Ree Elements Distribution

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Yucca Mountain Repository

Spent nuclear fuel is the radioactive by-product of electric power generation at commercial nuclear power plants, and high-level radioactive waste is the by-product from reprocessing spent fuel to produce fissile material for medical uses. The Yucca Mountain Nuclear Waste Repository in Nevada as designated by the NWPA Amendments of 1987, was to be a deep geological repository storage facility for spent nuclear reactor fuel and other high level radioactive waste. However, Current studies show that water moves quite rapidly through the rocks at Yucca Mountain and the spent nuclear fuel would be exposed to an oxidizing condition in the future. This can affect the nature of the spent nuclear fuel which is uraninite with impurities such as REE. The future of Yucca Mountain Repository can be compared to Oklo which is a natural repository in which alteration has happened.

Results and Discussion

Concentrations of REE and all elements within the range of the detector of the LA-ICP-MS were measured for both parts of the sample, dark and yellow, and showed in figure 5 and figure 6 respectively. In general, there is a trend showing both parts of the samples were enriched in light REE. The maximum concentration of REE in uraninite is around 12000 cps while it is about 800 cps in altered part of uraninite. There is a big difference between the counts per second of REE in these two parts. Although uraninite has a lot of impurities as REE in its structure, but because of high mobility of REE they will move and the altered uraninite will show no to little counts per second of REE in their structure.

[pic 4]

Fig.5: The concentration of REE in uraninite-dark part

[pic 5]

Fig.6: The concentration of altered uraninite-yeloow part

[pic 6]

Fig.7: The missing concentration- Maximum ppm in dark part- minimum ppm in yellow part

Figure 7 shows the missing concentration of each element which is calculated based on the maximum counts per second of each element in dark region is subtracted by the minimum counts per second of the same element. Nd shows the highest missing concentration and it can be concluded that it is highly mobile element. The data obtained from Nd can be approved by a study done by Bros et al. (1995) who found fissiogenic Nd and Sm with concentration more than the natural concentration in Oklo region as far as 7 m above the core of the reactor zone.

Conclusion

Based on isotopic studies and the ratio of 235U/238U which is much lower than other U deposits in nature, Oklo was called a natural reactor zone. The location therefore represents a natural analogue of a nuclear waste disposal site. Based upon preliminary results obtained from this study, it can be concluded that REE enrichment shows a general trend towards decreased concentration as a function of alteration in fissiogenic products. The spent nuclear fuel in Oklo natural reactor zone showed high concentration of rare earth elements while the altered part of uraninite in yellow color id depleted in REE. Nd showed the maximum missing concentration and it can be concluded that it has the highest mobility between other REE as it was proved by previous studies that fissiogenic Nd was found in several meters above the core of reactor zone. Oklo can be an analogue to a

The spent nuclear fuel in Yucca Mountain nuclear repository which is below the groundwater level and exposed to oxidizing condition might become altered and all REE based on their high mobility can leave the spent fuel and spread out in the environment. Serious actions should be taken in order to prevent this hazardous event in advance.

Future Studies

The results of this study require additional analytical procedures as a means of determining validity. Sample from other reactor zones which are surface mine and not underground mine should also be analyzed.

Determining the REE source would serve to increase understanding since REE could exist in uraninite before natural fission and also as fissiogenic products.

Moreover two alterations, preliminary and secondary, have happened to the reactor zone. Determining the age of uraninite and altered part of it by Sr and/or Pb isotopic analysis can be helpful and give us better view on understanding the REE mobility.

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