Nuclear safety is a feature of reactor facility and nuclear power plant to prevent initiation of nuclear accident with a defined probability.
Nuclear accident is an accident leading to fuel elements’ damage that exceeds established safety operation limits and this accident is caused by nuclear-physical processes as a result of violation of monitoring and control of chain fission reaction in the core and/or formation of critical mass during refueling, transportation and storage of fuel elements as well as in case of insufficient heat removal from fuel elements.
The NPP safety is ensured due to consecutive implementation of the defense-in-depth concept based on the application of physical barriers system in the direction of spread of radioactive materials and ionizing radiation. The system of the power unit physical barriers includes a fuel pellet, cladding tube, primary circuit boundary, containment and biological shielding. When revealing the unavailability of any physical barrier or its protection means (provided by NPP design) the power operation of NPP unit is forbidden according to the safe operation conditions.
Basic state normative documents, defining requirements ensuring nuclear safety of NPPs:
- “General provisions on nuclear power plants’ safety” НП 306.2.141-2008 (NP 306.2.141-2008);
- “Regulations on nuclear safety of NPP reactor facilities with pressurized-water reactors” НП 306.2.145-2008 (NP 306.2.145-2008);
- “Safety rules for storage and transportation of nuclear fuel at nuclear power facilities” ПНАЭ Г-14-029-91 (PNAE G-14-029-91);
- “Technical operation of electric power stations and grids. Regulations” ГКД 34.20.507-2003 (GKD 34.20.507-2003);
- “Fuel handling. Refueling in WWER-1000 reactor. The nomenclature of operational neutronic calculations and experiments”, SE NNEGC “Energoatom” 2013. СОУ НАЕК 064:2013 (SOU NAEK 064:2013).
Based on the abovementioned documents at SU NPP they developed and agreed in established order the list of nuclear-hazardous works (i.e. the works, which can lead to a nuclear accident), as well as they developed corresponding regulations, programs and instructions. The observance of these regulations, programs and instructions guarantees an accident-free fulfillment of nuclear-hazardous works at stages of transportation, refueling, operation and storage of nuclear fuel. On a mandatory basis the documentation includes: purposes and conductions of nuclear-hazardous works, safety measures, distribution of duties and responsibilities during work performance, order of procedures fulfillment, as well as criterions and control of works successful execution.
Fresh fuel storage at the new fuel element receiving station
The new fuel element receiving station is designed for fresh fuel storage and preparation of its transportation to NPP reactor compartment.
The new fuel element receiving station is designed for the acceptance and incoming inspection of fuel assemblies and reactor core structural components, received from the manufacturing plant in the fuel transport container; handling operations on transfer of fuel assemblies and structural components from the fuel transport container into the fresh fuel casings; storage or preparation for transportation of fresh fuel casings to Separate Subdivision SU NPP units for refueling of reactor core during scheduled preventive maintenance.
The new fuel element receiving station is designed to store up to 258 fresh fuel assemblies. For each Separate Subdivision SU NPP unit we receive and/or store in the new fuel element receiving station at the average 42 fuel assemblies annually, additionally at new fuel element receiving station we stock fuel assemblies for refueling of one power unit. Besides, at the new fuel element receiving station we store such construction components of reactor core as absorber rods of reactor control and protection system, burnable poison rods, fuel assembly dummies (i.e. exact copies of fuel assemblies without the fissionable material - uranium-235).
The distinctive feature of the WWER fuel is a risk of spontaneous nuclear reaction in the case if several fuel assemblies located closely to each other get into multiplying medium – water. To prevent the possibility of this event we implement a complex of technical and administrative measures. The possibility of water ingress into the new fuel element receiving station is eliminated due to a combination of the following measures:
- location of the new fuel element receiving station above the zero level;
- absence of neighbouring premises from which the water can ingress into the new fuel element receiving station premise;
- absence of pipelines with water in the new fuel element receiving station premise;
- location of the new fuel element receiving station into the flood-free area in case of inundation;
- control of temperature and relative humidity in the new fuel element receiving station with record of sensors’ indications by automatic recorders;
- restricted access to the premise of the new fuel element receiving station.
Also, the new fuel element receiving station is equipped with a water detection system, security alarm system and fire alarm.
Radiation environment control while transportation and storage of fuel assemblies in the new fuel element receiving station is performed by the units for gamma radiation detection providing optico-acoustic alarm. New fuel element receiving station belongs to the second class storage facility (under PNAE G‑14‑029‑91 classification).
Nuclear safety during storage and transportation of fuel assemblies, fuel transport containers and fresh fuel casing is provided by their construction taking into account specified limitations as to the amount and location.
During scheduled preventive maintenance the fresh fuel installed in the fresh fuel casing is transported with compliance of all the provided procedures and limitations from the new fuel element receiving station to the central hall of the power unit reactor department and further into the reactor facility core for operation during charge core life.
Nuclear fuel operation
At SU NPP units we use three types of fuel: two types of fuel assemblies (TVS-M and TVS-A) are Russian and one type of fuel assembly (TVS-W) is produced by America.
The main nuclear fuel supplier for the SE NNEGC “Energoatom” is Joint-Stock Company “TVEL” concern (Russia). Since 2005 this concern supplies to Separate Subdivision SU NPP new generation fuel assemblies – TVS-A, so-called fuel assemblies of alternative construction.
In 2005 six experimental (pilot) fuel assemblies manufactured in the United States of America were loaded in the SU NPP unit-3 core. The supplier is “Westinghouse” company. “Westinghouse” production fuel is rendered in the frames of Ukraine-American executive agreement signed in 2000 in reference to Ukraine Nuclear Fuel Qualification Project (UNFQP). The aim of the project is to qualify the usage of fuels assemblies at Ukrainian NPPs as well as qualification of methodology of design and analysis of fuel loads safety and alternative supplier. During the operation period no remarks have been made to the 6 pilot fuel assemblies.
In 2009, according to the second stage of UNFQP, the “Westinghouse” company supplied a fuel batch for core reloading consisting of 42 fuel assemblies (TVS-W) for SU NPP Unit-3. Fabrication of TVS-W is relocated to Swedish subdivision of “Westinghouse” company. The fuel batch of TVS-W for core reloading is loaded into SU NPP Unit-3 core in 2010.
In the course of refueling in 2012 and in 2013 we revealed TVS-W with deformed fuel assembly grids (elements of TWS-W fuel assembly skeleton).
In spite of the fuel assembly grids’ damage the fuel elements in these TVS-W fuel assemblies were leak-tight and we recorded that there were no releases of radioactive fission products.
Based on the results of this event investigation the “Westinghouse” company in collaboration with specialists from SE NNEGC “Energoatom” and Scientific & technical complex “Nuclear fuel cycle” of national scientific center “Kharkov Institute of Physics and Technology” made a decision to strengthen and change elements of TVS-W fuel assembly structure eliminating the damage of elements during handling operations.
The decisions on improvement of TVS-W structure were stated in conceptual engineering solution, justifications of design decisions that have been taken as well as in the licensing plan on use of the fuel of improved design. Currently these documents are agreed with State Nuclear Regulatory Inspectorate of Ukraine and there are works performed under these documents.
At the beginning of May 2014 the “Westinghouse” company plans to finish all the cycle of bench tests of improved design (TVS-WR) with the following provision of tests results to SE NNEGC “Energoatom” and to the State Nuclear Regulatory Inspectorate of Ukraine.
The first core loading of TVS-WR fuel assemblies is expected in the course of scheduled preventive maintenance at SU NPP Unit-3 (pilot power unit within the framework of implementation of “Westinghouse” fuel project – correspondent note). It starts at the end of 2014 to 2015 that is actually shall be loaded in the core in 2015.
Reactor core composition with a nuclear fuel is conducted in accordance with engineering solutions developed by NPP and agreed with NNEGC “Energoatom” and State Nuclear Regulatory Committee of Ukraine after independent expertise in the State Scientific and Technical Center on Nuclear and Radiation Safety. To justify the safe operation of fuel produced by Russia one use calculating complex of neutronic codes “KASKAD” developed by national research centre “Kurchatov Institute” (Russia). For power units with fuel produced by “Westinghouse” the design-basis justification of safe operation is made by Reactor core designing center founded on the basis of the National scientific center “Kharkov Institute of Physics and Technology” (Kharkov); the calculations are made by complex of neutronic codes “ARA-N”.
The following software and hardware is applied to monitor fuel in the core:
- at the Unit-1 – in-core instrumentation system uses mathematic software “Kruiz” (developed by Joint-Stock Company “Innovation firm, Specialized research-and-development institute of instrument making “Atom”, Russia);
- at the Unit-2 – mathematic software “Kruiz” (Joint-Stock Company “Innovation firm, Specialized research-and-development institute of instrument making “Atom”, Russia) was additionally adapted in 2011 for the fuel manufactured by “Westinghouse”;
- at the Unit-3 – in 2004-2005 we modernized the reactor core monitoring system with application of program-technical complex “Vulkan-VRK” and calculation model “BEACON” (Westinghouse) to monitor the fuel of Joint-Stock Company “TVEL” and “Westinghouse”. It should be noted that during charge core life 2004 the calculation model “BEACON” (Westinghouse) was operated in test mode simultaneously with basic monitoring system and showed satisfactory result of calculation and restoration of reactor core power density field completely composed of Russian production fuel after this calculation model was put into pilot and commercial operation (in 2005) to monitor the reactor core including fuel assemblies manufactured by Joint-Stock Company “TVEL” and “Westinghouse”. Based on the results of successful completion of pilot and commercial operation the program-technical complex “Vulkan-VRK” with calculation subsystem “BEACON” (Westinghouse) was put into commercial operation in 2010.
The fuel assemblies are operated in the core during three charge core lives with use of certain number of the fuel assemblies for the fourth year. Core charge life, as a rule, lasts for about ten months, reactor core recomposition is performed during the scheduled preventive maintenance of power units. Usually the scheduled preventive maintenance lasts for about two months. When operation of fuel assemblies is completed the fuel assemblies are unloaded into the at-reactor spent fuel pool for storage.
Spent fuel storage
At least three years shall pass from the moment of spent fuel unloading from the reactor core before shipping to processing. All this time, the spent fuel stores in the special at-reactor spent fuel pools – ferroconcrete storage, lined with stainless steel.
Here discharged assemblies undergo the activity decay. Water level reliably protects from radiation and ensures cooling of fuel assemblies.
Further the spent fuel is transported to the Mining & Chemical Combine (Zheleznogorsk, Krasnoyarsk region, Russia) for the technological holding with the following processing. Eventually all the highly radioactive wastes get after spent fuel processing will be returned to Ukraine (obligatory feature of such procedure is regulated by the requirements of the world countries laws, including Ukraine and Russia). Additional funds will be needed for their storage and disposal. It is also important that the spent fuel contains nuclear materials which can be used in the reactors of future generation.
So, nowadays we consider the possibility to build in our country a Centralized spent fuel storage facility (CSFSF) intended for the storage of spent fuel assemblies of three NPPs: Separate Subdivision RNPP, Separate Subdivision KhNPP and Separate Subdivision SU NPP. February 4th, 2009 the Cabinet of Ministers of Ukraine approved the project of feasibility study for its construction.
The design life time of CSFSF is up to 100 years. The technology offered by the American corporation “Holtec International” shall be applied during this period. At current stage the infrastructure of SU NPP is adapting to this technology. As a result of performed works the NPP will be able to accept containers for spent fuel transportation both American company and Russian enterprises specializing in storage and spent fuel processing.