5Boiling Water Reactor Basic Flow Use this slide to show the general Rx design, flow of water, steam, heat. And show where the radioactivity is.Starting with feedwater pumped into reactorReactor provides heatFeedwater converted to steam and directed to steam turbinesTurbines spin and connected to electrical generatorSteam is directed downward to condenser, where it is condensed back into waterA separate loop of water (e.g., ocean water) cools the condenser and is discharged back to oceanCondensed water is heated and pumped back into reactor as feedwater.
6Boiling Water Reactor Design Reactor vessel is within the primary containment vessel (drywell). Reactor vessel is thick steel, and the drywell is a steel shell surrounded by reinforced concrete.The secondary containment structure has the function to maintain the components out of the weather, but it is not a robust containment structure.The wetwell, also called the torus or suppression pool, has a purpose to take the steam being created after shutdown and condense it.Describe a normal operating cycle and refueling: plant operates for months, then is shut down for refueling. The top of the reactor and the containment are removed and the fuel (some of the fuel) is moved from the reactor vessel into the spent fuel pool.Note that the spent fuel pool is located in the secondary containment, and contains spent fuel from previous refuelings under water.en.wikipedia.org/wiki/Browns_Ferry_Nuclear_Power_Plant
7Nuclear Reactor Fuel Design Fuel is important because it contains almost all of the radioactivity. The safety systems are designed to maintain cooling to the fuel – if the fuel is maintained cool, you cannot release large quantities of radioactivity.Describe the fuel as about feet long and less than a foot square. There is a large surface area to improve heat transfer.The fuel itself is uranium oxide and is slightly radioactive before it is put into the reactor and becomes extremely radioactive after it has fissioned in the reactor.The fuel is changed out about once per months, and 1/3 is removed, 1/3 new, and the remainder is “shuffled”.A Fukushima reactor (unit 2 or 3) had about 2740 MWth of power (at 100%) and this produced about 765 MWe.~96 rods per assembly~ assemblies per Rx
8Key PointsThe nuclear fuel contains almost all of the radioactivity (>99%).The nuclear fuel continues to generate heat after the reactor is shut down.19 MW after 1 day12 MW after 1 week7 MW after 3 monthsThe fuel must be cooled, or there is a risk of fuel damage and release of radioactivity.Fuel is important because it contains almost all of the radioactivity. The safety systems are designed to maintain cooling to the fuel – if the fuel is maintained cool, you cannot release large quantities of radioactivity.So, all the safety systems are designed to pump in water and keep the fuel cool – or else there could be a large release of radioactivity.
9Stainless Steel Melts Activity Available for Release (Curies) Fuel Temperature (oF)Stainless Steel MeltsActivity Available for Release (Curies)Reactors 2 and 3 at Fukushima are 2380 MWth reactors. Assumptions for release fractions from USNRC NUREG Decay to 7.5 days after shutdown. Values listed are those estimated to be released from the core to the coolant, and release from coolant to containment and release from containment to environment are not included in this estimate. Ci shown above are for 100% fuel melt, 100% fuel overheat, and 100% fuel gap. It is likely that a value less than 100% will occur – for example, there may be 100% fuel gap plus 50% fuel overheat plus 10% fuel melt.The temperatures listed are for when fuel clad damage is expected – about 1300 oF, with resultant release of some noble gases and volatile fission products.Fuel Overheat- about 3,000 degrees F, release of 50% of noble gases and volatile fission products and some less volatile fission products (Tellurium, Barium, Strontium)Fuel Melt: about 4,500 degrees F, release of 100% NG and volatile fission products, and larger percentages of non-volatiles such as barium and strontium.Note that the stainless steel that hold the fuel together, spacers, etc. melts at about 2,500 degrees F, so the cladding and steel will be damaged or melted resulting in fuel pellets in a pile in the reactor with molten materials – all before the fuel itself is melted.
10Prior to Accident Spent Fuel Pool Reactor Service Floor (Steel Construction)Concrete Reactor Building (secondary Containment)Reactor CoreReactor Pressure VesselContainment (Dry well)Containment (Wet Well) / Condensation ChamberSpent Fuel PoolFresh Steam lineMain FeedwaterNormal operation and just after shutdown – normal flow of water and steam, reactor is being cooled by flowing feedwater being heated into steam.From Areva Presentation The Fukushima Daiichi Incident – Dr. Matthias Braun
11Earthquake and Loss of Electric Power Earthquake causes loss of offsite power.Emergency Diesel Generators supply powerTsunami disables EDGsSteam dumps to wet wellWater level in reactor decreasesFuel heats upCladding is damaged and releases noble gases and volatile isotopes (cesium and iodine)~3/4 of the core exposed, Cladding exceeds ~1200°CZirconium in the cladding starts to burn under Steam atmosphereZr + 2H20 ->ZrO2 + 2H2Exothermal reaction further heats the coreGeneration of hydrogen Unit 1: kg Unit 2/3: kgHydrogen gets pushed via the wet-well, the wet-well vacuum breakers into the dry-well> 99.9% of radioactivity is in the fuelFrom Areva Presentation The Fukushima Daiichi Incident – Dr. Matthias Braun
12Accident ProgressionLarge volume in wet well eventually heats to boiling and no more pressure suppressionPressure increasesHydrogen created by high temperature reaction of cladding & steamOperators decide to vent primary containment gas to secondary containmentGas has fission products and hydrogenFrom Areva Presentation The Fukushima Daiichi Incident – Dr. Matthias Braun
13Hydrogen Explosion Unit 1 und 3 Hydrogen burn inside the reactor service floorDestruction of the steel-frame roofReinforced concrete reactor building seems undamagedLower Flammability Limit (LFL) for Hydrogen = 4% in air, UFL = 75% in airLower Explosive Limit (LEL) for Hydrogen = 17% in air, UEL = 56%From Areva Presentation The Fukushima Daiichi Incident – Dr. Matthias Braun
14Early Releases and Dose Rates Background is – 0.01 mR/hrFukushima Daiichi Main Gate dose rates dependent on wind direction & events:3/14: 50 mR/hr3/15: 300 mR/hr due to venting from Unit 23/15: 1200 mR/hr due to explosion & fire on Unit 43/16: 850 mR/hr explosion on Unit 23/17: 1100 mR/hr – releases from Units 2 and 3 of plantU.S. 7th Fleet ship contaminated helicopter crew.US news crews returning after 2 wks have contaminated equipment.The situation was very confusing in the first two weeks. It was uncertain why the dose rates were going up then down – although now it appears to all be explainable based on venting of the primary containments and hydrogen explosions – and the wind direction.
17Emergency Planning Actions Date, TimeEvacuateShelterMarch 11, 15:42 - Loss of power in Nuclear Power PlantsMarch 11, 21:233 km10 kmMarch 12, 05:44March 12, 18:2520 kmMarch 15, 11:0630 kmMarch 21 – First food restrictions: spinach, kakina, & milkApril 22 – Selected areas km termed Planned Evacuation Zones if estimated dose > 20 mSv (2 rem)On April 22, the area within 20 km officially declared as a “no-entry Zone”. The planned evacuation zones from km are generally to the northwest of Fukushima Daiichi. Those who are expected to get 20 mSv in a year must leave within a month.Other actions: April 30, interim policy on whether to use school buildings or outdoor areas.April 22: Radiation measurements in ports in Japan to provide info on shipping containers, decontamination criteria, reporting.Considering exchanging soil layers as a way to lower radiation levels in affected areas.
18Radiation Measurements This graph is in uSv/hr. If you multiply by 0.1, then it is converted to mR/hr.In Ibaraki prefecture, the dose rate (average) was about 0.04 uSv/hr on 3/24, which is about 5-10 times background.Early on, the results from Fukushima province were not shown on the public websites.The reason for showing this is to show the reduction over time – the dose rates are due to deposition on the ground, and the reduction is the decay of I-131, which was the major radionuclide in the first few weeks after the accident.Note –no significant deposition events noted after 3/19
19Water and Food Products I-131, Cs-134, Cs-137MilkProduce (leafy vegetables, spinach, etc.)Drinking water (peak at 3x 30 km, now below limits).Seawater, fish productsInitially prevented sale of food & seafood within 30 km radiusRecent identification of beef with Cesium contamination.
20Percent above Action Level for I-131 Type of SamplePercent above Action Level for I-131Percent above Action Level for Cs-134 and Cs-137Meat and Eggs0%Milk4%0.2%Produce2%5%Seafood0.4%7%Tea Products14%TotalData from April - July After this synopsis of data on the WHO website, some samples of beef were found above the 500 Bq/kg limit for meat.
21Food and Drinking Water Japan Limits (Bq/L or Bq/kg) NuclideWater, MilkVege-tablesMeat, FishIAEA for all foodI-131300*2,0003,000Cs-1342005001,000Cs-137IAEA values are those that will result in 10 mSv per year (1 rem)Limits are based on Notice No Article 3 of the Dept of Food Safety, March 17, 2011.Note that the effluent limit (release to the ocean) is less than drinking water limit.Note that Bq/kg x = pCi/g.2000 Bq/kg = 54 pCi/g; 500 Bq/kg = 13.5 pCi/gWater Effluent limits are 40 Bq/L I-131, 60 Bq/L Cs-134, and 90 Bq/L Cs-137IAEA GSG-2, Criteria for Use in Preparedness and Response for a Nuclear or Radiological Emergency*Infant water and milk limit is 100 Bq/kgIAEA Limits based on 1 rem per year to most restrictive individual (generally infant) if consuming food for 1 year at the limit
31Status according to World Health Org As of July 2011
32So How Bad Was It?As a nuclear or industrial accident, it was major – resulted in evacuation, loss of a major electricity source, and uncertainty in the public for months.It was not a major health catastrophe, and it is not likely that there will be significant health effects.Why? – The emergency plan actually worked. Despite the initial confusion, people were evacuated, controls were placed on food, etc.Note that this is my personal opinion.