30 – Nuclear Reactor and Safety 30. Nuclear Reactors and Safety April 30 – May 18, 2018 Albuquerque, New Mexico, USA Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under SAND2018-4015 PE contract DE-NA0003525. Reactors and Safety Learning Objectives After completing this module, you should be able to: • Describe key characteristics of pressurized water reactors (PWR) and research reactors • List five generic types of reactor safety systems • Recognize importance of protecting safety equipment 2 The Twenty-Seventh International Training Course Page 1 30 – Nuclear Reactor and Safety Reactors and Safety Reactor Steam Cycle • Fission heat transferred to water • Water heated to steam Water also is necessary to slow down (moderate) . Direct (1-Loop) – Within reactor neutrons for the chain . Indirect (2-Loop) – Outside reactor in reaction to occur heat exchanger / steam generator Boiling Water Pressurized Water Reactor (BWR) Reactor (PWR) Courtesy of Nuclear Engineering International 3 Reactors and Safety Reactor Steam Cycle (cont’d) • Fission heat transferred to liquid water • Water heated to steam • Steam turns turbine-generator to produce electricity • Condenser water removes heat to increase efficiency . Once-through – natural or artificial body of water . Cooling towers • Wet • Dry 4 The Twenty-Seventh International Training Course Page 2 30 – Nuclear Reactor and Safety Reactors and Safety Multiple Barrier Containment • Four major elements of reactor multiple-barrier containment for fission products 1. Pellet 2. Cladding 3. Primary System 4. Reactor Containment Building 5 Reactors and Safety BWR/PWR Fuel • Pellets: UO2 • Fissile: 235U 2-4 wt% • Cladding: Zircaloy Research Reactors • Fuel Material – UO2 • 20-90+wt% 235U • Clad rods or plates – aluminum, zirconium, steel, ... 6 The Twenty-Seventh International Training Course Page 3 30 – Nuclear Reactor and Safety Reactors and Safety PWR Fuel Bundles Research Reactors • Single elements, small rod or plate bundles 7 Reactors and Safety PWR Safety and Control Rods / Clusters Research Reactors • 12+ Safety / Control Rods • 1 Safety Rod / 2 Control Rods 8 The Twenty-Seventh International Training Course Page 4 30 – Nuclear Reactor and Safety Reactors and Safety PWR Primary System Research Reactors • Pressurized vessel / forced circulation • Open pool / natural circulation 9 Reactors and Safety Reactor Safety • Prevent accidents • Take protective actions . Identification and correction • Mitigate . Long-term response to and control of consequences • Conduct safety analyses 10 The Twenty-Seventh International Training Course Page 5 30 – Nuclear Reactor and Safety Reactors and Safety Reactor Energy Sources • 98% retention of radioactive products in fuel pellets . Provide cooling . Prevent fuel melting • Stored energy in fuel, coolant, and structures • Nuclear transients . Increased power level . Large power pulse • Decay heat from fission products • Chemical reactions among fuel, cladding, and coolant • External events (natural, such as floods; human caused) 11 Reactors and Safety Design-Basis Accidents • The basis for assessing the overall safety acceptability of a particular reactor design • Classifications include: . Overcooling and undercooling . Loss-of-flow (LOFA) and Loss-of-coolant (LOCA) . Reactivity increase / neutron multiplication . Failure to shut down (“SCRAM”) . Spent-fuel system-radioactivity release . External events (natural or human-caused events) • Beyond-design-basis accidents 12 The Twenty-Seventh International Training Course Page 6 30 – Nuclear Reactor and Safety Reactors and Safety Generic Safety Systems • Provide mitigation by . Prevent overheating, fuel melting, and other damage . Prevent large-scale dispersal of fission products . Reliability is enhanced through redundancy in subsystem function and location • Five generic safety systems 1. Reactor trip 2. Emergency core cooling 3. Post-accident heat removal 4. Post-accident radioactivity removal 5. Containment integrity 13 Reactors and Safety Generic Reactor Safety Systems 4. PARR – Removal of Radionuclides 5. CI – Prevention of Dispersal of from Containment Atmosphere Radionuclides to Environment 1. RT – Rapid Shutdown of Reactor to Limit Core Heat Production 3. PAHR – Removal of Heat from Containment to Prevent 2. ECC – Core Cooling to Prevent Overpressurization Release of Radionuclides from Fuel 14 The Twenty-Seventh International Training Course Page 7 30 – Nuclear Reactor and Safety Reactors and Safety Reactor Trip / Emergency Core Cooling 1. Reactor trip (RT) (“SCRAM”) • Control rod . Neutron poison control rods (also used for insertion routine control) • Moderator . Injection of soluble boric-acid poison dump 2. Emergency core cooling (ECC) . Injection of borated water (cooling and reactivity reduction) • Coolant • Multiple trains injection • High, intermediate, or low pressure • Pool natural • Coincides with needs versus event history circulation . Recirculation of coolant • From reactor building sump • Long-term coolant supply 15 Reactors and Safety Post-Accident Heat / Radioactivity Removal 3. Post-accident heat removal (PAHR) . Coolant temperature reduction • Heat exchangers for ECC water recirculation • Heat . Containment-building pressure control exchanger • Containment-atmosphere coolers • Steam-condensing water sprays 4. Post-accident radioactivity removal (PARR) . Filter chemically active iodine and aerosol / particulate constituents . Noble-gas constituents can only be contained – or released in a controlled manner • Filtration . Containment sprays to remove radioactivity • Water sprays remove chemically reactive radioactive material • Additives can increase removal, e.g., of elemental iodine 16 The Twenty-Seventh International Training Course Page 8 30 – Nuclear Reactor and Safety Reactors and Safety Containment Integrity 5. Containment integrity (CI) . Last line of defense against fission-product release . Building: Leak-tight steel liner (pressure vessel); thick reinforced concrete . Isolate penetrations, e.g., with remotely operated valves . Other safety features control overall pressure Research Reactors • Containment • Confinement (negative pressure and filter) • Open pool / water volume 17 Reactors and Safety PWR Research Reactors – Decreasing complexity 18 The Twenty-Seventh International Training Course Page 9 30 – Nuclear Reactor and Safety Reactors and Safety Case Study: Three Mile Island Nuclear Station 19 Reactors and Safety TMI-2: Accident Sequence • Before 4 a.m. on March 28, 1979 . Running at 97% power . Seemingly minor problems • Small loss of coolant through pressurizer valve to drain tank • Emergency feedwater valves closed – Post-maintenance – Unintentional, unknown to operators • Blockage in the demineralizer for steam-generator water 20 The Twenty-Seventh International Training Course Page 10 30 – Nuclear Reactor and Safety Reactors and Safety 4:00:36 a.m. March 28, 1979 • INITIATOR: Unable to clear demineralizer blockage • No main feedwater • Main feedwater pump tripped off-line • Turbine tripped off-line • Emergency feedwater pump auto-started • Pressurizer pilot-operated relief valve (PORV) opened to reduce pressure • Reactor tripped on overpressure signal - - - Normal system response (first 8 sec) - - - . Chain-reaction shutdown . Decay-heat source remains 21 Reactors and Safety 22 The Twenty-Seventh International Training Course Page 11 30 – Nuclear Reactor and Safety Reactors and Safety TMI-2: Accident Sequence • Emergency feedwater blockage Rumored to . Prevented steam-generator function be sabotage! . Distraction until valves re-opened . Overall effect on accident progression uncertain • PORV indicated “closed” (solenoid was de-energized) . PORV actually was stuck open . Unrecognized small-break loss of coolant accident (SBLOCA) in progress • High-pressure injection (HPI) auto-start • HPI throttled . Open PORV . Spurious indication of too much water 23 Reactors and Safety TMI-2: Accident Sequence • Control room situation . Included 1300 annunciator lights; single klaxon . 800 alarm initiations within 14 minutes • Decay heat . 200 MWt at shutdown • → 40 MWt at 1 hr . Electric arc furnace • 130 MW-hr ↔ 300 ton Steel . Fuel melting 24 The Twenty-Seventh International Training Course Page 12 30 – Nuclear Reactor and Safety Reactors and Safety Consequences • Environmental . Brought under control rapidly . Minimal damage . “Public apprehension” • Functional / Financial . Loss of TMI-2 reactor . Clean-up costs . 6.5-yr to restart TMI-1 • Nuclear industry . Backfit and license-related costs . Reactor orders cancelled 25 Reactors and Safety Research Reactor Sabotage Scenario Types • Direct Attack: Adversary brings energy to disperse radioactive material . Explosive or incendiary attack on inventories of radioactive material . Prevented by denial of access to radioactive material inventories • Indirect Attack: Adversary uses energy in facility process/ equipment to disperse radioactive material . Reactivity Insertion Accident (RIA) . Disable SCRAM system and heat removal . Disable decay heat removal . Disable other safety equipment required to prevent radioactive material release . Prevented by denial of access to safety equipment 26 The Twenty-Seventh International Training Course Page 13 30 – Nuclear Reactor and Safety Reactors and Safety