Molten Core

Verified Timeline

Lore & Background

The MSRE primarily used two fuels: first uranium-235 and later uranium-233. The latter 233UF4 was the result of breeding from thorium in other reactors. Since this was an engineering test, the large, expensive breeding blanket of thorium salt was omitted in favor of neutron measurements. The reactor's piping, core vat and structural components were made from Hastelloy-N, and its moderator was a pyrolytic graphite core. The fuel for the MSRE was LiF-BeF2-ZrF4-UF4 (65-29.1-5-0.9 mole %). The secondary coolant was FLiBe (2LiF-BeF2), and it operated as hot as 650 °C and operated for the equivalent of about 1.5 years of full power operation. The result promised to be a simple, reliable reactor.

In Their Own Story

The purpose of the Molten-Salt Reactor Experiment was to demonstrate that some key features of the proposed molten-salt power reactors could be embodied in a practical reactor that could be operated safely and reliably and be maintained without excessive difficulty. For simplicity, it was to be a fairly small, one-fluid (i.e. non-breeding) reactor operating at 10 MWth or less, with heat rejection to the air via a secondary (fuel-free) salt. After two months of high-power operation, the reactor was down for 3 months because of the failure of one of the main cooling blowers. The reactor experienced stable neutronic operation. If temperatures increased or bubbles formed, the volume of the fluid fuel salts would increase and some fluid fuel salts would be forced out of the core, thereby reducing the reactivity.

Reader's Guide

The MSRE operated for 5 years. The salt was loaded in 1964, and nuclear operation ended in December 1969, and all the objectives of the experiment were achieved during this period. Checkout and prenuclear tests included 1,000 hours of circulation of flush salt and fuel carrier salt. Nuclear testing of the MSRE began in June 1965, with the addition of enriched 235U as UF4-LiF eutectic to the carrier salt to make the reactor critical. After zero-power experiments to measure rod worth and reactivity coefficients, the reactor was shut down and final preparations made for power operation. Power ascension was delayed when vapors from oil that had leaked into the fuel pump were polymerized by the radioactive offgas and plugged gas filters and valves.

Did You Know?

• The MSRE was initially designed for 15 MWth but operated at 7.4 MWth because of imprecise nuclear cross section data.
• The reactor used two fuels: first uranium-235 and later uranium-233 bred from thorium in other reactors.
• The fuel pump sprayed about 50 US gallons per minute of fuel into a gas space to allow xenon and krypton to escape, removing the neutron poison xenon-135.
• Almost 200,000 lb (90,000 kg) of Hastelloy-N in a variety of shapes were produced commercially for the MSRE.
• The MSRE's pyrolytic graphite core, grade CGB, also served as the moderator.

Gallery