Safety Related Physical Phenomena for Coupled High-Temperature Reactors and Hydrogen Production Facilities

High-temperature reactors are a potential low-carbon source of high-temperature heat for chemical plants—including hydrogen production plants and refineries. Unlike electricity, high temperature heat can only be transported limited distances; thus, the reactor and chemical plants will be close to each other. A critical issue is to understand potential safety challenges to the reactor from the associated chemical plant events to assure nuclear plant safety. The U.S. Nuclear Regulatory Commission (NRC) recently sponsored a Phenomena Identification and Ranking Table (PIRT) exercise to identify potential safety-related physical phenomena for high-temperature reactors coupled to a hydrogen production or similar chemical plant. The ranking process determines what types of chemical plant transients and accidents could present the greatest risks to the nuclear plant and thus the priorities for safety assessments. The assessment yielded four major observations. Because the safety philosophy for most chemical plants (dilution) is different than the safety philosophy for nuclear power plants (containment), this difference must be recognized and understood when considering safety challenges to a nuclear reactor from coupled chemical plants or refineries. Accidental releases of hydrogen from a hydrogen production facility are unlikely to be a major hazard for the nuclear plant assuming some minimum separation distances. Many chemical plants under accident conditions can produce heavy ground-hugging gases such as oxygen, corrosive gases, and toxic gases that can have major off-site consequences because of the ease of transport from the chemical plant to off-site locations. Oxygen presents a special concern because most proposed nuclear hydrogen processes convert water into hydrogen and oxygen; thus, oxygen is the primary byproduct. These types of potential accidents must be carefully accessed. Last, the potential consequences of the failure of the intermediate heat transport loop that moves heat from the reactor to the chemical plant must be carefully assessed.