Outlet Temperature Measurement Comparison of Gd and Non Gd Fuel Assemblies at Dukovany NPP

OUTLET TEMPERATURE MEASUREMENT COMPARISION

OF GD AND NON-GD FUEL ASSEMBLIES

AT DUKOVANY NPP

Monika Juříčková

Nuclear Research Institute Rez, Czech Republic

ABSTRACT

In year 2006 we started data processing from the Dukovany NPP operating history database that contained data from the old measurement system VK3 and the new Scorpio-VVER. The work has been done in cooperation with the reactor physicists at Dukovany NPP. Obtained data from database were compared with calculated parameters from 3D diffusion macrocode MOBYDICK.

During the data processing it was found that the Gd fuel assemblies have different time plot of measured assembly outlet temperature compared to the non-Gd fuel assemblies. The consequent analyses were focused on the detailed outlet temperature comparison of Gd and non-Gd fuel assemblies during their whole in-core life cycle.

In near future it is planned to develop calculation methods for the correction of measured assembly outlet temperature. Consequently, the correction methods will be implemented to the modernized Scorpio-VVER for Dukovany NPP.

Introduction

Dukovany NPP is in operation since 1985. The operational history database contains data from the old measurement system VK3 and the new Scorpio-VVER. Operational data were stored by the VK3 system up to 2002. Digitalized data from system VK3 is available only from year 1990. The new core monitoring system Scorpio-VVER started operation gradually in years 1998-2000.

The data from operational history database is useful for validation of new or upgraded diffusion codes, for studying various measured data (for example temperatures) or for researching the behavior of Gd fuel assemblies. Reactor physicists at Dukovany NPP observed slightly different behavior of the Gd-fuel outlet temperatures. Simultaneously, the differences were observed also during our other purpose analyses.

The consequent analyses were focused on the detailed outlet temperature comparison of Gd and non-Gd fuel assemblies during their whole in-core life cycle.

Input data

For analyses the following data from Dukovany NPP are needed: Operational history database, detailed fuel reloads, and data calculated by 3D diffusion macrocode Mobydick.

The Dukovany NPP operational history database consists of operational parameters from each unit and cycle in selected timestamps (with period from 72 hours in the oldest version of VK3, to 12 hours in Scorpio). The assumed radial power distribution and assumed assembly burnup is calculated with 3D diffusion macrocode Mobydick using Dukovany NPP operating data history database inputs.

Data processing

Due to the necessary amount of processed data, a special program was developed, which is able to process vast quantities of source data and is able to display results in comprehensive graphs.

It is possible to plot various dependences: Measured and calculated assembly temperature and their difference, calculated assembly burnup, calculated relative assembly power, time, etc. Data can be filtered in accordance to various criteria, for example position of 60-degree symmetry, fuel type, fuel age, unit, cycle, etc., and their combinations. Colors and marks can represent fuel age, fuel type, fuel ID, unit or cycle.

Dependence the difference between measured and calculated temperature on burnup is shown on graphs (Fig. 1 - Fig. 3). The dependence on burnup is approximately constant for each cycle for the 3.8% enrichment fuel. For Gd-1 and Gd-2 fuel the dependence on burnup is approximately constant for the 2nd year in core and following cycles. Only for 1st year the temperature difference shows gradual decrease to burnup approx. 7000 – 9000 MWd/tU.

Fig. 1Calculated / measured temperature comparison - 3.8% Enrichment fuel assembly

Fig. 2Calculated / measured temperature comparison - Gd-1 fuel assembly

Fig. 3Calculated / measured temperature comparison - Gd-2 fuel assembly

Discussion

The previously described difference between the measured and calculated temperature for Gd FA was detected also in Kola NPP-3 and –4 and it was assessed in RRC KI. The main conclusion of the experimental studies was that there is insufficient coolant mixing in the region from the fuel bundle to the fuel assembly thermocouple. Due to this fact the thermocouple measure is systematically higher than real.

The difference between Gd and non-Gd fuel is demonstrated in following cartograms. The cells show the change of difference measured and calculated temperature from beginning to end of cycle. For the Gd-fuel the change is higher then non-Gd fuel or Gd fuel from 2nd cycles.

Fig. 4Difference changes during cycle - Non-Gd fuel, unit 1

Fig. 5Difference changes during cycle - 1st cycle Gd-1 fuel, unit 1

Fig. 6Difference changes during cycle - 1st – 2nd cycle Gd-1 fuel, unit 1

Fig. 7Difference changes during cycle - 1st – 3rd cycle Gd-1 fuel, unit 1

Fig. 8Difference changes during cycle - Non-Gd fuel, unit 3

Fig. 9Difference changes during cycle - 1st cycle Gd-2 fuel, unit 3

Fig. 10Difference changes during cycle - 1st – 2nd cycle Gd-2 fuel, unit 3

Proposed correction

To eliminate the mixing induced error of temperature measurement, a correction method of measured temperature will be developed.

For the development of the correction methods the selected unit and cycle data from Dukovany NPP were used accordingly to the use of Gd fuel on each unit. Only reliable data for consequent analyses must be used and therefore the “unreliable” data were filtered out.

The correction of thermocouple readings was defined as a difference between measured and calculated temperature.

It is possible to fit line in dependence of the correction on burnup. Line is fitted for burnup between 0 and BuHi. BuHi is burnup level, from which the correction is constant. BuHi is approximately 8000 MWd/tU and depends on Gd-1 or Gd-2 type. In the first iteration, the slope seems the same for Gd fuel in the 1st year in core. In future it is planned to assess other possible dependences, for example on relative power of several central fuel rods – we are inspired by the team from Kurcatov Institute and their results presented in AER symposium 2006.

Fig. 11Correction fitting examples – Gd-1, Unit 2, 1st year

Fig. 12Correction fitting examples – Gd-2, Unit 3, 1st year

where

is correction, is measured temperature, is calculated temperature, is asymmetry temperature and is burnup

With known BuHi and b the corrected value of temperature () can be calculated by formula:

This formula is valid for . For there is no correction.

This method corrects only the mixing induced error the asymmetry is not affected.

In the next step parameter b value is analyzed.

The slope was calculated in dependences on type of fuel, unit and position. Then its histograms were generated. It seems that there are two major values of parameter b. Now it is necessary to identify the cause.

Fig. 13Parameter b histogram – Gd-1

Fig. 14Parameter b histogram – Gd-2

Fig. 15Parameter b histogram – Gd-1, Unit 4

Fig. 16Parameter b histogram – Gd-1, Symmetry 2

Fig. 17Parameter b histogram – Gd-1, Symmetry 6

Fig. 18Two values of parameter b example

Conclusion

Using data from operational history database and calculated data from MOBYDICK, different behavior of the Gd-fuel outlet temperatures was exhibited. The insufficient coolant mixing in the region from the fuel bundle to the fuel assembly thermocouple explains the behavior. Due to this fact the thermocouple measure is systematically higher than real.

New calculation methods were outlined for the correction of measured assembly outlet temperature. Development of the calculation methods will be continued and the data from the latest cycles will be included. The developed corrections methods will be verified with the measured data and the successfully verified correction methods will be implemented in the modernized Scorpio-VVER for Dukovany NPP.