The Three Mile Island

Ta’Juan DutrieuilleNovember 4, 2009

Period 1

Reactors

• The Three Mile Island had an incident that started with both of the reactors; the TMI-2 reactor was running while the TMI-1 reactor was shut down for refueling. As a result of the mechanical or electrical failure, the pumps in the condensate polishing system and the main feedwater system stopped running. But the reactor continued to generate decay heat which wasn’t a good thing since the steam generators no longer removed that heat from the reactor.

TMI-2 Plant

Feedwater Pump System

• Once that had happened the primary feedwater pump system failed. That caused the valves to be shut down for routine maintenance but that was breaking a key NRC rule.

Pilot-Operated Relief Valve(PORV)

• Due to the loss of heat removal and the failure of auxiliary system to activate, it triggered the pilot-operated relief valve to automatically open when it should have of been closed. The open valve permitted coolant water to escape from the primary system, and this was the mechanical cause of the crisis that followed.

Result of a Stuck Valve

• Because of the stuck valve the pressure in the primary system continued to decrease, steam pockets began to form in the reactant cooler, causing the pressurizer water level to rise even though coolant was still being lost through the PORV.

False Readings

• Because the operators judged the level of water in the core solely by the level in the pressurizer, since the pressurizer was high they assumed that the core was properly covered with coolant, unaware that because of the voids forming in the reactor vessel. The indicator provided false readings.

Cause of Temperature Rise

• After slow temperature rise the primary loop pumps begin to cavitate as steam, rather than water. Steam in the system locked the primary loop. In so much time the top of the reactor core was exposed and the intense heat caused a reaction to occur between the steam forming in the reactor core and the zirconium nuclear fuel rod cladding.

TMI-2 Core End-State Configuration

Heat

• The fiery reaction burned off the nuclear fuel rod cladding, the hot plume of reacting steam and zirconium damaged the fuel pellets. So the damaged fuel pellets released more radioactivity to the reactor coolant and produced hydrogen gas, which caused a small explosion in the containment building.

The Plant Contaminated

• They tried to use a block valve to shut off the coolant venting via the PORV but by that time they were too late. So the plant was contaminated by radiation.

Harmless Radiation

• A maximum of 480 petabecquerels (13 million curies) of radioactive noble gases were released by the event, but they were relatively harmless.

Harmful Radiation

• About 481 to 629 GBq (13 to 17 million curies) of thyroid cancer-causing iodine-131 were released.

Total Radiation

• An estimated 37 EBq (10 billion curies) in the reactor.

Three Mile Island Is Still Here Today

Work Cited

• http://en.wikipedia.org/wiki/Three_Mile_Island_accident

• http://www.nukeworker.com/pictures/thumbnails-92.html