A nuclear catastrophe is potentially around the corner. After the earthquake affected the Japanese nuclear plant, three explosions and a fire have occurred in four days. Quite simply, the situation at Japan’s earthquake-stricken Fukushima Daiichi nuclear plant is dire. On Tuesday there was an explosion at plant’s No. 2 reactor and a fire in a cooling pond used for nuclear fuel at the No. 4 reactor.
The radiation threat has grown as the Japanese authorities have started warning the citizens to seal themselves indoors to save themselves from the dangerous levels of radiation.
radiation levels surrounding the plant are now at “levels that can impact human health,” urging people within a 30-kilometer radius — a population of 140,000 — to remain indoors.
Just as a matter of comparison – Chernobyl was rated at a maximum level 7 on the INES scale. The Fukushima Daiichi accident has just been declared by the France’s nuclear agencyto be at a level 6. This rating applied before the fire at the pools of water containing spent fuel rods. Why is that important?
The pools sit on the top level of the reactor building, with spent fuel rods submerged in water. But like the reactors, the pools have lost power to their cooling systems after the earthquake. If any of the spent fuel rods were to catch fire, high heat would “loft the radiation in clouds that would spread the radioactivity,” says the New York Times
Meanwhile the residents in even Tokyo are clearing all the shelfs of the supermarkets off in anticipation of severe impact on basic amenities and supplies.
Tokyo residents emptied store shelves of daily necessities and stocked up on gasoline as the risk of nuclear radiation leaks from a facility north of the Japanese capital escalated. Seven & I Holdings Co., Japan’s biggest retailer, said its Ito-Yokado supermarkets are being emptied daily of necessities such as water, rice and batteries as soon as fresh supplies arrive.
The companies have started evacuating their employees out of Japan. Two companies who have started the process are Alcatel-Lucent SA, ICAP Plc (IAP) and Infosys Technologies Ltd. (INFY). Tata Consultancy Services (TCS) is also working to get its employees out soon.
We currently – as per the authorities – have a “partial meltdown” and are looking at a “full meltdown” situation, where the core of the reactor is affected. The safe guards to stop that are:
- A limiting fault (or a set of compounded emergency conditions) that leads to the failure of heat removal within the core (the loss of cooling). Low water level uncovers the core, allowing it to heat up.
- Failure of the emergency core cooling system. The ECCS (Emergency Core Cooling System) is designed to rapidly cool the core and make it safe in the event of the maximum fault (the design basis accident) that nuclear regulators and plant engineers could imagine. There are at least two copies of the ECCS built for every reactor. Each division (copy) of the ECCS is capable, by itself, of responding to the design basis accident. The latest reactors have as many as four divisions of the ECCS. This is the principle of redundancy, or duplication. As long as at least one ECCS division functions, no core damage can occur. Each of the several divisions of the ECCS has several internal “trains” of components. Thus the ECCS divisions themselves have internal redundancy – and can withstand failures of components within them. Although no limiting fault has ever occurred in a Western LWR, ECCS systems have been called on to perform a limited number of times. The staff of each plant keeps the ECCS in peak condition at all times. No complete failures of the ECCS had occurred prior to the 2011 Sendai earthquake and tsunami.
Between the loss of cooling and the full meltdown, there are 6 stages, which are:
- Core uncovery. In the event of a transient, upset, emergency, or limiting fault, LWRs are designed to automatically SCRAM (a SCRAM being the immediate and full insertion of all control rods) and spin up the ECCS. This greatly reduces reactor thermal power (but does not remove it completely); this delays core “uncovery”, which is defined as the point when the fuel rods are no longer covered by coolant and can begin to heat up. As Kuan states: “In a small-break LOCA with no emergency core coolant injection, core uncovery generally begins approximately an hour after the initiation of the break. If the reactor coolant pumps are not running, the upper part of the core will be exposed to a steam environment and heatup of the core will begin. However, if the coolant pumps are running, the core will be cooled by a two-phase mixture of steam and water, and heatup of the fuel rods will be delayed until almost all of the water in the two-phase mixture is vaporized. The TMI-2 accident showed that operation of reactor coolant pumps may be sustained for up to approximately two hours to deliver a two phase mixture that can prevent core heatup.”
- Pre-damage heatup. “In the absence of a two-phase mixture going through the core or of water addition to the core to compensate water boiloff, the fuel rods in a steam environment will heatup at a rate between 0.3 K/s (.5 F/s) and 1 K/s (2.13 F/s) (3).”
- Fuel ballooning and bursting. “In less than half an hour, the peak core temperature would reach 1100 K (1520 F). At this temperature, the zircaloy cladding of the fuel rods may balloon and burst. This is the first stage of core damage. Cladding ballooning may block a substantial portion of the flow area of the core and restrict the flow of coolant. However complete blockage of the core is unlikely because not all fuel rods balloon at the same axial location. In this case, sufficient water addition can cool the core and stop core damage progression.”
- Rapid oxidation. “The next stage of core damage, beginning at approximately 1500 K (2240 F), is the rapid oxidation of the Zircaloy by steam. In the oxidation process, hydrogen is produced and a large amount of heat is released. Above 1500 K, the power from oxidation exceeds that from decay heat (4,5) unless the oxidation rate is limited by the supply of either zircaloy or steam.”
- Debris bed formation. “When the temperature in the core reaches about 1700 K (2600 F), molten control materials [1,6] will flow to and solidify in the space between the lower parts of the fuel rods where the temperature is comparatively low. Above 1700 K (2600 F), the core temperature may escalate in a few minutes to the melting point of zircaloy (2150 K, 3410 F)) due to increased oxidation rate. When the oxidized cladding breaks, the molten zircaloy, along with dissolved UO2 [1,7] would flow downward and freeze in the cooler, lower region of the core. Together with solidified control materials from earlier down-flows, the relocated zircaloy and UO2 would form the lower crust of a developing cohesive debris bed.”
- (Corium) Relocation to the lower plenum. “In scenarios of small-break LOCAs, there is generally a pool of water in the lower plenum of the vessel at the time of core relocation. Release of molten core materials into water always generates large amounts of steam. If the molten stream of core materials breaks up rapidly in water, there is also a possibility of a steam explosion. During relocation, any unoxidized zirconium in the molten material may also be oxidized by steam, and in the process hydrogen is produced. Recriticality also may be a concern if the control materials are left behind in the core and the relocated material breaks up in unborated water in the lower plenum.”
There are larger implications of this nuclear accident, which has sent the countries in Europe – specifically France – and others like India, Canada and US into a tizzy. Are these countries safe? Are the reactors built in a place that could withstand natural disasters?
Germany meanwhile will halt nuclear reactors contributing to 25% of its energy to do a safety review.
The Indian PM meanwhile indicated that the country will reveiw the safety at all its nuclear facilities right away.
“The Department of Atomic Energy and its agencies, including the Nuclear Power Corporation of India, have been instructed to undertake an immediate technical review of all safety systems of our nuclear power plants, particularly with a view to ensuring that they would be able to withstand the impact of large disasters such as tsunamis and earthquakes,”
(Image of the Nuclear Meltdown – courtesy HT)
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