(Jeffrey Mahn) USofA - Nuclear Power Plant Accidents

Nuclear Power Plant Accidents Jeffrey A. Mahn Nuclear Engineer (Retired) Albuquerque, NM USA jamahn47@gmail.com

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Outline • Nuclear power plant operating characteristics and accident concern • Three Mile Island accident • Chernobyl accident • Fukushima accident

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Nuclear Power Plant Operating Characteristics • Nuclear power plants – Designed to produce base-load electricity (24 hours per day, 365 days per year) just like coal-fired power plants – Use a steam cycle to convert heat into electricity just like coal-fired power plants – Generate heat from fission of Uranium-235 atoms in nuclear reactor fuel

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Coal-fired Power Plant

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Boiling Water Reactor Plant

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Pressurized Water Reactor Plant

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Power Plant Operation Comparison Fossil-fueled Power Plant

Nuclear Power Plant

Heat Source Fuel Supply

Fuel fed continuously into “firebox� and ignited by a flame (burner)

Each reactor core fuel loading contains sufficient uranium fuel for 1-2 years of operation

Heat Generation Rate Control

Heat generation controlled by fuel feed rate

Fission reaction rate controlled by neutron absorbing materials inserted into reactor core

Means of Heat Removal

Water flowing through tubes in firebox wall

Water flowing through coolant channels in nuclear fuel assemblies

Heat Generation Termination

Termination of fuel feed to firebox

Termination of fission process by full insertion of neutron absorbing materials into reactor core*

* Radioactive decay of fission products in reactor fuel continues to generate significant heat, which must be removed, following reactor shutdown

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Focus of Nuclear Reactor Safety • All energy forms have upsides and downsides – nuclear energy is no exception • Nuclear energy’s upside – Enormous amount of energy produced by U-235 fission (~1.5 million times more per gram of fuel than hydrocarbon fuels)

• Nuclear energy’s downside – Energy from decay of radioactive fission products continues to produce heat in reactor fuel even after fission reaction is terminated (i.e., reactor shutdown) – Radioactive decay energy (heat) must be removed from reactor fuel to prevent overheating, melting, and release of radioactive material to the environment 8

Nuclear Fission Energy Disposition Mev

Fission Fragment Kinetic Energy 168 Fission Neutron Kinetic Energy 5 Prompt Gamma Ray Energy 7 Fission Fragment Delayed Radiation Beta Particles 8 Gamma Rays 7 Radiative Capture Gamma Rays 5 Total 200

%

84 2.5 3.5 4 3.5 2.5 100

Approximately 7.5% of the total energy from U-235 fission is contained in beta particles and gamma rays emitted by radioactive fission fragments and their radioactive decay products.

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Reactor Decay Heat Generation • Radioactive fission fragments and their radioactive decay products have half-lives ranging from seconds to millions of years • While decay heat generation drops off quickly as shorter half-life radioisotopes disappear, one week after reactor shutdown decay heat power is still ~1.5% of reactor operating power

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Nuclear Power Plant Accidents

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Three Mile Island Accident • Date of accident: March 28, 1979 • Type of nuclear plant: Babcock & Wilcox 2-loop pressurized water reactor (PWR) with oncethrough steam generators • Initiating event: operational upset in secondary plant (loss of condensate flow) led to trip of main feedwater pumps, which terminated feedwater flow to primary plant steam generators

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TMI Condensate & Feedwater Systems Containment Building

Atmospheric Dump Valve

EFW Block •Valve B

Main Steam Isolation Valve A

Steam Generator B Main Steam Isolation EFW Block •Valve A Emergency Feedwater Pumps (3)

Reactor

Steam Generator A

Valve B

Atmospheric Dump Valve

Turbine

Generator

Condenser Hotwell

Make Up Valve

Condensate Storage Tank Air-operated Motor-operated

Main Feedwater Pumps (2)

Condensate Pumps (3) Condensate Booster Pumps (3)

Condensate Polisher (8)

Location of accident initiating event

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Three Mile Island Accident (cont.)

• Accident sequence – Loss of heat removal from primary system caused pilot-operated relief valve (PORV) on pressurizer to open as primary system pressure increased – Reactor control rods automatically inserted on high primary system pressure, which terminated fission reaction in reactor fuel – Primary system pressure dropped below relief valve set-point, but PORV failed to close resulting in lossof-coolant accident (LOCA) condition – Emergency feedwater pumps started but block valves to steam generators were closed (human error) 14

TMI Primary Coolant System Pilot-operated Relief Valve

Primary Coolant Secondary Coolant

Pressurizer M

Steam Generator B

Steam Generator A Reactor Coolant Pump A

Reactor Coolant Pump B

Reactor Vessel Quench Tank

Loop A

Loop B

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Three Mile Island Accident (cont.)

• Accident sequence (cont.)

– High-pressure injection (HPI) pumps automatically started pumping water into primary system from refueling water storage tank as primary system pressure continued to decrease – HPI reduced, in accordance with LOCA procedure, when pressurizer went solid (i.e., lost steam bubble) – Primary coolant pumps shutdown in response to indications of low system pressure, high vibration, and low coolant flow – Termination of primary system coolant flow caused steam bubbles to collect in reactor vessel upper-head region 16

Three Mile Island Accident (cont.)

• Accident sequence (cont.)

– Fission product decay heat continued to boil off water in reactor core region until fuel became uncovered and melted top half of core – Primary system coolant flow restored at 16 hours into accident, essentially terminating accident

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Three Mile Island Accident (cont.)

• Radiological consequences – Exposure to nearest known offsite individual believed to be no more than 37 mrem (0.37 mSv), or about one-tenth of average American’s annual exposure – Concentrations of I-131 found to be below FDA maximum levels in milk by factor of 300 – No detectable levels of other nuclides, such as Cs137, found in wide variety of food samples

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Three Mile Island Accident (cont.)

• Current plant status: – Fuel removed from reactor vessel, plant cleaned up, decontaminated, and placed in post-defueling monitored storage – Unit 2 to be decommissioned with Unit 1 at end of Unit 1’s useful lifetime

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TMI – Pre Accident

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TMI – Post Accident

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Chernobyl Accident • Date of accident: April 26, 1986 • Type of nuclear plant: Russian dual–purpose (electricity and plutonium production) pressuretube graphite reactor with on-line refueling and no containment building • Initiating event: performance of a faulty test in which some reactor safety systems were deliberately bypassed; reactivity excursioncaused steam explosion resulted in disruption of nuclear core 22

Chernobyl RBMK Reactor

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Chernobyl Accident (cont.)

• Accident sequence – Commencement of test caused rise in reactor power resulting in coolant voiding (localized boiling) – Positive reactivity feedback mechanism quickly accelerated power increase – Operator-initiated control rod insertion too slow to affect rate of power increase – Power “pulse” caused fuel to disintegrate – Fragmented fuel contact with water-steam coolant mixture caused steam explosion – Explosion destroyed reactor building concrete walls, dispersing burning graphite and nuclear fuel – Plume of radioactive gases and particulates sent high into atmosphere 24

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Chernobyl Accident (cont.)

• Radiological consequences – Radioactive material releases included fission product gases, other volatile elements (e.g., iodine), particulates, aerosols, and graphite-fuel debris from reactor core (contamination of local countryside) – 28 plant workers and first responders received lethal radiation doses – More than 200 plant personnel and first responders hospitalized for radiation injuries – Public dose was only a few times higher than the annual background in the area (Source: UN Scientific Committee on the Effects of Atomic Radiation) 26

Chernobyl Accident (cont.)

• Radiological consequences (cont.)

– 22 people who recovered from acute radiation exposure died in the following 19 years; only 5 of those deaths were cancer-related – a cancer death rate that is no different than that of the general population – Chernobyl workers and populace in 30-km evacuation zone most likely to show detectable increase in longterm health effects, if any

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Chernobyl Accident (cont.)

• Current plant status: – Unit enclosed in permanent confinement structure – Continuously monitored for radiation releases – 15-meter deep barrier wall constructed to prevent contamination of groundwater – Local croplands, forests, and orchards decontaminated – Contaminated soil decontaminated or removed and stored in drums for disposal

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Chernobyl Unit 4 – Post Accident

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Chernobyl Unit 4 Permanent Confinement Structure

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Fewer Cancers Observed from Chernobyl Radiation Exposures Dr. Zbigniew Jaworowski, MD PhD DSc, former Chairman of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) stated: “What is really surprising, however, is that data collected

by UNSCEAR and the Chernobyl Forum show 15% to 30% fewer cancer deaths among the Chernobyl emergency workers and about 5% lower solid cancer incidence among the people in the Bryansk district (the most contaminated in Russia) in comparison with the general population. In most irradiated group of these people (mean dose of 40 mSv) the deficit of cancer incidence was 31 17%.�

Fukushima Accident • Date of accident: March 11, 2011 • Type of nuclear plant: 4 boiling water reactors with Mark-1 containment buildings – – – –

Unit 1: Unit 2: Unit 3: Unit 4:

GE BWR-3 GE/Toshiba BWR-4 Toshiba BWR-4 Hitachi BWR-4

• Initiating event: beyond design basis tsunami

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Tsunami Size as Accident Cause

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Fukushima Accident Summary • Reactors automatically shut down based on detection of seismic activity • Earthquake resulted in loss of offsite power transmission line damage • Emergency Diesel Generators powered emergency cooling systems • An hour later, station struck by tsunami that rendered inoperable ➢ All multiple sets of Emergency Diesel Generators ➢ AC electrical buses ➢ DC batteries 34

Fukushima Accident Summary (cont.)

• Tsunami damaged service water systems that provide heat rejection to sea • Delayed cooling caused substantial fuel damage before portable power supplies and pumps brought on-site to re-establish cooling with fresh water and seawater • Pressure inside reactor containment structures increased steadily while continuously venting to atmosphere • Containment leakage occurred when fuel cladding oxidized, and released hydrogen combusted in reactor buildings (U1-3) 35

Fukushima Plant – Before Tsunami

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Fukushima Plant – After Tsunami

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Containment Building Before and After Hydrogen Explosion

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Fukushima Plant – After H2 Explosions

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Fukushima Accident (cont.)

• Radiological consequences – Accident caused no loss of life or radiation sickness – Rise in worker thyroid cancer or leukemia (two cancers most likely to result from accident) unlikely to be detectable – Risk to roughly 140,000 civilians living within few tens of kilometers of plant even lower – Latent cancers in local population estimated to be zero – Surrounding land contaminated with radioactive material – Radioactively contaminated water leaking from reactor building basements to sea 40

Fukushima Accident (cont.) • Current plant status: – Recovery activities focused on containment and cleanup of radioactively contaminated material (primarily water and soil)

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Fukushima Current Plant Status Unit 4 spent fuel removal cover

Contaminated water storage tanks

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Contact Information Jeffrey A. Mahn Nuclear Engineer (Retired) Albuquerque, NM USA jamahn47@gmail.com

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