It all got triggered by a massive earthquake followed by a Tsunami which swept huge structures not to speak of humans and now the threat is even greater. The country stands atop a volcano. Adil Rasheed makes a study of functioning of nuclear power plants in light of the disaster that hit Japan
Nuclear power plant is an electricity producing unit like a thermal power plant. Difference being that in a nuclear power plant fission energy is harnessed to produce electricity. Nuclear energy has a very high energy density as compared to any fossil fuel. Nuclear energy if used properly is a pollution free source of huge energy. As of now 15% of the world’s electricity comes from nuclear source. U.SA has the largest number of reactors, almost 78% of France’s electricity comes from nuclear reactors. Another credit to French nuclear industry is all this power produced has been accident free. Fission is a nuclear reaction whereby a neutron on hitting unstable nuclei (like uranium -235, plutonium-239) breaks the nuclei into small nuclei and releases energy and more neutrons. These neutrons produced cause the fission of neighboring atoms hence a continuous chain reaction is formed. When neutrons produced remains constant with time we say that reaction is critical. The byproducts of a nuclear reaction are highly unstable nuclei hence radioactive. Radioactivity refers to phenomena whereby an unstable nucleus of an atom emits particles as well as radiation in the form of electrons, protons and high frequency electromagnetic rays known as gamma radiation. These nuclear radiations are harmful to human body. In lieu of the accident happened in Japanese reactors, I will just give a brief description of a nuclear power plant.
In its simplest form nuclear reactor consists of Reactor pressure vessel (RPV) which in turn consists of core, coolant, control rods, besides that there is a heat exchanger, Turbine-generator unit and condenser which are outside of the reactor pressure vessel. All the Japanese reactor in news are boiling water reactors(BWR) meaning that steam is formed inside reactor vessel. Reactor Core usually occupies the bottom most place inside the reactor (RPV) where fuel is kept. A fuel rod consists of uranium dioxide pellets in a Zircaloy-2 cladding tube. The uranium dioxide pellets are manufactured by compacting and sintering uranium dioxide powder into cylindrical pellets and grinding to size. The nominal density of the pellets is -95% of the theoretical uranium dioxide density (usually uranium 238 with 0.7% to 4.1% of uranium-235).Inside the core, fission takes place and heat released by fission reaction is removed continuously by a coolant (usually water) to the heat exchanger, where steam is formed and this steam is sent to run the turbine which in turn runs the generator and produces the electricity. It is to be noted that a failure of a coolant system will result in continuous heat built up in the core and ultimately the melting of the core or what is called “melt down’’, hence fission reaction needs to be controlled all the times. Reactor Core is the place of highest radioactivity and high neutron density, fission reaction inside the core is controlled by using control rods which are made of neutron absorbing material like Cadmium, boron-10. Another method to control and stop the fission reaction is to add boron in the form of boric acid directly into the coolant. Reducing of number of neurons will reduce the number of fissions per unit time and hence the energy output.
In a nuclear industry the worst accident is the melting of the core. This happens due to the failure of coolant system to remove heat from the core. This type of accident has occurred in three mile island of US nuclear station. In three mile core melted but because of the presence of core catcher vessel the radioactivity release was controlled. Generally there are various systems designed which are provided to ensure heat removal from the core even in the worst case scenario called LOCA (loss of coolant accident) which can happen due to a major break in coolant piping. There are active (which runs on power ) and passive systems which ensure core cooling all the time.
North - East Coast of Japan experienced a severe earthquake on March 11, 2011 at 14:46 JST of a Magnitude of 8.9, followed by high intensity tsunami resulting in large scale devastation in the region. The 11 nuclear power reactors in the affected region were automatically shutdown as called for by design. (There are 54 nuclear power reactors in operation in Japan spread over the country) There has been a wide coverage, in press and media, of possible radiological consequences resulting out of interruption in cooling of the reactor core in two of the reactors at Fukushima Daiichi site. Shutdown of a nuclear power plant means making the fission reaction subcritical which basically means that number of fissions occurring reduces with time. The Fukushima Daiichi site has 6 Boiling Water Reactors (BWRs). The units- 1, 2 and 6 were supplied by General Electric, unit-4 by Hitachi and units-3 & 5 by Toshiba. Unit-1 is a 439 MWe .BWR type-3 with earlier version (Mark -1) of primary containment which is in operation since 1971. At the time of event on March 11, 2011 at 14:46 JST the units 1,2 and 3 were in operation whereas Units-4, 5 and 6 were under maintenance shutdown. An earth quake of magnitude 8.9 hit Japan, followed by a significant tsunami. Units 1, 2 and 3 were automatically shutdown thus
Terminating the nuclear chain reaction. However, the decay heat in the fuel which is about 2%.
Full power shortly within 90 seconds of shutdown, needs to be removed. The decay heat, is a heat produced from the radioactivity release from the fission by products, this heat is absorbed by the coolant. This heat gradually decreases, is removed by supplying cooling water to ensure integrity of the
Fuel. To achieve this, normal cooling water systems as well as emergency cooling water systems (Backup) are provided in nuclear power reactors. At the Fukushima Daiichi Nuclear Power Plant Unit-1 the electrical power supply from grid was lost consequent to tsunami. The Diesel Generators which provide backup power in such cases started as expected and fed the emergency power for decay heat removal from the core, but got disabled after about one hour due to tsunami. Hence there was a failure of decay heat removal which resulted in increase in pressure inside the reactor pressure vessel. There is provision to automatically relieve this high pressure steam to a large pool of water called suppression pool connected to the primary containment. In boiling water reactors a suppression pool is provided which is basically a tank having huge reservoir of coolant, during uncontrolled pressure rise in reactor pressure vessel part of the steam is vented into the suppression pool so as to decrease the reactor pressure.
As this was happening repeatedly to maintain the pressure in the vessel, the pressure in the containment started rising. At 14:40 hours JST on March 12, 2011, the pressure in the containment housing the reactors pressure vessel had risen significantly necessitating release of the steam from the containment. There are essentially four barriers to radio activity release 1. Fuel matrix (since fuel uranium dioxide is porous it absorbs most of the gasesous byproducts) 2. Fuel clad (which is a hollow cylindrical tube surrounding the fuel, made of zircalloy) 3.Coolant system 4.Primary containment (a structure usually of cylindrical shape with hemispherical top) this building houses the reactor pressure vessel and is the final barrier to radioactivity, in many modern nuclear power plants a secondary containment is also provided when a radioactive substance comes out of the primary containment we say that there is a radio activity release into the atmosphere. So technically speaking the moment the radioactive steam was released from unit-1 Fukushima Daiichi, there was a radioactivity release since it does not have any secondary containment. After that The radiation fields were constantly monitored and are available all through the incident. The values were well in control and quite low. However, at 15:29 Hrs JST on March 12 2011, the radioactivity at site boundary, as measured by a moving car showed a spike of 1015 MicroSievert per hour at one location. This localized short spike is equal to the dose permissible annually. At other locations around the same time, the field continued to be low (7 to 40 MicroSievert per hour).
On March 12, at 13:30 hours JST, presence of low level Cesium -137 and Iodine-131 was also detected near unit-1 reactor indicating possible overheating of the fuel elements and Zircalloy Cladding.Zircalloy at high temperatures reacts with steam and produces hydrogen.
Zr +2H2 ¬ ZrH2 + H2
This hydrogen generated made its way along with the steam into the building surrounding the main containment and accumulated near its ceiling. The explosion in this building at around 15:30 hours JST was caused because of the hydrogen-oxygen mixture. As a mitigative measure to limit the damage, cooling of the core using sea water mixed with boron was initiated at 20:00 hours JST. The recent reports have indicated that a similar situation has emerged in Unit 3 and it is also being handled by sea water cooling. Latest reports, indicate that Fukushima Dai-ichi Unit-3 also had similar explosion at 11.03 hrs JST on March 14, 2011 due to hydrogen accumulation in the reactor building. According to preliminary reports the containment vessel is intact and there is no significant release of radioactivity. Reports coming at 06:20 hrs of March 15, 2011 JST, an explosive sound was heard at the lower part of the primary containment of Fukushima unit-2 near the suppression pool. The radiation field at 08:31 hrs JST was measured to be 8217 microsievert / hour. At about 9:36 hrs JST on March 15,2011 a fire broke out in reactor building of unit-4 of Fukushima plant. This unit was in shutdown state at the time of the earthquake/tsunami incident and does not contain fuel in the reactor core. The evacuation of people has been done up to 20 Kms. Radius and sheltering has been advised between 20 and 30 Kms. At about 12:30 hrs, March 15, 2011 the fire which broke out in unit-4 is reported to be extinguished and the radiation fields upto400 millisieverts /hour has been reported at the Fukushima site. The reason for fire was same zirconium-steam reaction and liberation of hydrogen. The breach of primary containment releases various types of radioactive substances, out of these the important ones are iodine-131, and cesium-137. Radioiodines when inhaled gets attached to a thyroid and emits radiation, hence thyroid receives a dose of 20 millisevert in one year. Iodine -131 emits beta (high speed electron) and gamma radiations, in order to reduce the dose from iodine -131 people living near the nuclear reactor in Fukushima were given normal iodine in the form of potassium iodide pills, so that radioactive iodine will not get attached with thyroid gland as thyroid takes up only a small fraction of a milligram of iodine from blood each day. If we flood the blood with stable iodine just before an exposure or shortly afterwards, we can reduce the thyroid uptake of radioactive iodine. Another one is cesium-137, cesium once it enters the body is disguised as potassium as it is chemically similar and belongs to the same group. Cesium -137 has a half life of 200 years, hence it decays slowly and pounds the body with huge amount of radiation over many years ultimately resulting in cancer. So, first step taken at a possible chance of radioactivity release is the evacuation of the people to the safer areas.
Since unit-1 Fukushima Daiichi is a 40 year old reactor it did not have a passive decay heat removal system which would have ensured heat removal from core even after complete power loss usually termed as station blackout. A passive system generally works on gravity where by a emergency coolant is supplied to reactor pressure vessel on actuation of a signal.
In india Nuclear power corporation is the lead runner for operation and construction of nuclear power plants.There are 19 nuclear reactors in india.All Indian nuclear reactors are designed to handle a complete power loss scenario. During the tsunami event in Tamilnadu state in 2004, Madras Atomic Power Station (MAPS) was safely shutdown without any radiological consequences .The plant was restarted in a matter of days after regulatory review ,further one of the criteria’s of site selection is the seismic zoning. Nuclear power plant is not usually constructed in earth quake prone areas which come under zone 4 and zone 5. In all Indian reactors there is a very less chance of hydrogen accumulation inside the reactor building because even if all of the zirconium reacts with steam to form the hydrogen, this hydrogen form will be quickly converted into water by platinum recombiners. Further, all reactors have double containment design, numerous engineered safety systems passive as well as active to provide core cooling in all design based accidents and beyond design based accidents. Most of the modern nuclear reactors around are fail safe meaning that in case of failure of a component it will shut down. All nuclear reactors are inherently safe and with the advancement in design they are becoming safer, there is also a world association of nuclear operators (WANO) which monitors the operation of nuclear reactors around the world and helps as a cord for information sharing between nuclear operators. All said and done, it seems that authorities in Japan were handicapped by a primitive reactor design and were slow to act resulting in a major nuclear accident whose consequences are still unknown.
(Adil Rasheed is Scientific officer-D, Nuclear Power Corporation of India Limited, Department of Atomic Energy)
Lastupdate on : Wed, 16 Mar 2011 21:30:00 Makkah time
Lastupdate on : Wed, 16 Mar 2011 18:30:00 GMT
Lastupdate on : Thu, 17 Mar 2011 00:00:00 IST