Worth of control rod

Worth of control rods U-23S londing per dements -9.2% AKAC -7.0% AK/K... 

CONTROL RODS AND DRIVE MECHANISMS (cont.) 5.6. Worth of control rod... 

Describe the effects of moderator temperature on the worth of control rods. 

The control rod drive mechanisms of the first control rod system (traditional, active type) are placed on the vessel top head the passive control rod system is within the reactor vessel. A core map with the indication of control rod positions is shown in Figure IV-7, the worth of control rods is given in Table IV-2. 

A neutron absorber of the control rods is 90% enriched B4C, which is the design solution to increase the reactivity worth of control rods and reduce the number of Rod Cluster Controls (RCCs). 

The control engineer will be able to obtain data on the items above from the reactor designers in the first instance and then from reactor physicists employed to work on the reactor, who can be expected to refine the match to the operational reactor by collecting extensive operating data and performing experiments where necessary. For example, it is customary to perform experiments to calibrate the worth of each control rod. In this case the data is usually given in the form of a graph of control rod insertion distance... 

Nominal Reactivity Control Worths The calculation of control rod and reserve shutdown control (RSC) worths under both hot and cold conditions have been performed for both the initial cycle BOG conditions and the equilibrium cycle EOC condition. In addition, the worth of all 30 control rods has been calculated for other times in cycle for both the initial core and an equilibrium reload cycle to determine how the total control rod bank worth is expected to change over the cycle. Other specific rod pattern control worths for hot conditions for the selected withdrawal of groups of three rods each in the outer bank of control rods were analyzed to define the maximum group worth for use in the transients analyzed in Chapter 15. These calculations were only performed for the EOC equilibrium core loading since that cycle condition yields the minimum temperature coefficient of reactivity and the maximum rod group reactivity worth for a rod group withdrawal transient. No reduction in control rod poison worth due to burnup has been assumed in this or other EOC rod worth calculations discussed below, although this effect would be minimal. 

Reserve Shutdown Requirements and Reactivity Worths The primary reactivity requirement for the reserve shutdown control (RSC) is to maintain shutdown (k < 0.99) indefinitely at or below the refueling temperature 192 C (377 F). Any partially inserted control rods or other fully withdrawn control rods are assumed not to be inserted in determining the RSC capability to meet this requirement, i.e., the maximum core operating excess reactivity is assumed to be held down by control rods and this excess reactivity is not a component of the requirement for the RSC. The reserve shutdown control equipment (RSCE) is also required to trip following the trip of the outer bank of control rods, and after some delay, for transients initiated by moisture ingress during power operation. 

Results with the previous calculation scheme were giving C/E values of about +5 to 20%. The fact that the comparison is significantly improved indicates that both the method and the nuclear data have been improved and that there remain probably no compensating effects. The results obtained with the ERANOS calculation scheme are therefore satisfactory and this scheme can be considered to be reliable as an explicit treatment of all shadowing and transport effects is taken into account. This good behaviour is also observed for the prediction of control rod reactivity worth in the Phenix reactor. There is only a ring of 6 control rods in this reactor. The results are gathered in Table 5. 

Theoretically, a single rod may reach a worth of two per cent or more (as an example, a rod at the centre of the core with all the other rods inserted, which increases the worth of the rod) but the reactivity corresponding to the ejection of any rod (one of the DBAs) is always kept below the prompt reactivity value (0.6 per cent) typically a limit of 0.5 per cent is adopted. The integrated worth of a control rod has the shape shown in Figure 4-5. 

SEPARATION OF CONTROL ROD FROM ITS DRIVE AND BWR HIGH ROD WORTH EVENTS... 

The experiments that have been performed are standard. Critical configurations have been determined for many different compositions. Reactivity coefficients have been determined by period measurements or by use of control rods calibrated by period measurements. Reactivity worths have also been determined analysis of the response to oscillator and rod drop experiments. Rossl-o measurements have been made on a number of assemblies to determine the ratio (rf the effective delayed neutron fractlqn to the prompt neutron lifetime and thus indirect to give information on the neutron spectrum. Detector responses, both as a function of detector material and as a function of position, have been made to determine data relevant to power distributions, bucklings, reflector savings, and neutron spectra. Spectrum measurements have been made by use of ehiulslon plates. 

These two critical configurations provide an interesting variation on the control rod worth experiments. Being critical, they are more clearly defined than the subcritl-cal experiments. However, they require careful interpretation to give a check of the worth of the rods in terms of reactivity because the core sizes are different. They are clearly separate benchmark experiments. 

Of the conventionai cores, only ZPPR-9 was constructed for basic physics data, and such parameters as power distribution. reaction rate ratios, sodium-void reactivities, material and Doppler reactivity worths, and control rod worths were... 

With the normal control rod patterns required to maintain an acceptable power distribution in the operating core, an average control rod will be worth about 0.005 dk effective. The maximum worth of a rod in a typical power operation pattern will be about 0.01 dk effective. The notch increment dimensions and spacing of the rods are set to limit the reactivity insertion to about 0.0003 dk/k for any notch increment of control withdrawn. Preplanned withdrawal patterns and procedural patterns and procedural controls are used to prevent abnormal configurations yielding excessively high rod worth. 

It is of interest to mention that one of the important effects of a control rod is the increased leakage it produces at the outer surface of the reactor and that this, rather than the extra neutrons absorbed, may be the predominant factor in determining the worth of the rod. 

The reactivity worth of a rod cluster control assembly is limited to preclude rupture of file coolant pressure boundary or disruption of the core internals to a degree that would impair core cooling capability due to a rod withdrawal or an ejection accident (Section 4.3.1.4.1 of Reference 6.1). 

Following criticality and prior to operation at power levels greater than 5% of rated power, physics tests are performed to verify that the operating characteristics of the reactor core are consistent with design predictions. During these tests, values are obtained for the reactivity worth of control and shutdown rod banks, isothermal temperature coefficient and critical boron concentration for selected rod bank configurations. 

Rod shadow Is an effect in which the repositioning of one control rod changes the reactivity worth of adjacent rods or causes a change in power level indication on neutron detectors when power level has remained constant. Rod shadow is caused by flux redistributions within the core as a result of rod motion. 

Control rods at the center of the core are exposed to a higher thermal flux than those at the core periphery and, therefore, have a greater worth. It is possible for a thermal flux peak to exist anywhere radially within the core which would increase the worth of a rod within that flux peak even if the rod were near the edge of the core. Generally, any parameter change within the core that changes thermal neutron flux will change rod worth. 

The minimal core height is 66 cm, with a diameter of 358 cm, and includes a total of 192 fuel assemblies (Table 9.7). The small sodium void worth could be reduced by further pancaking the core, which would come at the expense of increases in the bumup reactivity loss. Initial evaluations of postulated accident sequences indicated attractive safety characteristics for the burner core, comparable to a conventional (breakeven) core design. Since the reactivity insertion from withdrawal of control rods is larger for the pancaked core, smaller incremental movements (rod stop adjustments) are required. 

Apart from the clad melt mentioned above, it is difficult to find plausible mechanisms for significant reactivity transients in a gas-cooled thermal reactor. The number of control rods tends to be large, so that the maximum reactivity worth of a single rod is usually of the order of 0.1 %, in comparison with the 2% or so possible in a light water reactor. Consequently, rapid... 

Type of control rod drive mechanism (CRDM) Number of CRDMs Absorber rods per control assembly Absorber material Control Rod Worth ... 

Zhou, X., Liu, G., 2013. Study of control rod worth in the TMSR. Nuclear Science and Techniques 1, 010. 

By 1 (X) am, on April 26, the operators were able to stabilize the power back at 2(K) MWt, but this was as high as they could get it due to the xenon poison buildup that had started during the excursion to lower power and was still continuing. To drag the reactor up to 200 MWt, the operators had pulled far too many of the manual control rods out of the reactor, and the neutron flux distribution in the core was such that the reactivity worth of those rods that would be effective in the first few centimeters of travel back into the core was limited to the equivalent of six to eight fully inserted rods. 

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