EVEN UNDER THE MOST SEVERE ACCIDENT CONDITIONS
AND SIMPLIFIES THE SAFETY EQUATION
A
Simpler, More Rational Way to
Think about Nuclear Safety:
FOUR LEVELS OF SAFETY*
LEVEL 0:
No hazardous materials or confined energy sources.
LEVEL 1:
No need for active systems in event of subsystem failure. Immune to major
structural failure and operator error.
LEVEL 2:
No need for active systems in event of subsystem failure. No immunity
to major structural failure or operator error.
LEVEL 3:
Positive response required to subsystem malfunction or operator error.
Defense in depth. No immunity to major structural failure.
The MHR is the only reactor that meets the criterion of Level 1 safety. Its design is derived from natural properties of materials and optimum choice of reactor size, geometry and power density. It can withstand the total loss of coolant without the possibility of a meltdown Š going beyond simply saying "it is safe enough."
The Chernobyl and Three Mile Island reactors fall in the Level 3 category.
The Chernobyl power runaway was initiated by human error which resulted in a steam explosion, followed by structural failure, loss of coolant, core melting, and radioactivity release.
The Three Mile Island core melt accident was caused by human error which resulted in loss of coolant. Core melt caused radioactivity release from the reactor vessel, but the containment building effectively confined radioactive release.
*Definition developed
by Professor Lawrence Lidsky,
Massachusetts Institute of Technology.
The MHR is the only reactor that meets the criterion of Level 1 safety.
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WHAT
A LARGE NEGATIVE TEMPERATURE COEFFICIENT
MEANS TO SAFETY |
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The picture has captured a power pulse in a TRIGA research reactor where the power increased 4,000 times over its normal operating range. This intentional power increase lasted only about one hundredth of a second because the reactor has a very large negative temperature coefficient which naturally shuts the reactor down . . . guaranteed by the laws of nature. Like other U.S. power reactors, the GT-MHR has a negative temperature coefficient. By contrast, Chernobyl had a positive reactivity coefficient; its temperature increase acted to intensify the fission reaction, thus causing a runaway. Safety:
The Effects of Decay Heat The decay heat at Three Mile Island and Chernobyl caused the reactor fuel to melt, even after the fission reaction had essentially stopped, because of the loss of cooling water. The Modular Helium Reactor's decay heat will not cause a meltdown even if the coolant is lost. The reactorÕs low power density and geometry assure that decay heat will be dissipated passively by conduction and radiation without ever reaching a temperature that can threaten the integrity of the ceramically-coated fuel particles. . . even under the most severe accident conditions.
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The MHR is the only reactor that meets the criterion of Level 1 safety. |
