Notes On Corium And Containment

My notes (Nancy’s) on the Reactor Safety Training Course (R-800) from the NRC http://pbadupws.nrc.gov/docs/ML0210/ML021080117.pdf
The notes are an abbreviated gathering of parts that may be relateable to Fukushima from sections 3 and 4.
Suppression Chamber:

Steam can be sent from the RPV via a safety valve and piping into the suppression chamber (torus).

If the pressure in the suppression chamber (wetwell/torus) is higher than that in the drywell vacuum breakers equalize the pressure.

There is a cooling spray system for the drywell portion of containment and also cooling system in the suppression chamber that runs through a heat exchanger to cool the water in the suppression chamber.

Mark 1 reactors also have a direct vent system to the outside in the suppression chamber to relieve pressure. (There were attempts to open these at Fukushima)

Containment:

Mark 1 containments were not specifically designed to handle these situations:
Reactor Vessel  Rupture
Steam  Generator  Rupture
Large  break  (LOCA) Loss Of Coolant Accidents

These kinds of accidents produce higher containment pressure and more fission products. High heat caused by these kinds of accidents can also lead to failing containment by way of cracks or ruptures.

Areas of containment prone to failures are piping  penetrations, electrical  penetrations,  hatches  and  airlocks.  Isolation  valves  are  provided  on  all  pipes and  ducts  that  penetrate  the  containment but can also fail.

Pressure suppression style containment designs are smaller and allowed a 1.0  wt.%/day leakage rate without having a significant safety impact.

Statistics for Peach Bottom are likely similar to units at Fukushima as they have similar designs.
Peach Bottom BWR Mark 1 Peak Design Basis Accident Pressure PSIG 49.1 (KPA 339)
Peach bottom design: 56psig, ultimate failure pressure 148 psig

Challenges  to  containments  can  occur  during four  time  regimes:
1.  at  the  start  of the  accident
2.  prior to  reactor  vessel  failure
3.  at  or  soon  after  reactor  vessel  failure
4.  long  after  reactor  vessel  failure.

These containment systems can build pressure over hours or days so they have cooling systems installed. Sprays, fan  coolers,  suppression  pool  cooling,  or emergency  core  cooling  recirculation  cooling  are installed and have to meet a “single failure” standard of durability.

These  active  systems  do  not  act  quickly enough  to  affect  the  initial  blowdown  during a  large-break  LOCA,  but  limit  further pressure  increases  and  are  also  beneficial during  slower  developing  accidents.

These major accidents can cause the heat removal systems to fail. In some situations the  design pressure  may  be  exceeded  early,  but  the ultimate  failure  pressure  would  not  be reached  for  many  hours  or  even  days. The point where the RPV is breached, usually by melted fuel burning a hole through the lower portion of the RPV is of specific risk.  Several things can happen at that point, even happening at the same time.
1.  Steam  spike
2.  Steam  explosion
3.  Direct  containment  heating
4.  Hydrogen  combustion
5.  Containment  shell  melt through
6.  Downcomer  failure  (Mark  II  BWR)

Steam spikes or explosions can happen if there is water in the pedestal region below the RPV as molten fuel hits the pool of water. Water can build up in the pedestal due to a broken reactor cooling pipe or from containment cooling spray systems. Steam spikes are more of a problem if containment is already at a high pressure. A steam explosion can send out shockwaves that can damage the containment or the RPV supports. If the RPV supports fail the RPV can move significantly. RPV movement can cause more problems for pipe penetrations in the containment wall. A steam explosion can bypass the suppression chamber (torus) and cause the containment to over pressurize.

A containment that leaks slowly can leave the radioactive substances inside containment or the building, a large fast leak will dump large amounts of radioactive substances into the environment.

In a severe accident the reactor vessel “blowdown” (of steam) can be beyond the design basis of the containment structure. As heat and pressure increase before RPV failure containment can deform and create leaks around penetrations. This can also cause ruptures in the steel containment of a steel/concrete containment like at Fukushima. This is the likely outcome according to recent studies. The concrete  structure of the containment is more  likely  to  crack as the steel ruptures.

Three things can significantly add to the pressure in containment.
1. Initial  reactor coolant  system  blowdown (into the torus)
2. Steam release when reflooding the reactor with water
3. Hydrogen release when reflooding the reactor with water.

If penetrations don’t leak steel  containments have  a  larger  margin  between  the  design  and ultimate  failure  pressures  than  concrete containments.

Direct  Containment  Heating  (DCH)  involves the  ejection  of  the  melt  from  the  vessel  at high  pressure,  thus  spraying  the  molten material  into  containment. The small particles of fuel being sprayed into containment can super heat the atmosphere inside containment. This can rapid heat the containment area faster than a steam spike.

Any coolant (water) still in the RPV will then be released into containment. This water can quickly produce hydrogen. If the oxygen and hydrogen mixture is of the right ratio and temperature, the molten fuel can act as an ignition source and cause a hydrogen explosion. If large enough it can threaten containment. Hydrogen explosions can happen in conjunction with other events to fail containment.

The likely melt through scenario in Mark 1 containment is for the melted fuel (corium) to reach the floor of the drywell, flow out of the concrete vertical pedestal tube and across to where the floor and steel liner on the wall meet. This type of failure can happen faster and is more likely than burning through the basemat concrete directly below the reactor.

Public risk  is  probably  dominated  by  accidents  in which  substantial  core  damage  occurs  and the  containment  fails  or  is  bypassed.

A disturbing note found in the document, that the initial mindset was locating a reactor in a low population area was considered a safety buffer and containment systems were created to protect the population nearby.
“containments began  to  evolve  when  designers  realized  that remote  siting  would  not  be  practical  in  all cases.  The  first  containments  were  provided for  the  Knolls  Atomic  Power  Laboratory  and Shippingport  experimental  reactors  in  order to  allow  them  to  be  sited  in  more  populated areas.”

 

 

Containment Cap:

The yellow containment cap bolt style used at Peach Bottom nuclear plant is the style used at Fukushima and is prone to bolt elongation (stretching) and flange separation (opening) under high temperatures. Accidents where high containment (drywell) temperatures occur are more likely to have the containment cap bolts stretch.

The silicon seal of the containment cap can also fail, it degrades significantly at temperatures over  about  600  K (620  ‘F). These two flaws can create an outlet for containment in situations where the pressure and temperatures are both high.

**This is certainly a potential scenario as part of the unit 3 explosion. Considerable steam and debris was ejected high in the air but the containment cap or parts of it have not been found in the debris.

 

Corium (melted fuel):

**This quote talks about the failure in a Mark II containment but could likely happen in a Mark 1 containment.

A  phenomenon  of  importance  for  Mark  II BWRs  is  downcomer  failure.  While  Mark  II designs  vary  significantly,  there  is  often  the potential  for  molten  material  to  flow  across the  floor  and  into  the  downcomers.  This molten  material  may  directly  fail  the downcomer  or,  possibly,  lead  to  a  steam explosion  that  fails  the  downcomer.  Downcomer  failure  does  not  lead  to immediate  containment  failure;  however,  the suppression  pool  is  bypassed,  thus  negating its  heat  removal  and  fission  product scrubbing  capabilities.
** This could potentially happen at a Mark 1 if the corium didn’t melt through the drywell wall at the downcomer vent connection first. The location of the downcomer vent to the floor of the drywell varies at different Mark 1 reactor builds. Some are closer to the drywell floor than others. Those that sit at the junction of the drywell floor would be a candidate for this.
If the melted core is in containment the heat must be removed to prevent temperature and pressure buildup. The heat and pressure can weaken structures and make them more likely to leak or fail at lower temperatures. There are also non-condensable gasses created by the reaction as the corium destroys concrete it comes in contact with and add to the total pressure inside containment.

Base mat melt through happens when the melted fuel is in contact with the concrete for an extended period of time. It can cause hydrogen, non condensible  gases, radioactive  and  nonradioactive  aerosols all in large amounts. During this phase hydrogen explosions can occur if the mixture of hydrogen and oxygen are in the right ratios. Excess amounts of steam can suppress a hydrogen explosion, so adding water cooling at this point can remove the steam creating conditions favorable for a hydrogen explosion. The melted fuel can also burn through the base mat if it is not cooled sufficiently.

Venting can be done through the wetwell (torus) or through the drywell. Wetwell venting is preferred as it can scrub radionuclide releases through the wetwell. If the reactor has hardened venting pipes they are less likely to fail under high containment pressure.

When melted fuel comes into contact with water the following things could happen.
1.  The  water  may  act  to  cool  and  quench (refreeze)  the  molten  core  debris,  and may  limit  the  spread  of  molten  core across  the  containment  floor.
2.  The  debris  may  form  a  molten  pool  under the  water,  probably  with  an  overlying crust  layer,  and  remain  molten.
3.  An  energetic  fuel-coolant  interaction  may occur

Water in contact with corium does not always mean it will cool it. If the fuel forms a debris bed across the surface the water may not reach the area that needs to be cooled. Large amounts of forcefully escaping gasses can prevent water from reaching areas of the corium it needs to cool. This allows the fuel to burn through the concrete or metal where water can’t reach.

 

The biggest threat to containment is a steam explosion.
1.  generation  of  dynamic  pressure  loads (shock  waves)  that  can  fail  the containment  structure
2.  generation  of pressures  and  shock  waves that  can  fail  vessel  support  structures, leading  to  movement  of  the  vessel  and failure  of  containment  piping penetrations
3.  generation  of  energetic  missiles  that  can  vessel  case  will  always  be  at  low  pressure, be  thrown  into  the  containment
4.  generation  of pressures  and  shock  waves that  can  fail  the  drywell  floor  of a  BWR

The magnitude of a steam explosion can be impacted by the following:
1.  the  amount  of  water  available  to participate
2.  the  composition  of  the  melt,  including the  amount  of  unoxidized  metals  that may  react  during  the  explosion
3.  cavity  or  pedestal  region  geometry as  it  may  lead  to  confinement of the  explosion or  focusing  of  shock waves
4.  transmission  of  shock  waves  through  a water  pool
5.  pouring  rate  and  contact  mode,  i.e.,  water on  corium,  corium  on  water,  or  jet ejection  into  water
6.  fraction  of the  core  participating.

3 ways the corium may create a steam explosion are falling into a pool of water, ejecting in a jet spray into a water pool or having water dumped on top.

Rapid quenching (watering) can also cause rapid oxidation of the metals in the fuel creating hydrogen.

Corium in the RPV pedestal can encounter a large amount of water if the  containment sprays have  operated or  large  quantities  of  water have flowed  out  through  a  break  in  the  reactor coolant  system.  Then,  if the  sump  and  floor design  allows,  some  of  this  water  will overflow  into  the  reactor  cavity (pedestal). If the drywell spray system has not been used and water pipes have not spilled down into containment or the pedestal it is less likely to have water buildup below the fuel melt.

BWR  containment  types  are  susceptible to  failure  of  the  vessel  supports,  with relatively  small  amounts  of  water  present.

There is also the  possibility  of  corium  flowing  across the drywell floor and down  the downcomers  into  the  suppression  pool, failing  the  downcomers  with  a  steam explosion  or  as  a  result  of meltthrough. For  the  three  BWR types,  drywell  failures  from steam  explosions  contribute  noticeably  to  the overall  containment  failure  probabilities.
Core Concrete Interactions (CCI)

In addition to melting through the basemat corium can destroy vessel  supports  and other local  structures  that  can  indirectly  lead to  containment  failure.

Corium can  penetrate  concrete  at  the  rate  of  several inches  (tens  of  cm)  per  hour.

The intense heat in corium is sufficient to melt concrete, release gasses and melt the remaining oxides into the corium mass. This can include the metal reinforcing structures like rebar that are in the concrete. All of this addition to the corium makes it harder to cool.

If the corium is stopped before it melts though the basemat is based on these factors.
1.  type  of  concrete  and  aggregate  used  in the  structure
2.  basemat  thickness
3.  cavity  size  and  geometry
4.  melt  mass  in  the  cavity
5.  melt  composition
6.  Presence of overlying  water

The  presence  of  an overlying  water  pool  does  not  guarantee  that the  corium will be cooled.  A  crust  may form  over  the  melt,  and  heat  transfer  may  be insufficient  to  remove  all heat from  the  melt.  A  number  of  experiments have  shown  minimal  effect  of  water  on concrete  ablation  rates. However, overlying  water  can  reduce  fission  product releases  even  if  it  does  not  cool  the  debris.

So all the water at Fukushima may help keep the radiation releases down but may not be doing much to stop the corium from burning through structures. It was also noted that the big steam shows seen at Fukushima after earthquakes could be due to cracks in the crust over the corium due to the shaking.

Concrete loses its structural integrity before it actually melts. The process of damage to melting takes place between 30-1000 celsius. The point where the concrete loses structural integrity varies based on the make up of the concrete and is of particular risk at failing vessel supports in the BRW reactor.

The large amounts of steel in the reactor cavity (pedestal) can add to the melted mass as does the portion of the RPV that may melt with the fuel. The gasses produced due to the core concrete interaction can exceed those produced by the zirconium cladding on the fuel. A few thousand pounds of combustible gas can be created by the core concrete interaction. These gasses can spontaneously ignite if there is not a water pool over the melted fuel. These  gases  are noncondensible,  they  can  lead  to  significant pressure  buildup  that  can  not  be  removed using  sprays  or  suppression  pool  cooling.

In  the  absence  of  an  overlying  water  layer, Core-Concrete  Interactions  produce  dense clouds  of  aerosols. Aerosols can be created in minutes based on the VANESA chart. Aerosols  released  from  CCIs can  significantly  impact  the  concentrations of  radionuclides  in  the  containment atmosphere. In a situation where there are aerosols being created and the containment fails, large amounts of radionuclides can be released into the environment. These aerosols can also plug air filtration systems. If containment failure is delayed long enough the aerosols can settle, releasing less to the environment.

 

Direct Containment Heating:

When vessel  failure  occurs  at  a  pressure  of  a  few hundred  psi  (several  hundred  kPa)  or  more, the  melt  will  be  ejected  as  a  jet  into  the reactor  cavity. The fuel can be fragmented  by  jet breakup  and  swept  out  of the  cavity  into  the containment  where  it  will  heat  the atmosphere  (direct  containment  heating [DCH]). This thermal  energy  can  be  transferred  to  the containment  atmosphere  through  radiative and  convective  heat  transfer.  This  heat transfer  will  be  very  rapid,  with  much  of  it occurring  in  a  matter  of  seconds  if particles remain  airborne  and  continue  to  encounter cool  atmospheric  gases.

This can  lead  to  very rapid  and  efficient  heat  transfer  to  the atmosphere,  possibly  accompanied  by oxidation  reactions  and  hydrogen  burning. It  is  clear  that in  extreme  cases  high  pressure melt  ejection  and  direct  containment  heating can  produce  pressures that  threaten  structural integrity  of  the  containments.

From the RPV molten  material  will  be  ejected  as  a  liquid stream,  as the  liquid corium  level  in  the  vessel  drops,  gas blowthrough  will  begin  to  occur. High velocity  expanding  gas  flow  provides  the motive  force for ejecting  corium  from  the  reactor  cavity (pedestal). Some  experiments indicate  that  the  gases  exiting  the  reactor cavity  can  contain  as  much  as  50%  hydrogen during  some  phases  of the  blowdown. The  fuel melt  is  fully  dispersed  from  the  vessel and  cavity  long  before  (gas) blowdown  is complete. Gases  exiting  the  reactor  cavity may  have  velocities  of  several  hundred  feet per  second. Any trapped corium  may  result  in  subsequent core  debris-concrete  interactions  within  the reactor  cavity.

Vessel  failure  may  occur  at  a  small  opening, such  as  an  instrument  tube,  or  as  a  result  of a  larger  rupture. This can be  important  for  ex-vessel  steam  explosions  or  for  the  gas blowdown  phase.

The  degree  to  which  the  debris can  be  transported  to  the  top  of  the containment  affects  the  resulting  pressure rise. Some  containments  have  a  fairly  open  path around  the  reactor  vessel  to  the  upper containment.  Melt  can  be  dispersed  upwards from  the  cavity  through  the  annulus  around the  RPV  into  the  refueling  canal  and  upper dome.

Melted fuel rich  in  metal  will  tend  to  result  in higher  DCH (Direct Containment Heating). As metals oxidize the combustion of any  hydrogen  produced  by  the oxidization adds to the DCH.

The  presence  of  water  in  the  reactor  cavity could  result  in  some  quenched  debris,  thus partially  mitigating  the  DCH  threat.  But a jet  of  molten  material  entering  a  pool  of water  will  often  lead  to  a steam  explosion.

With small  levels  of water,  the  experiments  show  that  the  initial contact  with  molten  debris  produces  a  steam explosion  that  blows  the  remaining  water  out of the  cavity. High temperature  melt  injection into  model  reactor cavities  filled  completely  with  water  have produced  dramatic  steam  explosions  of sufficient  magnitude  to  threaten  structural damage. Water also  provides  an additional  source  of hydrogen that could lead to rapid hydrogen production.

Though they  have  automatic  depressurization systems,  boiling  water  reactors  could  be susceptible  to  high  pressure  melt  ejection  in some  accidents.  Hydrogen  combustion  in the drywells  of  inerted  boiling  water  reactor containments  will  not  be  especially important,  but  pressurization  as  a  result  of heat  transfer  to  the  drywell  atmosphere might  be  significant  in  these  containments that  are  typically  much  smaller  than pressurized  water  reactor  containments.

In  reactor  accidents  the  conditions  inside containment  prior  to  hydrogen  combustion may  include  elevated  temperature,  elevated pressure,  and  the  presence  of  steam. With 75%  additional  nitrogen,  the  atmosphere is  inert. This  corresponds  to  5%  oxygen  at the  limit  of  the  flammable  region.

Dry hydrogen/air explosions: 

Common sources  of ignition  are  sparks  from  electrical equipment  and  from  the  discharge  of  small static  electric  charges.The  addition  of  a diluent,  such  as  steam,  will  increase  the required  ignition  energy  substantially.

Deflagrations  are  flames  that  generally  travel at  subsonic  speeds  relative  to  the  unburned gas.
Deflagrations  propagate  mainly  by thermal  conduction  from  the  hot  burned  gas into  the  unburned  gas,  raising  its temperature high  enough  for  a  rapid  exothermic  chemical reaction  to  take  place.

For  hydrogen  concentrations between  4.1  and  6.0%,  there  will  be  upward propagation  from  the  ignition  source.  Hydrogen  concentrations  between  6.0  and 9.0%  will  produce  both  upward  and horizontal  propagation,  and  hydrogen concentrations  above  9.0%  will  produce propagation  in  all  directions,  although  the upward  propagation  may  be  faster  than  the downward  propagation.

In  laboratory  experiments that  when  hydrogen:air  mixtures  with hydrogen  concentrations  in  the  range  4-8% were  ignited  with  a  spark,  some  of  the hydrogen  was  not  burned. The  range  of incomplete  combustion  corresponds  to  the range  in  which  the  mixture  is  above  the flammability  limit  for  upward  propagation, but  below  the  flammability  limit  for downward  propagation.

A  detonation  is  a  combustion  wave  that travels  at  supersonic  speeds  relative  to  the unburned  gas  in  front  of  it. For  near stoichiometric  hydrogen  air  mixtures  this speed  is  about  6600  fps  (2000  m/s).

The  transition  from deflagration  to  detonation  and  is  still  not completely  understood  after  more  than  50 years  of  investigation.  We  can  say  that,  in most  postulated  reactor  accident  scenarios, deflagrations  are  much  more  likely  than detonations.

Hydrogen air  mixtures  near  stoichiometric (about  29%  hydrogen,  two  parts  H2 to  one part  02)  are  known  to  be  detonable.  Mixtures  departing  from  stoichiometric, either  in  the  hydrogen-lean  or  hydrogen-rich direction  are  increasingly  more  difficult  to detonate.  It  has  been  observed  that “detonation  limits”  are  functions  of geometry and  scale,  and  not  universal  values  at  given mixture  concentrations,  temperatures  and pressures. A  detonable  mixture  requires  adequate hydrogen  and  oxygen,  but  not  too  much steam.

Detonations  can  also  start  from deflagrations  that  accelerate  to  high  speeds pushing  shock  waves  ahead  of the  burn  front until  at  some  point  shock  heating  is sufficient  to  initiate  the  detonation.  Sources of  such  highly  accelerated  flames  are  high speed jets  coming  from  semi-confined  regions and  flames  passing  through  fields  of obstacles.  Many  obstacles  that  might  potentially  cause flame  acceleration,  such  as  pipes  and pressure  vessels,  are  present  in  the  lower sections  of  most  containments.  Very  fast burns  may  also  occur  due  to  the  presence  of a  very  intense  ignition  source,  such  as  a  jet of  hot  combustion  products  formed subsequent  to  ignition  in  some  adjoining semi-confined  volume.  The  detonability  of  a  mixture  is  increased (cell  size  is  decreased)  with  increasing temperature

Because of  the  containment  geometry,  the  shock waves  may  be  focused  in  local  regions,  such as  the  top  center  of the  containment  dome, giving  rise  to  large  local  peak  pressures  and impulses.

Hydrogen concentrations can happen in these places
1.  near  the  hydrogen  release  point
2.  under  ceilings  or  in  the  dome  due  to  the rise  and  stratification  of  a  low  density plume,  or 3.  near  steam  removal  locations  such  as  ice condensers,  suppression  pools,  and  fan coolers.

Missiles  may  be  generated  when  combustion (deflagration  or  detonation)  occurs  in  a confined  region  or  when  a  propagating combustion  front  produces  dynamic  pressure loads  on equipment.  Such  missiles  may  pose a  threat  to  the  containment  structure  itself, as  well  as  representing  a  potential  threat  to safety  and  control  equipment.

For  the  hydrogen to  burn,  it  is  necessary  that  at  some  location the  hydrogen:air:steam  mixture  be  within flammability  limits. Hydrogen  that  enters  the  containment  may start  to  burn  as  a  turbulent  diffusion  flame.  A  diffusion  flame  is  one  in  which  the burning  rate  is  controlled  by  the  rate  of mixing  of  oxygen  and  fuel.

Combustion  can  begin  either  because  of  an outside  ignition  source,  or  because  the mixture  temperature  is  above  the spontaneous  ignition  temperature. The  spontaneous ignition  temperature  is  in  the  range  of  9591076  °F  (515-580  -C).  Above  this  temperature,  combustion  can  occur  without  external ignition  sources  such  as  electrical  sparks.

The same quantity of hydrogen at TMI could easily cause a detonable mixture in a BWR containment.

In  general,  there  are  very  few  regulations and  guidelines  dealing  with  beyond-design basis  accident  phenomena  in  reactor containments.  For  example,  there  are  no specific  rules  dealing  with  CCIs,  ex-vessel steam  explosions,  or  direct  containment heating.  Hydrogen  control  has  been  an exception  to  this  approach,  significant regulations  were passed  following  the  TMI-2 accident.

After TMI the NRC ordered BWR containments to be inerted with nitrogen. These  containments  are  small enough  that  relatively  low  levels  of zirconium  oxidation  could  produce  detonable mixtures  in  containment. This doesn’t prevent hydrogen issues outside of containment or due to leaks.

The  hydrogen  rule  was  set  up  to address  only  degraded  core  accidents  and  not full  scale  melting  and  vessel  breach. NRC rules assume watering will be resumed and won’t have a large burst of hydrogen so most design basis accidents are not addressed by NRC rules.

Because  of its relatively small enclosed volume and  drywell  floor  area,  the  BWR  Mark  I containment  structural  boundary  is  particularly vulnerable to failure by overpressure or by direct contact attack should molten core and structural debris  leave  the  reactor  vessel.

Numerous analyses of the potential  for early failure of the containment  pressure  boundary  due  to  direct interaction  with  corium  have  been  published. At Peach  Bottom, drywell meltthrough is the most important  mode of containment  failure.

As indicated, there  is  a  single  doorway to  direct any flow from the inpedestal  region toward the opposite  portion  of the  drywell  wall.  The distance  from the point directly underneath the reactor vessel centerline to the closest  point of the drywall wall is only about  7 m (22.85  ft.).
***Unit 1 could be smaller due to it being a very early BWR.

The  carbon steel  sump  cover,  which  occupies  a  central rectangular  section of the pedestal  region floor.  The  existence  of the  sumps  is  a  factor  with respect  to  the  potential  for  debris  spreading since  erosion  of the  thin  sump  cover  would permit  a  significant  fraction  of the  emergent debris  to  be  retained  in the  sumps.  The  sump volume is about 5.7 m3 (200 ft.3) at both Browns Ferry  and Peach Bottom.

Another  pertinent  plant-specific  difference involves  the entrances to the vent pipes, which lead  to  the  pressure  suppression  pool the  lower  lip  of the  vent  pipe  shielded opening is located on the  sloping drywell wall a short distance  (about  0.61  m  [2  ft.])  above the floor.  At Peach Bottom, one of these vent pipes lies  directly  opposite  the  pedestal  doorway.

Quick melt scenario:
The  liquid  zirconium oxide mixture then drains into the containment over  a period of about  five minutes. The same debris composition to be released into the containment over the next  150  minutes.

Slow melt scenario:
Debrisrelocating into the lower plenum is quenched by the water there.  The remaining water is boiled away under the impetus of decay heating and the lower plenum steel  structures are subsumed into the  surrounding  debris.  Penetration  failures occur  at  a  time  when  only  liquid  metals  are present as the debris temperature increases after lower  plenum  dryout.  Oxide  melting  (and release)  follows the initial release  of metals.

For the Mark I liner failure  study, the slow melt scenario probability density functions are such that the initial pour can be approximated  as 14 m3 (494 ft.3) of a mixture  of stainless  steel  with  30% zirconium  metal  at  100  K  (180°F)  superheat.  This initial pour, which  occurs over a period of 20  minutes,  is  followed  by  a  relatively  slow release  of  15  m3 (530  ft.3)  of oxides  (mixed with  15%  zirconium  metal)  over  a period  of 100 minutes.

The material super heat is critical to the containment wall (Debris liquids (metals) have a higher effective conductivity  for  heat  transfer  into  the  wall surface.). Upward radiation from the molten pool held in the  core  region (of the RPV) might  cause melting of the upper  reactor  vessel  internal  structures  so  that the molten liquids, when released into the lower plenum,  would  include  a  large  quantity  of stainless  steel.

The melted debris (and fuel)  falling from  the  reactor  vessel  would  first  fill  the drywell  sumps,  then  would  spill  over  the pedestal  region  floor.  As  sufficient  height  is accumulated  over the  floor, flow would  begin through the pedestal doorway.  Initial spreading as the flow enters the drywell  region would be  slight,  but  after  contact  with  the  drywell wall,  the  flow  would  separate  into  two branches,  each  flowing  along  the  wall  in  a nearly  one-dimensional  fashion.  These  two branches then meet at a position  diametrically opposite to the doorway.  During  the  spreading  process,  the  flowing debris radiates to the overlying atmosphere  (or water)  and  transfers  heat  to  and  ablates  the underlying  concrete.  Gases  released  from the concrete  promote  oxidation  of  the  metals carried  with  the  debris,  and  the  associated energy  release  serves  to  increase  the  debris temperature.

MELTSPREAD  and  CORCON analyses  support  the  idea  that  the  depth  of debris  at the  wall, the initial  superheat,  and the duration  of superheat  employed  for  the  shell failure  analysis  are  appropriately  conservative.  In other words, the values used in considerations of heat  transfer  to  the  shell  are higher than those that would be produced in a best-estimate  analysis.

Structural analysis of the  Mark  I  shell  in localized  contact  with debris, which is of composition  (oxidic) and depth (20 cm  [8 in.]) was assumed to be covered with water. Containment pressure was represented  as  remaining  constant  at 0.2 MPa (29 psia).  Creep rupture was predicted to fail the  shell at  1533  K  (2300  ‘F),  which  is  about 240  K  (430  ‘F)  lower  than  the  carbon  steel melting temperature.  This temperature of 1533 K  (2300  ‘F)  was  then  adopted  by the  Mark  I failure study for use as the best-estimate failure temperature  for the drywell  shell.

If a significant quantity of water overlies the drywell floor at the time of initial debris  release, then all three  of  these  parameters  would  be  affected favorably,  from the standpoint of promoting the survival of the shell. The depth  of debris  adjacent to the wall would be  reduced,  because  more  of the  debris  would freeze within the pedestal region, and less would reach the shell.

The  effectiveness  of overlying water  in  cooling  crusted  debris  is  not  well understood,  insulating  crusts  would  develop only after the superheat  is lost.  Since  heat loss mechanisms to water from a superheated corium melt are  straightforward  and the associated heat transfer is large the  effectiveness  of  water  in  reducing superheat  is not a matter of controversy.

The  portion  of the  shell  that is  in  contact  with water  above  the  debris  acts  as  an  efficient cooling  fin.  A  cooling pathway  is established from the debris into the shell and up through the fin  to the  overlying  water. This accelerates  the elimination  of the debris  superheat.

It  is  not  yet  possible  to  make  definitive resolution  as  to  whether  or  not  it  is  worth risking  the  destructive  potential  of  steam explosions (in the pedestal) in  order  to  reap  the  beneficial aspects  of  water  on  the  drywell  floor.

The study concludes that there is a  “virtual  certainty”  of  shell  failure  if the containment  floor  is  dry  at the  time  of initial release,  but  that  early  shell  failure  is “physically  unreasonable” if the  drywell  is flooded  with water to the  lower lip of the vent  pipe  openings,  a depth  of about  61  cm  (2 ft.)
 
 

 

 

 

 

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