Japan’s prime minister, Shinzo Abe, has brought one of the country’s rising political stars into his cabinet, sparking speculation over his successor as he prepares to mark a record-breaking period in office.

But Shinjiro Koizumi, the son of the former prime minister Junichiro Koizumi, faces an early test of his abilities after he was appointed environment minister on Wednesday.

Koizumi, 38, will be instrumental in deciding the fate of more than 1m tonnes of contaminated water stored at the wrecked Fukushima Daiichi nuclear power plant. His appointment comes one day after his predecessor, Yoshiaki Harada, said the plant’s operator had no choice but to dump the water into the Pacific Ocean.

Koizumi has been widely tipped as a future prime minister. In a Kyodo news agency survey in May, he ranked second after Abe on who should be prime minister, with 19.9% support compared with 20.1% for Abe.

But, according to some observers, he may lack the experience and support inside the ruling Liberal Democratic party to immediately succeed Abe.

Despite his inexperience, Koizumi’s family background has guaranteed him widespread media coverage throughout his four terms as an MP.

On Wednesday, he became the third-youngest cabinet minister in Japan since the end of the second world war. He made the news again recently with the announcement, made outside the prime minister’s office, that he will marry Christel Takigawa, a well-known TV personality with whom he is expecting a child.

Little is known, however, about Koizumi’s views on specific issues, including nuclear power. Under Abe, Japan has committed to restarting some of the nuclear reactors closed down in the aftermath of the March 2011 meltdown at Fukushima Daiichi – a crisis that turned his father into a prominent opponent of nuclear power.

“Until now, Shinjiro has been mostly performance and it is not clear how much policy knowhow he has,” said Yu Uchiyama, a professor at Tokyo University. “Now that will become clear.”

Shinzo Abe will become Japan’s longest-serving leader in November. Photograph: Mikhail Metzel/Tass

Abe turned to more experienced colleagues for senior ministerial posts, moving Taro Kono from the foreign ministry to defence and replacing him with the economic revitalisation minister, Toshimitsu Motegi, whose promotion was seen as a reward for his work in negotiating a trade deal with the US. Taro Aso, the gaffe-prone deputy prime minister, will stay on as finance minister.

The number of women in Abe’s 19-member cabinet stands at just two after he appointed Seiko Hashimoto, a former Olympic speed skater, as Olympics minister a year before the summer Games open in Tokyo. Sanae Takaichi, a rightwinger who regularly visits Yasukuni, a controversial war shrine in Tokyo, was made interior minister.

Some analysts interpreted Abe’s reshuffle as the start of a process to pick his successor. “Abe intends to start an open race to pick the next prime minister or even the one after that,” said the SMBC Nikko Securities chief market economist, Yoshimasa Maruyama.

Abe, who began his second stint as prime minister in December 2012, will become Japan’s longest-serving leader in November. His tenure as LDP leader ends in September 2021, so party rules would have to change if he decided to serve another term.

Koizumi, often referred to by his first name, graduated from a private Japanese university in Japan and has a master’s degree from Columbia University in New York. He became an MP in 2009, winning the lower house seat vacated by his father.

Agencies contributed to this report