Environmental Impact Statement - radioactive monticello

Environmental Impacts of Operation
Under the current Surface Water Appropriations Permit, Monticello is allowed to withdraw a
maximum of 645 cfs for once-through or helper-cycle mode cooling and plant process water
from the Mississippi River. Special operating conditions are required when the river flow is less
than 860 cfs. For example, plant water intake may not exceed 75 percent of river flow when the
river flow is less than 860 cfs but greater than 240 cfs (NMC 2005a). When river flow is less
than 240 cfs, there are further restrictions, including use of a closed-cycle mode of operation
(NMC 2005a). Withdrawn water is returned to the river except for that amount evaporated in
the discharge canal and cooling towers. During 2002, the mean river flow was 6351 cfs while
mean intake flow was approximately 529 cfs; similar means for 2003 were 4572 cfs and 491.9
cfs, respectively (Xcel Energy 2004).
The cooling water flow path for open-cycle or once-through cooling includes (1) the Mississippi
River, (2) the intake, (3) the condenser, (4) the discharge structure, (5) the discharge canal,
(6) the discharge weir, and (7) the Mississippi River. From the discharge canal, the cooling
water returns to the river at a point approximately 1500 ft downstream from the intake. For the
other modes of operation (see Table 2-2), all, part, or none of the cooled water from the cooling
towers can be sent back to the circulating water pump intake (Afzal et al. 1975). Approximately
5 to 6 percent of total cooling water flow needs to be replaced with makeup water from the
Mississippi River during closed-cycle operation (NMC 2005a). Makeup water is supplied by two
31 cfs pumps that replace water lost by.cooling tower evaporation, drift, and blowdown (Afzal et
al. 1975).
Entrainment samples were collected at Monticello from September 12, 1973, to
August 18, 1974, by Knutson et al. (1976). During that period, river flow used for cooling
ranged from 1.6 to 19.4 percent of river flow with a mean of 10.1 percent (Knutson et al. 1976).
Entrainment rates for young-of-the-year fish were estimated at 9.2/hr from September 12, 1973,
to March 13, 1974; 5986/hr from May 22, 1974, to June 28, 1974; and 35/hr from June 28 to
August 18, 1974. The entrainment rate was estimated to be 1617/hr or 38,805/day for all
fishes. The maximum estimated entrainment rate was 22,635/hr on June 12, 1974.
Approximately 98 percent of all entrainment occurred between May 22 and June 28, 1974
(Knutson et al. 1976).
Entrainment losses consisted of twenty-three fish species or species groups (Knutson et al.
1976). Catostomid (sucker) fry comprised 96.5 percent of the entrainment. Black crappie
(Pomoxis nigromaculatus) comprised 1.3 percent, walleye (Sander vitreus) comprised 0.5
percent, and cyprinids (minnows) comprised 0.8 percent of total fish entrained (Knutson et al.
1976). Entrainment of young-of-the-year shorthead redhorse (Moxostoma macrolepidotum),
silver redhorse (M. anisurum), and white sucker (Catostomus commersoni) during April to
August 1974 was estimated at 7.8 million individuals; while only a combined 8400 black
crappie, walleye, and smallrnouth bass (Micropterus dolomieu) were entrained (Knutson et al.
1976). Since regular fish surveys have been initiated at Monticello before the plant started
operation, suckers have been the predominant species collected in electroshocking samples
(Xcel Energy 2004). Thus, entrainment has not apparently had an impact on sucker species
production.
NUREG-1437, Supplement 26 4-14 August 2006 1