ML19345A645
| ML19345A645 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 05/02/1968 |
| From: | Skovholt D US ATOMIC ENERGY COMMISSION (AEC) |
| To: | |
| Shared Package | |
| ML19345A642 | List: |
| References | |
| NUDOCS 8011240408 | |
| Download: ML19345A645 (4) | |
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SAFETY EVALUATION BY THE DIVISION OF REACTOR LICENS QG W'
DOCKET NO. 50-29
'b YANKEE ATOMIC ELECTRIC CCMPANY h
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PROPOSED CHANGE NO. 79 TO TECHNICAL SPECIFICATIONS INTRODUCTION By letter dated Hay 24, 1967, and amended February 2,1968 February 5, 1968, and April 30, 1968, Yankee Atomic Electric Company requested approval of Proposed Change No. 79 to the Technical Specifications of Facility License No. DPR-3 for the Yankee Nuclear Power Station.
The proposed change would authorize the pH range of the reactor coolant to be between 4.5 and 10.5.
DISCUSSION The pH range of the coolant is a function of the boron concentra-tion when added as boric acid. The permissible boron concentration in the coolant until the end of Core V life was 1300 ppm, and the authorized pH range was 5.0 to 10.5.
The permissible boron concen-tration in the coolant for Core VI was increased to 1700 ppm. The pH of the coolant with 1700 ppm boron is less than 5.0, thus neces-sitating a change in the Technical Specifications.
The principal safety concerns with the proposed change involve the potential (a) for an increased corrosica rate, since corrosion rate increases with decreasing pH, and (b) the hydrogen embittlement of the A 302 B carbon steel of the reactor vessel, which may occur due to additional hydrogen pickup from corrosion of the steel.
The reactor vessel is made of A 302 B carbon steel, clad with 0.109 inch thick stainless steel plates. The minimum original thickness of the A 302 B carbon steel as measured after forming the reactor bottom section was 4.110 inches. This compares with a minimum required thickness of 3.774 inches according to Sectioa VIII, and 3.0 inches according to Section III of the ASME Code. During the Core IV-V refueling period in 1965, it was determined that approximately two square inches of the stainless steel cladding eroded exposing the base metal (A 302 B carbon steel) to the coolant. The maximum depth of the defect, measured from impressions taken in 1965, is not more than 0.115 inch.
If it is assumed conservatively that the
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the A 302 B steel is approximately 0.016 inch. This leaves a present thickness of 4.094 inch, which is at least 0.3 inch more material than Section VIII of the ASME requires, and approximately 1.1 inch more than Section III requirements.
EVALUATION Experience with operating reactors has shown that the corrosion rate of stainless steel in contact with boric acid solution is very low.
However, the small exposed area of A 302 B carbon steel may corrode t a much faster rate as the pH is decreased.
The existing literature ind! cates that the corrosion rate of carbon steels increases with decreasing pH.
The rate remains within acceptable bounds until the pH is decreased to 4.0.
The licensee performed a number of tests to establish the corrosion rate of A 302 B carbon steel for the proposed operating conditions. These tests indicate that the average penetration was 0.0025 incb after exposure to shutdown conditions for two months, i.e., oxygen present in the boric acid solution (the most corrosive condition) and the coolant at 140 F.
Calculations indicate that the penetration would be less than 0.001 inch per year during normal operation. Assuming 30 cold shutdowns of two months each during the remaining 30 years of opera-tion, the maximum expected corrosion would be 0.105 inch.
In our opinion, this is not an excessive amount of corrosion for this plant.
During the shutdown in April 1968, the licensee made additional impressions of the eroded area. When compared to the impressions taken in 1965, it was found that the average difference between the two was 0.002 inch and the maxic m difference measured is 0.008 inch.
These measurements indicate that the average corrosion rate has not exceeded the calculated average value of 0.007 inch. Thus we believe that the corrosion of the base metal in contact with coolant for the proposed operating conditions will not be a safety problem.
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the exposed area. These tests isdicated that there should be no general corrosion, crevice corrosit>n or galvanic corrosion and i ;
therefore no cladding integrity problems during the remaining operating life of the reactor.
In our judgment, carbon steels, like A 302 B, which have a yield strength lower than 80,000 psi, are not susceptible to hydrogen embrittlement.
The licensee performed tests to determin. the extent of hydrogen embrittlement of the reactor vessel metal.
Experimental evidence shows that no significant amounts of hydrogen were absorbed by steel specimens at simulated conditions of Yankee reactor operation and shutdown. Based on this experimental evidence, we agree with the licensee's conclusions that no hydrogen embrittlement of the A 302 B carbon steel of the Yankee reactc; vessel will occur due to additional hydrogen pickup from corrosion of the steel.
Since experimental data indicate low hydrogen uptake by the vessel steel during a simulated shutdown period of more than 4 months, we agree with the licensee's conclusion that the "saaking" of the reactor vessel at 400 F in the plant heat up procedure prior to hot hydrostatic testing is not necessary.
CONCLUSION We conclude that an operating pH range for the cooltat between 4.5 and 10.5 does not represent significant hazards considerations not described or implicit in the hazards summary report and that there is reasonable assurance that the health and safety of the public will not be endangered by operation of the facility in accordar.ce with the prorosed change.
s Donald J. Skovholt Assistant Director for Reactor Operations Division of Reactor Licensing Date:
May 2, 1968 Su 6
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