As promised, I have done some more research for you regarding nuclear reactors. Here is a section out of one of my old textbooks regarding the control rods I mentioned. (I had to blow the dust off the book first).
Reactor control is achieved by carefully balancing the neutron production rate with the neutron loss rate, most commonly by adjusting the amount of neutron absorber, or control, in the core. Control materials--such as silver, indium, and cadmium, which are all highly absorbant to neutrons--are placed in rods with the same dimensions as the fuel rods; the set of control rods is inserted in the middle of the fuel assembly. The control rods are attached to a drive mechanism that moves them in and out of the core region. When reactor shutdown is desired, or when unexpected conditions are detected, such as a power loss, the rods are inserted automatically into the core.
Here is a section on nuclear safety:
As the reactor operates, a large inventory of radioactive isotopes accumulates. A fundamental objective of nuclear reactor design is to prevent accidents that could allow the escape of the radioactivity. In order for fission products to reach the environment, several barriers must be overcome. For an LWR, the barriers are the fuel cladding, which is capable of withstanding pressures and temperatures that are well beyond normal conditions; the pressure vessel, which is exceedingly strong, but does have numerous penetrations for the cooling water to enter and exit; and the containment building, designed to withstand substantial pressure.
In order for any barrier to be breached, the system must first become overheated. There are two possible ways for this to occur. The fission rate may grow too rapidly for the coolant to remove all of the energy being created, or the coolant system may fail. Excessive fission energy production is monitored by numerous sensors throughout the core region; if they detect a rapid rate of growth in the fission process, the control rods are automatically lowered into the core to absorb the fission products. The reactor shuts down.
The greatest threat to reactor safety is the loss-of-coolant accident, or LOCA. The fission process itself ceases if a reactor loses its cooling water because the reactor goes subcritical. However, the fuel continues to heat up due to stored thermal energy as well as from the decay heat of radioactive fission products. Without any coolant the cladding heats up and ultimately melts. Safety systems prevent the clad from overheating by providing emergency cooling water. Such systems are collectively known as emergency core cooling systems, or ECCSs. All such systems have multiple pathways for introducing water into the vessel under both high-pressure and low-pressure conditions.
The design of safety systems begins by hypothesizing a number of different failures and then developing systems to mitigate the consequences of these failures. Such failures are known as design basis accidents, and in order to obtain a license, a plant must show that it is protected against them. The broad areas of concern include accidents within the plant as well as accidents involving the handling of radioactive spent fuel. Initiating events include hardware failures, operator failures, and external events such as tornadoes.
I also did some further research on the Three Mile Island accident and Chernobyl since you mentioned them in your first post:
With regard to Three Mile Island, the accident began when a value stuck open, allowing a small amount of coolant to escape the vessel. The ECCS (emergency core cooling system) operated as designed and provided makeup water for the core. Unfortunately, the operators misinterpreted the information and shut off the ESSC for several hours. This caused a sizable amount of gaseous fission products to escape through the open value into the containment building which prevented its release. A small amount was also released to the environment, but these were almost entirely noble gases which are chemically inert and not retained within the human body.
The health effects of this accident proved to be virtually undetectable against the normal incidence of the background radiation.
Bibliography: American Nuclear Society, The Safety of Next-Generation Power Reactors (1988); Bennet, D. J., and Thomson, J. R., The Elements of Nuclear Power, 3d ed. (1990); Johnson, J. W., Insuring against Disaster (1986); Knief, Ronald N., Nuclear Energy Technology (1988); Marples, David R., Chernobyl and Nuclear Power in the USSR (1986).
I can also post information about the cause and effect of the Chernobyl accident. Please indicate whether you would like me to do this as well.
Thanks for your question. It was fun getting out the old books and doing some research. Hopefully the data will give you more comfort regarding the safety of reactors and Y2K.
Jim |
