1. Probability To determine the likelyhood that such an accident would occur something called an event tree is constructed. This determines the consequences of a particular event occurring Each component (pump, valves etc) has a failure probability assigned to it Bottom line-most recent studies indicate that for all 104 reactors operating the US, over their 30 year operating lifetime, there is a 1% probability of a large release of radioactivity

2. Core meltdown

3. Nuclear accidents 4 (that we know of…….) Enrico Fermi reactor in Illinois Three Mile Island-PA-Worst US accident Chernobyl-Ukraine-Worst accident ever Fukushima Nuclear plant accident in Japan – May surpass Chernobyl

4. Enrico Fermi Reactor Enrico Fermi demonstration nuclear breeder reactor, in Monroe, MI Two fuel rod assemblies reached 700F(normal operating temp is about 580 F). Reactor was shut down, and it appeared fuel was melting. It was determined a piece of zirconium from the meltdown cone (a structure designed to direct the flow of fuels should a meltdown occur) came loose and was blocking the coolant nozzles. The fuel in a breeder reactor is highly enriched, thus in molten form it could “reassemble” into critical assembly that could result in a runaway fission reaction which would generate enough heat to melt the material into the Earth. Some engineer remarked it could go all the way to China, and the phrase China Syndrome was born. Publically used by a Nuclear Physicist Richard Lapp in 1971 No one was injured and no radiation was released.

5. Three Mile Island Perhaps the most famous nuclear accident in the US On March 16, 1979, the movie China Syndrome, based on the effect described in the last slide, was released. 12 days later, March 28, 1979, the worst civilian nuclear accident in the US occurred at the Three Mile Island Nuclear Power Plant on the Susquehanna River, south of Harrisburg, PA occurred.

6. Location

7. The accident Partial core meltdown as the result of a LOCA Main feedwater pumps failed, triggered a controlled shutdown (scram). But the decay heat (heat generated by the decay of radioactive material in the fuel) continued, with nothing to remove it Auxiliary systems could not pump water, as their valves had been closed for maintenance (which was a violation of NRC regulations) Pressure built up, which was released by a PORV valve (Pilot-operated relief valve) which opened automatically, but failed to close. This allowed coolant water to escape.

8. The accident Plant operators had a control light that only indicated if power was applied to the valve, not if it were open or closed. The light went out when the power was cut, the operators did not know this did not mean the valve was closed. Bad design. However, there were other instruments that told the operators something was wrong, in fact that suggested the valve was still open, but the light was out! Bad operators. As pressure was lost, some of the coolant turned to steam and formed steam pockets (remember the high pressure is used to keep the coolant liquid at high T). This caused the coolant level in the pressurizer to look higher than it was, and the operators turned off the emergency core pumps which came on after the initial pressure loss. The tank that collected the discharge from the PORV overfilled and the sump pump in the containment building filled and sounded an alarm. This, plus abnormally high PORV T and higher than normal containment building Temperature and Pressure readings were ignored by the operators. A failure in the quench tank caused radioactive coolant to be pumped into a building outside the containment building.

9. Accident con’t Steam bubbles in the cooling pumps caused them to cavitate and need to be shut down, with the operators believing the coolant would circulate naturally. It did not (they did not know there were steam cavities that blocked the water flow). The top of the reactor became exposed and the steam reacted with the zirconium cladding on the fuel rods and damaged the fuel pellets, releasing more radioactivity into the coolant water. Plant had become seriously contaminated, but it wasn’t until 165 minutes after it all began that contaminated water reached radioactivity detectors and the alarms went off. At this point a new shift of operators came on, who noticed a problem and shut off the coolant venting via the faulty PORV valve. Several hours later, new water was pumped into the primary cooling loop, and a backup valve was opened to relieve the pressure so the loop would fill up. Around 2pm an explosion rocked the containment building. This explosion was the result of H released when the zirconium cladding was burned off of the fuel rods. 16 hours after the start, the primary coolant loop was operating the core T began to fall.

10. Another illustration of the accident site

11. Effects of Three Mile Island Amount of radiation released is debated, the containment building held. Official figures indicate a small amount of radioactivity was released. Independent measures claim radiation of 3-5 times higher than normal were detected hundreds of miles downwind of the plant. Long term health effects on residents are hotly debated, pick your favorite interpretation. The valve had failed in the open position 9 previous times, and 2 other times in the closed position. It had also previously failed at another plant, but those operators diagnosed the problem in 2 minutes in a plant only operating at 9% ( as opposed to the 97%) output at Three Mile Island. The valve company never notified its customers of the previous failure. Often blamed for the demise of nuclear power in the US – Probably an overstatement, but it certainly soured public opinion

12. Chernobyl Accident at the Chernobyl Nuclear Power plant in the Ukraine in 1986 At the time it was part of the Soviet Union Worst Nuclear power plant accident in history 2 died in initial steam explosion Deaths from radiation exposure cannot be counted, Soviet Union covered up the numbers Best estimates are 56 direct deaths and 4000 additional cancer related deaths (2005 report of the Chernobyl Forum)

14. Chernobyl Plant experienced power excursion (chain reaction went out of control) Resulted in a steam explosion and a secondary hydrogen explosion which tore the top off of the reactor and its building and exposed the core. NO containment building! Released large amount of radioactive particles into the air

15. Chernobyl Began with a test of a backup cooling system In the event of an external power failure, the reactor would shut down, but there would be no power to run the plant cooling pumps. Backup diesel generators took 1 minute to reach full capacity, This one minute cooling gap was not acceptable It was proposed to use the rotational energy of the turbine as it was spinning down to generate electricity in this gap. Since the turbine was spinning down, a voltage regulator was needed to provide stable power to the to the cooling pumps The test was to take place as the fuel rods were to be replaced…the worst possible time as the decay heat and radioactive nuclei present would be at its maximum at the end of a fuel cycle. Test had already failed once Plan was to run the reactor at low power, but the turbine at full speed. The steam supply would be cut off and the turbines would be allowed to spin down, and see if the voltage were regulated.

16. Chernobyl Test was delayed many hours by an unexpected shut down of another power station. This resulted in an untrained night shift taking over the experiment When power was reduced, the control rods were inserted too far, resulting in the an almost complete rector shutdown Resulted in xenon poisoning, where high levels of Xenon 135 absorb neutrons an inhibit the fission process. Operators saw the power drop too low, but were not aware of the Xenon poisoning, assuming instead a power regulator failed To compensate, they pulled the control rods out of the reactor core, beyond the limits of safe operation. This would have had to be done via manual overrides. Extra water was pumped into the core to cool it and reduce steam voids, but it exceeded safe water level limits. Water acts as a moderator, so it further reduced the power output. So the control rods were pulled all the way out. Reactor was set up for a runaway reaction, but the extra water and xenon were acting as a moderator. Excess steam and other changes in nominal operation were occurring and the automatic control system should have shut the reactor down, but the operators had disabled this system.

17. Chernobyl Operators were not aware of the unstable condition, and proceeded to shut off the steam to the turbines As they spun down, the water flow decreased and steam voids formed Control rods were not completely removed, they blocked the heat from reaching the cooling water. A massive steam build up occurred, an the reactor power and neutron generation increased overcoming the xenon poisoning. A runaway situation was in progress

18. Chernobyl A SCRAM was ordered. But, the insertion of the control rods displaced coolant (design flaw), increasing the reaction rate. Core overheated, fracturing fuel rods and blocking further control rod insertion Cooling pipes ruptured, and fuel rods melted Steam explosion occurs, which rips the 2000 ton lid off of the reactor 2-3 seconds later a second hydrogen explosion occurred, either from the reaction of the steam with the zirconium fuel rods or by the reaction of hot graphite and steam

19. Chernobyl Hot debris started fires on the roofs of other reactors Steam and smoke were highly radioactive No public notice was made until radiation alarms at a nuclear plant in Sweden went off! The cloud spread over Russia, Belarus, Ukraine and Moldova, but also Turkish Thrace, the Southern coast of the Black Sea, Macedonia, Serbia, Croatia, Bosnia-Herzegovina, Bulgaria, Greece, Romania, Lithuania, Estonia, Latvia, Finland, Denmark, Norway, Sweden, Austria, Hungary, the Czech Republic and the Slovak Republic, The Netherlands, Belgium, Slovenia, Poland, Switzerland, Germany, Luxembourg, Italy, Ireland, France (including Corsica) the United Kingdom and the Isle of Man. Reactor was contained in a concrete sarcophagus which has 200 tons of highly radioactive material inside Entire plant shut down in 2000

20. Effect Short term effects on rivers and groundwater 4 square KM of pine forest in the vicinity of the reactor died Some animals died or stopped reproducing Since the abandonment by humans, many wildlife species have returned to the area, with reports of higher incidences of deformities etc compared to non contaminated areas A black melanin rich fungi is growing on the reactors walls It is difficult to assess the human impact

21. Extent of the radioactive cloud