My brother, an environmental auditor gave this surplus meter that his employer has decommisioned. Last calibration - 88 ish...

It is alive, it was making lots of the typical trttttrtt noise and the display liy up, scaring the shit out of me before I realised that it is ranged uRoentgen /h and readout was  less than 1 uR/h
 Not terrible, not great...

But apart from a faulty nuclear reactor, what can I check it with? I live in a part of Sweden where the bedrock is 30m below my feet, covered in clay, soil and sand.

Would it detect anything but baseline in a rocky formation with billion year old granite? Or do I have to find a sample of something?
Calibration is not likely to be done but I would like to see that it is responding to incomig ionizing radiation.

I have quite a large xformer pulled from  a forklift battery charger

It consists of an ordinary toroid of about 10+ kgs , and an inductor in series with primary.

What is so peculiar is that applying 230VAC ( it s probably old enough to be rated 220V) it goes into partial saturation, idle current is about 0,4 Amps.
Voltage across inductor is about 15VAC.

I presume it is designed this way to enable the charging of a fully empty battery, which pulls a lot of current, causing quite a voltage drop across the inductor impedance and hence limits current until battery has absorbed enough charge to raise the voltage and charging current decreases?

The charger only contained a thermal switch breaking primary voltage and a couple of beefy electrodes. And some on-lights and an amp meter of course.

Sadly this means that the transformer is useless as connecting it directly to 230VAC will probably saturate it fully or very closely so idle current is in the amps range. It hums and burrs...

I could always add primary turns to it outside the secondary but it is a bit of hassle.

Any other proposals apart from the obvious to use the inductor in series as designed?

On the topic of arguing on the internet, especially with strawmen, can only end one way...

I wish I had streamers coming of it, but I will have to be satisfied with a LED light l8er tonite.
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I have a couple of huge transformers (2kVA 20*2V) -C-core that is.

but it is the normal type, ie not possible to install magnetic shunts. I have  some large inductors (reactors)
I have humungs large rectifiers (400V 200A *16pcs) and heatsinks to match. Might even have some thyristors fitted for this...

A simple setup would be to use it as a current limited output for a MMA (stick) welder, AC or DC.

But it is difficult to adjust the output current this way.

 Also, the circuit needs to be designed with quite a large inductance in the reactor to compensate for the large open circuit voltage used for arc striking (about 50V+) compared to operating voltage on arc (about 15-25V)

One option is to use one of my even humungouser variacs (25A, ie 5kVA) to adjust output voltage and hence current draw as the welding arc voltage is not so affected by current through it.
This leads however to a less than mobile equipment. All together it packs up to 60 kg+ without casing. Or weelbarrow...

I never intended to have a inverter style portable welder but what options are there to replace the variac? How to build a variable  inductance for current limit easy way?
I have also some iron cores EI-150 size etc, and a good set of angle grinders

What ideas to use for increasing the voltage for arc striking but still having a low voltage output?

In your opinion , is it necessary to use equlizing resistors parallell to diodes if you series connect the diodes to give a higher voltage rating?

How much should one derate the diodes, is 50% sufficient?