Nine Mile Saves Major Dollars with CCI Retrofits

Constellation Nuclear’s Nine Mile Point, Unit 2 is an 1124 MWe BWR that was commissioned in mid-1987. By the mid-90s, a 2- x 2-in. water- temperature control valve had failed due to control problems and cavitation. Recently, a 4- x 4-in. flow reject valve (reactor-spill valve to the reactor water clean up system) had also failed – its control capability severely deteriorated.

In each case, minimization of personal radiation exposure and scrap material (solid radwaste) that required decontamination resulting from the replacement processes were critical considerations. Retrofit (rather than replacement) of these safety-related control valves was the key, since both of them were welded into their piping systems, and the cost of valve-body removal and installation was avoided.

Retrofit Project #1

In this BWR, both of these valves are “hot” – one because of its environment, and the other because of its environment and the fluid handled. Designing the retrofit trim without intruding into the “hot” area was the challenge. To do this, advantage was taken of spare parts on hand to create a dimensional “model” of a retrofit trim design that would properly fit into the existing valve body, see Figure 1. The required new bonnet was designed using a spare bonnet-to-body ring gasket. Enough margin was built into the bonnet design to allow for rapid in-plant machining to match the needs of the existing welded-in body. Additionally, documentation and other available data provided valuable information to broadly guide design parameters without the need for access to the existing valve.

Figure 1

(A) A sectional view of the retrofitted 2- x 2-in. cooling water temperature control valve for turbine lubrication-oil cooling.

(B) A sectional view of the retrofitted 4- x 4-in. reactor spill valve in the reactor water clean-up system.

Retrofit Project #2

Capacity and controllability through a very wide flow range was critical in this application. As a result of a previous modification by OEM, capacity was reduced, which CCI later restored, shortening startup. Extreme trim characterization was essential to permit rapid plug lift off under low-flow conditions and prevent erosion. It was erosion that had previously caused a severe leakage problem in this valve. To prevent erosion, the valves were characterized to provide a 30% stroke travel at only 10% flow, Figure 2. In addition, while the OEM-modified valve had three stages of pressure reduction, the retrofit provided 20 stages to hold trim exit velocity to less than 100 ft/sec (30 m/sec).

Figure 2

The control valve characterization (% Cv vs. % stroke) curve for the 4- x 4-in. reactor spill valve to the reactor water clean-up system to prevent integral-plug/ seat-ring erosion for the very low flow operation required.

Results

At the conclusion of the 2000 refueling outage, when Nine Mile Point, Unit 2 started up, the control-room operators reported better than expected results. The entire range of previously experienced operating problems, as well as startup delays, had been eliminated by these two retrofit projects. In addition, the costs associated with unit startup delays and the processing of solid radwaste (the old valve bodies) were eliminated. The personnel rad exposure and high costs associated with an extra shutdown to dismantle and measure these valves before their retrofits could be designed were eliminated.

Published in SOLUTIONS Fall 2001