Elimination of Consequences of the Fuel Destruction at the Paks NPP
The Paks NPP is located in the heart of Hungary on the Danube River. Over the years of operation of Russian VVER-440 units, the Paks NPP has demonstrated the best performance and efficiency. However, the NPP personnel have been concerned by the problem of magnetite deposits on the fuel rod cladding in the last few years.
For the SFA cleanup operations, Framatome ANP developed and fabricated a 30 SFA capacity tank that was installed in Pool 1 of Pask NPP Unit 2. Framatom ANP specialists performed all the activities with full credit given by the NPP personnel. Five successful SFA cleanup operations were carried out in the tank without any signs of damage, but during the sixth operation an unforeseen event happened.
Later, an official incident investigation report stated that on April 10, 2003 in the process of a cleanup operation during scheduled maintenance of Unit 2 several fuel assemblies were seriously damaged causing a radioactive release that exceeded a regular one for an operating unit. On attempted removal of the lid, a significant radioactive release into the ventilation system, an increase in dose rate in the vicinity of the pool and volumetric activity in the water of the cooling pool were detected. The nuclear safety situation was unclear, too. After the lid was opened, experts inspected the state of the fuel assemblies and classified the event as Level 3 (an incident). The Unit 2 reactor was shutdown.
On April 17, 2003, on behalf of the Paks NPP management CEO Istvan Kocsis addressed Russian TVEL Corporation for assistance. In two hours after receiving the letter, the Russian side confirmed its readiness to participate in the international tender for elimination of SFA damage consequences at the Paks NPP and started developing proposals.
For this purpose, a team of leading industry enterprises headed by TVEL Corporation was established including SSC RIAR, Gidropress Experimental Design Bureau (OKB Gidropress), RRC "Kurchatov Institute". An important part in the project was assigned to Sosny specialists headed by Valery Smirnov. In addition to the above-mentioned enterprises, the project involved VNIPIET, Ozersk Plant of Nonstandard Equipment, Electrostal Machine-Building Plant and PA "Mayak" as a potential SNF consignee.
The primary investigations showed almost all the SFAs to have failed to one extent or another. Fragments of the fuel elements and the pellets that had spilt out of the claddings piled up in the middle of the tank. The lower parts of the fuel assemblies had not suffered any damage.
On September 9, 2003, TVEL Corporation and the Paks NPP signed an agreement stating that with adhering to all radiation and nuclear safety requirements Russian specialists should remove all the SFAs fragments from the tank and load them into special canisters to be stored underwater at the Paks NPP for 5 years, as well as to transport them to reprocessing or dry storage facilities. To achieve the aim, the Russian and Hungarian counterparts had a heavy research and development workload.
R&D Company "Sosny" developed draft and final designs of a frame, canisters, equipment and tools. For the purposes of the technology tryout and personnel training, Sosny specialists fabricated, mounted and tested the working frame, all the required equipment and tools on the SSC RIAR site. In addition, a set of video surveillance and radiation monitoring equipment for the working frame was developed. The developed set of tools for handling various SFAs included more than 100 types.
The most complicated and cumbersome part of the technology was the working frame designed by OKB Gidropress. The working frame and the large-size equipment were fabricated and tested at the Ozersk Plant of Nonstandard Equipment.
Untight water-filled canisters were developed to repackage the SNF. The internal space of the canisters contacts the water of the cooling pool through an original pressurizer. The pressurizer ensured removal of gaseous products that are generated during water radiolysis from the canisters and prevented the radionuclides dissolved in the water of the canister from releasing into the cooling pool. The SNF canisters were fabricated and tested at Electrostal Machine-Building Plant JSC.
Personnel training was conducted on a mockup working frame at SSC RIAR first, and then, on the real working frame at the Paks NPP. SSC RIAR and R&D Company "Sosny" specialists participated in and controlled the entire process of the working frame mounting on the cleanup tank at Unit 2 and adjustment of the equipment.
The tank clear-out operations started on October 1, 2006, after the Hungarian regulatory authority issued a permit. Trained Russian specialists worked 24 hours a day in four shifts. Standing on the working frame in the tank at a height of 3 m above the upper flange of the tank, a team of three operators cleared out the tank from the SFA fragments using long-length tools. The entire process was continuously surveyed and recorded by TV cameras.
The SFA fragments, separate pieces of the fuel rods and the bulk SNF were put into canisters. The heads and tails were separated from the fuel columns and put into a solid radioactive waste (SRW) vessel. In the process, the SNF was thoroughly accounted by weighing before being loaded into the canisters.
A big complication was a huge spacer plate with the SFAs swollen after a heat stroke and sitting tightly in the plate. The plate had to be cut into several fragments and removed from the tank. For the final cleanout operation, the lower heavy plate had to be drilled through by a specially designed pneumatic crown driller.
The main operations were as follows:
- First, the fragments of the fuel assemblies and the fuel rods were cleared out from the upper plate of the tank and loaded into removable parts of the canisters. Smaller bulk SNF fragments were shoveled into the removable parts of the canisters, too. SFA heads were separated from the fuel column and loaded into solid radwaste vessels. The removable parts were put into the canisters.
- An automatic grab and manipulators were used to clear out a pile of the fuel rod fragments in the middle of the tank.
- Then, the fragments of the upper plate and the fuel assemblies under it were successively taken out.Thus, the operators gained access to the SFA lower parts and using a grapple installed them in a cutter to separate the tails from the fuel column.
- The bulk SNF and the SFA fragments were removed from the lower plate of the cleanup tank in the same way as from the upper plate.
- After the required space was freed, the central part of the lower plated was drilled through.
- Picking up the bulk SNF from under the lower plate followed by removal of remaining SFA fragments and the bulk SNF again. As a result, there remained an insignificant amount of SNF on the bottom of the tank that later was removed by a suction system.
Practical operations on elimination of the incident consequences in Unit 2 were completed by signing an acceptance report on March 30, 2007.
In the process of the operations, radiation monitoring of the personnel was performed in full compliance with Russian and Hungarian norms and regulations. The maximum individual dose over the entire campaign did not exceed 10% of the annual dose limit, and the average one made up 5%.
The incident consequences elimination operations were finished on March 30, 2007. The work was completed in full scale as scheduled and at a high technological level. As a result, more than 5 tons of SNF were removed from the tank. On December 29, 2006 at 13:07 Unit 2 of the Paks NPP was brought to the minimum controllable power, and on January 1, 2007 it was put to the rated power.
Drying was the next stage of preparation of the spent fuel for shipping. Simulations of the SNF storage in an air-tight canister were performed in shielded cells at SSC RIAR to investigate the behavior of the destructed fuel and determine the remaining moisture in the canister that would allow safe SNF handling throughout the campaign. It yielded experimental data for justification of safe transportation modes for the spent nuclear fuel.
Thermal vacuum drying of the SNF with no water pre-draining was decided upon to minimize the personnel exposure, the amount of radioactive waste generated, and the work time.
The analysis demonstrated nuclear and radiation safety of the proposed technology.
The working platform that had been used to dry the SNF was upgraded to be employed during removal of the damaged fuel from the cleanup tank.
Specialized equipment was developed for drying the SNF canisters, i.e.
- equipment for removal of the pressurizers from the SNF-containing canisters and replacement of canister lids,
- equipment for thermal vacuum drying, gas filling and tightening the canisters including a shielding cask, a canister winch, heaters, a filtering and compensation unit, a valve control unit, systems for steam removal and condensation, non-condensed gas evacuation, gas filling, and process control and monitoring,
- equipment for flushing and liquid resistance measuring of the filtering and compensation unit,
- canister leak tester,
- support systems (lighting, video surveillance, communication),
- baskets for installing the canisters in the TUK-6 shipping cask.
A shielding cask ensured protection from ionizing radiation during SNF-containing canister drying and tightening operations. The electric heaters inside the heating module heated up the canister body. A dedicated heating module was designed for each type of the canisters.
A steam-gas mixture came from the SNF canister to the condenser unit through the filtering and compensation unit. The filter stack in the filtering and compensation unit captured radioactive aerosols and SNF particles continuously carried away by gas stream from the canister during drying operations. Aerosols are volatile fission products (137Cs and 134Cs) and activation products (60Со). The retention rate for the steam-gas mixture was 99.999 % related to the spent fuel particles larger than 0.01 μm; the one related to cesium-137 was 99.95 % and cobalt-60 – 94 %.
Once dried, the canisters were filled up with inert gas, all joints and gaps were tested for leakage, namely, the bottom drain valve, the canister lid valve, and the lid-to-body joint. The leak tests by an ASM142 helium leak detector showed that the total leakage from all dry SNF canisters did not exceed 6.7х10-7 Pa·m3/s.
The main operations for preparing the canisters with the damaged SNF for transportation were as follows:
1) Dismantle the pressurizers from the canisters in the cooling pool.
2) Transfer the canisters from the cooling pool to the well for replacing the lids, drying and tightening.
3) Replace the canister lids.
4) Dry, fill with inert gas, air-tighten, and perform leak tests of the SNF-containing canisters.
5) Transfer dry canisters from the well to the cooling pool.
6) Interim storage of the canisters in the cooling pool with leak tests regularly performed.
7) Leak tests of the SNF canisters prior to loading into shipping casks.
8) Load the canisters into the shipping casks and prepare the packages for transportation.
In 2013, hands-on operations to prepare the SNF canisters for transportation were successfully completed at the Paks NPP. Sosny staff performed the activities under the direction of the Paks NPP personnel following the procedure approved by the Hungarian regulator. During the activities, no failures occurred affecting the overall rate of the fuel preparation campaign. In 2014, the SNF canisters were shipped to Mayak PA for reprocessing.
The main outcome of the joint Hungary/Russia Paks Project is that the threat of a nuclear and radiation hazard was eliminated and the nuclear fuel was put in safe controllable conditions due to the activities performed. The data obtained from the operations give a chance to see the nuclear fuel behavior under severe accident conditions in a new light.
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