system
Given the following post, respond as if you're commenting on Reddit:
human
Chernobyl physics questions. Why did they turn off the pumps?

This is a questions on physics and the events leading up to the Chernobyl disaster. Also concern nuclear energy etc. Sorry reddit keeps rejecting this post so trying to cover all bases.

Apologies if this has been asked before or this is the wrong subredit, but here goes:

From my reading on the events leading up to the disaster the plan was to check if the turbine had enough latent pressure to power the pumps in the event of power failure whilst waiting for the diesel generators to be up to speed. Given this, it seems in the buildup to the disaster they actually did cut steam to the turbine and see if the pumps worked until the diesels were online, and thus reduced coolant at the critical moment. I understand it wasn't the sole cause but it was a factor.

My question is since it was known how much power the generators produced (5.5mw if memory serves), and they must have had some means of measuring turbine output. Could they not have just carried out the test without actually cutting off the power to the pumps? As I say I know there were many other contributory factors but this seems an easy way to test the turbine capabilities without risking anything? My reading is listed to Wikipedia/various documentaries and Web sources, so hopefully someone can fill me in. I understand that no test like that is the same as real world, but surely carrying it out 'my way' then applying a risk factor would work? Thanks :)
gpt
So lets start from the top.

This test was to verify the coastdown effect on the reactor coolant pumps following a loss of offsite power with a load reject turbine trip. 

The test was specifically designed to ensure the reactor was in a safe operating region for the test. If the test plan were followed, there would have been no issues as it was completely pre analyzed.

The coastdown time of the RCPs is a critical input into reactor safety analysis. BWRs simply utilize natural inertia of the RCPs for safety. PWRs will use large flywheels to ensure sufficient coastdown time. In this case, the RBMK design was utilizing generator coastdown and decay to support the coastdown time.

You cannot simply look at the turbine output. After the turbine trip, you are going to have coastdown. All the major pumps on that bus will actually oppose the coastdown, and help extend the coastdown time, in addition to the generator coasting down using residual inertial from the turbine generator. This test data requires your loads to remain on the bus in order to get an accurate depiction for how voltage/frequency decay is going to occur. Today you can model it pretty closely using computer models, but in the 70s/80s you didn't have the same in depth models, and you had no physical data to validate the theoretical data, so you had to do the actual test. 

Cutting off power to the reactor coolant pumps did put the reactor into the restricted operating region. All water reactors where there is active boiling have instability regions when they are at high power with low reactor coolant flow. What happens is the water flashes to steam, and because of insufficient flow, the steam doesn't get pushed out of the core, and instead, acts to further lower coolant flow by creating back pressure. In a BWR, this causes power to go down dramatically, which causes boiling to stop. Then natural circulation picks up again and power rapidly increases. This creates core oscillations which can cause fuel damage if left unchecked.

In an RBMK reactor, because they have a positive void coefficient, the worst case is that the steam and back pressure cause power to further increase, which makes more steam, more heat, more power. To ensure this does not happen, the RBMK had a safety limit on a minimum number of inserted control rods to ensure that the reactor was slow enough that natural feedback mechanisms like fuel heatup could counteract the voiding effect, and buy time for an automatic or reactor operator scram if the condition didn't get better.

The reason this accident occurred, was because the operators violated the test plan and intentionally put the reactor into the restricted operating zone before even performing the test, while simultaneously violating their safety limit. Tripping the main turbine and offsite power would have been safe in the pre-analyzed region, but they weren't in the analyzed region, and it did make the event worse. Having all the rods out past the safety limit also made the scram worse, as if those rods had already been partially inserted as pre-analyzed, there would have been very little or no positive reactivity insertion.

The bottom line, is that multiple things together compounded the prompt reactivity spike by the graphite tipped control rods, leading to the power excursion and near immediate core damage.