Fifteen minutes after starting the test, reactor power decreased to nearly zero due to the negative reactivity feedback effect ^*. Subsequently, as heat was transferred to the outside through radiation and natural convection, core temperature decreased, and the concentration of xenon (Xe) gas, which absorbs neutrons, also decreased. As a result, the reactor temporarily increased approximately 14 hours after the start of the test, while the power stabilized at approximately 1.2 %. Although fuel temperature temporarily increased immediately after the start of the test, design limit of 1600 ℃ was not exceeded. Analysis confirmed that, as reactor power decreased, fuel temperature stabilized at approximately 800 ℃.

High-temperature gas-cooled reactors (HTGRs) use graphite as the core material and helium gas as the coolant, enabling a design in which core melting cannot occur and providing excellent safety. By taking advantage of this feature, installation of HTGRs near energy demand sites such as industrial facilities and chemical plants is expected. To demonstrate this inherent safety, the world’s first safety demonstration test was conducted at the High-Temperature Engineering Test Reactor (HTTR) at full power (100 %) for a block-type HTGR.

In this test, all helium gas circulators (HGCs) were shut down, and insertion of the control rods was intentionally prevented. This setup simulated the loss of the two most important safety functions of a reactor- “shutdown” and “cooling”. Immediately after the start of the test, core temperature temporarily increased. However, due to the negative reactivity feedback effect ^*, reactor power spontaneously decreased to nearly 0 %. Throughout the test, fuel temperature never exceeded the design limit of 1600 ℃, and the confinement function of radioactive materials was confirmed to remain in effect (Fig. 1).

These results clearly demonstrated that even without control rod insertion or forced cooling, HTGR can naturally stabilize the safety state and cool down, while safely containing radioactive materials. This verifies the excellent inherent safety of HTGRs.

 

^* Negative reactivity feedback effect (self-regulating characteristic of the reactor): As core temperature increases, fission reaction is suppressed, and reactor power spontaneously decreases.

Nagasumi, S. et al., Demonstration of the Inherent Safety Feature of HTGRs Through the Loss-of-Forced-Cooling Test in the HTTR, Nuclear Engineering and Design, vol.446, part A, 2026, 114542, 14p.

Paper URL: https://doi.org/10.1016/j.nucengdes.2025.114542

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