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. 

This concept means that the risk of core meltdown in case the heat from fission processes cannot be led away must be absent. Two examples of proposed inherently safe reactor designs are ... 

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) ... 

Background. Natural convection driven by internal heat sources is of interest in geophysics, and the heat transfer associated with such motion is important in the design of tanks in which fermentation or other chemical reactions occur and in the safety analysis of nuclear reactors where a core meltdown is postulated. The last of these applications has led to the intensive study of internally generating horizontal fluid layers. 

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],... 

The main threat to a reactor would involve the cooling aq>acity becoming insufficient because of a sudden power excursion of the reactor, a blockage of the cooling circuit, or a loss of coolant. In either case the core could become overiieated and a core meltdown might begin. 

Nuclear core meltdown would cause loss of power, and possibly radiation exposure. 

Nuclear reactor core meltdown would cause loss of power, and possibly radiation exposure. Surface operations must abort mission and evacuate. If abort is unsuccessful or unavailable at the time, the crew could be lost All surface equipment is lost No environmental Impact on Earth. 

Incident. While increasing power, erratic neutron monitoring readings eventually led to reactor Scram . There was a partial core meltdown due to intermittent blocking of reactor coolant flow by a loose baffle in the base of the vessel. Some radiation released. 

Result. Core meltdown and considerable release of radiation to containment. A small amount was released to the environment. Reactor written off—operation of TMI-1 suspended. Clean-up and investigations have cost billions of dollars. Since then, there has been extensive reappraisal of safety requirements and no further ordering of nuclear plant by utilities in the USA. Uncertainty of situation and media reaction led to large public concern. 

The sample is small, but in reviewing these cases, and other problems that have led to extensive plant damage (but no core meltdown), a number of observations can be made ... 

In the Three Mile Island core meltdown, it is known that there was also a Zircaloy-water chemical reaction leading to a considerable release of hydrogen. However, the reactor circuit pressure relief and containment system there prevented damage with no significant fission product release to the atmosphere. There was no free oxygen in the vessel so that hydrogen did not react chemically. 

Step 1 obtain a short term core meltdown risk level during mid-loop cold shutdown operation which is comparable to the one existing with a water level up to the vessel mating surface mate ... 

Step 2 check that the overall risk of core meltdown, taking into account all the initiators and the long term sequences, is acceptable (relooping). 

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. 

In a core meltdown of a nuclear reactor, the failure of cooling would allow temperatures to rise above the melting point of the metal rods containing the uranium fuel (about 1205°C). In the worst scenario, the resulting mass of highly radioactive molten metal would melt through the steel and concrete of the containment vessel beneath it. 

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. 

NEW APPROACH TO CORE MELTDOWN CORE RESCUE SYSTEM - FROM ITALY... 

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. 

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