Pressurized water reactor nuclear power plant safety and accident countermeasures

Author: Pu Jilong
Publisher:
Publish Date: 1995-06-01
Features: This book provides a comprehensive and systematic overview of the overall safety of pressurized water reactor (PWR) nuclear power plants, reflecting the new understanding and awareness of nuclear safety in the international nuclear energy community in the late 1980s, as well as summarizing some achievements and insights in nuclear safety research in China. The book is divided into ten chapters. After briefly introducing the basic knowledge of nuclear power and nuclear safety, as well as the systems of PWR nuclear power plants, it delves into deterministic accident analysis methods, probabilistic safety assessment techniques, severe accident processes, severe accident analysis methods and main results, accident management, operational safety and operational safety management, and design improvements of PWRs. It thoroughly introduces the principles, theories, methods, and results of nuclear safety analysis and management. The book is comprehensive in content, up-to-date in information, and well-argued with fluent language, making it a valuable reference for engineering technicians engaged in the design, operation, research, and management of nuclear power plants, as well as for faculty and students of relevant universities and colleges. Most chapters of the book can also serve as an excellent intermediate-level popular science reading material for nuclear safety for administrative personnel at all levels involved in nuclear power management and readers interested in nuclear safety.
Excerpt: During the normal operation of a nuclear power plant, power changes are relatively slow, and the temperature change of the moderator can be synchronized with the fuel temperature. For the convenience of operators, various factors affecting reactivity can be integrated, and the rate of reactivity change with power is defined as the reactivity-power coefficient. To ensure stable operation, the reactivity-power coefficient is always negative. The reactivity change caused by a reactor transitioning from one power level to another is called power loss. In addition to local instability, PWRs may also experience hydraulic instability due to the formation of voids in coolant channels, sometimes referred to as channel instability. Under two-phase flow conditions, if there is a slight disturbance in the coolant flow rate and steam content in a channel, the change in two-phase pressure drop will cause a sign-reversed equal change in single-phase pressure drop, as the total pressure drop of the channel remains constant. The thermal equilibrium within the channel will exert a feedback effect on the coolant flow rate and steam content, either damping or amplifying the initial disturbance. Appropriate throttling measures can be added at the channel inlet to prevent the amplification of disturbances and avoid channel hydraulic instability.
2.2.2.6 Reactivity Balance and Reactivity Control
Under all operating conditions, sufficient reactivity must be provided to maintain criticality. During startup, the temperature rise of the reactor is very slow, and the isothermal temperature coefficient (zero-power temperature coefficient) is of primary concern. The zero-power temperature coefficient of PWRs in the range from room temperature to operating temperature is very small, and may even be positive at room temperature, making it inadmissible to start from room temperature. Approximately 1–2% of reactivity is lost when the reactor is heated from room temperature to operating temperature. During operation, neutron-absorbing isotopes such as xenon (Xe) and samarium (Sm) in fission products gradually accumulate, causing significant reactivity loss. After stable operation for several days, this portion of toxicity-induced reactivity loss reaches a saturation value of about 3–5%. In subsequent long-term stable operation, the accumulation of fuel consumption, heavy isotopes, and other fission products (residue) gradually consumes about 11–12% of the reactivity. Therefore, to allow the reactor to operate for a longer period (e.g., one year) and reduce the frequency of refueling, 15–20% of reserve reactivity should be reserved during initial loading. In the early stages of operation, other negative reactivity factors must compensate for the reserve reactivity, which is then gradually released as negative reactivity is withdrawn during operation. There are three methods to control reserve reactivity in PWRs: control rods, boric acid solution, and burnable poisons. Control rods are bundles made of strong absorbers and can move vertically within the reactor to adjust neutron absorption. Due to the potential for neutron flux distortion caused by their movement, control rods are typically kept outside the reactor during operation. The primary reactivity control method in PWRs is boric acid dissolved in the coolant (i.e., moderator). As burnup increases, boron is continuously diluted. The critical boron concentration of a new reactor is approximately 1000 ppm, at which the temperature coefficient may be positive or zero. For newly refueled reactors, to reduce the critical boron concentration, burnable poison absorbers must be added. These are bundles made of materials such as boron carbide, boron steel, or gadolinium oxide (Gd?O?), inserted at appropriate positions within the reactor. Boron or gadolinium becomes a nuclide with weak absorption capacity after absorbing a neutron, similar to the behavior of fuel combustion, hence the term "burnable poison."
2.2.2.7 Xenon Poisoning and Iodine Pit
Iodine-135, a product of fission, is unstable and decays into xenon-135 with a half-life of 6.7 hours, which is also unstable and has a high neutron absorption capacity. If not absorbed by neutrons, xenon-135 decays with a half-life of 9.1 hours. Under normal operation, the toxicity of equilibrium xenon is about 3%. When power changes, the toxicity of xenon varies significantly. After a xenon-free reactor is started, iodine-135 begins to accumulate with fission and decays into xenon-135, causing a noticeable drop in reactivity, which must be compensated by adjusting the boron concentration. Since xenon-135 also disappears due to neutron absorption and decay, the reactor reaches an equilibrium xenon poisoning state after 10 hours of stable operation. After shutdown, iodine production ceases, and the process of xenon disappearance also stops. However, the iodine that has already accumulated continues to decay into xenon. Due to the longer half-life of xenon compared to iodine, the concentration of xenon builds up, reaching a peak of xenon-135 concentration around 10 hours after shutdown, at which point the reactor is in a deep subcritical state. During this period, even if all control rods are fully withdrawn from the core, the reactor cannot be restarted. Over time, the iodine concentration depletes, and no new production occurs, while xenon gradually decays, returning the reactor to a typical shutdown state. This process is called the iodine pit, and the xenon reactivity curve during the iodine pit is shown in Figure 2.7. Forcing the startup of a reactor during the iodine pit is highly dangerous. This may introduce a very large positive reactivity. After startup, the neutron flux rises rapidly, and xenon-135 is quickly "burned," releasing the reactivity it controls very quickly, leaving little time for control rods to compensate. For large-scale reactors operating at high neutron flux, xenon poisoning acts as a slow-response strong negative feedback, potentially causing spatial instability in the neutron flux (i.e., power), a phenomenon known as xenon oscillation. Xenon oscillation can lead to severe distortion in the flux distribution, degrading the operating conditions of fuel elements. The xenon oscillation phenomenon in PWRs is not severe, and control is not difficult.
2.2.3 Reactor Thermal-Hydraulics Basics
The power output of a nuclear power system has no theoretical upper limit in reactor physics, but its limiting factor is thermal. Specifically, the safety of PWRs primarily concerns whether the heat generated by fission can be promptly removed. The fundamental task of reactor thermal-hydraulics design is to safely remove the heat generated within the core and, as much as possible, use this heat to produce high-pressure steam for power generation. In addition to the theoretical upper limit of thermal power, nuclear power plants have the following thermal-hydraulic characteristics:
To improve economic efficiency, the reactor core has a very high power density, approximately ten times that of conventional thermal power plant boilers. During operation, the fuel elements and their cladding in any part of the reactor must not exceed safety limits. The heat generated in the core must be approximately equal to the heat removed, otherwise fuel elements may be damaged. To address accident scenarios, significant thermal margins must be designed into the system. PWRs use pressurized ordinary water as both a moderator and a coolant, operating at high pressure and temperature with high flow rates and relatively small temperature rises across the core.