In order to put a positively charged particle inside an atomic nucleus, there is need to perforate a Coulomb barrier impossible to be perforated under conditions of low temperature and pressure.

For the supposal that cold fusion is able to occur under low conditions of pressure and temperature, there is need to consider that Coulomb repulsion is not so strong as it was considered up to now by nuclear theorists.

On another hand, to consider that cold fusion can be promoted by quantum tunneling is also impossible, because it introduces other puzzle:  if Coulomb barrier can be crossed by quantum tunneling at room conditions of low pressure and temperature, then, in the conditions of extremely high of temperature and pressure in the stars, cold fusion had to occur through a tax very higher than that occurring by hot fusion in the stellar nucleosynthesis. And therefore the hydrogen fuel of the stars would had to be had consumed billions years ago, and the Universe would be populated by cold giants.

So, any theory candidate to explain cold fusion must be able to conciliate the cold fusion occurrence in the laboratories with the hot fusion occurrence in the stars.  

In another words, there is need a theory able to respond the following question:  what differs cold fusion from hot fusion, and why cold fusion does not occur in the stars?

The answer for this question lies in the concept of electric field.
Along a century, the theorists never proposed a physical model of electric field, where the lines of force should be composed by the fundamental particles of the quantum vacuum.

No one undertook such a challenge, because scientists do not look for physical models, and the reason is because physical models composed by speculative particles of the quantum vacuum is a conjecture not allowed in the Scientific Method.

A physical model, whose line forces are composed by gravitons and electricitons, is proposed in the paper “Qn how Bohr model of hydrogen atom is connected to nuclear physics”:
https://fundamentaljournals.org/ijfps/article/view/ijfps.2018.330113/144

This model has some very interesting properties, as shown in that paper, and they are:

1- First property:
 The electric repulsion between two positively charged particles follows a new Coulomb law F= K.Q.q/(d^x), where x<2 when the distance between the two particles is shorter than 1000fm, and x<1 for distances shorter than 1,0fm.

Therefore, inside the atomic nuclei, the Coulomb repulsion is not so strong as the nuclear theorists believed.  And this means that there is no need the strong nuclear force, so that to keep protons and neutrons bound inside the atomic nuclei.

2- Second property:
The Coulomb repulsion depends on the velocity of the particles.

3- Third property:
The Coulomb repulsion also depends in the direction of the motion. 

For instance, in the U238 decay the alpha particle is leaving the nucleus, and the Coulomb repulsion with the newborn 234Th is weaker than the Coulomb repulsion when an alpha particle emitted by Po212 moves toward the nucleus U238, as happens in the Rutherford experiment.

This third property of the electric field is put in the form of equations, in the paper "Mathematical Confirmation for the Nuclear Properties K(O)= K(Ca)= 1/2K(Si)= 1/2K(Fe)":
https://www.scifedpublishers.com/open-access/mathematical-confirmation-for-the-nuclear-properties-ko-kcarnksi-kfe.pdf

And the calculation shows that Gamow quantum tunneling theory is not necessary for the explanation of a paradox:

After leaving U238, an alpha particle enters into a potential with 23MeV. But when the particle crosses the electrosphere, it leaves the newborn atom of 234Th with kinetic energy 4,2MeV.

Such behavior of the alpha particle can be compared with the following situation: a brick with 1kg falls down from a building with 23 meters, and it hits your head as if it was falling down from a height of 4,2 meter.

The equations of the electric field composed by gravitons and electricitons solves the puzzle, without the need of the unacceptable quantum tunneling theory proposed by Gamow.

But the electric field model has a fourth property:

4- Fourth property:
The electric field is not spherical as the physicists believed.  It has the shape of rings, as those rings of Saturn.  But due to random spin motion, in normal conditions it takes the shape of an apple (almost spherical).

The electric field with Saturn Rings shape is shown in the Figure 27 of the paper "Mathematical Confirmation for the Nuclear Properties K(O)= K(Ca)= 1/2K(Si)= 1/2K(Fe)".

The electric field with Saturn Ring shape is tested by equations in the paper.
From eq. (27) up to eq. (49), the calculation shows that the fusion between two protons (with this sort of electric field) reproduces the same quantity of proton-proton fusion as they occur, by second, in the Sun.

And quantum tunneling is not considered in the calculation.

Up to now the nuclear theorists are yet refusing to accept that Rossi Effect is real.  
But when Rossi’s company starts to sell his Ecat, the nuclear theorists will be obliged to swallow their proud, and will recognize that cold fusion really exists.  
Then they will need to look for a theory.  

And the only theory with chance to explain cold fusion is the present theory, in which the strong nuclear force does not play any rule in the aggregation protons and neutrons inside the atomic nuclei.