The Precipitation and redistribution of alloying element in Zircaloy-4 cladding tube oxidized in high-temperature steam

天谷 政樹

High Temperature Corrosion of Materials, 15 Pages, 2024/00

https://doi.org/10.1007/s11085-023-10216-9

Zirconium (Zr)-based alloys are widely used as fuel cladding material for light water reactors. Under a loss-of-coolant accident (LOCA) condition, the oxidation of fuel cladding by high-temperature steam induces the degradation of mechanical properties of the cladding and would affect the integrity of fuel rods and/or assemblies, etc., during LOCA. In this study, the distribution of the elements (zirconium, oxygen, tin, iron and chromium) in Zircaloy-4 cladding specimens oxidized in the temperature range of 1350- 1700 K in steam was analyzed along the radial direction of the specimens by using SEM/EPMA, and the cause of element distribution in the specimens was discussed in consideration of the morphology of precipitates in the specimens and hypothesized phase diagrams related to the elements contained in the specimens. The form of the particles precipitated and the comparison between SEM/EPMA results and hypothesized phase diagrams of Zr-Sn-O system suggested that the liquefaction of tin-rich material and/or Zr-(Fe,Cr) compounds occurred during the oxidation test. The results obtained indicate that Zircaloy-4 cladding tubes would start melting at the melting point of tin-oxide and the eutectic point of Zr-(Fe,Cr)compounds, which is much lower than the melting point of Zr, -Zr(O), or zirconium oxide (ZrO).

Fracture-mechanics-based evaluation of failure limit on pre-cracked and hydrided Zircaloy-4 cladding tube under biaxial stress states

Li, F.; 三原 武; 宇田川 豊; 天谷 政樹

Journal of Nuclear Science and Technology, 57(6), p.633 - 645, 2020/06

https://doi.org/10.1080/00223131.2019.1709996

To better understand the failure limit of fuel cladding during the pellet-cladding mechanical interaction (PCMI) phase of a reactivity-initiated accident (RIA), pre-cracked and hydrided cladding samples with base metal final heat-treatment status of cold worked (CW) and recrystallized (RX) were tested under biaxial stress conditions (axial to hoop strain ratios of 0 and 0.5). Displacement-controlled biaxial-expansion-due-to-compression (biaxial-EDC) tests were performed to obtain the hoop strain at failure (failure strain) of the samples. The conversion of the failure strains to J-integral at failure by finite-element analysis involving data of stress-relieved (SR) cladding specimens from our previous study revealed that the failure limit in the dimension of J-integral at failure unifies the effects of pre-crack depth. About 30 to 50 percent reduction in the J-integral at failure was observed as the strain ratio increased from 0 to 0.5 irrespective of the annealing type, pre-crack depth, and hydrogen content. the rate of fractional decreases of J-integral at failure with increase of hydrogen content are in the order of CWSRRX, which are essentially independent of strain ratio for the CW and SR samples. The results were incorporated into the failure prediction model of the JAEA's fuel performance code in the form of a correction factor that considers the biaxial loading effect.

Application of Bayesian optimal experimental design to reduce parameter uncertainty in the fracture boundary of a fuel cladding tube under LOCA conditions

成川 隆文; 山口 彰*; Jang, S.*; 天谷 政樹

Proceedings of 14th International Conference on Probabilistic Safety Assessment and Management (PSAM-14) (USB Flash Drive), 10 Pages, 2018/09

The reduction of epistemic uncertainty for safety-related events that rarely occur or require high experimental costs is a key concern for researchers worldwide. In this study, we develop a new framework to effectively reduce parameter uncertainty, which is one of the epistemic uncertainties, by using the Bayesian optimal experimental design. In the experimental design, we used a decision theory that minimizes the Bayes generalization loss. For this purpose, we used the functional variance, which is a component of widely applicable information criterion, as a decision criterion for selecting informative data points. Then, we conducted a case study to apply the proposed framework to reduce the parameter uncertainty in the fracture boundary of a non-irradiated, pre-hydrided Zircaloy-4 cladding tube specimen under loss-of-coolant accident (LOCA) conditions. The results of our case study proved that the proposed framework greatly reduced the Bayes generalization loss with minimal sample size compared with the case in which experimental data were randomly obtained. Thus, the proposed framework is useful for effectively reducing the parameter uncertainty of safety-related events that rarely occur or require high experimental costs.

The Effect of oxidation and crystal phase condition on the ballooning and rupture behavior of Zircaloy-4 cladding tube-under transient-heating conditions

成川 隆文; 天谷 政樹

Journal of Nuclear Science and Technology, 53(1), p.112 - 122, 2016/01

https://doi.org/10.1080/00223131.2015.1025884

In order to investigate the effect of oxidation and crystal phase condition on the ballooning and rupture behaviors of cladding tube under simulated loss-of-coolant-accident (LOCA) conditions, laboratory-scale experiments were performed in which internally pressurized non-irradiated Zircaloy-4 (Zry-4) cladding specimens were heated to burst in steam and argon gas conditions. Values of the maximum circumferential strain were normalized by dividing them by engineering hoop stress at the time of rupture. The dependence of the normalized value on burst temperature and the relationship between the normalized value and the length, width and area of rupture opening were evaluated. The correlation between the normalized value and the burst temperature suggested that the fraction of the phase in Zry-4 cladding specimens affected the strain in the specimens and the oxidation of specimens suppressed the amount of ballooning of the specimens. The relationship between the normalized value and the length, width and area of rupture opening indicated that the length, width and area of rupture opening depended on the crystal phase condition in Zry-4 cladding specimens irrespective of atmosphere in the case of the heating rate of 3 K/s.

ジルカロイ-4とMDAの機械特性に及ぼす水素吸収と中性子照射の影響(共同研究)

永瀬 文久; 菅野 光照*; 小澤 正明*; 小松 和志*; 古田 照夫*

JAERI-Research 2001-008, 61 Pages, 2001/03

JAERI-Research-2001-008.pdf:2.62MB

水素吸収及び中性子照射が軽水炉高燃焼度燃料被覆管の機械的特性に及ぼす影響に関し系統的な知見を得るために、さまざまな水素濃度を有するジルカロイ-4とMDA被覆管をJMTRにて照射し、引張試験を実施した。水素濃度範囲は10~140wtppmとした。照射量は1.1~3.610n/m(E>1MeV)であった。293及び633Kにおける試験により、水素濃度及び照射量に依存する機械特性の変化が調べられた。それらの変化は、ジルカロイ-4とMDAではほぼ同等であった。いずれの試験温度でも、800wtppm以上の水素濃度を有し照射された被覆管は、著しく小さな破断伸びを示すことがあった。このような破断伸びの著しい低下は、水素吸収量や照射量からは予想されない。本試験結果は、非常に高い水素濃度を有する照射被覆管において、水素吸収と照射の重畳的な影響を考慮する必要があることを示唆している。

室温における水素吸収ジルカロイ-4燃料被覆管の高速加圧バースト試験

永瀬 文久; 大友 隆; 上塚 寛

JAERI-Research 98-064, 25 Pages, 1998/11

JAERI-Research-98-064.pdf:2.17MB

高燃焼度軽水炉燃料棒のRIA条件下における破損挙動を調べるために、NSRRパルス照射時に生じるPCMIを模擬し被覆管を急速加圧できる装置を製作した。最初の実験として、室温で水素吸収Zry-4管に内圧を最大1.9MPa/msの高速で負荷するバースト試験を行った。その結果、水素吸収被覆管には、NSRR実験で見られた破損と酷似した軸方向に長い破損開口が生じた。さらに、破損挙動に及ぼす水素の影響が明瞭に観察された。円周方向の残留ひずみ量は、水素濃度が高いほど明確に低下した。また、水素化物を被覆外表面に偏析させた試料は、極めて小さなひずみ量で破裂し、破裂圧力も相対的に低かった。一方、加圧速度の及ぼす影響は比較的小さかった。NSRR実験等で見られたパルス照射時の高燃焼度燃料棒の破損が、被覆管の水素吸収と外面への水素化物偏析と深く関連していることが示された。

Embrittlement of Zircaloy-4 due to oxidation in environment of stagnant steam

上塚 寛; 古田 照夫; 川崎 了

Journal of Nuclear Science and Technology, 19(2), p.158 - 165, 1982/00

https://doi.org/10.3327/jnst.19.158

軽水炉の冷却材喪失事故時に生ずると予想される内面酸化によるジルカロイ被覆管の脆化挙動を明らかにするために、破裂被覆管内部の雰囲気状態を模擬した滞留水蒸気雰囲気中においてリング状ジルカロイ-4試片を890~1194Cの温度範囲で等温酸化した。その結果、滞留水蒸気雰囲気中で酸化したジルカロイ-4の脆化は主に吸収水素量に影響されることが明らかになった。100Cにおけるリング圧縮試験で、約500wt.ppm以上の水素を吸収したジルカロイ試片は脆性を示した。本実験の結果は、著しい脆化の原因となる破裂被覆内面における水素吸収が破裂被覆内部の雰囲気状態と強く関係しているという前報で報告した考え方を指示するものである。

Zircaloy-4 cladding embrittlement due to inner surface oxidation under simulated LOCA condition

上塚 寛; 古田 照夫; 川崎 了

Journal of Nuclear Science and Technology, 18(9), p.705 - 717, 1981/00

https://doi.org/10.3327/jnst.18.705

軽水炉のLOCA時に燃料被覆管が破裂した場合に生じる内面酸化による被覆管脆化挙動を明らかにするために、水蒸気流中で模擬燃料棒破裂・酸化実験を行なった。 その結果、内面酸化した燃料被覆管の脆化は被覆中に吸収された水素による水素脆化であることが明らかになった。100Cにおけるリング圧縮試験の結果、ジルカロイ被覆管は酸化温度が1000C以上の場合、吸収水素量200~300wt.ppm以上で脆化することがわかった。 酸化温度が932~972Cの場合には500~750wt.ppm以上で脆化することがわかった。 被覆中に吸収された水素はprior-結晶粒界近傍に1m以下の微細な水素化物として析出していた。 破裂開口中心から軸方向に15~45mm離れた位置に水素吸収のピークがあった。 水素吸収のピーク位置は酸化温度と水蒸気流速の違いによって変化した。