That may prompt investors to bet on its resurgence by buying shares in nuclear power plant operators, equipment makers or fuel producers.

Synthetic fuel producers are anxiously waiting for the agency to affirm or amend its position on what they must do to earn the lucrative credits.

The fuel producers are hoping that the I.R.S. will resume issuing private letter rulings.

In these scenarios it is the fuel producer or distributor who is liable to register and account for duty.

"Iran will be a nuclear fuel producer and supplier within a decade."

But the number of independent fuel producers is growing, under a law guaranteeing them a minimum price for their product.

The rule was developed in collaboration with refiners, renewable fuel producers, and many other stakeholders.

This problem is largely absent in economies that do not regulate the ability of fuel producers to innovate or price based on market demand.

The proposed energy tax based on the heat content of fuels would alter costs for fuel producers more than it would change consumption patterns, analysts say.

Fuel standards increasingly require that fuel producers extract sulfur from fossil fuels to prevent acid rain formation.