Masafumi Domae

Numerical Simulation on Effect of Methanol Addition on Coolant Radiolysis in Pressurized Water Reactor

The addition of alcohol into the primary coolant of pressurized water reactors (PWRs) was proposed as an alternative of hydrogen addition. The radiolysis of the coolant in the presence of methanol was simulated, assuming a batch system and changing dose rate. At 300°C, the addition of 4.0 ppm methanol shows an equivalent scavenging capacity toward the OH radical to a typical dissolved hydrogen concentration in Japanese PWR plants. At low dose rate, the addition of 1.0 ppm methanol suppresses considerably the radiolysis of the coolant. At high dose rate, the addition of at least 10 ppm methanol is necessary to lower concentrations of the radiolytic oxidizing products. Concentrations of the radiolytic oxidizing products are determined not only by the scavenging of the OH radical but also by secondary reactions. In the presence of fast neutron irradiation, methanol acts to decompose the radiolytic oxidizing products. The fact that steady-state concentrations of the final organic products depend on dose rate means that complicated secondary reactions contribute to the decomposition of methanol. The major final organic product is formed through a reaction of ethylene glycol with OH. Methanol addition may have the same effect as ethylene glycol addition, and radiation chemical reactions of ethylene glycol systems should be elucidated in future.

Thermal Decomposition of 3-Methoxypropylamine as an Alternative Amine in PWR Secondary Systems

The secondary coolant of pressurized water reactors is buffered to slightly alkaline pH by ammonia or amines in order to suppress corrosion. 3-Methoxypropylamine (MPA) is one of the promising alternative amines. The thermal decomposition of MPA was studied under two conditions: (i) a dissolved oxygen (DO) concentration of less than 5 ppb at 280°C for 1.5 h and (ii) a DO concentration of 20 ppb at 70°C for 2 h. The initial MPA concentration was 10 ppm. After the tests, concentrations of MPA and carboxylic acids were measured. Approximately 9 to 15% of MPA was decomposed after the tests. Carboxylic acid concentrations were as follows: (i) formate 110 ppb, acetate 260 ppb and propionate 400 ppb at 280°C, and (ii) formate less than 2 ppb, acetate 60 ppb and propionate 1270 ppb at 70°C. The reaction mechanism of MPA decomposition was estimated from the present experimental results. At 280°C, the hydrolysis of the ether bond initiates the decomposition, and the subsequent bond cleavage of C-N and/or C-C occurs. At 70°C, hydrogen abstraction by an oxygen molecule is the initiation reaction. MPA radicals and HO2 or C1 compounds propagate a chain reaction and result in a relatively high yield of propionate.

Numerical Simulation of Influence of Hydrogen Peroxide Photolysis on Water Chemistry in BWR Plant

As a novel technique to enhance the effect of H2 addition for HWC (hydrogen water chemistry) in BWR coolant, UV light photolysis of H2O2 was proposed. The effect of the H2O2 photolysis on water radiolysis was simulated for a batch system at 288°C. It was demonstrated that this technique reduced the concentrations of H2O2 and O2 with less amount of H2 than that under ordinary HWC condition. It was found that the technique can reduce the H2O2 concentration even in the absence of H2 addition although the OH concentration increases concomitantly. The radiation chemical mechanism under ordinary HWC condition is discussed briefly. The UV laser power required to reduce concentrations of the oxidizing species was estimated for a dose rate of †-ray 100Gy-s-1 based on the simulated results. A potential role of the Cherenkov radiation in H2O2 decomposition in coolant of a BWR plant was pointed out.

Development of Alternative Reductant Application in PWR Primary Systems

In primary coolant of pressurized water reactors (PWRs), high concentration dissolved hydrogen (DH) has been added, to prevent generation of oxidizing species through radiolysis of coolant. Recently, number of ageing plants is increasing and utilities are concerned about primary water stress corrosion cracking (PWSCC). Although mechanism of PWSCC is not fully clarified, some researchers consider that occurrence of PWSCC and crack propagation rate are affected by DH concentration. The authors consider that one of possible mitigation methods toward PWSCC is use of alternative reductant for hydrogen. From the radiation chemical aspect, aliphatic alcohols are typical scavengers of oxidizing radical generated through the radiolysis of water. The aliphatic alcohols are promising candidates of the alternative reductant. In the present work, possible alternatives of hydrogen were screened, and methanol was selected as the best candidate. Corrosion tests of type 304 stainless steels were carried out at 320°C in two conditions: (1) DH 1.5 ppm (part per million) and (2) methanol 2.9 ppm. Under two conditions, electrochemical corrosion potential was measured during the immersion tests. After the immersion tests, surface morphology of the stainless steel specimens was observed by scanning probe microscope. Major component of oxide film was analyzed by X-ray diffraction. From comparison of the test results, it is concluded that addition of methanol 2.9 ppm has almost the same effect as addition of DH 1.5 ppm.Copyright

Water Chemistry Technology of Methanol Addition in PWR Primary Systems: Radiolysis of Methanol Solution at 320 °C

Primary Water Stress Corrosion Cracking (PWSCC) is one of important ageing issues in PWR (Pressurized Water Reactor) primary systems. It has been pointed out that high concentration dissolved hydrogen may lead to occurrence of PWSCC. The authors have proposed to substitute hydrogen by methanol as a fundamental countermeasure of PWSCC. So far corrosion tests of stainless steels and Zircaloy-4 in methanol solutions at 320 °C were conducted under γ-ray irradiation and without irradiation. The test results show that methanol is promising.In the present paper, γ-ray irradiation experiments of methanol solution at 320 °C were done up to 100 kGy. A study on the radiolysis of methanol solution is important from two aspects. One concerns corrosion of structural materials. The radiolysis of methanol may result in formation of harmful compounds to the structural materials, such as carboxylic acids. It is necessary to know the yields of such compounds. The other concern is possible polymerization of methanol and formation of organic polymer deposit on fuel claddings. Large amount of the deposit on fuel claddings should be avoided to keep integrity of fuel claddings. Therefore, it should be clarified whether gaseous species are major products and whether polymerized species of methanol such as ethylene glycol is formed.After the γ-ray irradiation of methanol solution, following species were analyzed: CO2 and H2, methanol, formaldehyde, formate and acetate, and ethylene glycol and glycerin.Without γ-ray irradiation, the major process of the thermal decomposition of methanol at 320 °C is oxidation of methanol by water and generation of one CO2 molecule and three H2 molecules. Under γ-ray irradiation, the decomposition of methanol is accelerated; little methanol remains after 10 kGy irradiation. The major product is CO2, and polymerization of methanol unlikely occurs. After methanol is completely decomposed, the hydrogen yield still increases. The reducing environment is maintained. Probably, transient organic species play important roles. The addition of low concentration methanol may be sufficient to maintain reducing environment of the PWR primary systems.© 2014 ASME

Corrosion Behavior of Zircaloy-4 in Methanol Solution at 320 °C Under Gamma-Irradiation

It has been pointed out that high concentration dissolved hydrogen is one of the important factors of PWSCC (primary water stress corrosion cracking) in the primary systems of pressurized water reactors. Application of a substitution for hydrogen may be a fundamental countermeasure of PWSCC. The authors are developing a new water chemistry technology of a hydrogen alternative to suppress PWSCC. In the present paper, corrosion tests of Zircaloy-4 were performed in deaerated 5 mmol dm−3 methanol solution at 320 °C in the absence and presence of gamma-irradiation. The nominal absorbed dose of the test water was 100 kGy. After the immersion tests, the specimens were analyzed. Weight gain per unit surface area, thickness of oxide film and hydrogen storage were measured. In addition, Raman spectroscopy was carried out, to investigate possible deposition of organic compounds on surface of the specimens. The corrosion behavior of Zircaloy-4 without irradiation agreed with literature data. It was concluded that the presence of methanol did not affect the corrosion behavior of Zircaloy-4. The corrosion behavior of Zircaloy-4 hardly depended on 100 kGy gamma-irradiation. On the Raman spectra of the specimens after the immersion tests, the Raman peaks ascribed to polyethylene or graphite were not found. The deposit of decomposition products of methanol would be negligible if any. It seems that polymerization is not the major process in thermal decomposition and radiolysis of methanol, but methanol decomposes into CO2 or carboxylic acids.Copyright

Corrosion Test of 304 Stainless Steel in Hydrazine and Methanol Solutions at 320°C Under Gamma-Irradiation and Gamma Radiolysis of Hydrazine and Methanol

It has been pointed out that dissolved hydrogen is one of the key factors of PWSCC (primary water stress corrosion cracking) in the primary systems of pressurized water reactors. The authors consider that application of alternative reductant for hydrogen may mitigate PWSCC. The corrosion tests of 304 stainless steels in 2 mmol dm−3 methanol solution and 2 mmol dm−3 hydrazine solution at 320 °C were carried out under γ-ray irradiation, and the corrosion environment was evaluated. Electrochemical corrosion potential (ECP) of the stainless steels was measured during the immersion tests. The ECP values were −605 mV and −643 mV vs. SHE in 2 mmol dm−3 hydrazine solution and 2 mmol dm−3 methanol solution at 320 °C, respectively. After the immersion tests, oxide films formed on the stainless steel specimens were analyzed with scanning probe microscope and X-ray diffraction. It is concluded that from the ECP measurement and comparison with previous results corrosion environment under following conditions is similar: (1) DH 1.5 ppm without irradiation, (2) methanol 2.9 ppm without irradiation, (3) hydrazine 2 mmol dm−3 under γ-ray irradiation and (4) methanol 2 mmol dm−3 under γ-ray irradiation. During the immersion tests, the test waters were sampled several times, and analyzed, in order to understand radiolysis of the methanol solution and the hydrazine solution at 320 °C. Hydrazine is decomposed predominantly through thermal decomposition, and an ammonia molecule is formed from a hydrazine molecule. Formaldehyde and ethylene glycol were detected in the methanol solution. But, carboxylates were not detected. The present results do not demonstrate oxidation of methanol to CO or CO2 . It is pointed out that irradiation experiments of higher absorbed dose are necessary.Copyright

An in Situ Raman Spectroscopy System for Long-term Corrosion Experiments in High Temperature Water up to 673 K

A Raman spectroscopy system has been developed, in order to identify metal oxides formed on the surfaces of metals and steels in high temperature water up to 673 K. A supercritical water loop system including a Raman cell was installed. The design of the loop system is up to 673 K and 40 MPa. The Raman cell has a diamond window without window-to-metal packing. Raman spectrum of alumina plate was measured at room temperature, at 523 K and at 673 K under pressure of 25 MPa. A long-term measurement was also performed at 523 K and 25 MPa for 117.5 h. In all cases intense Raman peaks attributed to alumina were observed. Raman spectrum of anatase particles in suspension was measured at 673 K and 25 MPa. The results show that the Raman spectroscopy system developed in the present study works well not only for plate sample but also for suspension. Raman spectra observed for titanium plate in high temperature water of 673 K and 25 MPa show growth of several Raman peaks with time up to 257 h. The peaks disappeared after cooled down to room temperature. The experimental results have demonstrated importance of in situ Raman spectroscopy.