Spent Nuclear Fuel Air Shipment: Safety Justification
S.V.Komarov , B.A.Kanashov, O.P.Barinkov, A.N.Dorofeev
Nuclear & Enviromental Safety, №1, 2010
Spent fuel assamblies (SFAs) has been traditionally shipped across Russia only by rail before. The International Program on Russian-Origin Research Reactor Fuel Return (RRRFR) has assigned new tasks to nuclear power enterprises making them involve almost all types of conveyances including air transport.
Most of the research reactors cannot receive heavy large-size containers on their sites. The use of Russia made TUK-19 casks weighing up to 5 tons does not require significant, upgrades to the research reactor (RR) equipment moreover, analyses have shown that it is good for RR SNF shipments by all conveyances including air transport. So, a choice was made in favor of the TUK-19 casks for SFA transportation from the IFIN-HH Institute (Magurele, Romania).
The task implementation involved the following activities:
— develop an ovcrpack for transportation of the TUK-19 packages by various conveyances;
— perform a safety analysis of the package design for air transportation;
— perform a safety analysis of the package shipment.
The project involved the consignor (FSUE "PA "Mayak"), the air carrier (Volga-Dnepr Airlines), the fabricator of the freight containers (Abakanvagonmash, JSC the fabricator of TUK-19 tie-downs (Ozersk Non-Standai Equipment Plant, QJSC), the Russian Maritime Register Shipping (RMRS), expert organizations (FSUE "RFNC–VNIIEF", FSUE "ETC SPb") and the developer oft technology and equipment (Sosny R&D Company). All the activities were performed under the auspices and control of nuclear and radiation safety and licensing department of Rosatom State Nuclear Energy Corporation. The customer was the National Commission for Nuclear Activities Control of Romania (CNCAN).
DEVELOPMENT OF OVERPACK
To ensure multi-modality of the shipment, an overpack was developed on the basis of a specialized freight large-capacity 20-foot ISO-container (the specialized ISO-container) that accommodates three TUK-19 casks (fig.1). The specialized ISO-container meets ISO standards in terms of handling оperations during transportation. As for strength characteristics the specialized ISO-container satisfies industrial norms for transportation of hazardous cargoes by various convey¬ances, i.e. by air, road, rail and sea. The TUK-19 packages were fastened with turnbuckles capable of withstanding acceleration and vibrations typical of all types of conveyances.
A set of tie-downs for each TUK-19 cask consists of a supporting ring and four turnbuckles. With their lower ends the turnbuckles are connected to the brack¬ets welded to the floor of the container, and with their upper end — to the clamp embracing the TUK-19 cask.
The specialized ISO-container was subject to expertise for compliance with standards at RMRS. After successful tests at test facilities certified by RMRS, the specialized ISO-container was allowed to ship cargoes with Certificate of Approval 08.11146.130.
JUSTIFICATION OF PACKAGE DESIGN SAFETY
Russian (NP-053-04) and international (TS-R-1) reg¬ulations allow for air transportation of radioactive fissile materials in Type В packagings with activity restrictions. Due to the fact that the activity of the "hottest" Romanian SFAs did not exceed 3000 A2 (3000* 1013 Bq) specified in the regulations, it was decided to certify the package as Type B(U). The most difficult task was to prove that an individual package remains subcritical under enhanced tests: a puncture (rupture), an enhanced thermal impact and a drop. The regulatory requirements state that the packages of fissile materials to be transported by air should ensure nuclear safety after two series of additional enhanced tests simulating an air crash. So, VNIIEF experts made calculations of the dynamic deformation and performed a safety analysis of the TUK-19 package under mechanical impacts simulating normal operating conditions and non-standard transport conditions including an air crash. The results demonstrate that the package retains its initial strength and containment after an impact on an individual package under accident conditions (a drop from a height of 9 m, falling onto a bar from a height of 3 m, a drop of a 500 kg mass from a height of 9 m). After the tests the cask still allows removal of the SFA-containing basket and the SFAs from the basket. An impact onto a rigid target at a velocity of 90 m/s under different angles destructs the lid bolts. But the cask body (the radiation shield) retains its integrity, the basket and the SFAs get deformed and the SFAs and the fuel rods get destructed. In most cases the basket and the SFAs do not spill out of the cask. When deformed, the lid can get stuck in the TUK-19 body. In compliance with the certification procedure cur¬rently used in the Russian Federation, experts of Nuclear Safety Department of SSC - IPPE performed an independent assessment of the package nuclear safety. In the worst case of an impact onto a rigid target Keff is 0.872 that does not exceed the regulated value of 0.95. Transportation of the TUK-19 casks containing S-36 SFAs by road and air was shown to satisfy nuclear safety regulatory requirements.
Since the maximum total residual heat release in the package containing four SFAs was less than 9 W, the temperature on the TUK-19 external surface under normal conditions did not exceed 40°C.
Calculated and measured radiation levels of the TUK-191 cask loaded with four SFAs with the maximum radiation characteristics demonstrate that u nder normal transport conditions the dose rate on the package surface (0.158 mSv/h) is much less than the allowable value (2 mSv/h). Since under accident conditions the radiation shield remains intact, the dose rate at a distance of 1 in from the package surface will not exceed 0.38 mSv/h that is much less than the allowable value (10 mSv/h).
The regulations set activity limits in case of a radioactive release from the package. Under normal transport conditions the maximum allowable value make up 10-6 A2/h, under accident conditions - 10*A2 per week for Kr-85 and A2 for all other radionuclides. According to the calculations, the loss of the radioactive contents from the TUK-19 cask for 1 hour under normal transport conditions can make up 5.7* 103 Bq that is equal to – 0.06% of the A2 value (1*1013 Bq), and during an accident - not more than 0.005%.
TRANSPORT SAFETY JUSTIFICATION
In radiation risk assessments during an SNF shipment, the TUK-19 cask was taken as a hazardous production object; so, NPP norms and regulations were applied. The accidents with a probability higher than 10-7 in anticipation of one shipment per year were considered to be design-basis, and those with a probability less than 10-7, but higher than 10-10 – beyond-design-basis (they can require public protective measures). The criterion for taking protective measures is 0.1 Sv; the same value is chosen as the maximum dose value for the personnel involved in the shipment and design-basis accidents.
Fig. 2 presents a tree of events (emergencies) during the SNF shipment from Bucharest to Ekaterinburg that include accidents and a probability of emergencies.
Under normal air transport conditions, the SFA-containing cask ensures safety of both the escort personnel, and the population. The acceptable risk criterion for the es¬cort personnel is satisfied due to the SNF package peculiarities and radiation monitoring, and for the population — due to observance of regulatory requirements.
The analysis of emergencies shows that there are no probable events (P>1*10-7) that can be assessed as accidents on the INES scale (Fig. 3). Probable events (1, 3, 8) are considered to be incidents, and events with harder consequences (4-7) are low probable. Event 2 is unlikely and do not have serious consequences.
In design, particular attention was paid to emergen¬cy response actions. With ETC SPb actively involved, Emergency Card 7-02 was developed to specify primary actions of the aircraft crew, the escort personnel and emer¬gency response services in case of an accident.
Certificate of Approval for Package Design and Shipment RUS/3104/B(U)F-96T was issued after all safety justification activities.
In the evening on 29 June 2009, a guarded vehicle convoy with overpacks left the IFIN-HH site and headed to the airport of Bucharest, where the containers were loaded into an AN-124-100 aircraft. The flight track passed by large populated localities and hazardous production facilities, and it lied over the Black Sea to avoid crossing of the airspace of third countries. After an refueling stop in Ulyanovsk, the aircraft successfully landed at the Koltsovo airport, from where the consignment was delivered to PA "Mayak".
The Romanian VVR-S reactor SNF removal project demonstrated that air transportation of SNF packages with limited activity is feasible and safe.
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