The Light Water Reactor (LWR) is a type of thermal-neutron nuclear reactor that uses ordinary water, or "light water," as both its coolant and its neutron moderator. This distinguishes it from the indigenous Pressurized Heavy Water Reactors (PHWRs), which use heavy water. The two main variants are the Pressurized Water Reactor (PWR) and the Boiling Water Reactor (BWR).
The concept originated in the United States, developed initially for the U.S. naval nuclear propulsion program under Admiral Hyman Rickover. The PWR prototype was developed for submarine propulsion in the early 1950s, leading to the launch of the USS Nautilus in 1954. The technology was commercialized to solve the problem of generating large-scale electricity, and the very first nuclear power reactor in India, a BWR at Tarapur, Maharashtra, was a turnkey project built in collaboration with the USA.
LWRs produce heat through controlled nuclear fission using slightly enriched uranium fuel, typically containing 3 to 5% of the fissile isotope U-235. The light water circulates through the core, absorbing the heat and acting as a moderator to slow down the fast neutrons, which is necessary to sustain the chain reaction with enriched fuel. In a PWR, the water is kept under high pressure to prevent boiling and transfers heat to a separate secondary loop to generate steam. A BWR allows the water to boil directly in the core, with the resulting steam driving the turbine.
LWRs are the most common reactor type globally, accounting for 379 out of 441 operational power reactors as of 2022. In India, LWRs are an integral part of the nuclear program, supplementing the PHWRs. Operational examples include the KKNPP 1&2 (2x1000 MW) set up with the Russian Federation. LWRs connect to India's three-stage nuclear power programme as they produce plutonium, which is the fuel for the second stage's Fast Breeder Reactors (FBRs). Recent changes include the development of advanced Generation III designs with enhanced safety features, and a major current trend is the utilization of LWR technology in Small Modular Reactors (SMRs).