The coolant temperature and cladding temperature in the fuel assemblies are estimated by subchannel analysis. An example of the subchannel model is illustrated in Fig. 4.10. There are three types of subchannel. The inner subchannel

is surrounded by three fuel elements, the peripheral subchannel is surrounded by two fuel elements and the inner wall of the wrapper tube, and the corner subchannel is surrounded by a fuel element and the wrapper tube corner. The three-dimensional thermal-hydraulic calculation is carried out in the subchannel analysis by modeling the fuel assembly as a set of parallel subchannels.

The coolant temperature distribution and cladding temperature distribution in each fuel assembly are calculated using the inlet flow rate determined by the flow allocation, and the power distribution among the fuel elements as the input conditions. In the subchannel analysis, the mixing of mass, momentum and energy between the neighboring subchannels is taken into account.

In the subchannel analysis, generally, the distributions of the coolant velocity and coolant temperature in the fuel assembly are obtained by solving mass, momentum and energy conservation equations in every axial node.

The design parameters used in the analysis are formulated depending on the analysis models. They are determined by experiments such as assembly flow tests using a simulated fuel assembly with the same dimensions as the actual one and coolant mixing tests. When the coolant temperature distribution in the assembly is obtained, the cladding temperature distribution is calculated according to the power distribution among the fuel elements. Consequently, the heat flux on the cladding surface is given by the power distribution; then the increase in the temperature from the coolant to the cladding surface is calcu­lated using the heat transfer coefficient.

The maximum fuel temperature and maximum cladding temperature are esti­mated considering the engineering safety factors in order to secure enough design margin from the temperature distribution obtained by the subchannel analysis.

In the core thermal-hydraulic design, the nominal (most probable without any error margin) temperature distribution in the fuel assembly is calculated by the subchannel analysis first. Then, the hot spot temperatures of fuel centerline and fuel cladding are obtained considering the uncertainties of the calculated temper­ature caused by the tolerances of the design parameters etc. The hot spot temper­atures are to be confirmed not to exceed the corresponding design limits. The temperature uncertainties are considered by multiplying the nominal tempera­tures by the engineering safety factors which are provided according to the errors of each parameter. The engineering safety factors are divided into two types. One is treated as a multiplication term by considering the systematic errors which must be considered cumulatively. The other is treated as a statistical term coming from random errors. An example of the former error is that of the power distri­bution. An example of the latter error is the fabrication tolerance of the fuel pellet.

Equation (4.14) is an example of the evaluation equation for the maximum temperature which includes consideration of the engineering safety factors.

In this equation, Gij, Fkj are the factors of multiplication treatment and statistical treatment, respectively. ATj is the nominal temperature rise of each position. while m and n are the numbers of the multiplying factor and statistical factors, respectively.

T1HS : Coolant T2HS : Clad outer surface T 3 HS : Clad inner surface T4HS : Pellet surface T 5 HS : Pellet centerline

AT1 = ATNa : From assembly inlet tocoolant AT2 = ATfum : From coolant to clad outer surface AT3 = ATclad : From clad outer suface to inner surface AT4 = ATgap : From clad inner suface topelletsurface AT5 = ATfuel : From pellet surface to pellet center

As described above, the engineering safety factors are for estimating the maximum temperature rise in each position. By considering them, the fabrication tolerances, uncertainties of thermal-hydraulic parameters, power distribution uncertainty, and measurement uncertainty of the reactor thermal power, etc. are taken into account.

Fig. 4.11 epts with low void reactivity

[3] Maximum fuel temperature

In the core thermal-hydraulic design, melting of the fuel pellets should be avoided at normal operation and anticipated operational occurrences. The maximum fuel centerline temperature with consideration of the engineer­ing safety factors is confirmed to satisfy the design limit. As for the evaluation of the maximum fuel centerline temperature, see the list [1](4) of Sect. 4.1.3.