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Off-Gas System

BWR reactorBWR reactorA boiling water reactor uses Light Water as both coolant and neutron moderator. The second most used reactor, next to the PWR reactors, there are approximately 75 plants in current operation. The efficiency of these reactors is about 46%, with 33-34% in practice. Enriched uranium is used as nuclear fuel, as light water absorbs too many neutrons to use Uranium that is natural. Light water is not as good of a moderator compared to Heavy Water or graphite, but it is good as in the event of a LOCA o offgas system is used for condensers as some gases are made that cannot be condensed. If the condenser has to vent to the offgas open due to high pressure this is the treatement process. Basically what happens is that it goes throughs ome treatments and then exits thorught the plant stack, and this is all monitored to comply with regulatory standards.

The following is directly made from chatgpt; not my ideal source, however this system has been sitting on my backlog for weeks, so I had to do it this way.

The BWR Off-Gas System is designed to collect, process, delay, and monitor non-condensable gases (including some radioactive isotopes), and then vent the cleaned gas to the atmosphere through the plant stack under controlled conditions.

  1. Source:
    Non-condensables (mostly H₂, O₂, N₂, and small amounts of radioactive gases like xenon, krypton, and tritium) come from:
    • Condenser (air in-leakage)
    • Steam radiolysis
    • Minor fuel leakage (fission product gases)
  2. Air ejectors or vacuum pumps:
    • Extract gases from the condenser shell side
    • Send them to the off-gas treatment system
  3. Off-Gas Treatment System (before venting)
    • Off-gas condenser: removes residual steam
    • Catalytic recombiner: converts explosive H₂ + O₂ → H₂O
    • Moisture separator / cooler
    • Charcoal beds or decay tanks (older systems) to delay noble gases
      • Decay tanks: 6–12 hours delay to let short-lived isotopes (e.g., Xe-133, Kr-88) decay
      • Charcoal beds: adsorb gases; breakthrough monitored
    • Monitors: radiation levels (particulate, noble gas, iodine channels)
  4. Stack Release
    • Final effluent is discharged via plant stack (often 200–300 ft tall)
    • Continuously monitored by radiation monitors
    • Released only if within Tech Spec and regulatory limits (10 CFR 20, 10 CFR 50 App I)

If the Off-Gas System Is Overwhelmed:

  • If there’s excess steam, or the charcoal beds saturate, or the recombiner is bypassed:
    • Gases may reach the stack faster
    • Still monitored and may trigger High Radiation Stack Alarm
    • In extreme cases (e.g., SBO, LOCA), off-gas vents may bypass full treatment (emergency mode)