Molten-Salt Reactor Experiment
Since this was an engineering test, the large, expensive breeding blanket of thorium salt was omitted in favor of neutron measurements.
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.
However, graphite with the desired pore structure was available only in small, experimentally prepared pieces, and when a manufacturer set out to produce a new grade (CGB) to meet the MSRE requirements, difficulties were encountered.
The fuel system was located in sealed cells, laid out for maintenance with long-handled tools through openings in the top shielding.
Conventional reactors may have to wait hours until xenon-135 decays after shutting down and not immediately restarting (so-called iodine pit).
The choice of Hastelloy-N for the MSRE was on the basis of the promising results of tests at aircraft nuclear propulsion conditions and the availability of much of the required metallurgical data.
Consequently, all major components were fabricated in U.S. Atomic Energy Commission-owned shops at Oak Ridge and Paducah, Kentucky.
[9] At the time that design stresses were set for the MSRE, the data that was available indicated that the strength and creep rate of Hastelloy-N were hardly affected by irradiation.
[13] Centrifugal pumps were developed similar to those used successfully in the aircraft reactor program, but with provisions for remote maintenance, and including a spray system for xenon removal.
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,[16] 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.
By this time, ample 233U had become available,[17] so the MSRE program was extended to include substitution of 233U for the uranium in the fuel salt, and operation to observe the new nuclear characteristics.
An unexpected consequence of processing the salt was that its physical properties were altered slightly so that more than the usual amount of gas was entrained from the fuel pump into the circulating loop.
The circulating gas and the power fluctuations that accompanied it were eliminated by operating the fuel pump at slightly lower speed.
A limited examination program was then carried out, including a moderator bar from the core, a control rod thimble, heat exchanger tubes, parts from the fuel pump bowl, and a freeze valve that had developed a leak during the final reactor shutdown.
Parameters and operational statistics:[2] Power: 8 MW (thermal) output: 92.8 GWh equivalent full-power: 11,555 h Fuel salt: fluoride cations: 65% Li-7, 29.1% Be, 5% Zr, 0.9% U weight: 11,260 lbs (5,107 kg) melting temp: 813 F (434 C) inlet temp: 1175 F (635 C) outlet temp: 1225 F (663 C) flow rate: 400 gal/min (1514 l/min) fuel pump circulating: 19,405 h Coolant salt: fluoride cations: 66% Li-7, 34% Be weight: 15,300 lbs (6,940 kg) coolant pump circulating: 23,566 h Moderator: nuclear graphite Container: Hastelloy-N First fuel: U-235 first critical: 1 June 1965 thermal output: 72,441 MWh critical hours: 11,515 h full-power output equivalent: 9,006 h Second fuel: U-233 critical: 2 October 1968 thermal output: 20,363 MWh critical hours: 3,910 h full-power output equivalent: 2,549 h Shutdown: December 1969 The broadest and perhaps most important conclusion from the MSRE experience was that a molten salt fueled reactor concept was viable.
It ran for considerable periods of time, yielding valuable information, and maintenance was accomplished safely and without excessive delay.
Heat transfer coefficients measured in the MSRE agreed with conventional design calculations and did not change over the life of the reactor.
Limiting oxygen in the salt proved effective, and the tendency of fission products to be dispersed from contaminated equipment during maintenance was low.
Post-operation examination of pieces of a control-rod thimble, heat-exchanger tubes and pump bowl parts revealed the ubiquity of the cracking and emphasized its importance to the MSR concept.
[22] But beginning in the mid-1980s, there was concern that radioactivity was migrating through the system, reported by an ORNL employee who was among 125 people working above the reactor, which had not been decontaminated or decommissioned.
Department of Energy Oak Ridge Operations Manager Joe Ben LaGrone ordered evacuation of 125 employees, based on findings reported to him inspector William Dan DeFord, P.E.
[citation needed] The ensuing decontamination and decommissioning project was called "the most technically challenging" activity assigned to Bechtel Jacobs under its environmental management contract with the U.S. Department of Energy's Oak Ridge Operations organization.
[25] Much of the high cost was caused by the unpleasant surprise of fluorine and uranium hexafluoride evolution from cold fuel salt in storage that ORNL did not defuel and store correctly, but this has now been taken into consideration in MSR design.
