# In Situ Neutron Radiography Investigations of Hydrogen Related Processes in Zirconium Alloys
## 3 Examples of In Situ Neutron Radiography Investigations
### Investigation of the Reaction of Zry-4 in Steam, Nitrogen/Steam, and Air/Steam Atmospheres
#### 3.3.3 Results and Discussion  
A large number of tests were performed. Describing the results of all of them exceeds the framework of this paper. As an example, Figure 11 shows the development of the sample annealed in a nitrogen/steam atmosphere at 900 \({}^{\circ}\)C with a total gas flow rate of 24 L/h. A neutron radiography video of this reaction is available online as supplement S3.  
The strong swelling is obvious. Macrocracks are visible after a 2 h reaction. The images are referenced by the image of the empty furnace. The sample holder rod is visible. Referencing with the sample holder rod but without the sample is not successful because the weight of the sample changes the sample holder position slightly but sufficiently that the sample holder appears in the image.  
A comparison of the reaction in the different atmospheres gives information about the influence of nitrogen on the reaction kinetics. In Figure 12, the sample appearances after reaction at 800 \({}^{\circ}\)C for 8 h and at 1000 \({}^{\circ}\)C for 40 min are compared for annealing in a pure steam, air/steam, and nitrogen/steam atmosphere, respectively. The samples are referenced by the initial sample state. Due to this referencing, the initial sample image appears brighter if the sample shape is strongly changed like in the image taken after 8 h reaction in nitrogen/steam.  
Whereas the neutron transmission of the Zry-4 sample does only marginally change after 8 h reaction at 800 \({}^{\circ}\)C in pure steam, it increases after reaction in an air/steam mixture. In addition, a slight swelling of the sample is visible (former lower edge of the sample appears bright). For the sample annealed in a nitrogen/steam mixture, a strong swelling and a large increase of the total neutron cross section can be observed. The formation of zirconium nitride (ZrN) precipitates in the oxide layer, as well as their reoxidation, is connected to strong volume changes. Similar to the breakaway effect discussed before, many cracks are formed in the oxide scale, resulting in an enhanced hydrogen uptake. In the air/steam mixture, additional oxygen is available, reducing susceptibility to reaction with nitrogen. In addition, less steam is consumed, and with it, less free hydrogen is produced.  
Figure 11: Radiographs of the sample reacted in a nitrogen/steam atmosphere with a total gas flow rate of 24 L/h.  
Figure 12: Comparison of neutron radiographs taken after 8 h reaction at 800 \({}^{\circ}\)C and 40 min at 1000 \({}^{\circ}\)C in pure steam, air/steam, and nitrogen/steam mixtures (flow rates of 2 g/h H\({}_{2}\)O and 10 L/h nitrogen or oxygen flux, respectively).  
Relations differ for reaction at 1000 \({}^{\circ}\)C. The changes in the neutron transmission were lowest after reaction in an air/steam atmosphere. Mainly, the oxygen in the air was consumed, and only little free hydrogen was produced. The increase in the total neutron cross section of the sample oxidized in pure steam was caused by the strong hydrogen uptake due to the breakaway effect discussed before. The strongest reaction was found in the nitrogen/steam atmosphere. Here, the breakaway of the oxide scale started much earlier than in pure steam. It was surprising and unknown before that the first hints of a breakaway of the oxide scale were already detected after a 2 min reaction time. As mentioned before, the breakaway of the oxide scale starts after an approximately 30 min oxidation time.  
More details of the experiments and the results obtained are given in [13] and will be published in [19].