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The BORAX-IV reactor, which was the successor to BORAX III, began operation in December of 1956. This reactor, with a design power of 20 MW(t), was used principally to test high thermal capacity fuel elements made from ceramics of uranium and thorium. Like the previous BORAX reactors, BORAX-IV was a boiling water reactor operating at 300 psi. It was capable of producing 2.5 MW of electricity.
The new ceramic core of uranium-thorium oxide fuel elements demonstrated the feasibility of stable operation with this fuel, which can operate at higher temperatures, is less reactive with water coolant in case of cladding rupture, is cheaper to manufacture, and has higher burnup posssibilities. It also produced measurable quantities of the artificial, thorium-derived fuel, uranium 233.
The BORAX-IV reactor was first brought to criticality on December 3, 1956 under atmospheric pressure, with water temperatures ranging from cold to boiling (93 °C at the elevation of BORAX-IV). It was operated with this core until April 17, 1957. Operation with a 59-element core at 300 psig and 216 degrees Celsius started in May 1957 and continued intermittently until December 5, 1957.
The 59-element core was then revised in order to increase the maximum power for continuation of stability studies. The boron steel poison rods were removed and 10 new elements were added to the periphery of the core. On February 19, 1958, the reactor was started with the revised core.
Subsequent operation of the reactor during the spring of 1958 was for the purpose of evaluating the effect of operating a direct-cycle boiling water reactor with a fuel element defect and for locating defective elements in the core. On March 11 and 12, 1958, the BORAX-IV reactor was operated at a power of 2.4 Mw, even though a large number of fuel elements contained defects through the cladding. Measurements made during the reactor operations included the following:
BORAX-IV operated until June of 1958, after which is was replaced by BORAX V.