Chernobyl Nuclear Accident Congressional Hearings Transcript

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capacity, and has long been considered a means for Improving
reactor performance and safety. Currently, as part of the U.S.
Department of Energy program, a Modular High Temperature Gas-
Cooled Reactor (MHTGR) is being designed which takes advantage of
these graphite properties to attain a passively safe system,
i.e., the safety of the public is assured under all conditions
without requiring any operator action or the operation of powered
systems (such as pumps, valves, motors, etc.). In supporting
this concept, the Congress has recognized that a system of this
type is essential to overcome the technical and institutional
barriers which have prevented the deployment of nuclear systems
in the U.S., for the past decade. The need for a passively safe
system, such as the Modular HTGR, is even more essential after
Chernobyl
The Chernobyl nuclear disaster has raised questions regarding
the design similarities and differences between the RBMK
(Chernobyl type) boiling water graphite moderated reactor and the
MHTGR. In particular, the reported graphite fire has drawn attention
to the MHTGR which also uses graphite as a moderator. In
this testimony, I will identify the numerous fundamental differences
in the safety of the two systems.
The CHernobyl RBMK reactor is a pressure-tube type of
reactor using zirconium alloy clad uranium oxide pellets as fuel,
boiling light water as the coolant, and graphite as the moderator
(Figure 1). The fuel assemblies consist of a cluster of 18 clad
fuel pins located in vertical zirconium alloy pressure tubes
through which the cooling water is circulated. The reactor is
designed to perform on-line refueling. The reactor coolant inlet
and outlet temperatures and pressures are consistent with boiling
water reactor technology. The approximately 1700 pressure tubes
which serve as the pressure boundary for the system pass through
stacked graphite moderator blocks. The graphite blocks are enclosed
in a thin stainless steel enclosure and blanketed with a
helium and nitrogen gas mixture. The safety system incorporates
an emergency core cooling system and a pressure suppression