Axial shims

With pole diameters of circa 10-20 cm, that is, well above the circumference of the resonator, magnets of this type produce an axial held at the sample in the resonator of sufficient homogeneity for all conceivable EPR purposes. Contrary to the situation in NMR spectroscopy, fine tuning of the held, or shimming, is not required. 

Transition Elements (Section 13.16-13.19) Generally, shims are inserted between elements with different numbers of flights to compensate for the axial clearance of the shafts. It is also possible to use transitional elements whose screw profiles alter continuously along the axis from o ne number of flights to another, while maintaining a complete self-cleaning function. 

Thrust rings/cones and shims are used in conjunction with pressure vessel end caps to minimize longitudinal movement of membrane modules within the pressure vessel. Movement of the membrane modules can cause the O-rings to wear as well as cause telescoping of the membranes and spacers during pressurization. Thrust rings/cones also serve to distribute the axial pressure load to the full end cap. 

Power distributions within the core following the withdrawal of a single RCCA are calculated using ANC and APOLLO. The peaking factors are then used by VIPRE-01 to calculate the DNBR for the event. The case of the worst rod withdrawn from the mechanical shim or axial offset bank inserted at the insertion limit, with the reactor initially at full power, is analysed. This incident is assumed to occur at beginning of life because this results in the minimiun value of moderator temperature coefficient. This assumption maximises the power rise and minimises the tendency of increased moderator temperature to flatten ihe power distribution. 

For cases of any one RCCA fully inserted, or the mechanical shim or axial offset banks inserted to their rod insertion limits with any single RCCA in one of those banks fiilly withdrawn (static misalignment), the DNBR remains greater than the safety analysis limit value. 

For the case of the accidental withdrawal of a single RCCA, with the reactor in the automatic or manual control mode and initially operating at full power with the mechanical shim or axial offset banks at their insertion limits, an upper bound of the number of fuel rods experiencing DNB is 5 percent of the total fuel rods in the core. 

Figure 9.6 is an illustration of a core which maintains reactivity by upward rod withdrawal and has no chemical shim. At the beginning of core life, peak flux is below the axial core centerline because of rods in the top of the core. By the end of core life,... 

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