  Magnetic reconnection plays a crucial role in powering solar flares,
production of energetic particles, and plasma heating. However, where the
magnetic reconnections occur, how and where the released magnetic energy is
transported, and how it is converted to other forms remain unclear. Here we
report recurring bi-directional plasma outflows located within a large-scale
plasma sheet observed in extreme ultraviolet emission and scattered white light
during the post-impulsive gradual phase of the X8.2 solar flare on 2017
September 10. Each of the bi-directional outflows originates in the plasma
sheet from a discrete site, identified as a magnetic reconnection site. These
reconnection sites reside at very low altitudes ($< 180$ Mm, or 0.26
$R_{\odot}$) above the top of the flare arcade, a distance only $<3\%$ of the
total length of a plasma sheet that extends to at least 10 $R_{\odot}$. Each
arrival of sunward outflows at the looptop region appears to coincide with an
impulsive microwave and X-ray burst dominated by a hot source (10-20 MK) at the
looptop, which is immediately followed by a nonthermal microwave burst located
in the loopleg region. We propose that the reconnection outflows transport the
magnetic energy released at localized magnetic reconnection sites outward in
the form of kinetic energy flux and/or electromagnetic Poynting flux. The
sunward-directed energy flux induces particle acceleration and plasma heating
in the post-flare arcades, observed as the hot and nonthermal flare emissions.
