Research on Ageing Degradation Phenomena in a Fuel Reprocessing Facility

Any licensee of spent nuclear fuel reprocessing facility has a duty to maintain its performance to meet the regulatory requirements based on the law.

In order to gain important technical viewpoints on ageing and apply them to the regulatory oversight of licensees’ maintenance activities, the Japan Atomic Energy Agency (JAEA) was entrusted to conduct studies on the following issues by the Nuclear Regulatory Authority:

(1) corrosion of stainless steel;

(2) stress corrosion cracking of zirconium; and

(3) hydrogen embrittlement cracking of Zr/Ta/Stainless steel explosion-bonded joints.

As for (1), a highly active liquid waste evaporator was selected, and corrosion experiments were carried out to understand the effects of corrosion-accelerative ions dissolved in the liquid concentrate in the evaporator. Based on the experiments, a corrosion rate model equation was proposed, and it was pointed out that operating conditions and the solution composition, which affect the re-oxidation of the corrosion-accelerative ions, are important.

As for (2), a plutonium concentrator was selected, and constant load tensile tests with controlled applied potentials and electrochemical tests were conducted in nitric acid and sodium nitrate solutions.
From the results, a map which shows the effect of nitric acid concentration to crack initiation potential and the importance of the total nitrate ion (nitric acid and sodium nitrate) concentration was discussed.

As for (3), hydrogen absorption experiments using four kinds of test pieces—zirconium, tantalum (a bonding material for zirconium and stainless steel joints), tantalum-zirconium alloy, and an explosion-bonded joint sample—were carried out in a nitric acid solution under gamma ray irradiation conditions with a dose rate of up to 7 kGy / h. The results showed that the hydrogen absorption behavior of zirconium is affected by gamma ray irradiation rate, i.e., threshold of irradiation rate for hydrogen absorption was observed. On the other hand, the hydrogen absorption to tantalum was scarcely observed even under gamma ray irradiation conditions.



Division of Research for Nuclear Fuel Cycle and Radioactive Waste
Regulatory Standard and Research Department,
Secretariat of Nuclear Regulation Authority, Japan