Void coefficient

All RBMK reactors have positive void coefficients which means that increasing the boiling rate increases the steam fraction in the core which increases reactivity causing more steam void which causes more reactivity and so on. Competing factors provide stability, but startup, shutdown and maneuvering below about 600 MWt are unstable, hence, there is a rule prohibiting extended operation below 700 Mwt. 

Binder Sulfur Marshall Stability Flow Voids (%) Coefficient of... 

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. 

The void coefficient of reactivity could be large enough to result in a positive power coefficient under some conditions... 

The positive void coefficient of the RBMK and its rapidly acting positive power coefficient, under certain... 

This defect is met in commercial BWR designs by additional circulation of the coolant around the reactor core, over and above the coolant removed as steam through the turbines. Controllers link the flow rate to the power demand as felt on the turbine, so that the increased coolant input sweeps out the steam bubbles and compensates for the void effect. The RBMK has a similar additional coolant circulation which is necessary, perhaps, for its satisfactory operation with a negative water void coefficient but would exacerbate the disadvantage of a positive coefficient. This again was a feature of the Chernobyl accident. 

Why should the coolant have a positive void coefficient under certain circumstances ... 

The effect was further exacerbated by the peculiarity of the rod design, displacing about 1 m of water in the control rod shaft tube, top and bottom of the graphite follower hanging below the control rod. With the control rod fully out, the two water gaps line up with the top and bottom of the core which have been described as being the most reactive sections. If there is a positive void coefficient, the initial penetration of the absorbing section at the top simultaneously removes (voids) water at the bottom and adds reactivity in this area. 

The Doppler effect in the fuel would be expected to stabilise the void coefficient with a negative effect. But the Doppler coefficient only operates if the fuel temperature is high. Operated below 30% power, the normally high temperature rise in the ceramic fuel is much decreased and the Doppler effect inoperative until, during the course of the accident, the fuel temperature rose markedly. Indeed it is possible that this was the mechanism that terminated the nuclear excursion before break-up. 

It has been said that as fuel burnup progresses, moderator and water void coefficients tend to become positive. Furthermore, a step that is proposed to be taken in the RBMKs since Chernobyl is to force a... 

The equivalent of 70-80 rods will be kept within the core at all times. This can be compared with the specified operating minimum at Chernobyl of the equivalent of 30 rods in the core and the 6-8 equivalent rods at the time of the accident. This extra provision will greatly reduce the value of the void coefficient of reactivity. 

In the longer term, fixed absorbers will be installed which will reduce the positive void coefficient, and consequently fuel enrichment will have to be increased from the present level of 2% to 2-4%. Work has been... 

In the meantime the reactor shuts down (even if the power could initially increase slightly if the void coefficient is positive) and the safety injection through the accumulators and then through the high and low pressure pumps, starts. 

The reactor normally contains a steam-water mixture so that any fast increase in pressure produces steam condensation, an increase of the water mass present and, because of the negative void coefficient for safety reasons, an increase in the core reactivity. It is easily seen that in a BWR the ATWS accident (transients with failure to scram) is particularly serious and represents one of the dominant severe accidents in overall risk evaluations. An accident caused by the spurious and complete closure of isolation valves on steam... 

Benchmark ULOF of Modified BN-800 reactor with near zero sodium void coefficient of reactivity was analysed and results were presented at IAEA Consultancy Meeting of December 1996. Based on this, improvements to the pre-disassembly phase accident calculation codes have been made. 

The investigation of safety and more particularly of severe accident conditions is important for accelerator driven systems (ADS). Subcritical ADS could be of particular interest for the actinide transmutation from the safety point of view, because fast reactors with Neptunium, Americium and Curium have a much smaller fraction of delayed neutron emitters (compared to the common fuels and U), a small Doppler effect and possibly a positive coolant void coefficient. This poses a particular problem of control since the fraction of delayed neutrons is essential for the operation of a nuclear reactor in the critical state. In addition, the IRC presented in the past a review of accelerator-driven sub-critical systems with emphasis on safety related power transients followed by a survey of thorium specific problems of chemistry, metallurgy, fuel fabrication and proliferation resistance. 

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