CHina's 4th gen "gas-cooled pebble bed" reactors

[dan_b]

Any nuclear engineers on here able to explain what these are and how they're different/better to the PWRs we're all kind of familiar with?

https://www.powermag.com/china-starts-u ... r-reactor/

[nowty]

Differences are uses Helium gas as the active coolant media, therefore inert and reduces chemical reactions within the reactor. No water in the reactor so less chance of turning into explosive hydrogen. Uses graphite as the moderator, that does burn and can crack easily though. Needs to operate at a higher temperature because your using a gas as the coolant media.

Lower power density so things go bad in slower time.

The major improvement in safety is that it does not require active cooling to prevent meltdowns in a shutdown state. Can cool through simple conduction and thermal radiation.

[Countrypaul]

Any nuclear engineers on here able to explain what these are and how they're different/better to the PWRs we're all kind of familiar with?

https://www.powermag.com/china-starts-u ... r-reactor/

A quick read suggests that they use Helium rather than water as the cooling/thermal transfer agent so, I guess, much more like an AGR but higher temperatures (750C or even 950C compared to 650C for an AGR). It seems an interesting approach if it works efficiently and can be scaled up, the higher proposed temperatures also claim to make hydrogen from water thermally a future possibility.

[dan_b]

DIdn't we do gas-cooled reactors in the 70s? They used carbon dioxide?
Helium's not exactly a hugely abundant gas though thought we were running out of the stuff globally?!

[Moxi]

Hi Dan,

Basically the main difference between the pebble bed and the PWR is the coolant temperature is much higher on the gas cooled circuit. The same higher temperatures are seen in the AGR fleet that is operating in the UK, however, where we used CO2 for the coolant in AGR's which we subsequently discovered, damages the moderator and the reactor gas baffles over time at the higher operational temperatures the Chinese are planning to use an inert gas (Helium not sure what the daughter products would be for Helium under Neutron bombardment - I will look it up later).

PWR's and BWR's use water as the coolant and moderator and can only operate at low temperatures relative to the boiling point of water at a given overpressure.

In a water moderated reactor if you get the control wrong you can potentially flash the water to steam and get an explosion like Chernobyl.

Pebble beds have eluded the engineers and scientists for many many decades now and its highly likely that these experimental reactors will have their ups and downs. I can see issues with coating degradation of the fuel if the gas flow is too high / turbulent and I am sure there's much more that they will be interested in monitoring at the scale.

of course higher temperature coolant does mean you can burn the fuel hotter and that's got a different purpose, not sure what international checks and balances are in place for that ?

Moxi

[Paul_F]

Any nuclear engineers on here able to explain what these are and how they're different/better to the PWRs we're all kind of familiar with?

https://www.powermag.com/china-starts-u ... r-reactor/

Essentially it's a big hopper with fuel pebbles dumped in at the top and coming out the bottom. Two key advantages over PWRs:

1. No water. Quite apart from the safety benefits of not having a coolant which changes phase, this also means you get quite a bit more electricity out for the same amount of heat generated.
2. You remove individual fuel elements one at a time for refuelling, rather than all at once. This means more uptime on the reactor, higher burnup on the fuel (less high level waste) and potentially a way to dispose of actinide (long-lived) waste through burning it up with excess neutrons.

Naturally it also has a bunch of it's own problems, and PWRs have the huge advantage of lots of reactors out there with vast numbers of running hours so the problems are very well understood. I really like the Pebble Bed type of reactor, but don't get too excited about it yet - it's still fundamentally a nuclear reactor and still has most of the same problems as others.

[Moxi]

Interestingly in the article it looks like the fuel is loaded in to trays within a fuel assembly ? They note that individual modules can be taken offline for refuelling etc.

It will be interesting to follow their progress.

Moxi