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Coolant level monitoring is necessary for nuclear reactor safety and reliability, and the current need is to monitor small modular reactor (SMR) designs.  Safety and reliability are compromised by using current invasive sensors, such as pressure gauges, that penetrate the reactor and containment vessels (RPV and CNV).  ISL has invented a non-invasive sensor (GraviSense™) that can measure coolant level to 3 cm accuracy using super-sensitive gravimeters that sense the mass distribution of the coolant from outside of both vessels.  Because GraviSense™ will be outside of the CPV in a SMR reactor pool, it can be maintained and replaced without disturbing the pressure vessels, contrary to the operation of invasive sensing methodologies.

GraviSense™ will be able to determine if a Loss-of-Coolant accident (LOCA) has occurred (Figure 1).




Figure 1 – Concept for using GraviSenseTM for measuring loss of coolant (LOCA) in a small modular reactor (SMR).  The coolant in the inner reactor pressure vessel (RPV) changes its mass distribution during a leak, which the two gravity sensors measure as a change in gravity with time. 


  • Doesn’t measure a point field such as pressure or thermocouple, but measures the effect of the entire mass distribution and records changes
    over time!
  • Non-invasive to the internal pressure spheres of both the RPV and CNV!
  • Entirely passive and non-threatening to employees, in contrast to methods such as gamma rays!

Use of GraviSense™ will reduce reactor down-time and lead to cost savings, because the sensors can be replaced one at a time for maintenance without interrupting the power cycle!

ISL researched GraviSense™ in a DOE STTR Phase 1 using the NuScale NIST facility at Oregon State University, with excellent results that confirm its ability to resolve coolant level changes (Figure 2).


Figure 2 – STTR Phase 1 experiment from NuScale NIST facility, showing outstanding gravimetric sensitivity to coolant fill/drain in a large vertical cylinder.