TECHNIQUES AND RESULTS OF EXAMINATION OF FISSION PRODUCT RELEASE FROM VVER FUEL RODS WITH ARTIFICIAL DEFECTS AND A BURNUP OF ~60 MWd/kgU AT THE MIR LOOP FACILITY РНЦ КИ 8th International Conference on VVER Fuel Performance, Modelling and Experimental Support 26 September – 04 October 2009, Helena Resort near Burgas, Bulgaria А.V. Burukin, А.V. Goryachev, S.А. Ilyenko, A.L. Izhutov, V.V. Konyashov, V.Yu. Shishin, V.N. Shulimov JSC “SSC RIAR”, Dimitrovgrad L.M. Luzanova, V.N. Miglo RRC “Kurchatov Institute”, Moscow, Russian Federation

2 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria CONTENTS Introduction 1. EQUIPMENT AND TECHNIQUES 1.1 PV-1 loop facility of the MIR reactor, equipment used for radiation measurements and sampling 1.2 Procedure of fission product sampling and activity measurement 1.3 Irradiation rig and experimental fuel rods 2. RESULTS OF IRRADIATION TESTS 3. MAIN RESULTS OF POST-IRRADIATION EXAMINATION 4. RESULTS AND DISCUSSIONS Conclusions

3 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria PURPOSE OF EXAMINATION On the basis of experimental data on radioactive fission product (RFP) release from the defective high-burnup VVER fuel rods under conditions simulating typical design-basis operation scenarios of a power reactor, it is possible to justify radiation safety of NPP taking into account the effect of burnup level on RFP release from defective fuel rods into the primary coolant of the operating reactor and to verify calculation codes for prediction of radiation environment in the primary circuit of the VVER reactor. INTRODUCTION

4 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria INTRODUCTION IRRADIATION TEST TASK During the first test, which was conducted at the PV-1 loop facility of the MIR reactor, release of gaseous and main dose-forming volatile RFP from an experimental defective re-fabricated fuel rod was measured; this fuel rod was prepared using a full-size VVER fuel rod operated in the NPP power reactor. The re-fabricated fuel rod was irradiated under quasi-steady-state conditions which correspond to operating parameters of the VVER-1000 fuel rods. That was a methodical test which aimed at testing of new equipment and techniques in order to assess their appropriateness for the further test run.

5 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria Preparatory stages before testing involved the following activities: -analysis of technical characteristics of the MIR loop facilities in order to select the most appropriate one taking into account predicted coolant activity and implementation of different design-basis scenarios for fuel operation providing compliance with safety requirements; -analysis of characteristics of the modern spectrometric equipment in order to assess whether measurements of predicted coolant activities are sufficient taking into account acceptable measurement intervals which allow tracing of activity change kinetics during the test; -design, manufacture and installation of equipment for irradiation test at the MIR loop facility; the equipment should provide application of required examination techniques; - development of equipment for application of calibrated artificial through defects on claddings of re-fabricated fuel rods. 1. EQUIPMENT AND TECHNIQUES

6 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 1.1 PV-1 loop facility of the MIR reactor, equipment used for radiation measurements and sampling 1 – loop channels; 2 – circulating pumps; 3 – pressure compensators; 4 – gas tanks; 5 – heat exchanger of PVP-1 loop facility; 6 – main heat exchanger; 7 – mechanical filter; 8 – purification heat exchanger; 9 – ion-exchange filter Layout of PV-1 loop facility of the MIR reactor

7 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 1 – ion-exchange filter; 2 – portable sample container; 3 – chamber wall of PV-1 loop facility; 4 – flow-through measuring tank; 5 – collimator; 6 – HPGe detector Equipping of the PV-1 loop facility primary circuit with auxiliary radiation measuring instruments 1.1 PV-1 loop facility of the MIR reactor, equipment used for radiation measurements and sampling 6

8 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 1.1 PV-1 loop facility of the MIR reactor, equipment used for radiation measurements and sampling 1 – ion-exchange filter; 2 – portable sample container; 3 – sample syringe; 4 – HPGe detector; 5 – buffer gas vessel; 6 – water vessel Flowsheet of coolant sampling system and sample handling operations 2 5 6

9 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 1.1 PV-1 loop facility of the MIR reactor, equipment used for radiation measurements and sampling 1 – gamma ray detector; 2 - radiation (background) shielding of detector; 3 - radiation collimator; 4 - shadow shielding of measuring tank; 5 – hole in the shielding of measuring tank; 6 - measuring tank; 7 - cutoff valves of measuring tank Gamma spectrometry measurement area for the coolant samples at PV-1 loop facility Coolant inlet Coolant outlet PC for control and primary processing DSPec-jr-2.0 instrument To MCS of PV-1

10 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 1.2 Procedure of fission product sampling and activity measurement Complementary procedure and system for measurement of specific activity of radionuclides can be applied simultaneously during irradiation testing: - on-line measurement system which provides detailed information about short-lived high-activity fission products with a short half-life period (5-10 min); -sampling technique which provides quantitative information about long-lived low-activity fission products. Sampling: Sampling frequency – every day; Separation of coolant phases – immediately after sampling; Measurement of radiation spectrum of coolant fractions after sampling: - in min; - in ~ 1.5 h; - in ~ 1 day; - in ~ 3 days; - in ~ 7 days. «On-line» measurements: Measurement of radiation spectrum of coolant – continuously during irradiation, exposure time min.

11 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 1.3 Irradiation rig and experimental fuel rods Irradiation rig and experimental fuel assembly For testing an irradiation rig was developed and manufactured; it provided thermophysical parameters and a coolant velocity diagram near the experimental fuel rods which are similar to the standard ones in a full-size VVER-1000 fuel assembly. The experimental fuel assembly, which contained seven fuel rods and was similar to the fragment of the VVER-1000 fuel assembly at the cross-section, formed a part of the irradiation rig. The experimental re-fabricated fuel rod was inserted into the central cell and surrounded by six fuel rods with unirradiated fuel. Fuel rod with an artificial defect Design of the experimental fuel rod was selected from considerations of its similarity to the full-size VVER-1000 fuel rod as to the active part and gas plenum volume. The active fuel length was ~ 1000 mm; it corresponds to the MIR core height. The artificial defect was applied on the cladding in form of a through hole 1 mm in diameter; it was applied in the active part with the maximum heat release at a height of ~ 240 mm from the fuel rod end plug.

12 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 2. RESULTS OF IRRADIATION TESTS Duration of irradiation of the re-fabricated fuel rod for the first test depended on a standard cycle of the MIR reactor and made up ~22 days; it allowed achieving equilibrium activity levels of most of the recorded RFP in the PV- 1 primary coolant Change in maximum fuel temperature, temperature (Тc) and pressure (Рout) of PV-1 primary coolant during testing Change in maximum (MLHGR) and average (ALHGR) LHGR of the fuel rod with artificial defect during testing

13 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 2. RESULTS OF IRRADIATION TESTS Change in specific activity of iodine radionuclides in the coolant during testing Change in specific activity of inert radioactive gases in the coolant during testing

14 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 3. MAIN PIE RESULTS Test performed by the above techniques did not reveal any noticeable changes of fuel rod state except for the strained area with a crack in the gas plenum area. Cladding surface of the active part, the artificial defect and the areas near this defect didn’t undergo any noticeable changes. Non-destructive examination Inspection and photographing of fuel rod Gamma scanning Profilometry measurements X-ray radiography Areas of the fuel rod cladding with the artificial defect after testing: (a) artificial defect, (b) through defect in the gas plenum area a) b)

15 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria No RFP redistribution and fuel wash-out through the artificial defect No change in fuel rod diameter over the full active fuel length No gaps between the fuel pellets; No damage of pellets Results of gamma scanning of the experimental fuel rods in the area of the artificial defect after testing Inspection and photographing of fuel rod Gamma scanning Profilometry measurements X-ray radiography Non-destructive examination 3. MAIN PIE RESULTS

16 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria Destructive examination Metallographic examination Electron probe microanalysis - fuel is fragmented into 4-6 parts by radial cracks mainly ; - diameter of central hole in the fuel column is 2.3–2.4 mm over the full length of the experimental fuel rod; - rim-layer is ~ 60 μm in width; - no fuel-cladding gap over the full length of the experimental fuel rod; - fuel-cladding interaction layer is ~ 10 μm in width Fuel microstructure of experimental fuel rod with artificial defect after testing at different cross-sections: (a)cross-section ~500 mm above end plug, (b) cross-section near the artificial defect 3. MAIN PIE RESULTS b) a) Boundary with central hole Center of pellet Boundary with cladding

17 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 1. PIE showed that the fuel-cladding gap of the experimental fuel rod was bridged. A close thermal contact between fuel boundary area and cladding caused the rim-layer temperature to be within limits specified for normal operation of an original full-size VVER fuel rod. This observed effect and absence of fuel surface oxidation, which is probably caused by a close contact between fuel and cladding, suggest that only gaseous and volatile fission products, which were accumulated in the central hole of the fuel column, released into the primary coolant of the PV-1 loop facility through the cladding defects (artificial defect and defect formed in the gas plenum area during testing). 4. RESULTS AND DISCUSSIONS

18 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 2. Defect in the gas plenum area as a result of cladding strain and rupture was formed in the earliest test stage due to thermal expansion of the coolant if the fuel rod reached a specified LHGR. Change in MLHGR of fuel rod and activity of PV-1 primary coolant in accordance with sensor readings in the course of bringing reactor to power 4. RESULTS AND DISCUSSIONS

19 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 4. RESULTS AND DISCUSSIONS Change in MLHGR of fuel rod and Pout before/after test completion in the course of MIR reactor shut-down and decrease of pressure in the PV-1 primary circuit Change in 131 I, 134 I and 137 Cs activity in the coolant under the above conditions

20 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 3. Significant size of the defect, which was formed in the gas plenum area, reduced appreciably (or practically eliminated) inhibition of volatile and gaseous fission product release from the free volume of fuel rod into the coolant. If we compare size and location of the artificial defect and the through defect formed on the cladding during testing, we can suppose that iodine from the central hole of the fuel column released completely and without interruption through the macrodefect in the gas plenum area. 4. RESULTS AND DISCUSSIONS

21 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria CONCLUSIONS 1. Complex of equipment and several techniques for examination of RFP release from defective fuel rods were developed, prepared and tested at the PV-1 loop facility of the MIR reactor. 2. During the first test, which was conducted at the PV-1 loop facility and aimed at testing of developed equipment and techniques, measurement of RFP release from an experimental re-fabricated fuel rod with a burnup of ~60 MWd/kgU and an artificial defect was performed under design-basis steady-state operating conditions of the VVER-1000 reactor.

22 РНЦ КИ 26 September – 04 October 2009, Burgas, Bulgaria 3. PIE of all main parameters of the experimental defective fuel rod did not reveal any state peculiarities which could be caused by the artificial defect, i.e. fuel and cladding characteristics in the defect area did not differ from the initial ones (before testing) as well as their characteristics in areas distant from the defect; they are typical for fuel rods with a similar irradiation history in the VVER NPP. The gap in the experimental fuel rod was bridged due to close contact between fuel and cladding at increased fuel burnup; it can appreciable reduce RFP release into the PV-1 primary coolant. This suggestion and quantitative characteristics of effect of gap bridging in a high-burnup fuel rod on RFP release should be investigated during the next tests performed at the PV-1 loop facility. 4. Values of RFP release measured during the first test at the PV-1 loop facility in the MIR reactor will be used for development of an empirical engineering model in order to take into account high burnup effects and their impact on fission product release from fuel and defective fuel rods. CONCLUSIONS

23 РНЦ КИ THANK YOU FOR ATTENTION! ACKNOWLEDGMENTS The authors would like to express profound gratitude to the specialists of JSC TVEL, especially to Mr. А.I. Sharikov, for his great assistance and active participation in management of the tests in the MIR reactor