Brief Description of Reactor Design: Encapsulated Nuclear Heat Source (ENHS)

The Encapsulated Nuclear Heat Source (ENHS) is a concept being developed under the Nuclear Energy Research Initiative (NERI) program by a consortium lead by University of California-Berkeley. It is a liquid metal-cooled reactor (LMR) that can use either lead (Pb) or a lead-bismuth (Pb-Bi) alloy as the reactor coolant. As opposed to sodium as the traditional liquid-metal coolant, the lead-based coolants are chemically inert with air and water, have higher boiling temperatures, and better heat transfer characteristics for natural circulation. The ENHS has a very long core life, and it uses natural circulation to cool the reactor core and to produce steam to drive the turbine. The ENHS concept relies on autonomous control, that is, after the reactor is brought to full power, variation in power output follow the electricity generating needs automatically (load following) by using temperature feedback from the varying steam pressure and feedwater flow.

The ENHS concept is based on the idea of encapsulating the reactor core inside its own vessel as a module, with no external piping connections. The core is located in a central vertical cylinder inside the vessel. The annular region between this cylinder and the outer wall of the reactor module is constructed as a counterflow heat exchanger. This ENHS module is inserted into a large pool of secondary molten metal. The heat generated in the core is carried upward by the primary molten metal coolant to the top of the vertical cylinder, where openings connect to the primary side of the annular heat exchanger region. The primary coolant flows downward and back through another set of openings under the reactor core. The molten metal in the pool enters the secondary side of the annular heat exchanger through openings in the reactor vessel at the bottom, and exits through another set of openings at the top. The steam generators are separate modules which are also inserted into the secondary pool, adjacent to the reactor vessel module. Water also circulates through the steam generator using natural circulation, so that no pumps are used in the entire reactor system. The concept can automatically load follow over a wide power range. The use of small steam generators makes it easier to design the power plant to use supercritical (very high temperature) steam and, thus, thermal-to-electrical energy conversion efficiencies exceed 42 percent. The secondary coolant pool design also offers the flexibility to connect to desalination plants or district heating systems.

The ENHS can operate at full power for 15 years. The ENHS module is manufactured and fueled in the factory, and shipped to the site as a sealed unit with solidified Pb (or Pb-Bi) filling the vessel up to the upper level of the fuel rods. With no mechanical connections between the reactor module and the secondary system, the module is easy to install and replace, similar to using a battery. At the end of its life, the ENHS module could be removed from the reactor pool and stored on site until the decay heat drops to a level that lets the coolant solidify. The module with the solidified coolant would then serve as a shipping cask. Its compact, sealed design combined with very infrequent refueling provides high proliferation resistance. These design characteristics are intended specifically for remote siting; however, the total weight of an ENHS module when fueled and when loaded with Pb-Bi to the upper core level is estimated to be 300 tons, which could pose a shipping challenge, especially to remote areas.

[Source: Argonne National Laboratory, "Small Modular Reactors", August 30, 2001]