MANAGEMENT OF DAMAGED SNF HANDLING OPERATIONS AT PAKS NPP Е.А. Zvir, V.P. Smirnov Research and...

MANAGEMENT OF DAMAGED SNF HANDLING OPERATIONS AT PAKS NPP

Е.А. Zvir, V.P. SmirnovResearch and Development Company “Sosny”, Moscow, Russian Federation

I. HamvasPaks NPP, Paks, Hungary

IAEA International Conference on Management of Spent Fuel from Nuclear Power Reactors Vienna, 31 May - 4 June 2010

IAEA-CN-178/09-01

IAEA International Conference on Management of Spent Fuel from Nuclear Power Reactors, Vienna, 31 May - 4 June 2010

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Handling leaky fuel is one of the crucial issues of nuclear power and engineering. This includes

transportation of the leaky spent nuclear fuel (SNF) stored in NPP cooling pools.

On 10 April 2003, during an outage period a chemical cleaning program for the fuel assemblies

was carried out at unit 2, in a specially designed cleaning tank. The tank was located in pit No.1

near the reactor. 30 fuel assemblies were severely damaged due to inadequate cooling.

In the period from 01.10.2006 to 30.03.2007 the Paks and Russian specialists were involved in

clearing out the tank. All the SNF (more than 5 tons) was loaded from the tank into specially

designed ventilated canisters of types 28 and 29 and now is temporarily stored in the spent fuel

cooling pool of the Paks NPP.

Taking into account the state of the assemblies, the optimum solution would be to reprocess the

spent fuel and dispose of the resulting waste.

On behalf of the Paks NPP, the Russian specialists performed much research and development

work to justify safety of the damaged spent fuel. This enabled developing of a technology for

preparation of the canisters containing the damaged spent fuel for shipment to Mayak (Russia)

for re-processing.

INTRODUCTION


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Preparation of the canisters for

shipment from the Paks NPP to

Mayak and further treatment

includes the following three

key stages:

handling of the canisters

at Paks;

transport of the canisters

from Paks to Mayak;

re-processing of the fuel

at Mayak.

GENERALISED FLOW SHEET OF CANISTERS HANDLING


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The main challenge to ensure and justify safety of shipment of wet damaged spent

fuel is the need to keep the canister leak-tight over a long period of time. This

includes the time of residence of the leak-tight canisters at the plant site, the time of

shipment and receipt at Mayak and the time of keeping the fuel at Mayak before

cutting up and reprocessing.

Transportation of the wet damaged SNF differs from the traditional way of handling

the intact SNF due to radiolysis of the residual water in the canisters. Radiolysis

increases pressure inside the tight canisters and generates an explosive gaseous

mixture. The experience in handling the damaged fuel at TMI-2 NPP shows that it is

one of the main processes influencing safety.

THE MAIN PROBLEM OF SHIPMENT


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When developing the basic technology, the Russian and international regulations

were analyzed to comply with safety requirements for handling damaged SFAs at

the re-processing plant, and transportation of the failed spent fuel from the Paks

site to Mayak was justified. All key safety aspects of transport were considered:

nuclear and radiation safety, structural integrity and leak-tightness of the canisters,

thermal conditions, explosion and fire safety.

The preliminary calculations performed by the Russian Institute of Experimental

Physics confirmed that defective spent fuel can be safely transported in gas-filled

canisters of types 28 and 29. Their design takes into account the Mayak

requirements to the pre-processing handling operations.

A gas-filled transport overpack TUK-6 can be used for the shipment. Four TUK-6

overpacks would be sufficient to transport all the canisters from the Paks NPP to

Mayak.

KEY SAFETY ASPECTS OF TRANSPORT


CHOICE OF CANISTER DRYING METHOD

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The residual water in the canisters must not exceed 5% of the fuel mass; this requirement

arises from the specificity of spent fuel re-processing at Mayak.

The canisters can be prepared at the Paks NPP for the shipment in three ways:

water drainage with no additional drying, filling with an inert gas and sealing of the

canisters;

water drainage, drying of the spent fuel, filling with an inert gas and sealing of the canisters;

drying of the spent fuel without prior water drainage, filling with an inert gas and sealing

of the canisters.

To select the optimum method for preparation of the canisters, experimental equipment was

designed and fabricated, and tests were carried out to try these three approaches. The tests

involved reduced-height simulators of type 29 canisters loaded with irradiated VVER-440 fuel,

and full-scale type 28 and 29 canisters loaded with dummy spent fuel assemblies.


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To determine the residual moisture in the canister that would guarantee safety at all

handling stages, experimental research was performed to model the storage of the

failed spent fuel in a leak-tight canister after water removal by various methods. The

fuel from a VVER-440 FA with the following parameters was used for the

experiments:

initial enrichment in U-235 – 3.6 %;

average burnup throughout FA – 27.6 MW*d/kgU;

maximum burnup along FA height – 32.0 MW*d/kgU;

total FA operation time – 649.1 eff.d;

date of discharge of FA from the reactor – 18 June,1999.

EXPERIMENTAL RESEARCH


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LOGICAL SEQUENCE OF EXPERIMENTS


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Justification of safety at all stages of handling the canisters sealed after water drainage is

impossible, because over six months the hydrogen volumetric concentration in a type 29 canister

will reach 8.7%.

If upon completion of thermal vacuum drying the residual pressure in the canister does not

exceed 6 mbar, in two years of storage in a leak-tight type 29 canister the hydrogen volumetric

concentration will not be greater than 0.05%, whereas in a type 28 canister it will not be higher

than 0.04%.

The amount of hydrogen accumulated in the canister will be two orders of magnitude below the

level of self-ignition of the oxygen-hydrogen mixture.

With class IV leak-tightness (permissible leakage of up to 6.7*10-7 Pa*m3/s), the excessive

pressure drop in a canister will take much longer than the interval between the on-set of sealing

the first canister and completion of processing the last canister.

Since there is lack of oxygen in the canisters tightened after thermal vacuum drying, it is not

efficient to install catalysts in them.

RESULTS OF EXPERIMENTS


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Taking into account the estimated duration and exposure, as well as the quantity of resulting

radwaste, thermal vacuum drying without prior drainage was identified as the optimum approach.

The experiments verified handling safety of the prepared and sealed canisters and nuclear safety of

the spent fuel accumulation on the filtering elements.

It was determined that a gaseous fission products release during drying the canisters will not exceed

the reference level established at the Paks NPP.

As a result of the research and development activities, experimental results and analytical

evaluations were obtained to license the canister drying technology with no prior drainage of the

boric acid solution.

RESULTS OF EXPERIMENTS


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The basic technology for preparation of the canisters with spent fuel for

shipping to the re-processing facility at Mayak shall ensure:

non-exceeding the limits of personnel exposure;

safe temporary storage of the canisters with spent fuel in the cooling pool;

observance of the Paks NPP safe operation limits;

a minimal release of nuclear materials and radioactive isotopes

from the canisters;

minimization of radwaste generation.

BASIC TECHNOLOGY OF CANISTERS PREPARATION


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The technology is based on the following:

to ensure the safe transport, the canisters with spent fuel must be dried, filled with an inert gas

and sealed;

prior to drying, the lids suitable for water-filled canisters must be replaced by those suitable for

gas-filled canisters;

during storage and transport of the dried gas-filled canisters, catalysts for oxidation of hydrogen

are not used;

the canisters are dried without prior water drainage;

drying, filling with an inert gas and sealing of the canisters is performed inside a shielded

technological module installed in pit №1;

before loading into the TUK-6 overpack, the gas-filled canisters are stored in the cooling pool;

for the preparation of the canisters for transport, the existing equipment used for removal of

spent fuel from the cleaning tank should be used to the maximum extent possible (it may be

upgraded to accommodate new tasks).

THE BASIC PROVISIONS


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The technology is feasible and has a sufficient level of safety.

Currently, there are no major insurmountable technical or legal obstacles in the way of

transporting the canisters with the failed fuel to Russia for re-processing.

Simulation of the handling operations with type 28 and 29 canisters at Mayak

demonstrated that they can be processed using the plant's standard technology.

The equipment and technology of the plant are ready to receive and re-process the failed

spent fuel from the Paks NPP.

CONCLUSIONS

Thank you for your attention!

MANAGEMENT OF DAMAGED SNF HANDLING OPERATIONS AT PAKS NPP Е.А. Zvir, V.P. Smirnov Research and...

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