Nuclear reactors fuel loss

In March 1979 one of the nuclear reactor plants at the Three Mile Island (TMI) site (in Pennsylvania) had an equipment failure that led to a loss of cooling water from the reactor core. Operator errors compounded this mechanical malfunction, resulting in the destruction of most of the reactor core and melting some of the fuel. In spite of the extensive damage, the nuclear reaction was stopped and very little radiation was released to the environment. In fact, no person off site received more than a few millirem (equivalent to a few days background exposure) and no person on site exceeded their annual dose limit of 5 rem. This was not known at first, however, and the governor of Pennsylvania ordered the evacuation of many people from the area. 

Cladding—material that covers the fuel elements in a nuclear reactor in order to prevent the loss of heat and radioactive materials from the fuel. Containment— Any system developed for preventing the release of radioactive materials from a nuclear power plant to the outside world. 

Rewetting of hot surface. In this case, the liquid phase contacts a hot surface and rewets it, with the accompanying formation of vapor. Examples here are the quenching of hot metal objects in metal forming processes and the rewetting of hot fuel elements in a nuclear reactor following a loss-of-coolant accident. 

As laid out in the Equation 21.1 definition of /c-effective, the basic neutron balance relationship is between fission neutron production and neutron loss mechanisms, the two neutron loss mechanisms being neutron absorption and neutron leakage. Nuclear reactors, for which criticality must be created and maintained for extended periods, tend to be large devices—both for neutron balance reasons and because of the large fuel inventory required to deliver substantial amounts of power for long periods of time. The result of this, from the point of view of the A -effective equation, is that neutron leakage is a relatively unimportant neutron... 

All three designs (BWR, PWR, and RBMK) rely on control rods to provide a safe shutdown of the nuclear reactor during emergency situations. Both the PWR and RBMK use control rods inserted from the top of the reactor, which also allows for gravity to help the lowering of the rods in the event of a power loss. This is not possible in BWR reactors, however, because the top areas of the reactor vessels are filled with steam separation equipment. Instead, BWRs use cruciform-shaped control rods energized by hydraulic pressure, which are pushed between the fuel assemblies and up into the reactor from the bottom (Fig. 1.6). 

In March 2011, a once-in-a-thousand-years offshore earthquake struck northern Japan. The earthquake was followed by a massive tsunami. The two events caused immense loss of life and damage to the nation s infrastructure. The earthquake and tstmami also caused serious damage to at least two of the nuclear reactors at the Fukushima Dai-ichi complex and one of the pools used for storing spent fuel rods. 

The length of time that fuel can be used in a reactor before it must be discharged depends on the characteristics of the reactor, the initial composition of the fuel, the neutron flux to which it is exposed, and the way in which fuel is managed in the reactor, as described in more detail in Chap. 3. Factors that eventually require fuel to be discharged include deterioration of cladding as a result of fuel swelling, thermal stresses or corrosion, and loss of nuclear reactivity... 

One of the attractive features of the fast reactor is its hard neutron spectrum. To expand this feature, a metallic fuel core is employed in the 4S. However, it is more difficult to reduce void reactivity for a core with a harder spectrum. It is very important to design the void reactivity to be negative in order to prevent a severe nuclear accident in the event of sudden loss of coolant, sudden loss of coolant flow or a large gas bubble entrainment in the core. 

In most reactors the amount of plutonium formed will not fully compensate for the loss of U235, and the ability of the fuel to sustain the nuclear reactions (its reactivity ) falls until it is necessary to replace the fuel to keep the plant producing adequate levels of power. 

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