauthorized access to the Reserve in order to conceal the effects of the accident. Eventually, it was revealed that supposed laboratory experiments on the effects of radiation on plants and animals published in Soviet scientific journals were actually reports on the impact of the accident on the environment. Finally, in 1976, Zhores Medvedev revealed what had really happened at Ozyosk. IN 1979, a Freedom of Information Act request filed with the CIA revealed that the CIA had learned about the accident in 1957 but had withheld the information in order to protect the new US nuclear industry. The Soviets finally declassified their records on the disaster in 1990.

            Ultimately, it is estimated that over eight thousand people died because of the accident at Ozyosk. The disaster is rated as a six on the International Nuclear Event Scale.

 

 

 

 

 

 

 

 

           The International Atomic Energy Agency (IAEA) introduced the International Nuclear and Radiological Event Scale (INRES ) in 1990 to categorized nuclear accidents to assist in communicating the seriousness and dangers of such events. The scale is logarithmic like the Richter scale for earthquakes. This means that each step up the scale in ten times the intensity of the prior step. Interpretation of severity in nuclear accidents is more subjective than earthquake scaling and a level on the INRES scale can only be assigned after the fact when analysis has been completed. This means that the INRES scale is of limited use in dealing with the immediate problems accompanying a nuclear accident.

            Level 0 is called to as a “deviation” and is not a threat to public safety.

            Level 1 is called an “anomaly” and may include over exposure of a member of the public in excess of statutory annual limits of radiation exposure, minor problems with nuclear reactor safety components and/or loss or stolen low level radioactivity source, device or transportation package.

            Level 2 is called an “incident” and may include exposure of a member of the public to more than ten microsieversts of radioactivity, exposure of a nuclear worker to more than the statutory annual limits, radiation levels of more than fifty microsieversts per hour in a nuclear facility, contamination of an area of the facility that was not designed for exposure, failure of safety provisions with no serious consequences, lost high radioactivity source, device or transport package found with seals intact, and/or inadequate packaging of a high radioactivity sealed source.

         Level 3 is called a “serious incident” and may include exposure of nuclear workers to more than ten times the statutory annual limit, non-lethal radiation burns, exposure of more than one Sieverts per hour in a nuclear facility, severe contamination in an area not designed for exposure with a low probability of significant public exposure, accident at a nuclear power plant with no safety provisions still operating, and/or delivery of highly radioactive sealed source without adequate procedures in place to handle it.

            Level 4 is called an “accident with local consequences” and may include minor release of radioactive material into the environment that require no countermeasures other than controls on local food production, one or more deaths from radiation, melting or damage to fuel which release more than one percent of fuel in the core and/or release of significant amounts of radioactive materials inside a facility which has a high probability of exposure to the public.

            Level 5 is called an “accident with wider consequences” and may include release of radioactive materials into the environment which will trigger planned countermeasures, several deaths from radiation, severe damage to the reactor core, release of large quantities of radioactive material inside a facility with a high probability of significant exposure of the public

            Level 6 is called a “serious accident” and consists of significant release of radioactive material into the environment requiring the implementation of serious planned countermeasures.

            Level 7 is call a “major accident” and consists of a major release of radio­active ­material into the environment with widespread health and environmental impacts requiring implementation of planned and extended ­countermeasures.

            Problems with the INRES were revealed when the rating of the 1986 Chernobyl accident in Ukraine was compared with the rating of the Fukushima disaster in Japan. The scale is a qualitative rating system which was designed more for public relations than for scientific classification. There is no definition for an event past Level 7. A new quantitative scale for nuclear accidents has been proposed to address these problems.

            The Three Mile Island Nuclear Generating Station is a commercial nuclear power plant. It is located on Three Mile Island in the Susquehanna River. The island is three miles downriver from the town of Middletown, Pennsylvania which is near the city of Harrisburg, Pennsylvania. There are two nuclear reactors referred to as Three Mile Island Unit 1(TMI-1) and Three Mile Island Unit 2 (TMI-2) at the station.

            TMI-1 is a pressurized water reactor with two cooling towers that draws water from the Susquehanna River. It has a net generating capacity of eight hundred megawatts of electrical energy. It began generating electricity in 1974 and was licensed to operate for fourty years. In 2009, its license was extended for another twenty years.

            TMI-2 is a pressurized water reactor with two cooling towers that draws water from the Susquehanna River and is similar to TMI-2 It has a net generating capacity of nine hundred megawatts of electrical energy. It began generating electricity in at the end of 1978 and was licensed to operate for fourty years.

            On March 28, 1979 TMI-2 suffered the worst accident to ever happen at a commercial nuclear reactor in the United States. At 4 A.M. there were failures in the secondary cooling system which is separate from the reactor core. Then a relief valve stuck in the open position in the primary cooling system which resulted in the release of large amounts of the steam from the reactor.

            The operators failed to recognize that the problem was a loss of coolant. The indicators in the control room had been poorly designed and the operators were not well trained. One of the operators mistakenly believed that there was too much water in the reactor causing the steam release. He prevented the automatic emergency cooling system from switching on.

            The employees of Met Ed, the company that operated the TMI power station and the Nuclear Regulatory Commission (NRC) spent the next five days trying to understand the full scope and cause of the accident. During that time, they worked with the media to communication the evolving situation to public, especially those who lived around the plant. One hundred and fourty thousand pregnant women and pre-school children were evacuated from the surrounding area. The NRC  ultimately authorized the release of fourty thousand gallons of radioactive waste water into the Susquehanna River. This led to a public outcry and loss of credibility for the NRC. The full complexity of the accident was not well understood until much later.

            Although there were radioactive gases and iodine-131released into the environment, the authorities claimed that subsequent epidemiological studies indicate that there was no impact on public health. This finding has been disputed up to the present day with claims that some people were made ill by the accident.

            Clean up started in the summer of 1979 and officially ended in December of 1993, nearly fifteen years later, and cost around one billion dollars. The seven level International Nuclear Event Scale rated the accident as an “Accident with Wider Consequences” known as a level five incident.

            Public reaction to the accident galvanized the anti-nuclear activists. New regulations were imposed on the nuclear industry and there was a decline in the construction of new power plants.

 

 

 

 

            The boiling water reactor (BWR) design is used in about one third of the commercial power generation reactors in use in the world today. It is the second most popular reactor design behind pressurized water reactors. The BWR design was the result of collaboration between the Idaho National Laboratory and General Electric in the mid-1950s. Today, GE Hitachi Nuclear Energy is the main manufacturer of the BWRs.

            In the BWR design, ordinary water that has been processed to remove minerals is used as the coolant and the neutron moderator. The heat from the fission reaction in the core of the reactor is used to turn the coolant water to steam. The water is kept under pressure so that the temperature at which it turns to steam rises from two hundred and twelve degrees Fahrenheit to five hundred and fifty degrees Fahrenheit.

            The steam from the reactor core is then used to turn a turbine to generated electric power. The steam then enters a condenser where cooling water flowing through a second system is used to turn the steam back into liquid water. This water is drawn from a river, lake or ocean and is returned at a higher temperature.

            Water returning to the core is draws heat from the steam in the system and fed into the bottom of the core where it rises and turns back to steam. Because the steam is part of the primary water system that includes the core, the water and the steam contain radionuclides leaked from the fuel rods. The part of the reactor that contains the steam turbine must be well sealed and shielded to prevent escape of radiation.

            The water in BWRs also acts as a moderator for the fission reaction in the core. As the fast neutrons generated by fission collide with water molecules, they are slowed down which increases their ability to trigger more fission events in the core. As the water heats, the molecules move apart and reduce the ability of the water to slow down the fast neutrons. This in turn reduces the activation of fission events which cools the core. This creates a negative feedback control system which keeps the core within a particular temperature range. The position of the control rods determines the center or set point of the temperature range. Light water is an excellent moderator and allows the construction of compact reactor cores.

            Controlling flow of water with recirculation pumps is a useful secondary control system that is present in some BWR designs. There are steam bubbles in the water around the core. If the water flow is increased, the steam bubbles are removed more quickly and the water is denser. This increases neutron moderation which increases fission and power generation. Slowing the flow of water decreases neutron moderation and decreases the fission reaction, lowering heat production and power generation.

            The fuel used in BWRs is uranium dioxide (UO₂) ceramic pellets in Zircaloy fuel rods. The BWR has evolved through 6 different designs which are divided into Generation I, Generation II, Generation III. The number of fuel rods in a bundle and the number of bundles in a core have changed during this evolutionary process.

            Boron control rods are used in BWRs to start the reactor, shut down the reactor and to control short terms changes in power demand.  The control rods can also compensate for changes in the fuel due to depletion or poisoning by isotopes generated during fission.

            The boiling water reactors operate at about half the pressure of the pressurized water reactors. The pressure vessel in the BWR gets less radiation than the pressure vessel in the PWR and does not become brittle as quickly. There are fewer components in a BWR compared to a PWR which reduces the possibility of a rupture and leaking of coolant. With natural convection circulation of water, the possibility of failure of cooling pumps is eliminated. Boric acid is not used in the coolant water eliminating corrosion. A single major manufacturer supports standardized design. BWRs are not used for propulsion systems and are not as useful for developing nuclear weapons. This makes BWRs desirable for export from the United States.

 

1. Reactor pressure vessel (RPV) 2. Nuclear fuel element 3. Control rods 4. Circulation pumps 5. Control rod motors 6. Steam 7. Feedwater 8. High pressure turbine (HPT) 9. Low pressure turbine

10. Generator 11. Exciter 12. Condenser 13. Coolant 14. Pre-heater 15. Feedwater pump 16. Cold water pump 17. Concrete enclosure 18. Connection to electricity grid