Reactor Types:	|	RBMK	|	VVER 440/213	|	VVER 440/230	|	VVER 1000

The RBMK

The former Soviet Union built 17 nuclear units based on the RBMK design used at the Chornobyl nuclear power plant. Today it is generally recognized that there are three generations of RBMK nuclear power plants.

Principal Strengths:

The low core power density of RBMKs provides a unique ability to withstand station blackout and loss of power events of up to an hour with no expected core damage.

Ability to be refueled while operating, permitting a high level of availability.

The graphite moderator design allows the use of fuel that is not suitable for use in conventional water-moderated reactors.

Principal Deficiencies:

The most significant difference between the RBMK nuclear plant design and most of the world's nuclear power plants is the RBMK's lack of a massive steel and concrete containment structure as the final barrier against large releases of radiation in an accident. The effectiveness of American-style reactor containments was shown in the 1979 Three Mile Island Unit 2 accident, when virtually all radiation was retained inside the containment building, despite considerable melting of the fuel. In the Chornobyl accident, the RBMK plant's accident localization system (the RBMK's version of containment) could not withstand the force of the accident.

Accident mitigation systems are limited and ineffective.

Reactor control systems are unforgiving to many potential system upsets, with a consequent potential difficulty of successful recovery.

Faster and less stable nuclear chain reactions--and power increases--when cooling water is lost. In tech nical terms, this characteristic is called a "positive void coefficient." Soviet engineers sought to mitigate this tendency by backfitting RBMKs with faster-acting control rods and other improvements. U.S.- style light water moderated plants, however, are designed with just the opposite characteristic--a "negative void coefficient"--so that the nuclear chain reaction automatically stops when coolant is lost. The design of the Kursk-5 RBMK has reportedly been modified, resulting in a negative void coefficient.

Inadequate fire-protection systems.

Limited capability for steam suppression in the graphite stack.

Flawed separation and redundancy of electrical and safety systems.

Complicated piping arrangements.