55. The previous United llations study on nucfear veapons contained an analysis of the cost of the acouisition and further development of these veapons. Since then some further stud.ies vhich provide data about the availability of nuclear technology have become available. notably from the Internationa"l Conference on Ivuclear Power and Its Fuel Cycle held at Sal-zburg " Austria, under the auspices of the Internationat Atomic Energy Agency (f977) ana the Tnternational Fuel Cycle Evaluation (IWFCE, 19BO). These studies have been utilized to update the nrerrinrr< qra l \fcac O / Q/ FiEr.ro" quoted from "Tactical lTucl-ear I'treapons: European Perspectives', edited ty STpnt (mndon, Taylor and- Francis" 19?8). 9/ Sone other ltcrks in the :bundar-t literature cn Lssues relzted to nt'-:i-edr prcliferaticn have ;lsc beicit r;-s':d in lrt'Ia-rir:t this section' Of particular irnportance is the report entitled 'T{ucl-ear Proliferation and- Safeguardstr by the Congress of the United 3tates (Office of TechnoloFy Assessnent l!97'l). 1.."al"sl",a, English Annex Page 23 57. To be a nuc lear-r"reapon State a nation must necessarily possess an expfosived.evice based on the nuclear fission of either uranium or prutoniur,r. urani,mu asfound in nature. is a mixt,re of several isotopes vhich differ rry only aboutl per cent in welght. but greatly in nucl"ear properties. The fissile isot,opeurariun-235 forns only o"T per cent of natural uraniurn, the rest being uran ium-238" Uraniurn-238 is iissile only by very high-energy neutrons and cannot beused to rnake a fission !'eapon. fhe uranium- 235 fraction must therefore beincreased in an isotope enrichment facility to in principre rnore than 5 per cent.
**Analysis**
The text appears to be a scientific or technical discussion about nuclear weapons, focusing on the cost of acquisition and development, as well as the properties of uranium isotopes. It seems to be a factual and informative piece, providing data and references to support its claims. Given the technical nature of the content, it is likely that the text is related to a field of study or research.

**Category**
Science and technology