1. State Scientific Center– Research Institute of Atomic Reactors
Study of VVER-1000 fuel rods behavior under Loss-of-Coolant-Accident (LOCA). In pile MIR-LOCA/45 and MIR-LOCA/69 tests
© A.V.Alekseev, А.V.Goryachev, О.I.Dreganov, A.L.Izhutov, L.V.Kireeva, I.V.Kiseleva, V.N.Shulimov (JSC «SSC RIAR»),
© V.I.Kuznetsov, V.V.Novikov, A.V.Salatov, P.V.Fedotov (JSC «VNIINM»),
XI Conference on Reactor Materials Science,
in commemoration of 55 th anniversary of material science department of JSC “SSC RIAR”
Dimitrovgrad, May 27-31, 2019

2. Цель экспериментов
Two tests designated as MIR-LOCA/45 and MIR-LOCA/69 were made in the channel of MIR reactor. Test fuel rods were refabricated from the spent VVER-1000 fuel rods irradiated at the 1st unit Kalininsky NPP up to burnup of 45 and 69 MWt d/kgU.
The object of the experiments was to investigate the behavior of the fuel rods with enhanced fuel load under the conditions of the loss-of-coolant-accident: cladding deformation, parameters of the fuel rod depressurization, cladding state, fuel fragmentation.

3. Main tasks
Experimental confirmation of calculated time/temperature parameters of the high burnup fuel rods depressurization in LOCA accident,
Determination of the fuel pin behavior in case of cladding deformation/depressurization,
Examination of the post-test cladding state.

4. The spent mother fuel rods were the fuel rods of new design:
Test fuel rods
The spent mother fuel rods were the fuel rods of new design:
Cladding Ø 9,1×7,93 mm from sponge zirconium based E110 alloy;
Pellet Ø 7,8 mm without the central hole with the grain size of 25 μs.
Test fuel rods were instrumented by pressure transducers and ТC on the cladding surface.

5. Fuel structure of the mother rods
MIR-LOCA/45
MIR-LOCA/69

6. Irradiation rig
The irradiation rig was placed in the channel shroud with the steam-argon medium. The fuel rod was cooled by natural convection of superheated steam and radiant heat exchange.
The irradiation rig provided for five spacers. The original design of spacers was different from the design of spacer grids for the VVER FA. Three central spacers pressed the thermocouples to the cladding surface.
The irradiation rig had an electrical heater to imitate thermal effect of surrounding fuel rods.
Cross-section of irradiation rig
Irradiation rig in the loop channel shroud
Fuel rod positioning in the irradiation rig

7. Tests scenario
The scenario of tests provided the maximal proximity of the fuel rod linear power, cladding temperature, heating rate and the gas pressure difference on the cladding to the calculated scenario of the LOCA second stage.
MIR-LOCA/45
MIR-LOCA/69

8. the peak cladding temperature: - (800 - 820)оС (MIR-LOCA/45)
Main test parameters
Test parameters:
the peak cladding temperature:
- ( )оС (MIR-LOCA/45)
- ( )оС (MIR-LOCA/69),
the average heatup rate of the fuel rod cladding to peak temperature (2 - 4) оС/s,
Imitation of core reflood by cooldown of the fuel rod by water from the bottom at a rate of water level rising of m/s.

9. On-line readings of the cladding TCs
MIR-LOCA/45
MIR-LOCA/69
Bottom spacer TC Top spacer TC Central spacer ТC

10. Calculated maximal cladding temperature Internal pressure (measured)
Calculated peak cladding temperature of the fuel rod and measured internal gas pressure
MIR-LOCA/45
MIR-LOCA/69
Pressure, MPa
Time, h:min
Calculated maximal cladding temperature
Internal pressure (measured)

11. Cladding state after MIR-LOCA-45 test
In the MIR-LOCA/45 test the fuel rod has become depressurized at temperature of 770 оС and the cladding pressure differential of 5 MPa.

12. Cladding state after MIR-LOCA-69 test
In the MIR-LOCA/69 test the peak cladding temperature has amounted to 748 оС. The test fuel rod remained intact.

13. Fuel fragmentation

14. Conclusions
МIR-LOCA/45 and МIR-LOCA/69 tests aimed at studying the behavior of high burnup rods in the conditions of the design basis loss-of-coolant-accident were carried out in the MIR reactor.
In МИР-LOCA/45 test the fuel rod has become depressurized at temperature of 770 оС and the cladding pressure differential of 5 MPa.
In МИР-LOCA/69 test the fuel rod remained intact at cladding temperature of 750 °С and the cladding pressure differential of 5,8 MPa (calculated values for VVER-1000 LOCA accident)
The factors able influence the cladding strain under LOCA conditions are:
High hydraulic fuel stack resistance in the fuel rods with pellets without the central hole in the absence of the fuel-cladding gap, slows down gas inflow to the heated rod segment and the cladding strain by filling gas pressure ;
Tight fuel-cladding contact hampers the cladding lift-off;
Oxide films on the external and inner-diameter surface generate the restraining effect on deformation of cladding surface layers.

15. Thank you for your attention!