Very High Temperature Reactor (VHTR)

The Very High Temperature Reactor (VHTR), one of six Gen IV nuclear system candidates, is designed for electricity and heat cogeneration, and its high outlet temperature is ideal for hydrogen production and use in the chemical, oil, and iron industries. Drawing on operational experience from past or operating gas-cooled reactors in five countries, the VHTR features TRi-structural ISOtropic (TRISO) fuel, helium coolant, and low power density that supports passive decay heat removal. While the original approach for VHTR focused on very high outlet temperatures for hydrogen production, research and market assessments suggest that modest outlet temperatures (700-850°C) are sufficient and limit technical challenges associated with deployment. It also promises inherent safety, high thermal efficiency, process heat capability, low operational costs, and modular construction.

Presentation of the Very High Temperature Reactor System

Among the 6 candidates of the Gen IV nuclear systems in the technical roadmap of Gen IV International Forum (GIF), the Very High Temperature Reactor (VHTR) is primarily dedicated to the cogeneration of electricity and heat. Its high outlet temperature makes it attractive for hydrogen production as well as for the chemical, oil and iron industries. 

The reference design considered by GIF utilizes the TRi-structural ISOtropic (TRISO) particles based fuel, helium coolant, as well as the dedicated core layout and lower power density to allow removal of decay heat through passive mechanisms.

The VHTR technology benefits from the operational feedback of 40 Gas Cooled Reactors (GCR, CO2 cooled) and 7 High Temperature Gas Reactors (HTGR, He cooled) gained in 5 different countries. The VHTR has potential for inherent safety, high thermal efficiency, process heat application capability, low operation and maintenance costs, and modular construction.

Schematic view of a Very High Temperature nuclear Reactor (VHTR)
Schematic view of the Gen. IV Very High Temperature Reactor (VHTR) Nuclear Energy System

Attributes of the VHTR

The VHTR is a next step in the evolutionary development of high-temperature gas-cooled reactors (HTGRs). It is a graphite-moderated, helium-cooled reactor with thermal neutron spectrum. It can supply nuclear heat and electricity over a range of core outlet temperatures between 700 and 950°C, and potentially more than 1 000°C in the future. 

The base elements of VHTR fuel are TRISO particles, each smaller than a millimeter in diameter. Surrounding this kernel are layers of carbon and silicon carbide, providing stable containment for fission products at temperatures of 1600°C or higher. These particles can be either integrated in cylindrical compacts integrated in blocks resembling hexagonal 'prisms' made of graphite (prismatic-block type core) or encased in silicon carbide within graphite in pebbles of about 60mm in diameter (pebble-bed type core).

Although the shapes of the fuel compounds for these two configurations are different, the technical basis for both is the same: TRISO coated particles fuel in a graphite matrix, fully ceramic (graphite) core structure, helium coolant, and low power density, to achieve high outlet temperature and the retention of fission production inside the coated particle under normal operation condition and accident condition. 

As mentioned before, to allow passive decay heat removal, each core options should be kept below the following power limits: 

  • Below 625 MWth for prismatic-block type 
  • Below 250 MWth for pebble bed type

A VHTR can be designed to use alternative and symbiotic fuel cycles such as U-Pu, MOX or Th-U.

For electricity generation, a direct cycle with a helium gas turbine system directly placed in the primary coolant loop, or, at the lower end of the outlet temperature range, an indirect cycle with a steam generator and a conventional Rankine cycle can be used. Today, the direct Brayton power conversion cycle is being pursued less aggressively in favour of an indirect Rankine cycle because of its lower technological risk and increased flexibility in terms of working fluid and mission of the reactor (electricity, process heat, and co-generation).

For nuclear heat applications such as process heat for refineries, petrochemistry, metallurgy and hydrogen production, the heat application process is generally coupled with the reactor through an intermediate heat exchanger (IHX), in what is often called an indirect cycle.

VHTR Reactor Parameters

Reference Value  

SpectrumThermal 
Core Outlet Temperature rangeGraphite
Coolant700–950°C with the goal to reach above 1000° temperatures at a later stage.
Primary pressureHe Gas
Power Range10 MWth to 842 MWth
FuelUranium containing (enriched in U-235) TRISO based pebbles or compacts. A VHTR can be designed to use alternative and symbiotic fuel cycles such as U-Pu, MOX or Th-U.

How do Very High Temperature Reactors meet Generation IV Criteria?

Benefits and Applications of Very High Temperature Reactors

While the original approach for VHTR at the start of the Generation IV program focused on very high outlet temperatures and hydrogen production, current market assessments have indicated that electricity production and industrial processes based on high temperature steam that require modest outlet temperatures (700-850°C) have the greatest potential for application in the next decade while also reducing the technical challenges associated with higher outlet temperatures. As a result, the focus has moved from higher outlet temperature designs such as in GT-MHR and PBMR reactors to lower outlet temperature designs such as HTR-PM in China and the NGNP in the US. These experiences should nevertheless pave the way for higher output temperature capable materials in the future. 

Most of the benefits and advantages of VHTR designs have been listed in the above description of how those reactors meet the Generation IV criteria. Yet the potential, opened by very high outlet temperatures of VHTRs, for an expanded range of applications of nuclear heat cannot be understated.

Main Challenges for the Deployment of VHTR

Demonstrating the viability of the VHTR core requires solving several significant technical challenges. Fuels and materials must be developed that: 

  • permit an increase of the core-outlet temperatures from around 800°C to more than 1 000°C for the entire plant lifetime; 
  • permit the maximum fuel temperature under accident conditions to reach levels approaching 1 800°C; 
  • permit maximum fuel burnup of 150-200 GWd/tHM; 
  • avoid power peaking and temperature gradients in the core, as well as hot streaks in the coolant gas; 
  • limit structural degradation from air or water ingress. 

An important amount of work is needed regarding the use of the reactor heat for industrial processes and the coupling of a nuclear plant with other industrial facilities. 

Process-specific R&D gaps need to be filled to adapt the chemical process and the nuclear heat source to each other regarding temperatures, power levels and operational pressures. Heating of chemical reactors by helium is a departure from current industrial practice and needs specific R&D and demonstration. The development of an intermediate heat exchanger, ducts, valves and associated heat transfer fluid is needed to deliver process heat to many of the chemical processes. The viability of using nuclear process heat to produce hydrogen needs further study. Any contamination of the product will have to be avoided. Development of heat exchangers, coolant gas ducts and valves will be necessary for isolation of the nuclear island from the production facilities. This is especially the case for isotopes like tritium, which can easily permeate metallic barriers at high temperatures. Over the past two decades, significant advances have been made in the key technologies necessary to deploy a VHTR. Further R&D is needed, and GIF is actively participating to deploy VHTR.

How is GIF working to solve those

In it’s 2014 Technology Roadmap GIF identified the following areas of R&D as priorities for the VHTR system:

  • Fuels and Materials
  • Reactor Systems
  • Balance of Plant
  • Fuel Cycle
  • Economics
  • Safety Objectives

To know more please consult the 2014 GIF Technology Roadmap.

GIF has established System Steering Committees (SSC) to implement the research and development (R&D) for each Generation IV Reactor Concept, with participation from GIF Members interested in contributing to collaborative R&D efforts on the said system. Each SSC plans and integrates R&D projects contributing to the development of a system. 

Each SSC is governed by a System Arrangement (SA), provisional SSCs are created under a Memorandum of Understanding (MoU). 

Several projects can be carried within an SSC, each project is government by a Project Arrangement (PA) and managed by a Project Management Board (PMB).

The GIF VHTR SSC was established in 2006. To effectively support the development of the VHTR technology/system and to overcome jointly the challenges identified on the path to do so, the GIF coordinates several international R&D activities related to VHTR system. They are now arranged by the VHTR SSC Signatories into four joint R&D projects.

VHTR Projects History - Past projects

The VHTR technology benefits from the operational feedback of 40 Gas Cooled Reactors (GCR, CO2 cooled) and 9 High Temperature Gas Reactors (HTGR, He cooled) gained in 7 different countries (China, France, Germany, Japan, Spain, UK, US). 

The High-Temperature Gas-Cooled Reactor (HTGR) has a history spanning over 50 years, marked by successes and challenges. The initial experimental plants included a 20 MWe unit in the UK and a 15 MWe unit in Germany, operational in 1966. The German project, intended to burn thorium, faced political and technical obstacles, leading to its abandonment in 1988.

In the U.S., two commercial power-producing HTR plants were constructed. The first, a 200 MWth/115MWth experimental unit at Peach Bottom, operated from 1967 to 1974, providing valuable insights. These findings contributed to the development of a larger 330 MWe plant at Fort St Vrain, active from 1979 to 1989. While this reactor demonstrated the technical feasibility of HTRs, further development was needed to establish their economic viability.

Slide presenting past HTGR VHTR nuclear reactors
Test Reactors and Prototypes - HTGRs/VHTRs

Project/Program

Country

Power Rating

Timeframe

Notes

High-Temperature Gas-Cooled Reactor (HTGR)University of Chicago, USN/A1944 - 1947Initial proposal of HTGR
Dragon ReactorUnited Kingdom21.5 MWth1965 - 1976Exploring use of TRISO fuel
Peach Bottom Unit 1 ReactorUnited States200 or 115 MWth1966 - 1974First HTGR to produce electricity
AVR ReactorGermany46 MWth1967 - 1988Exploring use of carbide BISO, and TRISO particle fuel
Fort St. Vrain Generating StationUnited States842 MWth1979 - 1989Load-following power plant
THTR-300Germany750 MWth1983 - 1989Prototype reactor using TRISO fuel
HTR-10China10 MWthFirst criticality in 2000 - Stopped in 2019Prototype reactor of pebble-bed HTGR

VHTR Current Developments

Operating VHTR Reactors

Three VHTR/HTGRs plants are currently operating with the HTR-PM being a commercial scale nuclear power plant with two reactors feeding one turbine. 

Project NameCountryPower RatingExpected Deployment DateNotes
High-Temperature Engineering Test ReactorJapan30 MWthFirst criticality in 1998, operated 50 days in 2010, restarted in July 2021.Outlet temperature of 950°C was reached at full thermal power in 2004 for the first time in the world.
One of the current focuses is demonstrating elements related to coupling with H2 production. 
HTR-PM (Shidao Bay 1)China250 MWth/reactor

200MWe net output for the whole plant with two reactors feeding one turbo-generator.
Criticality 2021 / Commercial operation December 2023

Scaled-up version of

HTR-10
Two reactors feeding one turbine. 

Status of VHTR designs

Project/ProgramCountryPower RatingExpected Deployment DateNotes
GTHTR300CJapan Atomic Energy Agency, Japan

600 MWth

274 MWe

Conceptual45~50% thermal efficiency
PMBRPebble Bed Modular Reactor (Pty) Limited / Eskom, South Africa 400 MWth / 200MWeOn HoldInternational Cooperation design team, aimed for industrial application
Prismatic HTRGeneral Atomics, US

350 MWth

150 MWe

Under DesignTRISO particles, assembling in 2 steps.
SC-HTGRFramatome, France/USA

625 MWth

272 MWe

Not knownBased on HTR-MODUL
U-BatteryUrenco, UK-Germany-The Netherlands

10 MWth

4 MWe

Cancelled in 2023Based on Dragon reactor/Fort St. Vrain reactor
USNC MMRUltra Safe Nuclear Corporation , US

10~45 MWth

3.5~15 MWe

Pre-licensing steps carried out in Canada (VDR 1 done, VDR 2 ongoing, application for a license to prepare the site)

Micro reactors,

Ceramic core design,

Heat storage to allow for flexibility of output without having to cycle the reactor

Holos-Quad HTRHologen, US Military

22 MWth

10 MWe

Under development with ANLTransportable in a 40-foot shipping container
Adams EngineAdams Atomic Engines10 MWeDesign shelvedLiquid nitrogen coolant
HTMR-100Steenkampskraal Thorium Limited, South Africa

100 MWth

35 MWe

ConceptualVery strong negative temperature coefficient
GT-MHRCooperation between the USA and Russia

600 MWth

285 MWth

On hold before constructionMulti-layer ceramic coating fuel
MIGHTR - Modular Integrated Gas High Temperature ReactorBoston Atomics, US20-600MWthIn development since 1984 

References

2023, Handbook of Generation IV Nuclear Reactors, Second Edition, Dr. Igor L. Pioro, Handbook of Generation IV Nuclear Reactors | ScienceDirect

International Atomic Energy Agency Advanced Reactors Information System (ARIS) Online Database

News related to the VHTR System

December 2023, China's demonstration HTR-PM enters commercial operation - https://www.world-nuclear-news.org/Articles/Chinese-HTR-PM-Demo-begins-commercial-operation

August 2023, Fuelling high temperature SMRs - https://www.neimagazine.com/features/featurefuelling-high-temperature-smrs-11110962/ -

May 2021, UK / Nuclear Company Announces HTGR Collaboration With Japan - https://www.nucnet.org/news/nuclear-company-announced-htgr-collaboration-with-japan-5-4-2021 ,  

August 2020, USNC, Korean companies to develop micro modular reactorshttps://www.world-nuclear-news.org/Articles/USNC-Korean-companies-to-develop-micro-modular-rea

November 2015, VHTR cooling system performance verifiedhttps://www.world-nuclear-news.org/Articles/VHTR-cooling-system-performance-verified