PWR vs. BWR: Pressurized & Boiling Water Reactors Explained

Generation IV nuclear reactors, developed by the Generation IV International Forum (GIF) established in 2001, represent the next generation of nuclear technology designed to achieve sustainability, safety, economic competitiveness, and proliferation resistance through six major reactor designs: Supercritical Water Reactor, Very High Temperature Reactor, Gas-cooled Fast Reactor, Sodium-cooled Fast Reactor, Lead-cooled Fast Reactor, and Molten Salt Reactor. These designs progressively address limitations of earlier generations by utilizing fast neutron spectra for fuel breeding, operating at higher temperatures for greater efficiency, and incorporating passive safety features to minimize accident risks.

The Generation IV International Forum (established 2000 with 10 founding countries, later joined by Russia and China) identified six reference systems for sustainable nuclear energy by 2040: Sodium Fast Reactor (SFR), Lead-cooled Fast Reactor (LFR), Gas-cooled Fast Reactor (GFR), Very High Temperature Reactor (VHTR), Supercritical Water Reactor (SCWR), and Molten Salt Reactor (MSR). The first three operate with fast neutrons; the latter three with thermal neutrons. Fast neutron reactors achieve breeding ratios of 1.25 versus 0.5-0.6 for thermal reactors, utilizing 99% of uranium resources versus 0.5-0.6% in PWRs. Fast reactors excel at transmuting minor actinides through favorable fission-to-capture ratios. Advanced fuel forms include carbide, nitride, and metallic fuels offering higher densities, better thermal conductivity, and improved radiation resistance. Coolant options expand beyond water to include helium, CO2, liquid metals (sodium, lead), and molten salts. The molten salt reactor uniquely dissolves fuel components in salt mixtures, creating liquid fuel that can continuously process and remove fission products. Countries pursue different systems based on national priorities and existing expertise, with France focusing on SFR and MSR, China advancing VHTR, and others exploring multiple pathways.

Four Generation 4 reactor concepts are being developed: Sodium-cooled Fast Reactor (SFR) for breeding and waste burning at 500-600°C; Pebble Bed Reactor (PBHR) with TRISO fuel providing inherent safety at 700-900°C; Molten Salt Reactor (MSR) using liquid fuel and graphite moderator for thorium breeding; and Dual Fluid Reactor (DFR) combining breeding with high temperatures. Each concept addresses different priorities, with approximately 80 total concepts under development worldwide.

Current nuclear reactors worldwide are outdated, belonging to second and third generations conceived in the 1950s-1970s. Fourth generation (Gen 4) reactors represent a completely new class of technology with six main designs: molten salt, gas cooled fast, sodium cooled fast, lead cooled fast, supercritical water cooled, and very high temperature reactors. Gen 4 reactors share four core goals: dramatically safer through passive safety features, far more fuel-efficient (over 90% vs. less than 5% in traditional reactors), produce far less long-lived radioactive waste (hundreds of years vs. tens of thousands), and can operate at extremely high temperatures for clean hydrogen production and industrial processes.

Generation IV reactors represent revolutionary designs pursuing multiple objectives: extracting up to 300 times more energy from nuclear fuel through advanced recycling, consuming existing nuclear waste as fuel, achieving improved operational safety, providing proliferation resistance, enabling hydrogen production for transportation, and reducing overall costs through modular design and simplified construction. Six reactor types have been selected for development worldwide: High-Temperature Gas-Cooled Reactors use graphite moderators and helium/CO₂ coolants with pebble fuel for process heat and hydrogen production; Sodium-Cooled Fast Reactors use liquid sodium coolant with proven Russian technology; Lead-Cooled Fast Reactors offer advantages with lead coolant; Molten Salt Reactors use liquid fluoride salts with inherent safety features; Supercritical Water-Cooled Reactors aim for higher thermal efficiencies through supercritical steam conditions. China has recently commenced construction of high-temperature gas-cooled reactors, continuing research that began decades earlier.

The nuclear fuel cycle encompasses the entire journey of nuclear fuel from extraction to disposal: (1) Mining - extraction of uranium ore from the earth; (2) Milling - crushing ore to extract uranium, producing yellowcake (concentrated uranium powder); (3) Conversion - converting yellowcake to uranium hexafluoride gas for enrichment; (4) Enrichment - increasing uranium-235 concentration for reactor use; (5) Fuel fabrication - converting enriched uranium to fuel rods for reactors; (6) Reactor operation - nuclear fission generates heat; (7) Spent fuel management - used fuel becomes highly radioactive and generates heat; (8) Storage - interim storage in water pools or dry casks; (9) Reprocessing (optional) - extracting remaining fissile material; (10) Final disposal - deep geological repositories for long-term storage.

The nuclear fuel cycle encompasses uranium mining, enrichment, fuel fabrication, reactor operation, and waste management. Australia has the world's largest uranium reserves but is only the fourth-largest producer. Mining methods include open pit and underground mining, with In-Situ Leaching (ISL) being the dominant technology worldwide. After mining, uranium undergoes treatment to produce 'yellow cake' (uranium oxide concentrate). Natural uranium contains only 7% uranium-235 (the fissile isotope), while the rest is uranium-238. Light water reactors require enrichment because water absorbs too many neutrons. Enrichment increases the proportion of uranium-235 through centrifuge separation of uranium hexafluoride gas. Fuel fabrication creates pellets loaded into tubes and assembled into fuel assemblies. Fuel stays in the reactor for about six years before removal. Spent fuel is initially stored in cooling ponds (3-4 meters of water) for several years, providing both shielding and cooling. After cooling, there are three main disposal options: reprocessing (extracting uranium and plutonium for reuse, as done in France where 96% of fuel can be recovered), dry cask storage (steel and concrete containers for up to 100 years), or deep geological disposal (in stable rock formations like Finland's Onkalo facility, 500 meters underground in granite).

The nuclear fuel cycle involves mining uranium ore, processing and enrichment to increase U-235 concentration, fabrication into fuel pellets and assemblies, reactor use for several years, interim storage in water pools or dry casks, and final disposal or reprocessing. Used fuel can be reprocessed to recover plutonium for reuse or potential weapons development. Nuclear waste remains hazardous for 10,000 to 240,000 years, requiring careful containment. Disposal methods include geological disposal in stable underground formations, though deep ocean disposal is largely prohibited.

The nuclear fuel cycle encompasses three phases: front-end preparation, reactor operation, and back-end management. Open cycles dispose of spent fuel; closed cycles reprocess it. Nuclear power relies on fissile materials (U-235, Pu-239) that sustain chain reactions, with moderators (graphite, heavy water) enabling fission in natural uranium. Light water reactors require enriched uranium (~3-5% U-235). Uranium exploration identifies extractable reserves; natural uranium consists of ~99.28% U-238 (fertile) and 0.71% U-235 (fissile). Mining uses conventional or in-situ leaching methods. Milling produces yellow cake (U3O8), which converts to uranium hexafluoride gas for enrichment. Enrichment increases U-235 concentration using gaseous diffusion or centrifuge methods, producing depleted uranium (~470,000 tons stored in US) as byproduct. Fabrication converts enriched UF6 to uranium dioxide pellets loaded into zirconium alloy tubes. Reactor cores contain hundreds of fuel assemblies with non-uniform fuel depletion requiring optimal reloading strategies—a computationally intensive discrete optimization problem. Post-irradiation examination studies used fuel to understand accident scenarios. Radioactive releases differ between normal operations and accidents. IAEA criteria categorize releases: normal operation, coolant spikes, pressure loss, cladding failure, or core melting. Environmental migration depends on soil binding—cesium tightly binds to clay minerals (KD=1,000), while strontium binds weakly (KD=80-150). Medical countermeasures like Prussian Blue reduce cesium biological half-life. Reprocessing separates fissile and fertile materials from spent fuel. Mixed oxide (MOX) fuel blends reprocessed uranium and plutonium with depleted uranium. Europe and Japan reprocess commercially; US policy prohibits commercial reprocessing due to proliferation concerns. Advanced partitioning separates minor actinides for transmutation. On-load refueling (CANDU, PHWR reactors) enables continuous fuel management through hundreds of individual pressure tubes. Interim storage uses water pools or dry casks after initial cooling. Deep geological repositories (Yucca Mountain) provide long-term isolation. Once-through cycles store spent fuel without reprocessing. Plutonium recycling creates MOX fuel but increases non-fissile isotope ratios limiting reuse passes. Minor actinide recycling in fast reactors consumes long-lived waste. Accelerator-driven systems use proton/electron beams to generate fast neutrons for transmutation without sustaining chain reactions.

The nuclear fuel cycle encompasses uranium mining, conversion, enrichment, fuel fabrication, reactor operation, and spent fuel management. Uranium mining employs three methods: underground in-situ leaching (environmentally reconstructable within 10-15 years), open-pit, and underground mining. Processing produces uranium tailings containing concentrated radium and radon gas decay products, posing significant environmental hazards that can contaminate groundwater. Conversion transforms uranium to uranium hexafluoride for enrichment. Two primary enrichment methods exist: gas diffusion using porous membranes where lighter U-235 diffuses faster, and gas centrifugation spinning at 100,000 RPM creating centrifugal forces separating isotopes. Modern facilities use thousands of centrifuges in cascades. The process produces depleted uranium hexafluoride (mostly U-238), which is chemically toxic but not truly waste—fast reactors can utilize it as fuel. Fuel fabrication creates uranium dioxide tablets pressed into zirconium cladding tubes for reactor loading.

Existing French safety guidance documents for PWRs are largely applicable to SMRs: (1) Safety case methodology and rules for safety studies are fully technology-neutral; (2) Design safety principles addressing redundancy, diversification, and component qualification apply to any reactor type; (3) Specific examples from PWR experience remain relevant but do not limit application to other reactor types. This finding suggests that SMR regulatory adaptation may be minimal, with the primary challenge being the need to assess innovative features on a case-by-case basis against established safety principles.

Four top-ranked SMR designs emerge from comparative assessment: (1) NuScale PWR modules (60 MW each, 12-module pool configuration) achieve triple crown safety (no operator action, no backup power, no water inventory for 30 days), load following to 20%, and factory-built construction; undergoing US NRC licensing with Idaho construction planned for 2026; (2) BWRX-300 compact boiling water reactor achieves passive safety for 7 days, complete underground containment, and simple design; undergoing dual US and Canadian regulatory review; (3) Terrestrial Energy IMSR uses graphite-moderated replaceable cores every 7 years for corrosion management, representing most mature molten salt design; (4) Moltex SSR-W operates as fast spectrum waste burner, recycling spent fuel to reduce radioactive waste lifetime. Each design balances technical innovation, regulatory maturity, and commercial viability differently.

Small Modular Reactors (SMRs) represent a transformative approach to nuclear energy with electrical outputs of 300 MW or less, achieving sizes 1/10 to 1/100 of conventional reactors. This miniaturization enables integration of cooling systems into single pressure vessels, eliminating pipeline vulnerabilities and significantly enhancing safety. Key innovations include natural circulation cooling that maintains functionality during power outages and modular manufacturing that reduces costs by 50% and construction timelines by 50%. Applications span remote mountainous regions, polar areas, seawater desalination (40,000 tons/day), and hydrogen production. The global SMR market is projected to grow from 400 GW to 1,000 GW by 2035. South Korea achieved a historic milestone in 2012 by receiving the world's first design approval for the SMART reactor, which can power cities of 100,000 people while demonstrating the viability of SMR technology for international markets.

Small Modular Reactors (SMRs) are new types of nuclear energy that are modular, more efficient, safer, and produce less waste. The President referred to two models of SMRs licensed: New Scale Power (which has design certification) and General Electric Hitachi BWRX 300 (which does not yet have design certification). Both are Light Water Reactors (LWRs), the same technology as BNPP (PWR design). SMRs are smaller, modularized, quicker to construct, and cheaper, but use the same fundamental technology. This allows nuclear power to be located in fringes where power is lacking, enabling more efficient distribution in the grid.

Small Modular Reactors are based on Pressurized Water Reactor (PWR) technology, which is the same technology used in approximately 300 out of 440 nuclear reactors operating worldwide. This technology has a proven track record of over 60 years of safe operation. SMRs represent one of the most heavily regulated industries in the energy sector, with strict regulatory oversight from bodies like the U.S. Nuclear Regulatory Commission.