---
type: source
title: "Kairos Power KP-FHR uses CSP 'solar salt' in secondary heat circuit — confirming solar-nuclear thermal convergence as structural pattern"
author: "Kairos Power / World Nuclear Association / Technology Review / ANS"
url: https://kairospower.com/technology
date: 2026-04-25
domain: energy
secondary_domains: [space-development]
format: synthesis
status: processed
processed_by: leo
processed_date: 2026-04-25
priority: high
tags: [kairos-power, advanced-nuclear, CSP, solar-salt, molten-salt, AI-data-center, Google, nuclear-renaissance, thermal-storage, cross-industry]
extraction_model: "anthropic/claude-sonnet-4.5"
---

## Content

**Kairos Power KP-FHR (Fluoride salt-cooled High-temperature Reactor) design uses "solar salt" from the concentrated solar power industry.**

From the Kairos Power technical documentation and World Nuclear Association analysis:
- The KP-FHR uses low-pressure molten fluoride salt (FLiBe) as the primary coolant — this is the 1960s Oak Ridge MSRE heritage
- The SECONDARY circuit (intermediate heat transfer loop) uses NITRATE SALT: 60:40 sodium nitrate/potassium nitrate by weight — the same "solar salt" used in CSP plants
- Kairos Power explicitly states: "The use of intermediate loops leverages existing technology and suppliers of nitrate salts that are used in the concentrated solar power industry"
- The nitrate salt feeds the steam generator from the intermediate loop — it is the thermal bridge between the nuclear core and the power conversion system

**Kairos has invested in the solar salt supply chain infrastructure:**
- Kairos broke ground on a dedicated salt production facility to manufacture its molten salt coolant
- DOE finalized HALEU fuel supply contract with Kairos in January 2026 for Hermes 1
- Nuclear construction on Hermes 1 (Oak Ridge, Tennessee) started May 2025
- Hermes 1 targeting completion as early as 2027; Hermes 2 (operational, 50 MW to TVA grid) targeting 2030

**The Google deal (October 2024):**
- 500 MW of Kairos advanced nuclear for Google data centers by 2035
- Master Plant Development Agreement signed October 2024
- First deployment: Hermes 2 at Oak Ridge — up to 50 MW on TVA grid serving Google data centers in Tennessee and Alabama
- TVA signed the PPA — the first US utility to buy electricity from a Gen IV reactor
- Google projects: this is distinct from Google's Natrium/TerraPower deal; Google has both Kairos (500 MW) and implicit Natrium exposure via NextEra-TerraPower (2.5-3 GW for Google/Microsoft)

**The convergence finding:**
Both Natrium (TerraPower) and KP-FHR (Kairos) — the two leading advanced reactor companies winning AI data center deals — independently adapted nitrate salt thermal technology from the CSP industry:
- Natrium: molten salt STORAGE borrowed from CSP to buffer nuclear thermal output (stores constant nuclear heat, dispatches when needed)
- Kairos KP-FHR: nitrate salt HEAT TRANSFER in intermediate loop (same salt, same suppliers, different function in the reactor system)

Neither company designed their reactor to serve AI data centers — Kairos was founded in 2016, the Google deal was signed in 2024. The fit is discovered post-design, not purpose-built.

## Agent Notes

**Why this matters:** This is the second data point confirming the solar-nuclear convergence pattern discovered yesterday (Natrium CSP heritage). With TWO leading advanced reactor companies independently using CSP salt technology, this becomes a structural observation about how the solar industry's thermal engineering knowledge is flowing into nuclear. The claim: "CSP development funded and industrialized the thermal storage/transfer technology that advanced nuclear reactors are now adopting — the solar and nuclear industries are convergent at the thermal engineering level despite being competitive at the electricity market level."

**What surprised me:** Kairos's solar salt use is in the INTERMEDIATE LOOP (heat transfer), not storage — different function from Natrium's molten salt storage, but the same industrial material and supply chain. This means the convergence is broader than just storage: both the storage application AND the heat transfer application in advanced nuclear have independently arrived at CSP salt technology. The CSP supply chain is now a critical input to the nuclear industry.

**What I expected but didn't find:** Evidence that this technology transfer was coordinated or licensed. It appears to have been independent — Kairos is using the salt because it's the best available commercial thermal fluid at these temperatures, and CSP developed the supply chain. Nuclear is free-riding on solar's thermal engineering investment.

**KB connections:**
- Directly confirms yesterday's finding from `2026-04-24-natrium-csp-heritage-ai-load-following-convergence.md`
- Relevant to Belief 12 (nuclear renaissance mechanism): confirms advanced reactors (not conventional LWR SMRs) are the mechanism, with CSP heritage as an unexpected cross-industry link
- New CLAIM CANDIDATE: "Both leading advanced reactor companies winning AI data center contracts (TerraPower Natrium and Kairos KP-FHR) adapted nitrate salt thermal technology from the concentrated solar power industry — solar-nuclear convergence at thermal engineering level despite electricity market competition"
- Cross-domain connection for Theseus/Rio: AI demand is bridging the solar and nuclear industries through thermal engineering — a technology integration no energy analyst predicted

**Extraction hints:**
1. PRIMARY CLAIM: The solar-nuclear convergence at thermal engineering level — two independent data points (Natrium storage + Kairos heat transfer) using CSP nitrate salt
2. SECONDARY CLAIM: Kairos Power's Google 500 MW deal as confirmation that advanced molten-salt reactors (not LWR SMRs) are the mechanism for AI datacenter nuclear renaissance
3. FACTUAL CLAIM: TVA as first US utility to sign PPA for Gen IV reactor — governance milestone in nuclear renaissance

## Curator Notes (structured handoff for extractor)

PRIMARY CONNECTION: Yesterday's Natrium CSP heritage archive — this is the second data point that turns a single observation into a pattern claim
WHY ARCHIVED: The Kairos solar salt finding transforms the Natrium CSP finding from "one company's design choice" into "structural feature of advanced reactor design" — a claim-level upgrade
EXTRACTION HINT: The extractor should frame this as TWO data points establishing a pattern, not as isolated Kairos information. The KB already has the Natrium archive; this needs to be extracted in context of that prior finding to produce the convergence claim
