ML23331A686
| ML23331A686 | |
| Person / Time | |
|---|---|
| Site: | 07103103 |
| Issue date: | 07/01/2006 |
| From: | Secured Transportation Services |
| To: | Boyle R Office of Nuclear Material Safety and Safeguards, US Dept of Transportation (DOT), Office of Hazardous Materials Safety |
| Shared Package | |
| ML23331A280 | List: |
| References | |
| CAC 001794, EPID L-2023-DOT-0004 | |
| Download: ML23331A686 (11) | |
Text
Nuclear Research Institute ez a.s.
Division of Reactor Services
ÚJV Report: Z - 1336, Revision 2
Decontamination Effect on Structural Materials of
-KODA VPVR/M Cask
Prepared by: Ing. Vladimír -varc, Miroslav Picek
Department Manager: Ing. Milan Zmítko, CSc
DRS Manager: Ing. Jan Kysela, CSc
ez, July 2005
Abstract
The Report deals with the assessment of decontamination solutions and water effect on corrosion resistance of -KODA VPVR/M cask structural materials. The Report also includes some recommendations aimed at prevention or deceleration of corrosion attack of materials by more aggressive decontamination agents.
Table of Contents
Abstract 2
Table of Contents 3
- 1. Introduction 4
- 2. Objectives 4
- 3. Decontamination conditions 4
- 4. Decontamination solutions 4
- 5. Materials and coatings 5
- 6. Decontamination solutions effect on corrosion resistance of the above materials and coatings 5
- 7. Conclusion 7
- 1. Introduction
The Skoda VPVR/M cask is intended for transport and storage of spent nuclear fuel from research reactors of eastern type. The cask body is made of steel according to the SN 422707. Cask surface is designed smooth and provided with coating according to the DIN 55928, with no possibility to retain water, and is easy to decontaminate, if needed. Trunnions are made of stainless steel. Condensing of water is impossible with respect to the operational temperature (39°C).
The cask internal space is of a cylindrical shape with smooth s urface provided with aluminium spray application according to the SN 038551. In the upper and lower part of the internal space there are recesses made for the primary and secondary lids. The recess surface is provided with stainless cladding. In front surfaces there are threaded holes made to allow mounting of the secondary lids. The lids are made of stainless steels.
Removable internals of atabor sheets, a so called basket, intended for the placement of spent fuel assemblies is inserted into the cask internal space. The basket is filled with spent fuel on the pool bottom. After filling with fuel the basket is withdrawn and inserted in the cask body. When the cask internal space is completely dried, the lids are tightly closed, the space vacuumed and filled with helium with an underpressure of 0,07 MPa.
- 2. Objectives
To assess the effect of proposed decontamination solutions and water under specified conditions on degradation of cask structural materials and external coating. Technical specifications of the below materials and namely Corrosion Proceedings compiled by a team in the State Research Institute of Material Protection Corrosion Resistance of Structural Materials and Coatings were used for this purpose. The Proceedings was developed based on in-house testing, long-term operational experience and according to data in literature.
- 3. Decontamination conditions
atmospheric pressure temperatures to 60°C
- 4. Decontamination solutions
service water demi water oxalic acid 10g/l citric acid 10g/l mixture of 10g/l H2C2O4 + 1g/l HNO3 mixture of 10g/l NaOH + 5g/l KMnO4 sodium tripolyphosphate 3,5g/l oxalic acid 5g/l
- 5. Materials and coatings (exposed to decontamination solutions)
cast steel according to SN 422707.9 (in threaded holes) sheets of steel ATABOR plates of steel acc. to SN 17247.4 trunnions + primary lids of steel X3CrNiMo 13-4 pull rods of steel X2 CrNiMoN 22-5-3 suspension of steel 14Ch17N2 + 1.4310 (spring )
external coating BN-Acryl-Decke R hot aluminium coating zinc-coated bolts secondary lids of steel X3 CrNiTi 18-10 ( 1.4541 )
- 6. Decontamination solutions effect on corrosion resistance of the above materials and coatings It should be stated at the beginning that all structural materials will mainly be exposed to common atmospheric conditions and during the transport of fuel the cask internal surfaces will be exposed to helium - in dry condition. However, the exposure of materials to decontamination solutions is in hours in order, and during the year approx. in tens of hours. Water is only used for rinsing. It means that even higher corrosion rates during decontamination are acceptable since the corrosion resistance or rate indicated in the Proceedings considers more continual or permanent effect of the above environment.
I.e. of the factors dangerous during chemical decontamination by respective agents it is first of all the possibility of corrosion attack namely in places where the coating or sprayed layer is damaged.
However, the surface quality is regularly checked (see Rules for Safe Handling) and hence the local corrosion attack is practically eliminated.
6.1. Steel acc. to SN 422707.9
This material is used for the cask body which is from the outsi de protected by acrylic coating and from the inside by aluminium sprayed layer, in the area of the primary and secondary lids recess it is protected by stainless steel cladding. Threaded holes are the only places which are unprotected. As these places are rather difficult to check and pitting corrosion is likely to occur due to the contact with decontamination agents and water, we recommend to protect the threaded holes against any contact with these agents. After removing the lids the threaded holes shall be sealed, e.g. using a rubber plug or plastic bolts sealed eventually with a filler paste.
6.2. Stainless chromium-nickel steels
the following steels can be included in this group:
steel ATABOR steel acc.to SN 17247.4 steel X3 CrNiTi 18-10 steel X2 CrNiMoN 22-5-3 steel X6 CrNiTi 18-10 steel 14 Ch17N2 + 1.4310 steel X3 CrNiMo 13-14
Corrosion resistance in water In general, it can be characterized that these steels have, for the given purpose of application, very good corrosion resistance in water media, which is applicable not only for clean waters but also river, cooling and hot waters.
Corrosion resistance in solutions of citric and oxalic acids Although according to the Proceedings the resistance of chromium and chromium-nickel stainless steels in water solutions of organic acids is not acceptable at all, the usage of them for cleaning has spread approximately since the sixties as in comparison with st rong mineral acids they show lower corrosion rate which is advantageous when cleaning not very oxidized or contaminated surfaces.
The most often used organic acids also include citric and oxalic acids. According to the Proceedings the applicability of stainless chromium-nickel steels in oxalic as well as citric acid is up to the concentrations of 15% and temperatures up to 38°C, for maximum allowable loss of 0,5 mm/year (graph XII-6, page 286 and XII-8, page 278 in the Proceedings). Based on experience also a short-term exposure of these materials to the above acids is possible under higher temperatures (100 - 150°C).
The content of molybdenum substantially increases the resistance of steels against organic acids.
However, above 150°C austenitic steels fail as well.
Corrosion resistance in solutions of H2C2O4 + HNO3 and NaOH + KMnO4 Corrosion resistance of the above steels in these solutions results from some tests carried out at the University (V-CHT). Based on the tests performed to determine corrosion resistance of stainless austenitic steels of type 18-8 (by means of a method to determine corrosion losses) in the above mentioned as well as other mixtures it was detected that corrosion rates were very low and negligible with respect to corrosion. Therefore, attention was paid to possible increase of steel susceptibility to some type of local corrosion attack due to decontamination. Tests were carried out with samples of steel 17 248 which were subject to metallographic evaluation of point corrosion attack.
The samples exposed in alkaline potassium permanganate proved to be the most resistant, both at 22°C and at 50°C. Also other methods (e.g. cyclic voltammetry methods ) used for the determination of susceptibility to local corrosion proved that the resistance is maximum when applying alkaline potassium permanganate.
Corrosion resistance in the solution of sodium tripolyphosphate These salts contain anions in which there is a respective number of PO 4 groups. It is penta-sodium triphosphate. When assessing behaviour of these steels to sodium phosphate in general, they are highly resistant under all concentrations and temperatures.
6.3. External coating BN-Acryl-Decke R
Acrylic polymers are highly resistant against atmospheric effects. They resist water, diluted solutions of acids and alkali and solutions of inorganic salts. As regards organic compounds, they are aliphatic hydrocarbon as well as mineral and vegetable oils resistant They are also micro-organism resistant.
The same coating is used for CASTOR and CONSTOR casks of GNS Essen company intended for spent fuel from power reactors.Decontaminability of this coating is documented by a Report issued by the Testing Laboratory of Forschungszentrum Julich,SRN dated 25.10.1999.
6.4. Aluminum hot coating
Aluminium of 99,9% purity was used for spraying. Aluminum hot coating is very suitable namely for long-term protection of structures against atmospheric corrosion and corrosion caused by water even at higher temperatures (180°C).
The use of oxalic and citric acids is possible, namely up to the temperatures of 38°C, and even slightly higher for a short period of time. We recommend not to use solutions with nitric acid or hydroxide.
Sodium phosphate cannot be used for aluminum either. Decontami nation Manual will be developed for the assessed cask which will include decontamination regimes and composition of particular decontamination solutions.
6.5. Zinc-coated bolts
Zinc is relatively resistant against atmospheric corrosion. It is not resistant against acids and alkali. In distilled water and demineralized water the zinc corrosion is low but when saturating with air with the presence of CO2 and at higher temperature the corrosion rate increases. Chlorides, sulfates and nitrates accelerate corrosion.
6.6. Used the decontamination means and methods at reprocessing plant (Mayak)
In accordance with the standard technology of transport packagi ng preparation, casks and wrappers (fuel assemb lies) undergo rad ioactive contamination control and de-contamination procedures.
A decontamination process is car ried out in a chain of rooms and by means of different equipment, each responsib le for a certain process sta ge: cask preparation, de-contamination, radiation monitori ng, drying and painting. The a rea is equipped for re-ception, storage, preparation a nd release of decontaminating so lutions.
Decontamination provides for the step-by-step realization of th e following opera-tions:
Decontamination of the inside ca sk surfaces by means of special ly equipped false covers that are connected through hosepipes with quick-de tachable joints to water, vapor, decontaminating solutions, compressed air, vacuum and bl owing communica-tions. Decontamination is carried out with the help of hydraulic monitor or vapor ejec-tion mouthpieces (VEM). It is possible to decontaminate the insi de cask surfaces manu-ally with the help of vapor ejection sprayers (VES) or decontaminating solutions.
Decontamination of the outside c ask surfaces and cask covers in special tanks equipped with a VEM set. De contamination is realized by means of vapor ejection or with the help of a decontaminating solution that is exposed in the tank during heating and agitation in accordance with the norms.
Manual decontamination of local cask contamination at the point of final manual treatment (hereinafter - FMT) by means of pumps with dec ontaminating solu-tions for VES, as well as by scrubbers, rags, etc.
Decontamination of cask co vers in tanks or manually.
Cask decontamination is fulfilled in accordance with the following norms:
For inside surfaces decontamination with the use of a false cover:
Vapor, pressure not more than 0,6 P (6 gf/sm2)
Decompressed air, pressure 0,5 P (5 gf/sm2)
Hot water, pressure 0,9 P (9 gf/sm2) and temperature (70+/-10)
Temperature of the supplied so lution not more than 65
Solution consumption from 0,1 to 0,15 m3/h Vapor consumption from 5,0 to 7,5 m 3/h Duration of the solution supply 40 min
Vacuum gage pressure in the cask not less than 3 P (300 mm w.lm.)
For inside surfaces d econtamination with t he use of a solution:
Solution temperature not more than 65°C Time of solution exposure during weak agitation from 0,5 till 8,0 h Compressed air agitation is possible if vacuum gage pressure is provided in the cask 1 P (100 mm w.lm.)
For outside surfaces with VEM use:
Temperature of the supplied solution not more than 65°C Solution consumption from 0,1 to 0,15 m3/h Solution volume requir ed for decontamination from 5,0 to 7,5 m3 Vapor consumption from 5 to 7,5 m3/h Decontamination duration from 60 till 480 min
For outside surfaces decontamin ation by a immersion method:
Temperature of the supplied solution not more than 65 Vacuum gage pressure in the tank 3 P (300 mm w.clm.)
Exposure duration during agitation 480 min
Solution volume in -85510/1,2 10,8 m3 - 94,0 m3 Air consumpti on for agitation 15 m3/h - 40 m3/h After decontamination and solution release the cask is washed with hot water dur-ing 15 minutes.
For final manual treatment of the outside surfaces with VED use:
Decontaminating solutions, pressure 0,3 P (3 gf/sm2)
Hot water, pressure Not more than 0,9 P (9 gf/sm2) temperature (70+/-10) 0 Vapor, pressure Not more than 0,6 P (6 gf/sm2)
Compressed air, pressure Not more than 0,5 P (5 gf/sm2)
For decontamination of cask covers:
Temperature of the supplied solution no more than 65 Solution consumption from 0,1 to 0,15 m3/h Solution volume required for decontamination from 8 to 14 m3 Vapor consumption from 5 to 7,5 m3/h Decontamination du ration with the use of vapor ejection mo uth pi ec es 120 min Vacuum gage pressure in the cask 3 P (300 mm w.clm.)
After decontamination and soluti on release the cover is washed with hot water during 15 minutes.
Composition of the main d econtaminating solutions a) Solution -1 has the following composition:
sulphanole (or -10), mass concentration... (7,5+/-2,5) g/l b) Solution -2 has the following composition:
sulphanole (or -10), mass concentration... (7,5+/-2,5) g/l Oxalic acid, mass conc entration. 5 g/l Natrium tripoliphosphate, mass concentratio n.. 3,5 g/l Comment
- It is possible to use trilon B instead of oxalic acid with the same concentration.
Recommendations and warnings
During of decontamination vent-holes of the cask should be protected (by temporary coating of a mastic type) from any contact with decontaminating agents.
This will help to avoid local corrosion.
It is not recommended to use a mixture of oxalic and nitric acids for chrome nickel stainless steels. A mixture of hydroxide with potassium permanganate seems to be more suitable. It should be settled between the user (manufacturer) and spe-cialists of the processing plant in the process of Decontamination Manual devel-opment.
It is not recommended to use solutions containing nitric acid or hydroxide in the process of aluminum coating decontamination. Sodium phospha te cannot be used for decontamination.
Decontamination Manual will be based on decontamination of the inside surfaces up to accept able norms only with the use of hot condensate.
To provide the required conditions for -KODA VPVR/M casks decontamination with the help of condensate during standard operation, it is necessary to create a sepa-rate decontamination area at Mayak plant supplied with the special equipmen t pre-sumably including the following devices:
Device Quantity 2
High pressure washer without wat er heating 2 Type HD 895 S
1 High pressure washer with wa ter heating 1 Type HDS 801 1 High pressure washer with water heating and a vaporization mo de Type SB-HDS 695 1
- 7. Conclusion
Several recommendations result from the above outline. Namely during decontamination the threaded holes shall be protected against any contact with decontamination agents and water with respect to the possibility of local corrosion attack.
For the same reason we do not recommend to use a mixture of oxalic and nitric acids for chromium-nickel stainless steels but first to test other solutions. A mixture of hydroxide with potassium permanganate seems to be more favourable. The Decontamination Manual will be modified accordingly - it shall be agreed upon between the user (manufacturer) with the transporter and re-processing plant specialists.
As regards the aluminium spray, we do not recommend to use solutions with nitric acid or hydroxide for decontamination. Sodium phosphate cannot be used for decontamination.
Therefore, the Decontamination Manual will be prepared so that the internal surfaces can be decontaminated in an acceptable way, e.g. using hot condensate only.
The Decontamination Manual will be supported by particular corrosion tests results in accordance with users' requirements.
References:
- 1. Technical specifications of respective materials
- 3. Hluchá V.: Decontamination effect on corrosion resistance of steel, V-CHT, Praha, 1985