After spending more than $5 billion to build and operate a giant laser installation the size of a football stadium, the Energy Department has not achieved its goal of igniting a fusion reaction that could produce energy to generate power or simulate what happens in a nuclear weapon.


The latest deadline for achieving ignition was last Sunday, Sept. 30, the end of fiscal year 2012, but it passed amid mounting concerns that the technical challenges were too great to be mastered on a tight time schedule.


Congress will need to look hard at whether the project should be continued, or scrapped or slowed to help reduce federal spending.


The idea of using lasers to trigger fusion reactions to produce energy dates back many decades, but the idea of using laser fusion for weapons research became more important when underground nuclear testing was curtailed by treaty in the 1990s.


The new laser facility, built between 1997 and 2009 and known as the National Ignition Facility, uses 192 lasers to fire light beams at tiny targets, smaller than peppercorns, filled with hydrogen atoms. The resulting compression and heat are supposed to fuse the atoms into helium, releasing bursts of thermonuclear energy. But technical reviews this year of the experiments conducted so far have made it clear that the scientists in charge do not fully understand how the process is working and may not be able to achieve ignition quickly.


Scientists at the Lawrence Livermore National Laboratory, in Livermore, Calif., which operates the facility, cite numerous technical advances gained in more than 1,000 experiments, including firing the world’s most powerful laser bursts and developing unsurpassed diagnostic instruments to measure what happens under intense heat and pressure. The review panels cite scientific and technological progress but also say that progress has been slower than anticipated.


As William Broad reported in The Times last Sunday, there is a sharp split among experts on whether the project — one of the most expensive federally financed projects ever — is worth the money. Just operating it costs roughly $290 million a year.


The laboratory’s supporters say the facility deserves continued funding because it conducts advanced research and will play an important role in assessing whether fusion will someday become a feasible energy source. They also say that it keeps highly talented weapons designers at work on important national security issues.


If the main goal is to achieve a power source that could replace fossil fuels, we suspect the money would be better spent on renewable sources of energy that are likely to be cheaper and quicker to put into wide use.


Even if ignition is achieved in the laboratory in the next several years, scaling up to a demonstration plant will cost billions and may ultimately show that fusion is not a practical source of power.


The fallback argument — that laser fusion allows scientists to simulate conditions at the core of a nuclear explosion and verify the reliability of the nation’s nuclear stockpile without having to test a weapon — is disputed by some experts who think the stockpile will be reliable for decades.


Congress will need to look hard at whether these “stockpile stewardship” and long-term energy goals can be pursued on a smaller budget.