Fuelling floating reactors13 August 2004
ABV and VBER reactors are designed to be deployed in remote regions on floating platforms. They can operate without refuelling for many years, increasing safety, reliability and economy. How are their reactor cores adapted for these conditions? By OB Samoilov, VS Kuul, OA Morozov, VI Alexeev and MA Bolshukhin
Floating (barge-mounted) power stations are very attractive as autonomous power sources for remote region due to their high reliability, radiation and environmental safety, acceptable cost effectiveness and long fuel life of 8 to 12 years.
The proven technologies of marine nuclear power reactors have been employed to develop such floating NPPs. Russian marine-type PWRs have more than 6000 reactor-years of total operating experience in navy and civil ships.
The first design is a barge-mounted power station with an ABV-type integral PWR which uses natural coolant circulation in the primary circuit and has capacity of about 10MWe while the barge's displacement is about 2500t.
The second design has a 110MWe VBER-type modular reactor on a barge with a displacement of about 12,000t.
Such floating units have the following essential advantages:
- The plant is factory-produced and supplied as a ready-made installation so, plant construction period is much shorter.
- Plant siting is considerably simplified.
- The plant is continuously operated over 8 to 12 years without refueling and maintenance (its total service life is about 50 years).
- The plant is environmentally friendly: refueling, radwaste management and maintenance are provided off-site at a special maintenance centre.
- After decommissioning the 'green field' concept is easily realised at its site area.
The ABV reactor plant
The 10MWe ABV reactor plant uses an integral PWR with natural coolant circulation (see Figure 1). It is inherently safe due to the reactor core's negative feedbacks and enhanced thermal inertia. Once-through steam generators are used. The main characteristics of the ABV reactor are given in Table 1.
In many respects the ABV reactor core characteristics have been determined from research and development performed for icebreaker reactor cores which gave a reliable design basis proven by engineering analysis and testing. Also, the updated safety requirements have been met.
The ABV reactor's cassette-type core (see Figure 2) is unified in respect of its structure, fuel as-sembly (FA) design (Figure 3) and structural items with the core being developed for the pilot nuclear cogeneration power plant with KLT-40S reactors.
The following solutions developed for icebreaker reactor cores are used in the ABV-reactor core:
- Gadolinium is used in rods to compensate for fuel burnup reactivity margin.
- Improved core power shaping is provided by fuel and gadolinium distribution.
- Two independent mechanical systems are employed for reactivity control without boron so-lution in the primary coolant.
- The central shim rod group is only used to compensate for reactivity changes during operation.
An optimised fuel lattice with improved neutron moderation permits increased fuel burnup in the reactor core.
Fuel with high thermal conductivity based on uranium dioxide granules dispersed in Al-alloy matrix is employed, that provides high reactor power manoeuvreability. Development of the fuel is provided by the Bochvar All-Russian Scientific Research Institute for Inorganic Materials (VNIINM). The fuel is currently in-pile tested. Corrosion-proof zirconium alloy is used as the fuel cladding material. Power changing rate within the entire power operation range amounts to 0.1%Nnom/s.
Uranium enrichment of less than 20% is sufficient and meets IAEA non-proliferation requirements.
A combination of low core power density (33MW/m3), 'soft' fuel operating conditions and reduced fuel heating rates ensure increased thermal margins and extended core life.
Refuse from soluble boron control together with adopted parameters of the fuel lattice provided negative reactivity coefficients on fuel and coolant temperatures, as well as negative steam and integral power coefficients of reactivity in the entire range of operating parameters, that provides for the core inherent safety characteristics. The core's inherent safety ensures power self-control at steady-state reactor operation, power rise self-limitation at positive reactivity perturbations, reactor self-shutdown, primary coolant pressure and temperature limitation, as well as limitation of heating-up rate in reactivity-initiated accidents.
Very efficient core physical shaping by means of special fuel and gadolinium distribution throughout the core, developed for the icebreaker reactors, provides for the minimisation of power fields and fuel burnup non-uniformities and the effective use of fuel.
The core's low power density combined with fuel characteristics and some other adopted design solutions allow refuelling intervals to extend as far as 10 -12 years. The core's life is as much as about 70,000 hours.
The main characteristics of the ABV reactor's cassette-type core are summarised in Table 2.
To realise the core design with extended fuel life, appropriate research and development needs to be carried out to provide for the required lifetime characteristics of core structural items and fuel assemblies. Reduced core power density and fuel heating rates facilitate the successful solution of this problem. As compared with VVER-type reactor fuel, specific core power density is reduced by a factor of 2.5-3 and fuel heating rate is about four times less.
VBER reactor plant
The VBER reactor plant was also developed based on the experience of the development, con-struction and operation of marine nuclear reactors. The modular marine-type PWRs along with VVER-type power reactors are the most developed reactor technology proven by long-term suc-cessful operation at civil and navy ships. At present the total fault-free operating record of these reactors exceeds 6000 reactor-years.
Modular arrangement of the NSSS represents a key feature of the reactor design concept (see Figure 4). The reactor pressure vessel, two once-through steam generators and hydraulic chambers of two reactor coolant circulating pumps are integrated into a single vessel system by welded short coaxial pipes through which coolant is circulated. The reactor plant capacity is about 110MWe. The reactor plant represents a two loop modification of the more powerful VBER-300 reactor design.
The combination of proven engineering solutions developed for the marine reactor plants and recent designs of VVER-type reactor plants allows the essential requirements on safety, reliability and efficiency, which are currently imposed to the future generation of power plants to be met. The main VBER reactor characteristics are summarised in Table 3, VBER reactor core main data is summarised in Table 4.
The accepted concept predetermines selection of a cassette-type structure for the core (See Figure 5), operability of which has been verified by operation in VVER-type reactors at various nuclear power plants.
Ductless AFA-type fuel assemblies with a rigid skeleton structure developed by OKB Mechanical Engineering are used in the core (see Figure 7). The results of AFA fuel operation in VVER-1000 core of Kalininsk unit 1 confirmed its high load-bearing capacity and resistance to form changing under operating conditions.
The accepted core and fuel structure has provided geometrical stability of the VBER reactor core and therefore stability of its neutronic and thermal-hydraulic parameters. Thus, engineering so-lutions on the fuel and VBER core structure are completely proven by positive operating experience of VVER-1000 reactor.
The low power density of the core (33MW/m3), 'soft' conditions of the fuel operation and reduced fuel heating rates provide for the increased thermal margins and extended fuel life of the core. Taking into account the current licence limit on uranium enrichment for VVER reactor fuel (not more than 5%) a fuel cycle with the entire core reloading has been accepted, that gives the refueling interval of 7 to 8 years. The core life is about 50,000 hours.
A high performance electromechanical reactivity control system is used in VBER reactor. It con-sists of 72 control rod assemblies and provides the cooled down core subcriticality taking into account the most reactive assembly failure (stuck-rod event).
To compensate for the fuel burnup reactivity margin, fuel rods with gadolinium poison contained in uranium dioxide pellets are employed, the same as those used in VVER-1000 reactor, as well as boric acid solution in the primary coolant. Boric acid is gradually removed from the coolant by special filters as fuel gradually burns up.
The reactivity margin and reactivity changes during the reactor power operation are compensated by means of electromechanical system of the reactor control and protection assemblies. Both fuel lattice and the core parameters provide xenon stability that simplifies the control system operation algorithm in transients.
Low boric acid concentration in the coolant and the fuel lattice parameters provide negative re-activity coefficients on fuel and coolant temperatures, negative steam and integral power reactivity coefficients within the entire range of parameters variation that predetermines the core inherent safety features.
Maximal fuel burnup in discharged fuel assemblies is 41.6MWd/kgU, much lower than the value validated for VVER-1000 (55MWd/kgU). The margin gives a potential for the core life further extension in future, when uranium enrichment above the licensed 5% value will become possible. Improved fuel with reduced cladding thickness and enlarged fuel pellet diameter being currently developed for VVER reactors will also enable the core life to be extended.
OB Samoilov, VS Kuul, OA Morozov, VI Alexeev, MA Bolshukhin, II Afrikantov OKB Mechanical Engineering, Burnakovsky Projezd 15, Nizhny Novgorod, 603074, RussiaTablesTable 2. Main data of ABV reactor cassette core Table 1. Main ABV reactor characteristics Table 3. Main data of VBER reactor Table 4. Main data of VBER reactor core