А Е Ц К О З Л О Д У Й - Е А Д N P P K O Z L O D U Y – P L C 17 th Symposium of AER Y alta, Crimea, September 24-28, 2007 WWER-1000 START UP PHYSICS TESTS AT THE KOZLODUY NPP BY COMPARISON WITH THE CALCULATED DATA I. Stoyanova, A. Antov NPP Kozloduy, Bulgaria

Subject This paper presents the results of the start up physics test program carried out after the stage of the reactor core refueling. The aim of the start up physics test program is to prove that the reactor core can be operated as designed. Codes used The Russian code system KASKAD [1,2] has been used for neutron- physics calculations of the WWER-440/1000 reactors at Kozloduy NPP since 2002.

Introduction According to the start up physics test procedures the following neutron physics characteristics are compared with the calculated ones: critical boric acid concentration at Hot Zero Power; isothermal temperature coefficient (ITC); 10 th control rods bank worth. Critical boric acid concentration at HZP A comparison between the measured and calculated data for the last 6 cycles of the Units 5 and 6 is presented in the Table 1. The average difference is lower than 0.3g/kg.

Unit/ cycle Number of Assemblies TVSA Т, С Р 1к, kg/сm 2 Height of control rods bank, % Снзвоз [g/kg] Calc.- Meas. [g/kg] Н 1 9 Н 10 MeasuredCalculated Unit 6/ cycle Unit 6/ cycle Unit 6/ cycle Unit 5/ cycle Unit 5/ cycle Unit 5/ cycle Table 1. Measured and calculated boric acid concentrations at HZP, BOC

Isothermal temperature coefficient (ITC) The difference between measured and calculated ITC for the Unit 6 is in the range of (-6.2÷4.8)pcm/°C as presented in the Table 2a. For the Unit 5 the difference is in the range of (-1.5÷2.4)pcm/°C as presented in the Table 2b. If a good look is taken at the Tables 2a and 2b it can be concluded that the ITC depends on the direction of the temperature changes – whether it increases or decreases. This is due to the different temperatures and even boric acid concentrations into the reactor core and into the pressurizer. The methodical uncertainty of the calculation is about ±3pcm/°C. But there is an uncertainty of the measured values, too. In general, it can be concluded that the results are in good agreement for both – Units 5 and 6.

Table 2a. Measured and calculated ITC at HZP, BOC (Unit 6)

Table 2b. Measured and calculated ITC at HZP, BOC (Unit 5)

10th control rods bank worth The relative errors between measured and calculated integral efficiencies are less than 10%. Table 3. Measured and calculated 10 th control rods bank worth at HZP, BOC

Conclusions The results of the start up physics test program are presented and discussed. It is concluded, that the results of the start up physics tests are in good agreement with the calculated ones.

References [1] Комплекс программ нейтронно-физических расчетов РНЦ КИ Комплекс программ КАСКАД. Инструкция по използованию графического интерфейса комплекса КАСКАД - Бычкова Н.А., Томилов М.Ю., Москва [2] Комплекс программ нейтронно-физических расчетов РНЦ КИ Комплекс программ КАСКАД. Программа БИПР-7А - Суслов А.А., Шишков Л.К., Большагин С.Н., Москва 2002.