High temperature gas cooled reactors (HTGRs, see a brief description in the box 4.2) were operated in the past in the United Kingdom, the United States and Germany, and there are currently two small operating experimental reactors of this type in China (HTR-10) and in Japan (HTTR). The previous operating experience, cumulatively stretching from 1965 to 1989 [4.1], is probably too dated to be judged according to the current regulatory norms or safety standards. In 2010 there were no operating commercial reactors of this type anywhere in the world.
Basic characteristics of the HTGR designs considered in this report are given in Table 4.4[24] [25]. All HTGRs are helium cooled reactors. The PBMR appeared to be a promising concept in an advanced development stage, with targeted deployment date in South Africa set for 2013. However in 2010 the vendor company — PBMR Pty — suffered from financial difficulties with the government no longer supporting the project. By that stage they had started to develop an indirect cycle HTGR similar to the Chinese HTR-PM.
As will be discussed in more detail in Section 8.5, all HTGR safety design concepts provide for passive decay heat removal to the outside of the reactor vessel. In view of this, with the currently known reactor vessel materials it appears that ~600 MWth is an upper limit of the unit size for HTGRs, which means that all HTGRs would fall into the SMR category.
Plant configuration with direct Brayton cycle is employed in all of the designs of Table 4.3, except the Chinese HTR-PM which is an indirect cycle HTGR employing the steam generators and a
Rankine cycle with reheating for power conversion. The indirect cycle efficiency of the HTR-PM is also remarkably high, 42%, due to steam reheating.
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Table 4.4. Basic characteristics of advanced SMR designs — high temperature gas cooled reactors
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SMR Design Principal designer, Country
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Development status (2010)
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Thermal/Electric output, MW (gross)
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Availability/
Plant
lifetime
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Construction period/ Land-based or floating
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Mode of refuelling/ Refuelling interval
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Mode of
deployment/ Plant configuration
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HTR-PM,
INET, Tsinghua University, China [4.1]
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In licensing, in
construction
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250/105 per module
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85%/ 40 years
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48 months
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On line pebble transport
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Concentrated/ Two-module plants, Multi-module plants as an option
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PBMR
(previous design) PBMR Pty, South Africa [4.1]
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Stalled
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400/182 per module
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> 95%/35 years
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FOAK plant: 30-34 months; Commercial plant: 24 months
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On line pebble transport
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Concentrated/ Four — and 8- module plants
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GT-MHR GA, USA, OKBM Afrikantov, Russia [4.1,4.11]
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Design development in progress (at a slow pace)
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600/287.5
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>85%/ 60 years
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First module: 36 months
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In batches/15 months
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Distributed or concentrated/ Single or multimodule plants
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GTHTR300 JAEA, Japan [4.10]
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Design development in progress
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600/274
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90%/ 60 years
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Not specified
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In batches/24 months
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Distributed or concentrated/ Single or multimodule plants
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Because the high-power Brayton cycle gas turbines are currently not available from the industry, the indirect cycle HTR-PM appears today as a leader among all HTGRs, with the construction related actions and licensing started in China, see Section 4.4.
When high temperature non-electric applications are targeted, the HTGR design includes an intermediate heat exchanger to deliver heat to process heat application systems. Because of high temperatures (up to 850-900oC), HTGRs appear to be the only SMR technology line for which complex co-generation is considered, such as, for example, electricity generation with co-production of hydrogen and use of reject heat for seawater desalination.
The main technical characteristics of HTGR SMRs considered are the following:
• All HTGR designs target availability factors of more than 85%. The plant lifetime is typically 60 years for HTGRs with pin-in-block (non-moveable) fuel design and 35-40 years for those with pebble bed (moveable) fuel design.
• On-line refuelling is used in the pebble bed designs (HTR-PM and PBMR [previous design]), while the pin-in-block designs use partial refuelling in batches. [26]
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• The diameter and height of the reactor vessels for all HTGRs are typically within the ranges 6.5-8 m and 23-31 m, correspondingly. In all designs the containment is provided by a single or double walled citadel of the reactor building. The containment secures a path for helium release as a safety action in overpressure accidents, see Section 8.5.
• The plant surface area, specified only for the PBMR (previous design), is remarkably small — 11 639 m2 for an 8-module plant of 1 320 MWe.
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Box 4.2. High temperature gas cooled reactors
Historically, HTGRs have been considered primarily for high temperature non-electrical applications, such as hydrogen production or coal gasification, etc. For this purpose, all HTGR designs employ tri-isotropic (TRISO) fuel: Tiny (typically, less than 1 mm in diameter) ceramic fuel kernels with multiple ceramic coatings (typically, several pyrocarbon layers and a silicon carbide layer). TRISO fuel has a proven capability to confine fission products at high temperatures (up to 1 600oC in the long-term) and operate reliably at very high fuel burn-ups up to 120 MWday/kg [4.1].
There are two basic modes of TRISO fuel used in HTGRs. In one case coated particles are embedded in graphite matrix to form spherical fuel elements continuously moving through the core (pebble bed fuel used in the HTR-PM and the PBMR [previous design]), in another — similar coated particles are embedded in graphite matrix to form fuel pins to be fixed in dedicated holes located in the graphite moderator (“pin-inblock” fuel used in the GTHTR300 and the GT-MHR). In both cases the core has an annular shape with central and radial graphite reflectors. This configuration improves the power distribution allowing for a higher thermal output and a higher average fuel burn-up.
The use of TRISO fuel in HTGRs of any fuel design contributes to a low volumetric power density in the reactor core, 6-7 MW/m3 [4.1], which is a factor negatively affecting the economy of the plant. To face this, a direct Brayton cycle is being traditionally considered for HTGRs, employing a compressor and horizontal or a vertical shaft gas turbine (see Figure below). Energy conversion with Brayton cycle may offer cycle efficiencies of up to 45-48% (against 32-34% in PWRs) at 750-950oC core outlet helium temperature, contributing to an improved plant economy.
Conceptual layout of the PBMR (previous design) primary system [4.10]
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