Chernobyl: RBMK vs LWR

Could it happen here?

This is the chief concern of U.S. citizens. The U.S. has over 100 operating commercial nuclear power plants, more than any other country, and many of them are near large population centers. The vast majority of nuclear engineers would answer this question with an emphatic "NO". There are a number of significant design and operational differences between the Chernobyl-type reactors (RBMK) and U.S. commercial light water reactors (LWR) that make a Chernobyl-style disaster essentially impossible in the U.S. In this section, we address the differences in design between these types of reactors and explain how the RBMK's unique features made the accident possible, perhaps even likely.

To understand the differences, one needs to know a bit about how reactors work.

Once one understands the basics of reactor design, one can then identify the main similarities and differences between the reactor types and assess the prospects for a "Chernobyl" in the U.S.

What is A Nuclear Reactor?

A nuclear reactor a device designed to release, under controlled circumstances, the energy stored in the nucleus of an atom. Although engineers talk of 'burning' nuclear fuel, the process is fundamentally different than the chemical process of combustion. In combustion, the atoms or molecules of the fuel (coal, oil, wood, etc.) are joined to molecules of oxygen in a process that releases heat. In a nuclear reactor, the fuel atoms themselves are split into smaller atoms in a process called fission. Pound for pound, the amount of energy released in a fission reaction is over a million times larger than that released during a combustion reaction. A single thimble sized pellet of uranium dioxide typically remains in a reactor for up to 3 years and produces the energy equivalent of a ton of coal.

The energy produced in the fission process is also in the form of heat. Once this heat is generated, the process of converting it into electricity is identical to that of fossil fuel plants. A coolant flows through the reactor and absorbs the heat. It can either turn to steam inside the reactor core, as in the RBMK and U.S. Boiling Water Reactors (BWR), or be sent to a steam generator in which its heat is used to turn water into steam in a secondary loop, as in a U.S Pressurized Water Reactor. Either way, the steam is used to drive turbines and produce electricity.

The fuel is a form of uranium called U-235, so-called because it contains 235 protons and neutrons in the nucleus. Naturally-occurring uranium consists of only 0.7% U-235 which is too low for use in most reactor types. Therefore, the uranium is 'enriched' to about 3% U-235.

Most reactors in operation (including the RBMK, the PWR, and BWR) are thermal reactors. That means that the neutrons produced during the fission process must be slowed down to low (thermal) energies before they are able to cause more fissions in the chain reaction.

The following components can be found in all thermal nuclear reactors.

  1. Fuel
    Thimble-sized pellets of enriched uranium dioxide. Uranium dioxide (UO2) has a higher melting point than pure uranium so it can withstand higher operating temperatures. The pellets are stacked inside tubes of Zircaloy and assembled vertically in the reactor core.
  2. Moderator
    This is the material that slows down the neutrons. This can be ordinary water, 'heavy' water, or graphite (in solid blocks).
  3. Control rods
    These are rods made of neutron absorbing materials, such as cadmium or boron. They are moved into or out of the reactor core to control the rate of its reaction or to stop it completely.
  4. Coolant
    This can be either liquid (water, heavy water) or gas (as is used in some European designs). It is pumped through the reactor core to remove the heat.
  5. Containment
    Reactors are contained inside a casing that acts as a radiation shield and is designed to prevent the release of radioactivity into the environment. All reactors contain a minimum amount of shielding to protect the reactor operators and equipment from radiation damage. Most designs also include a thick steel-reinforced concrete shell that prevents the release of radioactive materials in the event of a severe accident.

Some Reactor Types

There are two types of reactors used in the U.S. for the production of electricity: the Pressurized Water Reactor (PWR) and the Boiling Water Reactor (BWR). Both of these types use ordinary water as both coolant and moderator and therefore are known as Light Water Reactors (LWR). Chernobyl is a type of reactor called an RBMK (Russian acronym) which uses a graphite moderator and water coolant.


The RBMK and a typical U.S. LWR are both thermal reactors that burn uranium dioxide fuel. By thermal it is meant that the neutrons that are emitted by the fissioning atoms must be slowed down to low (thermal) energies so that they can cause more fissions. The heat that is generated in this process is carries away by the coolant which, for both reactor types) is ordinary water. This heat is used to make steam which is then used to drive turbo-generators to make electricity. The rate of the reaction is controlled by inserting control rods into the core. The large amounts of radiation produced under normal operation are contained within the core by heavy concrete and steel shielding.


There are a number of major and minor differences between the RBMK and U.S. Light Water Reactors. For the purposes of this discussion, only the major differences which are relevant to the accident are highlighted.

The fuel assemblies in the RBMK are contained in individual pressure tubes, whereas one pressure vessel contains all of the assemblies in an LWR. The reason for the RBMK design is so that assemblies can be loaded and unloaded individually without shutting down the reactor. This is an advantage if the reactor is to be used for both plutonium and electricity production. LWR's must be shut down for re-fueling and therefore the fuel is kept in as long as is economical. Water acts as both coolant and moderator in LWR's so that a loss of coolant also stops the fission reaction. In the RBMK, the moderator is solid graphite and the water coolant acts as a poison. That means that the presence of water absorbs neutrons and slows the reaction. If coolant is lost or is converted to steam, reactor power may increase. This is known as a positive void coefficient and it represents a serious design flaw. Under certain operating conditions, the power can increase uncontrollably until the reactor disintegrates. This is what happened at Chernobyl. No power reactor in the U.S. can be licensed for construction or operation if it possesses this feature.

The graphite blocks are also flammable at high temperatures. A number of Soviet citizens died in the process of putting out the fire caused by the explosion.

In addition to the shielding, LWR's have an even thicker wall of steel- reinforced concrete surrounding the reactor structure. This structure, called a containment vessel, prevents radioactive release in the event of an accident. Because of this feature, no member of the public was injured or killed when the reactor core melted at Three Mile Island in 1979. The Soviet RBMK does not possess a containment vessel.

In addition to these fundamental differences in design, U.S. reactors are operated under strict regulations. Unlike Chernobyl, U.S. reactor operators are unable to disable the safety systems which prevent dangerous situations from developing. Although equipment can malfunction and operators can make errors, the design of U.S. light-water reactors prevents these mishaps from leading to dangerous releases of radiation.

What is arguably the most significant difference between what was the Soviet nuclear industry and that of the U.S. is the culture of safety that exists here.

Every analysis performed, every decision that is made, and every action taken is done so in the context of the safety of the plant, its personnel, and the local community. Contrary to what many people may believe, this safety culture does not reduce the profitability of the electric utility. Ask any plant manager and he or she will tell you that a safe plant is an efficient plant. Equipment failure or operator mistakes can cost the utility millions of dollars in revenue in addition to regulatory fines. More importantly, however, is the fact that plant employees and their families are members of the local community and have a personal interest in the economic and safe operation of the plant.

Back to Main Page