Scram rods

Unit 125. SPl Unit 125 was used at NS 3105 at the second phase of its running. In 1997 the NS reactor was shutdown, coolant drained, cut out of the primary circuit and "frozen". All absorber rods were completely inserted and made immobile via electric cable dismantlement, scram rods and automatic control rods being in the "frozen" alloy. The operational time of the reactor was about 15% of its lifetime. 

The reliability of the information on the submarine and icebreaker SGI structures and materials centers on the reactor cores, the thermal shields, the RPVs, and their associated support structures. Core details for the submarine LMRs is the most comprehensive, with information on the fuel rod materials, dimensions, and pitch, CCR and EPR materials and locations, and overall materials distribution. For the icebreaker PWRs, the information is essentially limited to die fuel rod materials, dimensions, and pitch and the configuration of the control rods (OCRs) and scram rods (EPRs) within the cores. In both cases, the core details are substantially more reliable than the core details for the submarine PWRs, where assumptions were made. Information for the thermal shields and the RPVs is reasonably complete with regard to the materials, dimensions, and locations of one with respect to another. The most detail is associated widi the submarine LMRs. Information on the support structures for the reactor cores, thermal shields, and RPVs has not been provided to any significant degree. As such, it is the area where the largest number of assumptions have been made and where the data is the least reliable. With respect to future concerns of potential reactor criticality and remedial actions, the lack of details on the support structures may impact future decisions. 

The 3.1% Ak ( 10.6) worth of total swing was calculated for a six-month equilibrium fuel cycle at an 80% load factor. This reactivity change can be compensated by one-half of the control rods (shim rods). The remaining half of the control rods serve as scram rods and are withdrawn from the core during full-power operation (maximum withdrawal... 

The fuel subassemblies of the core and of the blanket are placed in the discharge header mounted on the pressure chamber of the reactor vessel (fig.5). The core loading of 220 subassemblies contains fuel elements with UO2 fuel and a blanket material. More recently some MOX experimental fuel elements were also loaded into the core. There are 12 positions in the core for the rods of the control and safety system. Of the 12 control rods 2 are automatic control rods, 6 are reactivity compensators, 3 are scram rods and 1 is a temperature effect compensator. The core is surrounded by the blanket, containing depleted uranium dioxide. The axial blanket is mounted in the subassemblies of the core (Fig. 6) and the radial blanket is formed by fuel subassemblies containing blanket material. Above the core is a central column containing the control rod drives and thermocouples. The column is only as wide as necessary for the absorber rod systems it does not cover the full width of the active core zone. 

Emergency protection Six scram rods in leak-tight sleeves placed in the centres of six fuel assemblies and eight control rods of the compensation group (101 absorber elements)... 

On the total, the core has 101 absorber rods. In the centres of six fuel assemblies, sealed sleeves are welded to the reactor cover where scram rods are moved. The scram rods are similar to those used in icebreaker reactors. 

Fuel assembly with scram rod. FIG. VI-7. Cartogram of the KLT-20 core... 

The LIM performance was also demonstrated by neutron radiography as shown in Fig. XVn-13. The time required for the reactivity insertion by LIM is 0.4 s, which is much shorter than the free drop of conventional scram rods (by as much as 2 s). 

Control rod material - the material of rods used to control reactor reactivity and power (including coarse and fine power control, bumup compensation and scram rods). Data providers should choose the appropriate option from the multiple-choice menu boron stainless steel, cadmium stainless steel, boron carbide stainless steel, carbon steel, boron carbide, boron carbide/hafnium, boron carbide/cadmium alloy, cadmium alloy, Ag-ln-Cd alloy. 

Figure 7 6.2 1 Is a plot of the scram rod characteristic and Is Identical to Figure A-2 of HW-71 8 V0L2.

The stabilizing reactivity feedback caused by negative reactivity temperature coefficients for the fuel and coolant as well as the void reactivity coefficient mean that heating up the core structural components, including fuel, or water boiling in the core would eventually result in a spontaneous reduction or self-limitation of the reactor power irrespective of the positions of control rods, including scram rods. 

The scram mechanism is mounted between the upper and lower plates. It consists of a double form bar linkage. One of the members carries a set of two rollers and another member carries a set of two rollers and a slot to accommodate the scram rod eccentric shaft. The rollers are held in place by the plates and the two members of the four bar linkage. 

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