The Senate is expected to vote early this week on starting up production of neutron versions of the 8-inch artillery shell and Lance warhead - key steps, according to Pentagon officials, in a modernization of U.S. tactical nuclear weapons deployed in Europe.

The debate may focus less on the neutron warheads themselves and more on amendments that would give Congress, or one house, a veto over President Carter's decision to go ahead with the weapons.

The President, who until news stories appeared was unaware that neutron weapon production funds were in the fiscal 1978 budget, has promised to review the program and make his decision after Aug. 15.

Meanwhile, in a routine letter, the Pentagon, on Carter's behalf, asked Congress last month go approve the funds so he would have "maximum flexibility" in making his decision.

Yesterday, a key Pentagon official said no decision had been made on how many neutron weapons eventually would be purchased. He made clear, however, that the Defense Department would press for production of not only the 8-inch but also the smaller 155 mm shells.

There are some 450 8-inch tubes in Europe and 2,196 155s, according to testimony given a House Appropriations subcommittee in March. Both the United States and its allies have these weapons.

"The overall number (of neutron shells) depends on the mix we decide upon with NATO allies," the official said.

The House approved the neutron weapon production money without debate, but it deleted research funds for the neutron 155 mm shell.

The Senate bill contains all the money requested by the administration.

Unlike current nuclear weapons that destroy enemy installations, equipment and personnel by blast and heat, the planned neutron shells and missiles are specifically designed to kill enemy personnel primarily by radiation and to limit the collateral damage of blast and heat.

Proponents of the new weapons argue that because they limit potential damage to areas surrounding the battlefiled, they are more usable than warheads now in the stockpile. And, they add, a usable weapon makes a more credible deterrent to Soviet military men.

Opponents, however, say that if the neutron weapons are more usable, they in effect lower the vague threshold between conventional and all-out nuclear war. They also consider the neutron concept - killing enemy personnel through radiation - a more inhuman form of warfare than even traditional nuclear weapons.

There was some opposition voiced against the neutron Lance on the grounds that an arms control impact statement had not been sent Congress as required by law.

The Arms Control and Disarmament Agency recently completed such a statement on the neutron weapons and according to informed White House sources, though it projected some "negative" reactions by the Soviet Union, they were termed "trivial."

The pro and con arguments on the weapons art only one element in the current Senate debate.

Sen. Mark O. Hatfield (R-Ore.), who has led opposition to funding the new weapons are only one element in the should not vote the money until Carter decides whether he wants to go ahead with production.

Hatfiled, making that argument, narrowly lost out in the Senate Appropriations Committee on a 10-to-10 vote. On July 1, he lost another vote, 43 to 42, as Sen. John C. Stennis (D.-Miss.) watered down an attempt to strike the neutron production money.

The Pentagon has put out a chart showing that the proposed radiation kill radius of a neutron shell would be about 200 to 300 yards. Defense officials also note that since neutron radiation does not travel far and dissipates quickly, it would cause less destruction and fewer casualties among those outside the immediate target area.

Defense officials said yesterday, however, that the neutron weapons, whether Lance warheads or artillery shells, would not be fired alone, but as part of a package of perhaps 30 to 50 nuclear shells and warheads, possibly including some not of the enhanced radiation variety.

In an interview with a Washington Post reporter yesterday, Lt. Gen. Howard H. Cooksey, Army research director, said neutron warheads would permit added nuclear coverage of European invasion routes. Tactical nuclear weapons now deployed are limited to where they can be fired because of the danger of killimg civilians and destroying property.