Environmental Impact Statement - radioactive monticello

Environmental Impacts of Operation
Monticello has a once-through heat dissipation system. However, due to a potential
combination of low flows and high ambient water temperatures, Monticello has two cooling
towers that can be used to meet thermal discharge standards through helper cycle mode or by
complete or partial recirculation of the cooling water (NMC 2005a). The Surface Water
Appropriations Permit requires that the cooling towers must be operated in partial recirculation
mode when the river flow is between 240 and 860 cfs, or closed-cycle mode when the river flow
is less than 240 cfs, and in accordance with allowable thermal discharge limits set forth by the
MPCA in the NPDES Permit. NPDES Permit stipulations specify that the maximum daily
average temperature at the end of the discharge canal cannot exceed the following limiting
temperatures: 95 0 F from April through October; 85 0 F in November and March; and 80°F from
December through February (MPCA 2002). However, discharge temperatures in excess of
these limits is allowed, on a limited basis, when the plant is required to operate in a partial
recirculation or closed-cycle mode to meet the Surface Water Appropriations Permit limitation
(NMC 2005a). When reduced river flows and/or elevated ambient river temperatures limit the
ability for Monticello to meet thermal discharge limits, plant procedures require a reduction in
power output to maintain compliance with the State of Minnesota NPDES permit (NMC 2005a).
The cooling towers are normally used when the ambient river temperature reaches 68°F or
when the discharge canal temperature approaches permitted temperature limits (NMC 2005a).
In partial recirculation mode, a portion of the cooled water from the cooling towers is
recirculated to the intake, while the remainder is discharged to the river. When river flow is less
than 860 cfs, a maximum of 75 percent of river flow at the intake may be withdrawn. Partial
recirculation may used to comply with this restriction (NMC 2005a). During extreme low flow
conditions, Monticello can operate as a closed system. During this operational mode, the
makeup requirement is only 54 cfs (NMC 2005a). Occasionally, one cooling tower may be
used during winter to provide a flow path for heated water to the intake structure when
suspended ice is present in the river (NMC 2005a).
Monticello has operated in a once-through or helper mode approximately 98 percent of the time
(NMC 2005). The remainder of the time Monticello has operated in a partial recirculation or
closed-cycle mode.
In 2002, the mean ambient river temperature was 49.7 0 F while the discharge canal temperature
was 77.9 0 F. In 2003, these temperatures were 50.9 0 F and 78.2 0 F, respectively (Xcel Energy
2004). In winter, when ambient river temperatures were near freezing (32 0 F), the discharge
temperatures generally ranged between 70 and 75°F; whereas in summer when ambient river
temperatures range from approximately 70 to 80 0 F, the discharge temperatures ranged from
approximately 84 to 91 *F (NSP 2004). The main body of the thermal plume is generally
confined to the south bank of the river; and 30 to 70 percent of the river was unffected by the
thermal plume (NMC 2005a). The 9 0 F plume is considered the immediate discharge zone,
while the 3°F plume is considered the intermediate discharge zone (Afzal et al. 1975). During
approximately 70 percent of the period from June through September, the 9 0 F isotherm
extended less than half the river width and less than 700 ft downstream of the discharge. The
August 2006 4-19 NUREG-1437, Supplement 26 1