13.58. Since there is only slight neutron absorption in the coolant and the moderator is kept cool, there is little decrease in reactivity from shut­down to power in contrast to that for LWRs. Continuous refueling also avoids reactivity burnup changes. The core reactivity inventory requiring compensation by the control system is therefore small. The coolant tem­perature and void coefficients are slightly positive; nevertheless, the power coefficient is negative, although small, as a result of the overriding effect of the negative Doppler fuel coefficient. Thus, the system is inherently stable.

13.59. Several types of control elements are provided in the CANDU reactor. Primary shutdown capability is provided by 28 vertical absorber (shutdown) rods with secondary shutdown, if necessary, by high-pressure injection of gadolinium nitrate solution into the moderator. Since the core is large and the negative power coefficient is small, xenon oscillations, which are slow, require control (§5.76 et seq.). The neutron flux level is therefore measured continuously at many points (—100) in the core and the flux level adjusted as necessary. Flux flattening is provided by 21 absorber rods called “adjusters.” In addition, there are 14 “zone control” absorbers for bulk reactivity control and local suppression of flux oscilla­tions. These absorbers consist of vertical chambers which can be filled with ordinary water to any desired level. Four vertical (solid) absorber rods supplement the zone control absorbers.