The first nuclear reactor in India is the Apsara reactor. Let's dive into the details of this answer:

Introduction:
India's journey in nuclear energy started in the 1950s with the establishment of the first nuclear reactor, named Apsara. This marked the beginning of India's pursuit of nuclear technology for peaceful purposes.

Background:
India's interest in nuclear energy emerged shortly after gaining independence in 1947. The country recognized the potential of nuclear power for electricity generation, as well as other applications such as medicine, agriculture, and industry.

Apsara Reactor:
The Apsara reactor, also known as the APSARA (Atomic Power Station Research Reactor), was a research reactor that became operational on August 4, 1956. It was located in Trombay, Mumbai, and was designed and built by the Bhabha Atomic Research Centre (BARC).

Key Features:
The Apsara reactor had several notable features, including:
- It was a swimming pool-type reactor, where the nuclear core was submerged in water.
- The core was fueled by natural uranium and moderated by heavy water.
- It had a thermal output of 1 MW (megawatt), which was primarily used for research and experimentation.

Significance:
The Apsara reactor played a crucial role in establishing India's capabilities in nuclear technology. It facilitated research in various fields, including reactor physics, radioisotope production, and nuclear medicine.

Legacy:
The success of the Apsara reactor paved the way for further advancements in India's nuclear program. It led to the development of subsequent research reactors like CIRUS (Canadian-Indian Reactor-United States), Dhruva, and many others. These reactors have contributed significantly to India's nuclear research and development over the years.

Conclusion:
The Apsara reactor holds immense historical and technological significance as India's first nuclear reactor. It laid the foundation for the country's nuclear energy program and paved the way for further advancements in the field. The establishment of the Apsara reactor marked a significant milestone in India's pursuit of peaceful nuclear technology.