With a drip and a bubble and a plop and a boom, up goes Tank 701 at the Citichem chemical plant in Somewheresville, U.S.A. Soon all the air for miles around has been poisoned with K-34, and you know what that means. Or can guess. "Acceptable Risks," the "ABC Sunday Night Movie" at 9 on Channel 7, conjures a lethal leak of toxic gas in a typical American town, and should therefore be much more harrowing than it is.

Obviously inspired by the tragic occurence at Bhopal, India, where at least 2,000 people died after a toxic leak in 1984, "Acceptable Risks" unfortunately lets off very little steam of its own. The film is nearly three-quarters over before the tank has finally blown its top and sent deadly vapor into the surrounding vicinity. Until that happens, the talky-talky film proves awfully wearying with its environmental debates and cookie-cutter romances.

Two of the very best character actors around, Brian Dennehy and Kenneth McMillan, are convincingly distraught as the head of the plant and the maintenance chief; Christine Ebersole, as a new employe, gives another of her casually vivacious performances; Cicely Tyson pours heart and soul into the role of a concerned councilwoman; and Beah Richards strikes sentimental sparks as her mother.

But for all that, the film seems weak and watered down, the script by Norman Strum reportedly having suffered the heavy hand of network meddling. The corporation that owns the chemical plant is, for instance, let off the hook; the employes are all depicted as scrupulously vigilant about safety standards. When an AY-47 level transmitter gauge fails to arrive from Pittsburgh, it sets the dire course for the plant, and it's depicted as being virtually nobody's fault.

If anybody is a bad guy, it's the city's mayor, who wouldn't listen to Tyson's warnings and hastily approved a new housing development right next door to the plant. Dennehy's plant manager gets heavy pressure from the home office to up production of K-34, but the home office escapes being a heavy just the same.

When the tank blows, director Rick Wallace gets to stage "Day After"-like scenes of human calamity, mostly people choking and swooning. Despite the occasional unconvincingly gasping extra, it is a frightening sequence, perhaps in part because the little California town had seemed so Spielbergily idyllic. But 15 minutes of shock value doesn't quite redeem all the shuffling and stalling that has preceded it. "Acceptable Risks" takes too few risks itself. 'Outrage'

Shamelessly preposterous on nearly every conceivable level, "Outrage," the CBS Sunday Night Movie at 9 on Channel 9, may be the looniest vigilante movie of the year. Oh yes -- another vigilante movie. The networks apparently are going to exploit the vigilante theme until at last viewers and critics rise up in a mob and take communications law into their own hands. Now there's a credible plot for you.

But it is not the plot of "Outrage." The plot of "Outrage" is this: a man whose daughter was murdered and raped and who has seen the killer go free on a technicality (are there no end to these cursed technicalities?), takes the law into his own hands, shoots and kills the man, and then turns himself into the police. A brash young attorney takes his case and decides the only chance he has of winning is "by putting the system of justice on trial," which he does. The system loses.

Robert Preston, his theatrical gentility making him totally wrong for the role, plays the avenging father, Beau Bridges plays the idiot attorney and Burgess Meredith makes a spectacle of himself as the world's crustiest judge. Linda Purl plays Bridges' wife, a role given excessive prominence for a courtroom drama. Indeed, so much attention is lavished on the couples' home life, their hopes and dreams, that the film seems a pilot for a series. Maybe, "Joe Jerk, Lousy Lawyer."

The attorney's courtroom tactics are patently ridiculous even in the context of courtroom-movie theatrics. Writer Henry Denker puts didactics first, dramatic credibility last. He has his chess-piece characters behave in unprofessional and implausible ways.

For example, the accused father insists he will not cooperate in defending himself against the murder charge; but then he does cooperate, sitting silently during the trial and his attorney's mad scenes. The lawyer wants to introduce the rape and murder into the case but can't; then the prosecution does introduce it by playing the father's videotaped confession to the jury, a ploy that does the defense, and the screenwriter, a huge clumsy favor.

One of the few cliche's that the judge does not sputter in the course of the trial is the one about how he isn't going to let the courtroom be turned into "a circus." That's perhaps because it becomes a circus almost at once. All it needs is three dancing elephants and a fat lady.

We are supposed to like the young lawyer played by Bridges even when he stoops to a gratuitous racist insult directed at the prosecutor, who is black and who is played with a simmering dignity by William Allen Young (never has there been a courtroom drama in which a sensible viewer's sympathies are bound to be so at odds with the author's). The film gets into racial matters with which it cannot cope and which it therefore should have avoided. The prosecutor forgives the racist remark. We don't. The film stomps on shaky ground.

Denker enjoys playing J'accuse, however. He picks on a whipping boy already riddled with welts, the media. Anthony Newley plays an unscrupulous literary agent who offers big bucks for rights to the vengeful daddy's tale. The character says of television, "Unless you're on it, you may as well not exist."

After turning down the offer, Preston's character laments to his young lawyer, "What kind of world are we living in, kid? Is the almighty dollar so important that people will buy and sell anything?" Answer: apparently yes, else how would a bungled mess of a movie like this get made and shown on TV? 'Valerie'

As Mary Tyler Moore has attempted a sitcom comeback this season and failed, her old sidekick Valerie Harper (Rhoda on "The MTM Show") now resurfaces and, alas, proceeds to sink. "Valerie," an NBC comedy getting a "sneak preview" at 8:30 tonight on Channel 4, then moving to its regular time slot Monday at 8:30, is a vehicle that lumbers in on about 3.5 flat tires.

Harper has had her wings clipped and been domesticated for the role of an airline pilot's wife (he disappears at the start of each episode) raising three teen-age sons in Oak Park, Ill. Both the first and second episodes focus not on Harper but on the oldest of the sons, 16-year-old David, played by Jason Bateman.

Shades of "Family Ties." Except that Bateman is no Michael J. Fox; he is even less interesting. Bateman did a snappy job of playing a youthful con man on last year's short-lived "It's Your Move," but a year in the life of a kid can be devastating to his charisma. Bateman lost his.

Harper is reduced to traipsing around after the kid and his amorous embarrassments. In the premiere tonight he has decided to date a 24-year-old woman but mom straightens him out and he learns a valuable lesson about relationships. In the second episode he is ashamed to be seen in the company of this nice girl he really likes because she isn't pretty enough but mom straightens him out. And he learns a valuable lesson about relationships.

That "The Cosby Show" has brought wholesomeness back to situation comedy is fine, but people forget the program is also cheeringly funny. "Valerie," written by Charlie Hauck and directed by James ("Cheers") Burrows, is mired in old-style comedy writing that attempts to disguise itself with yuppie-stroking references to Trivial Pursuit, rock 'n' roll, and the "Attica, Attica" chant from "Dog Day Afternoon."

Harper was able to make a whiny, long-suffering urban kvetch appealing and funny on Moore's show and on "Rhoda," but she can't do much with the relatively colorless role of Hip, Caring, All-American Mom. Perhaps, though, if the show's spotlight ever shifts away from the boring son and back to her, Valerie and "Valerie" may yet get a chance to shine. If not shine, at least glow a little.