Can the amount of heavier elements in the Sun's core be approximated? Can the amount of heavy and other types of elements as impurities inside the Sun's core be approximated?
If these elements are present, do they disturb the process of hydrogen fusion, since they occupy the center, which is the place of maximal pressure necessary for the fusion process? In particular, do these elements affect the rate of hydrogen fusion?
 A: The most recent effort to establish current elemental quantities in the sun’s photosphere that Im aware of is this article from 2003: https://iopscience.iop.org/article/10.1086/375492/pdf
Table 1 on page 3 shows the estimate for the current sun, and later tables are protosolar. The first number is roughly natural log (For anyone new to that, think of it as someone reporting the exponent to you, giving you something akin to order of magnitude). The method used is indicated in about the fourth column. Anything with an a or an s has been verified as being present. I was surprised to see something so heavy as Rn (atomic number 86) in there and verified, albeit in minuscule amounts.
The order of elements for the first eight by number of atoms is; H, He, O, C, Ne, N, Mg, Si. Anything beyond that is over five orders of magnitude down. The photosphere is about 3/4 H, and is estimated to be 98.7% H and He, and the core is roughly that same total, but the H is higher as we go toward the center. At the core’s radius (at r= 0.2R), it is still approximately 70% H and decreases to about 1/3 H at the center. Source: http://solar-center.stanford.edu/helio-ed-mirror/english/engmod-res.html
Of note is the fact that the core is plasma, so no molecules are present. This is important. Chemical and moleculardynamic interactions slow nuclear reactions more than plasma does. There are no covalent bonding dynamics and far less molecular collision as the diameter of an atoms nucleus is as low as a millionth of the diameter of the same atom with its electron clouds. That said, nuclear reactions can still occur.
Aside from the main hydrogen fusions in the core (p-p I and to a much lesser extent p-pII), there is one called CNO which is almost two percent of the He production, and greater even than p-p III. Its primary catalysts are O, C, and N. However, most of the elements other than H and He, even in the core, are from the early universe. Hence they will be close to the proportions in the earth and especially the photosphere.
Actually, beyond O, C, Ne, and N, the quantities are low. The Mg content is around 8% of the O content. So one key question is whether around O and C in a plasma mixture would impede fusion. Based upon the effort needed to collide a proton into an oxygen by these researchers: https://www.worldscientific.com/doi/abs/10.1142/S0218301316500609 I would say the proton cross-section of nuclei that size is negligible. The next step would be to check proton cross-sections for O16 etc as listed. Such as by accessing articles like this (paid): https://inspirehep.net/literature/107753
The reasoning here is that we know how much CNO is happening. We dont know how many other protons C and O are absorbing. Hence cross-sections could help. My guess is that they are only aiding fusion overall.
Similar sized nuclei are in the hundred millibarns range. While I havent done a calculation, at this point Im guessingg it wouldnt matter much, but “much” depends on research application and/or opinion, and is just an educated guess for now. And furthermore I think if anything the heavy elements overall are helping fusion.
