The SI units are a coherent and consistent set of units that can be used in calculations without the need for conversion factors. For the present purpose two classes of SI units may be distinguished: base units and derived units.

There are seven dimensionally independent base units, but only the following five are used in this book:

Base Units Quantity Unit Symbol Length meter (or metre) m Mass kilogram kg Time second s Thermodynamic temperature kelvin* К Amount of substance molet mol *For convenience, but not for calculations, temperatures may be expressed on the Celsius (formerly called the centigrade) scale, where tQC is equal to t + 273.15 K. The unit degree Celsius is equal to the unit kelvin. tThe mole is the amount of substance in a system that contains as many elementary entities (atoms or molecules) as there are atoms in 0.012 kg of carbon-12, i. e., the Avogadro number of entities.

periments, 3.157

[1]Although Ec should include the decay energies of all radioactive species formed as a result of nonfission neutron reactions, only those of relatively short half-life contribute to the energy release in the state of pseudo-equilibrium attained by the reactor soon after startup.

[2]Commercial power reactors have radial fuel regions with different enrichments in uranium — 235; as a result, the power distribution differs from that for uniform enrichment assumed here. Also, the axial power density is likely to be skewed for reload core loadings as a result of previous operational history (see Chapter 10).

[3]The quantity q/A, having the dimensions of heat/(time)(area), i. e., W/m2, is called the heat flux.

[4]For values of bla < 2, the quantity In (b/a), which appears here and in heat-transfer equations for clad fuel rods (§9.47 et seq.), is well approximated by 2(b — a)/(b + a).

[5] Certain fluids, e. g., suspensions, which do not obey the Newton equation, are referred to as non-Newtonian fluids.

[6]The saturation temperature is the temperature of the saturated vapor, i. e., saturated steam, at the existing pressure.

[7]The designation “W-3” arises from the fact that it is the third such correlation developed by the Westinghouse organization.

[8]If English units are used in equation (9.39) or in subsequent equations, gc, the factor relating force and mass, must be used as appropriate.

Tn the equivalent Darcy-Weisbach equation a friction factor equal to 4/is used.

[9] Since the velocity head is equal to the kinetic energy of a unit mass of flowing fluid, the term arises from the similarity with the potential energy of a height (or “head”) of a unit fluid mass.

[10]

[11]For further information, standard reference books and manufacturers’ catalogs should be consulted. The loss coefficients Kx are often expressed in terms of an “equivalent” length of straight pipe.

[12]In SI units, R = 8.314 J/K • mol, and the molecular weight (mass of one mole) is in kilograms (§1.12). Thus, the velocity of sound in helium at 1000 К is

[13]For a fuel rod of given dimensions and fuel composition, the heat flux, i. e., the heat flow per unit area q/A, is proportional to the local heat-generation rate (or heat source) per unit volume Q the latter is equivalent to the local power density which is determined by 2/ф [cf. equation (2.54)].

+Other parameters, e. g., fuel cladding temperatures, become limiting under emergency conditions (Chapter 12).

[14]This postulate implies that the following analysis gives a good approximation provided dtldx « dtldn, where n is a coordinate normal to x.

[15] Since the average core outlet temperature in a PWR is below the saturation temperature, little or no vapor leaves the reactor vessel. However, there is substantial subcooled (local) boiling in the flow core to the heated surface, and bulk boiling may occur in the “hot channels.”

[16]It can be readily shown that the heat flux at the outer surface of a clad fuel rod is Qa2l 2b, where Q is the average rate of heat generation (by fission) per unit volume of fuel, a is the radius of the fuel pellet, and b is the outer radius of the clad rod. These three quantities are affected by the parameters in Table 9.2.

Statistical subfactors Based on ± Зет

Rod pitch and bowing 1.045

Clad rod diameter 1.042

Statistical combination 1.062

Nonstatistical subfactors

Inlet flow maldistribution 1.03

Internal mixing, and boiling flow redistribution and diversion 1.10

Flow mixing 0.90

[17]The plant capacity factor is the ratio of the actual electrical energy (in kW • h) generated in a given period (usually one year) to the amount that could be produced if the plant operated continuously at its rated power during the whole period.

[18]A burnup of 2.85 TJ/kg U is equivalent to 33,000 megawatt-days/metric ton U (see Example 1.2).

[19]Another measure of relative biological hazard is the water (or air) dilution volume, sometimes called the toxicity, defined as the volume of water (or air) required to dilute a given amount of radioisotope to a concentration for drinking (or continuous exposure). However, this measure does not take into account solubility or ingestion processes.

[20]The NRC recommends that, unless a guaranteed and reliable source of water is available, e. g., an ocean, large river, or large lake, the ultimate heat sink should consist of two separate sources, such as two independent cooling towers or spray ponds, or a tower and a spray pond, or one of these together with some other water source.

[21]This tank of borated water is normally used for temporary storage of spent fuel elements during refueling operations.

[22]Some severe accident scenarios lead to containment failure (§12.99). However, contain­ment designs for future reactors are likely to include severe accident mitigation features.

[23]In a cold-leg break, the desired flow of injected water down the downcomer and up through the core would be resisted by the blowdown flow in the opposite direction. If the resistance is large, ECC water bypass could occur, in which the injected fluid flows around the downcomer annulus and out through the pipe break.

[24] According to the data in Table 6.1, the mass energy-absorption coefficient of gamma rays in soft body-tissue is roughly 1.1 times that in air over a wide range of energies.

[25]The scale developed by the International Atomic Energy Agency provides for seven levels, with each step representing a factor of 10 in severity. Thus a level 3 incident would be roughly 10,000 times less serious than the level 7 Chernobyl accident.

[26]The term fault as used here refers to a mechanical failure or malfunction but also includes any type of abnormal situation, such as deviation from operational procedures.

[27]HVAC = heating, ventilation, and air conditioning.

[28]For further information, see “The International System of Units (SI),” National Bureau of Standards Special Publication 330; “Standard for Metric Practice,” American Society for Testing and Materials.