Episode:
110
HDR Grosswelzheim
Country:
West Germany
Years of Operation:
1969–1971
Category:
Prototype & Demonstration
Reactor Type:
BWR
Coolant:
Light Water / Steam (superheated)
Fuel Type:
Enriched Uranium
Moderator:
Light Water
Thermal Power (MWth):
100
Electrical Power (MWe):
100
Status:
Prototype & Demonstration
timeline
First Criticality Year
1969
Commercial Op Year
Shutdown Year
1971
Lessons Learned
1. Efficiency Isn’t Free
- Chasing marginal efficiency gains with superheated steam looked brilliant on paper—but came with material limits, thermal stress, and complexity that erased the upside.
2. Simple Wins the Marathon
- While these hot rods flamed out, conventional BWRs and PWRs kept producing. Reliability, not elegance, wins in the long run.
3. Materials Set the Ceiling
- You can design for higher temperatures all day—but if your fuel and cladding can’t survive the environment, the reactor won’t either.
While the rest of the industry stuck with proven designs, these three chased superheated glory and delivered "meh" results.
No meltdowns. No headlines. Just three prototypes that quietly proved a hard truth: sometimes the simplest path is the reliable reactor that actually stays on the grid.
sources
ARTICLE
HDR Großwelzheim; The Bavarian Superheated Steam Hot Rod That Joined the Early Retirement Club (Forgotten Reactors Episode #110)
Germany tried to push a BWR to superheated glory for extra efficiency, but like its American cousins Pathfinder and BONUS, the fancy upgrade proved too hot to handle – and the whole club ran out of steam fast.
Picture a sleepy corner of Bavaria in the late 1960s: Karlstein am Main, right next door to Germany’s first commercial reactor. Engineers weren’t content with ordinary boiling water.
They built the Heißdampfreaktor (Hot Steam Reactor or HDR) Großwelzheim – a 25 MWe prototype that didn’t just boil water; it superheated the steam right in the core for that extra thermal kick. Think of it as taking a reliable sedan and bolting on a turbo: clever on paper, but the engine had other plans.
Construction began in 1965 with criticality on October 14, 1969 and grid connection the same day. Commercial operation followed in August 1970.
On paper, German engineering at its boldest. In reality? It lasted just 18 months before permanent shutdown on April 20, 1971. Total generation: 6.2 GWh—barely a blip.
The culprit? Fuel-element defects—cladding cracks, warping, the works. Full power wasn’t just difficult, it was risky. Like a gourmet oven that fails every time you turn up the heat.
It wasn’t alone. The U.S. chased the same superheated-steam dream with Pathfinder in South Dakota and BONUS in Puerto Rico.
All promised hotter, drier steam and better efficiency… and all hit the same wall: cladding erosion, corrosion, and fuel integrity breakdowns at elevated temperatures. Pathfinder limped along before costs and superheater issues killed it. BONUS shut down in 1968. Three reactors, same ending. Great in the brochure. Rough on the road.
After shutdown, HDR found a second life. From the mid-1970s into the early ’90s, it became a non-nuclear testbed for pipe ruptures, earthquake simulations, and safety experiments.
Dismantling wrapped up by the late 1990s. Today, the site is clean, quiet, and forgettable—a Bavarian hillside with no hint of its brief nuclear ambition.
SLIDE DECK
Related Reactors
BONUS BWR Puerto Rico
BWR
1964-1968
South of Rincón, Puerto Rico—tucked behind surf breaks and palm trees—sits a lonely concrete dome that once symbolized the atomic future of the Caribbean. Today, it’s a quiet relic. But in the early 1960s, that dome housed one of the most ambitious nuclear experiments ever attempted.
Prototype & Demonstration
