Odd Quirks About Nuclear Reactors - Xenon and Decay Heat

There are a couple of things that all nuclear reactors do that make them behave quite differently than other, more mundane, heat sources.  One is just odd, and the other is a little scary.



The first item, the odd one, is called the "Xenon Precluded Start-up". 

When a Uranium (or Plutonium) atom fissions, or splits, you end up with two much lighter atoms, called 'fission products' or sometimes 'daughter products'.  A fissionable nucleus can split into a myriad of combinations, but some combinations are more likely than others.

The curve below shows the percentage of fission products of U-235 by mass.  In nuclear power school, this was called the Mae West curve :)   Note that the more likely fission products have two peaks at a mass of about 95 and 135. 

This is important, because it turns out that a major neutron absorber (aka "Neutron Poison") is an unstable fission product daughter called Xenon-135.  Xenon-135 is the most powerful neutron absorber known.  There is an index that describes the likelihood that an atom will absorb a neutron.  The number for Uranium-235 is 504.81.  For Xenon-135 this number is 2,000,000. This is a significant removal mechanism for free neutrons in the reactor core.  However the situation is somewhat dynamic.

A reactor that has been shut down for a few days has no Xenon-135 in the core, because Xenon-135  has a fairly short half-life.  However, when a reactor is run up to power, Xenon-135 production begins, and it starts to accumulate in the reactor.

Only 5% of the Xenon-135 is produced directly as a fission product.  The remaining 95% is produced when the fission product Iodine-135 beta decays to Xenon-135.   After ramping the reactor to full power, the control rods have to be adjusted frequently (bumped slightly out of the core) to compensate for the Xenon-135 build-up.  Bumping the control rods exposes additional fuel to neutrons, increasing neutron population to compensate their loss due to the poison build-up.

After about 50 hours at full power, both Iodine-135 and Xenon-135 have built up to equilibrium; Xenon-135 is being created just as fast from I-135 decay as it is being removed by absorbing neutrons.  Neutron absorption is the major removal mechanism for Xenon-135 during reactor operation, because of its extremely large index of neutron absorption.

Here is where the reactor physics gets interesting!  Iodine-135 has a 6.7 hour half-life, meaning that in 6.7 hours, half of the I-135 will decay into Xe-135.  In 6.7 more hours, half of the remaining I-135 will decay to Xe-135, and so on.  The reactor poison Xenon-135 however, has a *longer* 9.7 hour half-life.

Suppose you are operating a reactor that has burned up most of its fuel, and it is almost time for refueling.  You operate this reactor at full power for 50 hours, and in doing so, build up an equilibrium level of Xenon-135, and have a massive inventory of I-135 that is continuously decaying to Xenon-135.

Now for some reason you need to reduce to half power.  The neutron population is now only half of what it was at full power, but the Iodine-135 load in the core is very high.  The Iodine-135 in the core is the 100% power equilibrium, because you were just running at full power.  This Iodine-135 inventory will decay to the neutron poison Xenon-135, only now there is only half the neutron flux in the reactor core to remove that Xenon-135.

What can (and does) happen at this point is that the Iodine-135 decays into Xenon-135 and shuts the reactor down.  There is nothing that can prevent this shutdown, including withdrawing all the control rods.  Remember the reactor's fuel is not fresh, so the control rods are probably mostly removed from the core already...  Also exposing high burn-up fuel to neutrons will not have as much positive effect as it would with a new core.  This transient reactor shutdown condition is called "Xenon Precluded Start-up".

This condition doesn't last forever.  Xenon-135 peaks about 11 hours after power is reduced, as the the core load of I-135 diminishes.  At 11 hours, the removal rate of Xenon-135 from beta decay exceeds the production rate of Xenon-135 due to beta decay of Iodine-135.

The core will eventually run out of Iodine-135, because it is a short-lived fission product, and fission is not happening due to poisoning by Xenon-135.  Eventually Xenon-135 diminishes to the point where the aging reactor core can re-start. 

Below is a curve showing the build-up of Xe-135 in a recently shutdown reactor core.  If the core does not have positive reactivity greater than 0.1 to 0.5∆k/k (depending on power level and time after shutdown) then the reactor will not be able to start again until the Xe-135 decays below the available reactivity in the aging reactor fuel.  The curve labeled "4" would be a very large core loading of I-135 decaying to Xe-135.

Xenon Precluded Start-up is only a significant problem for older cores that have high fuel burn-up, and don't have much available excess reactivity to compensate for a surge of such a powerful poison.

Xenon-135 also played an important role in the accident at Chernobyl (Чорнобиль) #4.  Reactor power had been reduced to 800 MW from 3000 MW to perform a turbine coast-down test.  Xe-135 began building up and further reducing reactor power.  As a result, more and more of the control rods were removed from the core in an attempt to maintain power at 800 MW.  Unfortunately once the test began, the core experienced a rapid positive reactivity insertion.  With most of the rods out of the core, it was not possible to regain control of the reactor again.  The reactor became prompt critical, and a steam explosion and fire followed, releasing millions of Curies of radioactive contaminants.

The other quirk that nuclear reactors have is decay heat.

Decay heat is what made Three Mile Island and Fukushima melt down and hit the front page of the news, although each accident occurred in different ways.

When a reactor trips, or scrams, the fission process pretty much stops immediately.  Neutron population (and fission) dwindles to a millionth of full power over a few minutes, and below the point where heat could be produced in a second or two.

However, in the core of a reactor that has been run at full power, there are a huge number of fission products that are highly unstable, having just been split.  They are emitting copious amounts of alpha, beta, gamma, and neutron radiation.  Decay heat is the result of all these types of radiation interacting with matter and causing molecular physical movement of atoms (friction), and expressed as heat.

Below is a pellet of Pu-238, glowing red-hot due to decay heat.  Pu-238 has an extremely high rate of alpha decay. 



Below are estimates of decay heat in a reactor over time, following a trip from full power. "Retran" and "Todreas" are two different analytical models used to create the graph.

Using this chart, it's easy to understand how decay heat can cause fuel to melt.  The fuel must be kept submerged or decay heat will cause the fuel to get hot enough to melt the fuel assemblies.  The cladding, being metal, melts first.  This is followed by the ceramic fuel itself.

Let's look at a typical large Nuclear Power Station.  The electrical output will be perhaps 1300 MW.  Because the efficiency of a nuclear power plant is only about 33%, the reactor has to generate about 3900MW of heat to get that 1300 MW of electricity.

Now look at the chart.  Immediately after shutdown, the reactor is generating 270 MW of decay heat.  Four hours after shutdown, our 3900 MW(thermal) reactor core is still generating 39 MW of decay heat!!  Ten days after shutdown, the decay heat is about 0.3% of full output or 12 Megawatts of heat.  That is still plenty of heat to melt the core if it were not removed.

This fuel dryout condition is what happened at both Three Mile Island and at Fukushima Daichi.  At Three Mile Island, there was a steam turbine trip, which was followed by a reactor trip.  The primary coolant pressure surged due to the sudden temperature transient.  A relief valve opened to release this excess pressure.  Unfortunately the relief valve did not close again when pressure returned to normal. 

The operators did not recognize that they were losing coolant due to a huge number of unusual alarms.  A steam bubble formed in the reactor vessel, and forced primary coolant into the pressurizer vessel.  They had was no understanding of why the pressurizer suddenly filled with water, and their desire was to return the pressurizer level to normal.  They were justifiably concerned about "taking the plant solid", where the pressurizer has no steam bubble to absorb pressure surges.

The emergency reactor water make-up pumps were turned off in an effort lower the water level in the pressurizer to normal.  Because emergency cooling water was shut off, the reactor core remained uncovered, and melted down due to decay heat building up in a dry core.  Sadly, had the operators not stopped the emergency water fill pumps which had started automatically, the core would very likely have remained undamaged.

With Fukushima, there was a total loss of power following an earthquake-generated tsunami.  The earthquake knocked out offsite power, and the tsunami flooded the electrical switchgear room, which was located in the basement.  This scenario occurred in three of the four reactors at the site.  Additionally, spent fuel pools that required constant circulation for cooling lost power, and those pools began steaming off.

With a complete loss of power, water could not be circulated through the core to remove heat.  Operators had to vent steam to prevent an overpressure condition, which threatened to blow the reactor pressure vessels apart.  Eventually each of three reactors ran out of water, and the dry cores melted down due to decay heat build-up.  Although it is not certain at this time, it is very likely that the molten fuel melted the bottom of the reactor vessels.  This would allow hydrogen to escape into the atmosphere and produce the impressive explosions that were so widely televised.  Because the primary containment systems were compromised, large amounts of radioactivity were released into the environment, particularly the ocean.

Eventually, out of desperation, seawater was pumped into the primary coolant loops.  It is likely that following the earthquake, no possible human action could have prevented the meltdown.  The errors made in this case were plant siting and electrical design, rather than operator error.