Hot channel

T4. Tong, L. S., Chelmer, H., and McCabe, E. A., Hot channel factors for flow distribution and mixing in core thermal design, WCAP-2211 (1963). 

The monitoring uncertainty and operational transient margin is to ensure that the minimum DNB ratio is calculated at the worst operating condition. The assumed worst operating condition consists of a power surge of 12% in a worst power distribution (power skew at top), accompanied by an inlet coolant temperature elevation of 4°F (2°C) and a pressure swing of 30 psi (0.2 MPa). A set of worst hot channel factors in core life should also be used in evaluation of the worst power distribution. Such an assumed worst operating condition is obviously overly... 

The worst operating condition in a common design practice consists of overly conservative assumptions on the hot-channel input. These assumptions must be realistically evaluated in a subchannel analysis by the help of in-core instrumentation measurements. In the early subchannel analysis codes, the core inlet flow conditions and the axial power distribution were preselected off-line, and the most conservative values were used as inputs to the code calculations. In more recent, improved codes, the operating margin is calculated on-line, and the hot-channel power distributions are calculated by using ex-core neutron detector signals for core control. Thus the state parameters (e.g., core power, core inlet temper-... 

The values of fjz) were taken as being constant for small changes of power level in a given channel at a given location. The values of fjz) were then used to determine the local hot channel conditions for the CHF prediction procedure. 

Figure 5.80 General Electric correlations for CHF. Curves are typical hot-channel mass flux of 1.0 X 106 lb/hr ft2). (From Roy, 1966. Copyright 1966 by Nucleionics Week, Washington, DC. Reprinted with permission.)... 

Adjacent subchannels are open to each other through the gap between two neighboring fuel rods flow in one channel mixes with that in the other. In addition, as observed previously, there is crossflow between channels because of the pressure gradient. Local turbulent mixing reduces the enthalpy rise of the hot channel. On the other hand, flow leaving the hot channel increases its enthalpy rise. Calculation of the net result is complicated, although the equation describing enthalpy rise can easily be written. 

The energy (Qreq) required increasing inlet feed solution temperamre to the hot channel (7hi) was expressed by Alklaibi and Lior [123] as follows ... 

Drop-wall interaction. Hie earlier models ignored the contribution of drop-wall interaction. The direct evaporation of droplets at the hot channel wall can play an important role when the wall temperatures are relatively low just upstream of the dryout point. Evans et al. [344] made measurements of vapor superheat just downstream of the dryout point and found that for approximately one-third of a meter downstream, the vapor remained at its saturation temperature, indicating that, in this region, the heat flux was being absorbed by... 

The plastic industry, like other sectors, also seeks to minimize waste production. For this, rationalization systems have been developed (hot channels removing carrots and channels supplying materials for injection, software preventing the wrong parts being made especially at startup, etc.) or systems for immediate recovery and immediate recychng after grinding in front of the press. 

Pressure oscillations as high-density liquid impinges upon hot channel walls were also recognized as a potential problem, not only in the two-phase flow regime, but also at pressures and temperatures higher than critical, since there is no assurance that phase equilibrium w ill always prevail throughout the flow channels. 

Fig. 3.12. Comparison of ORNL (lines) and INEEL (points) calculations of hot channel temperatures for 600 MW helium-cooled NGNP design.

The subassemblies are designed to reach a peak temperature rise of 300 X 1.177 = 353°F. The hot-channel factor of 1.29 results in a maximum temperature rise of 456°F. The average core temperature rise is 319°F. 

This accident was analyzed using shutdown coolant flows of 10 and 15% of full flow and no mechanical shutdown provisions. In the former case, core power began to level out near 850 MW, but the maximum coolant temperature in the hot channel exceeded the IbOO F criteria in 4.8 sec. With 15% full flow, core power approaches 1170 MW, with the main coolant temperature reaching I600°F in approximately 10 sec. Fuel temperatures in both cases were well below the design maximums. 

Measurement of hot channel factors (as allowed by facility design and operational limits and conditions) and effects of control rod positions on nuclear instrument indications. 

The RELAP5-3D MASLWR model was used to calculate the sequence of events for an ADS line break and a steam vent line break. The break cases serve to bound transients that involve inadvertent opening of the ADS or steam vent valves. Fig. I-8(a) shows the core hot channel collapsed liquid level and fuel cladding surface temperature at the core hot location for the ADS line break scenario. The results show that core collapsed liquid level is sufficient to provide cooling to the fuel and that neither the fuel nor its cladding experience a thermal excursion. 

The design criteria are to have no coolant boiling and no fuel melting and to ensure that temperature does nor exceed 650°C for the primary boundary structure. Temperatures are evaluated for the nominal hottest pin, which is assumed to have a nominal hot channel factor of 1.53 without the engineered safety factor. The outlet coolant temperature is 593°C in normal operation. 

Hot channel peak cladding outer surface temperature, °C 568/567 520/516... 

Hot channel peak cladding inner surface temperature, °C 579/579 531/527... 

In a channel with external heating (Figure 1.8a) we have the simplest flow model. The melt in the channel flows in laminar fashion at a very low flow rate next to the channel wall, and at the maximum rate in the centre of the channel. The differentiation in flow rates causes shear stresses in the melt. The greatest increase in flow rate, i.e., the greatest shear velocity, is attained by the melt near to the channel wall. However, there are relatively low shear stresses along the hot channel wall - lower than with flow along a cold wall -with a uniform flow in the channel centre. 

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