12.22. Nuclear power reactors are designed to produce heat to satisfy the demand for steam by a turbine-generator, up to a specified limit. The reactor control system, with its automatic and manual controls, serves to maintain safe operating conditions as the demand is varied (§5.185 et seq.). Because excess cooling capability is provided in the design of the reactor system, an overpower equal to about 118 percent (in a PWR) or 120 percent (in a В WR) of the rated (or design) power can be tolerated without causing damage to the fuel rods. If the thermal power should exceed the limiting value or if other abnormal conditions which might endanger the system should arise, the reactor protection system would cause reactor trip (or “scram”), as described in Chapter 5.

12.23. In reactor operations, the term transient describes, in general, any significant deviation from the normal value of one or more of the important operating parameters, e. g., system temperatures and pressures, thermal power level, coolant flow rate, turbine trip, equipment failure, etc. If the transient is a minor one, within the permissible operating limits of the system, the controls will be adjusted automatically to compensate for the deviation. A severe transient, however, will activate the reactor protection system.

12.24. The purpose of the protection system is to shut the reactor down and maintain it in a safe condition in the event of a system transient or malfunction that might cause damage to the core, most likely from over­heating. The protection system includes a wide variety of instruments for measuring operating variables and other characteristics of the overall nu­clear plant system. If the instruments indicate a transient that cannot be corrected immediately by the control system, the reactor is shut down automatically by the protection system. In addition, the reactor operator can cause an independent (manual) trip if there are indications that an unsafe condition may be developing.

12.25. When a reactor trip signal is received in a PWR, the electro­magnetic clutches holding up the control rods are deenergized by an au­tomatic cutoff of electric power. The rods then drop into the reactor core. Borated water (boric acid solution) can be injected from the chemical and volume control system or CVCS (§11.99) by manual action to provide a backup to the control rods if required. In a BWR, a rapid shutdown is achieved by forcing the control rods up into the core by hydrostatic pres­sure; at the same time, power to the recirculation pumps is cut off. The reactivity in a BWR can also be decreased by injection of an aqueous solution of sodium pentaborate.

12.26. An essential requirement of the reactor protection system is that it must not fail when needed; on the other hand, an error in the instru­mentation or other malfunction with the system should not cause an un­necessary (“false”) reactor trip. In order to avoid such false trips, three or more redundant channels, consisting of detector and actuator, are used to monitor operating variables. A reactor trip will occur only when two or more channels call for action simultaneously. The availability of several independent channels permits regular testings of the channels, one at a time, without impairing the effectiveness of the protection system.

12.122. An aerosol is a gaseous suspension of fine liquid or solid par­ticles. Spherical diameters may vary from 1 mm to as small as 1 nm, but the range of interest is normally from 0.01 to 100 |xm. Degraded core materials making their way into the containment as a result of a severe accident tend to form aerosols by several processes. For example, the volatile fission products, such as cesium, iodine, and tellurium, are likely to condense or chemically react to form lower-volatility compounds and then condense. Nonvolatile fission products and other materials also form solid aerosols or combine with the steam present to form liquid aerosols. Although solid aerosols can have various shapes, a spherical shape is first assumed in analysis and correlation factors applied.

12.123. The amount of fission products available for release from the containment depends greatly on aerosol formation and deposition pro­cesses that occur following a degraded core accident. However, the picture is complicated. The suspending gas is likely to be in turbulent motion while the particles change size and may deposit by several processes.

12.124. Deposition by sedimentation as a result of the action of gravity occurs in accordance with the Stokes law principle described in §12.121. However, both the particle density and diameter tend to change during
the course of an accident affecting the settling velocity. Particles having a diameter of about 1 |xm have a settling velocity of about 0.1 mm/s. A measure of fission product removal is the decontamination factor, defined as the ratio of activity before to that after a given process. In a PWR containment vessel having a volume of 70,000 m3, it would take 17 days to achieve a decontamination factor of 100 for such particles [5].

12.125. Modeling of the deposition of aerosols has been carried out by the TRAP-MELT code [9]. In addition to sedimentation, other deposition processes include thermophoresis, turbulent deposition, and diffusion. Thermophoresis is the result of a force in a nonisothermal gas arising from unequal molecular impacts on the particle. Turbulent deposition is a wall effect resulting from the particles not being able to follow the fluid eddy flow adjacent to the wall. Diffusion become significant for particles having a diameter of less than 0.1 |xm. It is the result of bombardment by the gas molecules, which results in a diffusive flux when the particle concentration varies in space. Aerosol behavior is the subject of continuing modeling development supported by experimental research.