In a nuclear reactor.

Options

A. the chain reaction is kept under control by rods of cadmium, which reduces the rate
B. the thick concrete shield is used to slow down the speed of fast neutrons
C. heavy water (or graphite )moderate the activity of the reactor
D. out of `U^(238)` and `U^(235)` natural uranium has less than `1%` of `U^(235)`

Explanation

To solve the problem regarding the functioning of a nuclear reactor, we will analyze each of the provided options step by step.

### Step-by-Step Solution:

1. **Understanding the Role of Control Rods**:
- In a nuclear reactor, the chain reaction is controlled using control rods, which are typically made of materials like cadmium or boron. These materials are effective at absorbing neutrons.
- **Conclusion**: The statement that "the chain reaction is kept under control by rods of cadmium which reduces the rate" is **True**.

2. **Function of the Thick Concrete Shield**:
- The thick concrete shield in a nuclear reactor is primarily used for radiation protection. It protects personnel and the environment from harmful radiation emitted during the nuclear fission process.
- **Conclusion**: The statement that "the thick concrete shield is used to slow down the speed of fast neutrons" is **False**. Its main purpose is to shield against radiation, not to slow down neutrons.

3. **Moderation of Neutrons**:
- Heavy water (D2O) and graphite are materials used as moderators in nuclear reactors. Their role is to slow down fast neutrons to thermal energies, which increases the likelihood of these neutrons causing fission in uranium-235.
- **Conclusion**: The statement that "heavy water or graphite moderates the activity of the reactor" is **True**.

4. **Composition of Natural Uranium**:
- Natural uranium consists mainly of uranium-238 (about 99.3%) and a small fraction of uranium-235 (about 0.7%). Thus, it is accurate to state that natural uranium has less than 1% of uranium-235.
- **Conclusion**: The statement that "out of uranium-238 and uranium-235, natural uranium has less than 1% of uranium-235" is **True**.

### Final Evaluation of Options:
- Option A: True (Control rods of cadmium reduce the rate of reaction)
- Option B: False (Thick concrete shield does not slow down fast neutrons)
- Option C: True (Heavy water or graphite moderates the reactor)
- Option D: True (Natural uranium has less than 1% uranium-235)

### Correct Options:
The correct options are A, C, and D.