Back to 2006 listCause and Countermeasures to Cracks Confirmed in Hafnium Control Rods at Hamaoka Nuclear Power Station Reactor No. 3
September 19, 2006
Chubu Electric Power Co., Inc.
In accordance with instructions*1 issued by the Nuclear and Industrial Safety Agency, Chubu Electric has performed external checks on used hafnium control rods that had been used up through the previous cycle at Hamaoka Nuclear Power Station Reactor No. 3 (boiling water reactor, rated electric output 1.1 million kW). The checks confirmed cracks in the sheath and tie rod sections of five of the 13 control rods.
This incident has not caused any leakage of radioactivity.
(Previous disclosures were made on August 7 and 16, 2006.)
Subsequently, the used hafnium control rods in which cracks were confirmed were subjected to detailed investigation. This press release summarizes the results of investigation of cause and measures that Chubu Electric will put into effect in response.
Investigation results were reported to the government of Japan today.
1. Results of investigation of cause
The investigation compared the hafnium control rods in which cracks were recently confirmed with the used hafnium control rods*2 in which cracks were confirmed previously and determined that they were basically the same in terms of part and location cracked, crack form, and the type of materials cracked. It is conjectured therefore that, as with the previously confirmed cracks, these cracks were the result of stress corrosion cracking stemming from three compounding factors: materials, stress and environment.
2. Countermeasures
The 13 hafnium control rods in question, including the five in which cracks were confirmed, are being replaced with boron carbide control rods.
*1 These instructions were issued by the Nuclear and Industrial Safety Agency on January 19, 2006 after cracks were confirmed in hafnium control rods in the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Station, Reactor No. 6. Companies with boiling water reactors were instructed to inspect hafnium control rods that had come out.
*2 Chubu Electric performed inspections on used hafnium control rods that were in storage at Hamaoka Nuclear Power Station awaiting disposal. Cracks were confirmed in 13 used hafnium control rods of Reactor No. 3. (Causes and countermeasures were previously disclosed on May 26, 2006.)
(Attachment)
1. Investigation results
(1) Hafnium control rods inspection results
• Of the 13 control rods, cracks were confirmed in five with thermal neutron exposure dose of 7.1 x 10 to the power of 21st/cm2 or greater.
• Most cracks were confirmed in the upper part of the control rods (especially Node 1).
• The cracks largely occurred in the horizontal direction across the sheath.
• In all five of the control rods in which cracks were confirmed in the sheath, cracks were also confirmed in the tie rod.
(2) Comparison with hafnium control rods in which cracks were previously confirmed
• The five hafnium control rods in which cracks were recently confirmed were compared with the 13 used hafnium control rods in which cracks were previously confirmed, and it was determined that they were basically the same in terms of part and location cracked, crack form, the type of materials cracked, the stress conditions, environmental conditions and fast neutron exposure dose.
| Item | Used hafnium control rods in which cracks were previously confirmed | Used hafnium control rods in which cracks were recently confirmed | |
| Part and location cracked | • Sheath cracks Occurred mainly between the first and second level spot welds and between the third and fourth level spot welds of Node 1. • Tie rod cracks Occurred near cracks on sheath. |
Same as left. | |
| Crack form | Development in horizontal direction | Same as left. | |
| Material conditions |
Sheath | Low carbon stainless steel (SUS316L) |
Same as left. |
| Tie rod | Low carbon stainless steel (SUS316L) |
Same as left. | |
| Stress conditions | It is conjectured that tensile stress up to approx. 300 MPa occurs near spot welds. | Same as left. | |
| Water quality conditions | Dissolved oxygen in reactor water approx. 300 ppb | Dissolved oxygen in reactor water • Approx. 50-100 ppb during hydrogen injection period • Approx. 300 ppb when hydrogen not injected |
|
| Fast neutron exposure dose | 7.9 x 10 to the power of 21st/cm2 - 8.9 x 10 to the power of 21st/cm2 (average of nodes with highest exposure dose) |
7.4 x 10 to the power of 21st/cm2 (four rods) 7.6 x 10 to the power of 21st/cm2 (one rod) (average of nodes with highest exposure dose) |
|
2. Conjectured cause
The investigation compared the hafnium control rods in which cracks were recently confirmed with the used hafnium control rods in which cracks were confirmed previously and determined that they were basically the same in terms of part and location cracked, crack form, and the type of materials cracked. It is conjectured therefore that, as with the previously confirmed cracks, these cracks were the result of stress corrosion cracking stemming from three compounding factors: materials, stress and environment.
< The cracking process >
• Initial cracks occurred because of radiation-induced stress corrosion cracking in the sheath as a result of materials exposed to fast neutron irradiation, the hot atomic reactor water environment in which oxygen was dissolved, and residual stress from welding near anchors, etc.
• Tensile stress occurred in the sheath and tie rod as a result of expansion due to irradiation growth of the hafnium panel from fast neutron irradiation and as a result of an increase in frictional resistance between the hafnium panel and sheath caused by the adherence of corrosion product. Radiation-induced stress corrosion cracking caused cracks to expand.
< Mechanism of stress corrosion cracking from the three compounding factors of materials, stress and environment >
3. Structural health evaluation
Concerning cracks confirmed in hafnium control rods, it has been confirmed that even in cases where emergency shutdown (scram) and earthquake stress acted simultaneously, the hafnium control rods were structurally healthy; therefore the cracks confirmed in the sheaths and tie rods of the five hafnium control rods were modeled conservatively and their healthiness evaluated.
The results confirmed that the control rods were structurally healthy and did not present a safety problem.
4. Measures to prevent recurrence
Either of the following countermeasures to hafnium control rod cracks can be used: replacing them with boron carbide control rods, or keeping hafnium control rods' thermal neutron irradiance within 4 x 10 to the power of 21st/cm2.
In Reactor No. 3, the 13 hafnium control rods in question, including the five in which cracks were confirmed, are being replaced with boron carbide control rods.
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