SMR, LLC
SMR LLC is a Holtec International affiliated company that specializes in the development of inherently safe small modular reactors. SMR's first foray into this area is called HI-SMUR 140 whose name is derived from HoltecInherently Safe Modular Underground Reactor. As the above words in HI-SMUR’s name indicate, its core is located completely underground, it is operated by gravity induced flow (no reactor coolant pump), it does not rely on off-site power for shutdown (Inherently Safe), and it can be installed as a single unit or a cluster at a site (Modular). Passive in every aspect of its operation, HI-SMUR’s principal technical mission is “safety and security first.” HI-SMUR’s principal safety credentials derive from locating the core underground in a reactor vessel that has no penetrations to provide a drain-down path for the reactor coolant. Eliminating the reactor coolant pump and the need for emergency or off-site power to cool the reactor core in the event of a forced shutdown, are among the distinguishing design features of HI-SMUR that define its mission of utmost safety and security. Other major features of HI-SMUR are its small footprint, minuscule site boundary dose, large inventory of coolant in the reactor vessel and its modularity, i.e., the freedom to build the number of units at a site to best suit the owner’s projected power needs. The expected duration of the construction life cycle is 24 months.
HI-SMUR
The Holtec Inherently-Safe Modular Underground Reactor (HI-SMUR) is a 145 MWe reactor designed to provide an economical and safe source of clean energy from nuclear fission. The core strengths of HI-SMUR are its inherent safety and simplicity of operation. The operational simplicity of HI-SMUR and the modest financial outlay required to establish and commission it will make it possible to deliver the fruits of pollution-free nuclear energy to the vast mass of humanity around the globe that does not presently have access to a reliable source of power or to a robust electrical energy delivery system. Competitive with large nuclear reactors on a per-megawatt basis, HI-SMUR is tailored to add generation capacity to the installed base incrementally with incremental capital outlays. Thanks to its inherent operational simplicity, HI-SMUR requires a minimal cadre of trained personnel to run the plant. Multiple units can be clustered at one location or geographically dispersed without a significant increase in the per-megawatt construction cost. Geographical dispersal and underground configuration serve as natural antidotes to post-9/11 concerns. The modest power output of HI-SMUR makes it a viable candidate source of reliable electrical energy or for providing heating steam to a city or process steam as a cogeneration plant serving an industrial plant.
Conceived as the ultimate foil to the natural disasters of Fukushima’s genré , HI-SMUR 140 is the very epitome of safety. Designed to withstand the strongest recorded earthquake in the United States, HI-SMUR 140 will also withstand a severe follow-on flood event without endangering the health and safety of the plant’s workers or the general public. Uncontrolled heating of the plant’s fuel is definitively precluded by HI-SMUR’s design features that cannot be defeated by a cataclysmic event of Fukushima’s severity.
As a passive small modular reactor of the pressurized water genre with supreme safety, ease of maintenance and superb security as its principal calling cards, HI-SMUR is also ideally suited to serve as a reliable power source to strategic national assets of any country. The main design features that define HI-SMUR and speak to its inherent safety and reliability are:
1. Core deep underground: The reactor core resides deep underground in a thick-walled reactor vessel (RV) made of an ASME Code material that has decades of proven efficacy in maintaining reactor integrity in large PWR and BWR reactors. All surfaces wetted by the reactor coolant are made of stainless steel or Inconel, which eliminates a major source of crud accumulation in the reactor vessel.
2. Gravity-driven circulation of the reactor coolant: HI-SMUR does not rely on any active components (Reactor Coolant pump) for circulating the reactor coolant through the core. Instead, the flow of the reactor coolant through the reactor vessel, the steam generators, and other miscellaneous equipment occurs by the pressure head created by density differences in the flowing water in the hot and cold segments of the primary loop. The reliability of gravity as a motive force in HI-SMUR underpins its inherent safety. The movement of the reactor water requires no pumps, valves, or moving machinery of any kind.
3. No reliance on off-site power: Offsite power is not essential for shutting down HI-SMUR. The rejection of reactor residual heat during the shutdown also occurs by gravity-driven circulation. Thus, the need for an emergency shutdown power supply at the site ? a major concern for nuclear plants ? is eliminated.
4. Assurance of a large inventory of water around and over the reactor core: The HI-SMUR reactor vessel (RV) has no penetrations in its bottom 120 feet, which means that the core will remain submerged in a large inventory of water. All penetrations in the RV are located in the top region of the RV and are small in size. The absence of large piping in the reactor coolant system precludes the potential of a “large break” LOCA event.
5. All critical components readily accessible: Both the steam generator and the control rod drive system are located outside the reactor vessel at a level that facilitates easy access, making their preventive maintenance and repair a conveniently executed activity. Each steam generator is a horizontal pressure vessel with built-in design features to conveniently access and plug tubes.
6. Demineralized water: The reactor coolant is demineralized water, which promotes criticality safety because of its strong negative reactivity gradient with rise in temperature. Elimination of borated water also simplifies the nuclear steam supply system (NSSS) by eliminating the systems and equipment needed to maintain and control boron levels in the primary coolant. Pure water and corrosion resistant primary coolant loop help minimize crud buildup in the RV.
7. Rapid refueling: The core, along with its support structure, is removed from the reactor as a single unit and delivered to the fuel pool after about 100 hours of decay in the core. The refueling of the reactor occurs by the simple expedient of installing the loaded core with its support structure as a single unit. Thus the refueling outage can be completed in less than a week.
After a few years of decay in the pool, the core, along with its support structure, will be canisterized underwater in a multi-purpose canister and transferred to the underground dry storage facility adjacent to the Containment Building.
8. HI-SMUR is truly modular: One can build only one system at a site or a large number. Clustering a number of units at one site will reduce the overall O&M costs.
9. Long operating cycle: HI-SMUR will operate for approximately 3 years before requiring refueling.
10. Short construction life cycle: Virtually all HI-SMUR components are shop fabricated. Site work is limited to reinforced concrete construction and a limited amount of welding to assemble the shop-built equipment and parts. As a result, it is possible to complete the construction of a reactor unit in 24 months from the first shovel in the ground.
11. Efficient steam cycle: A set of two horizontal steam generators are arranged in series and integrally welded to the reactor vessel. The efficiency of the HI-SMUR power cycle and compactness of the system is further enhanced by superheaters that are integrally welded to the reactor vessel. The superheaters, one attached to each steam generator, increase cycle efficiency and also protect both the H.P. and L.P. turbines from the deleterious effect of moist steam.
12. Pressurizer: The reactor coolant loop incorporates a standalone pressurizer in the manner of a typical PWR. The pressurizer serves to control the pressure in the reactor vessel by the application of controlled nitrogen pressure from high pressure nitrogen tanks.
13. Suitable for water-challenged sites: HI-SMUR can be installed at sites with limited water availability, such as creeks and small rivers that are inadequate for large reactors. HI-SMUR can be operated equally well in a water-challenged region by using Holtec’s air-cooled condenser technology to reject the plant’s waste heat. Using air in lieu of water, of course, results in a moderate increase in the plant’s cost.
14. System parameters in the safe and proven range: Reactor operating pressure and temperature is in the proven range for PWRs. Lower core power density than that used in large PWRs for improved thermal-hydraulic control (please see table below) and an improved margin to departure-from-nucleate boiling in the reactor core.
15. Onsite storage of used fuel for an extended period: The HI-SMUR facility will be equipped with Holtec's patented underground storage system that will provide storage for used fuel produced from 20 years of operation. The storage system can be extended for additional years of operation, as needed. The only limit on the onsite storage capacity is from the physical space available at the site. As a rule of thumb, each cycle of reactor operation will consume approximately 16 ft x 16 ft area (260 square feet of land) to place one core in dry storage.
16. Minimized piping runs and minimum use of active components to enhance reliability and cost competitiveness: The amount of piping in the primary and secondary systems in HI-SMUR is the least of any nuclear plant design on the market, as is the number of pumps and valves.
17. In-service inspection: All weld seams in the primary system including those in the reactor vessel are accessible for conducting NRC mandated inspections. The HI-SMUR in-service inspection capability meets or exceeds the inspection requirements of nuclear plants under Section XI of the ASME Code.
18. Earthquake hardened design: Virtually all major equipment in the Containment Building is either underground or horizontally mounted to withstand strong seismic motions. This includes the reactor vessel, the Fuel Pool, the Reactor water storage tank (all underground) and the steam generators, the superheaters, and the Kettle Reboiler (all horizontal) that are floor mounted.
19. Aircraft impact proof containment: The Containment structure can withstand the impact of a crashing fighter plane or a commercial liner without sustaining a thru-wall breach.