Physics 5 / History 177 Spring 2000

The Physics of the Nuclear Age
Prof. Gary R. Goldstein and Prof. Martin Sherwin

Nuclear Reactors

  1. How reactors work

    1. Controlled fission (core)
      1. Prevent exponential growth (2t/Tmean) in 235U or 239Pu
      2. Sustain linear growth ( r t )
        1. use delayed n's from decays of daughters, etc. - t~10-4sec -0.5% of n's
        2. get multiplication factor for prompt neutrons, (eta)hprompt <1 for t~10-8sec, but overall factor h=1 so have longer gen.time
      3. Absorb some neutrons: control rods (C,Be,Pb,...B,Cd) to keep h at 1 or below
      4. Slow down neutrons to thermal (Graphite, H2O, D2O,...) - moderator
        1. Graphite - Fermi at U.of Chicago, Hanford, Chernobyl
          1. Water cooled
          2. Gas cooled (exptal)
        2. H2O - Light Water
          1. Boiling Water Reactor - half of US commercial -Plymouth
          2. Pressurized Water Reactor - TMI, Rowe, subs
        3. D2O - heavy water reactor - CANDU - more efficient
      5. Keep U density just at criticality: Fuel rods with enriched U (1 yr operating)
    2. Heat transfer: Fission Energy->Heat (mostly KE of daughters in fuel rods)-->Steam (from coolant directly or pressurized water coolant to water)-->Mechanical Energy-->Electrical Energy + Waste (heat, spent fuel, radionuclides)
      1. Coolant to prevent core overheating, melting, igniting H2O,D2O,liquid Na,He gas
      2. Coolant or heat exchanger to remove heat energy
      3. Steam production
      4. Hot water needs to be cooled down to recirculate
    3. Electrical generators - conventional steam driven turbines - low efficiency

  2. Safety requirements - containment and emergency SCRAM system
    1. Containment vessel to withstand high pressure, heat and contain radionuclides and contaminated materials
      1. TMI worked
      2. Chernobyl failed
    2. Emergency shutdown system
      1. Control rods drop
      2. More coolant pours in
      3. Core is sealed off
      4. TMI and Chernobyl failed partially
  3. Accidents
    1. Overheating due to
      1. loss of coolant
      2. uncontrolled reactions due to
        1. failure of the moderators
        2. collapse of structures
          1. due to structural damage from radiation
          2. due to meltdown
    2. failure of emergency system to respond with extra coolant, shut off valves
    3. "Breach of containment" vessels
      1. From structural weakening
      2. from pressure buildup
      3. from explosions
    4. Release of contaminated material through failing shutoff valves, fractured pipes
      1. to air
      2. to site
      3. into ground
      4. into water table
    5. Release of contaminants via fallout
    6. Notification of affected areas and evacuation


Readings in American Atom: Chap.IX short history '54-'79
Doc.68 A.E. Act
Doc.69 AEC report postponing Fermi N.Power Plant '56 - concern over positive component of temperature coefficient from EBR-I and resulting instability
Doc.70 AFL-CIO opposition '56 to a fast breeder reactor in Mich. which was given preliminary approval in a "secretive and irregular manner" by the AEC.
Doc.71 AEC Report on fuel rod damage in Fermi reactor '66
A well handled accident. Irregularities in temperature and control rod position were noticed and acted upon when radioactivity in gases above core was seen to be quite high. Containment was not breached and "scram" procedure for shutdown proceeded successfully. Two fuel rods were damaged [why?].
Doc.72 Environmental considerations ruled paramount to financial committment to build a new plant.
Doc.73 TMI accident - a PWR

What happened to the TMI reactor according to W.J.Lanouette (Bull.At.Sci. June'79) and R.Wolfson, "Nuclear Choices".

  1. Reactor had been shut down for maintenance and testing a few days before Mar28'79. Two valves for auxiliary feedwater had been left shut and not reopened when reactor was restarted that morning.
  2. When restarted feedwater pump failed to work in feeding steam turbine, leaving primary coolant going through an insufficient heat sink and keeping too hot while auxiliary feedwater failed to come on.
  3. Coolant gets superheated and pressure builds up in both turbine and core setting off scram system (about 5 seconds after startup).
  4. High pressure coolant is vented out into containment vessel and out-valve sticks, leaving core uncovered, to heat up.
  5. Released coolant builds up on the floor and is pumped out to auxiliary tanks, which are overflowing. Contaminated water vents through ventilators to outside.
  6. Emergency Core Cooling System (ECCS) turns on but operators interpret the increasing water level in containment structure as sign that there is too much coolant in core.
  7. Operators turn down ECCS and turn off many alarm systems, not knowing they had Loss Of Coolant Accident (LOCA), while they ponder for ~2 hours. The core is uncovered, unbeknownst to the operators.
  8. Remaining coolant reacts with Zirconium fuel cladding to produce H bubble. H bubble and water vapor have sufficient pressure to block entry of extra coolant.
  9. At 3 hours radioactivity in containment vessel causes alarms leading to emergency notice.
  10. By 5 hours NRC and some operators realize core is uncovered.
  11. By 7 hours reactor is under control but overflowing water tanks continue to release radioactivity into environs.
  12. A meltdown had occurred in the process.

The Chernobyl disaster. RMBK reactor has graphite moderator and H2O coolant(+ n absorber).
  1. Apr.25,1986 operators prepare to test reactor ECCS components by lowering power and turning off ECCS. Power goes from 3200 MW down to 30 MW.
  2. 135Xe builds up. This isotope normally absorbs neutrons to transmute into other species, but with such low levels of neutrons being produced now, Xe "poisons" the reactor.
  3. Instead of waiting for the 9 hour half-life, operators remove control rods to restart reactor. Continuing test, they turn on water pumps.
  4. 20 minutes into restoring power, coolant pumping is reduced, causing danger signals.
  5. To continue test, operators vent steam from turbine to lower load on reactor. Lower water level in core causes steam and LOCA.
  6. Lower water level increases fission rate and reactor runs away (500 times maximum in 5 seconds).
  7. Rapid production of steam blows off concrete lid.
  8. Uncovered fuel rods react with water to produce H.
  9. H ignites and graphite burns carrying core material aloft.
  10. All gaseous material escapes and 3% of solids. Contamination through fallout is continental.
  11. Tens to hundreds of thousands extra cancer deaths.