Monticello (USA)

Map of Monticello

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540 MW BWR constructed by General Electric; grid connection in 1971.

Facilities in Monticello

plantreactor typconstruction startoperation startshut down

"The Monticello plant in Minnesota, USA was shut down after a beam supporting the turbine control valve enclosure in the plant's turbine hall broke loose from the building's overall structure, dropping onto a steam pipe. Pipe integrity was maintained and no steam escaped. Safety systems at the plant promptly shut it down as turbine control mechanisms in the enclosure malfunctioned, opening main steam isolation valves that subsequently activated sensors and triggered a full shutdown. All the plant's safety systems functioned normally and there was no release of radioactivity or threat to the health and safety of the workers and of the public."


Northern States Power Co. (NSP) hopes to have a 6.3% power uprate for Monticello approved in August and to have the unit at its increased power level this year.
NRC´s Office of Nuclear Reactor Regulation said the staff finds the proposal acceptable except for a couple of loose ends, particularly seismic qualification for the main steam lines and the main condenser. The Individual Plant Evaluation risk assessment shows only a small increase in risk connected with the power uprate The uprate could shrink the window available for operator action.
Replaced for the uprate were the high pressure turbine, condensate pumps, isophase bus cooler, and stator cooling water pumps. Monticello´s current licensed maximum power level is 1,670 MW (thermal). NSP wants that raised to 1,775 MW (th).


More BWRs may follow as Monticello shuts to replace suction strainers:
Northern States Power Co. has shut Monticello until mid-July to replace the suction strainers in the suppression pool after learning from new engineering calculations that the strainers are undersized and could impair flow to the emergency core cooling system (ECCS) under some accident conditions.
Commonwealth Edison is already making a similar backfit at Dresden, and NRC is about to order all BWR owners to see if they have similar problems.
Bill Hill, Monticello plant manager, said the repairs alone would cost $ 1,5-million, while replacement power costs for the two-month outage have not been estimated.
Managers at the 26-year old, 580 MW General Electric BWR asked a contract engineering firm that is modeling the plant's piping systems to check the function of the suction strainers after Commonwealth Edison reported to NRC that Dresden's strainers excessively impeded flow to the ECCS. ComEd is replacing the Dresden-2 and -3 suction strainers during the plant's current outage.
The strainers, in the bottom of BWR suppression pools, prevent debris from entering the ECCS as water is recirculated from the pools in an accident. NRC has been looking into suction strainer performance for some years, but concern in the past has focused on loss of suction if debris clogged the strainers during a loss-of-coolant accident.

NW, May 22, 1997


Failure of a nonseismic fire suppression pipe could cause loss of both emergency diesels; Problem existed for an undetermined period of time.