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Natural Convection in Nuclear Power Plants
Published in Robert E. Masterson, Nuclear Reactor Thermal Hydraulics, 2019
The normal flow of coolant is illustrated by the arrows in this figure. Now suppose that the power to the plant is lost at t = 0. At this time, all off-site power is assumed to be unavailable. In other words, the plant is no longer assumed to be connected to the power grid. The timeline for the events that occur next is illustrated in Figure 22.14. Within one minute, all modern reactors (even primitive ones) are designed to use the standby diesel generators that are present on the site to make up for this lost power. Thus, most of the time, the diesel generators automatically come online and restore the lost electrical power (so that the coolant pumps can continue to function). However, if the diesel generators do not start immediately (or if they are destroyed by an unanticipated event such as an earthquake or a tsunami), then the coolant pumps that feed the cooling water to the core and the steam generators cannot function. The control system senses this and immediately inserts the control rods into the core, causing the reactor to scram. The scram terminates the fission process and shuts the reactor down. Unfortunately, as we discussed in Chapter 5, the reactor core continues to produce a large amount of decay heat, and this decay heat must be removed by allowing the coolant to flow over the fuel rods. If there is no electrical power available to run the coolant pumps, the water begins to stagnate in the core, and the core flow goes from a state of forced convection to a state of natural circulation. (The water also begins to heat up because there is no way to remove the decay heat.)
Descriptive Statistics
Published in William M. Mendenhall, Terry L. Sincich, Statistics for Engineering and the Sciences, 2016
William M. Mendenhall, Terry L. Sincich
Unplanned nuclear scrams. Scram is the term used by nuclear engineers to describe a rapid emergency shutdown of a nuclear reactor. The nuclear industry has made a concerted effort to significantly reduce the number of unplanned scrams. The accompanying table gives the number of scrams at each of 56 U.S. nuclear reactor units in a recent year. Would you expect to observe a nuclear reactor in the future with 11 unplanned scrams? Explain.
Maintenance can Seriously Damage your System
Published in James Reason, Managing the Risks of Organizational Accidents, 2016
Two separate maintenance failures were implicated in the Three Mile Island nuclear power plant accident that occurred on 28 March 1979.3 One initiated the emergency; the other delayed its recovery. The first failure occurred when a maintenance crew was attempting to renew the resin for the treatment of the plant’s water. A small volume of water found its way into the plant’s instrument air system, tripping the feedwater pumps. This, in turn, cut the water flow to the steam generator, and tripped the turbine, preventing the heat of the primary cooling system from being transferred to the cooler water in the secondary system. At this point, the emergency feedwater pumps came on automatically. However, the pipes from the emergency feedwater storage tanks were blocked by closed valves, erroneously left shut during maintenance two days earlier. With no heat removal, there was a rapid rise in core temperature and pressure which caused the reactor to ‘scram’—the control rods were automatically lowered into the core, absorbing neutrons and stopping a chain reaction. However, because decaying radioactive materials still produce heat, the temperature and pressure within the core increased further, causing a pilot-operated relief valve to open. This was supposed to flip open and then close but it remained stuck in the open position. A hole was thus created in the primary cooling system through which radioactive water, under high pressure, poured into the containment area and then down into the basement. It took a further 16 hours to restore the plant to a safe state.
Basic consideration of a nuclear power monitoring system using neutron-induced prompt gamma rays
Published in Journal of Nuclear Science and Technology, 2020
Koichi Okada, Atsushi Fushimi, Shun Sekimoto, Tsutomu Ohtsuki
We are studying a new nuclear power monitoring system using ex-core detectors for boiling water reactors (BWRs). Nuclear power is monitored to ensure the proper operation of the nuclear reactor from its startup to its shutdown. One of the monitored items is the neutron flux in the reactor core. When reactor power becomes excessive, the neutron monitoring system emits a signal to scram or to block the control rod to be withdrawn in order to prevent damage to the fuel cladding. For rated power monitoring in BWRs, local power range monitors (LPRMs) are used. Typically, 52 LPRM assemblies are installed in an advanced BWR core. Four LPRM detectors are installed as a single LPRM assembly and they measure the local power at each position in the LPRM assembly.