Coolant enthalpy

Since a phase change can occur, the coolant enthalpy is used instead of the temperature. The coolant energy balance becomes... 

Coolant enthalpy at the core inlet. This quantity depends on feed-water temperature. Its influence is Important, although considerably slower than the pressure effect Just mentioned. 

The coolant enthalpy inside the primary coolant loops and the RPV in the CV is substantially smaller than that of LWRs. This makes the CV more compact and lower in height. The cOTistruction period will be shortened due to the decrease in the number of reactor building floors. 

To evaluate the cladding temperatures directly during abnormal transients, it was necessary to develop a database of heat transfer coefficients at various conditions of heat flux, flow rate, and coolant enthalpy. The database of heat transfer coefficients was prepared by numerical simulations that successfully analyzed the deterioration phenomenon itself. The database, Oka-Koshizuka correlation, has been used for safety analysis. 

MCST [12,13]. For high temperature reactors, the coolant enthalpy rise in the core is high and coolant flow rate is inevitably low. The gap between fuel rods is kept small to increase the coolant velocity in the core. 

LWRs (about 1/8 of that of a BWR with the same thermal output). The coolant enthalpy rise in the core is large and the coolant temperature and density changes are large. For inlet coolant temperature of 280°C and density of 0.8 g/cm, the average outlet coolant temperature is 500°C and density is less than 0.1 g/cm. Hence, fuel assembly design should be such that both the fuel rod cooling and neutron moderations are effectively achieved. 

Low coolant flow rate and small coolant inventory due to high coolant enthalpy rise... 

Although the coolant flow in the Super LWR is single-phase, the coolant enthalpy and therefore the density change substantially in the core because the coolant flow rate per thermal power in the Super LWR core is less than one eighth of LWR cores. Thus, the Super LWR can be susceptible to flow oscillations as the BWRs are. In Sect. 5.4, thermal hydraulic stability of the Super LWR is analyzed with the frequency domain approach. The analysis includes both supercritical and subcritical pressure conditions. 

The core outlet coolant enthalpy must be greater than that of the saturated steam from the flash tank to prevent the main steam temperature from decreasing through the line switching. The relation between the required core outlet temperature and the flash tank pressure is shown in Fig. 5.4 [3]. If the flash tank pressure is taken to be 6.9 MPa (the same as that of supercritical FPPs), the coolant temperature at the core outlet must be greater than 420°C. This core outlet temperature is readily achievable in the present design (1,000 MWe class) of the Super LWR. 

The properties of supercritical water are determined by utilizing the JSME 1980 Steam Table in SI units. The thermal-hydraulic properties at the outlet of one node are used to determine the corresponding properties at the inlet of the next downstream node. As for the boundary conditions, the chaimel inlet coolant enthalpy and flow rate are specified from the water rod outlet enthalpy and water rod outlet flow rate. 

In the present design of the Super LWR, the operating pressure is 25 MPa. The effect of this pressure is investigated while keeping the feedwater flow rate, core power, and inlet and outlet coolant enthalpies constant. The relation between the decay ratio and the pressure is shown in Fig. 5.41 [11]. The reason why the decay... 

The radial heat transfer model is almost the same as that of SPRAT-DOWN (see Fig. 4.2.6). The coolant enthalpy is calculated by solving an energy conservation equation. The heat fluxes on the fuel rods and the water rod walls that have been calculated at the previous time step are utilized the same as in SPRAT-DOWN. This does not cause a problem by keeping the time step reasonably short. 

There are two competitive phenomena associated with the MCST calculation. One is the subchannel heterogeneity as discussed above. The other is the flow mixing among the subchannels, which mitigates the flow and coolant enthalpy... 

The uncertainty of heat transfer correlation can be treated in two ways one is to use a corresponding engineering hot channel factor, the other is to treat this uncertainty separately with parametric uncertainties. The latter is applied to this study as is done for the Super LWR in Chap. 2. The uncertainty of the heat transfer correlation is evaluated by comparing with the Oka-Koshizuka correlation and the Dittus-Boelter correlation in the high coolant enthalpy region. The uncertainty is evaluated as 6.33°C. It is taken as the correlation uncertainty because the hot spot is always in the high coolant enthalpy region. 

The second term of (7.28) in Control system (A) tries to keep the enthalpy rise in the core equal to that in the initial condition. Since the core outlet temperature increases with the pressure when the outlet coolant enthalpy is kept constant, the main steam temperature settles to a slightly higher value than the initial one in this event. However, the difference is 0.7°C, and it does not seem to be a problem in practice. 

The recuperator is an essential component in the closed direct cycle Brayton system being considered for Prometheus. For the compressor pressure ratios being considered ( 2.0) for Prometheus applications the thermal load of the recuperator is a significant fraction of the coolant enthalpy rise from the compressor exit to the turbine inlet and enthalpy reduction from the turbine exit to compressor inlet. 

W-3 CHF correlation. The insight into CHF mechanism obtained from visual observations and from macroscopic analyses of the individual effect of p, G, and X revealed that the local p-G-X effects are coupled in affecting the flow pattern and thence the CHF. The system pressure determines the saturation temperature and its associated thermal properties. Coupled with local enthalpy, it provides the local subcooling for bubble condensation or the latent heat (Hfg) for bubble formation. The saturation properties (viscosity and surface tension) affect the bubble size, bubble buoyancy, and the local void fraction distribution in a flow pattern. The local enthalpy couples with mass flux at a certain pressure determines the void slip ratio and coolant mixing. They, in turn, affect the bubble-layer thickness in a low-enthalpy bubbly flow or the liquid droplet entrainment in a high-enthalpy annular flow. 

H enthalpy of steam relative to raw medium temperature, J/kg n flux of microorganisms due to axial dispersion, m 2s 1 k Boltzmann s constant, 1.3805 x 10 23 J/K or 1.3805 x l-16 erg/K kd specific death rate, s or kg/m s L length of holding section, m M initial mass of medium in batch sterilizer, kg Mw molecular weight of gas molecules, kg/kmol ms steam mass flow rate, kg/s mc coolant mass flow rate, kg/s... 

Controlled variables include product compositions (x,y), column temperatures, column pressure, and the levels in the tower and accumulator. Manipulated variables include reflux flow (L), coolant flow (QT), heating medium flow (Qb or V), and product flows (D,B) and the ratios L/D or V/B. Load and disturbance variables include feed flow rate (F), feed composition (2), steam header pressure, feed enthalpy, environmental conditions (e.g., rain, barometric pressure, and ambient temperature), and coolant temperature. These five single loops can theoretically be configured in 120 different combinations, and selecting the right one is a prerequisite to stability and efficiency. 

Because formaldehyde synthesis is exothermic, the reactor requires a coolant to remove the excess enthalpy of reaction. Thermodynamically, we should run the reaction at as low a temperature as possible to increase conversion, but at low temperatures, however, the rate of reaction decreases. At high reaction ten era-tures unwanted side reactions occur. Commercially, the reaction occurs fi om 600 °C (1110 °F) to 650 C (1200 °F), which results in a methanol conversion of 77 to 87 % when using a silver catalyst [24]. Because formaldehyde and methanol can form flammable mixtures with oxygen, we should carry out the reaction with mixture compositions outside of its flammability range. The oxygen used is less than the stoichiometric amount. 

Heat duty to coolant from enthalpy balance... 

After substituting Equation 3.4.3 into Equation 3.4.2, we obtain Equation 3.4.12 in Table 3.4.1. Physically, Equation 3.4.12 means that the enthalpy flowing into the reactor with the reactant stream plus part of the enthalpy released in the reactor by chemical reaction will raise the temperature of the products to 600 °C. The coolant removes the remaining enthalpy of reaction as heat. For simplicity, we again assume that we can use the mole fraction average of the pure component enthalpies for the enthapy of gas mixtures as given by Equations 3.4.13 and 3.4.14. Equations 3.4.15 to 3.4.22 are the pine component enthalpies we need for Equations 3.4.13 and 3.4.14. 

In semibatch operation Eq. (10.1.7) must be modified to include the enthalpy brought in by the addition of a fresh reaction mixture, a term which can often be absorbed into Q, Q itself will be governed by the same sort of equations as were given for the stirred tank. Thus, for a coolant jacket of constant temperature Teo, we would have Q = h(T — T ) and so on. This, of course, assumes that the rate of heat removal in the transient state would be instantaneously the same as in a steady state, which is not true. Here again, a more exact analysis with allowance for transient heat convection and conduction would be out of proportion to the problem. At best a lumped constant model with some time constants might be constructed for control purposes. 

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