The phenomenon of nuclear fission can be carried out both in a controlled and in an incontrolled way. Out of the following, the correct statements vis-à-vis these phenomens are:

Options

A. The fission energy released per reaction is much more than conventional nuclear reactions and one of the products of the reaction is that very particle which initiates the reaction.
B. It is the ''surface to valume'' ratio of the sample of nuclear fuel used which determines whether or not the reaction would sustain itself as a 'chain reaction''.
C. The ''control rods'' in a nuclear reactor must be made of a material that absorbs neutorns effectively.
D. The energy released per fission as well as energy released per unit mass of the fuel in nuclear fission are both greater than the corresponding quantities for nuclear fusion.

Explanation

To solve the question regarding the phenomenon of nuclear fission and to identify the correct statements about controlled and uncontrolled fission, we will analyze each statement step by step.

### Step-by-Step Solution:

1. **Understanding Nuclear Fission**:
- Nuclear fission is the process where a large nucleus splits into smaller nuclei, releasing a significant amount of energy. This can occur in both controlled environments (like nuclear reactors) and uncontrolled environments (like nuclear bombs).

2. **Analyzing Statement A**:
- **Statement A**: "The energy release per reaction is much more than conventional nuclear reactions and one of the products of the reaction is that very particle which initiates the reaction."
- **Evaluation**: This statement is correct. In nuclear fission, the energy released per reaction is indeed much greater than in conventional chemical reactions. Moreover, one of the products of fission is a neutron, which can initiate further fission reactions, thus sustaining a chain reaction.

3. **Analyzing Statement B**:
- **Statement B**: "The surface to volume ratio of the sample of nuclear fuel used determines if the reaction would sustain itself as a chain reaction."
- **Evaluation**: This statement is also correct. The surface to volume ratio is crucial in determining whether the fission reaction can sustain itself. A higher ratio can lead to a more effective chain reaction because it allows more neutrons to escape, while a lower ratio retains more neutrons for further fission.

4. **Analyzing Statement C**:
- **Statement C**: "The control rods in a nuclear reactor must be made up of a material that absorbs neutrons effectively."
- **Evaluation**: This statement is correct as well. Control rods are indeed made from materials like boron or cadmium, which are effective at absorbing neutrons to regulate the fission process and maintain a controlled reaction.

5. **Analyzing Statement D**:
- **Statement D**: "The energy release per fission as well as energy release per unit mass of the fuel in nuclear fission is greater than in nuclear fusion."
- **Evaluation**: This statement is incorrect. The energy released per reaction and per unit mass in nuclear fusion is significantly greater than that in nuclear fission. Fusion reactions, such as those occurring in the sun, release much more energy than fission reactions.

### Conclusion:
Based on the evaluations:
- **Correct Statements**: A, B, and C.
- **Incorrect Statement**: D.

### Final Answer:
The correct statements regarding nuclear fission are A, B, and C.

---