CONFORMITY BETWEEN LR0 MOCK UPS AND VVERS NPP PRV ATTENUATION

Transcription

1 CONFORMITY BETWEEN LR MOCK UPS AND VVERS NPP PRV ATTENUATION D. Kirilova, K. Ilieva, S. Belousov Institute for Nuclear Research and Nuclear Energy, Bulgaria address of main author: Abstract. The calculational results of neutron flux attenuation through the reactor pressure vessel () of VVER type of reactors and their Mock-ups have been compared. The neutron flux with energy above.5 MeV used for metal irradiation damage evaluation has been considered. The VVER-44 attenuation results of Kozloduy NPP Unit 4 with standard core loading and Unit 3 with dummy cassettes core loading have been used for the comparison. The results of Unit 6 have been considered for VVER-1. The Mock-ups for VVER-44 type of reactor, standard and dummy cassettes loadings, and VVER-1 type of reactor, created at the Czech critical assembly LR in NRI, Rez, have been addressed. For the VVER- 44 the neutron flux attenuation through the and its Mock-up has consisted within the calculation uncertainty. For the VVER-1 the neutron flux attenuation through the and its Mock-up has consisted also within the calculation uncertainty except the region behind where the attenuation difference has been significant. It has been demonstrated that this difference is due to the different biological shielding material used for the VVER-1 and its Mock-up. The obtained results have been basis to conclude that the Mock-ups of VVER-1 and VVER-44 type of reactors could be used for experimental simulating of the neutron flux attenuation in the nuclear power plants (NPP). However the NPP application of Mock-up results, especially for the VVER-1, has to be done taking into account the existing peculiarity of the biological shielding as well as the attenuation dependence on the azimuth. 1. Introduction The comparison between the attenuation results of the Reactor Pressure Vessel () of nuclear power plants (NPP) VVERs and their LR Mock-ups first of all is needed in order to qualify the application of the mock-ups as benchmarks only, or/and as experimental tool for simulating of the NPP irradiation conditions. The LR Mock-ups have been carried out at the Czech critical assembly LR in the Nuclear Research Institute in Rez near Prague [1; 2, 3]. The attenuation factor AF, equal to the ratio of the neutron flux/fluence value at the considered position through and behind the with thickness T, to its value at the position onto inner wall, has been used for the evaluation. The considered radial positions and azimuth directions have corresponded to the Mock-ups measurements. A flux/fluence with energy above.5 MeV has been used because this neutron fluence is applied in the Russian standard [4] for evaluation of the shift of radiation embrittlement temperature for VVER s steel. The neutron flux results have been obtained by three-dimensional calculations with discrete ordinate code TORT [5] and problem oriented multigroup neutron cross-section library BGL [6]. 1

2 2. Conformity study for VVER-44 The radial positions of VVER-44 considered for comparison of the attenuation through the are the following: point 3 - at the inner wall, point 4 at 1/3T, point 5 at 2/3T, and point 6 - behind the. The axial position has been taken on the level of the seam weld 4. The azimuth direction for which the comparison has been carried out is equal to º and corresponds to this one of the realized measurements at LR Mock-ups. The comparison has been carried out for two configurations of the core loading scheme: standard (Fig. 1) which has been modelled by LR Mock-up No 1 (Fig. 2) and core loading with dummy cassettes placed in the periphery of the core (Fig. 3) modelled by LR Mock-up No 2 (Fig. 4). In the LR Mock-ups of VVER-44 the simulator has consisted of three 1/3T thick steel slabs, which have been moved in radial direction so to provide an ability of consequent measurements in an gap of 65 mm width or between the separated slabs, or in front or behind the simulator. Four different calculation configurations for every Mock-up have been used for modelling of corresponding measurements. The configuration with the gap in front of simulator (Fig. 2) has been used for flux calculation at point 3. Every consequent configuration has been obtained by shifting of corresponding simulator slab in the active core direction. The configurations with gap for measurements at 1/3T, 2/3T and behind the simulator (Fig. 4) have been used for separate Mock-up calculation at points 4 to 6 correspondingly [3]. 2

3 I. Reactor Core II. Steel Shell III. Moderator (Water) IV. Core Basket V. Coolant (Water) VI. Barrel VII. Coolant (Water) VIII. Cladding IX. Vessel X. Air Cavty XI. Steel Shell XII. Thermal Insulation XIII. Steel Wall of Water Tank XIV. Water Tank XV. Steel Wall of Water Tank I V VII VIII IV VI II III XII X XI XIII IX XIV XV Fuel Assembly Control Rod FIG. 1. Geometry model of VVER-44, standard core loading FIG. 2. Mock-up No. 1 of VVER-44, standard core loading 3

4 The AF values at the considered positions (point 3 to 6) for LR Mock-up (Mockup) and VVER-44 (VVER), as well as the relative difference (Mockup/VVER-1) between the AF results are presented in Tables I-II. I. Reactor Core II. Steel Shell III. Moderator (Water) IV. Core Basket V. Coolant (Water) VI. Barrel VII. Coolant (Water) VIII. Cladding IX. Vessel X. Air Cavty XI. Steel Shell XII. Thermal Insulation XIII. Steel Wall of Water Tank XIV. Water Tank XV. Steel Wall of Water Tank I IV III II V VI VII VIII XII X XI XIII IX XIV XV Fuel Assembly Control Rod Dummy Cassette FIG. 3. Geometry model of VVER-44, dummy cassettes loading FIG. 4. Mock-up No. 2 of VVER-44, dummy cassettes loading 4

5 The flux attenuation has been evaluated basing on the neutron flux/fluence calculations carried out for Kozloduy NPP Unit 4, cycle 13&14, with VVER-44 standard core loading and for Unit 3, cycle 16&17, with VVER-44 core loading with dummy cassettes. Table I Comparison of attenuation factor AF for VVER-44 with standard core loading (Mock-up 1) Point No AF(Mockup) AF(VVER) (Mockup/VVER -1), % Table II Comparison of attenuation factor AF for VVER-44 with dummy cassettes core loading (Mock-up 2) Point No AF(Mockup) AF(VVER) (Mockup/VVER -1), % In both cases of core loading the relative difference is negative through the whole thickness and does not exceed 1%. This difference is in the limits of neutron fluence calculation uncertainty [7]. Based on this good consistency it could be concluded that the VVER-44 Mock-ups simulate well enough the irradiation conditions at NPP and the obtained results are appropriate for NPP reactor dosimetry application. 3. Conformity study for VVER-1 The radial positions in VVER-1 considered for comparison of the attenuation through the are the following: point 3 - at the inner wall, point 4 at 1/4T, point 5 at 2/4T, point 6 at 3/4T, and point 7 - behind the. The axial position has been taken on the level of the seam weld 3. The azimuth direction of comparison is equal to º and corresponds to this one of the realized measurements at LR Mock-up [8]. The comparison has been carried out for a standard core loading of VVER-1 (Fig. 5) and its LR Mock-up (Fig. 6). The Mock-up simulator has consisted of four 1/4T thick steel slabs, which as in case of VVER-44 Mock-up can be moved in radial direction so to provide an ability of consequent measurements in an gap of 65 mm width or between the separated slabs, or in front or behind the simulator. Five different calculation configurations for VVER-1 Mock-up have been used for modeling of corresponding measurements. The configuration with the gap in front of simulator (Fig. 6) has been used for the flux calculation at point 3. Every consequent configuration has been obtained by shifting of the corresponding simulator slab in the active core direction. The configurations with gap for measurements at 1/4T, 2/4T, 3/4T, and behind the simulator have been used for separate Mock-up calculation at points 4 to 7 correspondingly [1, 2]. 5

6 ф Concrete Shaft Dry (Biological) shield Thermal Insulation o Air Cavity Vessel Cladding Coolant Barrel 4 ф72 75 Reactor Core Coolant Baffle o 8 R Thermal Shield o 1533 Channel r, mm d, mm FIG. 5. Geometry model of VVER-1 FIG. 6. VVER-1 Mock-up 6

7 The AF values at the considered positions (point 3 to 7) for LR Mock-up (Mockup) and VVER-1 (VVER), as well as the relative difference (Mockup/VVER-1) between the AF results are presented in Tables III. Table III Comparison of attenuation factor AF for VVER-1 Point No AF(Mockup) AF(VVER) (Mockup/VVER -1), % The VVER-1 flux attenuation values have been taken from the calculations carried out for Kozloduy NPP Unit 6, cycle 5. The relative difference again, as in case of VVER-44, is negative and less than 1% that does not exceed the limits of neutron fluence calculation uncertainty [9] through the thickness up to 3/4T depth. However the relative difference changes the sign and reaches 18% behind the (point 7), which is demonstrated on Fig. 7. Relative Units Relative Units Mockup/VVER-1, % VVER Mock-up bhd 24 Radius, cm Fluence distribution normalized to the value in the cladding cladding cladding cladding T 1/4T 1/2T 3/4T concrete cladding concrete thermal insulation concrete Ratio FIG. 7 Fluence attenuation through of NPP and Mock-up (VVER-1) To understand the reason for this difference at point 7 (behind the ) an additional analysis has been carried out. The VVER-1 attenuation results have been compared with those obtained for a modified Mock-up geometry model which corresponded to the configuration 7 with the same positions of comparison but including the VVER-1 shielding (NPP concrete 7

8 covered with thermal insulation [1]) instead of the Mock-up biological shielding (Mock-up concrete with cladding). The results of this comparison are demonstrated on Fig. 8. In this case the relative difference behind the does not exceed 1%. Mockup/VVER-1, % Relative Units Relative Units Fluence distribution normalized to the value in the cladding cladding cladding T cladding 1/2T 1/4T 3/4T thermal insulation bhd concrete Radius, cm thermal insulation concrete VVER Modified Mock-up Ratio FIG. 8 Fluence attenuation through of VVER-1 and the modified Mock-up (VVER- 1) The last comparison let us to conclude that the main raison for the obtained inconsistency between the VVER-1 and the Mock-up attenuation (Tables III) is the difference between the VVER-1 and the Mock-up biological shielding. The density of the Mock-up concrete is g/cm3 [8]. The density of the VVER-1 thermal insulation homogenized over the steel layers and so-called staffol [1] (25 steel layers of.1 mm thickness each one, and gaps between layers) is about 1 g/cm3. Consequently the reflection of the neutron flux with energy above.5 MeV in the gap of VVER-1 from the thermal insulation is less than the reflection from the biological shielding simulator of the Mock-up. Additionally, measured and calculated Mock-up attenuation data are presented in the Table IV [1]. The comparison of the results (Cal/Exp-1) shows a consistency within 15 %. This result is in accordance with the requirement of the Russian standard limiting the uncertainty of the neutron fluence determination up to 15% [4]. 8

9 Table IV: Relative deviations of calculated from measured attenuation factors for the Mockup for neutrons with energy above.598 MeV E thr =.498 Exp. Exp. MeV AF Std. Cal. Cal/Exp-1 Point No dev. % % Azimuth dependence of the attenuation In the previous sections a good consistency of the Mock-ups and VVER-1 attenuation results has been shown. That is favorable for the justification of the Mock-up results application to the NPP. This consistency however has been demonstrated for definite azimuth directions specified by Mock-ups configurations. Study of the dependence of the attenuation results on the azimuth direction has been carried out. The attenuation factor AF for different azimuth positions behind the of VVERs has been calculated in order to quantify the azimuth dependence. The values of AF normalized to the AF averaged over the azimuth directions are presented on Fig VVER-44 standard VVER-44 dummy VVER-1 1 AF/AFm-1, % Azimuth, deg FIG. 9. Azimuth dependence of AF normalized to the mean azimuth value 9

10 In case of VVER-44 the variation of AF along the azimuth is less than 7%. This value does not exceed the calculation uncertainty and could not be considered as significant. This means that the measured VVER-44 Mock-up attenuation results could be applied for other azimuth directions within 1% uncertainty. In case of VVER-1 the variation of the AF along the azimuth is higher (more than 18%) than the calculation uncertainty and could not be neglected. Since the azimuth of maximal exposure is about 8º for VVER-1 the AF especially in this direction is of a particular interest for metal embrittlement assessment. That is why the AF azimuth dependence has to be taken into account when the VVER-1 Mock-up results are applied for NPP. 5. Conclusion The conformity between VVER Mock-ups carried out at the Czech critical assembly LR in Rez and NPP VVERs based on the attenuation through the of the neutron flux/fluence with energy above.5 MeV has been studied. The relative difference of the attenuation factor does not exceed 1% for VVER-44. In VVER-1 case the relative difference of the attenuation factor is negative and does not exceed 9% through the thickness. The flux attenuation behind the of VVER-1 is 18% higher than the attenuation behind its Mock-up. This inconsistency is a result of the difference in the biological shielding behind the for the Mock-up and VVER-1. Regarding the benchmarking of VVER-1 Mock-up the good consistency between calculated and measured attenuation factor is in accordance with the requirement of the Russian standard limiting the uncertainty for determination of the neutron fluence by 15%. The obtained results give us confidence that the Mock-up of VVERs could be used as experiment simulating the VVER neutron flux/fluence attenuation. This means that the measured Mock-up flux attenuation could be applied at NPP in the limits of the mentioned uncertainties. The Mock-up results for VVER-1 have to be applied more carefully i.e. to take into account the existing peculiarity of the biological shielding and attenuation azimuth dependence. The obtained results of the two comparisons, Mockup/VVER and cal/exp for the VVER-1 Mock-up, show that the validation of the neutron fluence calculational results carried out by ex-vessel activation detectors positioned behind the is reliable enough as these measurements are practically the only possible for validation of the neutron fluence. 1

11 REFERENCES [1] BALLESTEROS, A., et al., Reactor Dosimetry: Accurate Determination and Benchmarking of Radiation Field Parameters, relevant for Pressure Vessel Monitoring (REDOS), AMES report n.17, EUR EN (25), [2] OŠMERA, B., et al., Data Base for WWER-1 Pressure Vessel IAEA Regional Project RER 4/17, Reactor Dosimetry, ASTM STP 1398, John G. Williams, David W. Vehar, Frank H. Ruddy and David M. Gilliam, Eds., American Society for Testing and Materials, West Conshohoken, PA (21) [3] ZARITSKY, S., et al., Reactor Vessel Dosimetry Benchmarks for Commercial VVER-44 Plants, Transactions of ANS, Vol. 77 (1997) 347 [4] Regulations and Calculation Norms for Equipment and Tubing Strength of the Nuclear Energy Facilities, PNAE G , Gosatomenergonadzor, Moscow, Energoatomizdat (1989) (In Russian) [5] RHOADES, W.A., CHILDS, R.L., TORT Three Dimensional Discrete Ordinate Neutron/Photon Transport code with Space-Dependent Mesh and Quadrature, ORNL-6268 (1987) [6] BUCHOLZ, J. A., ANTONOV, S. A., BELOUSOV, S. I., BGL44 and BGL1, Broad Group Neutron/ Photon Cross-Section Libraries Derived from ENDF/B-VI Nuclear Data, IAEA INDC (BUL)-15, Distrib.:G (1996) [7] BELOUSOV, S. I., ILIEVA, K. D., MATUSHKO, V. L., Sensitivity Analysis and Neutron Fluence Adjustment for VVER-44, JAI 936, Reactor Dosimetry, 12th Volume (25) [8] OŠMERA, B., ZARITSKY, S., WWER-1 Mock-up Experiment In the LR- Reactor. Mock-up Description and Experimental Data, Nuclear Research Institute Řež, ÚJV R REDOS / R(1) / / Issue 1/ (22) [9] BELOUSOV, S. I., ILIEVA, K. D., Sensitivity Analysis and Neutron Fluence Adjustment for VVER-1, Reactor Dosimetry in the 21st Century, World Scientific, Jan Wagemans et al., Eds., New Jersey, London, Singapore, Hong Kong. (23) [1] Passport of VVER-1/32, Thermal shielding, Unit 5, Kozloduy NPP, (in Russian) 11