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SAFETY

Safety Levels*

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.

In 1979, 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. The Three Mile Island reactor was categorized as a Level 3.

In 1986, 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 Chernobyl reactor was categorized as a Level 3.

Today, 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. The GT-MHR is categorized as a Level 1.

*Definition developed by Professor Lawrence Lidsky, Massachusetts Institute of Technology.

Decay Heat

Decay heat, resulting from the decay of fission products, is a phenomenon in all reactors. The heating does not stop when the power is shut off, so having a negative temperature coefficient is good but not enough.

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 GT-MHR's decay heat will not cause a meltdown even if the coolant is lost. The reactor's 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.

Temperature

Like other U.S. power reactors, the GT-MHR has a negative temperature coefficient, which naturally shuts the reator down, guaranteed by the laws of nature.

Graphites

It is often incorrectly assumed that the combustion behavior of graphite is similar to that of charcoal and coal. Numerous tests and calculations have shown that it is virtually impossible to burn high-purity, nuclear-grade graphites. Graphite has been heated to white-hot temperatures (~1650°C) without incurring ignition or self-sustained combustion. After removing the heat source, the graphite cooled to room temperature. Unlike nuclear-grade graphite, charcoal and coal burn at rapid rates because:

• They contain high levels of impurities that catalyze the reaction.
• They are very porous, which provides a large internal surface area, resulting in more homogeneous oxidation.
• They generate volatile gases (e.g. methane), which react exothermically to increase temperatures.
• They form a porous ash, which allows oxygen to pass through, but reduces heat losses by conduction and radiation.
• They have lower thermal conductivity and specific heat than graphite.

In fact, because graphite is so resistant to oxidation, it has been identified as a fire extinguishing material for highly reactive metals.

The oxidation resistance and heat capacity of graphite serves to mitigate, not exacerbate, the radiological consequences of a hypothetical severe accident that allowed air into the reactor vessel. Similar conclusions were reached after detailed assessments of the Chernobyl event; graphite played little or no role in the progression or consequences of the accident. The red glow observed during the Chernobyl accident was the expected color of luminescence for graphite at 700°C and not a large-scale graphite fire, as some have incorrectly assumed.

Spent Fuel

On a per MWe-yr basis, GT-MHR spent fuel has a number of advantages over light-water reactor (LWR) spent fuel and is an ideal waste form for permanent geologic disposal:

• The lower decay heat load allows for efficient repository loading, requiring ~1/2 of the repository land area needed for LWR spent fuel.
• The high thermal efficiency results in a fission product inventory that is 50% lower for the GT-MHR.
• The high thermal efficiency and lower fertile fuel loading (U-238) result in plutonium and actinide inventories that are a factor of 2.5 lower for the GT-MHR.

GT-MHR spent fuel offers a greater resistance to diversion than LWR spent fuel. For canisters of equal volume, the plutonium content in a GT-MHR canister is more than a factor of 20 lower than that for an LWR canister, and plutonium in GT-MHR spent fuel has a lower percentage of Pu-239.

As shown in the figure, the TRISO coatings act as miniature containment barriers that are highly resistant to corrosion and pressure buildup over geologic time scales. The very low corrosion rates of silicon carbide and pyrocarbon have been confirmed during independent testing at national laboratories. The TRISO coatings provide the substantial benefit of long-term containment without having to rely on the waste package.