Department of Marine Nuclear Power Systems and Innovative Designs

NUCLEAR POWER PLANTS OF HIGH SAFETY AND COST EFFECTIVENESS WITH FAST LEAD-COOLED REACTOR BREST AND ON-SITE FUEL CYCLE FOR FUTURE LARGE-SCALE NUCLEAR POWER

A NATURALLY SAFE COST-EFFECTIVE REACTOR BREST WITH ELECTRIC POWER OF 300 MWe AND 1200 MWe

BREST is a nuclear power plant with a lead-cooled fast reactor fuelled with uranium-plutonium mononitride and using a two-circuit heat transport system to deliver heat to a supercritical steam turbine.

BREST offers:

• natural radiation safety in all credible accidents caused by internal or external impacts, including sabotage, with no need for people evacuation;
• long-term (practically unlimited) availability of fuel resources due to very efficient utilization of natural uranium;
• proliferation resistance owing to the absence of production of weapons-grade plutonium and on-site dry reprocessing of fuel without plutonium separation from uranium;
• environmentally safe energy production and radwaste management owing to the use of a closed fuel cycle which includes transmutation and in-pile burning of actinides, transmutation of long-lived fission products, radwaste purification from actinides, cooling and final radwaste disposal without disturbing the natural radiation balance;
• cost competitiveness due to naturally safe technologies of NPP and fuel cycle; absence of sophisticated engineered safety systems; reactor makeup with ²³⁸U alone; high parameters of lead providing for supercritical parameters in the steam-turbine circuit and high efficiency of the thermodynamic cycle; low construction costs.

Natural radiation safety relies on:

• high-boiling (T_boil=2024 K) radiation-resistant low-activated lead coolant which does not react with water and air and hence affords low-pressure heat removal while excluding the possibility of fire, chemical and thermal explosions in the event of circuit failure, steam generator leakage and any temperature surges in the coolant;
• high-density (g_theor=14.3 g/cm³), highly heat-conductive (l=20 W/m·K) mononitride fuel which operates at low temperatures (T_max<1150 K, with T_melt=3100 K), and is characterized by small radiation swelling (~1% per 1% burnup) and low release of fission gas (<10% of all gaseous fission products generated), which prevents contact effect of fuel on cladding so that the latter is loaded by gas pressure of <2 MPa only by the end of the campaign;
• use of shroudless fuel assemblies (FA) in a sparse fuel grid in the core with moderate power density (no more than ~200 MW/m³), which prevents loss of heat removal in case of local flow blockage in an FA and provides for intensive natural circulation of the coolant;
• core and lead reflector design (composition and geometry) which affords full breeding of fuel (CBR»1), small or negative power, temperature and void effects of reactivity, a small total amount of reactivity in the core (Dk/k<b_eff) which rules out uncontrollable prompt criticality excursion in the event of inadvertent withdrawal of all control rods, whatever the reactor state;
• use of passive protection systems of direct action, responding to abnormal coolant flow or temperature at the core inlet and outlet;
• passive emergency core cooling system using external air circulation to remove heat through the vessel;
• cooling circuit configuration with different coolant levels in the downcomer and riser legs, which affords smooth transition to natural circulation in case of a loss-of-flow accident;
• high heat accumulating capacity of the lead circuit.

Owing to a unique combination of the natural properties of lead coolant and mononitride fuel, fast reactor physics, core and cooling circuit design, the BREST reactor can boast of a radically higher level of inherent safety. Furthermore, it remains stable even in very severe accidents involving failure of active protection features, which no reactor now in operation or under design can cope with, in particular:

• spurious withdrawal of all control rods;
• trip (jamming) of all primary pumps;
• trip (jamming) of all secondary pumps;
• reactor vessel failure;
• any break in the secondary piping or steam generator tubes;
• coincidence of various accidents;
• blackout, in which reactor can be cooled for as long as necessary, without time restrictions, etc.

Even extreme accidents caused by sabotage and involving damage to outer barriers, i.e. reactor building, vessel cover and others, do not result in radioactivity releases requiring evacuation of local residents and land withdrawal from use for a long period of time.

Accident analysis looked in particular into an event involving failure of the reactor vessel (cover) and building, presumably as a result of sabotage. In this case, the reactor is shut down from full power with a temporary global rise of coolant temperature to ~1000 K. Fuel elements retain their integrity, with fuel leakage remaining within the design limits. Radionuclide release to the environment in this accident will amount to less than 1000 Ci (in terms of ¹³¹I), which corresponds to INES Level 5, i.e. the event not requiring evacuation of people. Lead purification from bismuth and other radionuclides would take the accident down to Level 4 or even Level 3.

By today, the conceptual designs have been prepared for the 300 MWe and 1200 MWe modifications of the BREST reactor (Fig. 1, Fig. 2), the associated design studies and analyses have been performed. Reactor physics was validated in experiments at U-Pu-Pb critical assemblies, and nuclear data were corrected. Long-term corrosion testing of steels was performed in Pb circulation loops and experiments were carried out to study interaction of Pb with air and water of high parameters, of nitride fuel with Pb and steel claddings, etc.

Economic assessments and comparative studies have confirmed the possibility of reducing the capital costs of the plant and the cost of its electricity as compared to those at VVER NPPs.

Given the experience with heavy-coolant reactors, numerous in-pile studies on nitride fuel, calculations and experiments performed in the course of the work on the concept, the designers felt confident enough about the basic aspects of the plant to embark on a detailed design of a demonstration BREST-OD-300 plant with on-site fuel cycle facilities at the Beloyarsk NPP site.

Technical Characteristics of BREST-300 and BREST-1200 Reactors

Figure 1. General view of BREST-300

Figure 2. BREST-1200 ( click to view full size )