Coolant Void Coefficient

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. 

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. 

A series of neutronics calculations were conducted to evaluate the coolant void coefficient and to understand its sensitivity to various core parameters. Both ORNL and SNL participated in the physics analysis using slightly different models and assumptions. The results are in good agreement, however. 

Fig. 3.2. Variation of coolant void coefficient with fnel volnme fraction.

An extensive series of measurements has been made with clusters of j-in. diam. natural TJOk rods clad in aluminum. A major portion of the series dealt with a 37-rod cluster on an ll.l-ln. lattice pitch, the NDA-S L design. Among the parameters studied were lattice pitch, rod-rod pitch, cluster size, insulation gap between housing tubes, and clad material. The coolant void coefficient was also determined. A corresponding series of lattices has been evaluated using the ROCLAND-A code. The ai ee-ment between the calculations and measurements is satisfactory however, the calculations yielded are generally lower values than the measurements by 0.5-1.0% in ko,. 

The coolant void coefficient is, in general, positive with a value of 0.025 in k for the standard 37-rod cluster on an 11.1-in. lattice pitch. 

With a representative tubular cluster having a fuel/ coolant volume ratio of 2.20, the coolant void coefficient increased from 0.007 to 0.014 as the lattice pitch was increased from 7.5 io 13.22 in. The major portion of the change was produced by the imefflcient in the outer coolant channel for which the void coefficient was a negative 0.0007 at a lattice pitch of 7.S-in. and increased to 4-0.006 at 13.22-in. lattice pitch. 

Avoidance of a positive coolant void coefficient of reactivity or... 

The changes in coolant void coefficient for the whole core during reactor lifetime are shown in Fig. XXV-4 the coefficients for lead coolant and metal fuel are also shown for comparison [XXV-6]. In all cases, the coolant void coefficient remains negative. It could be noted that it becomes positive, if the coolant is changed to sodium. 

Magnitude of coolant void coefficient (CVC) for LiF-BeF decreases with increasing uranium loading and increasing burnable poison loading... 

There are two features of contemporary fast reactors which have attracted particular attention on the grounds of safety core disruptive accidents (CDAs) caused by positive reactivity transients which may be exacerbated by the fact that coolant void coefficient of reactivity is under some circumstances positive (if the coolant is sodium) and sodium fires. There is no doubt that there will be continuing work during the coming decade to improve safety in both of these areas. 

While the coolant void coefficient is normally positive, the degree of subdivision of the piping allows any power excursion arising from a loss of coolant accident to be easily terminated by the control rods. An added advantage is that, with on-load refueling, the excess reactivity of the core can be maintained at a much lower level than is required for a light water reactor. 

Coolant void coefficient < -147 pcm/% in normal operation < -43 pcm/% in cold shutdown... 

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. 

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. 

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