Heat Pipe Heat Exchanger for Nuclear Reactor Cooling

Heat pipe heat exchangers (HPHX) are core passive thermal management equipment for small modular reactors (SMRs), micro nuclear reactors, conventional PWR/BWR passive safety systems, space nuclear reactors, and spent fuel storage cooling. They rely on phase-change heat transfer without pumps, delivering inherent safety against station blackout accidents post-Fukushima.

1. Core Working Principle for Nuclear Service

Nuclear-grade heat pipes are fully hermetic, radiation-resistant vacuum metal tubes loaded with high-temperature working fluid:

Evaporator Section (Reactor Heat Source)

Embedded inside reactor core / spent fuel pool / containment: fission decay heat vaporizes internal liquid working fluid, absorbing massive latent heat.

Vapor Transportation

High-temperature vapor flows to the condenser side with ultra-low thermal resistance.

Condenser Section (Heat Sink Side)

Vapor condenses, releasing heat to secondary coolant (air, SCO₂, molten salt, cooling water).

Liquid Return

Condensed liquid flows back to evaporator via capillary wick or gravity; fully passive circulation with zero external power, no rotating machinery.

Partition plates fully isolate radioactive primary medium from secondary cooling loop, minimizing tritium leakage risk.

2. Two Main Classification for Nuclear Cooling Scenarios

2.1 Integral In-Core Alkali Metal Heat Pipe (Heat Pipe-Cooled Micro/SMR)

Directly inserted into reactor monolith core, the mainstream design for solid-state micro-reactors (NASA Kilopower, LANL SPR, domestic small nuclear battery)

Operating temperature: 500–1100℃

Working fluid: Sodium, potassium, lithium (alkali metals)

Shell material: Alloy 800H, 316H stainless steel, tantalum cladding (radiation hardening)

Layout: Hexagonal core lattice, each heat pipe surrounded by fuel pellets; independent single-pipe operation

Function: Directly extract core fission heat to Brayton/Stirling power conversion cycle, eliminate primary coolant pumps entirely

2.2 Separated Loop Heat Pipe Heat Exchanger (Passive Safety for Conventional PWR/SMR)

Two separated chambers (radioactive evaporator + non-radioactive condenser) connected by vapor/liquid pipelines, widely used for residual decay heat removal (DHR) in Linglong One ACP100 and AP1000 passive containment cooling

Low-temperature type (spent fuel pool cooling, 80–250℃): Deionized water, ethanol as working fluid, titanium/304SS shell

Medium-temperature type (containment residual heat, 200–450℃): Ammonia, mercury substitute

Key advantage: Radioactive zone and heat sink zone are physically separated, easy maintenance without radiation exposure