Choice of Transport Routes For Bilibino Spent Fuel on the Basis of Radiation Risks Assessment

  

E.V. Suvorova, A.V. Khaperskaya (Rosatom Corporation), A.A. Stroganov, A.V. Kuryndin, A.S. Shapovalov (SEC NRS), A.V. Detkina, O.P. Barinkov, A. N. Dorofeyev (Sosny R&D Company)

Nuclear & Enviromental Safety, №3-4, 2013

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In connection with the decision to end operation of Bilibino NPP, its reactors are due to be shut down for good starting in 2019. Within the framework of the Federal Target Programme "Nuclear and Radiation safety Assurance for 2008 and Until 2015", Sosny Research and Development Company had in 2011 developed a technology to prepare the Bilibino spent fuel for shipment to a reprocessing plant or repository, and produced a safety justification for the technology. Some results and plans for Bilibino spent fuel management were discussed in the article Interim results and prospects on Bilibino spent fuel management published in the Nuclear and Environmental Safety edition 3 of 2012. Since then, Rosatom Corporation identified reprocessing at Mayak as the final stage of management for this kind of spent fuel. In order to select the route, mode of transport and transport overpacks to be used, an assessment and comparison had to be performed of radiation risks that may occur during the performance of associated operations.
Resulting from a series of technical and economic evaluations, two basic routes for spent fuel transport were selected: "sea" (using sea, automobile, and rail transport) and "air" (using air, automobile and rail transport). For the "air" route, two classes of spent fuel overpack were considered: B(U) and С.

Basic Data for Spent Fuel Transport Routes

The possibility of building a class C overpack for air transport of radioactive materials is confirmed by the experience of engineering, fabrication and use of TUK-145/C transport overpacks. That package was successfully used in 2013 for shipments of irradiated research reactor fuel from Vietnam and Hungary within the framework of the RRRFR programme. Calculations of radiation risks associated with spent fuel transports from Bilibino NPP were performed by Sosny Research and Development Company jointly with Scientific and Engineering Centre for Nuclear and Radiation Safety using the Intertran-2 computation code, which is recommended for use by the RB-039-07 safety guide and developed within the framework of the IAEA-sponsored research programme entitled "Coordinated Research Program on The Probabilistic Safety Techniques Related to the Safe Transport of Radioactive Material".
The following were used as inputs for calculations:
– spent fuel transport route lengths as a sum of individual sections;
– annual rates of expected spent fuel transport using various modes of transport;
– technical characteristics of transport overpacks;
– spent fuel characteristics (radionuclides composition and levels of activity);
– number of packages simultaneously loaded upon a transport vehicle;
– duration of spent fuel transport over each section of the route;
– conditions of transport and composition of transport vehicles, number of potential stops en route;
– number of personnel involved with spent fuel transport;
– population density within various sections of the route;
– number of potential accidents per 1 km of route [1, 2];
– accident severity category and share of potential accidents during transport with a certain severity category [3] pre-postulated in accordance with NP-053-04 classification [4];
– share of radionuclides released from the package depending on category of accident severity.

Methods Of Radiation Risks Asesment

For assessment of radiation risks associated with spent fuel transport, two approaches were used. The first is based on measuring radiation risk as per NRB-99/2009 [5] as probability that a person or his/her descendants will develop a harmful health condition as a consequence of exposure to radiation. For normal conditions of spent fuel transport, calculations were performed of maximum individual effective exposure doses to personnel and members of the public, and then to calculate maximum individual radiation risks these were multiplied by the averaged risk coefficient, assumed to be 0.05 Sv-1 in accordance with clause 2.3 in NRB-99/2009. The resulting values were compared to the NRB-99/2009 limits for individual lifetime risk (1*10-3 for Group A personnel, 2.5*10-4 for Group B personnel, and 5*10-5 for the general public).
Conditions of potential accidental exposure were evaluated as the sum of products of maximum individual effective doses multiplication by the corresponding probabilities of adverse events and by the averaged risk coefficient assumed to be 0.05 Sv-1. The resulting values were compared to generalised risk limits (2*10-4 year-1 for personnel, 1*10-5 year-1 for the public). It is important to note that the applicability of this approach is limited to the effective dose of 500 mSv.
The second approach is based on the definition of risk as adopted by the IAEA and implemented by the Intertran-2 code: "risk is the product of event probability times its consequence". For spent fuel transport in normal and accident conditions, consequence is understood as collective effective dose (man-Sv) to various groups of the public and personnel, which may be exposed to radiation emitted by the contents of transport overpacks.

Radiation Risks Asesment Results

In normal conditions during transport of Bilibino spent fuel for reprocessing following all considered routes, using all considered modes of transport and transport overpacks, calculated radiation risks for personnel and the general public are within the annual limits on radiation risks established by NRB-99/2009.
On the basis of obtained data that describe the consequences of potential accidents the conclusion was made that it is possible to ensure safe (within the provisions of NRB-99/2009 and NP-053-04) spent fuel transport using class B(U) packages and following the "sea" route. In this case, generalised annual radiation exposure risk is 5.1*10-7, which is slightly above 5% of the limiting value for the public.
When spent fuel is shipped inside class B(U) packages following the "air" route, maximum accidental radiation exposure risk is 4.82*10-6 (some 50% of the limiting value for the public).
Air transport of spent fuel inside class С packages offers the lowest generalised radiation risk of accidental exposure as compared to other suggested transport options – 3.01*10-7, which is far below the limits stipulated by NRB-99/2009. Collective effective dose as computed using the Intertran-2 software package for normal conditions of transport is 0.19 man-Sv for spent fuel transport inside class B(U) package by air; 0.113 man-Sv by sea; 0.075 man-Sv inside class C package by air. Intertran-2 computation results for accident conditions are summarised in Table 2. The values are obtained using dose coefficients built into the software by its developers [6], as well as the results of calculations using dose coefficients as per NRB-99/2009 [5].

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Therefore, on the basis of review of calculation results, the recommendation was issued in favour of the "air" route for shipment of Bilibino spent fuel. From the safety assurance viewpoint, the preferred option is air transport of spent fuel inside a class C package.

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