Performance Assessment of Spent Fuel Assemblies Removed from Shutdown RBMK Reactor Cores for Reburning in Operating Power Units

    

B. Kanashov, I. Kuzmin, A. Kostyuchenko, S. Perepelkin, V. Chesanov

International Conference on Management of Spent Fuel from Nuclear Power Reactors: An Integrated Approach to the Back End of the Fuel Cycle, IAEA Headquarters, Vienna, Austria, 15–19 June 2015

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Abstract. In 2018 – 2035, the design lifetime of 11 Russian RBMK-1000 reactors will expire. The fuel of shutdown for decommissioning reactor has some residual performance. In case of unit-by- unit decommissioning such fuel can be burnt in the RBMK-1000 units which are still in operation and its reburning will minimize fresh fuel needs. The poster concerns economical effect of shutdown reactor fuel reburning, describes milestones of the SFA handling procedure offered for the Leningrad NPP and provides some information about SFA performance assessment and procedure safety analysis. The assessments confirm good residual performance of the RBMK 1000 SFAs sufficient for their safe and cost effective reburning in operating units of the same type.

Key Words: RBMK, fuel of shutdown reactor, residual performance, SFA handling.

1.Introduction

Now 11 power units with the RBMK-1000 reactors are in operation at three NPPs in Russia. In 2018 – 2035, the design lifetime of the RBMK-1000 reactors will expire (Table 1). In case of unit-by-unit decommissioning, such fuel can be burnt in the RBMK-1000 units which are still in operation.

The fuel of shutdown for decommissioning reactor (here and after called as spent fuel or spent fuel assembly – SFA) has some residual performance and its reburning will minimize fresh fuel needs. The amount of SFAs which have more than a half of their residual performance is too large to be reburnt at the operating units of the Leninrgad NPP even if some SFAs are rejected as non-conforming by any reason. According to the analyses [2, 3], the reburning of the fuel removed from the shutdown reactors of the Leningrad NPP, in case of its decommissioning, will allow to save up to 1850 fresh fuel assemblies depending of the reburning mode.

2.SFA Handling Procedure

The procedure for handling the SFAs during their preparation for reburning includes the SFA storage in the reactor pool, the transportation to the operating unit, the SFA preparation for loading into the core and the SFA health checking (see FIG. 1).

3.SFA Performance Assessment and Procedure Safety Analysis

The results of the SFAs post irradiation examination in a wide range of their operation period were analyzed in order to assess the performance of candidate SFAs [4, 5]. The analysis of the corrosion characteristics and mechanical properties of the fuel rod claddings, state of the spacer grids and the elements of the SFA skeleton, and the SFA geometry has confirmed the performance of RBMK-1000 SFAs further burning after their irradiation for 1100 effective days (see FIG. 2). The analysis of the examinations of RBMK-1000 SFA after long-term storage in the reactor pools [6] has not revealed any serious changes in the SNF health.

The standard equipment should be used to transport the SFAs between the units, but it has to be modified in order to minimize any risk of the SFA damage during its loading in / unloading out of the transport cask. The analysis of the modified procedure for transport the SFAs between the units of one and the same NPP has proved its safety under normal and accident conditions of operation. The main restrictions to the modified procedure are similar to those imposed to the standard one, i.e. the velocity of the container railcar shall not exceed 5 km/h and the acceleration during transportation shall not exceed 4g.

4.SFA Requirements

Basing the analysis of the efficiency of the SFA further burning, and of the reliability and safety of the procedure for SFA storage, transport and handling at the operating power unit, the following requirements were specified:

Requirements to fuel burnup:
•max 20 MW×day/kgU or
•period of operation at rated power, max – 1100 effective days (about 4 years).

Requirements to SFA cooling period at least one year of cooling in the core or in the reactor storage pool.

Requirements to SFA integrity. All SFAs subject to further burning shall be leak tight. The cladding leak testing shall be performed for all SFA after their removal from the shutdown reactor and after their transportation to the operating power unit.

Requirements to SFA elements health. Each SFA shall be visually inspected in compliance with the following rejection criteria:
•any damage, displacement or absence of the spacer grid;
•any dents or damage (incl. corrosion) of the fuel rod claddings;
•no fuel rod plug;
•any foreign object between the fuel rods;
•any change of the FA geometry (distance between the fuel rod bundles, dimensions of the spacer grid, etc.) impeding the implementation of process operation during the SFA transport or irradiation.

All SFAs shall be twice examined: after their removal from the shutdown reactor and after their transportation to the operating power unit.

5.Conclusion

The nuclear fuel of the shutdown RBMK-1000 reactor has a good residual performance that can be used during the unit-by-unit NPP decommissioning. The reburning of the fuel removed from the shutdown reactors of the Leningrad NPP will allow saving up to 1850 fresh fuel assemblies. The feasibility to continue SFA operation is confirmed with the examinations of the fuel assemblies with burnup up to the maximum design value. At present the procedure for SFA reburning is being developed and the Leningrad NPP equipment is being prepared for further burning of the SFAs removed from the shutdown reactors.

6.References

[1] GABARAEV B., CHERKASHOV YU., PETROV A., et al., “Justification of Extension of RMBK Reactors Lifetime”, Atomic Energy 100, Iss. 4 (2006) 328-335 (in Russian).
[2] FEDOSOV А. “Optimal Use of Fuel in Process of Decommissioning of NPP with RBMK Reactor”, Atomic Energy 102, Iss. 5 (2007) 284–290 (in Russian).
[3] FEDOSOV A., KRAYUSHKIN A., NOVIKOV V., TISHKIN YU., “Fuel Reburning in Process of RBMK Reactor Decommissioning”, Safety, Efficiency and Economics in Atomic Energy Industry (Proc. 7th Int. Res. Conf. Moscow, 2012), 227–229 (in Russian).
[4] MARKOV D., ZVIR E., ZHITELEV V., STROZHUK А., MAERSHINA G., SMIRNOVA I., “Corrosion Failure of Fuel Rod Claddings of RBMK-1000 Fuel Assemblies Spent at Leningrad NPP Until Fuel Burnup in Range of 4–32 MWday/kgU” (Proc. TVEL Res. Conf. Moscow, 2010) (in Russian).
[5] KOSTYUCHENKO А, CHESANOV V., ZVIR Е., MARKOV D., MAERSHINA G., “Oxidation of RBMK 1000 Fuel Rods Claddings During Standard Operation”, Reactor Material Science (Proc. 9th Conf. Dimitrovgrad, 2009) (in Russian).
[6] RAZMASHKIN N., KRITSKIY V., BEREZINA I., “Problems Related Long-Term Storage of Spent Nuclear Fuel”. Reactor Material Science (Proc. 9th Conf. Dimitrovgrad, 2009) 478–485 (in Russian).

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