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Content archived on 2024-04-15

TESTING OF COBALT-FREE ALLOYS FOR VALVE APPLICATIONS USING A SPECIAL TEST LOOP

Objective

THE RADIATION LEVEL AROUND THE COMPONENTS OF PRESSURISED WATER REACTORS (PWR) PARTICULARLY GOVERNS THE RADIATION EXPOSURE OF THE WORKERS DURING THE PERIODIC MAINTENANCE OPERATIONS, AS WELL AS DURING DECOMMISSIONING OPERATIONS. SINCE THE ACTIVATION PRODUCT COBALT-60 IS ONE OF THE MAIN CONTRIBUTIONS TO THIS EXPOSURE, THE USE OF COBALT ALLOYS IN THE PRIMARY CIRCUIT SHOULD BE AVOIDED AS FAR AS POSSIBLE.

THE ALLOYS LIKELY TO REPLACE COBALT ALLOYS MAINLY USED IN NUCLEAR COCKS AND VALVES, E.G. STELLITE GRADE 6 AND GRADE 12, MUST COMPLY WITH FOLLOWING CRITERIA :

- GOOD WELDABILITY;
- HARDNESS EQUIVALENT TO THAT OF COBALT ALLOYS;
- RESISTANCE TO FRICTION AND WEAR EQUIVALENT TO THAT OF THE COBALT ALLOYS.

IN THE PAST FEW YEARS, FRAMATOME, JOINTLY WITH CEA, ASSESSED A NUMBER OF HARD COBALT-FREE ALLOYS CONSIDERED AS PROMISING, TWO OF THEM WERE SELECTED : EVERIT 50, SELECTED AS A RESULT OF THE FIRST COMMUNITY RESEARCH PROGRAMME (SEE FINAL REPORT EUR 9865), IF INFORMATION NECESSARY TO ITS REALISATION IS AVAILABLE.

THIS RESEARCH AIMS AT ESTABLISHING THE PERFORMANCES OF THESE THREE ALLOYS, COMPARATIVELY TO STELLITE GRADE 6, ON VALVES MOUNTED ON DOUBLEAU LOOP OF CEA, OPERATED IN CONDITIONS AS CLOSE AS POSSIBLE TO PWR WORKING CONDITIONS. THE SELECTED VALVES ARE GLOBE-VALVES AND SWING CHECK-VALVES. THE RESEARCH IS LED BY FRAMATOME.
Since the use of cobalt alloys should be avoided as far as possible in pressurized water reactor (PWR) components, a programme aimed at establishing the performance of cobalt free alloys has been performed for valve applications, where cobalt alloys are mainly used. Two types of cobalt free alloys were selected: nickel-chromium-boron-silicon and nickel-chromium-iron alloys.

Cobalt free valve behaviour has been compared to that of cobalt valves by implementation of a programme in a special PWR test loop. As a result of the loop test programme, which included endurance, thermal shock and erosion tests, cobalt free alloy candidates to replace cobalt alloys can be proposed in relation with valve type (globe valve and swing check valve). Colmonoy 4-26 (nickel-chromium-boron-silicon alloy) and Cenium Z20 (nickel-chromium-iron alloy) deposited by plasma arc process were found suitable for use in 3 inch swing check valves. For integral parts acting as guide rings, Nitronic 60 and Cenium Z20/698 were tested successfully. For small bore components such as 2 inch globe valves, no solution can yet be proposed; introduction of cobalt free alloys is dependent on the development of automatic advanced arc surfacing techniques applied to small bore components.
B.1. BASIC STUDY INCLUDING DESIGN AND SPECIFICATIONS OF THE SELECTED VALVES (FRAMATOME).

B.2. SYNOPSIS OF RESULTS OBTAINED ON HARD COBALT-FREE ALLOYS IN ORDER TO JUSTIFY THE SELECTION OF CENIUM Z 20 AND COLMONOY 5 (FRAMATOME).

B.3. COMMISSIONING OF THE VALVES WITH DEPOSITS OF CENIUM Z 20 AND COLMONOY 5 AND, IF POSSIBLE, EVERIT 50, COMPARED WITH STELLITE GRADE 6 HARD-FACED VALVES (FRAMATOME).

B.4. IMPLEMENTATION OF THE SELECTED HARD-FACED VALVES IN THE DOUBLEAU LOOP AND DEPOSITS TESTING (CEA) :
B.4.1. ENDURANCE TESTS UNDER PWR PRIMARY CIRCUIT CONDITIONS (320C, 160 BARS, PH=7, 1500 CYCLES).
B.4.2. RESISTANCE TO THERMAL SHOCKS TESTS (100C AND 250C).
B.4.3. EROSION TESTS AT 70C AND 320C DURING 10 MINUTES.

B.5. OBSERVATION OF VALVES BEHAVIOUR DURING TESTS AND EXAMINATION OF DEPOSITS AND PARTS (DYE-PENETRANT TESTING, INTERNAL TIGHTNESS, SURFACE STATE) AFTER EACH SERIES OF TESTS (CEA).

B.6. CONCLUSIONS AND RECOMMENDATIONS FOR USING HARD COBALT-FREE ALLOYS AS DEPOSIT IN THE VALVES (FRAMATOME).

Topic(s)

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Call for proposal

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Coordinator

SOCIETE FRAMATOME
EU contribution
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Address
Tour Framatome
92084 PARIS LA DEFENSE
France

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