Understanding the Nuclear Reactor: Principles, Components, and Types

Since its inception, the nuclear reactor has been a subject of debate, prompting continuous modifications and advancements. This evolutionary process persists today. Regrettably, during a full-scale invasion, the Zaporizhzhia NPP Units were unlawfully seized by Russian terrorists, and the global community lacked an effective mechanism to compel their evacuation. This occupation has resulted in widespread misinformation and diverse assumptions. One undeniable truth is that conflict escalates the risk of emergencies at nuclear power plants. To counter unwarranted fear and speculative claims, comprehending the operational principles and varying design types of reactors becomes crucial. The Uatom.org Editorial Board has set a clear objective: to present this complex information in simple terms, to convey it to the widest audience possible.

Reactor operation principle

The nucleus of Nuclear Power Plant (NPP) operation rests on a nuclear chain reaction. Within an NPP, energy undergoes a triple conversion process: initially, nuclear energy transforms into thermal energy; subsequently, thermal energy transitions into mechanical energy, culminating in the production of electrical energy.

This initial transformation from nuclear to thermal energy transpires within the reactor core, where substantial heat emerges through uranium fission. Typically, enriched uranium serves as the reactor fuel, housed within fuel rods organized into fuel assemblies. These assemblies collectively constitute the reactor core.

Uranium contained in TVELs is a radionuclide – an element with an unstable nucleus, thanks to this property it is capable of radioactive decay. As a result of decay, the nucleus splits into two fission fragments, releases energy and generates from 1 to 8 neutrons. At the same time, neutrons have a fairly high speed and, colliding with neighboring nuclei, provoke subsequent fissions, thanks to which the fission reaction occurs again, as a result of which there are more neutrons. If the particles that cause a nuclear reaction occur as products of this reaction, then this is a fission chain reaction.

The resulting fission fragments have a large kinetic energy, which, due to collision with atoms of other elements, is transformed into heat – the process of thermalization. This heat is transferred by the coolant, which is usually purified water, and is fed to the steam generator by the main circulation pumps. The primary circuit of the reactor coolant is a circuit together with the pressurizer system and the main circulation pumps designed to ensure the coolant circulation through the core in the operating modes and conditions established by the design. The coolant that comes into contact with the fuel assemblies becomes radioactive, so it is closed within the primary circuit and has no direct contact with the secondary circuit.

Water within the closed primary circuit of the system transmits its thermal energy to the secondary circuit coolant, which is ordinary purified water in the steam generator, causing it to reach boiling point. The cooled coolant returns to the reactor via the primary circulation pump thus closing the circuit. The steam generated in the steam generator travels through the secondary circuit piping, propelling the blades of the steam turbine. This process converts thermal energy into mechanical energy. Simultaneously, an electric generator, linked to the steam turbine, transforms this mechanical energy to produce electrical energy.

This illustrates the energy production process in the double-circuited nuclear power plants operating in Ukraine.

Types of reactors

Presently, approximately 440 nuclear reactors operate across 32 countries, collectively delivering about 390 GW of power. In 2022 alone, they generated 2,545 TWh, accounting for roughly 10% of global electricity generation. As of November 2023, about 60 reactors are under construction worldwide, with an additional 110 planned for construction, predominantly in Asia. Over the past two decades, 108 reactors have been decommissioned, surpassing the 97 reactors launched during that period. Despite numerous designs available, most reactor designs align with four primary classes:

Generation I reactors: These are initial industrial prototypes from the 1950s and 1960s, upgraded versions of military reactors employed in submarines or for plutonium production.

Generation II reactors: Representing the bulk of industrial reactors globally, prominent examples include the RVPK (in Russian: RBMK) and VVER-1000, which will be further discussed.

Generation III reactors have extended service life, estimated for up to 60 years, with potential extension to 100 years before requiring capital repair or reactor pressure vessel replacement, setting them apart from their Generation II reactors. Furthermore, Generation III+ reactors feature passive safety systems, eliminating the need for continual operator intervention or electronic feedback to safely shut down the plant in emergency scenarios. Notable among Generation III+ reactors is the AP1000 by Westinghouse.

And finally, Generation IV reactors are developed for industrial use within the next twenty to thirty years. These encompass diverse systems such as GFR (Gas Cooled Fast Neutron Reactor System), LFR (Lead Cooled Fast Neutron Reactor System), MSR (Molten Salt Reactor System), SCWR (Supercritical Water Cooled Reactor System), SFR (Fast Neutron Reactor System) with sodium cooling, and VHTR (Ultra-High-Temperature Reactor System).

The difference between RVPK and VVER-1000

RVPK, standing for high power channel reactor, represents a second-generation reactor featuring boiling water and a graphite moderator.

The core consists of a graphite cylinder positioned within a concrete shaft. This reactor design incorporates 1,661 vertical process channels, each 88×4 mm in diameter, housing fuel assemblies. These channels facilitate coolant flow, allowing for the cooling of the fuel elements containing uranium dioxide tablets. One noteworthy feature is the capability for fuel reloading without the need to shutdown the reactor.

Reactor systems of this kind were utilized at the Chornobyl nuclear power plant in Ukraine, the site of the world’s most catastrophic civilian nuclear accident in 1986. These reactors bear intrinsic design flaws: reactivity escalates with increased steam volume, leading to uneven energy release within the core. Moreover, the vast number of channels (1,693 in RVPK-1000) increases these deficiencies. Certain corrective measures were implemented after the Chornobyl accident, such as enhanced uranium enrichment and a tenfold reduction in the control system’s operational duration, negating the effect of a positive shutdown. Yet, certain issues persist, remaining economically or technically impractical to rectify. For instance, only two out of twelve reactors of this type have fully independent secondary containment systems, failing to meet International Atomic Energy Agency (IAEA) safety standards. Furthermore, RVPK reactors contain larger amounts of zirconium alloys compared to other reactor types, about 50% more than a conventional BWR2, along with substantial quantities of graphite (approximately 1,700 tons). The combustion of graphite can substantially increase emergency situations. Additionally, graphite reacts explosively with water at high temperatures, generating hydrogen. While the destruction of a single train in RBMK might not invariably lead to catastrophic outcomes, the extensive network of channels and pipes necessitates a proportional number of welds, complicating preventive inspections and maintenance within this expansive system.

The RVPK containment system has undergone enhancements to manage increased pressure, now capable of withstanding bursts in up to nine channels simultaneously. However, in the event of a coolant loss accident leading to the termination of flow within a channel, temperatures can soar to critical levels, resulting in the rupture of up to forty channels. This catastrophic scenario risks the complete destruction of the entire core. In light of these fundamental design flaws, these reactors are deemed “non-improved” by the international community, prompting efforts to decommission them. Notably, RVPK reactors in Lithuania and Ukraine have already ceased operations. Conversely, in russia, plans involve extending their operational lifespan rather than an early closure.

The Soviet VVER reactors were initially designed for military submarine use, hence their compact size and increased capacity in comparison to other reactor types. Water serves as both the coolant and moderator in these reactors. The core, situated within the reactor pressure vessel, heats the pressurized coolant in the primary circuit without causing it to boil. Heat transfer occurs through the steam generator’s heat exchange tubes, facilitating boiling and evaporation of the secondary circuit’s coolant. Subsequently, the steam produced in the generator is directed into the turbine via steam piping. This reactor type maintains stability due to its negative temperature coefficient of reactivity.

Presently, seven Eastern European countries house a total of 53 reactors categorized into three primary types. The oldest among these is the generation I VVER 440-230 reactor. The subsequent iteration, the 440-213 reactors, belonging to the VVER generation II, introduced an enhanced emergency cooling system for the core. While these reactors lack a complete secondary containment system, they incorporate a mechanism, the bubble condensation tower, to capture radioactivity release during accidents, although it does not protect the core from external hazards. The third design iteration, the VVER 1000-320, witnessed further modifications, ramping up the power output to 1000MW. Consequently, VVER-1000 reactors have enhanced safety features in comparison to RBMK reactors, significantly reducing the likelihood of a Chornobyl-like scenario at the Zaporizhzhia NPP.

Cold / hot shutdown

The VVER-1000 reactors, previously discussed, are operated at all six units of the Zaporizhzhia nuclear power plant (ZNPP). Contrary to the SNRIU requirements, the occupying authorities initiated a “hot” shutdown for Units 4 and 5. However, in November 2023, a primary to secondary leak in one of the Unit 5 steam generators compelled the occupiers to transition this unit into a “cold” shutdown state. According to licensing terms, all units at the ZNPP site are mandated to maintain a “cold” shutdown state.

During the reactor operation at power levels and occurrence of an accident, the main factor of radiation exposure is the isotope of radioactive iodine I-131 (a product of nuclear fuel fission). Due to the fact that neutron absorbers have been introduced into the reactor core and the reactors are in a cold/hot shutdown state, I-131 is not generated since September 2022, because all ZNPP units were shutdown. The half-life of iodine I-131 is about 8 days, so its activity in the of Zaporizhzhia NPP reactors decreased by approximately 17 billion times.

During a “hot shutdown”, as well as during a “cold shutdown”, the emergency protection rods are lowered into the core, the concentration of boric acid in the reactor and the primary circuit will not allow to maintain the chain reaction (“steady shutdown concentration of boron”), high pressure and temperature in the primary circuits are ensured by the operation of the main circulation pumps, and not by the chain reaction in the nuclear fuel. That is, with all the listed operating modes of power units, they do not produce electricity and they do not contain radioactive iodine (I-131).

That is why it can be stated that the iodine-131 isotope is not a radiation risk factor in the event of an accident at the Zaporizhzhia NPP at the moment. That is, possible consequences of an accident at the shutdown power units will be lower than at the Chornobyl NPP and Fukushima.

References:

1. K. Denysevych, Yu. Landau, V. Neiman, V. Suleymanov, B. Shylyaev. Development of Nuclear Energy and Unified Energy Systems
2. IAEA Reference Data Series “Nuclear Power Reactors in the World 2022”
3. V. Pavlovych. Physics of Nuclear Reactors
4. Plans For New Reactors Around The World
5. Types of Nuclear Energy Reactors Operated in the World
6. Antony Froggatt. Nuclear Reactor Hazards
7. How to Save Yourself From an Accident at a Nuclear Power Plant? General Rules

Editorial Board of the Uatom.org website