A terrorism plot that sparked last summer's Orange Alert envisioned turning the Citigroup Center into "cutthroat shrapnel," according to a report yesterday. In reconnaissance plans, accused terrorist Dhiren Barot, currently jailed in Britain, called the building a glass house whose panels could be turned into "a potential flying piece of cutthroat shrapnel," according to a CBS News report. Barot carefully cased the landmark tower on Lexington Ave. and E. 53rd St. down to the smallest detail - even describing how the toilets could be used as a place to assemble a bomb. "The documents were very detailed," NYPD Deputy Police Commissioner Paul Browne told the Daily News. "They included, for example, how many seats there were around the conference table in the board room of the New York Stock Exchange [which he also scouted]. The level of detail showed they had engaged in serious reconnaissance."Barot, 33, whose alleged Al Qaeda alias was Abu Eisa Al Hindi, checked out financial centers in Newark and Washington in addition to Citigroup and the Stock Exchange. A 50-page printout from his computer obtained by CBS revealed he was very thorough. "Restrooms ... do not have fully enclosed ... doors," he wrote. "If anything is being assembled there ... rest it on the toilet seat. So it can't be seen." Barot gave precise dimensions of support columns and mentioned they were coated with a "fire proof" material that was deemed "effective" except "for infernos such as ... the WTC," a reference to the 9/11 attacks.