Nuclear fuels for transient test reactors

Transient test reactors with the ability to test fissile specimens under extreme conditions have been crucial tools in the development of nuclear technologies. Less than 10 unique facility designs have ever been constructed, most of which remain operational today and still use the original nuclear fuel constructed for them more than 40 years ago. Historic fuel systems for transient test reactors vary in significant ways which have marked influences on reactor capabilities. Eventually, new fuel will be needed to support the longevity of transient test reactor missions. This paper reviews precedent transient reactor fuel systems in the context of their unique requirements. A few key conclusions are illustrated by comparing and contrasting these transient test reactors. Fuel composites which are mostly graphite can enable transient reactors with very high neutron fluence capability (>2E16 n/cm2) and are amenable to longer “shaped” transients but cannot achieve pulses <10 ms in duration. Reducing the graphite-to-uranium ratio can yield a very narrow pulse capability but delivers less fluence and requires cores with considerably more fissile material. Designs based on uranium dioxide (UO2) make use of readily available materials to create compact cores with narrow pulse width capabilities but with moderate neutron fluence capabilities (∼2E15 n/cm2). Uranium zirconium hydride (U-ZrHx) is a well-established fuel system for pulsing reactors which has been intermittently manufactured throughout the decades. U-ZrHx offers similar capabilities to UO2 designs in terms of nuclear kinetics, but with about half the fluence capability (∼1E15 n/cm2). An evolution of the UO2 system, termed “ternary ceramic” fuel, shows that dispersing UO2 in zirconium oxide and calcium oxide can increase fluence capability greatly (∼7E15 n/cm2), but is not presently a commonly available fuel form. A unique composite of UO2 and beryllium oxide (UO2-BeO) can be used to create a core with similar kinetics and compact core geometry as U-ZrHx designs, but with significantly higher fluence capability (∼6E15 n/cm2). Like ternary ceramic fuel, newly fabricated UO2-BeO would require reestablishing its historic manufacturing process which would be further complicated by the health hazards associated with beryllium. Like most engineering problems, there is no perfect solution, but this paper outlines the advantages and disadvantages of candidate fuel options to help guide detailed evaluations.