Identify the correct sequence of concentration in increasing order of the following uranium isotopes in naturally occurring uranium:

(A) U-233

(B) U-234

(C) U-235

(D) U-238

Choose the correct answer from the options given below:

Correct option is A

Introduction:

• Uranium (U), atomic number 92) is a primordial, naturally occurring radioactive element.
•  Natural uranium is a mixture of three major isotopes, all of which are radioactive: Uranium-238$$(3\text{}^{238}\text{U}), Uranium-235 (4\text{}^{235}\text{U}$$​$$(5\text{}^{234}\text{U})$$​, and a trace amount of Uranium-234.
• The difference in their natural abundance is primarily due to their respective half-lives and their role in the long-term radioactive decay chains.
• The question includes Uranium-233 ($$\text{}^{233}\text{U}$$​), which is not a naturally abundant isotope, making the sequence largely determined by comparing a synthetic isotope with three natural ones.
• The sequence is based on the approximate natural abundance (by atom percent) of each isotope:

• U-233 : Because $$\text{}^{233}\text{U}$$​ is an artificial isotope produced in the Thorium fuel cycle ($$\text{}^{232}\text{Th} \rightarrow \text{}^{233}\text{Th} \rightarrow \text{}^{233}\text{Pa} \rightarrow \text{}^{233}\text{U}$$, its concentration in naturally occurring uranium is essentially zero. It is therefore the lowest in the sequence.

• U-234: $$\text{}^{234}\text{U}$$​ is naturally present not because it is primordial, but because it is a decay product in the long-lived $$\text{}^{238}\text{U} decay chain (specifically, \text{}^{238}\text{U} decays to \text{}^{234}\text{Th}$$$$\text{}^{234}\text{U}). Since \text{}^{234}\text{U} is in secular equilibrium with its parent (\text{}^{238}\text{U}), its activity is equal to \text{}^{238}\text{U}, but due to its much shorter half-life (2.45 \times 10^5 years), its mass abundance is extremely low (approximately \text{1}/\text{17,325} of \text{}^{238}\text{U}$$​, which eventually decays to

• U-235 : As one of the two primordial isotopes,$$\text{}^{235}\text{U}$$​ has a much greater abundance than the trace decay product $$\text{}^{234}\text{U}$$​. Its concentration is critical as it is the fuel for most commercial nuclear reactors.

• U-238: This isotope, with a half-life nearly equal to the age of the Earth, is the dominant component of natural uranium.