In the thermohydraulic design of the HTTR (see Fig. 4.26), the fuel temperature is calculated based on the power distribution from the nuclear design, the primary coolant flow rate, the engineering hot spot factors and the geometry of the fuel block.

The core components such as the fuel blocks, replaceable reflector blocks and control rod guide blocks have different heat generation. The fuel blocks also have different powers according to their uranium enrichment and loading position. Thus, the adequate core coolant flow rate which directly contributes to cooling the fuel is ensured. The coolant is distributed to each fuel block to keep the maximum fuel temperature as low as possible during normal operation.

[1] Thermohydraulic design codes

The thermohydraulic design consists of the flow distribution calculation and the resulting fuel temperature calculation (see Fig. 4.26). Since the HTTR core is formed by piling up the hexagonal graphite blocks, it is necessary in the fuel temperature calculation to consider not only the coolant flow in the cooling hole which directly contributes to cooling the fuel but also the coolant flow not directly contributing to fuel cooling such as the horizontal cross flow between the piled blocks, the gap flow between the columns, etc.

(1) Flow distribution calculation

The FLOWNET code [47] based on the flow network model is used for the flow distribution calculation. The coolant flow paths in the core are modeled as channels having equivalent length, area and hydraulic diameter. The flow channels are connected by equivalent paths of thermal conduc­tivity. The channel data and heat transfer coefficient, etc. which have been measured beforehand by the hydraulic tests with the same scale as the actual core are conservatively corrected and adopted to the calculation.

The 1/6 core is modeled in the flow distribution calculation by consid­ering the symmetry of the core [28]. The radial and axial flow network models are shown in Fig. 4.37. A column is modeled as a single flow channel. The flow channels including the gap flow paths are connected by horizontal flow paths and thermal conduction. The graphite blocks shrink due to neutron irradiation, which is taken into account for the gap size.

(2) Fuel temperature calculation

The fuel temperature is calculated based on the power distribution from the nuclear design and the coolant flow distribution from the flow distribution calculation considering the thermal conductivity of the fuel blocks and the engineering hot spot factors [48, 49]. The fuel temperature analysis code TEMDIM [50] is used. The core is represented by multi cylindrical chan­nels, and the 2D temperature distribution and thermal deformation of each channel are calculated. Finally, the maximum fuel temperature is obtained by considering the engineering hot spot factors.

The calculation model of fuel rod is a cylindrical model which consists of fuel compacts, the gap between the fuel compact and the graphite sleeve,

Fig. 4.37 Calculation model for coolant flow distribution in core

Graphite block

Graphite sleeve

Radiation heat transfer

Fuel compact

Fig. 4.38 Fuel temperature calculation model

the graphite sleeve, the coolant flow paths and the graphite block as shown in Fig. 4.38. The fuel rod is divided by radial and axial meshes. The power distribution from the nuclear design is corrected by considering the local power peaking and used for the fuel temperature calculation.

The nominal maximum fuel temperature is calculated based on the power distribution from the nuclear design and the coolant flow distribu­tion. Based on this nominal temperature, the systematic maximum fuel temperature is evaluated using the engineering hot spot factors i. e. the random factors and the systematic factors [51]. The thermohydraulic design
must be made so that the systematic maximum fuel temperature does not exceed the allowable design limit (1,495 °C) for normal operation as introduced in the list [4] of Sect. 4.2.2.

The fuel temperature at any position is obtained by adding the temper­ature rises of each component to the inlet coolant temperature. Thus, the nominal temperature TN of an arbitrary position is calculated as:

5

(4.27)

i = l

where

Tin : Core inlet coolant temperature (° c)

ATiN : Norminal temperature rise (°C)

1 = 1: Coolant

2 : Flim

3 : Sleeve

4 : Gap

5 : Fuel compact

The evaluated maximum fuel temperature is the maximum value of the systematic fuel temperature. The systematic fuel temperature is obtained based on the nominal temperature rises ATiN, at an arbitrary burnup step and arbitrary region of interest (entire core or each column) considering the engineering hot spot factors, as shown in Eq. (4.28).

(4.28)

where

Tf : Systematic fuel temprature (° C)

ATi: Temperature rise considering engineering hot spot factors (° C) fs : Systematic factors associated with fuel rod n : Number of systematic factors fR : Random factors associated with fuel rod m : Number of random factor

The maximum fuel temperature at an arbitrary burnup step and arbitrary region of interest is evaluated by Eq. (4.29) based on the systematic temperature:

Tmax = Max {Tf)

where

Tmax: Maximum fuel temperature (° C)