  The projected performance and detector configuration of nEXO are described in
this pre-Conceptual Design Report (pCDR). nEXO is a tonne-scale neutrinoless
double beta ($0\nu\beta\beta$) decay search in $^{136}$Xe, based on the
ultra-low background liquid xenon technology validated by EXO-200. With
$\simeq$ 5000 kg of xenon enriched to 90% in the isotope 136, nEXO has a
projected half-life sensitivity of approximately $10^{28}$ years. This
represents an improvement in sensitivity of about two orders of magnitude with
respect to current results. Based on the experience gained from EXO-200 and the
effectiveness of xenon purification techniques, we expect the background to be
dominated by external sources of radiation. The sensitivity increase is,
therefore, entirely derived from the increase of active mass in a monolithic
and homogeneous detector, along with some technical advances perfected in the
course of a dedicated R&D program. Hence the risk which is inherent to the
construction of a large, ultra-low background detector is reduced, as the
intrinsic radioactive contamination requirements are generally not beyond those
demonstrated with the present generation $0\nu\beta\beta$ decay experiments.
Indeed, most of the required materials have been already assayed or reasonable
estimates of their properties are at hand. The details described herein
represent the base design of the detector configuration as of early 2018. Where
potential design improvements are possible, alternatives are discussed.
  This design for nEXO presents a compelling path towards a next generation
search for $0\nu\beta\beta$, with a substantial possibility to discover physics
beyond the Standard Model.
