Next Generation Nuclear Plant Phenomena Identification and Ranking Tables (PIRTs) (NUREG/CR-6944) - Volume 6: Process Heat and Hydrogen Co-Generation PIRTs

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Publication Information

Manuscript Completed: October 2007
Date Published: March 2008

Prepared by:
C.W. Forsberg – Panel Chair

Panel Members:
M.B. Gorensek (SRNL)
S. Herring (INL)
P. Pickard (SNL)

Oak Ridge National Laboratory
P.O. Box 2008
Oak Ridge, TN 37831-6170

S. Basu, NRC Project Manager

Prepared for:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code N6376

Availability Notice


A Phenomena Identification and Ranking Table (PIRT) exercise was conducted to identify potential safety-0-related physical phenomena for the Next Generation Nuclear Plant (NGNP) when coupled to a hydrogen production or similar chemical plant. The NGNP is a very high-temperature reactor (VHTR) with the design goal to produce high-temperature heat and electricity for nearby chemical plants. Because high-temperature heat can only be transported limited distances, the two plants will be close to each other. One of the primary applications for the VHTR would be to supply heat and electricity for the production of hydrogen. There was no assessment of chemical plant safety challenges.

The primary application of this PIRT is to support the safety analysis of the NGNP coupled one or more small hydrogen production pilot plants. However, the chemical plant processes to be coupled to the NGNP have not yet been chosen; thus, a broad PIRT assessment was conducted to scope alternative potential applications and test facilities associated with the NGNP. The major conclusions are as follows:

  • NGNP vs a commercial high-temperature reactor. The PIRT panel examined safety issues associated with the NGNP and a commercial plant. For the NGNP, only a small fraction of the heat is expected to be used to produce hydrogen or other chemicals, with most of the heat used to produce electricity. In contrast, for a commercial high-temperature reactor application, all of the heat might be used for production of hydrogen or other chemicals. Because the total chemical inventories determine the potential hazard to the nuclear plant from a chemical plant, the hazards of a small chemical plant associated with the NGNP may be significantly less. For the NGNP, there may be multiple generations of hydrogen production and other chemical technologies tested; thus, one must either envelope the safety implications of the different technologies or update the safety analysis with time.
  • Chemical plant safety, regulatory strategy, and site layout. The safety philosophies for most chemical plants and nuclear power plants are fundamentally different. For hazards such as hydrogen leaks, the safety strategy is dilution with air to below the concentration of hydrogen that can burn in air. For example, a small amount of hydrogen in an enclosed room is an explosion hazard. However, a large release of hydrogen to the environment is a relatively small hazard when outdoors. Consequently, most chemical plants are built outdoors to allow rapid dilution of chemicals with air under accident conditions. The reverse strategy is used for nuclear plants, where the goal is to contain radionuclides because their hazard does not disappear if diluted with air. Primary chemical plant safety strategies include outdoor construction (no containment), controlled chemical inventory sizes, site layout features, and adequate separation distances between process and storage facilities. These differences must be recognized when considering safety challenges to coupled nuclear and chemical plants.
  • Hydrogen. 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. This conclusion is based on several factors: (1) if hydrogen is released, it rapidly rises and diffuses, thus making it very difficult to create conditions for a large explosion and (2) a hydrogen burn does not produce high thermal fluxes that can damage nearby equipment. In addition to laboratory and theoretical analysis of hydrogen accidents, there is a massive knowledge base in the chemical industry with hydrogen accidents and, thus, a large experimental basis to quantify this hazard based on real-world experience.
  • Heavy gases. Many chemical plants under accident conditions can produce heavy ground-hugging gases such as oxygen, corrosive gases, and toxic gases. Industrial experience shows that such accidents can have major off-site consequences because of the ease of transport from the chemical plant to off-site locations. If the chemical plant or the stored inventories of chemicals are capable of releasing large quantities of heavy gases under accident conditions, this safety challenge requires careful attention. Oxygen presents a special concern. Most proposed nuclear hydrogen processes convert water into hydrogen and oxygen; thus, oxygen is the primary byproduct. Oxygen has some unique capabilities to generate fires. Equally important, these will be the first facilities that may release very large quantities of oxygen to the atmosphere as part of normal operations. There is a lack of experience. The phenomena associated with plume modeling and the effects of such plumes on the nuclear plant safety-related structures, systems, and components are of high importance.
  • Heat exchanger failure. The second major class of safety challenges with high importance is associated with the failure of the intermediate heat transport loop that moves heat from the reactor to the chemical plant. Several different heat transport media are being considered including helium, helium-nitrogen mixtures, liquid salt mixtures, and high-temperature steam. High-temperature steam is required as a process chemical for some processes, such as the production of hydrogen using high-temperature electrolysis, thus steam could be the intermediate heat transport fluid. The choice of heat transport fluid will partly depend upon distance. Over longer distances, liquid salts and steam are expected to have lower heat-transport costs. For gas-phase intermediate heat transport systems, there are several specific phenomena of high safety importance.
    • Blowdown of intermediate heat transport loop. In certain pressure boundary failures, the blowdown could accelerate fluid flow through the primary heat exchangers. Depending upon the failure location, this may result in accelerated fluid flow of the cold heat-transport fluid through the intermediate heat exchanger and result in overcooling the reactor coolant because of enhanced heat transfer in the primary heat exchanger. After blowdown, there will be a loss of the heat sink.
    • Leak into reactor primary system. The total gas inventory in the intermediate loop may be significantly larger than the total inventory of gas in the reactor primary system. A large or small leak from the intermediate heat transport loop into the reactor in accident scenarios where the primary system depressurizes could add large inventories of gas to the reactor, providing a sweep gas to move fission products from the reactor core.
    • Chemical additions to reactor core. If steam or other reactive gases from the intermediate heat transport loop enter the reactor because of a heat exchanger failure, there is the potential for fuel damage—particularly given the much higher temperatures proposed for some applications of high-temperature reactors.
    • Hot fluids. If the heat transport loop fluid escapes into the reactor building, the high temperatures could cause significant damage.

The hazards associated with various chemicals and methods to minimize risks from those hazards are well understood within the chemical industry. Much but not all of the information required to assure safe conditions (separation distance, relative elevation, berms) is known for a reactor coupled to a chemical plant. There is also some experience with nuclear plants in several countries that have produced steam for industrial applications. The specific characteristics of the chemical plant, site layout, and the maximum stored inventories of chemicals can provide the starting point for the safety assessments.

While the panel identified events and phenomena of safety significance, there is one added caveat. Multiple high-temperature reactors provide safety-related experience and understanding of reactor safety. In contrast, there have been only limited safety studies of coupled chemical and nuclear plants. The work herein provides a starting point for those studies; but, the general level of understanding of safety in coupling nuclear and chemical plants is less than in other areas of high-temperature reactor safety.

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