Chernobyl

1. 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)

3. 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

4. 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.

5. 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.

6. 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

7. 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

8. 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

9. 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

10. Extent of the radioactive cloud

11. Fukushima Nuclear plant accident • 11 March 2011 –9.0 magnitude undersea Earthquake and Tsunami hit northern Japan • Plant contains 6 boiling water reactors, 3 of which were offline at the time of the earthquake(4,5,6). • Reactors 1,2,3 were shut down after the earthquake, but the tsunami flooding caused electrical failure which knocked out the emergency pumps needed to cool and control water flowing into the reactors. • Initial concern is over the damaged storage pool for spent fuel from reactor 2 • Pressure is seen to rise in Unit 1, as cooling system fails

13. Fukashima Timeline • March 12 • Battery power for reactor 3 cooling system runs out and its fuel rods are exposed. • Unit 1 and 2 vent radioactive steam to relive the pressure building up • H explosion occurs in Unit 1 • March 14 • H explosion at Unit 3 • Unit 2 fuel rods are fully exposed to air, and seawater is pumped in to cool them

14. Fukushima Timeline • March 15 • Fire breaks out in Unit 4 • Unit 2 experiences another explosion. Fuel rods may be melting • Explosion occurs at Unit 4 • Fire in the spent fuel pond at Unit 4 is releasing radioactivity into the air and water may be boiling off in the pond • Unit 5 water level is falling • Unit 3 spent fuel pond has decreasing water levels

15. Fukushima Timeline • March 17 • Radiation levels are rising in cities within 75KM of the plant • March 19 • Power becomes available to Unit 5 and 6.Cooling of Unit 5 spent fuel pond begins. • Milk and spinach in areas near the power station are found to have increased levels of radioactivity. • Unit 6 spent fuel pond begins to be cooled • March 21 • Full cooling to reactor 5 is resumed

16. Fukushima Timeline • March 22 • Seawater near the plant shows elevated levels of radioactivity • March 23 • Power restored to Unit 3 • March 24 • 3 workers exposed to radiation in Unit 3 turbine building. 2 received radiation burns after standing in contaminated water for2 hours without protective boots.

17. Fukushima Timeline • March 25 • Possible breaches in the containment buildings of Units 1,2,3 reported. • March 26 • Radiation levels of 10 million times normal in the Unit 2 turbine room were reported, but later found to be inaccurate. It was only 100,000 times higher than normal. • Damage is now known to be: • Roof of reactor 2 damaged but intact • Reactor 3 roof is blown away • Walls of reactor 4 building have collapsed • Steam is emanating from reactors 2,3,4

18. Fukushima Timeline • March 28 • Radioactive water found in tunnels that house electrical wires form the turbines, and within meters over overflowing which would send this water into the sea. • March 29 • Radiation reported to have reached the UK and US, though not at unsafe levels.

19. 3 minutes after second explosion at Unit 3

23. Other accidents-1980s • March 13, 1980 –- Orléans, France - Nuclear materials leak • A brief power excursion in Reactor A2 led to a rupture of fuel bundles and a minor release (8 x 1010 Bq) of nuclear materials at the Saint-Laurent Nuclear Power Plant. The reactor was repaired and continued operation until its decommissioning in 1992. • March, 1981 - Tsuruga, Japan – Overexposure of workers • Overexposure of workers More than 100 workers were exposed to doses of up to 155 millirem per day radiation during repairs of a nuclear power plant, violating the company's limit of 100 millirems (1 mSv) per day. • September 23, 1983 Buenos Aires, Argentina - Accidental criticality • An operator error during a fuel plate reconfiguration in an experimental test reactor led to an excursion of 3×1017 fissions at the RA-2 facility. The operator absorbed 2000 rad (20 Gy) of gamma and 1700 rad (17 Gy) of neutron radiation which killed him two days later. Another 17 people outside of the reactor room absorbed doses ranging from 35 rad (0.35 Gy) to less than 1 rad (0.01 Gy). • April 26, 1986 — Prypiat, Ukraine (then USSR) - Power excursion, explosion, complete meltdown • A mishandled reactor safety test led to an uncontrolled power excursion, causing a severe steam explosion, meltdown and release of radioactive material at the Chernobyl nuclear power plant located approximately 100 kilometers north-northwest of Kiev. • May 4, 1986 – Hamm-Uentrop, Germany (then West Germany) - Fuel damage • A spherical fuel pebble became lodged in the pipe used to deliver fuel elements to the reactor at an experimental 300-megawatt THTR-300 HTGR. Attempts by an operator to dislodge the fuel pebble damaged its cladding, releasing radiation detectable up to two kilometers from the reactor. • November 24, 1989 — Greifswald, Germany (then East Germany) - Fuel damaged • Operators disabled three of six cooling pumps to test emergency shutoffs. Instead of the expected automatic shutdown a fourth pump failed causing excessive heating which damaged ten fuel rods. The accident was attributed to sticky relay contacts.

24. Other Accidents 1990s • April 6, 1993 — Tomsk, Russia – Explosion • A pressure buildup led to an explosive mechanical failure in a 34 cubic meter stainless steel reaction vessel buried in a concrete bunker under building 201 of the radiochemical works at the Tomsk-7 Siberian Chemical Enterprise plutonium reprocessing facility. The vessel contained a mixture of concentrated nitric acid, uranium (8757 kg), plutonium (449 g) along with a mixture of radioactive and organic waste from a prior extraction cycle. The explosion dislodged the concrete lid of the bunker and blew a large hole in the roof of the building, releasing approximately 6 GBq of Pu 239 and 30 TBq of various other radionuclides into the environment. The contamination plume extended 28 km NE of building 201, 20 km beyond the facility property. The small village of Georgievka (pop. 200) was at the end of the fallout plume, but no fatalities, illnesses or injuries were reported. The accident exposed 160 on-site workers and almost two thousand cleanup workers to total doses of up to 50 mSv (the threshold limit for radiation workers is 100 mSv per 5 years) • June, 1999 - Ishikawa Prefecture, Japan - Control rod malfunction • Operators attempting to insert one control rod during an inspection neglected procedure and instead withdrew three causing a 15 minute uncontrolled sustained reaction at the number 1 reactor of Shika Nuclear Power Plant. The Hokuriku Electric Company who owned the reactor did not report this incident and falsified records, covering it up until March, 2007. • September 30, 1999 — INES Level 4 - Ibaraki Prefecture, Japan - Accidental criticality • Workers put uranyl nitrate solution containing about 16.6 kg of uranium, which exceeded the critical mass, into a precipitation tank at a uranium reprocessing facility in Tokai-mura northeast of Tokyo, Japan. The tank was not designed to dissolve this type of solution and was not configured to prevent eventual criticality. Three workers were exposed to (neutron) radiation doses in excess of allowable limits. Two of these workers died. 116 other workers received lesser doses of 1 mSv or greater though not in excess of the allowable limit..

25. Other accidents -2000s • April 10, 2003 - Paks, Hungary - Fuel damaged • Partially spent fuel rods undergoing cleaning in a tank of heavy water ruptured and spilled fuel pellets at Paks Nuclear Power Plant. It is suspected that inadequate cooling of the rods during the cleaning process combined with a sudden influx of cold water thermally shocked fuel rods causing them to split. Boric acid was added to the tank to prevent the loose fuel pellets from achieving criticality. Ammonia and hydrazine were also added to absorb iodine-131. • April 19, 2005 — Sellafield, England, United Kingdom - Nuclear material leak • Twenty metric tons of uranium and 160 kilograms of plutonium dissolved in 83,000 literes of nitric acid leaked over several months from a cracked pipe into a stainless steel sump chamber at the Thorp nuclear fuel reprocessing plant. The partially processed spent fuel was drained into holding tanks outside the plant. • November 2005 — Braidwood, Illinois, United States - Nuclear material leak • Tritium contamination of groundwater was discovered at Exelon's Braidwood station. Groundwater off site remains within safe drinking standards though the NRC is requiring the plant to correct any problems related to the release. • March 6, 2006 — Erwin, Tennessee, United States - Nuclear material leak • Thirty-five liters of a highly enriched uranium solution leaked during transfer into a lab at Nuclear Fuel Services Erwin Plant. The incident caused a seven-month shutdown and a required public hearing on the licensing of the plant.

26. Fusion • Powers the sun • In fusion, we combine two atoms and release energy • Easiest to do this with H or its isotopes • We already talked about the proton-proton chain in the sun • D-T reaction-takes deuterium and tritium and creates He

27. Fusion • In order for these reactions to occur, one needs the deuterium and tritium at high temperatures • For the DT reaction, T = 40 x 106 K • For DD reaction, T = 100 x 106 K • DT is good for bombs, not so good for long term power, and tritium has a half life of 12 years

28. Thermonuclear devices • Also called H bombs, these are fusion bombs • Require fission to compress and heat the fusion fuel. • The fusion releases enormous amounts of high speed neutrons which are then often used to induce fission in matter that it is normally difficult to induce fission in (such as depleted Uranium, Uranium composed mostly of 238U). • This adds to the radioactive fallout of the bomb

29. Fusion reactors • Main problem is maintaining the fusion material at high enough T so that fusion produces the bulk of the energy (break even) • Confined plasma via a magnetic field so it does not contact the container walls (which would cool it and quench the reaction) • Not nearly as hazardous as fission reactors, most studies show severe failures to be contained within the plants themselves.

30. Barriers to fusion power • Scientific feasibility of the reactions • Economics • Damage to reactor components due to large flux of high energy neutrons • Availability of materials to build the reactors • Maybe in 50 years -but we have been saying that for 50 years!