Coolant Temperature Coefficient of Reactivity

The coolant coefficient is given in Equation 6.2.5 Hlhe term involving C is related to the coolant density variation. The terms involving. f. L. p and T are related to both the coolant density and temperature variations. 

The -coefficient is simplest to compute. Basically, what is Involved is a reduction in the shielding effcfct of the water within the fuel element to the Interaction of fast neutrons between the concentric fuel tubes. 

The calculation of is made by homogenising the fuel, cladding and coolant internal to the fuel assembly, and calculating the collision probabilities for the three fast neutron groups described in Section 2 5on the fast effect calculation. 

Share is no esqposure-dependence on the 6 -coeffielent. She theoretical value of the average value of this coefficient over the coolant temperature range is  

The p-coefflclent is also fairly straied tforward and again the effect is related to the water density change primarily although there is a slight effect associated directly with the water tenperature, which is connected with the Doppler effect. 

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Load change can be followed-up automatically by negative coolant temperature coefficient of reactivity. 

The relatively slow driving functions which can be applied to the reactor throuc(b primary coolant temperature changes are followed almost exactly by the reactor. In responding to changes in reactor coolant inlet tenperaturej the combination of the fuel and coolant temperature coefficients of reactivity is such that changes in inlet temperature are attenuated to approximately one-third their value, and appear at the reactor outlet with the opposite sign. 

Moderator (and coolant) temperature coefficient of reactivity Moderator void coefficient of reactivity Fuel temperature coefficient of reactivity Effective prompt neutron lifetime Delayed neutron fraction(s)... 

Core reactivity is controlled by means of chemical poison dissolved in the coolant, burnable poison rods and control rod assemblies. Soluble boron and burnable poison rods are utilized for shutdown and fuel bumup reactivity control. Control rod assemblies (37 clusters) are used for power regulation and hot shutdown. The core consists of 3 regions with enrichments of 2.4%, 2,67 % and 3. 0%, It has a negative temperature coefficient of reactivity. The core has a fuel cycle of 12 to 16 months with a discharge bumup of 30,000 MWd/tU. 

From the safety standpoint, the thermal capacity and strong negative temperature coefficient of reactivity also work to passively mitigate reactivity and loss of coolant accidents. Nevertheless, a safety-related reactor trip and safety features monitoring systems are included... 

The teoperature-dependent sources of reactivity variations In N Reactor Include> In addition to the usual fuel and graphite heating effects (which are nuclear) one associated with coolant heating (which Is both nuclear and physical). The fuel temperature and coolant tenparature coefficients of reactivity are negative the graphite temperature coefficient Is positive. Other temperature-associated physical effects such as fuel expansion are too small to produce any significant reactivity coefficient. 

Several inherent and passive safety features are incorporated in compact high temperature reactor. Due to negative temperature coefficient of reactivity, the power of the reactor comes down without necessitating any external control in case of increase in core temperature. The reactor also adopts passive systems like removal of core heat by natural circulation of liquid metal coolant in the main heat transport circuit, passive regulation and shut down systems. The reactor is also able to remove heat passively by way of conduction in the reactor block and by radiation and natural convection from the outer surface of the reactor during loss of heat sink. The paper deals with the details of passive systems incorporated in the AHWR and CHTR and the analysis performed for these systems. 

The temperature feedback mechanisms provide a link between the reactor s neutronics and its coolant systems independent of any action of the control system. The size and relative importance of the temperature effects will vary from reactor to reactor, but designers work hard to ensure that there is an overall negative coefficient of reactivity with temperature, which provides for automatic limiting or mitigation of temperature excursions. Some important temperature feedback mechanisms are listed below ... 

The negative reactivity contributions of the coolant temperature coefficient up to 210 C (saturation temperature at the 300 psla startup loop pressure) will be 2,2 per cent k. 

The effects of plutonium formation, uranium-235 burnout and fission product formation (other than xenon and samarium) are partially Included In the exposure-dependence of the graphite and coolant temperature coefficients (See Table 5-1) However, the reactivity of the cold reactor is also dependent on these effects ... 

Isothermal temperature coefficient at full power (pcm/°C) Total power coefficient of reactivity (pcm/MWth) at full power, constant inlet temperature Maximum coolant void effect (dollars), including only regions with a nositive coolant reactivity worth... 

Negative reactivity coefficients on the fuel and coolant temperature, on the specific volume of coolant negative steam density and power (integral) coefficients of reactivity ... 

The elimination of soluble boron control together with the adopted parameters of the fuel lattice provide negative reactivity coefficients on the fuel and coolant temperature negative steam and integral power coefficients of reactivity in the entire range of operating parameters, which altogether secures inherent safety features of the reactor core. These inherent safety features ensure power self-control in a steady state reactor operation, power rise self-limitation under positive reactivity insertions, self-control of the reactor power and primary coolant pressure and temperature self-limitation in transients, as well as the limitation of the heat-up rate in reactivity-initiated accidents. 

The total teo ratiire coefficient cannot he easily stated since none of the separate teoqperatures (graphite> fuel and coolant) are representative of a single reactor temperature (as vould he the case In a homogeneous reactor) The teiqperatures are all related to the power of the reactor however (assuming cozistant coolant flow and Inlet coolant temperature) and it Is sometimes useful to define a power coefficient of reactivity as follows ... 

In the solution of the reactor kinetics equations for short term transients there are two prompt reactivity feedback effects. These are related to the uranium and coolant temperature coefficients 3tenon variation and graphite heating have too long a time constant to affect the short-time kinetics calclatlons ... 

She most significant aspect of the reactor is that it is relatively last conqpared to the total loop She time constants of the fuel element and coolant transport time throui the reactor are on the order of one and one-half seconds vhereas the total coolant transport time is 60 seconds and the effective time constant of the heat exchanger is 20 seconds. Furthermore there is a very tight loqp in the reactor kinetics in the form of the two prompt negative temperature-coefficient feedbacks from the coolant water and fu. These two coefficients produce a power coefficient of reactivity of -2.6 x 10" per Mw. 

The ultimate application of the Doppler reactivity coefficients is a calculation of the reactivity change when the temperature of the reactor changes in either a controlled or accidental manner but in either case usually in a nonuniform manner. There are at least three important causes of non-uniform temperature changes. Two are the nonuniform neutron flux and coolant temperature distribution, slowly varying nonuniformities that can be accounted for by the procedures developed by Foussoul (72A). 

Autonomous reactor operation requires intrinsic self-regulation of reactor power. This behaviour can be obtained by negative reactivity feedback such as that provided promptly by Doppler broadening of neutron absorption resonances in resulting from increased fuel temperature, and quickly by efrects associated with coolant temperature increases and density decreases (i.e., negative void coefficient), and fuel thermal expansion. However, adequate safety consideration would also have to be given to possible reactivity insertion transients initiated by overcooling events. 

The reduced content of boric acid ensures negative values of void and coolant temperature reactivity coefficients in the whole range of temperature variation. This allows for reactor self-shutdown at loss of primary circuit int ty and power self limitation in emergencies with rise of power and temperature. 

Self-protection, self-regulation and self-limitation of reactor power due to the n ative reactivity coefficients (for power, void, fuel and coolant temperature) within the entire range of reactor operation parameters. 

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