Basic nuclear reactor safety (Jeffrey Mahn)

Basic Nuclear Reactor Safety Nuclear Safety Principles and Engineered Safety Systems Jeffrey A. Mahn Nuclear Engineer (Retired) Albuquerque, NM USA jamahn47@gmail.com

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 energy 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

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.

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

Decay Heat

One week after reactor shutdown decay heat power is ~1.5% of operating power.

Nuclear Reactor Safety • Basic purpose of nuclear reactor safety is to maintain integrity of the multiple barriers to fission product release; supported by a three-level defense-indepth approach • Control strategies developed to facilitate steady-power operations and to limit severity and/or mitigate consequences of potential accidents

Defense-in-Depth Level 1 • Prevention – seeks to avoid completely those operational occurrences that could result in system damage, loss of fuel performance, and abnormal releases of radioactivity; prevention measures include – inherently stable operating characteristics such as negative reactivity feedback (nuclear criticality control) – known materials in components/structures – design safety margins – periodic testing and inspections – operational instrumentation and automatic controls – plant personnel training – quality assurance – safety assessment

Defense-in-Depth Level 2 • Protection – seeks to halt or deal with unlikely, low-probability incidents and operational occurrences that cause reactor shutdown and that may lead to minor fuel damage and small releases of radioactivity; protection measures include – – – – – –

fast shutdown (reactor trip) primary coolant system pressure relief system/component operational interlocks automatic monitoring and safety-system initiation transient operating guidelines and procedures measurement and control of radiation levels, doses, and effluent radioactivity

Defense-in-Depth Level 3 • Mitigation – seeks to limit the consequences of accidents if they occur despite prevention and protection measures; mitigation measures include – emergency feedwater system – emergency core cooling systems – emergency electrical power (diesel generators, station batteries, and inverters) – radioactivity containment structures – emergency planning

Nuclear Power Plant Operational Safety Objectives • Core reactivity (nuclear criticality) control – neutron population control • Heat removal capability • Fission product containment

Light Water Reactor (LWR) Operational Reactivity Control

Reactor Neutron Population Neutron Balance Equation Rate of neutron Rate of Rate of Rate of population = neutron – neutron – neutron increase production absorption leakage or

Accumulation = Production – Losses

Neutron Multiplication Factor k =

neutron production neutron losses

Reactivity Defined • A nuclear system is “critical” when neutron

production and loss rates are equal (k = 1) • k – 1 represents “excess multiplication” of a system’s neutron population

• (k – 1)/k represents system “fractional excess multiplication,” or system “reactivity” (ρ)

Relationship Between k, ρ, and System Criticality Multiplication Factor Reactivity State of Criticality k>1

ρ>0

Supercritical

k=1

ρ=0

Critical

k<1

ρ<0

Subcritical

Reactivity Feedback Mechanisms • Cause reactor core reactivity to change as neutron level changes • Operate on short time scales and are very important to reactor operation and safety • Feedback mechanisms described as reactivity coefficients – rate of change of core reactivity with respect to rate of change of a feedback variable (e.g., fuel temperature, moderator temperature, moderator voids)

Too much neutron absorption; no sustained chain reaction (k < 1)

Constant neutron population (k = 1)

Too little neutron absorption; exponentially increasing neutron population (k > 1)

Pressurized Water Reactor (PWR) Reactivity Control • Short Term – Full-length control rods (within fuel assemblies) • Drive mechanisms on top of vessel • Provide routine power control • Contain neutron absorbing material – Boron carbide (B4C) – Au-In-Cd (80%-15%-5%)

• Intermediate/Long Term – Soluble neutron absorber (boric acid) in reactor coolant minimizes control rod use

PWR Fuel Assembly

PWR Reactivity Control (cont.)

• Shutdown [Safety] Rods – Held out of reactor core – Used to terminate unanticipated reactor excursions and for planned shutdowns

• Burnable Neutron Absorber (Shim) Rods – Match reactor core fuel burnup (create relatively uniform neutron flux across core) – Reduce core power peaking (flatten energy generation profile across core) – Offset positive increase in moderator temperature coefficient of reactivity as reactor coolant soluble boron is used up

Boiling Water Reactor (BWR) Reactivity Control • Short Term – Control rod motion – Reactor coolant flow rate adjustment (preferred) • Negative reactor coolant (moderator) void feedback • Power varies directly with coolant flow rate

• Long Term/Large Change Control – Full-length control rods (between fuel assemblies) • Drive mechanisms on bottom of reactor vessel (avoids steam dryers and separators at top of vessel) • Provide routine power control • Contain B4C material

BWR Fuel Assembly

BWR “Fuel Module” Fuel Rod

Water Rod Control Blade

BWR Reactivity Control (cont.)

• Long Term/Large-Change Control (cont.)

– Burnable neutron absorber (Gadolinia) in fuel pellets • Several burnable neutron absorber rods per fuel assembly • Positioned to reduce power peaking in fuel assembly

LWR Operational Heat Removal

Boiling Water Reactor Power Plant

(Animation)

Pressurized Water Reactor Power Plant

(Animation)

LWR Shutdown Heat Removal

Nuclear Fission Energy Disposition Fission Fragment Kinetic Energy Fission Neutron Kinetic Energy Prompt Gamma Ray Energy Fission Fragment Delayed Radiation Beta Particles Gamma Rays Radiative Capture Gammas Total

Mev 168 5 7

% 84 2.5 3.5

8 7 5 200

4 3.5 2.5 100

Delayed radiation from fission fragment radioactive decay accounts for ~7.5% of total fission energy and continues to generate heat in the nuclear fuel material even after the fission process is terminated. This heat must be removed from the fuel material to prevent overheating.

Decay Heat Production Following Reactor Shutdown (WASH-1400)

Examples of Medium Half-Life Fission Products Half Life (years)

Decay Energy MeV

Decay Mode

155Eu

4.76

0.252

β,γ

85Kr

10.76

0.687

β,γ

113mCd

14.1

0.316

β

90Sr

28.9

2.826

β

30.23

1.176

β,γ

121mSn

43.9

0.390

β,γ

151Sm

90

0.077

β

Fission Product

137Cs

Long Half-Life Fission Products Half Life (years)

Decay Energy MeV

Decay Mode

99Tc

211,000

0.294

β

126Sn

230,000

4.050

β,γ

79Se

327,000

0.151

β

93Zr

1,530,000

0.091

β,γ

135Cs

2,300,000

0.269

β

107Pd

6,500,000

0.033

β

15,700,000

0.194

β,γ

Fission Product

129I

PWR Residual Heat Removal (RHR) System

BWR Residual Heat Removal System

LWR Fission Product Containment

Fission Product Retention Barriers Defense-in-Depth • Fuel Pellet • Fuel Pin Cladding • Primary Coolant System Boundary • Reactor Containment Building

Multiple Barriers Close-up of steam generator showing the 3rd Barrier

1st & 2nd Barriers Pellet & Fuel Rod Cladding

3rd Barrier Primary System Boundary

4th Barrier Reactor Containment

Nuclear Power Plant Operational Occurrence/Accident Safety Objectives • • • • •

Core reactivity control Reactor coolant inventory control Core heat removal capability Containment heat removal capability Radioactivity containment

Engineered Safety Features and Functions • Reactor Protection System – reactor trip (RT) terminates nuclear chain reaction (core reactivity control) • Emergency Core Cooling Systems (ECCS) – inject coolant into reactor vessel upon loss of reactor coolant inventory and remove heat from nuclear fuel (reactor coolant inventory control & core heat removal capability) • Post-Accident Heat Removal (PAHR) – removes heat from containment building (containment heat removal capability)

Engineered Safety Features and Functions (cont.)

• Post-Accident Radionuclide Removal (PARR) – removes radionuclides from containment atmosphere and filters containment effluents (radioactivity containment) • Reactor Containment Building Integrity (CI) – prevents release of post-accident radioactivity (radioactivity containment)

Engineered Safety Features (ESFs)

Engineered Safety System Reliability • Reliability design principles and criteria assure protection and safety function availability at all times – System/component redundancy provides single-failure protection – System/component diversity provides common-mode failure protection – Physical separation provides protection against simultaneous loss (e.g., by fire or flood)

Fail-Safe Design Principle â&#x20AC;˘ Assures that systems and components designed to automatically return to safest condition upon failure or loss of power

Gen 2 vs. Gen 3+ Reactor Safety • Generation 2 reactor safety has inherent weakness – safety devices added to reactor plant design as result of accident considerations – safety dependent on active Defense-in-Depth Level 3 mitigation measures (see slide 9)

• Safety of Generation 3+ reactors is fundamental consideration of reactor plant design – passive safety makes use of gravity, natural circulation, high temperature resistance

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