“‘If you have the infernal charlatanry to tell me that a small container the size of a- of a walnut, blast it, holds a nuclear generator, I’ll have you before the Protector in three seconds.’
‘Explain it yourself then, if you can. I say it’s complete.’
The tech-man’s flush faded slowly as he bound the chain about his waist, and, following Mallow’s gesture, pushed the knob. The radiance that surrounded him shone into dim relief. His blaster lifted, then hesitated. Slowly, he adjusted it to an almost burnless minimum.
And then, convulsively, he closed circuit and the nuclear fire dashed against his hand, harmlessly.”
Foundations by Isaac Asimov, page 212
One of the main themes expressed in Foundation is the stagnation of the Empire, which sees no further need to innovate or develop technologically, having already reached its zenith of power and fallen. The Empire does not move on from its belief that bigger is inherently better, so the centuries used by the Foundation to advance technologically are simply used to maintain the enormous nuclear generators and ships created long before.
Mallow's personal force shield, powered by a nuclear generator the size "of a walnut" is representative of an enormous amount of necessity-driven development on metal-poor Terminus. It is not a great stretch of the imagination for today's society, though. Perhaps all modern society needs to accomplish this same type of feat is a similar motivation. If humanity continues guzzling irreplaceable resources, that day could be soon.
Nevertheless, Mallow's device seems fantastical for two reasons. Firstly, we are still in an international culture that believes size is proportional to perfection. We believe that bigger is better, like the First Galactic Empire. The exceptions to this general rule are our increasingly small personal devices, like laptops and phones. Such changes hint at the difference between our mindset and the Empire's: we are still innovating. The fact that we have not fully applied the ideas of shrinking and improving to nuclear generators does not mean it is impossible. A walnut-sized nuclear generator could well be possible- we certainly have ones small enough to safely power individual homes. The second piece that makes the device seem impossible is the force field aspect. Such gadgets are preeminent tools in many works of science fiction, but have yet to be created by humanity. We are not sure of how often this has been tried.
In conclusion, we believe that Mallow's device may well be feasible, but our knowledge of technology is not yet great enough for force fields, let alone personal force fields powered by nuclear generators. Whether we are accurate or not in this judgment can only be learned through research.