Ostensible reasons given are:
To save money;
To appear in the eyes of Japanese citizens that the national government is serious in eliminating the waste in bureaucracy.
So, the expertise in dealing with matters nuclear is considered "waste" in the post-Fukushima Japan.
And how much money this administration says it could save by forcing these engineers to retire? There are 170 of them, with the average annual salary of 13 million yen (US$149,000) according to Sankei Shinbun article linked below (on page 2 which is not quoted).
Let's see, politicians elected to the National Diet receive 22 million yen per year. They also receive generous benefits like:
12 million yen for transportation, communication;
27 million yen for hiring 3 secretaries (excluding their benefits)
7.8 million yen for maintaining the office
Many politicians say that's pittance, that without this kind of money people without resources cannot run for public office.
Personally, I'd rather have aging nuclear engineers than clueless politicians.
Anyway, here's from Sankei Shinbun (1/4/2013; part):
[Sankei Shinbun] talked to the government sources on January 3 and found out that the national government is set to demand about 170 nuclear engineers to retire all at once from the Japan Nuclear Energy Safety Organization (JNES), an independent administrative corporation, when JNES gets absorbed into the Nuclear Regulatory Agency. Under the Nuclear Regulatory Agency, these engineers would have to be treated as national public servant. However, under the current Government Officials Act, there is no system to hire a large number of older [above 60] engineers. Without a new legal action, it could lead to the loss of knowledge and expertise, which in turn could interfere with decommissioning and assessment of nuclear power plants.
The Nuclear Regulatory Agency, created in September last year to act as the secretariat to the Nuclear Regulatory Authority, lacks staff with the firsthand knowledge of nuclear power plants, and is scheduled to be combined with JNES which has staff with such knowledge. That should have happened when the Nuclear Regulatory Agency was set up, but it has been delayed as it supposedly runs counter to the government policy of reducing the number of national public servants.
The discussion will begin in earnest this year. The Office for Atomic Energy Regulation and Reorganization Promotion under the Cabinet Secretariat, which has been planning the consolidation of nuclear-related independent administrative corporations, recognizes [these older JNES engineers as] "Invaluable workforce who has experienced the peak construction period of nuclear power plants". However, according to the government sources, "As reduction of government personnel progresses, unless we totally streamline the bureaucracy the Japanese citizens won't be satisfied", and the government will demand that engineers 60 years old and above to retire all at once.
How much dumber these LDP politicians get? A whole lot, I'm afraid.
When the next big nuclear accident happens, whether it's Hamaoka or Monju or Rokkasho, guess who gets to be blamed? Japanese citizens, of course, for supposedly "demanding the cut in government spending on public servants".