- Last Updated: 29 January 2016 29 January 2016
In Pressurized Water Reactors (PWRs), such as the Babcock & Wilcox system at TMI, the reactor core itself is housed within a massive steel bottle known as the Pressure Vessel. Three coolant circuits, or loops,are used in moving heat from the reactor core to the turbines which drive the generators. Each loop uses water, in one phase or another, as its coolant.
The primary or RC (Reactor Coolant) loop begins in the reactor vessel. Water passing among the fuel assemblies in the core is heated to several hundred degrees Fahrenheit, under a pressure of over two thousand pounds per square inch. This is the ultimate pressure cooker; the high pressure is necessary to prevent the water from boiling into steam, which wouldn't adequately cool the fuel elements.
Hot water leaves the pressure vessel through the outlet nozzle, along a pipe known as a hot leg, and enters the steam generator. This is a heat exchanger, basically. In the diagram at right, you can see that the primary water passes through tubes inside the steam generator, in the process giving up most of its heat to the cooler secondary water flowing around the tubes. The secondary water, under far lower pressure, flashes quickly into steam.
An important feature is that the primary and secondary water never mix, or even touch. Primary coolant stays inside the tubes, and secondary water stays outside. This keeps any radioactive contaminants and activation in the primary water, and out of the atmosphere.
In reality, the internals of a steam generator are much more complex than this. Babcock and Wilcox, designers of TMI's reactor system, use a peculiar type of steam generator called an OTSG, or Once-Through Steam Generator. Highly efficient and reliable, the OTSG has one drawback. It holds less secondary water than other, more conservative designs. This means that should feedwater (the flow of secondary water) fail for any reason, the OTSG will boil dry very, very quickly. This made TMI's emergency feedwater systems very important.
Once the primary water has expended its heat in the steam generator, it's sent back to the reactor vessel, by way of the cold leg, for another trip. Primary coolant is constantly recirculated. Mammoth main coolant pumps, or RC pumps keep the water moving through the core. Each pump at TMI was two stories high, and delivered nine thousand horsepower. Each pump was capable of circulating 360,000 gallons of coolant, at rated pressure, per minute!
Attached to the primary coolant loop is a tall, slim tank called the pressurizer. The pressurizer is normally about half full of water, and half full of steam. A tube at the bottom connects the tank to the reactor vessel, and at the top are relief valves which open to protect the system when too much pressure builds up. By using powerful electric heaters and water sprays, the operators can, to a limited extent, control pressure within the primary system by controlling the size of the bubble. The steam bubble, the only steam ever allowed in the primary system, serves also to cushion the system against shocks, and prevents pipe ruptures from sudden pressure spikes.
The relief valves at the top of the pressurizer bear some further description, given their pivotal role in the accident. The main, large-diameter relief valve was known as a PORV, or Power Operated Relief Valve. The one used at TMI was an Electromatic Relief Valve manufactured by Dresser Industries. It's shown in orange on the diagrams.
The Electromatic valve was known to be somewhat unreliable. The valve was carefully designed by Dresser to never, ever fail to open, under any conditions. Unfortunately, there were quite a few conditions that might cause it to fail to close. In fact, a lesser-known, far less disastrous loss of coolant accident occurred at Davis-Besse Nuclear Power Station some years before TMI, due to this same valve's failure to close. So, at TMI, a manually operated block valve was installed, so that in the event that the relief valve failed to close, the outlet could still be blocked, and a disastrous loss of coolant could be prevented. The block valve is shown in violet in the diagrams.
The nuclear power industry prides itself on allowing for every "credible failure" that a plant might endure. In fact, the loss of great quantities of primary coolant, through a stuck valve, pipe break, or structural failure had been, everyone thought, well predicted and allowed for.
Two emergency core cooling systems served TMI in addition to the primary loop's heat removal capacity. The HPI (High Pressure Injection) system consisted of three pumps, two electric and one steam-driven. These pumps were designed to shove water into the core by brute force in huge quantities during a loss of coolant accident.
A depressurized system could also be cooled by the LPI (Low Pressure Injection) or makeup system, which is similarly equipped. Finally, if all else were to fail, a huge tank holding nearly a million gallons of borated water could be dumped, via gravity, into the core by the core flood system.
For safety's sake, all components of the primary cooling loop are located a huge concrete structure known as the containment dome. The walls of the containment are 12 feet thick, and reinforced with high- strength steel. It is designed to withstand the direct impact of a jet airliner, and can withstand incredible pressures from within as well.