The Chernobyl plant had four reactors - the fourth 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.

5 April: 1930 GMT

We are back in the UK after a 3.5-hour flight from Kiev, part of which we spent playing with our radiation dosimeters.

At one point we registered one of the highest readings of the whole trip - three micro-sieverts per hour.

This was higher than the two micro-sieverts we clocked on the road running through the once-forested area next to the Chernobyl plant (the Red Forest) though quite a bit less than the 11 micro-sieverts per hour radiating from a contaminated tank in one of the vehicle graveyards.

Overall, from the morning of Tuesday, 29 March to mid-afternoon on 5 April - about eight-and-a-half days - we soaked up 22 micro-sieverts of radiation.

During this short flight we soaked up four.

The operation on Anna Alexeyevets was a success, doctors say

Four micro-sieverts is also the dose we got on our first full day in Ukraine, when we spent several hours in and around the Chernobyl plant, and about an hour just a stone's throw from the sarcophagus.

No wonder it is said that airline crews are usually exposed to more radiation than staff at nuclear power plants.

Before we left Kiev, Phil photographed an operation at the Institute of Endocrinology, where Anna Alexeyevets, one of the patients we spoke to yesterday, had her cancerous thyroid gland removed.

The cancer was diagnosed early and the operation was a success.

Anna will need to take pills from now on to keep her hormone system running smoothly.

Apart from this - according to the surgeon, Professor Igor Komissarenko - she will lead a normal life.

5 April: 0800 (0500 GMT)

Anna Alexeyevets is awaiting an operation for Thyroid Cancer at the Institute of Endocrinology in Kiev

Twenty years after the Chernobyl disaster, there does not seem to be any consensus about the lessons to be drawn.

There are disputes about the cause of the explosion, and the amount of contamination, about the numbers killed, the list of illnesses that can be blamed on radiation, and how many people to compensate.

There are also disputes about how to make the sarcophagus safe and what to do with the contaminated lands.

And there is startling amount of cleaning up still to be done.

A huge operation was mounted in the first years after the disaster, then stagnation appears to have set in.

Take the graveyards of contaminated vehicles. Thousands of lorries stand in rows, usually with their bonnets open.

A helicopter at an equipment dump - hundreds of contaminated vehicles still await disposal

In many cases, the engine has been removed (for transplantation, one can only assume, into uncontaminated lorries) while the rest lies untouched. Salvaging engines has taken priority over decontamination or disposal.

As the main difference between the graveyards of 2006 and the photographs of 10 years ago is that the helicopters have lost their rotor blades, one cannot help wondering if they have some value as scrap.

These vehicles are not really a serious problem by the zone's standards, but they illustrate a general lack of urgency surrounding radioactive waste. At least, that is how it seems to us.

Both the Ukrainian Government and donor countries devote more attention to the sarcophagus and the problem of how to dispose of spent nuclear fuel. But delays and mistakes are being made in these areas too.

No-one would ever have expected there would be much to rejoice about on the 20th anniversary of the world's worst nuclear accident.

But people who like to look on the bright side can take comfort in the natural rebirth of the zone.

A worker at an equipment dump checks the levels of radiation from metal gathered in the zone

"By rendering this vast landscape uninhabitable for people, Chernobyl has allowed nature to make a comeback," the American-Ukrainian author of a natural history of the zone, Mary Mycio, told us last night. "Radiation is good for animals, in so far as it gets rid of people."

There is some hope that the incidence of thyroid cancer caused by the disaster will soon peak and begin to tail off. This is the one disease that everyone accepts has been caused by Chernobyl radiation.

Since 1992 there have been about 3,300 cases in Ukraine among children who were aged between 0 and 18 at the time of the disaster. That is between 30 and 50 times more than would have been expected.

According to received medical opinion, children who drank milk contaminated with radioactive iodine in 1986 generally ought to stop falling ill with thyroid cancer around 2006.

There is no guarantee that this will be the case - the same received opinion said they should not start falling ill until 1992, whereas in fact the process began earlier. But there is hope.

What is certainly good news, is that the vast majority of child thyroid cancers have been successfully treated.

"Hundreds of young girls that we have treated are already having their own children," says the Director of Ukraine's Institute of Endocrinology , Nikolai Tronko.

One of the last things we do before heading for the airport will be to watch one of these operations taking place.

4 April: 1530 (1230 GMT)

On the night the Chernobyl disaster occurred, an experiment was being carried out on the fourth reactor, before it was shut down for routine maintenance. Most explanations of the disaster say that the experiment caused the explosion.

The experiment - a test to see how long the turbines could continue powering the cooling system in the event of a loss of power from other sources - went wrong, the theory goes. The reactor slipped out of control, and when an operator hit the panic button the explosion followed instantly.

However, a former safety officer at the plant, Mykola Karpan, has written a book arguing that the experiment was a success. The reactor was not shut down in panic, it was shut down because the experiment was over.

Mykola Karpan says all water pressurised reactors are dangerous

Mr Karpan accepts that the experiment was a precursor to the accident, and that it was the shutting down of the reactor that triggered the explosion. But he says the design of the reactor was so flawed, it could have blown up during routine operations too.

One of the most sensational documents in the book has been published before. It is the transcript of a session of the Soviet Communist Party's top body, when it met more than two months after the accident, on 3 July 1986.

The politburo recognised that the reactor's poor design caused the disaster, and the designer begged for the chance to "correct my mistake".

His wish was granted, and a range of safety changes were subsequently made to all reactors of a similar design, known as the RBMK.

But two weeks later the party newspaper Pravda blamed the operators. It accused them of "serious violations" and said nothing about the design flaws.

RBMK reactors are still used in Russia and Lithuania, though the standard reactor in Russia is now a version of the pressurised water reactor.

As it happens, Mr Karpan is uneasy about this reactor too, describing it as a champagne bottle in constant danger of popping its cork. He doesn't just mean the Russian pressurised water reactors, but the Western versions as well.

3 April: 0000 (2100 GMT)

We spoke to a woman this evening who lived in Pripyat, the town next to the Chernobyl power station, and was five months pregnant when the disaster occurred. Because she was spending the weekend at her parents' house, in a village a few kilometres from the power station, she did not get evacuated with the rest of Pripyat's inhabitants.

Lena's daughter suffers from a rare blood disease

Pripyat was evacuated the day after the explosion, Sunday 27 April. Her parents' village, by contrast, was not evacuated until 4 May. The Soviet state loved workers, but had mixed feelings about villagers, or "peasants".

Lena heard about the accident less than 12 hours after it happened, but assumed it was no more serious than other accidents that had occurred at the plant in earlier years. No other information was available.

Only on the afternoon of the 27th did a policeman tell her family, privately, that pregnant women should get away, as quickly as possible. The police were not allowing people to use the main road, but her father-in-law knew all the back roads, and was able to drive her to a town some 30km away.

We'll tell the full story another time. Suffice it to say that she gave birth to an unusually large premature baby two months later. Her daughter, now aged 19, suffers from an extremely rare blood disease. Her immune system is weak, and almost any infection could lead to serious complications. In 2004, she almost died of meningitis.

Is there a link between the girl's illness and the pregnant mother's exposure to intense radiation? It would be hard to prove, though the girl's illness is so rare that Ukrainian doctors are inclined to think there is.

For the time being, she gets some financial support from the state. However, the Ukrainian government has been trying to curb the huge sums it pays to Chernobyl victims and liquidators, some of whom are fakes with false CVs and medical histories, engaged in benefit fraud.

The mother fears that once the 20th anniversary has come and gone her daughter, and many other genuine sufferers, will be quietly forgotten.

1630 (1330 GMT)

Yesterday we spoke to an expert on the wildlife of the Chernobyl zone, who surprised us by saying that animals did not seem to be too bothered by the present level of radiation.

He said he had searched for rodents in the sarcophagus, and had not found any - but he put this down to the absence of food rather than the presence of the reactor's highly radioactive remains. Birds nested inside the sarcophagus, he said, and did not appear to suffer any adverse effects.

Today we spoke to a doctor treating leukaemia patients. Is there a link between leukaemia and Chernobyl radiation? The jury is out.

A study is under way, which should shed light on the question, but it won't finish until 2009.

The patients in the hospital had no doubt, however. Of course there is a link, said an electrician who worked in the zone for two weeks in May 1986 and now has chronic lymphocytic leukaemia.

Students preparing to work in the nuclear sector had mixed views

We also went looking for students studying for degrees that will enable them to work at nuclear power stations.

I asked two of them, aged 17 and 18, if they were worried about safety at nuclear power stations, considering the appalling consequences of the Chernobyl disaster. One said no, the other said yes.

Curiously, the first student I stopped coming out of the faculty building was Iranian. He said 10 Iranians were taking the course, and that the main motivation was the good pay on offer at Iran's nuclear power station.

His parting shot was far more surprising: if he graduated with good marks, he said, he might try to get a job in Canada.

Stephen and Phil's Chernobyl diary started on Tuesday 28 March.