Impact of the Hypothetical RCCA Rodlet Separation on the Nuclear Parameters of the NPP Krško core

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1 International Conference Nuclear Energy for New Europe 2005 Bled, Slovenia, September 5-8, 2005 Impact of the Hypothetical RCCA Rodlet Separation on the Nuclear Parameters of the NPP Krško core ABSTRACT Marjan Kromar Jožef Stefan Institute Reactor Physics Division Jamova 39, 1001 Ljubljana, Slovenia Bojan Kurinčič Nuklearna elektrarna Krško Engineering Division - Nuclear Fuel & Reactor Core Vrbina 12, 8270 Krško, Slovenia Bojan.Kurincic@nek.si The separation of a single rodlet from the spider assembly is investigated in a typical NPP Krško cycle. Because the rodlet contains strong neutron absorption material, the neutron distribution in the core is altered. The effect on nuclear parameters of the reactor core is examined. It was determined that the changes in power distribution are not large enough to cause violation of safety limits. However, they are observable by the plant instrumentation system, but the diagnostic of the power anomaly is difficult. The detection of the event is easier at low power conditions. 1 INTRODUCTION A PWR rod cluster control assembly (RCCA) comprises a group of individual absorber rods fastened at the top end to a common spider assembly (Figure 1). The absorber material used is usually Ag-In-Cd alloy sealed into stainless steel tubing. The upper plug is threaded to spider assembly, which has a form of a central hub with radial vanes with fingers. The RCCAs are connected to the control rod drive mechanism controlled by the reactor operator. In the NPP Krško core there are 33 RCCAs, each cluster assembly consists of 20 rodlets. In this paper the separation of a rodlet from spider assembly is postulated. Because the rodlet contains strong neutron absorption material, the neutron distribution in the core is altered. In a NPP Krško typical reload core the effect on nuclear parameters of the reactor core is examined. The CORD-2 package [1], developed at Jožef Stefan Institute, is used to determine core power distribution and reactivity. The package has been validated for the nuclear design calculations of PWR cores. Results presented are obtained with the steady state approximation neglecting any possible transient effects

2 144.2 Figure 1: Rod cluster control assembly 2 RESULTS Power in the fuel assemblies near the inserted rod decreases. Consequently, the relative power in other assemblies increases. The highest increase occurs in the symmetrical positions across the core. Typical power distribution for the ARO (all rods out) and the rodded configuration is presented in Figure 2. The case with a single rod of the D bank inserted at hot full power (HFP) and the beginning of cycle (BOC) is shown. In the rodded assembly average power decreases for in power relative units, while the maximum increase on core symmetrical position is Change in the average power distribution causes also changes in the power peaking factors. In Figure 3 F H power peaking factors for the same case are presented. They are defined as a ratio of the highest integral rod power in the fuel assembly to the core average rod power. Changes in the power peaking factors are almost exactly the same as changes in the average power.

3 ARO Rod in Diff. Figure 2: Assemblywise power distribution for the ARO case and the case with inserted rod from D bank at HFP, BOC

4 ARO Rod in Diff. Figure 3: Assemblywise power peaking factor F H for the ARO case and the case with inserted rod from D bank at HFP, BOC Quadrant power tilt for the same case is presented in Figure 4. Tilt is defined as a ratio of average power in one core quadrant to the core average power. Tilt presented on the left side can be estimated directly by the excore power detectors and is available to the rector operator on line. This quarter power tilt ratio (QPTR) obtained from excore power detectors is limited by the plant Technical Specifications [2].

5 Figure 4: Quadrant power tilt for the case with inserted rod from D bank at HFP, BOC Summary results of all 12 BOC cases are presented in Table 1. Beside the ARO condition there are 11 different rod positions in one core quadrant (Figure 5). Initial quarter core symmetry was assumed in the calculation. In the ρ column the drop of the reactivity from the ARO case is listed. In F H column maximal core power peaking factors are presented. Core power tilts are shown in the last two columns. Maximal values in each column are highlighted. Table 1: Summary results for the HFP, BOC conditions Rod position ρ [pcm] F H Power Tilt 1 Power Tilt 2 ARO D D C C B B A A SA SA SB In Table 2 summary results for the entire cycle are presented. At each burnup step all 12 cases are calculated. The highest changes from the ARO case are given. For the compatibility reasons in the F H column increase from the corresponding ARO case is shown. Only a slight increase in the calculated parameters during the core burnout can be observed. We can see from the results that the maximal reactivity drop of 20 pcm, increase of the power peaking factor by 0.01 and a maximal quadrant power tilt of can be expected in the case of a rod drop event at the HFP conditions. A core designer designs the core loading pattern in such a way that there is at least 1.04 margin to the core power peaking factor limits in the Technical specifications (F H limit is 1.56 at HFP). During the core burnout the margin is most of the time even higher. However, even if we assume that there is a 0.04 margin, an increase of the power peaking factor by 0.01 does not violate the limit. Change in the quadrant power tilt ratio during the regular incore measurements is limited to 2%. Even a maximal rise of is well bellow the limit. We can conclude that a rod drop event is not strong enough to jeopardize core safety limits.

6 144.6 Figure 5: Control rod positions If the event happens during the hot full power operation, reactor operator would observe change in the core reactivity. A reactivity drop would result in the reactor core cooldown if the core thermal power is maintained. 20 pcm would lower average core temperature for approximately 0.8 C at BOC and 0.3 C at EOC. To compensate inserted reactivity operator would have to dilute reactor coolant for approximately 4 ppm. This amount is comparable to the one-day dilution performed by the reactor operator to compensate for core depletion. Change in the power distribution is not so pronounced. However, the differences are observable from the regular incore flux measurements and thermocouple mapping. In recent years NPP Krško uses Westinghouse BEACON-TM [3] system that has a 3-D on-line monitoring capability to allow the operator to make timely, accurate predictions about reactor operation, or analyze monitored power distributions. The BEACON-TM system uses available on line plant instrumentation including nuclear instrumentation and thermocouples to predict core power distribution in a real time. The virtually continuous monitoring of the reactor 3-D power distribution allows not only accurate assessment of safety margins but also anomalous core behaviour can be observed and diagnosed. The system would therefore help to detect anomalous power distribution but it would be difficult to discriminate the event from other plant parameter variations. The detection would be even harder if the fuel assembly with inserted rod is not instrumented or the inserted rod causes smaller power perturbation.

7 144.7 Table 2: Summary results for the HFP conditions at BOC, MOC and EOC. Burnup ρ [pcm] F H Power Tilt 1 Power Tilt 2 BOC MOC EOC Results presented so far were obtained at reactor full power (HFP). Since negative power feedback effects tend to compensate power changes, we expect higher power differences in the zero power range (HZP hot zero power). HZP summary results are presented in Table 3. While the reactivity changes are approximately the same as in the HFP case, power differences are two times larger. Power distribution with the inserted rod from the D bank for the HZP, BOC case is shown in Figure 6. Power peaking and QPTR limits are much higher at the low power conditions (F H limit is at zero power, QPTR is not limited bellow 50 % of the nominal power). Therefore, the violation of the limits is even less probable than in the HFP case. Nevertheless, HZP conditions are more suitable to detect dropped rod than at power conditions, since the power differences from the ARO configuration are larger. Table 3: Summary results for the HZP conditions at BOC, MOC and EOC. Burnup ρ [pcm] F H Power Tilt 1 Power Tilt 2 BOC MOC EOC CONCLUSIONS The separation of a rodlet from the spider assembly is investigated in a typical NPP Krško cycle. Inserted single rodlet would lower core reactivity by 20 pcm in a worst case. Core peaking factors would increase for less than at the HFP conditions and in the HZP case. Quadrant power tilt would increase for less than at the HFP and in the HZP. Changes in power distribution are observable but they are not large enough to cause violation of safety limits. They are detectable but it would be very difficult to discriminate the event from other plant parameter variations. The detection of the event seems to be easier at low power conditions.

8 ARO Rod in Diff. Figure 6: Assemblywise power distribution for the ARO case and the case with inserted rod from D bank at HZP, BOC REFERENCES [1] A. Trkov, M. Ravnik, CORD-2 Package for PWR Nuclear Core Design Calculations, Proceedings of the International Conference on Reactor Physics and Reactor computations, Tel-Aviv, Jan. 1994, Beer-Sheva, Ben-Gurion University of the Negev Press, (1994). [2] Nuclear Power Plant Krško, Technical Specifications. [3] BEACON TM, Westinghouse Best Estimate Analysis for Core Operation Nuclear.