ASME STP-NU-038-2010 — ASME Code Considerations for the Intermediate Heat Exchanger (IHX)

⚛️ ASME STP-NU-038-2010 is one of the “STP-NU” nuclear-technology reports published by ASME Standards Technology, LLC (ASME ST-LLC), prepared under sponsorship of the U.S. Department of Energy (DOE) and ASME ST-LLC. The report addresses ASME Code considerations for the Intermediate Heat Exchanger (IHX) intended for high-temperature gas-cooled reactor (HTR) and very-high-temperature reactor (VHTR) applications. It systematically reviews the IHX concepts that could be envisioned for service in the range from 850 to 950 °C, assesses their engineering maturity, identifies candidate materials, and outlines the issues that must be resolved before the IHX can be covered by formal ASME Code rules. The work was performed in support of the Next Generation Nuclear Plant (NGNP) project, with operation expected in the 2018–2021 timeframe.

The IHX is the critical heat-transfer link between the primary helium circuit of a VHTR and a secondary loop used for process-heat applications such as hydrogen production or electricity generation. Primary helium heated by the core to 850–950 °C enters the IHX and transfers heat to secondary helium at about 500 °C. Because the operating temperature lies at the upper end of the creep regime for most structural materials, the design, material selection, and code requirements for the IHX differ fundamentally from those of conventional heat exchangers — which is exactly the concern of this report.

1. Review of IHX Concepts 📌

Within the 850–950 °C range, the report reviews two broad families of IHX design: shell-and-tube (including helical-coil) arrangements and compact designs (including the plate-fin concept).

In the Tubular Helical Coil Heat Exchanger (THCHE), hot helium (850–950 °C) enters through the bottom of the IHX and heats the coiled tubes as it flows upward, then exits through a peripheral annulus, while relatively cold helium (about 500 °C) runs downward through the tubes from the top, is collected in the hot header (hot gas collector), and passes back out through an outlet at the top. The tubes are coiled to maximize heat-transfer area while minimizing the stress caused by thermal expansion. One end of each tube is attached to the tube plate and the other end to the hot header; spacer grids keep the pitch between tubes even. Representative examples include the AREVA KVK helical-coil IHX and the tubular IHX of Japan’s High Temperature Engineering Test Reactor (HTTR).

Compact designs — the plate-stamped heat exchanger (PSHE), the plate-machined heat exchanger (PMHE), and the plate-fin heat exchanger (PFHE) — offer very high heat-transfer surface per unit volume, which is attractive for large heat fluxes in high-temperature, high-pressure helium. A two-stage IHX concept is also discussed, splitting the temperature range to reduce the material and structural difficulty of any single unit.

2. Candidate Materials and Key Issues 🔬

The report first collects the material recommendations of reactor vendors and then reviews other alternatives. Because the service temperature is at the upper end of the creep regime, the primary selection criteria are long-term creep-rupture strength, creep-fatigue interaction, and microstructural stability at 850–950 °C. Typical candidates include nickel-based high-temperature alloys such as Alloy 617 and Hastelloy XR, as well as austenitic alloys such as Alloy 800H for the lower-temperature portions of the duty.

Key issues that must be addressed in material selection include:

• Creep rupture and creep-fatigue: the design life must simultaneously cover long-term creep damage and fatigue damage from startup–shutdown cycling, with due allowance for their interaction;
• Weldability: the quality of heavy-section, thick-wall, and dissimilar-metal welds governs IHX reliability, and welding must be followed by stringent nondestructive examination;
• Microstructural stability and thermal aging: long-term service precipitates phase evolution, grain-boundary weakening, and loss of ductility;
• Corrosion: trace impurities in the helium circuit (oxygen, moisture, hydrocarbons) can cause oxidation, carburization, or decarburization, which must be controlled by purification and online monitoring.

3. ASME Code (Codification) Considerations 📐

Part II discusses the key features a construction code must have: design rules, time-dependent material property data, fabrication, examination, testing, and quality assurance. The report reviews the history of high-temperature code development in the United States (ASME Section III Subsection NH evolved from earlier Code Cases such as 1592) as well as high-temperature code experience in Germany, and notes that the existing ASME NH was developed primarily for liquid-metal fast breeder reactor components. It may not directly cover IHX thin-wall helical coils, compact plate structures, and the helium service environment.

Specific codification issues to be addressed include: acceptable materials of construction, maximum temperature, maximum design life, nonclassical creep behavior, environmental effects, and possible new failure modes such as embrittlement and loss of ductility after long-term service.

4. Engineering Practice Essentials 🛠️

• 🔬 Long-term data first: before code adoption, complete the 850–950 °C creep-rupture, creep-fatigue, and aging database for candidate materials, with a unified data-evaluation methodology.
• 🧪 Component verification: prove coil forming, brazing/diffusion bonding, and weld quality through subscale or full-scale prototype fabrication and testing.
• 💨 Environmental control: maintain strict impurity limits in the helium circuit with periodic sampling, to prevent oxidation, carburization, or decarburization.
• 🔄 Combined creep-fatigue assessment: evaluate combined damage from startup–shutdown cycles, load-following, and steady high-temperature operation against the design life.
• 📋 Transitional provisions: until formal code rules are published, ASME Code Cases can provide interim design rules so that lead projects can proceed.

5. Frequently Asked Questions ❓