Dealing with legacy SNF21 July 2013
Plans to remove and store legacy spent nuclear fuel from marine applications in Russia are well-advanced. By S. A. Dushev and A. V. Timofeev.
?In compliance with the strategy adopted by Russian State Corporation Rosatom, planning and implementation of measures aimed at decommissioning nuclear and hazardous radiation-contaminated objects should follow the approaches and principles adopted by IAEA without shifting the burden of decommissioning costs onto future generations. In the coming years it is necessary to render such objects safe by removing their spent nuclear fuel.
To date, the Russian Federation and global partnership member-states have directed their efforts towards solving the problems related to the disposition of legacy nuclear materials. Major efforts are being performed by Rosatom enterprises with involvement of foreign donors aimed at developing technical and economic schemes, technologies and nonstandard equipment for fuel handling.
It is obvious that in the process of rehabilitation of various installations, in-house SNF handling flow charts providing SNF unloading and handling will be used in compliance with the storage types, storage conditions and fuel state. The options shall be thoroughly investigated and agreed upon by all the interested organizations (including a reprocessing plant) and regulatory authorities.
In the northwestern region of the Russian Federation there are several nuclear and radiation hazardous objects that have been decommissioned and that are at present subject to rehabilitation. One of them is the Maintenance Vessel Lepse (MV Lepse) (Figure 1) which has already entered into its comprehensive dismantling stage.
MV Lepse provided reactor plant refueling of Lenin, Arktika and Sibir nuclear icebreakers between 1963-1981. In 1988 the vessel was decommissioned, and up to the present day it has been used as a storage for SNF, radioactive waste and process tools.
The spent fuel assemblies (SFA), discharged from nuclear icebreaker reactors at different times, are placed in tubes and caissons of the hold beneath the waterline, in concrete and steel-lined containers.
A significant proportion of SNF has been damaged in some way, including the absence of grippers, which prevent them from being removed with standard methods. To provide safe handling of the SNF, a detailed analysis of the state of spent fuel assemblies was carried out, and based on its results an optimal SNF handling flow chart has been developed (Figure 2).
Activities for the comprehensive dismantling of MV Lepse will be carried out on a rigid base. First, the SNF storage unit will be cut out. A protective shelter and process module to provide the personnel safety and environmental protection in the process of SNF unloading is planned. Moreover, the shelter will include a Lepse hold sector simulator mockup.
SFA unloading technology includes the following three main stages:
- Unloading of the SFA removed from storage tubes with subsequent loading into transport canisters designed for five SFAs
- Unloading of storage tubes containing SFAs that are non-removable with subsequent loading into transport canisters designed for three tubes
- Unloading of SFAs from storage caissons, single-piece arrangement in special overpacks with subsequent loading into single-piece transport canisters.
All the removed SFAs can be scanned if needed to determine the state of fuel components using special equipment developed at Joint Stock Company N.A. Dollezhal Research and Development Institute of Power Engineering (JSC NIKIET). To provide complete unloading of SFAs a technology ensuring restoration of the gripper (top nozzle) has been proposed.
A full-scale mockup the of MV Lepse storage hold fragment has been developed and fabricated (Figure 3) with the aim of testing operability and adjustment of unloading equipment devices, mastering operations technologies for SNF unloading, teaching personnel needed to operate the equipment and simulating emergency situations arising in the process of operations. Technical specifications for remote-controlled SFA extraction devices have been developed.
Work has practically entered the production stage and is expected to finish in 2015-16.
Andreyeva Bay and Gremikha Village
Another environmental target to be solved in northwestern Russia is rehabilitation of the territories of the Navy former maintenance facilities located at Andreyeva Bay and Gremikha village.
A large amount of SNF is stored in Dry Storage Units (DSU) at Andreyeva Bay, in varying conditions (leak-tight SFAs and failed SFAs with different degrees of leakage). Abnormal storage conditions require a different approach to a SNF handling process.
DSU consists of three separate storage structures 2A, 2B and 3A fitted inside three existing partially-buried tanks, diameter 18m and depth of about 4m. Between 1983-1985 those tanks (Figure 4) were reconstructed to be used for interim storage of SNF.
Inside the storage cells there are several types of SFA canisters. Horizontal shielding with removable blocks has been installed to reduce the radiation levels immediately above the storage tanks.
To date, various options of stored SNF unloading and handling flow charts are under discussion. The most advanced option calls for all of the steps related to SNF withdrawal from DSU cells, loading of SFAs into ChT canisters and loading of SFA canisters into transport radiation-shielding casks TK-18 (TUK-108/1) to be performed with the help of a bridge crane and fuel-handling equipment in a purpose-built DSU tank shelter building.
A general chart of steps involved in unloading SNF from DSU cells and further handling in the DSU shelter is shown below (Fig. 5). The flow chart includes both channel-by-channel unloading and canister-by-canister unloading of SNF from DSU cells.
During channel-by-channel unloading, the SFA is removed from a DSU tube using a transfer cask held on the transfer carriage of the bridge crane, a visual inspection of the unloaded SFA is carried out, and then the SFA is loaded. Intact SFAs are loaded into a standard ChT canister; defective SFAs are loaded into an overpack that is installed in the ChT-11Sh canister (ChT-14Sh). Loaded canisters are then placed into TUKs.
Special-purpose and tube transfer casks enable the removal of stuck and damaged SFAs by encapsulating the entire DSU cell tube, which is cut out of the DSU. Damaged SFAs go to a tube unloading station for SFA repair; stuck SFAs are loaded into leak-tight canisters placed inside a redesigned TUK. This technology is being developed separately.
By implementing this approach, it is possible to transport the entire SNF inventory stored in DSU at Andreyeva Bay for reprocessing. Conceptual engineering and organizational work is required before work can begin. These decisions should be based on nuclear safety and radiation safety provisions as well as economics; the most important issues include early commencement of work, and reducing work duration and costs.
At Gremikha village, the entire SNF inventory, including defective SFA, unloaded from water-cooled reactors and stored, have been removed (Figure 6). At present, radwaste unloaded from nuclear-powered submarines is stored in special-purpose tanks.
In 2012 reactor core unloading activities were completed. A unique operation aimed at removing SFA cassettes from nuclear-powered submarines was carried out. The cassettes are placed in specially-modified canisters inserted in TUK-108/1 canisters for transport to a reprocessing plant. These activities required a tilting device and other specially-developed equipment. In future, it is planned to use these jigs, fixtures and tools to dismantle more radwaste and unload SFA cassettes according to a similar process directly at Gremikha.
?S. A. Dushev and A. V. Timofeev, JSC OKBM Afrikantov, 15 Burnakovsky Prospect, Nizhny Novgorod, Russia 603074
This article is based on the paper "Development of SNF Handling Technologies and Equipment to Decommission Nuclear and Radiologically Dangerous Objects" presented at the Fifth International Conference on Decommissioning Challenges, 7-11 April 2013, Avignon, France