Yantar is a fairly large area containing of a mobile lab, a swamp and a facility to the north. You'll come across alot of zombie stalkers and Snorks so be sure to bring plenty of firepower. Once the Brain Scorcher has been disabled more stalkers will move into the area and form a camp to the north-east; Duty members will also be sent in to protect the mobile lab.
Moving ahead you should soon notice a bunker-like structure with a wall around it - that's the mobile lab. Head there and dispatch of any zombie stalkers you run into on the way. Before you move inside take your time to investigate the nearby truck; it has a Wind of Freedom Suit in the back - jump onto the front bumper and further on into the back.
Inside the mobile lab you'll find Professor Sakharov and Kruglov (if you managed to save him). When you talk to Sakharov you'll be given your reward (SSP-99 Ecologist suit) given that you did manage to save Kruglov. When you inquire about X16 he'll say that they are trying to come up with a protection against the psi-emissions coming from the Brain Scorcher. He wants you to help them gather some more readings. Accept and you'll get the Take radiation measurements mission.
Sakharov functions as a trader so if you need to either supply yourself or get rid of some of your loot, now is the chance to do so; also notice that in the room next to Sakharov you'll be able to find a blue box stash that you can use for surplus equipment that you don't wish to either sell or drag along into X16. Sakharov will also provide optional missions. One of them, called Bring the unique bodysuit, requires you to obtain an armor found inside X16. It's a unique Stalker Suit, the Ghost Suit, so if you're already using armor that's superior to a basic Stalker Suit then be sure to pick up this mission - and even if you're still using eg. a Stalker Suit the reward for the mission is a much better armor than the Ghost Suit so there's really no reason why you would wish to keep it.
Inside the walls that surrounds the mobile lab you'll be able to find a number of containers; one of them has two crates inside it which may spawn some goodies. There are also some crates on top of the lab itself (use the ladder on the southern side to get up there) which also contain some items - I personally found an artifact and a RGD-5 grenade.
When you're ready to move on, go talk to Kruglov. You'll need to escort him outside as he takes readings. He'll head towards the tunnel to the north-east, then through it to the other side of the hill; try to stay ahead of him and deal with any nearby zombie stalkers. Kruglov, if caught by zombies, will go down very easily so it's urgent that you take them down before Kruglov runs into them. However, it's fortunately not the end of the world if Kruglov should die, but you'll miss out on a Stone Flower artifact that Sakharov will give you as a reward for keeping him alive.
Kruglov will make three stops on his way - just before the tunnel, just after, and next to the bus at the other end of the tunnel. At the final spot, he'll warn you but don't bother running or anything - you'll pass out and shortly after wake up again inside the bus. Kruglov is still outside, wounded and in need of a medkit - this part can be tricky because even though you may have killed all the zombie stalkers in the vicinity some may still appear and shoot Kruglov before you can act. If you're very keen on keeping him alive, and/or earning the Stone Flower artifact, be sure to create a new savegame before you reach the end of the tunnel.
Going back to Sakharov you'll be given a a functional psi-helmet (that will protect you from the Brain Scorcher) along with a new mission: Get inside the lab. He'll tell you about an associate of his, Vasiliev, that tried to enter the X16 lab along with Ghost not long ago. Vasiliev never made it back, at least not entirely, but his corpse is close by near the swamp - why they didn't bother to go look for him may be a good question - but then again, neither Sakharov nor Kruglov look like the Rambo type, and with all the zombie stalkers and Snorks about... I guess there is a good reason after all.
You can find Vasilievs body near the downed chopper to the west of the mobile lab and the swamp. There's bound be alot of zombie stalkers and Snorks about so be careful when you get close to the swamp. Once you search his body, you'll receive a recording that shed a little light on what has happened. You will also complete the mission Find out what happened to Ghost. Once you've heard the recording it's time to get to the lab. Go back to the mobile lab if you need to either get some more equipment or lose some items. You'll find alot of artifacts and general loot inside the lab so travel as light as possible.
When you enter through the gate move south to a hangar, it has some Burnt Fuzz anomalies inside along with a number of crates and a metal box. The metal box contains some ammunition, including the rare 7.62x54 mm for for the SVDm2 and the SVUmk2 sniper rifles. When you come out on the other side of the hangar you should be able to see a body (white dot) on your minimap - be sure to check it out as you'll can find the Healing Beril, a unique armor that has lacking protection against elements but a great bulletproof cap. Just north of the facility gate there is another hangar containing a metal box with some bandages and medkits inside it. Just south of the building with the entrance to X16 you'll find a small house (almost a shed) containing a Stalker Suit along with some boxes containing a few goodies.
When you've searched the entire place, or you just wish to head on, move into the building near the map marker; before you enter the lab, down the stairs, you may wish to go up first and retrieve a Goldfish artifact. Now head down and into X16.