Accidents core meltdown

As described in Chapter 1, the three largest radiological accidents of the last twenty years tire tlie explosion at Chernobyl, the partial core meltdown at Three Mile Island Unit 2, tuid the mishandling of a radioactive source in Brazil. The least publicized, but perhaps tlie most appropriate of tliese accidents, witli respect to waste management, was tlie situation in Brazil. 

The most serious accident tliat Ciui occur in a nuclear plant is a reactor core meltdown. In a core meltdown, the enclosed gases physically melt through tlie reactor vessel, and once contacting with cooler liquids or vapors either in a cooling jacket or in the outer enviromnent, cause a physical e. plosion to occur. However, tlie hazard caused by the e. plosion itself is minimal and more localized compared with the release of radioactive material that accompanies such an accident. 

A much more serious nuclear accident occurred at Chernobyl in the USSR on April 26, 1986, when one of the Chernobyl units experienced a full-core meltdown. The Chernobyl accident has been called the worse disaster of the industrial age. An area comprising more than 60,000 square miles in the Ukraine and Belarus was contaminated, and more than 160,000 people were evacuated. However, wind and water have spread the contamination, and many radiation-related illnesses, birth defects, and miscarriages have been attributed to the Chernobyl disaster. 

Even if terrorists succeeded in detonating an explosive at a reactor site, the health consequences would be limited. The reactor accident at the Three Mile Island, Pennsylvania nuclear power plant caused a small release of radiation, insufficient to cause any radiation injuries. Bypassing several safety systems caused the Chernobyl reactor incident, involving two explosions, fires and reactor core meltdown. This accident caused the following early phase health effects (1) ... 

A part of the US Nuclear Regulatory Commission s (NRC) severe accident research program was dedicated to hydrogen issues in LWR containment designs under core meltdown conditions. The analysis included the in-vessel and ex-vessel hydrogen generation as well as its mixing and distribution in the containment. 

The Sandia code CONTAIN is a lumped parameter code with mechanistical models for simulating the physical and chemical conditions in the nuclear containment to predict hydrogen and steam concentration distribution as well as the consumption of H2 by respective combustion. Assuming a core meltdown accident and no vessel breach, i.e., no corrosion/concrete interaction, the code has predicted a thermally stratified containment atmosphere with relatively low temperatures in the central and lower regions which would permit steam condensation. Concerning H2 deflagration, CONTAIN predicts respective bums, if sprays are used for steam removal [56],... 

Generally speaking, all these improvements lead to a significant reduction in the overall risk of core meltdown in cold shutdown conditions, and to uniform accident sequences and relatively moderate impact on long term accident phases. Specifically, the design of automatic makeup adequately reduces shutdown mode risks (aim of risk reduction linked to short term sequences achieved), and other preventive hardware improvements have proved to be adequate. However, discussions are currently taking place with the French safety authorities to definitively conclude this issue. 

Certainly the most catastrophic nuclear accident occurred on April 26, 1986, at the Chernobyl unit 4 reactor near Kiev, Ukraine. The accident resulted in a core meltdown, explosion, and fire. 

Mitigation of severe accidents up to core meltdown accidents in order to restrict offsite emergency response actions (evacuation or relocation of the population) to the nearby plant vicinity for a very limited duration — No food restriction at the site limits for the second harvest following a severe accident. 

There is the prospect that loss of coolant and hence core meltdown can be made incredible and that eventually designs will be released from these serious limitations. However, in the current work, the aim has been to determine how seriously the design is handicapped even if you allow for the unbelievable. Public acceptability of the first large, fast reactors may well require designs that meet the consequences of the meltdown accident. In this context, then, we must strive to understand the reactivity implications of voiding in sodium-cooled reactors. The first reactor of this class will establish failure statistics of control drives, demonstrate independence of different sources of failure, and thereby permit subsequent assertion that failure to scram is impossible. 

The type of accidents in which the Doppler effect plays a crucial role are those resulting from very high rates of reactivity increase, say l/sec or greater. Normal safety control systems are adequate to cope with accidents resulting from lesser rates of insertion. In the safety studies for the earlier small alloy fuel reactors, the main type of accident leading to the reactivity increase was core meltdown (8), which was in turn considered to result from either coolant failure or some less severe type of reactivity transient. In many of the large breeder reactor concepts of the future, the coolant has a positive void coefficient of reactivity (77, 12a) which could conceivably lead to an autocatalytic expulsion of the coolant and consequently high rates of reactivity increase. 

Some attempts were made to obtain more detailed information on the chemical conditions prevailing inside the reactor pressure vessel during the core meltdown phase, in particular on the redox conditions. Post-accident examinations showed that the oxide layers on the steel structural surfaces consisted exclusively of Fe304, with neither FeO nor FeaOs being detected. From these and other results it was concluded that over long periods of the accident sequence the ratio H2 H2O had been somewhat below 1.0. 

Probabilistic safety analysis has shown that the ultimate beyond design basis accident in the ELENA NTEP does not lead to core meltdown. Radioactivity release in the event of a failure of fuel element claddings and a loss of the primary circuit integrity during routine operation has a very low frequency of occurrence (lO ") personnel and the population in the territory near the plant would not be exposed to hazards in excess of the existing standards. 

The inherent safety features and passive operation capability of the SPINNOR and VSPESINOR are targeted to eliminate core meltdown and, therefore, to avoid adverse environmental impacts in accidents. Because their conversion ratio is about one, the fuel self-sustainable regime may be established, in which only fertile fuel material, e.g. depleted uranium, will be consumed to produce energy. If higher breeding ratios become necessary, they could be achieved just by placing an external blanket in the reflector position. 

The evaluated core meltdown probability for the VBER-300 is low. Nevertheless, in accordance with the regulations and considering the design experience of similar domestic and foreign new-generation reactors, the problems of safety in postulated severe accidents were considered in the VBER-300 design. 

Since the KAMADO is designed to have a negligible possibility of core meltdown, a potential radiation exposure in accidents could be essentially reduced or eliminated. 

Though the detailed safety and accident analyses have not been performed yet, the preliminary evaluations indicate that the KAMADO might be designed to essentially exclude a core meltdown accident. 

Maschek, W., Munz, C.D., Meyer, L., 1992. Investigation of sloshing motions in pools related to recriticalities in liquid-metal fast breeder reactor core meltdown accidents. Nuclear Technology 98, 27. 

---