Negative coefficient of reactivity

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 ... 

Self-limitation of power due to negative coefficients of reactivity (reactor "hot" condition), at the balance point between core power and emergency heat removal capacity. 

Negative temperature and Doppler reactivity coefficients. The negative coefficients of reactivity provide stable plant neutronics ... 

The design of the Chernobyl plant was flawed in other ways as well. Western reactors are designed when operating to maintain negative power coefficients of reactivity that prevent such runaway accidents. The Chernobyl plant would not have been issued a license to operate in the U.S. or other Western countries. The Chernobyl accident was in many ways the worse possible scenario having an exposed reactor core and roofless building. Thirty-one plant workers and firemen died directly from the radiation exposure and it is projected that over 3,400 local residents will eventually acquire and die of cancer due to radioactive exposure. 

This hydrogen density in e zirconium hydride is as high as in water at room temperature and is appreciably higher than in water at the 300°C used in power reactors. Another advantage of the uranium-zirconium hydride fuel-and-moderator mixture is its high prompt negative temperature coefficient of reactivity, a consequence of the intimate thermal contact between and hydrogen atoms. 

The RCSS and NCSS must provide the capability to control heat generation with moveable poisons and to control heat generation with inherent feedback. The moveable poison control function is accomplished both with a primary and a diverse secondary moveable poison control, while control with inherent feedback requires a negative temperature coefficient of reactivity. The NCSS and the RISS within the RS, also perform the function of heat generation control by maintaining the geometry for insertion of moveable poisons into the core. The NCSS monitors the neutron flux. 

Figure 4.2-6 shows the calculated temperature coefficient of reactivity for the BOC-IC condition. Curve A is the fuel prompt doppler coefficient due to heatup of the fuel compact matrix as a function of the assumed fuel temperature. Curve B is the active core isothermal temperature coefficient and is the Siam of the doppler coefficient and the moderator temperature coefficient of reactivity which is also strongly negative, due in large measure to the presence of LBP in the BOC condition. The moderator coefficient, not shown in Figure 4.2-6, would be the difference between Curve B and Curve A and would be -4.0 x 10" / C at 800 C (1472 F), for example. Curve C is the total reactor isothermal coefficient and includes the positive contribution of the reflector heatup to the estimated inner and outer reflector temperatures that would result when the fuel reaches the indicated temperature. 

The effectiveness of delayed neutron detectors for detecting clad failure was tested by operating the reactor with vented fuel SA in the core. The void coefficient of reactivity at various core locations were measured using two special SA fabricated for this purpose. The void coefficient was found to be negative. 

Partial disruption of the core could inhibit the insertion of some control rods under this accident situation, causing a local criticality condition as the core cools down, due to the negative temperature coefficient of reactivity of the fuel. Modifications were made to supplement a number of control rods with a facility to inject boron beads from storage hoppers above the core into in-core thimbles. This secondary shutdown system is automatically triggered by differential pressure sensors the beads can be recovered from the thimbles and returned to the hoppers in the event of a spurious operation. 

An anticipated difference between the ATHR and helium-cooled reactors is the coolant void coefficient of reactivity, since the relevant nuclear cross sections for molten salts are larger than those for helium. The void coefficient corresponds to the amount of reactivity that is added or subtracted by complete removal of the coolant. Since initial AHTR calculations indicated that the void coefficient could be positive or negative depending on the precise design of the core, the focus of the physics analysis effort was to characterize this effect more carefully. 

The expansion of the areas of application for carbon fibers is stimulated by their attractive properties, not found in other materials, such as strength, electrical conductivity, stability on exposure to reactive media, low density, low-to-negative coefficient of thermal expansion, and resistance to shock heating. The most representative applications of carbon fibers and element carbon fibers are as sorption materials, electrostatic discharge materials, catalysts, and reinforcement materials in composites. 

The quantity J drG r) is the negative of the power coefficient of reactivity, and the condition (12) is not in practice very restrictive. The connection with the definitions of the preceding paper are obvious. The detailed variation of G(t) can be very complex, and positivity is too restrictive a condition. We address ourselves in this paper to the problem of obtaining criteria for the stability of the equilibrium defined by (13), without making unduly specialized assumptions as to the form of D(r) and G(r). 

A negative power coeflBcient of reactivity is required under all credible circumstances. In addition, the local power coefficients of reactivity must be negative everywhere in the core to ensure stability of the power distribution. 

These features combined with a negative temperature coefficient of reactivity, large heat capacity of the graphite and die large design margins make the reactor safety extremely difficult to challenge. 

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. 

Self-shutdown of the reactor in emergency conditions due to negative void coefficient of reactivity. 

Burnable poison (Gd203) is used to partly compensate the fuel bum up reactivity, and soluble boron is utilized for reactor shutdown only. This results in a negative temperature coefficient of reactivity over the complete core life. 

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... 

On the one hand, this is achieved by the fact that there is a temperature span of approx. 700 C between the maximum permissible fuel element temperature of 1600°C and the maximum operating temperamre of the fuel elements. This temperature span ensures that the reactor core shuts itself down via the negative temperature coefficients of reactivity, even after accident-incurred introduction of any existing surplus reactivity. 

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

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