Presentation in Three Parts:
Nuclear Reactors, Three Mile Island, The Accident
Presented by David Y. Fishfeld
Nuclear Power Plants are always plagued with start up problems because
the system is very complex and the technology is new. It is really a hybrid
of two systems, one –the reactor, complex and technologically new to the
world, and the second – a conventional, older system for drawing away heat
to turn turbines.
The fission of atoms generates extreme heat that is used to boil water
into steam to drive turbines which generate electricity. The reaction is
in essence a controlled nuclear bomb. Fission occurs when an atom breaks
down to smaller atoms and particles. The control rods, which absorb neutrons,
control the rate of fission and the temperature is controlled by a cooling
system, which removes heat from the reactor, without this the reactor will
overheat, causing more fission to occur, and thus release more heat, etc.
The accident began in the secondary cooling system. There are two cooling
systems, as can be seen in the above picture, the primary cooling system
contains water under high pressure and temperature that circulates through
the core where the nuclear reaction is taking place. This hot water goes
to a steam generator and heats water in a secondary system, see the simpler
diagram on the cover. This water is kept at high pressure, so it remains
as water, until steam is needed. The steam drives the turbines to generate
electric power. This water is not radioactive, the water of the primary
system is. To drive the precision turbines the water must be pure. Resins
get into the water and are removed by a condensate polisher system; this
removes particles that are precipitated out. This polisher system may have
leaked and began the cascading effects that led to the accident.
Since there was no cooling system cooling the core, the reactor “scrammed” which means the graphite control rods which control the reaction are dropped into the core to absorb the neutrons and stop the fission process. The decaying radioactive heat can still produce enough electricity to power 18,000 homes. This heat builds up high temperature and pressure that would normally have been cooled by the cooling system. The ASDs to handle this were as follows: A pilot operated relief valve (Porv), relieves the pressure from the core by channeling the water from the core through a pressurizer and then through its top into a drain pipe into a sump. After the core’s pressure was sufficiently relieved the valve should have closed, it didn’t. This created a hole in the core system, the water from the Primary cooling system was pouring out. This valve had been faulty on previous occasions so an indicator for it was installed on the control board. This indicator was broken.
With all that water leaving, pressure dropped in the coolant system. this caused another ASD to turn on, the HPI, High Pressure Injection, which injects water at a high pressure into the core in order to cool it down, since the secondary system wasn’t cooling the water from the Primary system and hence the core. There are dangers to doing this. One is that you can ‘shock’ the core, and cause cracks in it and its containment vessel. Another, is that the all this water will flood the pressurizer. This is a vessel that has water on the bottom and steam on the top and is used to control the pressure in the core by heating the water at the bottom. If it is flooding there would be no steam, only water, this is called ‘going solid’, pressure would surge in the core and coolant pipes can burst leading to a loss of coolant accident, a LOCA, possibly causing a meltdown. So the operators new that going solid was bad and were warned by both the vendor and user of the system, so they cut down on the HPI. They didn’t realize they already had a Loss Of Coolant Accident because of the stuck open valve. There were two indicators which measured pressure in the reactor, one in the core and one in the pressuriser, however one said pressure was low and the one in the pressurizer said it was very high. They chose to believe the one that said pressure was high, since the HPI was on and the coolant pumps were on. They didn’t know about the two closed valves and the PORV that was open, still emptying water from the system. It took a few hours before the computer printed its message that there may be a problem with the PORV. Eventually the stuck valve was discovered, and closed. Much damage had already been done with the partially uncovered core, another half hour would have meant a complete core melt down. (Later it was found that up to 50% of the core melted. Source: Pbs web page)
The fuel rods are comprised of enriched uranium in pills stacked in a thin liner. Water circulates through this so that it would not melt. With temperature so high, it reacts with the water producing hydrogen. This creates pockets of hydrogen, which with oxygen and a spark can lead to an explosion. There was a jump in an indicator that told the pressure of the containment building. The pressure reached half the tolerance level for the building. This meant that a there was probably a hydrogen explosion. The hydrogen bubble is also dangerous without exploding, since it can prevent the flow needed for cooling.
Eventually the open Porv was discovered and closed, and a complete meltdown
was averted. The complexity of the system made it impossible to follow
in real time what was happening. Perhaps the tolerance level for many components
was too low. If there was no leak in the condensate polisher system then
the accident would have been averted –however something else would have
come up that the closed valves on the emergency feedwater pumps may have
come into play. Perhaps proper maintenance would have averted the disaster.
The underlying fact is that there were just so many valves and pipes and
interacting systems that it was impossible to cover all those possibilities
and examine each one. The more complex any device is, the more apt it is
to break down.