Positive void coefficients

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. 

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

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. 

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... 

The type of accidents in which the Doppler effect plays a crucial role are those resulting from very high rates of reactivity increase, say l/sec or greater. Normal safety control systems are adequate to cope with accidents resulting from lesser rates of insertion. In the safety studies for the earlier small alloy fuel reactors, the main type of accident leading to the reactivity increase was core meltdown (8), which was in turn considered to result from either coolant failure or some less severe type of reactivity transient. In many of the large breeder reactor concepts of the future, the coolant has a positive void coefficient of reactivity (77, 12a) which could conceivably lead to an autocatalytic expulsion of the coolant and consequently high rates of reactivity increase. 

HWRs using natural uranium have a positive void coefficient, which leads to positive power coefficients. This is accommodated in the design by employing independent fastacting SDSs based on poison injection into the moderator and spring assisted shut-off rods. 

The positive void coefficient, while it must be compensated for in an accident by the SDSs, has the advantage of resulting in fast and responsive neutronic trips for a number of accidents. It also ensures an inherent power reduction for rapid cool-down accidents such as steam main failure. 

Temperature coefficient of reactivity -17.5 pcm/K —14.6 pcm/K -4.3 pcm/K blanket salt positive void coefficient -2.4 pcm/K recent indications maybe positive -6.7 pcm/K... 

As breeders, such designs can have impressive doubling times but typically are rather impractical. They remove the large advantage of other MSR designs wherein the fuel is also the coolant. Thus, as for solid-fueled reactors, the flow of coolant fluid must be assured in all cases. As well, these designs typically would have large positive void coefficients of the coolant in a loss of coolant scenario. 

The thermal absorption of thorium is three times that of Due to this, the deleterious effects of parasitic absorption are less in thorium systems, and one can consider the use of light water coolant. This opens the way to in-core boiling. The reactor then has to be vertical, and then it becomes possible to design for 100% heat removal by natural circulation and passive safety. The possibility of positive void coefficient of reactivity has been countered by the lattice being under moderated with a burnable absorber in the fuel cluster. 

A reactor core of such a large volume has the disadvantage that load and load distribution in the reactor are difTicult to control. Another problem of this reactor type is the positive void coefficient of reactivity which may occur under certain circumstances. With increasing concentration of steam bubbles in the coolant, the absorption of neutrons in the coolant decreases while moderation of the fast neutrons by the graphite moderator remains unchanged. For this reason, an increase in reactor power is not limited by inherent physical properties of the reactor core and, therefore, the reactor load has to be controlled by extensive active measures, i. e. by complicated instrumentation. 

Implications of AHTR Designs With Positive Void Coefficients... 

Positive void coefficients can occur when the neutron absorption to scattering ratio is higher in the liquid than the graphite... 

There is one significant difference. Depending upon design, a positive void coefficient is possible with a molten salt coolant. Several other reactor types have this characteristic liquid-metal fast reactors and heavy water reactors. A positive void coefficient implies that if the core looses all coolant, the power level increases. 

Since Pb-Bi has a much higher boiling point (1670°C) than sodium, the positive void coefficient is expected to pose no serious problem for the PEACER. 

An array of so-called moderator displacement or Interlattice tubes located between the pressure tubes within the calandrla were provided. These tubes could be emptied if necessary to bring an excessively positive void coefficient into the desired range. An excessively negative void coefficient could be corrected by increasing the H2O impurity in the D2O moderator. In the event, the calculations of void coefficients have proved to be very accurate and it has not been necessary to resort to either of these measures. Interlattice... 

The coupled code JOSHUA is of particular Importance in the natural uranium SGHWR project. It has been used to show that the fuel management scheme may be designed to offset some of the consequences of the positive void coefficient. Since steam voids Increase reactivity there is a tendency for the power to peak towards the channel exit. This may be corrected by loading fresh fuel at the lower end of the channel and moving the parts of ein axially sub-divided element upwards during irradiation. 

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