A BBC News website team is in Ukraine to assess the legacy of the nuclear disaster at Chernobyl, 20 years after it spewed radiation across Europe.
The Chernobyl plant had four reactors - it was the fourth that exploded
Reporter Stephen Mulvey and photographer Phil Coomes will publish their reports and picture galleries closer to the anniversary itself on 26 April.
This diary records their impressions as they gather facts and interview people affected by the tragedy.
They will be in Ukraine until 5 April.
2 April: 2300 (2000 GMT)
The effects of the Chernobyl disaster were felt far beyond the limits of the 30km (19 mile) zone around the plant.
Reindeer as far away as the Nordic countries and sheep in some British uplands became unfit for human consumption.
The land rendered uninhabitable also stretches out beyond the 30km mark, mainly to the north, in Belarus, and to the west, in Ukraine.
At the same time there are areas inside the 30km zone (mainly in the south) which are less contaminated than parts of Kiev.
Blame the weather for this. The wind blew fallout in various directions before it fell to earth or was chucked down in rainstorms.
It took a while to draw up a good contamination map, but people needed to be evacuated quickly.
There was at least one case where people were moved from one of the cleaner villages inside the zone, to a more severely contaminated area outside it.
We met some people today who experienced a different problem, in some ways the reverse.
Children we met while asking for directions to the village
They lived outside the zone in a village that was only recognised as a radiation hotspot five years after the accident.
They were evacuated very late, to a newly-built village 40km (25 miles) away.
The new homes were thrown up quickly and without sufficient care. Many suffer from subsidence. Sometimes it can be seen with the naked eye that what was once a horizontal line of bricks is now slanting down at one end. Cracked walls abound.
The fine wooden houses the villagers left behind were bulldozed to ensure they were never inhabited again.
Only the cemetery there remains intact. Every family returns to it, the week after Easter, for a picnic with their ancestors.
1 April: 2100 (1800 GMT)
We talked today to two people whose lives have been touched by the Chernobyl disaster, in completely different ways.
One founded a youth movement, aimed at teaching young people to care for the environment, before she had even entered her teens.
A professor gave a talk about Chernobyl at her school, and she decided there and then that something should be done. Now 19, she has already been doing it for several years.
Pictures show those who risked their lives trying to clean up Chernobyl
She and her fellow activists visit schools to spread their message and organise out-of-school activities in their own suburb of Kiev.
So, for example, if you ever come across a group of uniformed children cycling through woods on the outskirts of Kiev on litter patrol, don't be surprised.
The other person we spoke to arrived for his shift at Chernobyl's fourth reactor on 26 April 1986, and found it no longer existed.
He spent three days following instructions to pump water into the ruins, by which time he had soaked up so much radiation he would never be allowed to work in the nuclear industry again.
He too has sought to "do something" about Chernobyl - by joining a group of artists, which draws attention to society's self-destructive potential.
Road signs recall the scores of villages that had to be evacuated
We also visited Kiev's Chornobyl Museum ("Chornobyl" being the Ukrainian version of the Russian "Chernobyl", which has become standard in English).
The museum's entrance hall and stairs are hung with the road signs that once told drivers they were entering or leaving 74 of the 112 evacuated villages in the zone.
A video shows, fleetingly, one of the six weddings that took place in Pripyat on 26 April, hours after the explosion.
There is also footage recording the heroic efforts of miners to tunnel under the reactor so that a heat exchanger could be built to halt the progress of the molten reactor core, if it burned its way through the plant's lower depths.
There are no postcards or souvenirs. Maybe it would be disrespectful if there were. However, I suspect there would be a market for them.
MYSTERIOUS GAS MASKS
31 March: 2230 (1930 GMT)
The story of the gas masks in the nursery schools turns out to be more complicated than I thought.
The explosion at the power station happened at 0123 on a Saturday morning. Life in Pripyat continued pretty much as normal for the rest of the day, but the town was evacuated on the Sunday. There were no children left to go back to school on Monday. So...where did the gas masks come from?
Vladimir Usatenko, a former member of parliament who spent much of the 1990s drafting laws governing nuclear power, tells me that the gas masks were held in the school - as they were in all schools - for use in case of attack by the US and its allies.
They were dragged out of cupboards and scattered around the classrooms by people who visited the school long after the evacuation. Possibly even by unscrupulous journalists.
Stephen Mulvey undergoes a radiation check after visiting the control room of the reactor
The guides who show visitors around the zone are working flat out as the 20th anniversary approaches. We bumped into Danish, Finnish and German radio and TV crews, and saw others from a distance.
One of these other crews was covered from head to toe in protective gear, with only their eyes exposed. This, surely, was for theatrical effect.
We wore disposable gloves if we touched anything that could have been contaminated, washed before eating and after returning to the hotel, and left our muddy wellington boots behind when we left. We would have worn masks had the weather been dry and dusty. We were checked for contaminants on leaving the zone, and found to be clean.
NO SHOW HORSES
1430 (1130 GMT)
No Przewalski's horses were posing for photographers today, unfortunately. A couple of weeks ago they were paying regular visits to a big haystack on the outskirts of town, provided by the Chernobyl fire brigade. But the weather has improved and they can find plenty to eat elsewhere.
The horses, which originated in Mongolia, became extinct in the wild in the 1960s, but have been successfully bred on a reserve in southern Ukraine.
Part of a doll left behind at the nursery in Pripyat by a child
The Chernobyl zone provided another area big enough for them to occupy comfortably, so they were introduced here too. Three years ago, according to Wormwood Forest, a natural history of the zone by American-Ukrainian author Mary Mycio, there were 65 of them.
Visiting Pripyat is an unforgettable experience. The nursery school was the most moving place we called at. Tiny shoes, dolls, a class photo album. I came across a teaching plan for one class for the year 1985-86.
It was filled in right up to 25 April, the day before the disaster. The teachers had developed the children's speaking ability by discussing a poem, The Little Flag, and gone on to practise counting, while at the same time consolidating their knowledge of colours, shapes and size.
Child-sized gas masks were also littered around, implying, perhaps, that some children came back to school on the Monday after the explosion, before they were evacuated.
The harbour in the town of Chernobyl
A wide variety of animal species has benefited from 20 years of peace and quiet in the zone. Wild boar, wolves, lynx, red and roe deer and moose, for example. We saw two moose yesterday and are hoping to see a species introduced by scientists - Przewalski's horse - today.
European bison have been reintroduced in the Belarusian section of the zone.
Wild boar are sometimes seen in the town of Chernobyl where we are staying, where they churn up the orchards of unoccupied houses. This method of feeding - basically, using their snout as a plough - can make them highly radioactive, as most contaminants are now located in the soil.
We have not seen boars or wolves, but have heard a few stories about them. A pack of wolves is sometimes seen on the territory of the power station itself. Workers at a nuclear fuel storage site, currently under construction, say wolves killed a boar right by their perimeter fence just two weeks ago, and sat there eating it for days.
In another story, told by a driver, the killer was a man - a friend of his who went hunting in the zone and shot a 180kg boar. It's hard to get a licence to hunt in the zone but, according to our driver friend, poaching is widespread.
A moose spotted in the zone
Black storks and white-tailed eagles are among the birds that have made a comeback. We hope to talk to at least one expert studying animals in the zone this weekend.
On our agenda today: an exploration of the town of Chernobyl and the abandoned town of Pripyat, built for workers at the nuclear plant, but uninhabited for 20 years. We hear it's turning, literally, into an urban jungle.
30 March: 1900 (1600 GMT)
We find some elderly people who were evacuated from the zone in 1986 but insisted on returning.
In theory, the zone of alienation is also a zone of "mandatory resettlement" but the Ukrainian authorities recognise it's hard to keep old people out of homes they have lived in all their lives.
One 78-year-old woman says she wishes for just one thing - that her family would follow her back.
This woman would like her family to return to the zone, too
She has not seen her son's family for more than a year - it's hard for them to visit her from Kiev, because they have no car and there is no public transport.
We decline the offer of something to eat, though not because the food would be contaminated - she buys most of it from grocers' vans that visit her village of 30 people twice a week.
A slightly younger couple seem more content, and gladly show us their smallholding: chickens, a pig, some land sown with winter wheat, and other patches for potatoes and oats.
Officials check their crops, and the water from their well, and have apparently given them a clean bill of health.
Of more concern may be the wood they burn to heat their cottages. Some trees absorb a lot of radiation, and it is said to get concentrated in the ash.
On the way back to the hotel we spot two moose resting in the snow about 50 yards from the road, half hidden by birch saplings and the odd fir tree.
We stop the car and look at them. They look back, hoping we'll go away. We don't, so they get to their feet, and a few moments later they bound off into the dusk.
1700 (1400 GMT)
What to do with more than a million tonnes of radioactive waste is one of the many problems confronting the Chernobyl authorities. A lot of it was buried quickly in the aftermath of the disaster.
About 460 burial sites have been properly identified and studied. Some of these have to be moved because they are too close to the Pripyat river, and occasionally get covered by spring floods.
Some of the radioactive machinery will be decontaminated, some buried
Another 500 or so burial sites have yet to be found, 20 years after the disaster.
Experts know roughly where they are, but not precisely, and they don't know what they contain.
We walk through a graveyard for contaminated machinery, armoured personnel carriers, cranes and lorries.
Some of these, ultimately, will be decontaminated and disposed of. Others will be buried - but the one site where they can be safely buried without contaminating the groundwater is nearly full.
1500 (1200 GMT)
First stop, the Red Forest. So called because the pine trees went red and died in response to a massive blast of radiation.
A wide variety of animal species has benefited from 20 years of peace and quiet in the zone
They were scooped up and buried, along with tonnes of contaminated soil, but the area - which has now sprouted a new generation of trees - is still very "hot" in places.
Standing on the road, rather than inside the forest, our dosimeters momentarily register a dose rate of two micro-sieverts an hour.
You soak up more radiation than that flying in an aeroplane at a height of 1,000m.
LIFE IN THE ZONE
29 March: 1200 (0900 GMT)
The director of the new plant, Nikolai Fridman, talks about the future for the industry
We are back in the zone for the next two days, aiming to pursue some of the stories readers suggested.
We will try to talk to someone who refused to live outside the zone, and who defied the government by returning home to live on contaminated land.
We will see how nature is reclaiming the town of Pripyat and the evacuated villages, and we will see what the contamination has done to the environment. We also hope to see some of the wildlife that, we hear, is flourishing here in the absence of human interference.
Our hotel in the town of Chernobyl where all visitors to the zone are put up is a yellow building brought from Finland in 1986. It is warm and there is a comforting smell of cooking food. Outside, snow is in full meltdown... Lucky we brought our Wellington boots.
SHINY NEW REACTOR
1600 (1300 GMT)
View from the road en route to the Rivne nuclear power station
We travel for 350km along the most direct road from Kiev to Warsaw - just one lane in each direction, pockmarked with potholes. On either side, birch and pine and a forest floor of snow. Many villages seem not to have gas, judging from the stacks of firewood.
The older cottages are made of logs, and we pass the odd horse and cart, but finally we arrive at the nuclear power station.
If the Chernobyl control room had something of a 60s retro feel - grey enamel surfaces, big plastic buttons, dim lighting - the bright magnolia-coloured control room of Rivne's fourth reactor is unmistakeably 21st Century. Several computer screens, covered in pink and green symbols, stand in front of every desk. (The software is Ukrainian, we were told.)
Security at Rivne is pretty tight - no photography is allowed - and the staff exuded an air of efficiency and discipline. None of the operators was smoking at the controls, anyway, as at least one was on our visit to Chernobyl.
The director of the plant, Nikolai Fridman, does not want to knock the kind of reactor which blew up at Chernobyl, the RBMK as it is known.
He points out that Russia is successfully using 10 of them to this day. Yes, his pressurised water reactors at Rivne are safer, but this is also a fairly old design. The next two reactors built at the plant will be of a new generation, possibly a type he refers to as "Fast Neutron".
Stephen and Phil at the Chernobyl plant
He's so busy thinking about the future, I ask if he ever remembers Chernobyl. "We think about it, we talk about it, we draw lessons from it. We study it in institutes, and we are not the only ones," he says.
For a moment I see Chernobyl from the perspective of the nuclear power community - not (or not only) as a environmental and humanitarian disaster - but as a threat to the future of the industry.
When the world's atomic power people get together they discuss Chernobyl, Three Mile Island and Windscale, Mr Fridman says, and agree that such incidents must never be repeated.
0800 (0500 GMT)
Entering the zone
Today will be a vision of the future, unlike yesterday's trip back in time. We're off to visit a new reactor that started generating last autumn. Foundations were laid in the 1980s, but work stopped after Chernobyl - the last thing most Ukrainians wanted was more nuclear power stations.
But a few years later, after the collapse of the USSR, Russia announced that oil and gas would no longer be supplied free of charge. Ukraine decided nuclear power was good after all, and the reactor was completed. At least a dozen more reactors are planned in the next 25 years.
ZONE OF ALIENATION....
28 March: 1900 (1600 GMT)
We'll return to the zone for two days and a night later this week, and we are likely to meet some of the staff when we visit Slavutych, the plant's new dormitory town built from scratch after the disaster. But we won't be going inside the plant again.
One strong impression: decommissioning a nuclear power station is not easy, especially when you have nowhere to put spent fuel and have to rely on international donors for cash. But it's as nothing compared with the task of making safe the wreckage of a ruined reactor.
The task at the moment is actually not so much making it safe, as making it safe enough for future generations to deal with. The hope is that in 50 years' time, people will have a better idea of what to do with the mess.
1500 (1200 GMT):
The Chernobyl plant had four reactors. It was the fourth that exploded, and which was later covered by a sarcophagus of steel and concrete. We have just visited the control room of the first reactor, and talked to staff manning the controls. The reactor has not generated electricity for 10 years, but the nuclear fuel has not been removed, and the safety and cooling systems are still in operation. It's like a patient on life support.
In a viewing room overlooking the forbidding gunmetal-grey sarcophagus, we get our first noticeable dose of radiation. Actually, we don't notice it, but our dosimeters do, notching up about two micro-sieverts in the space of 40 minutes. We had almost begun to think they were not working.
1000 (0700 GMT):
We're in the zone, heading for the plant. In our pockets we have radiation metres (dosimeters), a bit smaller than a packet of cigarettes.
The zone is an area the size of Greater London where ordinary life came to an end 20 years ago. Inhabitants were evacuated, checkpoints and fences went up, and nature took over. It's full name, literally translated from Ukrainian, is Zone of Alienation.
When I last did this drive in 1993, the overgrowth was beginning to engulf an abandoned village on the main road. It was probably waist high. Since then birch trees have shot up everywhere. The village is already becoming woodland again. In a month, when the trees are in leaf, the houses will not be visible at all.