Hazards

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Hazards and Mitigating Design Choices

A nuclear reactor contains many hazards that can ultimately lead to a Fission Product (FP) release including decay heat, excess reactivity, chemical reactions, and high pressure. Hazards can be considered the potential gradients that can or could become accessible and who equilibration can lead to fission product release.

The fission products in the fuel are the underlying hazard. Fission product gases are concentrated in the fuel at high pressure and temperature, but nonexistent in the low temperature and low-pressure environment. This form a chemical difference that would like to equilibrate to the disadvantage of lifeforms outside.

Each hazard is listed in the table below with the mitigating approach taken in the HTGR architecture. Each in their own, the hazards can be mitigated through passive mechanisms and inherent characteristics. The International Atomic Energy Agency’s (IAEA) definition of inherent safety being “safety achieved by the elimination of a specified hazard by means of the choice of material and design concept” and their logic on the various categories of passive systems is followed.[1] The second Table discusses various design choices, and their effects on the hazards.

HAZARD	Description
Chemical Hazard	Core components can interact with each other, the coolant, or external water and air to release energy or cause corrosion. At a minimum, this exacerbates maintenance and inspection needs during normal operations. During accidents, these features escalate conditions beyond the point of no return such as when Zirconium and water react at elevated temperatures to release energy and hydrogen which can then explode resulting in loss of control functions or LOCA. Similar problems occur with sodium coolant and heat pipes.
Reactivity Hazard	Control rod extraction or other mechanisms can cause the reactor to become dangerously supercritical and dramatically increase the power output of the reactor beyond what it is capable of handling. Too much power and the fuel will melt itself, releasing fission products. Most reactors can somewhat tolerate the partial ejection of one or two control rods but will fail catastrophically when more are extracted.
Terrorist Hazard	Bad actors may intentionally sabotage reactor components by explosive or kinetic means. Bad actors could intentionally withdraw all reactor control rods, cause intentional coolant depressurization or flooding.
Pressure Hazard	High pressure constitutes a stored potential energy that can be released explosively leading to impacts and damage to reactor components or the release of radioactive coolant. Pressure can escalate accidents.
Decay Heat Hazard	When a reactor shuts down, it continues to produce some fraction of the power as decay heat for hours and days. This decay heat is what leads to reactor melting, when the reactor operators are unable to cool the reactor down, such as in the case of pipe clogging, loss of power for pumps, or loss of coolant.

According to several studies, a TRISO fueled HTGR reactor is the highest TRL Generation IV technology [2], the most able to mitigate chemical and reactivity insertion hazards, and has sufficient acceptance from regulators and nuclear skeptics [3]. This perspective is becoming mainstream in academic [4] and industrial circles with HTGR being the most heavily invested advanced reactor technology besides LWR SMRs. It is postulated that this class of reactors can be characterized by small unit power systems connected to a large balance of plant, with few or no safety systems or safety grade equipment and lower operating costs. For HTGRs, this risk reduction is principally achieved by limiting power rating and using refractory ceramic materials so that fuel temperatures do not exceed limiting temperatures during simultaneous Beyond Design Basis Accidents (BDBA), assuming an appropriate degree of security and safeguards is provided to the reactor, and all while using only Class A Passive safety mechanisms. This reactor technology is called the Class A HTGR (CA-HTGR), which more or less mirrors the design architecture of the commercially developed USNC MMR.

Hazard	HTGR Approach
Chemical Hazard	Avoid water, sodium, and metals that react with target environment and RX components at high temperatures. Keep the fission products contained and solid to reduce reaction rates and state space. All core materials chemically compatible across radiation and temperature space Core materials have high chemical and radiation tolerance. Minimize use of water, keep it away from the core, provide excess drainage. Use helium coolant which is totally unreactive with fuel and graphite. Graphite has some reaction potential with water/air, which can be mitigated by reducing its surface area, reducing water ingress potential using separate steam exchanger or intermediate non-water loop, fuel-moderator inversion concepts
Reactivity Hazard	Strong negative temperature feedback in the fuel. As the power or temperature increases, the power is forced backdown by physical characteristics of the materials (no actions or mechanisms) Minimize voids in and around the RX that can be filled with foreign substances (e.g., water or flowing air) Control rods are used for startup, core power controlled by coolant flow. Properly design reactivity margin to withstand water submersion and other reactivity insertions. Tolerate full extraction of all control rods
Terrorist Hazard	Other hazards are drastically minimized compared to other reactors, reducing the potential impact of any terrorist action. Buried cartridges, fully below ground level, with limited access volumes. TRISO subdividing the nuclear fuel by 4-5 orders of magnitude compared to other fuel eliminates single point failure of fuel claddings and pressure vessel. Particles small and durable enough that has potential to remain intact during explosive fractures.
Pressure Hazard	Other hazards are drastically minimized compared to other reactors, reducing the potential impact of any pressure release. Unlike water or sodium, helium has limited damage potential, especially if clean from fission product gases or dust. Keep the helium clean using TRISO-based fuels. Reduce pressure and helium inventory when possible.
Decay Heat Hazard	Refractory all-ceramic cores (metals on the periphery) with high 2800 °C melting points. 1600°C tolerable temperatures in the fuel vs 800°C for UO2 and 400 °C for Zircaloy in conventional fuel forms. Materials with high melting temperature, high k, high c which enhance heat transfer and reduce temperature changes. Passive cooling systems to remove decay heat from the core without coolant, power, or operators and despite changes in the geometry of reactor surroundings. This is achieved by maximizing surface area to power ratio. Appropriate power rating for Class A passive safety (no moving parts or fluids). That is, reduce the decay heat burden and BDBA power so that the reactor geometry and physical materials can withstand the rise in temperatures.

[1] IAEA, “Safety Related Terms for Advanced Nuclear Plants.”

[2] “Advanced Modular Reactors Technical Assessment.”

[3] Lyman, “‘Advanced’ Isn’t Always Better.”

[4] Buongiorno, “Japan’s Next Nuclear Energy System (JNext).”

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