april 30 –may 18, 2018 albuquerque, new mexico, usaalbuquerque, new mexico, usa ... ultimately,...

30 – Nuclear Reactor and Safety

The Twenty-Seventh International Training Course

30. Nuc lear Reac tors and Sa fe ty

April 30 – May 18, 2018Albuquerque, 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 undercontract DE-NA0003525.

SAND2018-4015 PE

Reactors and Safety

Learn ing Object ives

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

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Reactors and Safety

Reactor Steam Cyc le

• Fission heat transferred to water• Water heated to steam

Direct (1-Loop) – Within reactor Indirect (2-Loop) – Outside reactor in

heat exchanger / steam generator

3Courtesy of Nuclear Engineering International

Water also is necessary to slow down (moderate)

neutrons for the chain reaction to occur

Boiling Water Reactor (BWR)

Pressurized Water Reactor (PWR)

Reactors and Safety

Reactor Steam Cyc le (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

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Reactors and Safety

Mul t ip le Barr ier Conta inment

• Four major elements of reactor multiple-barrier containment for fission products1. Pellet

2. Cladding

3. Primary System

4. Reactor Containment Building

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Reactors and Safety

BWR/PWR Fuel

• Pellets: UO2• Fissile: 235U 2-4 wt%• Cladding: Zircaloy

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Research Reactors• Fuel Material – UO2• 20-90+wt% 235U• Clad rods or plates –

aluminum, zirconium, steel, ...



Reactors and Safety

PWR Fuel Bundles

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Research Reactors• Single elements, small rod

or plate bundles

Reactors and Safety

PWR Safety and Contro l Rods / C lusters

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Research Reactors• 12+ Safety / Control Rods• 1 Safety Rod / 2 Control Rods



Reactors and Safety

PWR Pr imary System

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Research Reactors• Pressurized vessel /

forced circulation• Open pool / natural

circulation

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

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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)

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Reactors and Safety

Des ign-Bas is Acc idents

• 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

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Reactors and Safety

Gener ic 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 trip2. Emergency core cooling3. Post-accident heat removal4. Post-accident radioactivity removal5. Containment integrity

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Reactors and Safety

Gener ic Reactor Safety Systems

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4. PARR – Removal of Radionuclides from Containment Atmosphere

1. RT – Rapid Shutdown of Reactor to Limit Core

Heat Production

2. ECC – Core Cooling to PreventRelease of Radionuclides from Fuel

3. PAHR – Removal of Heat from Containment to Prevent

Overpressurization

5. CI – Prevention of Dispersal of Radionuclides to Environment



Reactors and Safety

Reactor Tr ip / Emergency Core Cool ing

1. Reactor trip (RT) (“SCRAM”) Neutron poison control rods (also used for

routine control) Injection of soluble boric-acid poison

2. Emergency core cooling (ECC) Injection of borated water (cooling and

reactivity reduction) • Multiple trains • High, intermediate, or low pressure• Coincides with needs versus event history

Recirculation of coolant • From reactor building sump• Long-term coolant supply

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• Control rod insertion

• Moderator dump

• Coolant injection

• Pool natural circulation

Reactors and Safety

Post-Acc ident Heat / Radioact iv i ty Removal3. Post-accident heat removal (PAHR)

Coolant temperature reduction • Heat exchangers for ECC water recirculation

Containment-building pressure control• 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 Containment sprays to remove radioactivity

• Water sprays remove chemically reactive radioactive material

• Additives can increase removal, e.g., of elemental iodine16

• Heat exchanger

• Filtration



Reactors and Safety

Conta inment Integr i ty

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

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Research Reactors• Containment • Confinement (negative pressure

and filter)• Open pool / water volume

Reactors and Safety

PWR

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Research Reactors – Decreasing complexity



Reactors and Safety

Case Study: Three Mi le Is land Nuc lear S ta t ion

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Reactors and Safety

TMI-2: Acc ident 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

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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

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Reactors and Safety

22



Reactors and Safety

TMI-2: Acc ident Sequence

• Emergency feedwater blockage Prevented steam-generator function 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

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Rumored to be sabotage!

Reactors and Safety

TMI-2: Acc ident 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

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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

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Reactors and Safety

Research Reactor Sabotage Scenar io 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

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Reactors and Safety

Acc ident vs. Sabotage

TMI Accident Sabotage Equivalent

Condenser blocked Condenser disabled

Valves closed Close / damage valves

Pressurizer open Any major coolant loss

Injection throttled Injection disabled

Pumps turned off Pumps disabled

Hydrogen ignition Ignite hydrogen

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Research Reactors• Disable cooling• Uncover fuel

Uncover Fuel

Reactors and Safety

Protect Safety Equipment

• Protecting safety equipment Engineering Safety Aspects of the Protection

of Nuclear Power Against Sabotage (NSS-4) Anything that can happen by accident can be

made to happen• Consider threat capabilities• Consider events with radioactive release or

where one more failure (sabotage) would cause radioactive material release

• Identifying Vital Areas Identification of Vital Areas at Nuclear Facilities

(NSS-16) Detailed guidance for identification of vital areas

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Reactors and Safety

Case Study: React iv i ty Inser t ion Acc ident a t SL-1 Reactor

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Reactors and Safety

S tat ionery Low Power No. 1 (SL-1) Reactor

• 3-MWT prototype for proposed arctic military application• Complex design for research reactor

Direct, boiling water cycle 200 kW electric, 400 kW heat (disposed of directly to the

environment)• 14 kg HEU (93%) in 40 aluminum fuel-plate assemblies• 9 T-shaped control rod assemblies• Steel vessel 1.4-m diameter × 4.4-m high

(4.5-ft diameter × 14.5-ft high)• Turbine, condenser, piping, pumps• Multiple shielding types

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Reactors and Safety

SL-1 Schemat ic

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Reactors and Safety

Conf inement and Support Bui ld ings

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C o n d e n s e r & Ve n t i l a t i o n

O p e r a t i n g F l o o r L e v e l

Ve s s e l & S h i e l d i n g

Support BuildingConfinement Building



Reactors and Safety

Acc ident Informat ion

• January 3, 1961 – Three shifts Maintenance of auxiliary systems Reactor core measurement preparation (required vessel head

and control rod removal) Control rod drive reinstall – 3 operators

• Site central alarm with cause unknown Fire crew responded: Support Building radiation level 25 r/hr Health Physics responded: Reactor Building (Confinement

Building) radiation level 200 r/hr• Operator withdrew central (highest worth) control rod

“as rapidly as he was able”• Caused Reactivity Insertion Accident (RIA) (“excursion”)

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Reactors and Safety

Acc ident V ideo

<https://www.youtube.com/watch?v=yjljS0aQbCc>

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Reactors and Safety

Acc ident vs. Contro l led-Excurs ion Example• SL-1 accident scenario

System supercritical Power increased in a 4-ms period Coolant steam voids and fuel element expansion – and

ultimately, destruction – terminated the excursion• Controlled-excursion example

Sandia’s Annular Core Research Reactor (ACRR)• Special control rode fired (N2 pressure) out of the core• System begins a “prompt supercritical” excursion

https://www.youtube.com/watch?v=pa0Fmcv83nw• Temperature feedback first slows chain

reaction, then causes a self-shutdown before any fuel damage occurs . . .

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Reactors and Safety

Consequences and Lessons Learned• Consequences

Steam explosion scattered the core Water hammer lifted vessel ~3 meters Total energy release equivalent to ~28 kg of TNT Two operators killed immediately, and the third died soon after

from head injury Reactor room highly contaminated with fission products

• Dose rate 0.5 to 1 Sv/h (50 to 100 rem/h) • Only slight radiological release outside building

Three lessons for similar (prototype) reactors More than a single control rod for critical Minimize manual control rod operation Pressure-tight containment required

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Reactors and Safety

Damage

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Steel Plate and Cooling Lines

Core, Vessel, Control Rods

Reactors and Safety

Protect Safety Equipment

• Protect Control Rod Safety Identify decay heat removal requirements from safety analysis If a Probabilistic Safety Analysis (PSA) has been prepared, it

likely documents the consequences of loss of decay heat removal• Protect against Disabled Decay Heat Removal

Locate required safety equipment and identify disablement approaches

• Insider tampering• Local sabotage• Sabotage controls / power• Attack on primary coolant boundary for water-cooled reactors • Vehicle bombs / standoff attacks (if within the DBT) may disable

both decay heat removal and confinement

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Reactors and Safety

Protect Other Safety Equipment

• Operating Experience (IAEA-TECDOC-1762)

• Anything that can happen by accident can be made to happen Consider threat capabilities Consider events with radioactive

release or where one more failure (sabotage) would cause radioactive material release

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Reactors and Safety

IAEA TDL-004

• Nuclear Security Management for Research Reactors and Related Facilities

• Single source guidance for all aspects of nuclear security Nuclear materials and radioactive materials Sabotage and theft protection

• Identifies synergies between theft and sabotage protection

• Provides detailed guidance for establishing security systems, measures, and programs meeting NSS-13 recommendations

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Reactors and Safety

Des ign for Unique Secur i ty Chal lenges

• Design for easy experiment / core reconfiguration More difficult to control access to equipment that:

• Reconfigures core configuration• Controls reactivity

• Broad visual access for teaching More difficult to secure information about potential equipment

sabotage targets• Need for multi-national experimenter access to critical

areas More difficult to verify trustworthiness of individuals with access

to critical areas

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Reactors and Safety

Summary• Safety systems provide mitigation by

Preventing overheating, fuel melting, and other damage Preventing large-scale dispersal of fission products Reliability is enhanced through redundancy in subsystem

function and location• Five generic safety systems

1. Reactor trip2. Emergency core cooling3. Post-accident heat removal4. Post-accident radioactivity removal5. Containment integrity

• Protect safety equipment … anything that can happen by accident can be made to happen

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april 30 –may 18, 2018 albuquerque, new mexico, usaalbuquerque, new mexico, usa ... ultimately,...

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