Today's inflation note concerns what the federal government, in these days of ever rising costs, is charging for radioactive plutonium, one type of which is a key material in the making of nuclear bombs.

It's something of a surprise that the Department of Energy (DOE), which produces plutonium in its reactors, sells any of it outside the government. But DOE officials quickly add that only persons or companies licensed by the Nuclear Regulatory Commission (NRC) can buy the substance, and they are only permitted to buy in quantities set by the NRC.

DOE, according to the June 2 Federal Register (page 29496), has decided to increase the price of plutonium 238 for the first time since December, 1976.

For the past 4 1/2 years, the price of a milligram of 40 percent-enriched plutonium 238 has been $1.10. Now it is to be raised to $1.70 a milligram, or $17,000 a gram.

Actually, 40 percent-enriched plutonium 238 has never been sold, according to one DOE official, because a major part of the 60 percent remaining material is plutonium 239, the type used in bomb making. "It's on the price list," he said, "because we have some in the stockpile."

The more enriched the plutonium 238, the less bomb-making material it contains but the more useful it is for research, power generation and medical applications, which are the main reasons people are permitted to purchase it.

DOE officials have some reassurance for anyone worried about the DOE being a source for bomb makers. The kind of plutonium they sell isn't good for bombs, they insist, and even that is going out primarily in milligram amounts. In a single year, DOE only sells "on the average, total, five grams," an official said last week. Unofficially, it is said, you need at least two kilograms, or 2,000 grams of plutonium 239, to make the critical mass necessary for a bomb.