Experimental and Calculation Investigation of Fire and Explosion Safety during Handling RBMK-1000 Spent Fuel

   

S. Amosov, A. Detkina, A. Leshchenko, S. Semenovykh (Sosny R&D Company), P. Ilyin, V. Teplov (SSC RIAR)

VII International Nuclear Forum ATOMTRANS-2012, S.-Petersburg, Russian Federation, 17-21 September, 2012

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Introduction

To date the NPP with RBMK-1000 reactors have accumulated a great quantity of spent fuel. The dry storage
with its subsequent removal to the centralized storage is considered as the main technology for the RBMK-1000 spent
fuel handling.

This solution was proved for the defective spent fuel assemblies, i.e. spent fuel assemblies suitable for the
longterm dry storage. The methods of handling defective fuels assemblies are being developed now. One of the ways to
solve the problem of handling the defective spent fuel is its reprocessing at the RT-1 plant at Mayak.

In 2010 Rosatom State Corporation initiated the project “Safe Handling of the RBMK Spent Nuclear Fuel.
Reprocessing Feasibility Evaluation”. This project provides for the preparation for the regular transportations of the
defective RBMK spent fuel assemblies for reprocessing.

Transportation of the defective (mainly, leaky) spent fuel assemblies must be performed in the leaktight
capsules in the TUK-11shipping casks. During the temporary storage and transportation of the wet RBMK spent fuel
assemblies in TUK-11cask there is a hazard of accumulation of the radiolysis products, in particular, hydrogen.
The feasibility evaluation of the fire and explosion safety that was made during the successful pilot removal in
2011 showed that handling of the leaktight canisters containing two leaky fuel rods (without the preliminary drying)
was admissible for no longer than five months. But this period is too short for the regular transportations. Therefore, the
calculation results were verified during the experimental investigations into the accumulation of hydrogen in the
capsules containing real leaky RBMK fuel rods.

The purpose of this paper is determine experimentally the kinetics of the hydrogen accumulation in the
conditions simulating the transportation of the RBMK-1000 spent fuel assemblies with water found under the leaky fuel
rod claddings and compare them with the calculation results of accumulation of radiolytic hydrogen in the leaktight
capsules containing RBMK-1000 fuel rods.

1. Subjects of research

The experiments for determination of the hydrogen accumulation kinetics in the leaktight capsules containing
the RBMK-1000 leaky fuel rods with water under claddings were carried out using the fuel rods the parameters of
which provided the conservative evaluation of the hydrogen accumulation kinetics. The experiments were carried out in
the shielded cell conditions.

The RBMK-1000 spent fuel assemblies intended for transportation and reprocessing are enriched to 2.0-2.4 %.
With respect to the radiation safety, the cooling period before transportation and reprocessing must be no less than
10 years. With respect to the cost effectiveness of the spent fuel reprocessing, the fuel burnup must not exceed
24 MWd/kgU.

Therefore, four RBMK-1000 spent fuel rods (enrichment 2.4%, burnup 24 MWd/kgU, cooling time 10 years as
of the beginning of the experiment) were selected for the experiment. Before placing into the capsule the upper plugs of
fuel rods were cut off.

The kinetics of the hydrogen accumulation was determined at the shielding cell temperature (30–40 °С) and at
80 °С that corresponds to the maximum calculated temperature of the fuel claddings during transportation of the
RBMK-1000 spent fuel assemblies with the given parameters in TUK-11.

2. Experimental facility and processing of measurement results

The equipment for filling fuel rods with water and simulation of the transportation conditions consisted of the
following components:
− capsule for location of the fuel rods;
− auxiliary system for the capsule preparation;
− sampling system.

After operation and storage in the cooling pool the plenum of the leaky fuel rod can be completely filled with
water. The capsule with fuel rods (the plenum upward) were previously evacuated and then filled with water along the
entire height of the fuel column. Water from the capsule spare volume was drained out under the argon pressure that
created an inert atmosphere in the capsule. The fuel rods, including the plenum, were completely filled with water.
After sealing the capsule the fuel rods were in an argon atmosphere at a pressure of about 1 bar.

The schematic diagram of the sampling facility is shown in Fig. 1. The volume of the sampling system
(including the sampler) is about 100 less than the capsule volum that provides a minimum sampling effect on the
processes inside the capsule. Analysis of the sample composition in the gas phase from the capsule was performed out
using an MI-1201 mass spectrometer.

The amount of hydrogen generated by the water electrolysis was determined considering the decrease of the
amount of hydrogen in the capsule due to sampling by the formula:

The relative error of the hydrogen amount in the capsule is practically fully determined by the mass
spectrometry error of the gas composition and makes up no more than 20.1 % of the measured value in the
concentration range of up to 0.1 vol.% and no more than 10.1 % of the measured value in the concentration range
exceeding 0.1vol. %.

An in-cell dry storage testing facility was used (Fig.2) to maintain the temperature of fuel rods at about 80 °С
for a long time.

On completion of the investigation of the hydrogen accumulation at the shielded cell temperature of 30–40 °С
the capsule cover was opened. Water was removed from all four fuel rods using a syringe fitted with an extension. Then
the fuel rods were reloaded from the RBMK spent fuel transportation simulation facility into the dry storage facility
capsule. The fuel rods in the capsule were located vertically (plenum upwards). The capsule was tightly closed with a
cover.

The temperature control error of the heating elements in the dry storage facility (±4 °С) causes vibrations
inside the capsule that hinders the calculation of the gas phase in the fuel rod capsule. In this connection a method was
developed to calculate the quantity of hydrogen accumulated in the capsule based on possibility to calculate the quantity
of argon in the capsule at each sampling. The amount of argon as of the moment of the i -th sampling is determined as:

3. Discussion of results

Kinetics of radiolytic generation of hydrogen in the capsule containing leaky RBMK-1000 fuel rods at 30 and
80° is shown in Fig. 3. The highest hydrogen accumulation rate was obtained in the experiment at 80°С in the absence
of water in the fuel rod plenum and was as high as 3.03·10-6 mol/hr for four fuel rods.
The significant difference between the hydrogen generation rate at 30 and 80 °С cannot be explained by only
the temperature dependence of the primary hydrogen release. Probably, the great effect is made by the presence or
absence of water in the plenum.

In accordance with the Henry’s law, hydrogen concentration in the capsule gas volume is directly proportional
to its concentration in water found in the leaky fuel rod plenum. Water radiolysis in the plenum is caused only by the
fuel gamma radiation (the path of alpha and beta particles in water is ~0.04 and ~0.85 mm, respectively, for a plenum of
170 mm height), i.e. the absorbed dose rate for water in the plenum is several times less than for water in the fuel part of
the fuel rod, and concentration of hydrogen in the plenum water will be always lower. The removal of the molecular
hydrogen by diffusion to the liquid/gas phase interface and desorption from the water surface into the gas phase is in
competition with the back chemical reactions running in water and leading to the recombination of the radiolysis
products with formation of water molecules. The higher is the water column, the less is the probability of its
recombination in the back reactions, for instance: H2 + OH → H + H2O.

The hydrogen accumulation rate inside the leaktight capsule with one fuel rod bundle was calculated based on
the following radiation characteristics of the RBMK-1000 fuel assembly [1]:
− initial enrichment in 235U 2%;
− fuel burnup 24.9 MWd/kgU;
− cooling time 10 years.

The temperature of the fuel rod claddings and gas inside the capsule was taken equal to 80 °С based on the
results of the thermal calculation. It was assumed that the inner volume of the leaky fuel rod was fully filled with water.
In this case the hydrogen generation is caused by the water vapor radiolysis in the capsule gas volume due to the gamma
radiation and from radiolysis of water in the leaky fuel rod due to alpha, beta and gamma radiation.

Evaluation of the hydrogen generation rate from radiolysis of water or water vapor was performed using
equation [2], which does not take into account the back reactions:

The calculated hydrogen generation rate only due water radiolysis inside one leaky fuel rod was
3.17·10-6 mol/h that is more than four times exceeds the hydrogen generation rate obtained experimentally at 80 °С
(3.03·10-6 mol/h for a capsule containing four fuel rods). The difference can be obviously explained by the factors
described above:
− the dependence of the hydrogen concentration in the gas capsule volume on its concentration in water of
the leaky fuel rod plenum (Henry’s law);
− the presence of the back reactions running in water and leading to the recombination of the radiolysis
products with production of water molecules.

Another important factor effecting the reduction of the radiolytic hydrogen release is related to physicalchemical
processes of the uranium dioxide oxidation-reduction at the fuel/water boundary of contact.

Oxidizing agents produced in the process of radiolysis (mainly, peroxide, oxygen and free radicals) react with
U(IV) at the fuel boundary forming U(VI). When the stoichiometric matrix achieves the U3O7 composition, the uranium
matrix is destroyed and dissolved in water [3]. This process does not reduce the probability of the molecular hydrogen
recombination by the oxidants. Peroxide makes the main contribution (98-99%) to oxidation of uranium dioxide in the
reaction chain generating hydroxyl radicals.

However, the multiple experiments revealed a significant effect of suppression of the water dissolution process
in the presence of the molecular hydrogen [3, 4].
Besides the recombination reactions the other mechanisms are available: hydrogen reduction of the dissolved
U(IV) or U(VI) in the solid phase on the fuel surface. It was proposed that the reduction of the solid uranium phase can
be catalyzed by nanometric particles of noble metals available in spent fuel [5] by the reaction:

The noble metal particles, called ε-particles, consist of the mixture of the metal fission products (Mo, Pd, Ru,
Nc и Rh) [6]. The reaction between H2 and radiolytic peroxide catalyzed by ε-particles is also possible [7].
Nevertheless, in any case molecular hydrogen is used for suppression of the fuel dissolution by hydrogen
peroxide.

Conclusion

The experimental data were obtained for the radiolytic hydrogen accumulation rate in the leaktight capsule
containing the leaky RBMK-1000 fuel rods filled with water. The results were compared with the evaluation of the fire
and explosion safety during transportation of the defective RBMK-1000 fuel assemblies in the leaktight capsules
without the preliminary drying.

The significant difference between the calculation results and the experimental data can be explained by the
ignorance of the following factors:
− the dependence of the hydrogen concentration in the gas phase on its concentration in liquid;
− the presence of the back reactions leading to the recombination of the radiolysis products with production
of water molecules.

While ensuring the transportation safety of the leaky RBMK-1000 fuel, in case, if the leaky fuel rod, including
its plenum, is fully filled with water, the rate of the hydrogen release into the capsule gas volume is determined by the
water radiolysis conditions in the plenum due to gamma radiation and the hydrogen mass transfer conditions between
the liquid and gas phases. Alpha and beta radiation of the leaky fuel rod has an insignificant impact on the hydrogen
accumulation rate in the capsule gas volume, and the plenum filled with water acts as a “gas lock”. This was
demonstrated by about an n-th order difference of the hydrogen release rate in the experiments with and the plenum
filled with water and without it given the fact that the dependence of the radiation hydrogen release on the temperature
was much lower.

References

[1] Radiation characteristics of irradiated nuclear fuel: Reference book/ V. Kolobashkin, P. Rubtsov, P.
Ruzhansky, V. Sidorenko. – M.: Energoatonizdat, 1983.
[2] S. Kabackchi, G. Bulgakova, Radiation chemistry in nuclear fuel cycle. M.: D. Mendeleev RUCT, 1997.
[3] J. Bruno, E. Cera, M. Grivé, Experimental determination and chemical modeling of radyolitic processes at
the spent fuel/water interface. SKB Technical Report TR-99-26, 1999, Svensk Kärnbränslehantering AB.
[4] P. Carbol, J. Cobos-Sabate, J. P. Glatz, B. Grambow, B. Kienzler, A. Loida, A. Martinez, E. Valiente, V.
Metz, J. Quiones, C. Ronchi, V. Rondinella, K. Spahiu, D. H. Wegen, T. Wiss, The effect of dissolved hydrogen on the
dissolution of 233U doped UO2(s), high burn-up spent fuel and MOX fuel. SKB Technical Report TR-05-09, 2005,
Svensk Kärnbränslehantering AB.
[5] Cera E, Bruno, Duro L, Eriksen T E. Experimental determination and chemical modeling of radiolytic
processes at the spent fuel/water interface. SKB Technical Report TR 06-07, Stockholm, 2006, Svensk
Kärnbränslehantering AB.
[6] Nilsson S., Jonsson M., On the catalytic effect of Pd(s) on the reduction of UO2 2+ with H2 in aqueous
solution, J. Nucl. Mater., 2008, V. 374, P. 290-292.
[7] Nilsson S, Jonsson M., On the catalytic effects of UO2(s) and Pd(s) on the reaction between H2O2 and H2 in
aqueous solution, J. Nucl. Mater., 2008, V. 372. – P. 160-163.

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