Which of the following is/are correct for nuclear reactor? a)A typical fission is represented by
`""_(92)""^(235)U=""_(0)""^(1)nto""_(54)""^(140)Ba+""_(36)""^(93)Kr+` Energy b)Heavy water (`D""_(2)O`) is used as moderator in preference to orduinary water (`H""_(2)O`) because hydrogen may capture neutrons, while D would not do that c)Cadmium rods increse the reactor power when they go in and decrease when they go outwards d)Slower neutrons are more effective in causing fission than faster neutrons in the case of `""_(235)U`

Options

A. A typical fission represented by `._(92)U^(235)+._(0)n^(1) rarr._(56)Ba^(143)+._(36)Kr^(93)+"energy"`
B. Heavy water is used as moderator in preference to ordinary water because H may capture neultrons. While D would not
C. Cadmium rod increase the reactor power when they go in, decrease when they go outward
D. Slower neutron are more effective in causing fission than faster neutrons in case of `U^(235)`

Explanation

To determine which statements about nuclear reactors are correct, let's analyze each statement one by one.

### Step 1: Analyze Statement A
**Statement A:** A typical fission is represented by
`""_(92)""^(235)U=""_(0)""^(1)nto""_(54)""^(140)Ba+""_(36)""^(93)Kr+ Energy`

In a fission reaction, the atomic number of the reactants must equal the sum of the atomic numbers of the products plus any neutrons involved. Here, the atomic number of Uranium-235 (U-235) is 92. The products listed are Barium-140 (Ba-140) with an atomic number of 54 and Krypton-93 (Kr-93) with an atomic number of 36.

Calculating the sum of the atomic numbers of the products:
- Atomic number of Ba-140 = 54
- Atomic number of Kr-93 = 36
- Total = 54 + 36 = 90

Since 90 (sum of products) does not equal 92 (atomic number of U-235), this statement is incorrect.

### Step 2: Analyze Statement B
**Statement B:** Heavy water (`D""_(2)O`) is used as a moderator in preference to ordinary water (`H""_(2)O`) because hydrogen may capture neutrons, while D would not do that.

This statement is true. Heavy water has a lower probability of absorbing neutrons compared to ordinary water. In a nuclear reactor, it is essential to have as many free neutrons as possible to sustain the chain reaction, which is why heavy water is preferred.

### Step 3: Analyze Statement C
**Statement C:** Cadmium rods increase the reactor power when they go in and decrease when they go outwards.

This statement is incorrect. Cadmium rods are neutron absorbers. When they are inserted into the reactor, they absorb neutrons, which decreases the reaction rate and thus decreases the reactor power. Conversely, when they are pulled out, the reaction rate increases, leading to an increase in reactor power.

### Step 4: Analyze Statement D
**Statement D:** Slower neutrons are more effective in causing fission than faster neutrons in the case of `""_(235)U`.

This statement is true. U-235 is more likely to undergo fission when it captures slower (thermal) neutrons. Fast neutrons are less likely to be absorbed by U-235, making slower neutrons more effective for sustaining the fission reaction.

### Conclusion
- Statement A: Incorrect
- Statement B: Correct
- Statement C: Incorrect
- Statement D: Correct

Thus, the correct statements are B and D.