Operating Reactor Temperature Coefficients

A separate description of the temperature coefficients in tezms more meaningful to the operator of the reactor is given belov. The temperature coefficients are separated as foUovs  

The net reactivity effeot of ir -humout and plutonium formation is positive in spite of a less-ttaan-one-for-one replacement of fissionable atoms by the conversion px ocess. This increase reeuUa.from tbe hi r fission cross section and neutron yield of compared to u The net eacposure effect, including 

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The operation of a reactor in the practical sense requires a small temperature coefficient in order that criticality can be easily maintained. Usually in addition to its small magnitude, the overall reactor temperature coefficient is designed to be negative to increase stability and to enhance the safety of the reactor. 

No inherent instability in plant response to planned and unplanned transients has been revealed, with either a single Brecon or multiple units, assuming the overall reactor temperature coefficient is negative and the Braytons are operating with active speed control. 

Equation (8) provides a general relationship between the reactor temperature profile and the operating parameters. In relating the system heat transfer to the conversion-molecular weights relationship for a reactor of fixed size, the heat transfer coefficient emerges as the correlating parameter. 

The above equation then represents the balanced conditions for steady-state reactor operation. The rate of heat loss, Hl, and the rate of heat gain, Hq, terms may be calculated as functions of the reactor temperature. The rate of heat loss, Hl, plots as a linear function of temperature and the rate of heat gain, Hq, owing to the exponential dependence of the rate coefficient on temperature, plots as a sigmoidal curve, as shown in Fig. 3.14. The points of intersection of the rate of heat lost and the rate of heat gain curves thus represent potential steady-state operating conditions that satisfy the above steady-state heat balance criterion. 

In the new designs, if coolant were lost, the nuclear chain reaction would be terminated by the reactor s negative temperature coefficient after a modest temperature rise. Core diameter of the modular units would be limited so that decay heat could be conducted and radiated to the environment without overheating the fuel to the point where fission products might escape. Thus, inherent safety would be realized without operator or mechanical device intervention. 

Figure 7.21 Evolution of the reactor temperature in the case of switching between operating points II and III, in the presence of a 10-K unmeasured increase in the coolant inlet temperature and an unmodeled 20% drop in the heat transfer coefficient U. Both disturbances occur at t = 60 min.

Figure 7.27 Evolution of the reactor temperature and temperature setpoint for a 10% rise in the production rate at operating point I, under plant-model parameter mismatch. The heat transfer coefficient U in the controller model is overestimated by 10% compared with its value in the plant.

To solve Eq. 7.1.16, we have to specify the value of HTN. However, its value depends on the heat-transfer coefficient, U, which depends on the flow conditions in the reactor, the properties of the fluid, and the heat-transfer area per unit volume, S/V). These parameters are not known a priori. Therefore, we develop a procedure to estimate the range of HTN. For isothermal operation (dQ/di = 0), we can determine the local HTN from Eq. 7.1.16 (taking the reactor temperature as the reference temperature, 6 = 1) ... 

The practical implementation of the above policies is not necessarily as straightforward as solving the above equations. As can be deduced from Equations 6.70-6.76, Pjjjj is a function of the propagation rate coefficients, the monomer concentrations, and most importantly, the total radical concentration. Hence, to precalculate the optimal monomer feed rates, the radical concentration must be specified in advance and kept constant via an initiator feed policy and/or a heat production policy. This is especially important considering that a constant radical concentration is not a typical polymer production reality. This raises the notion that one could increase the reactor temperature or the initiator concentration over time to manipulate the radical concentration rather than manipulate the monomer feed flowrates, that is, keep P j constant for simpler pump operation. Furthermore, these semibatch policies provide the open-loop or off-line optimal feed rates required to produce a constant composition product. The online or closed-loop implementation of these policies necessitates a consideration of online sensors for monomer... 

BOG conditions and about +3 x 10" / C for EOC conditions over the normal operating temperature range. In the calculation of the total reactor isothermal temperature coefficient of reactivity, the fuel and moderator temperatures up to about 1700 C (3092 F) have been varied isothermally. The inner and outer reflector temperatures on which the reflector contributions to the temperature coefficient calculations are based, are assumed to be in equilibrium with the respective fuel temperatures as discussed later. Table 4.2-12 lists the assumed temperature conditions used to determine the temperature coefficients of reactivity that have been plotted as a function of the active core temperature in Figures 4.2-6 to 4.2-8. A nine neutron group radial diffusion calculational model with cross sections based on the temperatures indicated in Table 4.2-12, was utilized to determine the temperature coefficients of reactivity. 

The isothermal temperature coefficients were measured by taking the difference of reactivity at approximately 250 and 370°C during reactor start-up. The measured isothermal temperature coefficients were constant through the MK-II operation because they were determined mainly by radial expansion of the core support plate, which is independent of bumup. However, when the core region was gradually extended from the 32 cycle, the isothermal temperature coefficients were decreased as predicted with the mechanism. Table 4 shows these values. 

Further reactivity coefficients are important, particularly the power coefficient, the change of reactivity with power. The size and indeed sign of this coefficient vary with the power level at which the reactor operates. These power coefficients are thus a fimction of temperature and express the effect of a temperature or power change. 

The first type of experiment involves the application of a flow reactor designed to operate over the temperature range -25 to 80 °C and over the pressure range 100 to 760 Torr. This reactor has been used to study competitive OH reactions with pairs of aliphatic ethers or with an ether plus 2,3-dimethylbutane as the reference compound. The temperature coefficients of the OH reactions with diethyl ether, methyl -butyl ether, ethyl n-butyl ether and di-n-butyl ether have been determined. This system has also been used to study competitive OH reactions with pairs of aromatic compounds or with an aromatic compound plus a reference compound. The temperature coefficients of the OH reactions with benzene, toluene, phenol, benzaldehyde, f7-cresol, m-cresol and p-cresol have been determined. 

The reactor control system consists of four rods located in the radial reflector and in the lower movable end reflector. Two rods are used for automatic and manual control, whereas the other two, together with the movable reflector, are used for the protection in case of emergency. The negative temperature coefficient of the reactor reactivity allows operating for a long time without the interference of the control system. Only some deterioration of electric power necessitated increasing of thermal power up to a new level. 

A key feature of LEADIR-PS, shared with the Modular High Temperature Gas-Cooled Reactor (MHTGR) under development by General Atomics, is that radionuclide releases are prevented by retention of the radionuclides within the fuel particles under all design basis events without operator action or the use of active systems. Thus, the control of radionuclide releases is achieved primarily by reliance on the inherent characteristics of the coolant, core materials, and fuel. Specifically, the geometry and size of the reactor core, its power density, coolant, and reactor vessel have been selected to allow for decay heat removal from the core to the ultimate heat sink through the natural processes of radiation, conduction and convection, while the negative temperature coefficients of the fuel and moderator assure reactor shutdown. 

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