Positive graphite temperature coefficient

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. 

In spite of these changes, the reactor was left with the possibility of a fast positive power coefficient, with a graphite temperature that was too high, and without a secondary shutdown system. In addition, it is now clear that the speed of insertion of the control rods under emergency conditions was totally inadequate. 

MOFM calculations averaged over a graphite temperature range of 20 to 5 0 C and with equilibrium coolant teoperature and density (232 C and 0.02 gm/cm ) is -1.0 X 10"5/oc in the reactor with green fuel and -5. x 10 /<>C In the reactor with an average fuel exposure of TOO MHD/T. The coefficient is less negative for the exposed fuel due to the positive tenperature coefficient of the plutonium port of... 

The variation in the Fermi age, T, vith graphite temperature arises from the fact that the neutrons have to slow down over a smaller energy range as the neutron temperature increases. The coefficient is small and positive and for a hetereogeneous reactor is difficult to calculate. For these reasons it can be neglected. 

Li Q, Kim JW, Shim TH, Jang YK, Lee JH. Positive temperature coefficient behaviour of the graphite nanofibre and carbon black filled high-density polyethylene hybrid composites. Adv Mater Res 2008 47 226-9. 

Ordinary carbon and pure polycrystalline graphite having small crystallites have negative temperature coefficients of resistance (semiconductor), while single crystals or graphite have positive temperature coefficients. The resistance-temperature relations for ordinary carbon resistors widely used in radio circuitry are of particular interest in low-temperature work because of their usefulness as sensitive thermometers and liquid level sensing devices. 

Other reactors do not have this feature. The Russian developed reactor type RBMK-1000, for instance, is based on neutron moderation in graphite and heat transport by water. Under certain conditions of low power it has a positive temperature reactivity coefficient and this was - among many other reasons - also responsible for the largest accident in reactor history (Chernobyl, 1986). 

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