Void coefficient SGHWR

Steam Generating HWR (SGHWR) started operation in 1968 with a designed output of 100 MWe. The use of light water coolant and heavy water moderator means that with the choice of the appropriate fuel-to-moderator ratio, the void coefficient could be made to... 

It is shown by comparison with experimental data from zero energy assemblies that the interplay of these parameters is well understood and that in particular the coolant void coefficient of reactivity can be predicted accurately. The representation of the complete core of the Winfrith SGHWR is discussed and it is shown that the validity of the techniques used have been experimentally demonstrated. Finally the nuclear design aspects of larger SGHWRs including those designed for operation with natural uranium fuel are reviewed. 

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. 

In principle, any third independent input could be used to complete the control, for example recirculation flow rate, but its effect on reactivity is proportional to the void coefficient and it is, therefore, not suitable for an enriched SGHWR. A direct control of reactivity would be excellent as the third control input since it is a mode of disturbance very different from the other two. 

For zero or slightly negative void coefficients (as in enriched SGHWRs) pressure control is more rapid in the decoupled system this is because the coupled system control loop includes the lag due to fuel conductivity in addition to the drum time constant, and matters discussed in paras 7 to 9, accentuate the difference for negative void coefficients. 

There Is little to choose between these systems for load following and the slightly better pressure control of the decoupled system at the void coefficient expected on Wlnfrlth SGHWR, led to choice of the decoupled system for the autocontrol of this reactor. 

Design studies are also being made of a commercial SGHWR, using non-enrlched fuel. From the point of view of station control the most important differences from enriched fuel SGHWRs lie in the significant positive void coefficient and the comparative lack of reactivity for start-up. 

To ensure reactor stability with the large positive void coefficient, a rapid mechanism for control of reactivity is required. The principal methods being studied include variation of primary circuit flow and absorber rods. Primary circuit flow control was briefly mentioned as a possible degree of freedom for control in paras 16 to 18 the range of void coefficient expected on the natural fuel SGHWR makes it potentially more useful than on the enriched fuel SGHWRs. 

SGHWRs fuelled with natural uranium will require radically different fuel management schemes, in order to maximize burn-up and correct the distorting Influence of the positive void coefficient on the axial power profile, axially sub-divided fuel elements are required. There is a much stronger incentive to use an on-load charge machine than in the enriched design. 

Dr Hicks mentioned that the Inter-lattloe tubes in the SGHWR have not been used. Does this mean that the calculations of void coefficient were unduly pessimistic or that control was easier than envisaged ... 

Taking into account the positive void coefficient in natural uranium fuelled SGHWRs, what are the problems with the dynamic behaviour of this type of reactor, especially in the ease of hydrodynamic instability ... 

A basic difference between CIRENE and the SGHWR consists in reactor physics. The former has to accept operation with a considerable coolant void coefficient this fact has to be duly taken into account in the design of the control and safety systems, which are described in some detail. 

This is due to the fact that CIRENE was basically conceived as a natural uranium fuelled reactor. Particular care has been therefore taken in order to minimize the light water content in the core the coolant does not contribute substantially to moderation, as happens in the SGHWR, but acts mainly as a heutron absorbing medium. A considerable coolant void coefficient had to be therefore accepted. 

In para. 2 the Authors state that due to the positive void coefficients of natural uranium SGHWRs, control devices with high dynamic and mechanical performances are required. The usual noise of the reactor might result in almost permanent rod oscillations. Are these predictions based on some quantitative analysis or experimental data, and are they common to the Canadian, Italian and British developments of such reactor systems ... 

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