Gravity
About
The Gravity is a microreactor based on PWR technology that will be deployed in a borehole approximately one mile underground. This innovation allows for robust containment, constant pressure, and embedded waste management. The reactor operates at 15 MWe and is designed for small-scale electricity production, but it can also be used for cogeneration, including desalination or district heating.
| Developer | Deep Fission |
|---|---|
| Country of Origin | United States |
| Size | Micro |
| Type | Pressurized Water Reactor (PWR) |
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Analysis
3
Deployment Timescale
Score Justification
The Gravity is in the pre-application stage of the regulatory process. It has a fully developed supply chain because it is based on PWR technology. Once the reactor is licensed, construction would be shaped by the need to drill and prepare a deep borehole, which introduces site-specific scheduling considerations. Deep Fission explicitly states that the Gravity is deployed only in stable crystalline basement rock.
By indicator
- 1/4 Regulatory Engagement
To what extent has the reactor developer engaged with a recognized nuclear regulatory authority in the licensing process? (30% of total score) - 4/6 Technology Precedent
Has the reactor design, or a sufficiently similar design, been certified anywhere in the world? (10% of total score) - 2/3 Modularity
What share of total reactor systems can be manufactured off-site in controlled factory environments rather than constructed on-site? (15% of total score) - 4/4 Specialization
To what extent do construction activities and components require lengthy qualification processes? (15% of total score) - 5/5 Supply Chain
How mature and available are suppliers for key reactor components and fuel services? (30% of total score)
5
Overnight Cost
Score Justification
Overnight costs for the Gravity are shaped less by large reactor components and more by site preparation. The fueled reactor module is manufactured off-site and deployed underground, and surface balance-of-plant requirements are minimal. However, the need for deep borehole drilling represents a specialized and site-intensive construction activity that may offset some of the savings from the compact, factory-built design.
By indicator
- 4/4 Component Cost
What is the expected cost of the reactor’s major components? (40% of total score) - 5/6 Construction Cost
To what extent does the design reduce construction cost and risk through modular fabrication and limited nuclear-grade specialization? (60% of total score)
5
Operational Cost
Score Justification
The Gravity’s high Operational Cost score is driven by simplified waste management because the core and its waste could remain in the borehole at the end of the reactor’s life. This design also has the potential to lead to lower maintenance, staffing, and decommissioning costs.
By indicator
- 3/3 Fuel Cost
What is the estimated cost of nuclear fuel per unit of electricity generated, including enrichment, fabrication, and back-end costs? (15% of total score) - 4/4 Maintenance Cost
What is the expected annual maintenance cost for the reactor and balance of plant systems, including consumables? (25% of total score) - 5/5 Staffing Level
How many full-time personnel are required to safely operate and maintain the reactor unit? (40% of total score) - 4/5 Spent Fuel & Radioactive Waste Management Cost
What are the expected operational costs associated with managing spent fuel, including interim storage, transport, disposal, or recycling? (10% of total score) - 5/5 Decommissioning Cost
What are the total lifetime contributions required for decommissioning, regardless of funding mechanism? (10% of total score)
2
Cost Predictability
Score Justification
While Cost Predictability for the Gravity remains limited due to the lack of a prototype, Deep Fission intends to complete a demonstration reactor through the Reactor Pilot Program. Siting costs associated with drilling the borehole could vary.
By indicator
- 0/5 Prototype
To what extent has the reactor design been built, demonstrated, or commercially deployed in practice? (75% of total score) - 2/3 Modularity
What share of total reactor systems can be manufactured off-site in controlled factory environments rather than constructed on-site? (25% of total score)
5
Security
Score Justification
Deep Fission has incorporated security by design by placing the reactor in a sealed vessel in a borehole about one mile underground. The reactor uses LEU fuel, and its thermal spectrum is not optimized to produce weapons-usable nuclear material.
By indicator
- 3/3 Fuel
What is the enrichment level and composition of the reactor fuel? (40% of total score) - 4/4 Nuclear Material Production
What is the potential for the reactor to produce weapons-usable nuclear material? (40% of total score) - 1/1 Security by Design
Has the reactor developer built in security by design? (20% of total score)
2
Safety
Score Justification
Gravity does not have an approved safety case for commercial operation. Shutdown is achieved through control rod insertion supported by inherent negative reactivity feedback. The design does not rely on fuel with additional inherent safety characteristics beyond those typical of light-water reactor technology. It operates at high pressure with a single containment boundary in the form of the borehole. Passive decay heat removal is enabled for extended durations through geologic heat dissipation.
By indicator
- 0/2 Safety Case
How mature and publicly established is the reactor’s safety case with the regulator? (40% of total score) - 1/2 Shutdown Mechanism
How diverse, independent, and passive are the reactor’s shutdown systems? (20% of total score) - 0/1 Fuel With Safety Characteristics
Does the reactor use fuel with accident tolerance or inherent safety characteristics? (10% of total score) - 3/4 Pressure & Containment
How well does the reactor’s containment strategy protect from the release of radioactive material? (10% of total score) - 3/3 Passive Heat Removal
How long can the reactor remove core heat without operator intervention? (10% of total score) - 3/4 Coolant Reactivity
How chemically reactive is the reactor coolant? (10% of total score)
4
Spent Fuel & Radioactive Waste Management
Score Justification
The Gravity benefits from simplified spent fuel management due to emplacement in the borehole, which also minimizes the necessity for on-site storage. Its spent fuel form has been licensed and qualified for disposal in multiple countries.
By indicator
- 1/1 Spent Fuel Licensing Precedent
Has the spent fuel form been previously licensed for disposal? (20% of total score) - 3/4 Waste Streams
How many distinct waste streams require separate conditioning or handling pathways? (20% of total score) - 3/3 On-Site Storage
How much on-site area is required for interim spent fuel storage? (10% of total score) - 2/3 Spent Fuel Volume
What volume of spent fuel is produced per unit of electricity generated? (15% of total score) - 1/2 Decay Heat
What is the decay heat output of spent fuel at the 50-year interim storage milestone? (20% of total score) - 2/2 Time to Interim Storage
What is the average time until spent fuel can be transferred to interim storage? (15% of total score)
5
Supply Chain
Score Justification
The Gravity’s supply chain relies on commercially established LWR fuels and materials, allowing it to tap into existing global manufacturing capacity rather than requiring new, purpose‑built supply chains.
By indicator
- 2/2 Key Component Availability
To what extent are commercial or pilot-scale suppliers available for the reactor’s major components? (60% of total score) - 4/4 Fuel Availability
Are suppliers available for both fuel fabrication and enrichment required by the reactor design? (40% of total score)