What is an Advanced Reactor?

By Nick Touran, Ph.D., P.E., 2023-04-16 , Reading time: 5 minutes

The term advanced in nuclear is used loosely to mean “reactors that are better than ones you’re worried about”. Due to the fact that a huge number of somewhat exotic reactor types were built and tested in the 1950s and 1960s, the term is somewhat misleading and complex. Depending on who you talk to, advanced reactors can mean a number of quite different things:

Definition 1: Reactors that incorporated hard-learned lessons from the past into the design

These advanced reactors have incorporated real-world, experience-informed lessons learned from previous versions of any given reactor make into newer, better, more optimized models. They generally have been built before, have existing supply chains, have licenses from the regulators, and are sitting around ready for people to purchase them. Examples:

  • GEH’s Advanced Boiling Water Reactor (ABWR)
  • Westinghouse’s Advanced Passive 1000 (AP1000)
  • KEPCO’s Advanced Power Reactor 1400 (APR-1400)

Definition 2: Reactors that, once built, could theoretically have extended capabilities beyond modern LWRs

These advanced reactors have characteristics designed to enhance the performance beyond the (already stellar) capabilities of the workhorse reactor fleet of light-water reactors (LWRs). Such advancements may include:

  • Ability to get more energy out of any given mass of nuclear fuel by breeding plutonium or U-233
  • Ability to reach higher coolant temperatures to:
    • Have higher heat-to-electricity conversion (thermal efficiency)
    • Supply industrial heat to replace fossil heat sources in hard-to-decarbonize chemical/industrial processes
  • Remove afterglow heat indefinitely after shutdown without any backup power required to improve safety in extreme conditions (e.g. large regional power outages). This generally requires better connections to ambient heat sinks and/or special coolants like liquid metal or molten salt.
  • Ability to be constructed without as many schedule delays or cost overruns, e.g. by incorporating elements of modular construction
    • Large equipment module built in factory and installed at site (e.g. AP1000 modular construction)
    • Reactor systems built in factory and shipped to site and installed in field-constructed buildings (e.g. NuScale SMR)
    • Entire plant systems built in factory and rail/truck shipped to site for turnkey operation (e.g. microreactors like the Army’s ML-1)

Generation-IV vs. Advanced

Related to definition 2, in 2000, experts in the international nuclear power community came together in the Generation-IV International Forum (GIF) to help guide the industry’s path forward. Over 100 experts looked through 130 reactor types proposed and chose the 6 most promising reactor types that they agreed had high potential to reach higher performance in 8 specific technology goals, listed here.

Advanced reactors vs. advanced reactor concepts

The discussion around nuclear reactors types has struggled since the early 1950s to differentiate between the purported benefits of conceptual reactors and the actual performance of built-and-operating real reactors. Admiral Rickover most famously summarized the situation in his paper reactors vs. practical reactors memo, which sharply criticized people who promoted the hypothetical benefits of reactors that were not yet built over reactor projects that were in construction or operation.

In many senses, advanced reactors as used in modern discussions are basically identical to the academic reactors Rickover referred to way back in 1953.

Academic reactor

  1. It is simple.
  2. It is small.
  3. It is cheap.
  4. It is light.
  5. It can be built very quickly.
  6. It is very flexible in purpose (“omnibus reactor”)
  7. Very little development is required. It will use mostly “off-the-shelf” components.
  8. The reactor is in the study phase. It is not being built now.

Practical reactor

  1. It is being built now.
  2. It is behind schedule.
  3. It is requiring an immense amount of development on apparently trivial items. Corrosion, in particular, is a problem.
  4. It is very expensive.
  5. It takes a long time to build because of the engineering development problems.
  6. It is large.
  7. It is heavy.
  8. It is complicated.

Basically, anyone can say that their reactor is way better than others that people have tried, but there’s no reason to believe them even a little until they can point to one that’s operating and show you how well it works. This is easy to forget in a world with significant VC energy funding.

Legal definitions of Advanced Reactors

Some countries have written legal definitions of advanced reactors as part of providing government support to some reactors but not others. For example, the USA’s Nuclear Energy Innovation and Modernization Act defines advanced nuclear reactors as follows:

USA NEIMA’s definition of advanced

The term advanced nuclear reactor means a nuclear fission or fusion reactor, including a prototype plant (as defined in sections 50.2 and 52.1 of title 10, Code of Federal Regulations (as in effect on the date of enactment of this Act)), with significant improvements compared to commercial nuclear reactors under construction as of the date of enactment of this Act, including improvements such as

  • additional inherent safety features;
  • significantly lower levelized cost of electricity;
  • lower waste yields;
  • greater fuel utilization;
  • enhanced reliability;
  • increased proliferation resistance;
  • increased thermal efficiency; or
  • ability to integrate into electric and nonelectric applications.

Chart of reactor design choices

As described in the reactors page, there are literally millions of different types of reactors. Which ones are advanced and which ones are not is pretty subjective.

graph TD

    TRL([Choose\ntechnology\nreadiness\nlevel]) --> High --> Construction
    TRL --> MedT[Medium] --> Construction
    TRL --> Low --> Construction
    
    Construction([Choose\nconstruction\nstyle]) --> Modular --> Size
    Construction --> Stick --> Size

    Size([Choose\nsize]) --> Micro --> Cycle
    Size --> Small --> Cycle
    Size --> Medium --> Cycle
    Size --> Large --> Cycle
    Size --> Gargantuan --> Cycle

    Cycle([Choose\npower cycle]) --> Rankine --> Moderator
    Cycle --> Brayton -->Moderator
    Cycle --> Stirling -->Moderator
    Cycle --> Piston --> Moderator
    Cycle --> Chemical --> Moderator
    Cycle --> Thermionic --> Moderator
    Cycle --> Ion[Ion capture] --> Moderator
    Cycle --> Inturbine[In-\nturbine\ncombustion] --> Moderator
    Cycle --> Other --> Moderator

    Moderator([Choose\nmoderator])  --> MWater[Water] --> FFM
    Moderator --> MHW[Heavy Water] --> FFM
    Moderator --> None --> FFM
    Moderator --> Graphite --> FFM
    Moderator --> Beryllium --> FFM
    Moderator --> MOrganic[Organic] --> FFM
    Moderator --> Hydride --> FFM

    FFM([Choose\nfuel\nform]) --> Oxide --> FC
    FFM --> Metal --> FC
    FFM --> Nitride --> FC
    FFM --> Carbide --> FC
    FFM --> TRISO --> FC
    FFM --> MSF[Molten salt] --> FC
    FFM --> LMF[Liquid metal] --> FC

    FC([Choose\nfuel\ncycle]) --> LEU[LEU converter] --> Coolant
    FC --> NU[Natural uranium\nconverter] --> Coolant
    FC --> HALEU[HALEU converter] --> Coolant
    FC --> HEU[HEU burner] --> Coolant
    FC --> PU[Plutonium burner] --> Coolant
    FC --> UPU[U-Pu breeder] --> Coolant
    FC --> THU[Th-U breeder] --> Coolant

    Coolant([Choose\ncoolant]) --> Gas --> CO2
    Gas --> Nitrogen
    Gas --> He
    Gas --> Air
    Coolant --> Water --> LW["Light\nwater"] --> BWR
    LW --> PWR
    LW --> Steam
    Water --> HW["Heavy\nwater"]
    Coolant --> LM[Liquid Metal] --> Na["Sodium/\nNaK"]
    LM --> Lead["Lead/\nPbBi"]
    LM --> Mercury
    Coolant --> Salt["Molten\nSalt"]
    Salt --> Fluoride
    Salt --> Chloride
    Coolant --> Organic
    Coolant --> Sulfur
    Coolant --> LH["Liquid\nHydrogen"]
    Coolant --> HP["Heat\nPipe"]
    Coolant --> OTH["Other\n(Sulfate,\nplasma,\ndust)"]

Useful reactor data

Given these, many useful quantities can be computed for comparison, such as mined uranium and SWUs needed per kWh. It’d be nice if people got serious about putting these all on Reactor Consumer Labels.

Metric Definition Purpose Units Examples
General
Model Trade name of the reactor type APR-1400, AP1000, ABWR, BWR-4
Fuel form What the fuel is made of To know details of fuel fabrication and supply chain, fabrication cost, capabilities UO2, carbide, molten salt, metal
Primary coolant What material transfers heat between the splitting atoms and the first heat exchanger/steam generator? Basic Water (PWR and BWR), sodium metal, helium, enriched chloride salt
Moderator What moderator (if any) is used in the core? To know if it's a fast or thermal reactor, and what the basic power density will likely be based on migration length Water, heavy water, graphite, beryllium, none
Rated thermal power Peak thermal power of the core To know how much heat it can make, roughly correlated with size MW (thermal)
Rated electric power Peak electric power of the power conversion cycle (if any) To know how much electricity it can make, important for end use MW (electric)
Utilized heat rate How much thermal power is utilized for a productive process (e.g. industrial heat, desalination, hydrogen) To know how much of the total power is being actually used rather than wasted to ambient BTU/hour (thermal)
Technology Readiness Level How mature the reactor product is. To differentiate between basic reactor concepts, partially-developed reactor products, and fully mature fleet products 1-9
Core
Structure Material used for core structural members To understand the contents and capabilities of the core Zircaloy, SS316, Graphite
Control Material(s) used for control To understand the control characteristics B4C, Ag-In-Cd, soluble boron
Average discharge burnup Amount of thermal energy extracted from fuel Essential quantity that determines fuel cycle economics, high level waste generation, and sustainability MWd/kg 7, 50, 200
Core power density How much heat is generated in each unit of core volume Describes how much power can fit in a volume, determines core size and decay heat removal requirements kW/l 104
Conversion ratio Fraction of fissile material at end of cycle divided by initial Explains to what degree the core breeds or converts fissile material unitless 0.5, 1.2
Initial heavy metal mass Total heavy metal mass loaded into reactor at startup Combined with fissile fraction, determines cost and resource needs of a new unit metric tonnes of heavy metal (e.g. of uranium) 20 MT
Average initial fissile fraction Average fissile fraction of initial core (enrichment) Important factor in fuel cycle practicality, sustainability, and economics Mass percent 4.5%
Average reload HM mass Rate at which heavy metal mass is loaded into the core. Sustainability and economics assessments metric tonnes of heavy metal per full-power year 3 MT
Average reload fissile fraction Average fissile fraction of reload fuel in equilibrium (enrichment) Important factor in fuel cycle practicality, sustainability, and economics Mass percent 3.2%
Core data
Average core height Average height of the core To know the size and power density m 4.2 m
Average core radius Average radius of the core To know the size m 4.2 m
Cycle length Average time between startup and shutdown (e.g. for maintenance and/or refueling) Needed for economics comparisons and operational assessments. Short cycle lengths are often associated with lower capacity factors months 18 months
Reflector material To know full neutronic capability, replacement rate, cost, shielding, and reflector heat removal needs Water, MgO, Beryllium
Reflector dimensions (Same as above) 20 cm average thickness
Fuel cycle
Initial fuel source Where the initial fuel is coming from in the fuel cycle To understand the fuel cycle Mined, recycled, downblended
Reload fuel source Where the reload fuel is coming from in the fuel cycle To understand the fuel cycle Mined, recycled, downblended
Thermal data
Flow rate Primary coolant flow rate To understand the energy balance and pumping requirements kg/s 17000
Primary coolant pressure The average pressure of the primary coolant. To understand the containment/cooling needs after depressurization kPa 16000 kPa
Average coolant outlet temperature Average coolant temperature at outlet of core To know which end uses of the heat are available, and to know material choices °C 500 °C
Core delta-T Change of temperature across the core To understand the heat balance, and thermal strains °C 32 °C
Shielding
Shield material(s) To know dose rates outside core, to compute activation of ex-core materials, cost, shielding cooling needs, gas generation, etc. Lead, borated water, B4C, LiH, tungsten, U-238, sand
Shield dimensions (Same as above) Multiple layers consisting of 2" of lead, 2 feet of 2% borated water, 2" of tungsten, 5 feet of concrete
Shield mass (Same as above)
Heat deposition rate in shield To determine whether or not you need to include shielding coolant lines 1 MWt
Dose rate outside shield To know how effective the shield is and how close people can be mSv/hr 6 mSv/hr
Plant data
Secondary coolant What material transfers heat between the first heat exchanger and the second heat exchanger/steam generator (if applicable)? Basic Water (PWR), sodium metal, fluoride salt
Tertiary coolant What material transfers heat between the second and third heat exchanger/steam generator (if applicable)? Basic Water, nitrate salt
Quaternary coolant What material transfers heat between the third and fourth heat exchanger/steam generator (if applicable)? Basic Water
Power cycle What kind of power conversion system is in place? To know overall capabilities Steam Rankine, Gas Brayton, Stirling, thermoelectric, endothermic chemical reaction
Safety data
Safety related generators? Whether or not the diesel generators are safety-related Determines level of passive safety Yes, No
SBO survival time How long the plant can cool itself in a station blackout without operator intervention Determines level of passive safety 2 hours, 72 hours, indefinitely
Administrative
Make Organization that designed the plant KEPCO, EDF, Westinghouse, GEH
Plant Individual plant name (if applicable), N/A if just talking about a design Diablo Canyon
Unit Individual unit name in a given plant (if applicable), N/A if just talking about a design Unit 2
Date announced When the design/reactor was first announced To know what era it's from, to make timelines 1962-05-12
Date licensed The date that license to operate was first granted (blank if none) To know if and when it has regulatory certification 1962-05-12
Date operational The date that the first of these first entered production To know if and when it started operating 1966-05-13
Availability factor Fraction of the time that the plant is not in an outage (e.g. at any power). Not to be confused with capacity factor. To know how long outages are percent 93%
Average Power Fraction Average fraction of total rated power that is generated when operational. To know how much load following is occurring, and to compute total capacity factor percent 96%

Special thanks to Brett Rampal and Adam Stein for discussing this with “us” here.

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