☢️РБМК против ВВЭР: Достаточно ли модернизаций после Чернобыля,

In 1986, the Chernobyl Nuclear Power Plant (Unit 4) disaster exposed the critical, inherent flaws of RBMK-1000 reactors—pressure-tube, water-graphite reactors operated in the USSR. This reactor type, unlike the pressure-vessel VVER, proved to be a dynamically unstable system with respect to power and steam quality disturbances. In this video, we provide a detailed comparison of these two main types of Russian nuclear reactors and analyze how successfully the extensive safety improvements implemented in RBMK reactors were implemented to eliminate their fatal design flaws. What you will learn: 1. Inherent flaws of RBMK reactors (The cause of Chernobyl) Before the accident, design calculations erroneously assumed that with complete coolant evaporation, reactivity would only increase to +2βeff and then become negative, leading to reactor shutdown. Post-accident studies and experiments, as well as earlier calculations (1980, 1985), showed that the reactor had a large positive void effect reactivity (PER), which, with complete coolant evaporation, reached +5β_{eff} or more. This caused uncontrolled reactor runaway. Furthermore, a problem with the reduced effectiveness of the emergency protection system (EPS)** was identified: when the emergency protection system (EPS) was triggered, the movement of the reactor rods could add positive reactivity for several seconds (from 0.5% to 7% of the weight of a single reactor rod). 2. Contrast with WWERs Unlike the RBMK, which uses graphite as a moderator, WWERs (Water-Water Reactors) are pressure-cooled reactors and use ordinary water as both a coolant and a moderator. This ensures the reactor shuts down automatically (negative reactivity coefficient) when steam appears or the temperature rises, which is a characteristic of inherent self-protection. VVER reactors are also equipped with a secondary cooling system, which reduces atmospheric emissions. 3. Five Key Areas of RBMK Modernization Immediately after the accident, a large-scale program was launched to address design flaws and modernize all 14 operating RBMK power units. Key areas of work: 1. Reducing the void coefficient of reactivity: The goal was to reduce the void coefficient of reactivity to a level not exceeding 1.0β. This was achieved by increasing the number of additional absorbers (AP) (80–90 for RBMK-1000) and increasing the operating reactivity margin (ORM). A decision was also made to load fuel enriched with 2.4% U-235 into the RBMK-1000 units. Measurements in 1995 showed that the void coefficient at operating units ranged from 0.1 beta to 0.97 beta. 2. Improving the efficiency of the control rods: The design of the manual control rods was modified to eliminate water columns under the displacer, eliminating the positive reactivity overshoot during their insertion. The rod movement speed was reduced from 18 to 12–14 seconds, and the initial velocity efficiency increased to 0.9 beta/s. 3. Implementation of the Fast Emergency Protection System (FASS): An independent system capable of introducing 2.5 beta of negative reactivity in 2 seconds was developed. 4. Tightening metal inspection: Metal inspection of large-diameter headers and pipelines was strengthened, allowing the probability of a major rupture to be estimated at $0.75 times 10^{-6} 1 reactor-year. 5. Increasing the capacity of steam bleeders (SB): The capacity of the steam bleeder systems was increased to ensure the integrity of the reactor compartment (RC) during the simultaneous rupture of up to four fuel channels (according to the initial solution) or even nine fuel channels (a more complex solution, implemented, for example, at Smolensk-3). 4. Conclusions: Has the risk been completely eliminated? The modernization has eliminated the design flaws revealed during the accident. However, challenges remain in upgrading first-generation reactors to meet modern requirements. Although the critical neutron fluence for graphite (one of the stack performance criteria) can be increased by taking into account the effects of gamma radiation, thereby extending the service life beyond the design 30 years, the safety approach for RBMK reactors differs from that of VVER reactors: while for a pressure vessel reactor the ultimate limit state is strictly deterministic, for RBMK reactors, graphite stack degradation is considered a process that can be controlled and extended over time. Were engineers able to overcome the inherent instability of the pressure-tube reactor? Let's delve into the details of nuclear physics and safety history!