Among the following one is wrong

Options

A. The energy of thermal neutrons is about `25 me V`
B. In a nuclear reaction, when neutrons multiplication factor, `K=1` then the reaction is said to be critical
C. `._(92)U^(235)` undergoes fission by bombardment of high energy neutron
D. On average `2.5` neutrons are emitted per fission of `._(92)U^(235)`

Answer

To determine which statement is incorrect among the provided options regarding thermonutrons and nuclear reactions, we will analyze each statement step by step.

### Step-by-Step Solution:

1. **Understanding Thermonutrons**:
- The first statement mentions that the energy of thermonutrons is about 25 MW. This is incorrect because thermonutrons (thermal neutrons) typically have much lower energy, around 0.025 eV, which is equivalent to about 0.000025 MW.

2. **Characteristics of Thermonutrons**:
- The second statement claims that thermonutrons are free neutrons. This is correct; thermonutrons are indeed free neutrons that have been slowed down to thermal energies.

3. **Speed of Thermonutrons**:
- The third statement states that their speed is very low. This is also correct, as thermal neutrons move at much lower speeds compared to fast neutrons due to their low kinetic energy.

4. **Neutron Multiplication Factor (k)**:
- The fourth statement discusses the neutron multiplication factor (k) and states that when k = 1, the reactor is in a critical state. This is correct. In a critical reactor, the number of neutrons produced in one generation is equal to the number of neutrons lost, which maintains a steady state of the nuclear reaction.

5. **Uranium-235 and Neutron Bombardment**:
- The statement regarding uranium-235 undergoing fusion-y bombardment of high-energy neutrons is misleading. While uranium-235 can undergo fission when bombarded by neutrons, it does not require high-energy neutrons; thermal neutrons can also initiate fission in uranium-235.

6. **Average Neutrons in Fission**:
- The final statement about the average number of neutrons produced in a fission reaction being about 2.5 is correct. This is a well-established fact in nuclear physics.

### Conclusion:
The incorrect statement among the options is the first one, which claims that the energy of thermonutrons is about 25 MW.

### Final Answer:
The wrong statement is: "The energy of the thermonutrons is about 25 MW."

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