Ampoule and Basket for Transportation of Non-Conforming Tight RBMK SFAs in TUK-109 Cask from NPP to FSUE Mayak PA

P. Bogatov, B. Kanashov, A. Samsonov (Sosny R&D Company), A. Zubkov, T. Listratova, A. Rodin, V. Sokolov (OJSC ECNC)

VIII International Nuclear Forum "Safety Of Nuclear Technologies: Nuclear Installation Safety Culture", St. Petersburg, Russian Federation, 9-13 September, 2013

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Introduction

By now, the reactor cooling pools and SNF storage facilities of the NPPs operating RBMK reactors have accumulated more than 100 000 spent fuel assemblies (SFAs). Most of them will be loaded into special double-purpose metal concrete TUK-109 casks or metal transport TUK-109T casks and sent for long-term storage in the well-type storage facility of the Mining and Chemical Combine. However, part of the SFAs is unfit for standard long-term storage in ampoules (dwg. 1621-28-0007-01) for fuel rod bundles for various reasons: loss of tightness, damaged structural elements, corrosion, etc. So, in November 2012, Rosatom State Corporation decided to transport non-conforming SFAs to FSUE Mayak PA for reprocessing and further use of the conversion products.
FSUE Mayak PA, in its turn, proposed that load components for the TUK-109 cask be developed (a basket and special transport ampoules that can be reprocessed together with the fuel rod bundle) to facilitate the transportation of the fuel rod bundles from non-conforming RBMK SFAs.
The analysis shows that the requirements for the ampoules for the transport of untight fuel rod bundles differ significantly from those for tight fuel rod bundles. Besides, the technologies of handling tight and untight fuel rod bundles differ in kind at NPPs and at FSUE Mayak PA. Taking into account the specified requirements, two designs of the ampoules were proposed:
– tight ampoule (design 1) for untight fuel rod bundles of RBMK-1000 SFAs,
– untight ampoule (design 2) for tight fuel rod bundles of RBMK-1000 SFAs.
This article describes a design 2 untight ampoule (dwg. СТВА.А.31.120) intended for the transport of fuel rod bundles of tight and conventionally tight RBMK-1000 SFAs from the NPP to FSUE Mayak PA. The primary purpose of the ampoule is to provide safety during the transport and process operations with the SFA fuel rod bundles, as well as during preparation for the transport, preparation for reprocessing and the transportation itself. To transport the ampoules in the TUK-109 cask, a new basket (dwg. 02-155) was developed, that provided orderly arrangement of the ampoules in the cask and ensured their safe transportation in compliance with the NP-053-04 Rules.

1. Radioactive Contents of the Cask

As it has been said before, the untight ampoule and the basket are intended for transportation of the fuel rod bundles of tight and conventionally tight non-conforming RBMK-1000 SFAs, from the NPP to FSUE Mayak PA. According to the SFA inspection procedure [1], the SFAs are considered as non-conforming, if they have the following defects:
– lack of more than two spacer grids in succession,
– lack/destruction of spacer grid 10 of the lower fuel bundle or spacer grid 11 of the upper fuel bundle,
– local diameter increase exceeding 87 mm,
– gap between the fuel rods of the upper and lower fuel rod bundles less than 9 mm.
SFAs with central fastening of the fuel rods, pilot SFAs with 7-meter fuel rods and badly damaged SFAs (twisted, deformed, fractured etc.) are considered as non-conforming. Dry storage of the non-conforming SFAs is allowed only after preparation of the SFAs according to a special technology, which has not been developed yet. Defects of the structural elements are an obstacle to disassembling of the SFAs and loading of the fuel rod bundles into ampoules (dwg. 1621-28-0007-01). All non-conforming SFAs, found during the operation of the cutting division, are returned to the SNF storage facility.
Thus, a task was set to develop an ampoule for transportation of fuel rod bundles from tight SFAs, in which the cladding of the outer row of the fuel rods was intact, but the SFA elements had the following defects:
– mechanical damage to the spacer grids (dents, scars, chips, ruptures, cracks) which can increase the outer diameter of the fuel rod bundle, but not more than 96 mm,
– partially destroyed, crumbling spacer grids or spacer grids lacking on their design positions (minimum two lower or upper spacer grids in succession or minimum three central spacer grids in succession).
Besides, the untight ampoule was intended for transportation of the fuel rod bundles from conventionally tight SFAs having the fuel rods with through microdefects of the cladding, gaseous fission products penetrated, but excluding the contact between the fuel composition and the environment (the coolant, the canister water during wet storage, the ampoule atmosphere during dry storage, etc.).
Tightness of the SFAs is determined from the results of the Cladding Leak Test and from the results Cs-137 measurement in the sample of canister water (isolated from the cooling pool water) after infusing the SFAs in it.

2. Requirements for the Ampoules

The untight ampoule is intended for use:
– in the shielded cells in the cutting division of the container maintenance building for SNF storage facility (CMB SNF SF) at the NPP with RBMK reactors (during SNF preparation for shipment),
– as a part of the TUK-109 cask together with a special basket (during the transport),
– in the cooling pool and cutting preparation division (CPD) (during reprocessing at FSUE Mayak PA.
Consequently, the ampoule shall meet the requirements of NP-001-97 (OPB-88/97) [2], NP-016-05 (General Provisions of Safety Assurance of Nuclear Fuel Cycle Facilities) [3], NP-061-05 [4] and NP-053-04 [5]. Besides, the untight ampoule shall meet the following special requirements:
– If possible, the ampoule shall be compatible with the existing SFA cutting equipment of the CMB SNF SF at the NPP and the equipment of FSUE Mayak PA.
– The ampoule can be easily dried and/or filled with inert gas as a part of TUK-109 cask.
– The ampoule shall prevent the escape of solid fuel particles or fragments of structural elements of the fuel rod bundles.
– The design shall provide remote installation of the lid and transfer of the ampoule.
– The ampoule is an expendable item and is reprocessed together with its contents (fuel rod bundle of RBMK-1000 SFAs) at FSUE Mayak PA. That is why, the ampoule shall not contain any bulky elements that can impede cutting in the cutting facility. Total length of the ampoule with fuel rod bundle, fed into the cutting facility, shall not exceed 3765 mm.
– The cost of the ampoule shall be low.

3. Requirements for the Basket for the Ampoules

The basket is intended to handle the ampoules with fuel rod bundles both at the NPP and at FSUE Mayak PA, so its design shall meet the requirements of NP-001-97 (OPB-88/97) [2], NP-016-05 (General Provisions of Safety Assurance of Nuclear Fuel Cycle Facilities) [3], NP-061-05 [4]. Safety during transportation shall comply with NP-053-04 [5]. Besides, the basket shall meet the following special requirements:
– Its dimensions shall fit into the TUK-109 cask, the shielded cell of the container maintenance building for SNF storage facility as well as make it possible to load the ampoules with fuel rod budles into the basket using the existing and newly-installed equipment of the shielded cell.
– The mass of the basket with SNF-loaded ampoules is not more than 22 tons, which is required for loading/unloading the cask. – The mass of the basket with empty ampoules is not more than 8 tons.
– The design of the basket shall enable loading of 102 ampoules with fuel rod bundles of non-conforming RBMK-1000 SFAs at the NPP in compliance with the SNF handling flow charts applicable the the cutting division of the CMB SNF SF.
– The design of the basket shall provide unloading of the ampoules with fuel rod bundles at FSUE Mayak PA.
– The design of the basket shall provide remote transfer of the basket with grapples.
– The design of the basket shall comply with the requirements for safe SNF handling at all stages of handling (at the NPP, during the transport of the basket with ampoules as a part of a TUK-109 cask, at FSUE Mayak PA).
– The basket shall be compatible with the existing equipment of the CMB SNF SF at the NPP and the equipment of FSUE Mayak PA. – The basket shall withstand multiple decontaminations.
– During normal operation, damage to the basket pintle shall be avoided.
– Under accident conditions, the basket can be easily taken out of the TUK-109 cask.

4. Design and Parameters of the Ampoule

The ampoule (dwg. СТВА.А.31.120) belongs to the equipment important to safety. Classification identification according to NP-016-05 [2] and NP-001-97 [3] is 2Н. Fig. 1 presents the general view of the untight ampoule (dwg. СТВА.А.31.120). The ampoule has two main parts - a body and a lid. The lid is secured onto the ampoule body by means of a lock based on a lock ring in an annular groove in the ampoule lid. When the lid is installed, the lock ring gets into the annular groove in the ampoule body, thus fixing the lid.
The ampoule body is a welded structure that consists of a throat, a shell, two ribs and a bottom. The shell is made of a tube 102 mm in diameter and a wall thickness of 2 mm. The bottom is welded to the lower end of the shell and bears a fuel rod bundle. The bottom has special drain holes 6 and 10 mm in diameter. A 1 mm mesh filter covers the holes to prevent the release of the large SNF particles. The drain holes serve to drain water after the basket with ampoules has been withdrawn from the cooling pool for further transfer to the CPD of FSUE Mayak PA. The ampoule containing water must not be fed to the cutting facility for fuel rod bundles. The drain holes 6 and 10 mm in diameter covered with the 1 mm mesh provide enough time to drain water from the ampoule (approx. 12 min) when the basket with ampoules is taken out of the cooling pool in building 101A of FSUE Mayak PA.
The length of the ampoule is 3765 mm, the mass is 20.6 kg. The length of a fuel rod bundle to be loaded into the ampoule shall not exceed 3743 mm.
In compliance with the strength analysis, the ampoule performs its functions as during normal operation under operating loads, so as under normal conditions of transport at acceleration up to 10g.
The strength of the ampoule under possible accident conditions was analyzed during a vertical drop of the ampoule onto a rigid target from a height of 5 m with a turnover (a drop onto the bottom of a shielded cell was modeled) and a vertical drop of the ampoule onto a shock absorber from a height of 6.5 m (a drop onto the bottom of the basket 02-155 and of the clamping device). At all variants of the design drop, the ampoule retains its strength, can be withdrawn from the basket, no fuel fragments is spilt.
The strength analysis confirms the compliance of the chosen design of the ampoule for transportation of the fuel rod bundles of non-conforming RBMK-1000 SFAs with the requirements of the current regulations and demonstrates that the ampoule design ensures its safe operation.
Performance and strength tests of the ampoule mockup were performed to verify technical solutions and to confirm the calculations. In particular, the joint of the lid and the body was tested for reliability by 30 cycles of installation/removal of the lid. The results demonstrated reliable functioning of the joint. The lock and the pintle of the ampoule lid were tested for strength by stressing the mockup with a load of 180 kg during 10 min (Fig. 2). The tests demonstrated that the ampoule withstands the loads without damage.
The following calculations were performed to determine the operation conditions and characteristics of the untight ampoule during transportation in the TUK-109 cask and interim technological storage:
– calculation of the nuclide composition of RBMK SNF for different U-235 enrichment, different burn-up values and different cooling time;
– calculation of decay heat for a fuel rod bundle and the dose rate of the transport and storage package UKKh-109 with 102 fuel rod bundles;
– analysis of thermal conditions of the transport and storage package UKKh-109 with 102 fuel rod bundles;
– predictions of the generated radiolysis products within the transport and storage package UKKh-109 with 102 fuel rod bundles; – strength analysis of the ampoule components under static load and for different initiating events.
The calculations show that if the cask is loaded with 102 fuel rod bundles containing uranium dioxide with the enrichment of 2.4 % in U-235, the burn-up of 25 MW•day/kgU and 10 years of cooling:
– Decay heat per one fuel rod bundle will not exceed 43.5 W. The total decay heat in the cask will be 4437 W, that does not exceed the decay heat limit for the TUK-109 cask (5800 W).
– The maximum temperature of the hottest fuel rod cladding reaches 160ºС, that is significantly lower than the allowable value of 300ºС.
– During 11 months of interim storage and 1 month of transportation, inflammable and explosive concentration of hydrogen is not generated in the air-filled inner volume of the TUK-109 cask.

5. Design and Parameters of Basket

The basket (dwg. 02-155) belongs to the equipment important to safety. Classification identification according to NP-016-05 [3] and NP-001-97 [2] is 2Н. Ampoules with fuel rod bundles are loaded into the basket and unloaded from it only when the basket is upright. The specifications of the basket are as follows:
Design mass of loaded basket, max, kg 17 500
Design mass of empty basket, max, kg 4100
Capacity (quantity of ampoules with fuel rod bundles), pcs 102
Height, max, mm 4067
Maximum outer diameter, mm 1465
Cell diameter, mm 108 Space between ampoules, mm 123
Design service life, min, years 50.
The basket consists of a body and bearing and shock absorbing elements. The basket body is a weldment and contains the following main elements:
– an upper grid and five grids under it, perforated with round holes: each grid has 102 holes for ampoules and one central hole for the central post;
– a base with a central hole for the central post and 102 holes for installation of shock absorbers;
– the central post with a bushing on the top to screw in the pintle for the grapple;
– 12 channels (tubes 121×6 mm). Passing through the holes in the grids, the channels are welded to the grids and to the base. The channels provide a distance between the grids and make the whole structure rigid. The channels and other holes of the grids (the basket seats) are intended to house the ampoules with fuel rod bundles;
– 12 stoppers welded to the upper grid.
The strength of the basket containing ampoules with fuel rod bundles was confirmed with the calculations of load impact under the following conditions:
– under normal and abnormal conditions of transport as a part of the TUK-109 cask (a drop from a height of 9 m onto a solid target, a drop from a height of 0.3 m, and a drop from a height of 1 m at the NPP). The loads on the basket were modeled with the acceleration 150 g;
– under abnormal conditions at a drop during loading of the basket with fuel rod bundles into TUK-109 cask (vertical drop from the height not more than 7 m);
– under abnormal conditions at a drop of an ampoule with fuel rod bundles into a basket seat from a height of 5 m);
– during abnormal handling of the basket at the NPP under seismic loads (the basket is hanging on the crane grapple);
– during normal operation - lift of the basket with a jerk;
– under normal conditions of transport by rail with acceleration impact on TUK-109 cask in compliance with NP-053-04 [5].
The calculations demonstrated that when loaded under abnormal conditions all principal structural elements of the basket met the strength conditions, the integrity of the basket pintle was not affected, withdrawal of the basket from TUK-109 cask remained possible. The compliance of the chosen design of the basket for transportation of the fuel rod bundles from non-conforming RBMK-1000 SFAs with the requirements of the current regulations.
Nuclear safety analysis was performed in compliance with the requirements of NP-061-05 Rules [4]. The basket, loaded with 102 ampoules of fuel rod bundles from non-conforming RBMK-1000 SFAs and immersed under water in the cooling pool of FSUE Mayak PA, ensures nuclear safety: effective neutron multiplication factor of the package will not exceed the value of 0.95 during normal and abnormal operation, including design-basis accidents. The resulted analysis of the accident beyond design basis demonstrates no self-sustained chain reaction in the system.

Conclusion

The special ampoule (dwg. СТВА.А.31.120) and basket (dwg. 02-155), developed according to the Technical Assignment of FSUE Mayak PA, allow transporting the fuel rod bundles of tight and conventionally tight RBMK SFAs based on uranium dioxide with initial enrichment 1.8, 2.0, 2.4 % in U-235 and based on uranium-erbium fuel with initial enrichment 2.6, 2.8, 3.2/2.5 % in U-235 (fuel assemblies with profiled enrichment), with maximum burn-up to 37.3 MWday/kgU after cooling for minimum 10 years. A Certificate was obtained for the design of TUK-109 package and for transport of 102 fuel rod bundles from non-conforming RBMK SFAs with initial U-235 enrichment up to 2.4 %, fuel burn-up to 22.8 MWday/kgU and 10 years of cooling.
The design of the ampoule (dwg. СТВА.А.31.120) ensures its safe operation under normal conditions (the body-lid joint is reliable, the ampoule cam be reprocessed together with the fuel rod bundles), as well as meets the safety requirements under abnormal conditions (integrity of the ampoule after drops, can be withdrawn from the basket, no fuel spillage). The design of the basket (dwg. 02-155), in its turn, meets strength conditions during normal operation, the integrity of the basket pintle is not affected, withdrawal of the basket from TUK-109 cask remains possible even after the impact of loads, characteristic for abnormal operation. The basket ensures nuclear safety during normal and abnormal operation, including design-basis accidents.
The suggested technical solutions on the ampoule and basket design allow, having eight TUK-109 casks and providing one shipment per year, removing 408 non-conforming SFAs/year from the NPP to FSUE Mayak PA for reprocessing, which corresponds to approximately 47 tons/year.

References

1 RBMK-1000 SFA Condition Control before Placing for Dry Storage МТ 1.2.1.02.999.0002-2010. Approved by Rosenergoatom Concern on February 10, 2010.
2 General Regulations on Ensuring Safety of Nuclear Power Plants (OPB-88/97): NP-001-97 (PNAE G-01-011-97): approved by Gosatomnadzor of Russia on November 14, 1997: enacted on July 7, 1998.
3 General Provisions of Safety Assurance of Nuclear Fuel Cycle Facilities; NP-016-05. – to replace NP-016-2000: approved by Federal Environmental, Industrial and Nuclear Supervision Service on December 2, 2005: enacted on May 1, 2006. – Moscow, 2006. 4 Regulations for the Safe Storage and Transport of Nuclear Fuel at Nuclear Power Facilities, NP-061-05: approved by Federal Environmental, Industrial and Nuclear Supervision Service on December 12, 2005: enacted on May 1, 2006.
5 Safety Guidelines for Radioactive Materials Transportation: NP-053-04. - To replace PBTRV-73 and OPBZ-83: approved by Federal Environmental, Industrial and Nuclear Supervision Service on October 04, 2004: enacted on May 1, 2005 – Moscow, 2005. – 105 pages.